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This document provides endorsements for the book "Thinking in C++" by Bruce Eckel. It contains quotes praising the book from four people: Al Stevens, Andrew Binstock, Gary Entsminger, and Richard Hale Shaw. All endorse the book for clearly explaining object-oriented programming concepts in C++ and providing answers to difficult questions about the C++ language.
![While static, when applied to a data member, definitely changes the way the data is
created (only one vs. the non-static one for each object), when applied to a method it’s not
so dramatic. An important use of static for methods is to allow you to call that method
without creating an object. This is essential, as we shall see, in defining the main( )
method which is the entry point for running an application.
Like any method, a static method may create or use named objects of its type, so a static
method is often used as a “sheperd” for a flock of instances of its own type.
Your first Java program
Finally, here’s the program5 . It prints out information about the system that it’s running
on using various methods of the System object from the Java standard library. Note that
an additional style of comment is introduced here: the ‘//’ which is a comment until the
end of the line:
// Property.java
import java.util.*;
public class Property {
public static void main(String args[]) {
System.out.println(new Date());
Properties p = System.getProperties();
p.list(System.out);
System.out.println("--- Memory Usage:");
Runtime rt = Runtime.getRuntime();
System.out.println("Total Memory = "
+ rt.totalMemory()
+ " Free Memory = "
+ rt.freeMemory());
}
}
At the beginning of each program file, you must place the import statement to bring in
any extra classes you’ll need for the code in that file. Notice that I said “extra.” That’s
because there’s a certain library of classes that are automatically brought into every java
file: java.lang. Start up your web browser and look at the documentation from Sun (if
you haven’t downloaded it from java.sun.com or otherwise installed the Java
documentation, do so now). If you look at the packages.html file, you’ll see a list of all
the different class libraries that come with Java. Select java.lang. Under “Class Index”
you’ll see a list of all the classes that are part of that library. Since java.lang is implicitly
included in every Java code file, these classes are automatically available. In the list, you’ll
5 Some programming environments will flash programs up on the screen and close them before
you've had a chance to see the results. You can put in the following bit of code at the end of
main( ) to pause the output:
try {
Thread.currentThread().sleep(5 * 1000);
} catch(InterruptedException e) {}
}
This will pause for 5 seconds. This code involves concepts that will not be introduced until
much later in the book, so you won't understand it until then, but it will do the trick.
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![see System and Runtime, which are used in Property.java. However, there’s no Date
class listed in java.lang, which means you’ll need to import another library to use that. If
you don’t know the library where a particular class is (or if you want to see all the
classes) you can select “Class Hierarchy” in the Java documentation. In a web browser, this
takes a while to construct, but you can find every single class that comes with Java. Then
you can use the browser’s “find” function to find Date, and when you do you’ll see it
listed as java.util.Date, which tells you it’s in the util library and that you must import
java.util.* in order to use Date.
Again looking at the documentation starting from the packages.html file (which I’ve set
in my web browser as the default starting page), if you select java.lang and then System,
you’ll see that the System class has several fields, and if you select out you’ll discover
that it’s a static PrintStream object. Since it’s static you don’t have to create anything,
the out object is always there and you can just use it. But what can you do with this out
object? That is determined by what type it is – it’s a PrintStream. Conveniently,
PrintStream is shown in the description as a hyperlink, so if you click on that you’ll see a
list of all the methods you can call for PrintStream. There are quite a few and these will
be covered later in the book, but for now all we’re interested in is println( ), which in
effect means “print out what I’m giving you to the console, and end with a new line). Thus
in any Java program you write you can say System.out.println(“things”) whenever you
want to print things to the console.
The name of the class is the same as the name of the file. When you’re creating a stand-
alone program like this one, one of the classes in the file must have the same name as the
file (the compiler complains if you don’t do this) and that class must contain a method
called main( ) with the signature shown:
public static void main(String args[]) {
The public keyword means the method is available to the outside world (described in detail
in Chapter 5). The argument to main( ) is an array of String objects. The args won’t be
used in this program, but they have to be there because they hold the arguments invoked
on the command line.
The first line of the program is quite interesting:
System.out.println(new Date());
Look at the argument: a Date object is being created just to send its value to println( ). As
soon as this statement is finished, that Date is unnecessary, and the garbage collector can
come along and get it anytime. We don’t have to worry about cleaning it up.
The second line calls System.getProperties( ). Again consulting the on-line documentation
using your web browser, you’ll see that getProperties( ) is a static method of class
System. Since it’s static, you don’t have to create any objects in order to call the method; a
static method is always available whether an object of its class exists or not. When you
call getProperties( ), it produces the system properties as an object of class Properties.
The handle that comes back is stored in a Properties handle called p. In Line three, you
can see that the Properties object has a method called list( ) that sends its entire contents
to a PrintStream object that you pass as an argument.
The fourth and sixth lines in main( ) are typical print statements. Notice that to print
multiple String values, we simply separate them with ‘+’ signs. However, there’s
something strange going on here: the ‘+’ sign doesn’t mean “addition” when it’s used with
String objects. Normally you wouldn’t ascribe any meaning at all to ‘+’ when you think
of strings. However, the Java String class is blessed with something called “operator
overloading.” That is, the ‘+’ sign, only when used with String objects, behaves differently
than it does with everything else. For Strings, it means: “concatenate these two strings.”
72 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-72-320.jpg)
![Documentation example
Here is the first Java program again, this time with documentation comments added:
//: Property.java
import java.util.*;
/** The first example program in "Thinking in Java."
* Lists system information on current machine.
* @author Bruce Eckel
* @author http://www.EckelObjects.com
* @version 1.0
*/
public class Property {
/** Sole entry point to class & application
* @param args Array of string arguments
* @return No return value
* @exception exceptions No exceptions thrown
*/
public static void main(String args[]) {
System.out.println(new Date());
Properties p = System.getProperties();
p.list(System.out);
System.out.println("--- Memory Usage:");
Runtime rt = Runtime.getRuntime();
System.out.println("Total Memory = "
+ rt.totalMemory()
+ " Free Memory = "
+ rt.freeMemory());
}
} ///:~
The first line:
//: Property.java
uses my own technique of putting a ‘:’ as a special marker for the comment line
containing the source file name. The last line also finishes with a comment, and this one
indicates the end of the source-code listing, which allows it to be automatically extracted
from the text of the book and checked with a compiler. This is described in detail in
Chapter 17.
Coding style
The unofficial standard in Java is to capitalize the first letter of a class name. If the class
name consists of several words, they are run together (that is, you don’t use underscores
to separate the names) and the first letter of each embedded word is capitalized, such as:
class AllTheColorsOfTheRainbow { // ...
For almost everything else: methods, fields (member variables) and object handle names,
the accepted style is just as it is for classes except that the first letter of the identifier is
lower case. For example:
class AllTheColorsOfTheRainbow {
int anIntegerRepresentingColors;
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![Keep this in mind any time you’re running a program that’s in a package.
Here’s the example:
//: Assignment.java
// Assignment with objects is a bit tricky
package c03;
class Number {
int i;
}
public class Assignment {
public static void main(String args[]) {
Number n1 = new Number();
Number n2 = new Number();
n1.i = 9;
n2.i = 47;
System.out.println("1: n1.i: " + n1.i +
", n2.i: " + n2.i);
n1 = n2;
System.out.println("2: n1.i: " + n1.i +
", n2.i: " + n2.i);
n1.i = 27;
System.out.println("3: n1.i: " + n1.i +
", n2.i: " + n2.i);
}
} ///:~
The Number class is very simple, and two instances of it (n1 and n2) are created within
main( ). The i value within each Number is given a different value, and then n2 is
assigned to n1, and n1 is changed. In many programming languages you would expect n1
and n2 to be independent at all times, but because you’ve actually assigned a handle here’s
the output you’ll see:
1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 27
Changing the n1 object appears to change the n2 object as well! This is because both n1
and n2 contain the same handle, which is pointing to the same actual object (the original
handle that was in n1 that pointed to the object holding a value of 9 was overwritten
during the assignment and, effectively, lost – it’s object will be cleaned up by the garbage
collector).
This phenomenon is often called aliasing and it’s a fundamental way that Java works with
objects. But what if you don’t want aliasing to occur in this case? You could forego the
assignment and say :
n1.i = n2.i;
This retains the two separate objects instead of tossing one and tying n1 and n2 to the
same object, but you’ll soon realize that manipulating the fields within objects is pretty
messy and goes against good object-oriented design principles. Because this is a non-trivial
topic, it is left for Chapter 12, which is devoted to the issue of aliasing. In the meantime,
you should keep in mind that assignment for objects can add surprises.
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![Aliasing during method calls
Aliasing will also occur when you pass an object into a method:
//: PassObject.java
// Passing objects to methods can be a bit tricky
class Letter {
char c;
}
public class PassObject {
static void f(Letter y) {
y.c = 'z';
}
public static void main(String args[]) {
Letter x = new Letter();
x.c = 'a';
System.out.println("1: x.c: " + x.c);
f(x);
System.out.println("2: x.c: " + x.c);
}
} ///:~
In many programming languages, the method f( ) would appear to be making a copy of its
argument Letter y inside the scope of the method. But once again a handle is being passed
so the line
y.c = 'z';
is actually changing the object outside of f( ). The output shows this:
1: x.c: a
2: x.c: z
Aliasing and its solution is a complex issue and, although you must wait until Chapter 12
for all the answers, you should be aware of it at this point so you can watch for pitfalls.
Mathematical operators
The basic mathematical operators are the same as the ones available in most programming
languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%,
produces the remainder from integer division). Integer division truncates, rather than
rounds, the result.
Java also uses a shorthand notation to perform an operation and an assignment at the
same time. This is denoted by an operator followed by an equal sign, and is consistent
with all the operators in the language (whenever it makes sense). For example, to add 4 to
the variable x and assign the result to x, you say: x += 4;.
This example shows the use of the mathematical operators:
//: MathOps.java
// Demonstrates the mathematical operators
import java.util.*;
public class MathOps {
// Create a shorthand to save typing:
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![static void prt(String s) {
System.out.println(s);
}
// shorthand to print a string and an int:
static void pInt(String s, int i) {
prt(s + " = " + i);
}
// shorthand to print a string and a float:
static void pFlt(String s, float f) {
prt(s + " = " + f);
}
public static void main(String args[]) {
// Create a random number generator,
// seeds with current time by default:
Random rand = new Random();
int i, j, k;
// '%' limits maximum value to 99:
j = rand.nextInt() % 100;
k = rand.nextInt() % 100;
pInt("j",j); pInt("k",k);
i = j + k; pInt("j + k", i);
i = j - k; pInt("j - k", i);
i = k / j; pInt("k / j", i);
i = k * j; pInt("k * j", i);
i = k % j; pInt("k % j", i);
j %= k; pInt("j %= k", j);
// Floating-point number tests:
float u,v,w; // applies to doubles, too
v = rand.nextFloat();
w = rand.nextFloat();
pFlt("v", v); pFlt("w", w);
u = v + w; pFlt("v + w", u);
u = v - w; pFlt("v - w", u);
u = v * w; pFlt("v * w", u);
u = v / w; pFlt("v / w", u);
// the following also works for
// char, byte, short, int, long,
// and double:
u += v; pFlt("u += v", u);
u -= v; pFlt("u -= v", u);
u *= v; pFlt("u *= v", u);
u /= v; pFlt("u /= v", u);
}
} ///:~
The first thing you will see are some shorthand methods for printing: the prt( ) method
prints a String, the pInt( ) prints a String followed by an int, and the pFlt( ) prints a
String followed by a float. Of course, they all ultimately end up using
System.out.println( ).
To generate numbers the program first creates a Random object. Because no arguments
are passed during creation, Java uses the current time as a seed for the random number
generator . The program generates a number of different types of random numbers with
the Random object simply by calling different methods: nextInt( ), nextLong( ),
nextFloat( ) or nextDouble( ).
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![The modulus operator, when used with the result of the random number generator, limits
the result to an upper bound of the operand minus one (99, in this case).
Unary minus and plus operators
The unary minus (-) and unary plus (+) are the same operators as binary minus and plus
– the compiler figures out which usage is intended by the way you write the expression.
For instance, the statement
x = -a;
has an obvious meaning. The compiler is able to figure out:
x = a * -b;
but the reader might get confused, so it is clearer to say:
x = a * (-b);
The unary minus produces the negative of the value. Unary plus provides symmetry with
unary minus, although it doesn’t do much.
Auto increment and decrement
Java, like C, is full of shortcuts. Shortcuts can make code much easier to type, and either
easier or harder to read.
Two of the nicer shortcuts are the increment and decrement operators (often referred to as
the auto-increment and auto-decrement operators). The decrement operator is -- and
means “decrease by one unit.” The increment operator is ++ and means “increase by one
unit.” If A is an int, for example, the expression ++A is equivalent to (A = A + 1).
Increment and decrement operators produce the value of the variable as a result.
There are two versions of each type of operator, often called the prefix and postfix
versions. Pre-increment means the ++ operator appears before the variable or expression,
and post-increment means the ++ operator appears after the variable or expression.
Similarly, pre-decrement means the -- operator appears before the variable or expression,
and post-decrement means the -- operator appears after the variable or expression. For
pre-increment and pre-decrement, (i.e., ++A or --A), the operation is performed and the
value is produced. For post-increment and post-decrement (i.e. A++ or A--), the value is
produced, then the operation is performed. As an example:
//: AutoInc.java
// Demonstrates the ++ and -- operators
public class AutoInc {
public static void main(String args[]) {
int i = 1;
prt("i : " + i);
prt("++i : " + ++i); // Pre-increment
prt("i++ : " + i++); // Post-increment
prt("i : " + i);
prt("--i : " + --i); // Pre-decrement
prt("i-- : " + i--); // Post-decrement
prt("i : " + i);
}
static void prt(String s) {
System.out.println(s);
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![}
} ///:~
The output for this program is:
i : 1
++i : 2
i++ : 2
i : 3
--i : 2
i-- : 2
i : 1
You can see that for the prefix form you get the value after the operation has been
performed, but with the postfix form you get the value before the operation is performed.
These are the only operators (other than those involving assignment) that have side effects
(that is, they change the operand rather than using just its value).
The increment operator is one explanation for the name C++, implying “one step beyond
C.” In an early Java speech, Bill Joy (one of the creators), said that "Java=C++--" (C plus
plus minus minus), suggesting that Java is C++ with the unnecessary hard parts removed
and therefore a much simpler language. As you progress in this book you'll see that many
parts are simpler, and yet Java isn't that much easier than C++.
Relational operators
Relational operators generate a boolean result. They evaluate the relationship between the
values of the operands. A relational expression produces true if the relationship is true,
and false if the relationship is untrue. The relational operators are less than (<), greater
than (>), less than or equal to (<=), greater than or equal to (>=), equivalent (==) and
not equivalent (!=). Equivalence and nonequivalence works with all built-in data types,
but the other comparisons won’t work with type boolean.
Testing object equivalence
The relational operators == and != also work with all objects, but their meaning often
confuses the first-time Java programmer. Here’s an example:
//: Equivalence.java
public class Equivalence {
public static void main(String args[]) {
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1 == n2);
System.out.println(n1 != n2);
}
} ///:~
The expression System.out.println(n1 == n2) will print out the result of the boolean
comparison within. Surely the output should be true and then false, since both Integer
objects are the same. But while the contents of the objects are the same, the handles are not
the same and the operators == and != compare object handles. So the output is actually
false and then true. Naturally, this surprises people at first.
What if you want to compare the actual contents of an object for equivalence? You must
use the special method equals( ) that exists for all objects (not primitives, which work fine
with == and !=). Here’s how it’s used:
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![//: EqualsMethod.java
public class EqualsMethod {
public static void main(String args[]) {
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1.equals(n2));
}
} ///:~
The result will be true, as you would expect. Ah, but it’s not so simple as that. If you
create your own class, like this:
//: EqualsMethod2.java
class Value {
int i;
}
public class EqualsMethod2 {
public static void main(String args[]) {
Value v1 = new Value();
Value v2 = new Value();
v1.i = v2.i = 100;
System.out.println(v1.equals(v2));
}
} ///:~
You’re back to square one: the result is false. This is because the default behavior of
equals( ) is to compare handles, so unless you override equals( ) in your new class, you
won’t get the desired behavior. Alas, you won’t learn about overriding until Chapter 7, but
being aware of the way equals( ) behaves might save you some grief in the meantime.
Most of the Java library classes implement equals( ) so that it compares the contents of
objects, rather than their handles.
Logical operators
The logical operators AND (&&), OR (||) and NOT (!) produce a boolean value of true or
false based on the logical relationship of its arguments. This example uses the relational
and logical operators:
//: Bool.java
// Relational and logical operators
import java.util.*;
public class Bool {
public static void main(String args[]) {
Random rand = new Random();
int i = rand.nextInt() % 100;
int j = rand.nextInt() % 100;
prt("i = " + i);
prt("j = " + j);
prt("i > j is " + (i > j));
prt("i < j is " + (i < j));
prt("i >= j is " + (i >= j));
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![public class ShortCircuit {
static boolean test1(int val) {
System.out.println("test1(" + val + ")");
System.out.println("result: " + (val < 1));
return val < 1;
}
static boolean test2(int val) {
System.out.println("test2(" + val + ")");
System.out.println("result: " + (val < 2));
return val < 2;
}
static boolean test3(int val) {
System.out.println("test3(" + val + ")");
System.out.println("result: " + (val < 3));
return val < 3;
}
public static void main(String args[]) {
if(test1(0) && test2(2) && test3(2))
System.out.println("expression is true");
else
System.out.println("expression is false");
}
} ///:~
Each test performs a comparison against the argument and returns true or false. It also
prints information to show you that it’s being called. The tests are used in the expression:
if(test1(0) && test2(2) && test3(2))
You might naturally think that all three tests would be executed, but the output shows
otherwise:
test1(0)
result: true
test2(2)
result: false
expression is false
The first test produced a true result, so the expression evaluation continues. However, the
second test produced a false result. Since this means that the whole expression must be
false, why continue evaluating the rest of the expression? It could be expensive. The
reason for short-circuiting, in fact, is precisely that: you can get a potential performance
increase if all the parts of a logical expression do not need to be evaluated.
Bitwise operators
The bitwise operators allow you to manipulate individual bits in an integral primitive data
type. Bitwise operators perform boolean algebra on the corresponding bits in the two
arguments to produce the result.
The bitwise operators come from C’s low-level orientation; you were often manipulating
hardware directly and had to set the bits in hardware registers. Java was originally
designed to be embedded in TV set-top boxes and so this low-level orientation still made
sense. However, you probably won’t use the bitwise operators that much.
The bitwise AND operator (&) produces a one in the output bit if both input bits are one;
otherwise it produces a zero. The bitwise OR operator (|) produces a one in the output bit
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![if either input bit is a one and produces a zero only if both input bits are zero. The bitwise,
EXCLUSIVE OR, or XOR (^) produces a one in the output bit if one or the other input bit is
a one, but not both. The bitwise NOT (~, also called the ones complement operator) is a
unary operator – it takes only one argument (all other bitwise operators are binary
operators). Bitwise NOT produces the opposite of the input bit – a one if the input bit is
zero, a zero if the input bit is one.
Since the bitwise operators and logical operators use the same characters, it is helpful to
have a mnemonic device to help you remember the meanings: since bits are “small,” there
is only one character in the bitwise operators.
Bitwise operators can be combined with the = sign to unite the operation and assignment:
&=, |= and ^= are all legitimate (since ~ is a unary operator it cannot be combined with
the = sign).
The boolean type is treated as a one-bit value so it is somewhat different. You can
perform a bitwise AND, OR and XOR, but you can’t perform a bitwise NOT (presumably
to prevent confusion with the logical NOT). For booleans the bitwise operators have the
same effect as the logical operators except that they do not short circuit. Also, the bitwise
operators on booleans gives you a XOR logical operator not included under the list of
"logical" operators. You’re prevented from using booleans in shift expressions (described
next).
Shift operators
The shift operators also manipulate bits. They can be used only on primitive, integral
types. The left-shift operator (<<) produces the operand to the left of the operator shifted
to the left by the number of bits specified after the operator (inserting zeroes at the lower-
order bits). The signed right-shift operator (>>) produces the operand to the left of the
operator shifted to the right by the number of bits specified after the operator. The signed
right shift >> uses sign extension: if the value is positive, zeroes are inserted at the higher-
order bits, if the value is negative, ones are inserted at the higher-order bits. Java has also
added the unsigned right shift >>> which uses zero extension: regardless of the sign,
zeroes are inserted at the higher-order bits. This operator does not exist in C or C++.
If you shift a char, byte, or short, it will be promoted to int before the shift takes place,
and the result will be an int. Only the five low-order bits of the right-hand side will be
used. This prevents you from shifting more than the number of bits in an int. If you’re
operating on a long, long will be the result. Only the six low-order bits of the right-hand
side will be used so you can’t shift more than the number of bits in a long. There is a
problem, however, with the unsigned right shift: if you use it with byte or short you
might not get the right results (it’s broken in Java 1.0 and Java 1.1). These are promoted
to int and right shifted, but the zero extension does not occur, so you get -1 in those cases.
The following example can be used to test your implementation:
//: URShift.java
// Test of unsigned right shift
public class URShift {
public static void main(String args[]) {
int i = -1;
i >>>= 10;
System.out.println(i);
long l = -1;
l >>>= 10;
System.out.println(l);
short s = -1;
Chapter 3: Controlling Program Flow 89](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-89-320.jpg)
![s >>>= 10;
System.out.println(s);
byte b = -1;
b >>>= 10;
System.out.println(b);
}
} ///:~
Shifts can be combined with the equal sign (<<= or >>= or >>>=). The lvalue is
replaced by the lvalue shifted by the rvalue.
Here’s an example that demonstrates the use of all the operators involving bits:
//: BitManipulation.java
// Using the bitwise operators
import java.util.*;
public class BitManipulation {
public static void main(String args[]) {
Random rand = new Random();
int i = rand.nextInt();
int j = rand.nextInt();
pBinInt("-1", -1);
pBinInt("+1", +1);
int maxpos = 2147483647;
pBinInt("maxpos", maxpos);
int maxneg = -2147483648;
pBinInt("maxneg", maxneg);
pBinInt("i", i);
pBinInt("~i", ~i);
pBinInt("-i", -i);
pBinInt("j", j);
pBinInt("i & j", i & j);
pBinInt("i | j", i | j);
pBinInt("i ^ j", i ^ j);
pBinInt("i << 5", i << 5);
pBinInt("i >> 5", i >> 5);
pBinInt("(~i) >> 5", (~i) >> 5);
pBinInt("i >>> 5", i >>> 5);
pBinInt("(~i) >>> 5", (~i) >>> 5);
long l = rand.nextLong();
long m = rand.nextLong();
pBinLong("-1L", -1L);
pBinLong("+1L", +1L);
long ll = 9223372036854775807L;
pBinLong("maxpos", ll);
long lln = -9223372036854775808L;
pBinLong("maxneg", lln);
pBinLong("l", l);
pBinLong("~l", ~l);
pBinLong("-l", -l);
pBinLong("m", m);
pBinLong("l & m", l & m);
pBinLong("l | m", l | m);
pBinLong("l ^ m", l ^ m);
90 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-90-320.jpg)
![Notice that you don’t have to use the trailing character when the compiler can figure out
the appropriate type. With
long n3 = 200;
there’s no ambiguity, so an L after the 200 would be superfluous. However, with
float f4 = 1e-47f; // 10 to the power
the compiler normally takes exponential numbers as doubles, so without the trailing f it
will give you an error telling you that you must use a cast to convert double to float.
Promotion
You’ll discover that if you perform any mathematical or bitwise operations on primitive
data types that are smaller than an int (that is, char, byte, or short), those values will be
promoted to int before performing the operations, and the resulting value will be of type
int. So if you want to assign back into the smaller type, you must use a cast (and, since
you’re assigning back into a smaller type, you might be losing information). In general,
the largest data type in an expression is the one that determines the size of the result of
that expression; if you multiply a float and a double, the result will be double; if you add
an int and a long, the result will be long.
Java has no “sizeof”
In C and C++, the sizeof( ) operator satisfies a specific need: it tells you the number of
bytes allocated for data items. The most compelling need for sizeof( ) in C and C++ is for
portability. Different data types might be different sizes on different machines, so the
programmer must find out how big those types are when performing operations that may
be sensitive to size. For example, one computer might store integers in 32 bits, whereas
another might store integers as 16 bits, and therefore programs could store larger values
in integers on the first machine. As you might imagine, portability is a huge headache for
C and C++ programmers.
Java does not need a sizeof( ) operator for this purpose because all the data types are the
same size on all machines. You do not have to think about portability on this level – it is
designed into the language.
Precedence revisited
Upon hearing me complain about the complexity of remembering operator precedence
during one of my seminars, a student suggested a mnemonic that is simultaneously a
commentary: “Ulcer Addicts Really Like C A lot.”
Mnemonic Operator type Operators
Ulcer Unary + - ++ – [[ rest…]]
Addicts Arithmetic (and shift) * / % + - << >>
Really Relational > < >= <= == !=
Like Logical (and bitwise) && || & | ^
C Conditional (ternary) A>B?X:Y
A Lot Assignment = (and compound assignment like *=)
96 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-96-320.jpg)
![// Bitwise operators:
//! x = ~y;
//! x = x & y;
//! x = x | y;
//! x = x ^ y;
//! x = x << 1;
//! x = x >> 1;
//! x = x >>> 1;
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
//! x <<= 1;
//! x >>= 1;
//! x >>>= 1;
//! x &= y;
//! x ^= y;
//! x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
byte B = (byte)x;
short s = (short)x;
int i = (int)x;
long l = (long)x;
float f = (float)x;
}
} ///:~
Note that boolean is very limited: you can assign to it the values true and false, and you
can test it for truth or falsehood. However, you cannot add booleans or perform any other
type of operation on them.
In char, byte, and short you can see the effect of promotion with the arithmetic
operators: each arithmetic operation on any of those types results in an int result, which
must be explicitly cast back to the original type (a narrowing conversion which might lose
information) to assign back to that type. With int values, however, you do not need to
cast, because everything is already an int. Don’t be lulled into thinking everything is safe,
though: if you multiply two ints that are big enough, you’ll overflow the result. The
following example demonstrates this:
//: Overflow.java
// Surprise! Java lets you overflow.
public class Overflow {
public static void main(String args[]) {
int big = 0x7fffffff; // max int value
prt("big = " + big);
int bigger = big * 4;
prt("bigger = " + bigger);
}
static void prt(String s) {
System.out.println(s);
}
104 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-104-320.jpg)
![The conditional must produce a Boolean result. The statement means either a simple
statement terminated by a semicolon or a compound statement, which is a group of
simple statements enclosed in braces. Anytime the word “statement” is used, it always
implies that the statement can be simple or compound.
As an example of if-else, here is a test( ) method that will tell you whether a guess is
above, below, or equivalent to a target number:
static int test(int testval) {
int result = 0;
if(testval > target)
result = -1;
else if(testval < target)
result = +1;
else
result = 0; // match
return result;
}
It is conventional to indent the body of a control flow statement so the reader might easily
determine where it begins and ends.
return
The return keyword has two purposes: it specifies what value a method will return (if it
doesn’t have a void return value) and it also causes that value to be returned immediately.
The test( ) method above can be rewritten to take advantage of this:
static int test2(int testval) {
if(testval > target)
return -1;
if(testval < target)
return +1;
return 0; // match
}
There’s no need for else because the method will not continue after executing a return.
Iteration
while, do-while and for control looping, and are sometimes classified as iteration
statements. A statement repeats until the controlling Boolean-expression evaluates to false.
The form for a while loop is
while(Boolean-expression)
statement
The Boolean-expression is evaluated once at the beginning of the loop, and again before each
further iteration of the statement.
Here’s a simple example that generates random numbers until a particular condition is
met:
//: WhileTest.java
// Demonstrates the while loop
public class WhileTest {
public static void main(String args[]) {
106 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-106-320.jpg)
![double r = 0;
while(r < 0.99d) {
r = Math.random();
System.out.println(r);
}
}
} ///:~
This uses the static method random( ) in the Math library, which generates a double
value between 0 and 1 (it includes 0, but not 1). The conditional expression for the while
says “keep doing this loop until the number is 0.99 or greater.” Each time you run this
program you’ll get a different-sized list of numbers.
Do-while
The form for do-while is
do
statement
while(Boolean-expression);
The only difference between while and do-while is that the statement of the do-while
always executes at least once, even if the expression evaluates to false the first time. In a
while, if the conditional is false the first time the statement never executes. In practice,
do-while is less common than while.
For
A for loop performs initialization before the first iteration. Then it performs conditional
testing and, at the end of each iteration, some form of “stepping.” The form of the for loop
is:
for(initialization; Boolean-expression; step)
statement
Any of the expressions initialization, Boolean-expression, or step may be empty. The
expression is tested before each iteration, and as soon as it evaluates to false execution will
continue at the line following the for statement. At the end of each loop, the step executes.
for loops are usually used for “counting” tasks:
//: ListCharacters.java
// Demonstrates "for" loop by listing
// all the ASCII characters.
public class ListCharacters {
public static void main(String args[]) {
for( char c = 0; c < 128; c++)
if (c != 26 ) // ANSI Clear screen
System.out.println(
"value: " + (int)c +
" character: " + c);
}
} ///:~
Chapter 3: Controlling Program Flow 107](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-107-320.jpg)
![Notice that the variable c is defined at the point where it is used, inside the control
expression of the for loop, rather than the beginning of the block denoted by the open
curly brace. The scope of c is the expression controlled by the for.
Traditional procedural languages like C require that all variables be defined at the
beginning of a block so when the compiler creates a block it can allocate space for those
variables. In Java and C++ you can spread your variable declarations throughout the
block, defining them at the point that you need them. This allows a more natural coding
style and makes code easier to understand.
You can define multiple variables within a for statement, but they must be of the same
type:
for(int i = 0, j = 1;
i < 10 && j != 11;
i++, j++)
/* body of for loop */;
The ability to define variables in the control expression is limited to the for loop. You
cannot use this approach with any of the other selection or iteration statements.
The comma operator
Earlier in this chapter I stated that the comma operator (not the comma separator, which is
used to separate function arguments) has only one use in Java: in the control expression of
a for loop. In both the initialization and step portions of the control expression, you can
have a number of statements separated by commas, and those statements will be evaluated
sequentially. The previous bit of code uses this ability. Here’s another example:
//: CommaOperator.java
public class CommaOperator {
public static void main(String args[]) {
for(int i = 1, j = i + 10; i < 5;
i++, j = i * 2) {
System.out.println("i= " + i + " j= " + j);
}
}
} ///:~
Here’s the output:
i= 1 j= 11
i= 2 j= 4
i= 3 j= 6
i= 4 j= 8
You can see that in both the initialization and step portions the statements are evaluated
in sequential order. Also, the initialization portion can have any number of definitions of
one type.
Break and continue
Inside the body of any of the iteration statements you can also control the flow of the loop
using break and continue. break quits the loop without executing the rest of the
statements in the loop. continue stops the execution of the current iteration and goes back
to the beginning of the loop to begin a new iteration.
108 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-108-320.jpg)
![This program shows examples of break and continue within for and while loops:
//: BreakAndContinue.java
// Demonstrates break and continue keywords
public class BreakAndContinue {
public static void main(String args[]) {
for(int i = 0; i < 100; i++) {
if(i == 74) break; // Out of for loop
if(i % 9 != 0) continue; // Next iteration
System.out.println(i);
}
int i = 0;
// An "infinite loop":
while(true) {
i++;
int j = i * 27;
if(j == 1269) break; // Out of loop
if(i % 10 != 0) continue; // Top of loop
System.out.println(i);
}
}
} ///:~
In the for loop the value of i never gets to 100 because the break statement breaks out of
the loop when i is 74. Normally you’d use a break like this only if you didn’t know when
the terminating condition was going to occur. The continue statement causes execution to
go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly
divisible by 9. When it is, the value is printed.
The second portion shows an “infinite loop” that would, in theory, continue forever.
However, inside the loop there is a break statement that will break out of the loop. In
addition, you’ll see that the continue moves back to the top of the loop without
completing the remainder (thus printing happens only when the value of i is divisible by
10). The output is:
0
9
18
27
36
45
54
63
72
10
20
30
40
The value 0 is printed because 0 % 9 produces 0.
A second form of the infinite loop is for(;;). The compiler treats both while(true) and
for(;;) in the same way so which one you use is a matter of programming taste.
Chapter 3: Controlling Program Flow 109](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-109-320.jpg)
![// Java’s "labeled for loop"
public class LabeledFor {
public static void main(String args[]) {
int i = 0;
outer: // Can't have statements here
for(; true ;) { // infinite loop
inner: // Can't have statements here
for(; i < 10; i++) {
prt("i = " + i);
if(i == 2) {
prt("continue");
continue;
}
if(i == 3) {
prt("break");
i++; // Otherwise i never
// gets incremented.
break;
}
if(i == 7) {
prt("continue outer");
i++; // Otherwise i never
// gets incremented.
continue outer;
}
if(i == 8) {
prt("break outer");
break outer;
}
for(int k = 0; k < 5; k++) {
if(k == 3) {
prt("continue inner");
continue inner;
}
}
}
}
// Can't break or continue
// to labels here
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
This uses the prt( ) method that has been defined in the other examples.
Notice that break breaks out of the for loop, and that the increment-expression doesn’t
occur until the end of the pass through the for loop. Since break skips the increment
expression, the increment is performed by hand in the case of i == 3. The continue outer
statement in the case of I == 7 also goes to the top of the loop and also skips the
increment, so it too is incremented by hand.
Here is the output:
i = 0
Chapter 3: Controlling Program Flow 111](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-111-320.jpg)
![continue inner
i = 1
continue inner
i = 2
continue
i = 3
break
i = 4
continue inner
i = 5
continue inner
i = 6
continue inner
i = 7
continue outer
i = 8
break outer
If not for the break outer statement, there would be no way to get out of the outer loop
from within an inner loop, since break by itself can break out of only the innermost loop
(the same is true for continue).
Here is a demonstration of labeled break and continue statements with while loops:
//: LabeledWhile.java
// Java's "labeled while" loop
public class LabeledWhile {
public static void main(String args[]) {
int i = 0;
outer:
while(true) {
prt("Outer while loop");
while(true) {
i++;
prt("i = " + i);
if(i == 1) {
prt("continue");
continue;
}
if(i == 3) {
prt("continue outer");
continue outer;
}
if(i == 5) {
prt("break");
break;
}
if(i == 7) {
prt("break outer");
break outer;
}
}
}
}
static void prt(String s) {
112 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-112-320.jpg)
![default: statement;
}
Integral-selector is an expression that produces an integral value. The switch compares the
result of integral-selector to each integral-value. If it finds a match, the corresponding
statement (simple or compound) executes. If no match occurs, the default statement
executes.
You will notice in the above definition that each case ends with a break, which causes
execution to jump to the end of the switch body. This is the conventional way to build a
switch statement, but the break is optional. If it is missing, the code for the following case
statements execute until a break is encountered. Although you don’t usually want this
kind of behavior, it can be useful to an experienced programmer. Notice the last statement,
for the default, doesn’t have a break because the execution just falls through to where the
break would have taken it anyway. You could put a break at the end of the default
statement with no harm, if you considered it important for style’s sake.
The switch statement is a very clean way to implement multi-way selection (i.e., selecting
from among a number of different execution paths), but it requires a selector that
evaluates to an integral value such as int or char. If you want to use, for example, a string
or a floating-point number as a selector, it won’t work in a switch statement. For non-
integral types, you must use a series of if statements.
Here’s an example that randomly creates letters and determines whether they’re vowels or
consonants:
//: VowelsAndConsonants.java
// Demonstrates the switch statement
public class VowelsAndConsonants {
public static void main(String args[]) {
for(int i = 0; i < 100; i++) {
char c = (char)(Math.random() * 26 + 'a');
System.out.print(c + ": ");
switch(c) {
case 'a':
case 'e':
case 'i':
case 'o':
case 'u':
System.out.println("vowel");
break;
case 'y':
case 'w':
System.out.println(
"Sometimes a vowel");
break;
default:
System.out.println("consonant");
}
}
}
} ///:~
Since Math.random( ) generates a value between 0 and 1, you need only to multiply it by
the upper bound of the range of numbers you want to produce (26 for the letters in the
alphabet) and add an offset to establish the lower bound.
114 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-114-320.jpg)
![Although it appears you’re switching on a character here, the switch statement is actually
using the integral value of the character. The singly-quoted characters in the case
statements also produce integral values which are used for comparison.
Notice how the cases can be “stacked” on top of each other to provide multiple matches
for a particular piece of code. You should also be aware that it’s essential to put the break
statement at the end of a particular case, otherwise control will simply drop through and
continue processing on the next case.
Calculation details
The statement:
char c = (char)(Math.random() * 26 + 'a');
deserves a closer look. Math.random( ) produces a double, so the value 26 is converted to
a double to perform the multiplication, which also produces a double. This means that
‘a’ must be converted to a double to perform the addition. The double result is turned
back into a char with a cast.
First, what does the cast to char do? That is, if you have the value 29.7 and you cast it to
a char, is the resulting value 30 or 29? The answer to this can be seen with an example:
//: CastingNumbers.java
// What happens when you cast a float or double
// to an integral value?
public class CastingNumbers {
public static void main(String args[]) {
double
above = 0.7,
below = 0.4;
System.out.println("above: " + above);
System.out.println("below: " + below);
System.out.println(
"(int)above: " + (int)above);
System.out.println(
"(int)below: " + (int)below);
System.out.println(
"(char)('a' + above): " +
(char)('a' + above));
System.out.println(
"(char)('a' + below): " +
(char)('a' + below));
}
} ///:~
The output is:
above: 0.7
below: 0.4
(int)above: 0
(int)below: 0
(char)('a' + above): a
(char)('a' + below): a
So the answer is that casting from a float or double to an integral value always
truncates.
Chapter 3: Controlling Program Flow 115](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-115-320.jpg)
![The second question has to do with Math.random( ). Does it produce a value from zero to
one, inclusive or exclusive of the value ‘1’? In math lingo, is it (0,1) or [0,1] or (0,1] or
[0,1)? (The square bracket means “includes” whereas the parenthesis means “doesn’t
include”). Again, a test program provides the answer:
//: RandomBounds.java
// Does Math.random() produce 0.0 and 1.0?
public class RandomBounds {
static void usage() {
System.err.println("Usage: nt" +
"RandomBounds lowernt" +
"RandomBounds upper");
System.exit(1);
}
public static void main(String args[]) {
if(args.length != 1) usage();
if(args[0].equals("lower")) {
while(Math.random() != 0.0)
; // Keep trying
System.out.println("Produced 0.0!");
}
else if(args[0].equals("upper")) {
while(Math.random() != 1.0)
; // Keep trying
System.out.println("Produced 1.0!");
}
else
usage();
}
} ///:~
To run the program, you type a command line of either:
java RandomBounds lower
or
java RandomBounds upper
In both cases, you are forced to break out of the program manually, so it would appear
that Math.random( ) never produces either 0.0 or 1.0. But this is where such an
experiment can be deceiving. If you consider that there are 2128 different double fractions
between 0 and 1, the likelihood of reaching any one value experimentally might exceed the
lifetime of one computer, or even one experimenter. It turns out that 0.0 is included in the
output of Math.random( ). Or, in math lingo, it is [0,1).
Summary
This chapter concludes the study of fundamental features that appear in most
programming languages: calculation, operator precedence, type casting, and selection and
iteration. Now you’re ready to begin taking steps that move you closer to the world of
object-oriented programming. In the next chapter, the important issues of initialization
and cleanup of objects will be covered, followed in the subsequent chapter by the essential
concept of implementation hiding.
116 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-116-320.jpg)
![constructor, Java automatically calls that constructor when an object is created, before
users can even get their hands on it. So initialization is guaranteed.
The next challenge is what to name this method. There are two issues. The first is that any
name you use could clash with a name you might like to use as a member in the class. The
second is that because the compiler is responsible for calling the constructor, it must
always know which method to call. The C++ solution seems the easiest and most logical,
so it’s also used in Java: The name of the constructor is the same as the name of the class.
It makes sense that such a method will be called automatically on initialization.
Here’s a simple class with a constructor (see page 80 if you have trouble executing this
program):
//: SimpleConstructor.java
// Demonstration of a simple constructor
package c04;
class Rock {
Rock() { // This is the constructor
System.out.println("Creating Rock");
}
}
public class SimpleConstructor {
public static void main(String args[]) {
for(int i = 0; i < 10; i++)
new Rock();
}
} ///:~
Now, when an object is created:
new Rock();
storage is allocated and the constructor is called. It is guaranteed that the object will be
properly initialized before you can get your hands on it.
Note that the coding style of making the first letter of all methods lower case does not
apply to constructors, since the name of the constructor must match the name of the class
exactly.
Like any method, the constructor can have arguments to allow you to specify how an
object is created. The above example can easily be changed so the constructor takes an
argument:
class Rock {
Rock(int i) {
System.out.println(
"Creating Rock number " + i);
}
}
public class SimpleConstructor {
public static void main(String args[]) {
for(int i = 0; i < 10; i++)
new Rock(i);
}
}
120 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-120-320.jpg)
![Here’s an example that shows both overloaded constructors and overloaded ordinary
methods:
//: Overloading.java
// Demonstration of both constructor
// and ordinary method overloading.
import java.util.*;
class Tree {
int height;
Tree() {
prt("Planting a seedling");
height = 0;
}
Tree(int i) {
prt("Creating new Tree that is "
+ i + " feet tall");
height = i;
}
void info() {
prt("Tree is " + height
+ " feet tall");
}
void info(String s) {
prt(s + ": Tree is "
+ height + " feet tall");
}
static void prt(String s) {
System.out.println(s);
}
}
public class Overloading {
static Random rand = new Random();
static int pRand(int mod) {
return Math.abs(rand.nextInt()) % mod;
}
public static void main(String args[]) {
for(int i = 0; i < 5; i++) {
Tree t = new Tree(i);
t.info();
t.info("overloaded method");
}
// Overloaded constructor:
new Tree();
}
} ///:~
A Tree object may be created either as a seedling, with no argument, or as grown in a
nursery, with an existing height. To support this, there are two constructors, one that
122 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-122-320.jpg)
![takes no arguments (we call constructors that take no arguments default constructors1) and
one that takes the existing height.
You may also want to call the info( ) method in more than one way: with a String
argument if you have an extra message you want printed, and without if you have
nothing more to say. It would seem strange to have to give two separate names to what is
obviously the same concept. Fortunately, method overloading allows you to use the same
name for both.
Distinguishing overloaded methods
If the methods have the same name how can Java know which method you mean? There’s
a very simple rule: Each overloaded method must take a unique list of argument types.
If you think about this for a second, it makes sense: how else could you as a programmer
tell the difference between two methods that have the same name, other than by the types
of their arguments?
Even differences in the ordering of arguments is sufficient to distinguish two methods:
//: OverloadingOrder.java
// Overloading based on the order of
// the arguments.
public class OverloadingOrder {
static void print(String s, int i) {
System.out.println(
"String: " + s +
", int: " + i);
}
static void print(int i, String s) {
System.out.println(
"int: " + i +
", String: " + s);
}
public static void main(String args[]) {
print("String first", 11);
print(99, "Int first");
}
} ///:~
The two print( ) methods have identical arguments, but the order is different, and that’s
what makes them distinct.
Overloading with primitives
Primitives can be automatically promoted from a smaller type to a larger one and this can
be slightly confusing in combination with overloading. The following example
demonstrates what happens when a primitive is handed to an overloaded method:
//: PrimitiveOverloading.java
1 In some of the Java literature from Sun they instead refer to these with the clumsy but
descriptive name “no-arg constructors.” The term “default constructor” has been in use for many
years and so I shall use that.
Chapter 4: Initialization & Cleanup 123](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-123-320.jpg)
![f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testShort() {
short x = 0;
prt("short argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testInt() {
int x = 0;
prt("int argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testLong() {
long x = 0;
prt("long argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testFloat() {
float x = 0;
prt("float argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testDouble() {
double x = 0;
prt("double argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
public static void main(String args[]) {
PrimitiveOverloading p =
new PrimitiveOverloading();
p.testConstVal();
p.testChar();
p.testByte();
p.testShort();
p.testInt();
p.testLong();
p.testFloat();
p.testDouble();
}
} ///:~
If you view the output of this program, you’ll see that the constant value 5 is treated as
an int, so if an overloaded method is available that takes an int it is used. In all other
cases, if you have a data type that is smaller than the argument in the method, that data
type is promoted. char produces a slightly different effect, since if it doesn’t find an exact
char match, it is promoted to int.
What happens if your argument is bigger than the argument expected by the overloaded
method? A modification of the above program gives the answer:
//: Demotion.java
// Demotion of primitives and overloading
public class Demotion {
static void prt(String s) {
System.out.println(s);
}
Chapter 4: Initialization & Cleanup 125](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-125-320.jpg)
![void f1(char x) { prt("f1(char)"); }
void f1(byte x) { prt("f1(byte)"); }
void f1(short x) { prt("f1(short)"); }
void f1(int x) { prt("f1(int)"); }
void f1(long x) { prt("f1(long)"); }
void f1(float x) { prt("f1(float)"); }
void f1(double x) { prt("f1(double)"); }
void f2(char x) { prt("f2(char)"); }
void f2(byte x) { prt("f2(byte)"); }
void f2(short x) { prt("f2(short)"); }
void f2(int x) { prt("f2(int)"); }
void f2(long x) { prt("f2(long)"); }
void f2(float x) { prt("f2(float)"); }
void f3(char x) { prt("f3(char)"); }
void f3(byte x) { prt("f3(byte)"); }
void f3(short x) { prt("f3(short)"); }
void f3(int x) { prt("f3(int)"); }
void f3(long x) { prt("f3(long)"); }
void f4(char x) { prt("f4(char)"); }
void f4(byte x) { prt("f4(byte)"); }
void f4(short x) { prt("f4(short)"); }
void f4(int x) { prt("f4(int)"); }
void f5(char x) { prt("f5(char)"); }
void f5(byte x) { prt("f5(byte)"); }
void f5(short x) { prt("f5(short)"); }
void f6(char x) { prt("f6(char)"); }
void f6(byte x) { prt("f6(byte)"); }
void f7(char x) { prt("f7(char)"); }
void testDouble() {
double x = 0;
prt("double argument:");
f1(x);f2((float)x);f3((long)x);f4((int)x);
f5((short)x);f6((byte)x);f7((char)x);
}
public static void main(String args[]) {
Demotion p = new Demotion();
p.testDouble();
}
} ///:~
Here, the methods take narrower primitive values. If your argument is wider then you
must cast to the necessary type using the type name in parentheses. If you don’t do this
the compiler will issue an error message.
You should be aware that this is a narrowing conversion which means that you might lose
information during the cast. This is why the compiler forces you to do it – to flag the
narrowing conversion.
126 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-126-320.jpg)
![Overloading on return values
It is common to wonder “Why only class names and method argument lists? Why not
distinguish between methods based on their return values?” For example, these two
methods, which have the same name and arguments, are easily distinguished from each
other:
void f() {}
int f() {}
This works fine when the compiler can unequivocally determine the meaning from the
context, as in int x = f( ). However, you can call a method and ignore the return value;
this is often referred to as calling a method for its side effect since you don’t care about the
return value but instead want the other effects of the method call. So if you call the
method this way:
f();
how can Java determine which f() should be called? And how could someone reading the
code see it? Because of this sort of problem, you cannot use return value types to
distinguish overloaded methods.
Default constructors
As mentioned previously, a default constructor is one without arguments, used to create a
“vanilla object.” If you create a class that has no constructors, the compiler will
automatically create a default constructor for you. For example:
//: DefaultConstructor.java
class Bird {
int i;
}
public class DefaultConstructor {
public static void main(String args[]) {
Bird nc = new Bird(); // default!
}
} ///:~
The line
new Bird();
Creates a new object and calls the default constructor, even though one was not explicitly
defined. Without it we would have no method to call to build our object. However, if you
define any constructors (with or without arguments), the compiler will not synthesize one
for you:
class Bush {
Bush(int i) {}
Bush(double d) {}
}
Now if you say:
new Bush();
Chapter 4: Initialization & Cleanup 127](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-127-320.jpg)
![return this;
}
void print() {
System.out.println("i = " + i);
}
public static void main(String args[]) {
Leaf x = new Leaf();
x.increment().increment().increment().print();
}
} ///:~
Because increment( ) returns the handle to the current object via the this keyword,
multiple operations can easily be performed on the same object.
Calling constructors from constructors
When you write several constructors for a class, there are times when you’d like to call
one constructor from another, to avoid duplicating code. You can do this using the this
keyword.
Normally, when you say this, it is in the sense of “this object” or “the current object,” and
by itself it produces the handle to the current object. In a constructor, the this keyword
takes on a different meaning when you give it an argument list: it makes an explicit call to
the constructor that matches that argument list. Thus you have a straightforward way to
call other constructors:
//: Flower.java
// Calling constructors with "this"
public class Flower {
private int petalCount = 0;
private String s = new String("null");
Flower(int petals) {
petalCount = petals;
System.out.println(
"Constructor w/ int arg only, petalCount= "
+ petalCount);
}
Flower(String ss) {
System.out.println(
"Constructor w/ String arg only, s=" + ss);
s = ss;
}
Flower(String s, int petals) {
this(petals);
//! this(s); // Can't call two!
this.s = s; // Another use of "this"
System.out.println("String & int args");
}
Flower() {
this("hi", 47);
System.out.println(
"default constructor (no args)");
}
void print() {
//! this(11); // Not inside non-constructor!
System.out.println(
"petalCount = " + petalCount + " s = "+ s);
Chapter 4: Initialization & Cleanup 129](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-129-320.jpg)
![}
public static void main(String args[]) {
Flower x = new Flower();
x.print();
}
} ///:~
The constructor Flower(String s, int petals) shows that, while you can call one
constructor using this, you cannot call two. In addition, the constructor call must be the
first thing you do, otherwise you’ll get a compiler error message.
This example also shows another way you’ll see this used. Since the name of the argument
s and the name of the member data s are the same, there’s an ambiguity. You can resolve it
by saying this.s to refer to the member data. You’ll often see this form used in Java code,
and it’s used in numerous places in this book.
In print( ) you can see that the compiler won’t let you call a constructor from inside any
method other than a constructor.
The meaning of static
With the this keyword in mind, you can more fully understand what it means to make a
method static. It means there is no this for that particular method. Thus you cannot call
non-static methods from inside static methods2 (although the reverse is possible), and it’s
possible to call a static method for the class itself, without any object. In fact, that’s
primarily what a static method is for. It’s as if you’re creating the equivalent of a global
function (from C). Except that global functions are not permitted in Java, and putting the
static method inside a class allows it access to other static methods and to static fields.
Some people argue that static methods are not object-oriented, since they do have the
semantics of a global function – with a static method you don’t send a message to an
object, since there’s no this. This is probably a fair argument, and if you find yourself
using a lot of static methods you should probably consider rethinking your strategy.
However, statics are pragmatic and there are times you genuinely need them so whether
or not they are “proper OOP” should be left to the theoreticians. Indeed, even Smalltalk
has the equivalent in its “class methods.”
Cleanup: finalization &
garbage collection
Programmers know about the importance of initialization, but often forget the importance
of cleanup. After all, who needs to clean up an int? But with libraries, simply “letting go”
of an object once you’re done with it is not always safe. Of course, Java has the garbage
collector to reclaim the memory of objects that are no longer used. Now consider a very
special and unusual case. Suppose your object allocates “special” memory without using
new. The garbage collector knows only how to release memory allocated with new, so it
won’t know how to release the object’s “special” memory. To handle this case, Java
2 The one case where this is possible occurs if you pass a handle to an object into the static method.
Then, via the handle (which is now effectively this), you can call non-static methods and access
non-static fields. But typically if you want to do something like this you’ll just make an ordinary,
non-static method.
130 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-130-320.jpg)
!["Beginning to finalize after " +
created + " Chairs have been created");
}
if(i == 47) {
System.out.println(
"Finalizing Chair #47, " +
"Setting flag to stop Chair creation");
f = true;
}
finalized++;
if(finalized >= created)
System.out.println(
"All " + finalized + " finalized");
}
}
public class Garbage {
public static void main(String args[]) {
if(args.length == 0) {
System.err.println("Usage: n" +
"java Garbage beforen or:n" +
"java Garbage after");
return;
}
while(!Chair.f) {
new Chair();
new String("To take up space");
}
System.out.println(
"After all Chairs have been created:n" +
"total created = " + Chair.created +
", total finalized = " + Chair.finalized);
if(args[0].equals("before")) {
System.out.println("gc():");
System.gc();
System.out.println("runFinalization():");
System.runFinalization();
}
System.out.println("bye!");
if(args[0].equals("after"))
System.runFinalizersOnExit(true);
}
} ///:~
The above program creates many Chair objects, and at some point after the garbage
collector begins running, the program stops creating Chairs. Since the garbage collector
can be run at any time, you don’t know exactly when it will start up, so there’s a flag
called gcrun to indicate whether the garbage collector has started running yet. A second
flag f is a way for Chair to tell the main( ) loop that it should stop making objects. Both
of these flags are set within finalize( ), which is called during garbage collection.
Two other static variables, created and finalized, keep track of the number of objs created
vs. the number that get finalized by the garbage collector. Finally, each Chair has its own
(i.e.: non-static) int i so it can keep track of what number it is. When Chair number 47 is
finalized, the flag is set to true to bring the process of Chair creation to a stop.
All this happens in main( ), in the loop
Chapter 4: Initialization & Cleanup 133](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-133-320.jpg)
![Member initialization
Java goes out of its way to guarantee that any variable is properly initialized before it is
used. In the case of variables that are defined locally to a method, this guarantee comes in
the form of a compile-time error. So if you say:
void f() {
int i;
i++;
}
You’ll get an error message that says that i may not have been initialized. Of course, the
compiler could have given i a default value, but it’s more likely that this is a programmer
error and a default value would have covered that up. Forcing the programmer to provide
an initialization value is more likely to catch a bug.
If a primitive is a data member of a class, however, things are a bit different. Since any
method may initialize or use that data, it might not be practical to force the user to
initialize it to its appropriate value before the data is used. However, it’s unsafe to leave it
with a garbage value, so each primitive data member of a class is guaranteed to get an
initial value. Those values can be seen here:
//: InitialValues.java
// Shows default initial values
class Measurement {
boolean t;
char c;
byte b;
short s;
int i;
long l;
float f;
double d;
void print() {
System.out.println(
"Data type Inital valuen" +
"boolean " + t + "n" +
"char " + c + "n" +
"byte " + b + "n" +
"short " + s + "n" +
"int " + i + "n" +
"long " + l + "n" +
"float " + f + "n" +
"double " + d);
}
}
public class InitialValues {
public static void main(String args[]) {
Measurement d = new Measurement();
d.print();
/* In this case you could also say:
new Measurement().print();
*/
}
Chapter 4: Initialization & Cleanup 135](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-135-320.jpg)
![// When the constructor is called, to create a
// Tag object, you'll see a message:
class Tag {
Tag(int marker) {
System.out.println("Tag(" + marker + ")");
}
}
class Card {
Tag t1 = new Tag(1); // Before constructor
Card() {
// Indicate we're in the constructor:
System.out.println("Card()");
t3 = new Tag(33); // Re-initialize t3
}
Tag t2 = new Tag(2); // After constructor
void f() {
System.out.println("f()");
}
Tag t3 = new Tag(3); // At end
}
public class OrderOfInitialization {
public static void main(String args[]) {
Card t = new Card();
t.f(); // Shows that construction is done
}
} ///:~
In Card, the definitions of the Tag objects are intentionally scattered about to prove that
they’ll all get initialized before the constructor is entered or anything else can happen. In
addition, t3 is re-initialized inside the constructor. The output is:
Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()
Thus, the t3 handle gets initialized twice, once before and once during the constructor call
(the first object is dropped, so it may be garbage-collected later). This might not seem very
efficient at first, but it guarantees proper initialization – what would happen if an
overloaded constructor were defined that did not initialize t3 and there wasn’t a “default”
initialization for t3 in its definition?
Static data initialization
What happens when the data is static? Exactly the same thing: if it’s a primitive and you
don’t initialize it, it gets the standard primitive initial values. If it’s a handle to an object,
it’s null unless you create a new object and attach your handle to it.
If you want to place initialization at the point of definition, it looks the same as for non-
statics. But since there’s only one piece of storage for a static, regardless of how many
objects are created, a question comes up: when does that storage get initialized? An
example makes this question clear:
138 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-138-320.jpg)
![//: StaticInitialization.java
// Specifying initial values in a
// class definition.
class Bowl {
Bowl(int marker) {
System.out.println("Bowl(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Table {
static Bowl b1 = new Bowl(1);
Table() {
System.out.println("Table()");
b2.f(1);
}
void f2(int marker) {
System.out.println("f2(" + marker + ")");
}
static Bowl b2 = new Bowl(2);
}
class Cupboard {
Bowl b3 = new Bowl(3);
static Bowl b4 = new Bowl(4);
Cupboard() {
System.out.println("Cupboard()");
b4.f(2);
}
void f3(int marker) {
System.out.println("f3(" + marker + ")");
}
static Bowl b5 = new Bowl(5);
}
public class StaticInitialization {
public static void main(String args[]) {
System.out.println(
"Creating new Cupboard() in main");
new Cupboard();
System.out.println(
"Creating new Cupboard() in main");
new Cupboard();
t2.f2(1);
t3.f3(1);
}
static Table t2 = new Table();
static Cupboard t3 = new Cupboard();
} ///:~
Bowl allows you to view the creation of a class, and Table and Cupboard create static
members of Bowl scattered through their class definitions. Notice that Cupboard creates a
non-static Bowl b3 prior to the static definitions. The output shows what happens:
Chapter 4: Initialization & Cleanup 139](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-139-320.jpg)
![}
// . . .
So it looks like a method, but it’s just the static keyword followed by a method body. This
code, like the other static initialization, is executed only once, the first time you make an
object of that class or you access a static member of that class (even if you never make an
object of that class). For example:
//: ExplicitStatic.java
// Explicit static initialization
// with the "static" clause.
class Cup {
Cup(int marker) {
System.out.println("Cup(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Cups {
static Cup c1;
static Cup c2;
static {
c1 = new Cup(1);
c2 = new Cup(2);
}
Cups() {
System.out.println("Cups()");
}
}
public class ExplicitStatic {
public static void main(String args[]) {
System.out.println("Inside main()");
Cups.c1.f(99); // (1)
}
static Cups x = new Cups(); // (2)
static Cups y = new Cups(); // (2)
} ///:~
The static initializers for Cups will be run when either the access of the static object c1
occurs on the line marked (1), or if line (1) is commented out and the lines marked (2) are
un-commented. If both (1) and (2) are commented out, the static initialization for Cups
never occurs.
Non-static instance initialization
non-static instance initialization Java 1.1 provides a similar syntax for initializing non-
static variables for each object. Here’s an example:
//: Mugs.java
// Java 1.1 "Instance Initialization"
class Mug {
Mug(int marker) {
System.out.println("Mug(" + marker + ")");
Chapter 4: Initialization & Cleanup 141](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-141-320.jpg)
![}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
public class Mugs {
Mug c1;
Mug c2;
{
c1 = new Mug(1);
c2 = new Mug(2);
System.out.println("c1 & c2 initialized");
}
Mugs() {
System.out.println("Mugs()");
}
public static void main(String args[]) {
System.out.println("Inside main()");
Mugs x = new Mugs();
}
} ///:~
You can see that the instance initialization clause:
{
c1 = new Mug(1);
c2 = new Mug(2);
System.out.println("c1 & c2 initialized");
}
looks exactly like the static initialization clause except for the missing static keyword.
This syntax is necessary to support the initialization of anonymous inner classes (see
Chapter 7).
Array initialization
Initializing arrays in C is error-prone and tedious. C++ uses aggregate initialization to
make it much safer5 . Java has no “aggregates” like C++, since everything is an object in
Java. However it does have arrays, and these are supported with array initialization.
An array is simply a sequence of objects, all the same type and packaged together under
one identifier name. Arrays are defined and used with the square-brackets indexing
operator [ ]. To define an array you simply follow your identifier with empty square
brackets:
int a1[];
However, you can also put the square brackets after the type name to produce exactly the
same meaning:
int[] a1;
5 See Thinking in C++ for a complete description of aggregate initialization.
142 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-142-320.jpg)
![This might be considered a more sensible syntax, since it says that the type is “an int
array.” But the former style of definition conforms to expectations from C and C++
programmers.
The compiler doesn’t allow you to tell it how big the array is. But we’re back to that issue
of “handles”: all you have at this point is a handle to an array, and there’s been no space
allocated for the array itself. To create storage for the array you must write an
initialization expression. For arrays, initialization can appear anywhere in your code, but
you can also use a special kind of initialization expression that must occur at the point the
array is created. This special initialization is a set of values surrounded by curly braces.
The storage allocation (the equivalent of using new) will be taken care of by the compiler
in this case. For example:
int a1[] = { 1, 2, 3, 4, 5 };
So why would you ever define an array handle without an array?
int a2[];
Well, it’s possible to assign one array to another in Java, so you can say:
a2 = a1;
What you’re really doing is copying a handle, as demonstrated here:
//: Arrays.java
// Arrays of primitives.
public class Arrays {
public static void main(String args[]) {
int a1[] = { 1, 2, 3, 4, 5 };
int a2[];
a2 = a1;
for(int i = 0; i < a2.length; i++)
a2[i]++;
for(int i = 0; i < a1.length; i++)
prt("a1[" + i + "] = " + a1[i]);
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
You can see that a1 is given an initialization value while a2 is not; a2 is assigned later – in
this case, to another array.
There’s something new here: all arrays have an intrinsic member (whether they’re arrays
of objects or arrays of primitives) that you can query – but not change – to tell you how
many elements there are in the array: length. Since arrays in Java, like C and C++, start
counting from element zero the largest element you can index is length - 1. If you go out
of bounds C and C++ quietly accept this and allow you to stomp all over your memory,
the source of many infamous bugs. However, Java protects you against such problems by
causing a run-time error (called an exception, the subject of Chapter 9) if you step out of
bounds. Of course, checking every array access costs time and code and there’s no way to
turn it off, which means that array accesses may be a source of inefficiency in your
program if they occur at a critical juncture. For Internet security and programmer
productivity, the Java designers felt this was a worthwhile tradeoff.
Chapter 4: Initialization & Cleanup 143](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-143-320.jpg)
![What if you don’t know how many elements you’re going to need in your array while
you’re writing the program? You simply use new to create the elements in the array.
Here, new works even though it’s creating an array of primitives (new won’t create a
non-array primitive):
//: ArrayNew.java
// Creating Arrays with new.
import java.util.*;
public class ArrayNew {
static Random rand = new Random();
static int pRand(int mod) {
return Math.abs(rand.nextInt()) % mod;
}
public static void main(String args[]) {
int a[];
a = new int[pRand(20)];
prt("length of a = " + a.length);
for(int i = 0; i < a.length; i++)
prt("a[" + i + "] = " + a[i]);
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
Since the size of the array is chosen at random (using the pRand( ) method defined
earlier), it’s clear that array creation is actually happening at run-time. In addition, you’ll
see from the output of this program that array elements of primitive types are
automatically initialized to ”empty” values (if they’re numeric, this is zero, if they are
char, it’s null).
Of course, the array could also have been defined and initialized in the same statement:
int a[] = new int[pRand(20)];
If you’re dealing with an array of non-primitive objects, you must always use new. Here,
the handle issue comes up again because what you create is an array of handles. Consider
the wrapper type Integer which is a class and not a primitive:
//: ArrayClassObj.java
// Creating an array of non-primitive objects.
import java.util.*;
public class ArrayClassObj {
static Random rand = new Random();
static int pRand(int mod) {
return Math.abs(rand.nextInt()) % mod;
}
public static void main(String args[]) {
Integer a[] = new Integer[pRand(20)];
prt("length of a = " + a.length);
for(int i = 0; i < a.length; i++) {
a[i] = new Integer(pRand(500));
prt("a[" + i + "] = " + a[i]);
}
}
144 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-144-320.jpg)
![static void prt(String s) {
System.out.println(s);
}
} ///:~
Here, even after new is called to create the array:
Integer a[] = new Integer[pRand(20)];
it’s only an array of handles, and not until the handle itself is initialized by creating a new
Integer object is the initialization complete:
a[i] = new Integer(pRand(500));
If you forget to create the object, however, you’ll get an exception at run-time when you
try to read the empty array location.
Take a look at the formation of the String object inside the print statements. You can see
that the handle to the Integer object is automatically converted to produce a String
representing the value inside the object.
It’s also possible to initialize arrays of objects using the curly-brace-enclosed list. There
are two forms, the first of which is the only one allowed in Java 1.0. The second
(equivalent) form is allowed in Java 1.1:
//: ArrayInit.java
// Array initialization
public class ArrayInit {
public static void main(String args[]) {
Integer[] a = {
new Integer(1),
new Integer(2),
new Integer(3),
};
// Java 1.1 only:
Integer[] b = new Integer[] {
new Integer(1),
new Integer(2),
new Integer(3),
};
}
} ///:~
This is useful at times, but it’s more limited since the size of the array is determined at
compile time. The last comma in the list of initializers is optional (this feature makes for
easier maintenance of long lists).
The second form of array initialization, added in Java 1.1, provides a convenient syntax to
create and call methods that take variable argument lists, including, if you choose,
unknown size as well as unknown type. Since all classes are ultimately inherited from the
common root class Object, you can create a method that take an array of Object and call
it like this:
//: VarArgs.java
// Using the Java 1.1 array syntax to create
// variable argument lists
Chapter 4: Initialization & Cleanup 145](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-145-320.jpg)
![class A { int i; }
public class VarArgs {
static void f(Object[] x) {
for(int i = 0; i < x.length; i++)
System.out.println(x[i]);
}
public static void main(String args[]) {
f(new Object[] {
new Integer(47), new VarArgs(),
new Float(3.14), new Double(11.11) });
f(new Object[] {"one", "two", "three" });
f(new Object[] {new A(), new A(), new A()});
}
} ///:~
At this point, there’s not much you can do with these unknown objects, and this program
uses the automatic String conversion to do something useful with each Object. In Chapter
11 (Run-Time Type Identification, or RTTI) you’ll learn how to discover the exact type of
such objects so you can do something more interesting with them.
Mulitdimensional arrays
Java allows you to easily create multidimensional arrays:
//: MultiDimArray.java
// Creating multi-dimensional arrays.
import java.util.*;
public class MultiDimArray {
static Random rand = new Random();
static int pRand(int mod) {
return Math.abs(rand.nextInt()) % mod;
}
public static void main(String args[]) {
int a1[][] = {
{ 1, 2, 3, },
{ 4, 5, 6, },
};
for(int i = 0; i < a1.length; i++)
for(int j = 0; j < a1[i].length; j++)
prt("a1[" + i + "][" + j +
"] = " + a1[i][j]);
// 3-D array with fixed length:
int a2[][][] = new int[2][2][4];
for(int i = 0; i < a2.length; i++)
for(int j = 0; j < a2[i].length; j++)
for(int k = 0; k < a2[i][j].length;
k++)
prt("a2[" + i + "][" +
j + "][" + k +
"] = " + a2[i][j][k]);
// 3-D array with varied-length vectors:
int a3[][][] = new int[pRand(7)][][];
for(int i = 0; i < a3.length; i++) {
146 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-146-320.jpg)
![a3[i] = new int[pRand(5)][];
for(int j = 0; j < a3[i].length; j++)
a3[i][j] = new int[pRand(5)];
}
for(int i = 0; i < a3.length; i++)
for(int j = 0; j < a3[i].length; j++)
for(int k = 0; k < a3[i][j].length;
k++)
prt("a3[" + i + "][" +
j + "][" + k +
"] = " + a3[i][j][k]);
// Array of non-primitive objects:
Integer[][] a4 = {
{ new Integer(1), new Integer(2)},
{ new Integer(3), new Integer(4)},
{ new Integer(5), new Integer(6)},
};
for(int i = 0; i < a4.length; i++)
for(int j = 0; j < a4[i].length; j++)
prt("a4[" + i + "][" + j +
"] = " + a4[i][j]);
Integer[][] a5;
a5 = new Integer[3][];
for(int i = 0; i < a5.length; i++) {
a5[i] = new Integer[3];
for(int j = 0; j < a5[i].length; j++)
a5[i][j] = new Integer(i*j);
}
for(int i = 0; i < a5.length; i++)
for(int j = 0; j < a5[i].length; j++)
prt("a5[" + i + "][" + j +
"] = " + a5[i][j]);
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
The code used for printing uses length so that it doesn’t depend on fixed array sizes.
The first example shows a multi-dimensional array of primitives. You delimit each vector
in the array with curly braces:
int a1[][] = {
{ 1, 2, 3, },
{ 4, 5, 6, },
};
Each set of square brackets moves you into the next level of the array.
The second example shows a three-dimensional array allocated with new. Here, the whole
array is allocated all at once:
int a2[][][] = new int[2][2][4];
But the third example shows that each vector in the arrays that make up the matrix can
be of any length:
Chapter 4: Initialization & Cleanup 147](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-147-320.jpg)
![int a3[][][] = new int[pRand(7)][][];
for(int i = 0; i < a3.length; i++) {
a3[i] = new int[pRand(5)][];
for(int j = 0; j < a3[i].length; j++)
a3[i][j] = new int[pRand(5)];
}
The first new creates an array with a random-length first element and the rest
undetermined. The second new inside the for loop fills out the elements but leaves the
third index undetermined until you hit the third new.
You will see from the output that array values are automatically initialized to zero if you
don’t give them an explicit initialization value.
You can deal with arrays of non-primitive objects in a similar fashion, which is shown in
the fourth example, demonstrating the ability to collect many new expressions with curly
braces:
Integer[][] a4 = {
{ new Integer(1), new Integer(2)},
{ new Integer(3), new Integer(4)},
{ new Integer(5), new Integer(6)},
};
The fifth example shows how an array of non-primitive objects can be built up piece by
piece:
Integer[][] a5;
a5 = new Integer[3][];
for(int i = 0; i < a5.length; i++) {
a5[i] = new Integer[3];
for(int j = 0; j < a5[i].length; j++)
a5[i][j] = new Integer(i*j);
}
The i*j is just to put an interesting value into the Integer.
Summary
The seemingly elaborate mechanism for initialization – the constructor – provided by Java
should give you a strong hint about the critical importance placed on initialization in the
language. As Stroustrup was designing C++, one of the first observations he made about
productivity in C was that a very significant portion of programming problems are caused
by improper initialization of variables. These kinds of bugs are very hard to find, and
similar issues apply to improper cleanup. Because constructors allow you to guarantee
proper initialization and cleanup (the compiler will not allow an object to be created
without the proper constructor calls), you get complete control and safety.
In C++, destruction is quite important because objects created with new must be
explicitly destroyed. In Java, memory for all objects is automatically released by the
garbage collector, so the equivalent cleanup method in Java isn’t necessary much of the
time. Thus (in cases where you don’t need destructor-like behavior) Java’s garbage
collector greatly simplifies programming, and adds much-needed safety in managing
memory. Some garbage collectors are even cleaning up other resources like graphics and
file handles. However, the garbage collector does add a run-time cost, the expense of which
is difficult to put into perspective because of the overall slowness of Java interpreters at
148 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-148-320.jpg)
![// Creating a package
package com.eckelobjects.util;
public class Vector {
public Vector() {
System.out.println(
"com.eckelobjects.util.Vector");
}
} ///:~
When you create your own packages, you’ll discover that the package statement must be
the first non-comment code in the file. The second file looks much the same:
//: List.java
// Creating a package
package com.eckelobjects.util;
public class List {
public List() {
System.out.println(
"com.eckelobjects.util.List");
}
} ///:~
Both of these files are placed in the subdirectory on my system:
C:DOCJavaTcomeckelobjectsutil
If you walk back through this, you can see the package name com.eckelobjects.util, but
what about the first portion of the path? That’s taken care of in the CLASSPATH
environment variable, which is, on my machine:
CLASSPATH=.;D:JAVALIB;C:DOCJavaT
You can see that the CLASSPATH can contain a number of alternative search paths.
Now, the following file can be placed in any directory (see page 80 if you have trouble
executing this program):
//: LibTest.java
// Uses the library
package c05;
import com.eckelobjects.util.*;
public class LibTest {
public static void main(String args[]) {
Vector v = new Vector();
List l = new List();
}
} ///:~
When the compiler encounters the import statement, it begins searching at the directories
specified by CLASSPATH, looking for a subdirectory comeckelobjectsutil, then seeking the
compiled files of the appropriate names (Vector.class for Vector and List.class for List).
Note that both the classes and the desired methods in Vector and List must be public.
Chapter 5: Hiding the Implementation 155](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-155-320.jpg)
![Automatic compilation
The first time you create an object of an imported class (or you access a static member of
a class), the compiler will go hunting for the .class file of the same name (so if you’re
creating an object of class X, it looks for X.class) in the appropriate directory. If it finds
only X.class, that’s what it must use. However, if it also finds an X.java in the same
directory, the compiler will first compare the date stamp on the two files, and if X.java is
more recent than X.class, it will automatically recompile X.java to generate an up-to-date
X.class.
If a class is not in a .java file of the same name as that class, this behavior will not occur
for that class.
Collisions
What happens if two libraries are imported via * and they include the same names? For
example, suppose a program does this:
import com.eckelobjects.util.*;
import java.util.*;
Since java.util.* also contains a Vector class, this causes a potential collision. However, as
long as the collision doesn’t actually occur, everything is OK – which is good because
otherwise you might end up doing a lot of typing to prevent collisions that would never
happen.
The collision does occur if you now try to make a Vector:
Vector v = new Vector();
Which Vector class does this refer to? The compiler can’t know, and the reader can’t know
either. So the compiler complains and forces you to be explicit. If I want the standard Java
Vector, for example, I must say:
java.util.Vector v = new java.util.Vector();
Since this (along with the CLASSPATH) completely specifies the location of that Vector,
there’s no need for the import java.util.* statement unless I’m using something else from
java.util.
A custom tool library
With this knowledge in hand, you can now create your own libraries of tools to reduce or
eliminate duplicate code. Consider, for example, creating an alias for
System.out.println( ) to reduce typing. This can be part of a package called tools:
//: P.java
// The P.rint & P.rintln shorthand
package com.eckelobjects.tools;
public class P {
public static void rint(Object obj) {
System.out.print(obj);
}
public static void rint(String s) {
System.out.print(s);
}
public static void rint(char s[]) {
156 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-156-320.jpg)
![System.out.print(s);
}
public static void rint(char c) {
System.out.print(c);
}
public static void rint(int i) {
System.out.print(i);
}
public static void rint(long l) {
System.out.print(l);
}
public static void rint(float f) {
System.out.print(f);
}
public static void rint(double d) {
System.out.print(d);
}
public static void rint(boolean b) {
System.out.print(b);
}
public static void rintln() {
System.out.println();
}
public static void rintln(Object obj) {
System.out.println(obj);
}
public static void rintln(String s) {
System.out.println(s);
}
public static void rintln(char s[]) {
System.out.println(s);
}
public static void rintln(char c) {
System.out.println(c);
}
public static void rintln(int i) {
System.out.println(i);
}
public static void rintln(long l) {
System.out.println(l);
}
public static void rintln(float f) {
System.out.println(f);
}
public static void rintln(double d) {
System.out.println(d);
}
public static void rintln(boolean b) {
System.out.println(b);
}
} ///:~
All the different data types can now be printed out either with a newline (P.rintln( )) or
without a newline (P.rint( )).
Chapter 5: Hiding the Implementation 157](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-157-320.jpg)
![You can guess that the location of this file must be in a directory that starts at one of the
CLASSPATH locations, then continues com/eckelobjects/tools. After compiling, the P.class
file can be used anywhere on your system with an import statement:
//: ToolTest.java
// Uses the tools library
import com.eckelobjects.tools.*;
public class ToolTest {
public static void main(String args[]) {
P.rintln("Available from now on!");
}
} ///:~
So from now on, whenever you come up with a useful new utility, you can add it to the
tools directory (or to your own personal util or tools directory).
Classpath pitfall
The P.java file brought up an interesting pitfall. Especially with early implementations of
Java, setting the classpath correctly is generally quite a headache. During the development
of this book, the P.java file was introduced and seemed to work fine, but at some point it
began breaking. For a long time I was certain this was the fault of one implementation of
Java or another, but finally I discovered that at one point I had introduced a program
(CodePackager.java, shown in Chapter 17) that also used a different class P. Because it
was used as a tool, it was sometimes placed in the classpath, and other times it wasn’t.
When it was, the P in CodePackager.java was found first by Java when executing a
program where it was looking for the class in com.eckelobjects.tools, and the compiler
would say that a particular method couldn’t be found. This was very frustrating, because
you can see the method in the above class P, and no further diagnostics were reported to
give you a clue that it was finding a completely different class (that wasn’t even public).
At first this could seem like a compiler bug, but if you look at the import statement it
only says “here’s where you might find P.” However, the compiler is supposed to look
anywhere in its classpath so if it finds a P there it will use it, and if it finds the “wrong”
one first during a search then it will stop looking. This is slightly different than the case
described on page 156 because there the offending classes were both in packages, and here
there was a P that was not in a package, but could still be found during a normal
classpath search.
If you’re having an experience like this, check to make sure there’s only one class of each
name anywhere in your classpath.
Package caveat
It’s worth remembering that anytime you create a package, you implicitly specify a
directory structure when you give the package a name. The package must live in the
directory indicated by its name, which must be a directory that is searchable starting from
the CLASSPATH. This means that experimenting with the package keyword can be a bit
frustrating at first because unless you adhere to the package-name to directory-path rule,
you’ll get a lot of mysterious run-time messages about not being able to find a particular
class, even if that class is sitting there in the same directory. If you get such a message,
try commenting out the package statement, and if it runs you’ll know where the problem
lies.
158 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-158-320.jpg)
![programmer who is using the library. Suppose you define a package dessert containing
the following compilation unit (see page 80 if you have trouble executing this program):
//: Cookie.java
// Creates a library
package c05.dessert;
public class Cookie {
public Cookie() {
System.out.println("Cookie constructor");
}
void foo() { System.out.println("foo"); }
} ///:~
Remember, Cookie.java must reside in a subdirectory called dessert, in a directory under
C05 (indicating Chapter 5 of this book) which itself must be under one of the CLASSPATH
directories. Don’t make the mistake of thinking that Java will always look at the current
directory as one of the starting points for searching: if you don’t have a ‘.’ as one of the
paths in your CLASSPATH, Java won’t look there.
Now if you create a program that uses Cookie:
//: Dinner.java
// Uses the library
import c05.dessert.*;
public class Dinner {
public Dinner() {
System.out.println("Dinner constructor");
}
public static void main(String args[]) {
Cookie x = new Cookie();
//! x.foo(); // Can't access
}
} ///:~
You can create a Cookie object since its constructor is public and the class itself is public
(we’ll look more at the concept of a public class later). However, the foo( ) member is
inaccessible inside Dinner.java since foo( ) is friendly only within package dessert.
The unnamed package
You might be surprised to discover that the following code compiles, even though it would
appear that it breaks the rules:
//: Cake.java
// Accesses a class in a separate
// compilation unit.
class Cake {
public static void main(String args[]) {
Pie x = new Pie();
x.f();
}
} ///:~
In a second file, in the same directory:
160 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-160-320.jpg)
![//: Pie.java
// The other class
class Pie {
void f() { System.out.println("Pie.f()"); }
} ///:~
You might initially view these as completely foreign files, and yet Cake is able to create a
Pie object and call its f( ) method! You’d normally think that Pie and f( ) are friendly and
therefore not available to Cake. They are friendly – that part is correct. The reason they
are available in Cake.java is because they are in the same directory and have no explicit
package name. Java treats files like this as implicitly part of the “unnamed package” for
that directory, and therefore friendly to all the other files in that directory.
private : you can’t touch that!
The private keyword means no one can access that member except that particular class,
inside methods of that class. Other classes in the same package cannot access private
members, so it’s as if you’re even insulating the class against yourself. On the other hand,
it’s not unlikely that a package might be created by several people collaborating together,
so private allows you to freely change that member without concern that it will affect
another class in the same package. The default “friendly” package access is often an
adequate amount of hiding – remember, a “friendly” member is inaccessible to the user of
the package. This is nice, since the default access is the one that you normally use. Thus,
you’ll typically think about access primarily for the members that you explicitly want to
make public for the client programmer, and as a result you might not initially think
you’ll use the private keyword very often since it’s tolerable to get away without it (this
is a distinct contrast with C++). However, it turns out that the consistent use of private
is very important, especially where multithreading is concerned (as you’ll see in Chapter
14).
Here’s an example of the use of private:
//: IceCream.java
// Demonstrates "private" keyword
class Sundae {
private Sundae() {}
static Sundae makeASundae() {
return new Sundae();
}
}
public class IceCream {
public static void main(String args[]) {
//! Sundae x = new Sundae();
Sundae x = Sundae.makeASundae();
}
} ///:~
This shows an example where private comes in handy: you might want to control how an
object is created, and prevent anyone from directly accessing a particular constructor (or
Chapter 5: Hiding the Implementation 161](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-161-320.jpg)
![all of them). In the above example, you cannot create a Sundae object via its constructor –
instead you must call the makeASundae( ) method to do it for you.3
Any method that you’re certain is only a “helper” method for that class can be made
private to ensure that you don’t accidentally use it elsewhere in the package and thus
prohibit you from changing or removing the method. Making a method private
guarantees that you retain this option.
protected : “sort of friendly”
The protected access specifier requires a jump ahead to understand. So first, you should
be aware that you don’t need to understand this section to continue through the book up
through the inheritance chapter. But for completeness an example using protected will be
briefly described.
The protected keyword deals with a concept called inheritance, which takes an existing
class and adds new members to that class without touching the existing class, which we
refer to as the base class. You can also change the behavior of existing members of the
class. To inherit from an existing class, you say that your new class extends an existing
class, like this:
class Foo extends Bar {
The rest of the class definition looks the same.
If you create a new package and you inherit from a class in another package, the only
members you have access to are the public members of the original package (of course, if
you perform the inheritance in the same package you have the normal package access to
all the “friendly” members). Sometimes the creator of the base class would like to take a
particular member and grant access to derived classes but not the world in general. That’s
what protected does. If you refer back to the file Cookie.java on page 159, the following
class cannot access the “friendly” member:
//: ChocolateChip.java
// Can't access friendly member
// in another class
import c05.dessert.*;
public class ChocolateChip extends Cookie {
public ChocolateChip() {
System.out.println(
"ChocolateChip constructor");
}
public static void main(String args[]) {
ChocolateChip x = new ChocolateChip();
//! x.foo(); // Can't access foo
}
} ///:~
One of the interesting things about inheritance is that if a method foo( ) exists in class
Cookie, then it also exists in any class inherited from Cookie. But since foo( ) is “friendly”
in a foreign package, it’s unavailable to us in this one. Of course, you could make it
3 There’s another effect in this case: since the default constructor is the only one defined, and it’s
private, it will prevent inheritance of this class (a subject that will be introduced in Chapter 6).
162 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-162-320.jpg)
![package c05.foreign;
import c05.*;
public class Foreign {
public static void main (String args[]) {
PackagedClass pc = new PackagedClass();
}
} ///:~
Explain why the compiler generates an error. Would making the Foreign class part of the
c05 package change anything?
168 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-168-320.jpg)
![objects, a couple of primitives and an object of another class. For the non-primitive
objects, just put handles inside your new class, and for the primitives just define them
inside your class (see page 80 if you have trouble executing this program):
//: SprinklerSystem.java
// Composition for code reuse
package c06;
class WaterSource {
private String s;
WaterSource() {
System.out.println("WaterSource()");
s = new String("Constructed");
}
public String toString() { return s; }
}
public class SprinklerSystem {
private String valve1, valve2, valve3, valve4;
WaterSource source;
int i;
float f;
void print() {
System.out.println("valve1 = " + valve1);
System.out.println("valve2 = " + valve2);
System.out.println("valve3 = " + valve3);
System.out.println("valve4 = " + valve4);
System.out.println("i = " + i);
System.out.println("f = " + f);
System.out.println("source = " + source);
}
public static void main(String args[]) {
SprinklerSystem x = new SprinklerSystem();
x.print();
}
} ///:~
One of the methods defined in WaterSource is special: toString( ). You will learn later
that every non-primitive object has a toString( ) method, and it’s called in special
situations when the compiler wants a string but its got one of these objects. So in the
expression:
System.out.println("source = " + source);
The compiler sees you trying to add a String object (“source = “) to a WaterSource. This
doesn’t make sense to it, because you can “add” a String only to another String, so it says
“I’ll turn source into a String by calling toString( )!” After doing this it can combine the
two Strings and pass the resulting String to System.out.println( ). Any time you want to
allow this behavior with a class you create you need only to write a toString( ) method.
At first glance, you might assume – Java being as safe and careful as it is – that the
compiler would automatically construct objects for each of the handles in the above code,
for example calling the default constructor for WaterSource to initialize source. The
output of the print statement is in fact:
valve1 = null
valve2 = null
170 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-170-320.jpg)
![System.out.println("s2 = " + s2);
System.out.println("s3 = " + s3);
System.out.println("s4 = " + s4);
System.out.println("i = " + i);
System.out.println("toy = " + toy);
System.out.println("castille = " + castille);
}
public static void main(String args[]) {
Bath b = new Bath();
b.print();
}
} ///:~
Notice that in the Bath constructor a statement is executed before any of the
initializations take place. When you don’t initialize at the point of definition, there’s still
no guarantee that you’ll perform any initialization before you send a message to an object
handle – except for the inevitable run-time exception.
Here’s the output for the program:
Inside Bath()
Soap()
s1 = Happy
s2 = Happy
s3 = Joy
s4 = Joy
i = 47
toy = 3.14
castille = Constructed
When print( ) is called it fills in s4 so all the fields are properly initialized by the time
they are used.
Inheritance syntax
Inheritance is such an integral part of Java (and OOP languages in general) that it was
introduced in Chapter 1 and has been used occasionally in chapters before this one, since
certain situations required it. In addition, you’re always doing inheritance when you
create a class, because if you don’t say otherwise you inherit from Java’s standard root
class Object.
The syntax for composition is obvious, but to perform inheritance there’s a distinctly
different form. When you inherit, you are saying, “This new class is like that old class.”
You state this in code by giving the name of the class, as usual, but before the opening
brace of the class body, you put the keyword extends followed by the name of the base
class. When you do this, you automatically get all the data members and methods in the
base class. Here’s an example:
//: Detergent.java
// Inheritance syntax & properties
class Cleanser {
private String s = new String("Cleanser");
public void append(String a) { s += a; }
public void dilute() { append(" dilute()"); }
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![public void apply() { append(" apply()"); }
public void scrub() { append(" scrub()"); }
public void print() { System.out.println(s); }
public static void main(String args[]) {
Cleanser x = new Cleanser();
x.dilute(); x.apply(); x.scrub();
x.print();
}
}
public class Detergent extends Cleanser {
// Change a method:
public void scrub() {
append(" Detergent.scrub()");
super.scrub(); // Call base-class version
}
// Add methods to the interface:
public void foam() { append(" foam()"); }
// Test the new class:
public static void main(String args[]) {
Detergent x = new Detergent();
x.dilute();
x.apply();
x.scrub();
x.foam();
x.print();
System.out.println("Testing base class:");
Cleanser.main(args);
}
} ///:~
This demonstrates a number of features. First, in the Cleanser append( ) method, Strings
are concatenated to s using the += operator, which is one of the operators (along with
‘+’) that the Java designers “overloaded” to work with Strings.
Second, both Cleanser and Detergent contain a main( ) method. You can create a main( )
for each one of your classes, and it’s often recommended to code this way so that your test
code is wrapped in with the class. Even if you have lots of classes in a program, only the
main( ) for the public class invoked on the command line will be called (and you can have
only one public class per file). So in this case, when you say java Detergent,
Detergent.main( ) will be called. But you can also say java Cleanser to invoke
Cleanser.main( ), even though Cleanser is not a public class. This technique of putting a
main( ) in each class allows easy unit testing for each class. In addition, you don’t have to
remove the main( ) when you’re finished testing; you can leave it in for later testing.
Here, you can see that Detergent.main( ) calls Cleanser.main( ) explicitly.
It’s important that all the methods in Cleanser are public. Remember that if you leave off
any access specifier the member defaults to “friendly,” which allows access only to
package members. Thus, within this package, anyone could use those methods if there
were no access specifier. Detergent would have no trouble, for example. However if a class
from some other package were to inherit Cleanser it could access only public members. So
to plan for inheritance, as a general rule make all fields private and all methods public
(protected members also allow access by derived classes; you’ll learn about this later). Of
course, in particular cases you’ll need to make adjustments, but this is a useful guideline.
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![Cartoon() {
System.out.println("Cartoon constructor");
}
public static void main(String args[]) {
Cartoon x = new Cartoon();
}
} ///:~
The output for this program shows the automatic calls:
Art constructor
Drawing constructor
Cartoon constructor
You can see that the construction happens from the base “outward,” so the base class is
initialized before the derived-class constructors can access it.
Even if you don’t create a constructor for Cartoon( ), the compiler will synthesize a
default constructor for you that calls the base class constructor.
Constructors with arguments
The above example has default constructors – that is, they don’t have any arguments. It’s
easy for the compiler to call these because there’s no question about what arguments to
pass. But what if your class doesn’t have default arguments or you want to call a base-
class constructor that has an argument? You must explicitly write the calls to the base-
class constructor using the super keyword and the appropriate argument list:
//: Chess.java
// Inheritance, constructors and arguments
class Game {
Game(int i) {
System.out.println("Game constructor");
}
}
class BoardGame extends Game {
BoardGame(int i) {
super(i);
System.out.println("BoardGame constructor");
}
}
public class Chess extends BoardGame {
Chess() {
super(11);
System.out.println("Chess constructor");
}
public static void main(String args[]) {
Chess x = new Chess();
}
} ///:~
If you don’t call the base-class constructor in BoardGame( ), the compiler will complain
that it can’t find a constructor of the form Game( ). In addition, the call to the base-class
constructor must be the first thing you do in the derived-class constructor (the compiler
will remind you if you get it wrong).
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![super(i);
System.out.println("Knife constructor");
}
}
// A cultural way of doing something:
class Custom {
Custom(int i) {
System.out.println("Custom constructor");
}
}
public class PlaceSetting extends Custom {
Spoon sp;
Fork frk;
Knife kn;
DinnerPlate pl;
PlaceSetting(int i) {
super(i + 1);
sp = new Spoon(i + 2);
frk = new Fork(i + 3);
kn = new Knife(i + 4);
pl = new DinnerPlate(i + 5);
System.out.println(
"PlaceSetting constructor");
}
public static void main(String args[]) {
PlaceSetting x = new PlaceSetting(9);
}
} ///:~
While the compiler forces you to initialize the base classes, and requires that you do it
right at the beginning of the constructor, it doesn’t watch over you to make sure you
initialize the member objects, so you must remember to pay attention to that.
Guaranteeing proper cleanup
Java doesn’t have the C++ concept of a destructor, a method that is automatically called
when an object is destroyed. The reason is probably that in Java the practice is simply to
forget about objects rather than destroying them, allowing the garbage collector to reclaim
the memory as necessary.
Often this is fine, but there are times when your class may perform some activities during
its lifetime that require cleanup. As mentioned in Chapter 4, you can’t rely on when the
garbage collector will be called, or if it will ever be called. Thus, if you want something
cleaned up for a class, you must write a special method to do it explicitly, and make sure
that the client programmer knows they must call this method. On top of this, as described
in Chapter 9 (exception handling), you must guard against an exception by putting such
cleanup in a finally clause.
Consider an example of a computer-aided-design system that draws pictures on the
screen:
//: CADSystem.java
// Ensuring proper cleanup
import java.util.*;
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![class Shape {
Shape(int i) {
System.out.println("Shape constructor");
}
void cleanup() {
System.out.println("Shape cleanup");
}
}
class Circle extends Shape {
Circle(int i) {
super(i);
System.out.println("Drawing a Circle");
}
void cleanup() {
System.out.println("Erasing a Circle");
super.cleanup();
}
}
class Triangle extends Shape {
Triangle(int i) {
super(i);
System.out.println("Drawing a Triangle");
}
void cleanup() {
System.out.println("Erasing a Triangle");
super.cleanup();
}
}
class Line extends Shape {
private int start, end;
Line(int start, int end) {
super(start);
this.start = start;
this.end = end;
System.out.println("Drawing a Line: " +
start + ", " + end);
}
void cleanup() {
System.out.println("Erasing a Line: " +
start + ", " + end);
super.cleanup();
}
}
public class CADSystem extends Shape {
private Circle c;
private Triangle t;
private Line[] lines = new Line[10];
CADSystem(int i) {
super(i + 1);
for(int j = 0; j < 10; j++)
lines[j] = new Line(j, j*j);
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![c = new Circle(1);
t = new Triangle(1);
System.out.println("Combined constructor");
}
void cleanup() {
System.out.println("CADSystem.cleanup()");
t.cleanup();
c.cleanup();
for(int i = 0; i < lines.length; i++)
lines[i].cleanup();
super.cleanup();
}
public static void main(String args[]) {
CADSystem x = new CADSystem(47);
try {
// Code and exception handling...
} finally {
x.cleanup();
}
}
} ///:~
Everything in this system is some kind of Shape (which is itself a kind of Object since it’s
implicitly inherited from the root class). Each class redefines Shape’s cleanup( ) method
in addition to calling the base-class version of that method using super. The specific
Shape classes Circle, Triangle and Line all have constructors that “draw,” although any
method called during the lifetime of the object could be responsible for doing something
that needs cleanup. Each class has its own cleanup( ) method to restore non-memory
things back to the way they were before the object existed.
In main( ), you can see two keywords that are new, and won’t officially be introduced
until Chapter 9: try and finally. The try keyword indicates that the block that follows
(delimited by curly braces) is a guarded region, which means that it is given special
treatment. One of these special treatments is that the code in the finally clause following
this guarded region is always executed, no matter how the try block exits (with exception
handling, it’s possible to leave a try block in a number of non-ordinary ways). Here, the
finally clause is saying “always call cleanup( ) for x, no matter what happens.” These
keywords will be explained thoroughly in Chapter 9.
Note that in your cleanup method you must also pay attention to the order in which the
base-class and member-object cleanup methods get called, in case one subobject may
depend on another. In general you should follow the same form that is imposed by a C++
compiler on its destructors: first perform all the work specific to your class (which may
require that base-class elements still be viable) then lastly call the base-class cleanup
method, as demonstrated here.
There may be many cases where the cleanup issue is not a problem; you just let the
garbage collector do the work. But when you must do it explicitly, diligence and attention
is required.
Order of garbage collection
There’s not much you can rely on when it comes to garbage collection. The garbage
collector might never be called. If it is, it can reclaim objects in any order it wants. In
addition, implementations of the garbage collector in Java 1.0 often don’t call the
finalize( ) methods. It’s best not to rely on garbage collection for anything but memory
reclamation, and if you want cleanup to take place, make your own cleanup methods and
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![don’t rely on finalize( ) (as mentioned earlier, Java 1.1 can be forced to call all the
finalizers).
Name hiding
Only C++ programmers might be surprised by this, since it works differently in that
language. If a Java base class has a method name that’s overloaded several times,
redefining that method name in the derived class will not hide any of the base-class
versions. Thus overloading works regardless of whether the method was defined at this
level or in a base class:
//: Hide.java
// Overloading a base-class method name
// in a derived class does not hide the
// base-class versions
class Homer {
char doh(char c) {
System.out.println("doh(char)");
return 'd';
}
float doh(float f) {
System.out.println("doh(float)");
return 1.0f;
}
}
class Milhouse {}
class Bart extends Homer {
void doh(Milhouse m) {}
}
class Hide {
public static void main(String args[]) {
Bart b = new Bart();
b.doh(1); // doh(float) used
b.doh('x');
b.doh(1.0f);
b.doh(new Milhouse());
}
} ///:~
As you’ll see in the next chapter, it’s far more common to override methods of the same
name using exactly the same signature and return type as in the base class. It can be
confusing otherwise (which is why C++ disallows it, to prevent you from making what is
probably a mistake).
180 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-180-320.jpg)
![Choosing composition
vs. inheritance
Both composition and inheritance allow you to place subobjects inside your new class. You
may now be wondering what the difference is between the two, and when to choose one
over the other.
Composition is generally used when you want the features of an existing class inside your
new class, but not its interface. That is, you embed an object so that you can use that
object to implement features of your new class, but the user of your new class sees the
interface you’ve defined rather than the interface from the embedded object. For this effect,
you embed private objects of existing classes inside your new class.
Sometimes it makes sense to allow the class user to directly access the composition of
your new class, that is, to make the member objects public. The member objects use
implementation hiding themselves, so this is a safe thing to do and when the user knows
you’re assembling a bunch of parts, it makes the interface easier to understand. A car
object is a good example:
//: Car.java
// Composition with public objects
class Engine {
public void start() {}
public void rev() {}
public void stop() {}
}
class Wheel {
public void inflate(int psi) {}
}
class Window {
public void rollup() {}
public void rolldown() {}
}
class Door {
public Window window = new Window();
public void open() {}
public void close() {}
}
public class Car {
public Engine engine = new Engine();
public Wheel wheel[] = new Wheel[4];
public Door left = new Door(),
right = new Door(); // 2-door
Car() {
for(int i = 0; i < 4; i++)
wheel[i] = new Wheel();
}
public static void main(String args[]) {
Car car = new Car();
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![car.left.window.rollup();
car.wheel[0].inflate(72);
}
} ///:~
Because the composition of a car is part of the analysis of the problem (and not simply
part of the underlying design), making the members public assists the client
programmer’s understanding of how to use the class and requires less code complexity for
the creator of the class.
When you inherit, you take an existing class and make a special version of it. Generally,
this means you’re taking a general-purpose class and specializing it for a particular need.
With a little thought, you’ll see that it would make no sense to compose a car using a
vehicle object – a car doesn’t contain a vehicle, it is a vehicle. The is-a relationship is
expressed with inheritance, and the has-a relationship is expressed with composition.
protected
Now that you’ve been introduced to inheritance, the keyword protected finally has
meaning. In an ideal world, private members would always be hard-and-fast private, but
in real projects there are times when you want to make something hidden from the world
at large and yet allow access for members of derived classes. The protected keyword is a
nod to pragmatism; it says, “This is private as far as the class user is concerned, but
available to anyone who inherits from this class or anyone else in the same package.”
That is, protected in Java is automatically “friendly.”
The best tack to take is to leave the data members private – you should always preserve
your right to change the underlying implementation. You can then allow controlled access
to inheritors of your class through protected methods:
//: Orc.java
// The protected keyword
import java.util.*;
class Villain {
private int i;
protected int read() { return i; }
protected void set(int ii) { i = ii; }
public Villain(int ii) { i = ii; }
public int value(int m) { return m*i; }
}
public class Orc extends Villain {
private int j;
public Orc(int jj) { super(jj); j = jj; }
public void change(int x) { set(x); }
} ///:~
You can see that change( ) has access to set( ) because it’s protected.
Incremental development
One of the advantages of inheritance is that it supports incremental development by allowing
you to introduce new code without causing bugs in existing code. This also isolates new
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![bugs to the new code. By inheriting from an existing, functional class and adding data
members and methods (and redefining existing methods) you leave the existing code – that
someone else may still be using – untouched and unbugged. If a bug happens, you know
it’s in your new code, which is much shorter and easier to read than if you had modified
the body of existing code.
It’s rather amazing how cleanly the classes are separated. You don’t even need the source
code for the methods in order to reuse the code. At most, you just import a package. (This
is true for both inheritance and composition.)
It’s important to realize that program development is an incremental process, just like
human learning. You can do as much analysis as you want, but you still won’t know all
the answers when you set out on a project. You’ll have much more success – and more
immediate feedback – if you start out to “grow” your project as an organic, evolutionary
creature, rather than constructing it all at once like a glass-box skyscraper.
Although inheritance for experimentation can be a useful technique, at some point after
things stabilize you need to take a new look at your class hierarchy with an eye to
collapsing it into a sensible structure. Remember that underneath it all, inheritance is
meant to express a relationship that says, “This new class is a type of that old class.” Your
program should not be concerned with pushing bits around, but instead with creating and
manipulating objects of various types to express a model in the terms that come from the
problem space.
Upcasting
The most important aspect of inheritance is not that it provides methods for the new
class. It’s the relationship expressed between the new class and the base class. This
relationship can be summarized by saying, “The new class is a type of the existing class.”
This description is not just a fanciful way of explaining inheritance – it’s supported
directly by the language. As an example, consider a base class called Instrument that
represents musical instruments and a derived class called Wind. Because inheritance
means that all the methods in the base class are also available in the derived class, any
message you can send to the base class can also be sent to the derived class. If the
Instrument class has a play( ) method, so will Wind instruments. This means we can
accurately say that a Wind object is also a type of Instrument. The following example
shows how the compiler supports this notion:
//: Wind.java
// Inheritance & upcasting
import java.util.*;
class Instrument {
public void play() {}
static void tune(Instrument i) {
// ...
i.play();
}
}
// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
public static void main(String args[]) {
Wind flute = new Wind();
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![static final Value v3 = new Value();
//! final Value v4; // Pre-Java 1.1 Error:
// no initializer
// Arrays:
final int a[] = { 1, 2, 3, 4, 5, 6 };
public void print(String id) {
System.out.println(
id + ": " + "i4 = " + i4 +
", i5 = " + i5);
}
public static void main(String[] args) {
FinalData fd1 = new FinalData();
//! fd1.i1++; // Error: can't change value
fd1.v2.i++; // Object isn't constant!
fd1.v1 = new Value(); // OK -- not final
for(int i = 0; i < fd1.a.length; i++)
fd1.a[i]++; // Object isn't constant!
//! fd1.v2 = new Value(); // Error: Can't
//! fd1.v3 = new Value(); // change handle
//! fd1.a = new int[3];
fd1.print("fd1");
System.out.println("Creating new FinalData");
FinalData fd2 = new FinalData();
fd1.print("fd1");
fd2.print("fd2");
}
} ///:~
Since i1 and I2 are final primitives with compile-time values, they can both be used as
compile-time constants and are not different in any important way. However, I3 is the
more typical way you’ll see such constants defined: public so they’re usable outside the
package, static to emphasize that there’s only one, and final to say it’s a constant. Notice
that final static primitives with constant initial values (that is, compile-time constants)
are named with all capitals by convention. Note that i5 cannot be known at compile time
so it is not capitalized.
Just because something is final doesn’t mean its value is known at compile-time. This is
demonstrated by initializing i4 and i5 at run-time using randomly generated numbers.
This portion of the example also shows the difference between making a final value static
or non-static. This difference shows up only when the values are initialized at run-time,
since the compile-time values are treated the same by the compiler (and presumably
optimized out of existence). The difference is shown in the output from one run:
fd1: i4 = 15, i5 = 9
Creating new FinalData
fd1: i4 = 15, i5 = 9
fd2: i4 = 10, i5 = 9
Note that the values of i4 for fd1 and fd2 are unique, but the value for i5 is not changed
by creating the second FinalData object. That’s because it’s static and is initialized once
upon loading and not each time a new object is created.
The variables v1 through v4 demonstrate the meaning of a final handle. As you can see in
main( ), just because v2 is final doesn’t mean you can’t change its value. However, you
cannot re-bind v2 to a new object, precisely because it’s final. That’s what final means
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![for a handle. You can also see the same meaning holds true for an array, which is just
another kind of handle. Making handles final seems much less useful than making
primitives final.
Blank finals
Java 1.1 allows the creation of blank finals, which are fields that are declared as final but
which are not given an initialization value. In all cases, the blank final must be initialized
before it is used, and the compiler ensures this. However, blank finals provide much more
flexibility in the use of the final keyword since, for example, a final field inside a class can
now be different for each object and yet still retains its immutable quality. Here’s an
example:
//: BlankFinal.java
// "Blank" final data members
class Poppet { }
class BlankFinal {
final int i = 0; // Initialized final
final int j; // Blank final
final Poppet p; // Blank final handle
// Blank finals MUST be initialized
// in the constructor:
BlankFinal() {
j = 1; // Initialize blank final
p = new Poppet();
}
BlankFinal(int x) {
j = x; // Initialize blank final
p = new Poppet();
}
public static void main(String args[]) {
BlankFinal bf = new BlankFinal();
}
} ///:~
You’re forced to perform assignments to finals either with an expression at the point of
definition of the field, or in every constructor. This way it’s guaranteed that the final field
is always initialized before use.
Final arguments
Java 1.1 allows you to make arguments final by declaring them as such in the argument
list. All this means is that inside the method you cannot change what the argument handle
points to:
//: FinalArguments.java
// Using "final" with method arguments
class Gizmo {
public void spin() {}
}
public class FinalArguments {
void with(final Gizmo g) {
//! g = new Gizmo(); // Illegal -- g is final
g.spin();
Chapter 6: Reusing Code & Classes 187](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-187-320.jpg)
![}
void without(Gizmo g) {
g = new Gizmo(); // OK -- g not final
g.spin();
}
public static void main(String args[]) {
FinalArguments bf = new FinalArguments();
bf.without(null);
bf.with(null);
}
} ///:~
Note that you can still assign a null handle to an argument that’s final without the
compiler catching it, just like you can with a non-final argument.
Final methods
There are two reasons for final methods. The first is to put a “lock” on the method to
prevent any inheriting class from changing its meaning. This is done for design reasons,
when you want to make sure that the behavior of a method is retained during inheritance
and cannot be overridden.
The second reason for final methods is efficiency. If you make a method final, you are
allowing the compiler to turn any calls to that method into inline calls. That is, when the
compiler sees a final method call it may (at its discretion) skip the normal approach of
inserting code to perform the method call mechanism (push arguments on the stack, hop
over to the method code and execute it, hop back and clean off the stack arguments, deal
with the return value) and instead replace the method call with a copy of the actual code
in the method body. This eliminates the overhead of the method call. Of course, if a
method is big then your code begins to bloat, and then you probably won’t see any
performance gains from inlining since any improvements will be dwarfed by the amount
of time spent inside the method. It is implied that the Java compiler is able to detect these
situations and choose wisely whether to actually inline a final method. However, it’s
better not to trust that the compiler is able to do this and only make a method final if it’s
quite small or if you want to explicitly prevent overriding.
Any private methods in a class are implicitly final. Because you can’t access a private
method, you can’t override it. You can add the final specifier to a private method but it
doesn’t give that method any extra meaning.
Final classes
When you say that an entire class is final (by preceding its definition with the final
keyword) you state that you don’t want to inherit from this class or allow anyone else to
do so. For some reason the design of your class is such that there is never a need to make
any changes, or for safety or security reasons you don’t want to allow subclassing.
Alternatively you might be dealing with an efficiency issue and you want to make sure
that any activity involved with objects of this class is as efficient as possible.
//: Jurassic.java
// Making an entire class final
class SmallBrain {}
final class Dinosaur {
188 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-188-320.jpg)
![int i = 7;
int j = 1;
SmallBrain x = new SmallBrain();
void f() {}
}
//! class Further extends Dinosaur {}
// error: Cannot extend final class 'Dinosaur'
public class Jurassic {
public static void main(String args[]) {
Dinosaur n = new Dinosaur();
n.f();
n.i = 40;
n.j++;
}
} ///:~
Note that the data members can be final or not, as you choose. The same rules apply to
final for data members regardless of whether the class is defined as final. Defining the
class as final simply prevents inheritance, no more. However, because it prevents
inheritance this means that all methods in a final class are implicitly final, since there’s
no way to override them. Thus the compiler has the same efficiency options as it does if
you explicitly declare a method final.
You can add the final specifier to a method in a final class, but it doesn’t add any
meaning.
Final caution
It can seem to be very sensible to make a method final while you’re designing a class. You
might feel that efficiency is very important when using your class and that no one could
possibly want to override your methods anyway. Sometimes this is true.
But be very careful with your assumptions here. In general, it’s very hard to anticipate
how a class can be reused, especially a general-purpose class. If you define a method as
final you might prevent the possibility of reusing your class through inheritance in some
other programmer’s project simply because you couldn’t imagine it being used that way.
The standard Java library is a good example of this. In particular, the Vector class is
commonly used and might be even more useful if, in the name of efficiency, all the
methods hadn’t been made final. It’s easily conceivable that you might want to inherit and
override with such a fundamentally useful class, but the designers somehow decided this
wasn’t appropriate. This is ironic in two places. First, Stack is inherited from Vector,
which says that a Stack is a Vector, which isn’t really true. Second, many of the most
important methods of Vector, such as addElement( ) and elementAt( ) are synchronized,
which as you shall see in Chapter 14 incurs a significant performance overhead that
probably wipes out any gains provided by final. This lends credence to the theory that
programmers are consistently bad at guessing where optimizations should occur. It’s just
too bad that such a clumsy design made it into the standard library, where we all have to
cope with it.
It’s also interesting to note that Hashtable, another important standard library class, does
not have any final methods. As mentioned elsewhere in this book, it’s quite obvious that
some classes were designed by completely different people than others (note the brevity of
the method names in Hashtable compared to Vector). This is precisely the sort of thing
that should not be obvious to consumers of a class library – when things are inconsistent it
Chapter 6: Reusing Code & Classes 189](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-189-320.jpg)
![static int prt(String s) {
System.out.println(s);
return 63;
}
public static void main(String args[]) {
prt("Beetle constructor");
Beetle b = new Beetle();
}
} ///:~
The output for this program is:
static Insect.x initialized
static Beetle.x initialized
Beetle constructor
i = 9, j = 0
Beetle.k initialized
k = 63
j = 39
The first thing that happens when you run Java on Beetle is that the loader goes out and
finds that class. In the process of loading it, the loader notices that it has a base class
(that’s what the extends keyword says), which it then loads. This will happen whether or
not you’re actually going to make an object of that base class (try commenting out the
object creation to prove it to yourself).
If the base class itself has a base class, that second base class would then be loaded, and so
on. Next, the static initialization in the root base class (in this case, Insect) is performed,
then the next derived class and so on. This is important because the derived-class static
initialization might depend on the base class member being initialized properly.
At this point, the necessary classes have all been loaded so the object can be created. First,
all the primitives in this object are set to their default values and the object handles are set
to null. Then the base-class constructor will be called; in this case the call is automatic but
you may also specify the constructor call (as the first operation in the Beetle( )
constructor) using super. The base class construction goes through the same process in
the same order as the derived-class constructor. After the base-class constructor
completes, the instance variables are initialized, in textual order. Finally, the rest of the
body of the constructor is executed.
Summary
Both inheritance and composition allow you to create a new type from existing types.
Typically, however, you use composition to reuse existing types as part of the underlying
implementation of the new type and inheritance when you want to reuse the interface.
Since the derived class has the base-class interface, it can be upcast to the base, which is
critical for polymorphism as you’ll see in the next chapter.
Despite the strong emphasis on inheritance in object-oriented programming, when
starting a design, you should generally prefer composition during the first cut, and use
inheritance only when it is clearly necessary (as you’ll see in the next chapter).
Composition tends to be more generally flexible. In addition, by using the added artifice of
inheritance with your member type, you can change the exact type, and thus the behavior,
of those member objects at run-time, therefore you can change the behavior of the
composed object at run-time.
Chapter 6: Reusing Code & Classes 191](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-191-320.jpg)
![You also saw a problem arise, which is embodied in the following (see page 80 if you have
trouble executing this program):
//: Music.java
// Inheritance & upcasting
package c07;
class Note {
private int value;
private Note(int val) { value = val; }
public static final Note
middleC = new Note(0),
cSharp = new Note(1),
cFlat = new Note(2);
} // Etc.
class Instrument {
public void play(Note n) {
System.out.println("Instrument.play()");
}
}
// Wind objects are instruments
// because they have the same interface:
class Wind extends Instrument {
// Redefine interface method:
public void play(Note n) {
System.out.println("Wind.play()");
}
}
public class Music {
public static void tune(Instrument i) {
// ...
i.play(Note.middleC);
}
public static void main(String[] args) {
Wind flute = new Wind();
tune(flute); // Upcasting
}
} ///:~
The method Music.tune( ) accepts an Instrument handle, but also without complaint
anything derived from Instrument. In main( ), you can see this happening as a Wind
handle is passed to tune( ), with no cast necessary. This is acceptable; the interface in
Instrument must exist in Wind, because Wind is inherited from Instrument. Upcasting
from Wind to Instrument may “narrow” that interface, but it cannot make it any less
than the full interface to Instrument.
Why upcast?
This program might seem strange to you. Why should anyone intentionally forget the type
of an object? This is what happens when you upcast, and it seems like it could be much
more straightforward if tune( ) simply takes a Wind handle as its argument. This brings
up an essential point: if you did that, you’d have to write a new tune( ) for every type of
194 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-194-320.jpg)
![Instrument in your system. Suppose we follow this reasoning and add Stringed and
Brass instruments:
//: Music2.java
// Overloading instead of upcasting
class Note2 {
private int value;
private Note2(int val) { value = val; }
public static final Note2
middleC = new Note2(0),
cSharp = new Note2(1),
cFlat = new Note2(2);
} // Etc.
class Instrument2 {
public void play(Note2 n) {
System.out.println("Instrument2.play()");
}
}
class Wind2 extends Instrument2 {
public void play(Note2 n) {
System.out.println("Wind2.play()");
}
}
class Stringed2 extends Instrument2 {
public void play(Note2 n) {
System.out.println("Stringed2.play()");
}
}
class Brass2 extends Instrument2 {
public void play(Note2 n) {
System.out.println("Brass2.play()");
}
}
public class Music2 {
public static void tune(Wind2 i) {
i.play(Note2.middleC);
}
public static void tune(Stringed2 i) {
i.play(Note2.middleC);
}
public static void tune(Brass2 i) {
i.play(Note2.middleC);
}
public static void main(String[] args) {
Wind2 flute = new Wind2();
Stringed2 violin = new Stringed2();
Brass2 frenchHorn = new Brass2();
tune(flute); // No upcasting
tune(violin);
tune(frenchHorn);
Chapter 7: Polymorphism 195](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-195-320.jpg)
![again, you might expect that Shape’s draw( ) is called because this is, after all, a Shape
handle so how could the compiler know to do anything else? And yet the proper
Circle.draw( ) is called because of late binding (polymorphism).
The following example puts it a slightly different way:
//: Shapes.java
// Polymorphism in Java
class Shape {
void draw() {}
void erase() {}
}
class Circle extends Shape {
void draw() {
System.out.println("Circle.draw()");
}
void erase() {
System.out.println("Circle.erase()");
}
}
class Square extends Shape {
void draw() {
System.out.println("Square.draw()");
}
void erase() {
System.out.println("Square.erase()");
}
}
class Triangle extends Shape {
void draw() {
System.out.println("Triangle.draw()");
}
void erase() {
System.out.println("Triangle.erase()");
}
}
public class Shapes {
public static Shape randShape() {
switch((int)(Math.random() * 3)) {
default: // To quiet the compiler
case 0: return new Circle();
case 1: return new Square();
case 2: return new Triangle();
}
}
public static void main(String args[]) {
Shape s[] = new Shape[9];
// Fill up the array with shapes:
for(int i = 0; i < s.length; i++)
s[i] = randShape();
// Make polymorphic method calls:
198 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-198-320.jpg)
![for(int i = 0; i < s.length; i++)
s[i].draw();
}
} ///:~
The base class Shape establishes the common interface to anything inherited from Shape –
that is, all shapes can be drawn and erased. The derived classes override these definitions
to provide unique behavior for each specific type of shape.
The main class Shapes contains a static method randShape( ) that produces a handle to a
randomly-selected Shape object each time you call it. Notice that the upcasting is
happening in each of the return statements, which take a handle to a Circle, Square or
Triangle and send it out of the method as the return type, Shape. Thus when you call this
method you never get a chance to see what specific type it is, since you always get back a
plain Shape handle.
main( ) contains an array of Shape handles which is filled through calls to randShape( ).
At this point you know you have Shapes, but you don’t know anything more specific than
that (and neither does the compiler). However, when you step through this array and call
draw( ) for each one, the correct type-specific behavior magically occurs, as you can see
from one output example:
Circle.draw()
Triangle.draw()
Circle.draw()
Circle.draw()
Circle.draw()
Square.draw()
Triangle.draw()
Square.draw()
Square.draw()
Of course, since the shapes are all chosen randomly each time, your runs will have
different results. The point of choosing the shapes randomly is to drive home the
understanding that the compiler can have no special knowledge that allows it to make the
correct calls at compile time. All the calls to draw( ) are made through dynamic binding.
Extensibility
Now return to the musical instrument example. Because of polymorphism you can add as
many new types as you want to the system without changing the tune( ) method. In a
well-designed OOP program, most or all of your methods will follow the model of tune( )
and communicate only with the base-class interface. Such a program is extensible because
you can add new functionality by inheriting new data types from the common base class.
The methods that manipulate the base-class interface will not need to be changed at all to
accommodate the new classes.
Consider what happens if you take the instrument example and add more methods in the
base class and a number of new classes. Here’s the diagram:
Chapter 7: Polymorphism 199](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-199-320.jpg)
![public String what() { return "Percussion3"; }
public void adjust() {}
}
class Stringed3 extends Instrument3 {
public void play() {
System.out.println("Stringed3.play()");
}
public String what() { return "Stringed3"; }
public void adjust() {}
}
class Brass3 extends Wind3 {
public void play() {
System.out.println("Brass3.play()");
}
public void adjust() {
System.out.println("Brass3.adjust()");
}
}
class Woodwind3 extends Wind3 {
public void play() {
System.out.println("Woodwind3.play()");
}
public String what() { return "Woodwind3"; }
}
public class Music3 {
// Doesn't care about type, so new types
// added to the system still work right:
static void tune(Instrument3 i) {
// ...
i.play();
}
static void tuneAll(Instrument3[] e) {
for(int i = 0; i < e.length; i++)
tune(e[i]);
}
public static void main(String args[]) {
Instrument3[] orchestra = new Instrument3[5];
int i = 0;
// Upcasting during addition to the array:
orchestra[i++] = new Wind3();
orchestra[i++] = new Percussion3();
orchestra[i++] = new Stringed3();
orchestra[i++] = new Brass3();
orchestra[i++] = new Woodwind3();
tuneAll(orchestra);
}
} ///:~
The new methods are what( ), which returns a String handle with a description of the
class, and adjust( ), which provides some way to adjust each instrument.
In main( ), when you place something inside the Instrument3 array you automatically
upcast to Instrument3.
Chapter 7: Polymorphism 201](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-201-320.jpg)
![You can see the tune( ) method is blissfully ignorant of all the code changes that have
happened around it, and yet it works correctly. This is exactly what polymorphism is
supposed to provide: your code changes don’t cause damage to parts of the program that
should not be affected. Put another way, polymorphism is one of the most important
techniques that allow the programmer to “separate the things that change from the things
that stay the same.”
Overriding vs. overloading
Let’s take a different look at the first example in this chapter. In the following program,
the interface of the method play( ) is changed in the process of overriding it, which means
you haven’t actually overridden the method, but instead overloaded it. The compiler allows
you to overload methods so it gives no complaint. But the behavior is probably not what
you want. Here’s the example:
//: WindError.java
// Accidentally changing the interface
class NoteX {
public static final int
MIDDLE_C = 0, C_SHARP = 1, C_FLAT = 2;
}
class InstrumentX {
public void play(int NoteX) {
System.out.println("InstrumentX.play()");
}
}
class WindX extends InstrumentX {
// OOPS! Changes the method interface:
public void play(NoteX n) {
System.out.println("WindX.play(NoteX n)");
}
}
public class WindError {
public static void tune(InstrumentX i) {
// ...
i.play(NoteX.MIDDLE_C);
}
public static void main(String[] args) {
WindX flute = new WindX();
tune(flute); // Not the desired behavior!
}
} ///:~
There’s another confusing aspect thrown in here. In InstrumentX, the play( ) method
takes an int which has the identifier NoteX. That is, even though NoteX is a class name it
can also be used as an identifier without complaint. But in WindX, play( ) takes a NoteX
handle that has an identifier n (although you could even say play(NoteX NoteX) without
an error). Thus it appears the programmer intended to override play( ) but just mistyped
the method a little bit. The compiler, however, assumed that an overload and not an
202 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-202-320.jpg)
![System.out.println("Wind4.play()");
}
public String what() { return "Wind4"; }
public void adjust() {}
}
class Percussion4 extends Instrument4 {
public void play() {
System.out.println("Percussion4.play()");
}
public String what() { return "Percussion4"; }
public void adjust() {}
}
class Stringed4 extends Instrument4 {
public void play() {
System.out.println("Stringed4.play()");
}
public String what() { return "Stringed4"; }
public void adjust() {}
}
class Brass4 extends Wind4 {
public void play() {
System.out.println("Brass4.play()");
}
public void adjust() {
System.out.println("Brass4.adjust()");
}
}
class Woodwind4 extends Wind4 {
public void play() {
System.out.println("Woodwind4.play()");
}
public String what() { return "Woodwind4"; }
}
public class Music4 {
// Doesn't care about type, so new types
// added to the system still work right:
static void tune(Instrument4 i) {
// ...
i.play();
}
static void tuneAll(Instrument4[] e) {
for(int i = 0; i < e.length; i++)
tune(e[i]);
}
public static void main(String args[]) {
Instrument4[] orchestra = new Instrument4[5];
int i = 0;
// Upcasting during addition to the array:
orchestra[i++] = new Wind4();
orchestra[i++] = new Percussion4();
orchestra[i++] = new Stringed4();
Chapter 7: Polymorphism 205](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-205-320.jpg)
![orchestra[i++] = new Brass4();
orchestra[i++] = new Woodwind4();
tuneAll(orchestra);
}
} ///:~
You can see that there’s really no change except in the base class.
It’s helpful to create abstract classes and methods because they make the abstractness of a
class explicit and tell both the user and the compiler how it was intended to be used.
Interfaces
The interface keyword takes the abstract concept one step further. You could think of it as
a “pure” abstract class. It allows the creator to establish the form for a class: method
names, argument lists and return types, but no method bodies. An interface can also
contain data members of primitive types, but these are implicitly static and final. An
interface provides only a form, but no implementation.
An interface says: “this is what all classes that implement this particular interface shall
look like.” Thus, any code that uses a particular interface knows what methods might be
called for that interface, and that’s all. So the interface is used to establish a “protocol”
between classes (some object-oriented programming languages have a keyword called
protocol to do the same thing).
To create an interface, you use the interface keyword instead of the class keyword. Like a
class, you can add the public keyword before the interface keyword (but only if that
interface is defined in a file of the same name) or leave it off to give “friendly” status.
To make a class that conforms to a particular interface (or group of interfaces) you use
the implements keyword. You’re saying “the interface is what it looks like, here’s how it
works.” Other than that, it bears a strong resemblance to inheritance. The diagram for the
instrument example shows this:
206 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-206-320.jpg)
![public String what() { return "Wind5"; }
public void adjust() {}
}
class Percussion5 implements Instrument5 {
public void play() {
System.out.println("Percussion5.play()");
}
public String what() { return "Percussion5"; }
public void adjust() {}
}
class Stringed5 implements Instrument5 {
public void play() {
System.out.println("Stringed5.play()");
}
public String what() { return "Stringed5"; }
public void adjust() {}
}
class Brass5 extends Wind5 {
public void play() {
System.out.println("Brass5.play()");
}
public void adjust() {
System.out.println("Brass5.adjust()");
}
}
class Woodwind5 extends Wind5 {
public void play() {
System.out.println("Woodwind5.play()");
}
public String what() { return "Woodwind5"; }
}
public class Music5 {
// Doesn't care about type, so new types
// added to the system still work right:
static void tune(Instrument5 i) {
// ...
i.play();
}
static void tuneAll(Instrument5[] e) {
for(int i = 0; i < e.length; i++)
tune(e[i]);
}
public static void main(String args[]) {
Instrument5[] orchestra = new Instrument5[5];
int i = 0;
// Upcasting during addition to the array:
orchestra[i++] = new Wind5();
orchestra[i++] = new Percussion5();
orchestra[i++] = new Stringed5();
orchestra[i++] = new Brass5();
208 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-208-320.jpg)
![orchestra[i++] = new Woodwind5();
tuneAll(orchestra);
}
} ///:~
The rest of the code works the same. That is, it doesn’t matter if you are upcasting to a
“regular” class called Instrument5, an abstract class called Instrument5, or an interface
called Instrument5. The behavior is the same. In fact, you can see in the tune( ) method
that there isn’t even any evidence about whether Instrument5 is a “regular” class, an
abstract class or an interface. This is the intent: each approach gives the programmer
different control over the way objects are created and used.
“Multiple inheritance” in Java
The interface isn’t simply a “more pure” form of abstract class. It has a higher purpose
than that. Because an interface has no implementation at all – That is, there is no storage
associated with an interface – there’s nothing to prevent many interfaces from being
combined. This is valuable because there are times when you need to say: “an x is an a and
a b and a c.” In C++, this act of combining multiple class interfaces is called multiple
inheritance, and it carries with it some rather sticky baggage because each class can have
an implementation. In Java, you can perform the same act but only one of the classes can
have an implementation, so the problems seen in C++ do not occur with Java when
combining multiple interfaces:
Abstract or concrete interface 1
base class
interface 1
interface 1
base class functions interface 1 interface 2 .… interface n
Although you aren’t forced to have an abstract or “concrete” (one with no abstract
methods) base class, if you do you can have a maximum of only one. All the rest of the
base elements must be interfaces. You place all the interface names after the implements
keyword and separate them with commas. You can have as many interfaces as you want,
and each one becomes an independent type that you can upcast to. The following example
shows a concrete class combined with several interfaces to produce a new class:
//: Adventure.java
// Multiple interfaces
import java.util.*;
interface CanFight {
void fight();
}
interface CanSwim {
void swim();
}
Chapter 7: Polymorphism 209](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-209-320.jpg)
![interface CanFly {
void fly();
}
class ActionCharacter {
public void fight() {}
}
class Hero extends ActionCharacter
implements CanFight, CanSwim, CanFly {
public void swim() {}
public void fly() {}
}
public class Adventure {
static void t(CanFight x) { x.fight(); }
static void u(CanSwim x) { x.swim(); }
static void v(CanFly x) { x.fly(); }
static void w(ActionCharacter x) { x.fight(); }
public static void main(String args[]) {
Hero i = new Hero();
t(i); // Treat it as a CanFight
u(i); // Treat it as a CanSwim
v(i); // Treat it as a CanFly
w(i); // Treat it as an ActionCharacter
}
} ///:~
You can see that Hero combines the concrete class ActionCharacter with the interfaces
CanFight, CanSwim and CanFly. When you combine a concrete class with interfaces this
way, the concrete class must come first, then the interfaces (the compiler gives an error
otherwise).
Notice that the signature for fight( ) is the same in the interface CanFight and the class
ActionCharacter, and that fight( ) is not provided with a definition in Hero. The rule for
an interface is that you can inherit from it (as you shall see shortly) but then you’ve got
another interface. If you want to create an object of the new type, it must be a class with
all definitions provided. But even though Hero does not explicitly provide a definition for
fight( ), the definition comes along with ActionCharacter so it is automatically provided
and it’s possible to create objects of Hero.
In class Adventure, you can see there are four methods which take as arguments the
various interfaces and the concrete class. When a Hero object is created, it can be passed
to any of these methods, which means it is being upcast to each interface in turn. Because
of the way interfaces are designed in Java, this works without a hitch and without any
particular effort on the part of the programmer.
Keep in mind that the core reason for interfaces is shown in the above example: to be able
to upcast to more than one base type. However, a second reason for using interfaces is the
same as using an abstract base class: to prevent the client programmer from making an
object of this class and to establish that it is only an interface. This brings up a question:
should you use an interface or an abstract class? Well, an interface gives you the benefits
of an abstract class and the benefits of an interface, so if it’s possible to create your base
class without any method definitions or member variables you should always prefer
interfaces to abstract classes. In fact, if you know something is going to be a base class,
210 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-210-320.jpg)
![your first choice should be to make it an interface, and only if you’re forced to have
method definitions or member variables should you change to an abstract class.
Extending an interface with inheritance
You can easily add new method declarations to an interface using inheritance, and you
can also combine several interfaces into a new interface with inheritance. In both cases
you get a new interface, as seen in this example:
//: HorrorShow.java
// Extending an interface with inheritance
interface Monster {
void menace();
}
interface DangerousMonster extends Monster {
void destroy();
}
interface Lethal {
void kill();
}
class DragonZilla implements DangerousMonster {
public void menace() {}
public void destroy() {}
}
interface Vampire
extends DangerousMonster, Lethal {
void drinkBlood();
}
class HorrorShow {
static void u(Monster b) { b.menace(); }
static void v(DangerousMonster d) {
d.menace();
d.destroy();
}
public static void main(String[] args) {
DragonZilla if2 = new DragonZilla();
u(if2);
v(if2);
}
} ///:~
DangerousMonster is a simple extension to Monster which produces a new interface.
This is implemented in DragonZilla.
The syntax used in Vampire works only when inheriting interfaces. Normally, you can use
extends with only a single class, but since an interface can be made from multiple other
interfaces, extends can refer to multiple base interfaces when building a new interface.
As you can see, the interface names are simply separated with commas.
Chapter 7: Polymorphism 211](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-211-320.jpg)
![Grouping constants
Because any fields you put into an interface are automatically static and final, the
interface is a convenient tool for creating groups of constant values, much as you would
with an enum in C or C++. For example:
//: Months.java
// Using interfaces to create groups of constants
package c07;
public interface Months {
int
JANUARY = 1, FEBRUARY = 2, MARCH = 3,
APRIL = 4, MAY = 5, JUNE = 6, JULY = 7,
AUGUST = 8, SEPTEMBER = 9, OCTOBER = 10,
NOVEMBER = 11, DECEMBER = 12;
} ///:~
Note the Java style of using all uppercase letters (with underscores to separate multiple
words in a single identifier) for static final primitives that have constant initializers – that
is, for compile-time constants.
The fields in an interface are automatically public, so it’s unnecessary to specify that.
Now you can use the constants from outside the package by importing c07.* or
c07.Months just as you would with any other package, and referencing the values with
expressions like Months.JANUARY. Of course, what you get is just an int so there isn’t
the extra type safety that C++’s enum has, but this (commonly-used) technique is
certainly an improvement over hard-coding numbers into your programs (this is often
referred to as using “magic numbers” and it produces very difficult-to-maintain code).
Initializing fields in interfaces
Fields defined in interfaces are automatically static and final. These cannot be “blank
finals” but the can be initialized with non-constant expressions. For example:
//: RandVals.java
// Initializing interface fields with
// non-constant initializers
import java.util.*;
public interface RandVals {
int rint = (int)(Math.random() * 10);
long rlong = (long)(Math.random() * 10);
float rfloat = (float)(Math.random() * 10);
double rdouble = Math.random() * 10;
} ///:~
Since the fields are static, they are initialized when the class is first loaded, upon first
access of any of the fields. Here’s a simple test:
//: TestRandVals.java
public class TestRandVals {
public static void main(String args[]) {
System.out.println(RandVals.rint);
212 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-212-320.jpg)
![System.out.println(RandVals.rlong);
System.out.println(RandVals.rfloat);
System.out.println(RandVals.rdouble);
}
} ///:~
The fields, of course, are not part of the interface itself but instead are stored in the static
storage area for that interface.
Inner classes
In Java 1.1 it’s possible to place a class definition within another class definition. This is
called an inner class. The inner class is a useful feature because it allows you to group
classes that logically belong together and to control the visibility of one within the other.
However, it’s important to understand that inner classes are distinctly different from
composition.
Often, the need for inner classes isn’t immediately obvious as you’re learning about them.
At the end of this section, after all the syntax and semantics of inner classes have been
described, you’ll find an example that should make clear the benefits of inner classes.
You create an inner class just as you’d expect: by placing the class definition inside a
surrounding class (see page 80 if you have trouble executing this program):
//: Parcel1.java
// Creating inner classes
package c07.parcel1;
public class Parcel1 {
class Contents {
private int i = 11;
public int value() { return i; }
}
class Destination {
private String label;
Destination(String whereTo) {
label = whereTo;
}
String readLabel() { return label; }
}
// Using inner classes looks just like
// using any other class, within Parcel1:
public void ship(String dest) {
Contents c = new Contents();
Destination d = new Destination(dest);
}
public static void main(String args[]) {
Parcel1 p = new Parcel1();
p.ship("Tanzania");
}
} ///:~
The inner classes, when used inside ship( ), look just like the use of any other classes.
Here, the only practical difference is that the names are nested within Parcel1. You’ll see
in a while that this isn’t the only difference.
Chapter 7: Polymorphism 213](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-213-320.jpg)
![More typically, an outer class will have a method that returns a handle to an inner class,
like this:
//: Parcel2.java
// Returning a handle to an inner class
package c07.parcel2;
public class Parcel2 {
class Contents {
private int i = 11;
public int value() { return i; }
}
class Destination {
private String label;
Destination(String whereTo) {
label = whereTo;
}
String readLabel() { return label; }
}
public Destination to(String s) {
return new Destination(s);
}
public Contents cont() { return new Contents(); }
public void ship(String dest) {
Contents c = cont();
Destination d = to(dest);
}
public static void main(String args[]) {
Parcel2 p = new Parcel2();
p.ship("Tanzania");
Parcel2 q = new Parcel2();
// Defining handles to inner classes:
Parcel2.Contents c = q.cont();
Parcel2.Destination d = q.to("Borneo");
}
} ///:~
If you want to make an object of the inner class anywhere but inside a non-static method
of the outer class, you must specify the type of that object as
OuterClassName.InnerClassName, as seen in main( ).
Inner classes and upcasting
So far, inner classes don’t seem that dramatic. After all, if it’s hiding you’re after, Java
already has a perfectly good hiding mechanism – just allow the class to be “friendly”
(visible only within a package) rather than creating it as an inner class.
However, inner classes really come into their own when you start upcasting to a base
class, and in particular an interface (the effect of producing an interface handle from an
object that implements it is essentially the same as upcasting to a base class). That’s
because the inner class can then be completely unseen and unavailable to anyone; all you
get back is a handle to the base class or the interface and it’s possible that you can’t even
find out the exact type, as shown here:
//: Parcel3.java
// Returning a handle to an inner class
214 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-214-320.jpg)
![package c07.parcel3;
abstract class Contents {
abstract public int value();
}
interface Destination {
String readLabel();
}
public class Parcel3 {
private class PContents extends Contents {
private int i = 11;
public int value() { return i; }
}
protected class PDestination
implements Destination {
private String label;
private PDestination(String whereTo) {
label = whereTo;
}
public String readLabel() { return label; }
}
public Destination dest(String s) {
return new PDestination(s);
}
public Contents cont() {
return new PContents();
}
}
class Test {
public static void main(String args[]) {
Parcel3 p = new Parcel3();
Contents c = p.cont();
Destination d = p.dest("Tanzania");
// Illegal -- can't access private class:
//! Parcel3.PContents c = p.new PContents();
}
} ///:~
Now Contents and Destination represent interfaces available to the client programmer
(the interface, remember, automatically makes all its members public). For convenience,
these are placed inside a single file, but ordinarily Contents and Destination would each
be public in their own files.
In Parcel3, something new has been added: the inner class PContents is private so no one
but Parcel3 can access it. PDestination is protected, so no one but Parcel3, classes in the
Parcel3 package (since protected also gives package access; that is, protected is also
“friendly”), and the inheritors of Parcel3 can access PDestination. This means that the
client programmer has restricted knowledge and access to these members. In fact, you
can’t even downcast to a private inner class (or a protected inner class unless you’re an
inheritor), because you can’t access the name, as you can see in class Test. Thus, the
private inner class provides a way for the class designer to completely prevent any type-
coding dependencies and to completely hide details about implementation. In addition,
extension of an interface is useless from the client programmer’s perspective since the
client programmer cannot access any additional methods that aren’t part of the public
Chapter 7: Polymorphism 215](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-215-320.jpg)
![} ///:~
Although it’s an ordinary class with an implementation, Wrapping is also being used as a
common “interface” to its derived classes:
//: Wrapping.java
package c07.innerscopes;
public class Wrapping {
private int i;
public Wrapping(int x) { i = x; }
public int value() { return i; }
} ///:~
You’ll note that Wrapping has a constructor that requires an argument, to make things
interesting.
The first example shows the creation of an entire class within the scope of a method
(instead of the scope of another class):
//: Parcel4.java
// Nesting a class within a method
package c07.innerscopes;
public class Parcel4 {
public Destination dest(String s) {
class PDestination
implements Destination {
private String label;
private PDestination(String whereTo) {
label = whereTo;
}
public String readLabel() { return label; }
}
return new PDestination(s);
}
public static void main(String args[]) {
Parcel4 p = new Parcel4();
Destination d = p.dest("Tanzania");
}
} ///:~
The class PDestination is part of dest( ) rather than being part of Parcel4 (also notice
that you could use the class identifier PDestination for an inner class inside each class in
the same subdirectory without a name clash). Therefore PDestination cannot be accessed
outside of dest( ). Notice the upcasting that occurs during the return statement – nothing
comes out of dest( ) except a handle to the base class Destination. Of course, the fact that
the name of the class PDestination is placed inside dest( ) doesn’t mean that
PDestination is not a valid object once dest( ) returns.
The next example shows how you can nest an inner class within any arbitrary scope:
//: Parcel5.java
// Nesting a class within a scope
package c07.innerscopes;
public class Parcel5 {
private void internalTracking(boolean b) {
Chapter 7: Polymorphism 217](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-217-320.jpg)
![if(b) {
class TrackingSlip {
private String id;
TrackingSlip(String s) {
id = s;
}
String getSlip() { return id; }
}
TrackingSlip ts = new TrackingSlip("slip");
String s = ts.getSlip();
}
// Can't use it here! Out of scope:
//! TrackingSlip ts = new TrackingSlip("x");
}
public void track() { internalTracking(true); }
public static void main(String args[]) {
Parcel5 p = new Parcel5();
p.track();
}
} ///:~
The class TrackingSlip is nested inside the scope of an if statement. This does not mean
that the class is conditionally created – it gets compiled along with everything else.
However, it’s not available outside the scope in which it is defined. Other than that, it
looks just like an ordinary class.
The next example looks a little strange:
//: Parcel6.java
// A method that returns an anonymous inner class
package c07.innerscopes;
public class Parcel6 {
public Contents cont() {
return new Contents() {
private int i = 11;
public int value() { return i; }
}; // Semicolon required in this case
}
public static void main(String args[]) {
Parcel6 p = new Parcel6();
Contents c = p.cont();
}
} ///:~
The cont( ) method combines the creation of the return value with the definition of the
class that represents that return value! In addition, the class is anonymous – it has no
name. To make matters a bit worse, it looks like you’re starting out to create a Contents
object:
return new Contents()
but then, before you get to the semicolon you say “but wait, I think I’ll slip in a class
definition”:
return new Contents() {
private int i = 11;
218 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-218-320.jpg)
![public int value() { return i; }
};
What this strange syntax means is “create an object of an anonymous class that’s
inherited from Contents.” The handle returned by the new expression is automatically
upcast to a Contents handle.
In the above example, Contents is created using a default constructor. The following code
shows what to do if your base class needs a constructor with an argument:
//: Parcel7.java
// An anonymous inner class that calls the
// base-class constructor
package c07.innerscopes;
public class Parcel7 {
public Wrapping wrap(int x) {
// Base constructor call:
return new Wrapping(x) {
public int value() {
return super.value() * 47;
}
}; // Semicolon required
}
public static void main(String args[]) {
Parcel7 p = new Parcel7();
Wrapping w = p.wrap(10);
}
} ///:~
That is, you just pass the appropriate argument to the base-class constructor, seen here as
the x passed in new Wrapping(x). An anonymous class cannot have a constructor where
you would normally call super( ).
In both the previous examples, the semicolon doesn’t mark the end of the class body (as it
does in C++). Instead, it marks the end of the expression that happens to contain the
anonymous class. Thus it’s identical to the use of the semicolon everywhere else.
What happens if you need to perform some kind of initialization for an object of an
anonymous inner class? Since it’s anonymous, there’s no name to give the constructor so
you can’t have a constructor. You can, however, perform initialization at the point of
definition of your fields:
//: Parcel8.java
// An anonymous inner class that performs
// initialization. A briefer version
// of Parcel5.java.
package c07.innerscopes;
public class Parcel8 {
// Argument must be final to use inside
// anonymous inner class:
public Destination dest(final String dest) {
return new Destination() {
private String label = dest;
public String readLabel() { return label; }
};
}
Chapter 7: Polymorphism 219](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-219-320.jpg)
![public static void main(String args[]) {
Parcel8 p = new Parcel8();
Destination d = p.dest("Tanzania");
}
} ///:~
If you’re defining an anonymous inner class and want to use an object that’s defined
outside the anonymous inner class, the compiler requires that the outside object be final.
This is why the argument to dest( ) is final. If you forget, you’ll get a compile-time error
message.
As long as you’re simply assigning a field the above approach is fine, but what if you need
to perform some constructor-like activity? With Java 1.1 instance initialization, you can
effectively create a constructor for an anonymous inner class:
//: Parcel9.java
// Using "instance initialization" to perform
// construction on an anonymous inner class
package c07.innerscopes;
public class Parcel9 {
public Destination
dest(final String dest, final float price) {
return new Destination() {
private int cost;
// Instance initialization for each object:
{
cost = Math.round(price);
if(cost > 100)
System.out.println("Over budget!");
}
private String label = dest;
public String readLabel() { return label; }
};
}
public static void main(String args[]) {
Parcel9 p = new Parcel9();
Destination d = p.dest("Tanzania", 101.395F);
}
} ///:~
Inside the instance initializer, you can see code that couldn’t be executed as part of a field
initializer (that is, the if statement). So in effect, an instance initializer is the constructor
for an anonymous inner class. Of course, it’s limited: you can’t overload instance
initializers so you can have only one of these constructors.
The link to the outer class
So far, it appears that inner classes are just a name-hiding and code-organization scheme,
which is helpful but not totally compelling. However, there’s another twist. When you
create an inner class, objects of that inner class have a link to the enclosing object that
made them, and so can access the members of that enclosing object – without any special
220 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-220-320.jpg)
![qualifications. In addition, inner classes have access rights to all the elements in the
enclosing class1 . The following example demonstrates this:
//: Sequence.java
// Holds a sequence of Objects
interface Selector {
boolean end();
Object current();
void next();
}
public class Sequence {
private Object[] o;
private int next = 0;
public Sequence(int size) {
o = new Object[size];
}
public void add(Object x) {
if(next < o.length) {
o[next] = x;
next++;
}
}
private class SSelector implements Selector {
int i = 0;
public boolean end() {
return i == o.length;
}
public Object current() {
return o[i];
}
public void next() {
if(i < o.length) i++;
}
}
public Selector getSelector() {
return new SSelector();
}
public static void main(String args[]) {
Sequence s = new Sequence(10);
for(int i = 0; i < 10; i++)
s.add(Integer.toString(i));
Selector sl = s.getSelector();
while(!sl.end()) {
System.out.println((String)sl.current());
sl.next();
}
}
} ///:~
1 This is very different from the design of nested classes in C++, which is simply a name-hiding
mechanism. There is no link to an enclosing object and no implied permissions in C++.
Chapter 7: Polymorphism 221](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-221-320.jpg)
![private int i = 11;
public int value() { return i; }
}
protected static class PDestination
implements Destination {
private String label;
private PDestination(String whereTo) {
label = whereTo;
}
public String readLabel() { return label; }
}
public static Destination dest(String s) {
return new PDestination(s);
}
public static Contents cont() {
return new PContents();
}
public static void main(String args[]) {
Contents c = cont();
Destination d = dest("Tanzania");
}
} ///:~
In main( ), no object of Parcel10 is necessary; instead you use the normal syntax for
selecting a static member to call the methods that return handles to Contents and
Destination.
Referring to the outer class object
If you need to produce the handle to the outer class object, you name the outer class
followed by a dot and this. For example, in the class Sequence.SSelector, any of its
methods can produce the stored handle to the outer class Sequence by saying
Sequence.this. The resulting handle is automatically the correct type (this is known and
checked at compile time, so there is no run-time overhead).
Sometimes you want to tell some other object to create an object of one of its inner classes.
To do this you must provide a handle to the other outer class object in the new expression,
like this:
//: Parcel11.java
// Creating inner classes
package c07.parcel11;
public class Parcel11 {
class Contents {
private int i = 11;
public int value() { return i; }
}
class Destination {
private String label;
Destination(String whereTo) {
label = whereTo;
}
String readLabel() { return label; }
}
public static void main(String args[]) {
Parcel11 p = new Parcel11();
// Must use instance of outer class
Chapter 7: Polymorphism 223](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-223-320.jpg)
![// to create an instances of the inner class:
Parcel11.Contents c = p.new Contents();
Parcel11.Destination d =
p.new Destination("Tanzania");
}
} ///:~
To create an object of the inner class directly, you don’t follow the same form and refer to
the outer class name Parcel11 as you might expect, but instead you must use an object of
the outer class to make an object of the inner class:
Parcel11.Contents c = p.new Contents();
Thus it’s not possible to create an object of the inner class unless you already have an
object of the outer class. This is because the object of the inner class is quietly connected to
the object of the outer class that it was made from. If you make a static inner class, then
it doesn’t need a handle to the outer class object.
Inheriting from inner classes
Because the inner class constructor must attach to a handle of the enclosing class object,
things are slightly complicated when you inherit from an inner class. The problem is that
the “secret” handle to the enclosing class object must be initialized, and yet in the derived
class there’s no longer a default object to attach to. The answer is to use a syntax provided
to make the association explicit:
//: InheritInner.java
// Inheriting an inner class
class WithInner {
class Inner {}
}
public class InheritInner
extends WithInner.Inner {
//! InheritInner() {} // Won't compile
InheritInner(WithInner wi) {
wi.super();
}
public static void main(String args[]) {
WithInner wi = new WithInner();
InheritInner ii = new InheritInner(wi);
}
} ///:~
You can see that InheritInner is extending only the inner class, not the outer one. But
when it comes time to create a constructor, the default one is no good and you can’t just
pass a handle to an enclosing object. In addition, you must use the syntax
enclosingClassHandle.super();
inside the constructor. This provides the necessary handle and the program will then
compile.
224 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-224-320.jpg)
![Can inner classes be overridden?
What happens when you create an inner class, then inherit from the enclosing class and
redefine the inner class? That is, is it possible to override an inner class? This seems like it
would be a powerful concept, but “overriding” an inner class as if it were another method
of the outer class doesn’t really do anything:
//: BigEgg.java
// An inner class cannot be overriden
// like a method
class Egg {
protected class Yolk {
public Yolk() {
System.out.println("Egg.Yolk()");
}
}
private Yolk y;
public Egg() {
System.out.println("New Egg()");
y = new Yolk();
}
}
public class BigEgg extends Egg {
public class Yolk {
public Yolk() {
System.out.println("BigEgg.Yolk()");
}
}
public static void main(String args[]) {
new BigEgg();
}
} ///:~
The default constructor is synthesized automatically by the compiler, and this calls the
base-class default constructor. You might think that since a BigEgg is being created the
“overridden” version of Yolk would be used, but this is not the case. The output is:
New Egg()
Egg.Yolk()
The above example simply shows that there isn’t any extra inner class magic going on
when you inherit from the outer class. However, it’s still possible to explicitly inherit from
the inner class:
//: BigEgg2.java
// Proper inheritance of an inner class
class Egg2 {
protected class Yolk {
public Yolk() {
System.out.println("Egg2.Yolk()");
}
public void f() {
System.out.println("Egg2.Yolk.f()");
}
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![}
private Yolk y = new Yolk();
public Egg2() {
System.out.println("New Egg2()");
}
public void insertYolk(Yolk yy) { y = yy; }
public void g() { y.f(); }
}
public class BigEgg2 extends Egg2 {
public class Yolk extends Egg2.Yolk {
public Yolk() {
System.out.println("BigEgg2.Yolk()");
}
public void f() {
System.out.println("BigEgg2.Yolk.f()");
}
}
public BigEgg2() { insertYolk(new Yolk()); }
public static void main(String args[]) {
Egg2 e2 = new BigEgg2();
e2.g();
}
} ///:~
Now BiggEgg2.Yolk explicitly extends Egg2.Yolk and overrides its methods. The method
insertYolk( ) allows BigEgg2 to upcast one of its own Yolk objects into the y handle in
Egg2, so when g( ) calls y.f( ) the overridden version of f( ) is used. The output is:
Egg2.Yolk()
New Egg2()
Egg2.Yolk()
BigEgg2.Yolk()
BigEgg2.Yolk.f()
The second call to Egg2.Yolk( ) is the base-class constructor call of the BigEgg2.Yolk
constructor. You can see that the overridden version of f( ) is used when g( ) is called.
Inner class identifiers
Since every class produces a .class file which holds all the information about how to create
objects of this type (this information produces a meta-class called the Class object), you
might guess that inner classes must also produce .class files to contain the information for
their Class objects. The names of these files/classes have a strict formula: the name of the
enclosing class, followed by a ‘$’ followed by the name of the inner class. For example, the
.class files created by InheritInner.java include:
InheritInner.class
WithInner$Inner.class
WithInner.class
If inner classes are unnamed, the compiler simply starts generating numbers as inner
class identifiers. If inner classes are nested within inner classes, their names are simply
appended after a ‘$’ and the outer class identifier(s).
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![action( ) is the method that’s called when the Event is ready( ), and description( ) gives
textual information about the Event.
The next file contains the actual control framework that manages and fires events. The
first class is really just a “helper” class whose job is to hold Event objects. You could
replace it with any appropriate collection, and in Chapter 8 you’ll discover other
collections which will do the trick without requiring you to write this extra code:
//: Controller.java
// Along with Event, the generic
// framework for all control systems:
package c07.controller;
// This is just a way to hold Event objects.
class EventSet {
private Event events[] = new Event[100];
private int index = 0;
private int next = 0;
public void add(Event e) {
if(index >= events.length)
return; // (Should throw exception)
events[index++] = e;
}
public Event getNext() {
boolean looped = false;
int start = next;
do {
next = (next + 1) % events.length;
// See if it has looped to the beginning:
if(start == next) looped = true;
// If it loops past start, the list
// is empty:
if((next == (start + 1) % events.length)
&& looped)
return null;
} while(events[next] == null);
return events[next];
}
public void removeCurrent() {
events[next] = null;
}
}
public class Controller {
private EventSet es = new EventSet();
public void addEvent(Event c) { es.add(c); }
public void run() {
Event e;
while((e = es.getNext()) != null) {
if(e.ready()) {
e.action();
System.out.println(e.description());
es.removeCurrent();
}
}
}
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![addEvent(new WaterOn(tm + 3000));
addEvent(new WaterOff(tm + 8000));
addEvent(new Bell(tm + 9000));
addEvent(new ThermostatDay(tm + 10000));
// Can even add a Restart object!
addEvent(new Restart(tm + 20000));
}
public String description() {
return "Restarting system";
}
}
public static void main(String args[]) {
GreenhouseControls gc =
new GreenhouseControls();
long tm = System.currentTimeMillis();
gc.addEvent(gc.new Restart(tm));
gc.run();
}
} ///:~
Notice that light, water, thermostat and rings all belong to the outer class
GreenhouseControls, and yet the inner classes have no problem accessing those fields.
Also, most of the action( ) methods also involve some sort of hardware control, which
would most likely involve calls to non-Java code.
Most of the Event classes look very similar, but Bell and Restart are special. Bell rings,
and if it hasn’t yet rung enough times it adds a new Bell object to the event list, so it will
ring again later. Notice how inner classes almost look like multiple inheritance: Bell has all
the methods of Event and it also appears to have all the methods of the outer class
GreenhouseControls.
Restart is responsible for initializing the system, so it adds all the appropriate events. Of
course, a more flexible way to accomplish this is to avoid hard-coding the events and
instead read them from a file (an exercise in Chapter 10 asks you to modify this example
to do just that). Since Restart( ) is just another Event object, you can also add a Restart
object within Restart.action( ) so that the system regularly restarts itself. And all you
need to do in main( ) is create a GreenhouseControls object and add a Restart object to
get it going.
This example should move you a long way towards appreciating the value of inner classes,
especially when used within a control framework. However, in the latter half of Chapter
13 you’ll see how elegantly inner classes are used to describe the actions of a graphical
user interface. By the time you finish that section you should be fully convinced.
Constructors & polymorphism
As usual, constructors are different from other kinds of methods. This is also true when
polymorphism is involved. Even though constructors themselves are not polymorphic
(although you can have a kind of “virtual constructor” as you will see in Chapter 11), it’s
important to understand the way constructors work in complex hierarchies and with
polymorphism. This understanding will help you avoid unpleasant entanglements.
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![public static void main(String[] args) {
new Sandwich();
}
} ///:~
This example creates a complex class out of other classes, and each class has a constructor
that announces itself. The important class is Sandwich, which reflects three levels of
inheritance (four, if you count the implicit inheritance from Object) and three member
objects. When a Sandwich object is created in main( ), the output is:
Meal()
Lunch()
PortableLunch()
Bread()
Cheese()
Lettuce()
Sandwich()
This means that the order of constructor calls for a complex object is as follows:
1. The base-class constructor is called. This step is repeated recursively such that
the very root of the hierarchy is constructed first, followed by the next-derived
class, etc., until the most-derived class is reached.
2. Member initializers are called in the order of declaration.
3. The body of the derived-class constructor is called.
The order of the constructor calls is important. When you inherit, you know all about the
base class and can access any public and protected members of the base class. This means
you must be able to assume that all the members of the base class are valid when you’re in
the derived class. In a normal method, construction has already taken place, so all the
members of all parts of the object have been built. Inside the constructor, however, you
must be able to assume that all members that you use have been built. The only way to
guarantee this is for the base-class constructor to be called first. Then when you’re in the
derived-class constructor, all the members you can access in the base class have been
initialized. “Knowing all members are valid” inside the constructor is also the reason that,
whenever possible, you should initialize all member objects (that is, objects placed in the
class using composition) at their point of definition in the class (e.g.: b, c and l in the
example above). If you follow this practice, you will help ensure that all base class
members and member objects of the current object have been initialized. Unfortunately,
this doesn’t handle every case, as you will see in the next section.
Inheritance and finalize( )
When you use composition to create a new class, you never worry about finalizing the
member objects of that class. Each member is an independent object and thus is garbage
collected and finalized regardless of whether it happens to be a member of your class.
With inheritance, however, you must override finalize( ) in the derived class if you have
any special cleanup that must happen as part of garbage collection. When you override
finalize( ) in an inherited class, it’s important to remember to call the base-class version
of finalize( ), since otherwise the base-class finalization will not happen. The following
example proves this:
//: Frog.java
// Testing finalize with inheritance
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![System.out.println("Amphibian finalize");
if(DoBaseFinalization.flag)
try {
super.finalize();
} catch(Throwable t) {}
}
}
public class Frog extends Amphibian {
Frog() {
System.out.println("Frog()");
}
protected void finalize() {
System.out.println("Frog finalize");
if(DoBaseFinalization.flag)
try {
super.finalize();
} catch(Throwable t) {}
}
public static void main(String args[]) {
if(args.length != 0 &&
args[0].equals("finalize"))
DoBaseFinalization.flag = true;
else
System.out.println("not finalizing bases");
new Frog(); // Instantly becomes garbage
System.out.println("bye!");
// Must do this to guarantee that all
// finalizers will be called:
System.runFinalizersOnExit(true);
}
} ///:~
The class DoBaseFinalization simply holds a flag that indicates to each class in the
hierarchy whether to call super.finalize( ). This flag is set based on a command-line
argument, so you can view the behavior with and without base-class finalization.
Each class in the hierarchy also contains a member object of class Characteristic. You will
see that regardless of whether the base class finalizers are called, the Characteristic
member objects are always finalized.
Each overridden finalize( ) must have access of at least protected since the finalize( )
method in class Object is protected and the compiler will not allow you to reduce the
access during inheritance (“friendly” is less accessible than protected).
In Frog.main( ) the DoBaseFinalization flag is configured, and a single Frog object is
created. Remember that garbage collection and in particular finalization might not happen
for any particular object so to enforce this, System.runFinalizersOnExit(true) adds the
extra overhead to guarantee that finalization takes place. Without base-class finalization,
the output is:
not finalizing bases
Creating Characteristic is alive
LivingCreature()
Creating Characteristic has heart
Animal()
Creating Characteristic can live in water
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![If you do this inside a constructor, you call a method that might manipulate members that
haven’t been initialized yet: a sure recipe for disaster.
You can see the problem in the following example:
//: PolyConstructors.java
// Constructors and polymorphism
// don't produce what you might expect.
abstract class Glyph {
abstract void draw();
Glyph() {
System.out.println("Glyph() before draw()");
draw();
System.out.println("Glyph() after draw()");
}
}
class RoundGlyph extends Glyph {
int radius = 1;
RoundGlyph(int r) {
radius = r;
System.out.println(
"RoundGlyph.RoundGlyph(), radius = "
+ radius);
}
void draw() {
System.out.println(
"RoundGlyph.draw(), radius = " + radius);
}
}
public class PolyConstructors {
public static void main(String[] args) {
new RoundGlyph(5);
}
} ///:~
In Glyph, the draw( ) method is abstract, so it is designed to be overridden. Indeed, you
are forced to override it in RoundGlyph. But the Glyph constructor calls this method, and
the call ends up in RoundGlyph.draw( ), which would seem to be the intent. But look at
the output:
Glyph() before draw()
RoundGlyph.draw(), radius = 0
Glyph() after draw()
RoundGlyph.RoundGlyph(), radius = 5
When Glyph’s constructor calls draw( ), the value of radius isn’t even the default initial
value 1. It’s zero. This would probably result in either a dot or nothing at all being drawn
on the screen, and you’d be staring, trying to figure out why the program won’t work.
The order of initialization described in the previous section isn’t quite complete, and that’s
the key to solving the mystery. The actual process of initialization is:
1. The storage allocated for the object is initialized to binary zero before anything else
happens.
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![}
class Stage {
Actor a = new HappyActor();
void change() { a = new SadActor(); }
void go() { a.act(); }
}
public class Transmogrify {
public static void main(String[] args) {
Stage s = new Stage();
s.go(); // Prints "HappyActor"
s.change();
s.go(); // Prints "SadActor"
}
} ///:~
A Stage object contains a handle to an Actor, which is initialized to a HappyActor object.
This means go( ) produces a particular behavior. But since a handle can be re-bound to a
different object at run time, a handle for a SadActor object can be substituted in a and
then the behavior produced by go( ) changes. Thus you gain dynamic flexibility at run
time. In contrast, you can’t decide to inherit differently at run time; that must be
completely determined at compile time.
A general guideline is “use inheritance to express differences in behavior, and member
variables to express variations in state.” In the above example, both are used: two different
classes are inherited to express the difference in the act( ) method, and Stage uses
composition to allow its state to be changed. In this case, that change in state happens to
produce a change in behavior.
Pure inheritance vs. extension
When studying inheritance, it would seem that the cleanest way to create an inheritance
hierarchy is to take the “pure” approach. That is, only methods that have been established
in the base class or interface are to be overridden in the derived class, as seen in this
diagram:
Shape
draw()
“is a” erase()
Circle Square Line
draw() draw() draw()
erase() erase() erase()
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![class Useful {
public void f() {}
public void g() {}
}
class MoreUseful extends Useful {
public void f() {}
public void g() {}
public void u() {}
public void v() {}
public void w() {}
}
public class RTTI {
public static void main(String args[]) {
Useful x[] = {
new Useful(),
new MoreUseful()
};
x[0].f();
x[1].g();
// Compile-time: method not found in Useful:
//! x[1].u();
((MoreUseful)x[1]).u(); // Downcast/RTTI
((MoreUseful)x[0]).u(); // Exception thrown
}
} ///:~
As in the diagram, MoreUseful extends the interface of Useful. But since it’s inherited, it
can also be upcast to a Useful. You can see this happening in the initialization of the
array x in main( ). Since both objects in the array are of class Useful, you can send the
f( ) and g( ) methods to both, and if you try to call u( ) (which exists only in MoreUseful)
you’ll get a compile-time error message.
If you want to access the extended interface of a MoreUseful object, you can try to
downcast. If it’s the right type, it will be successful. Otherwise, you’ll get a
ClassCastException. You don’t have to write any special code for this exception, since it
indicates a programmer error that could happen anywhere in a program.
There’s more to RTTI than a simple cast. For example, there’s a way to see what type
you’re dealing with before you try to downcast it. All of Chapter 11 is devoted to the study
of different aspects of Java run-time type identification.
Summary
Polymorphism means “different forms.” In object-oriented programming, you have the
same face (the common interface in the base class) and different forms using that face: the
different versions of the dynamically-bound methods.
You’ve seen in this chapter that it’s impossible to understand, or even create, an example
of polymorphism without using data abstraction and inheritance. Polymorphism is a
feature that cannot be viewed in isolation (like a switch statement, for example), but
instead works only in concert, as part of a “big picture” of class relationships. People are
often confused by other, non-object-oriented features of Java, like method overloading,
which are sometimes presented as object-oriented. Don’t be fooled: If it isn’t late binding,
it isn’t polymorphism.
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![element access very fast, but you pay for this speed: when you create an array object, its
size is fixed and cannot be changed for the lifetime of that array object. You might suggest
creating an array of a particular size and then, if you run out of space, creating a new one
and moving all the handles from the old one to the new one. This is the behavior of the
Vector class which will be studied later in the chapter. However, because of the overhead of
this size flexibility, a Vector is measurably less efficient than an array.
The vector class in C++ does know the type of objects it holds, but it has a different
drawback when compared with arrays in Java: the C++ vector doesn’t do bounds
checking, so you can run past the end (however, it’s possible to ask how big the vector is).
In Java, you get bounds checking regardless of whether you’re using an array or a
collection – you’ll get a RuntimeException if you exceed the bounds. As you’ll learn in
Chapter 9, this type of exception indicates a programmer error and thus you don’t need to
check for it in your code. As an aside, the reason the C++ vector doesn’t check bounds
with every access is speed – in Java you have the constant performance overhead of
bounds checking all the time for both arrays and collections.
The other generic collection classes that will be studied in this chapter, Vector, Stack and
Hashtable, all deal with objects as if they had no specific type. That is, they treat them as
type Object, the root class of all classes in Java. This works fine from one standpoint: you
need to build only one collection, and any Java object will go into that collection (except for
primitives – these can be placed in collections as constants using the Java primitive
wrapper classes, or as changeable values by wrapping in your own class). This is the
second place where an array is superior to the generic collections: when you create an
array, you create it to hold a specific type. This means you get compile-time type checking
to prevent you from putting the wrong type in, or mistaking the type that you’re
extracting. Of course, Java will prevent you from sending an inappropriate message to an
object one way or another, either at compile-time or at run-time, so it’s not as if it is
riskier one way or another, it’s just nicer if the compiler points it out to you, faster at
run-time, and there’s less likelihood that the end user will get surprised by an exception.
For both of the aforementioned reasons – efficiency and type checking – it’s always worth
trying to use an array if you can. However, when you’re trying to solve a more general
problem arrays can be too restrictive. After looking at arrays, the rest of this chapter will
be devoted to the collection classes provided by Java.
Arrays are first-class objects
Regardless of what type of array you’re working with, the array identifier is actually a
handle to a true object that’s created on the heap. The heap object can be created either
implicitly, as part of the array initialization syntax, or explicitly with a new expression.
Part of the heap object (in fact, the only field or method you can access) is the read-only
length member that tells you how many elements can be stored in that array object. The
‘[]’ syntax is the only other access that you have to the array object.
The following example shows the various ways an array can be initialized, and how the
array handles can be assigned to different array objects. It also shows that arrays of
objects and arrays of primitives are almost identical in their use; the only difference is
that arrays of objects hold handles while arrays of primitives hold the primitive values
directly (see page 80 if you have trouble executing this program).
//: ArraySize.java
// Initialization & re-assignment of arrays
package c08;
class Weeble {} // A small mythical creature
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![public class ArraySize {
public static void main(String args[]) {
// Arrays of objects:
Weeble a[]; // Null handle
Weeble b[] = new Weeble[5]; // Null handles
Weeble c[] = new Weeble[4];
for(int i = 0; i < c.length; i++)
c[i] = new Weeble();
Weeble d[] = {
new Weeble(), new Weeble(), new Weeble()
};
// Compile error: variable a not initialized:
//!System.out.println("a.length=" + a.length);
System.out.println("b.length = " + b.length);
// The handles inside the array are
// automatically initialized to null:
for(int i = 0; i < b.length; i++)
System.out.println("b[" + i + "]=" + b[i]);
System.out.println("c.length = " + c.length);
System.out.println("d.length = " + d.length);
a = d;
System.out.println("a.length = " + a.length);
// Java 1.1 initialization syntax:
a = new Weeble[] {
new Weeble(), new Weeble()
};
System.out.println("a.length = " + a.length);
// Arrays of primitives:
int e[]; // Null handle
int f[] = new int[5];
int g[] = new int[4];
for(int i = 0; i < g.length; i++)
g[i] = i*i;
int h[] = { 11, 47, 93 };
// Compile error: variable e not initialized:
//!System.out.println("e.length=" + e.length);
System.out.println("f.length = " + f.length);
// The primitives inside the array are
// automatically initialized to zero:
for(int i = 0; i < f.length; i++)
System.out.println("f[" + i + "]=" + f[i]);
System.out.println("g.length = " + g.length);
System.out.println("h.length = " + h.length);
e = h;
System.out.println("e.length = " + e.length);
// Java 1.1 initialization syntax:
e = new int[] { 1, 2 };
System.out.println("e.length = " + e.length);
}
} ///:~
Here’s the output from the program:
b.length = 5
b[0]=null
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![b[1]=null
b[2]=null
b[3]=null
b[4]=null
c.length = 4
d.length = 3
a.length = 3
a.length = 2
f.length = 5
f[0]=0
f[1]=0
f[2]=0
f[3]=0
f[4]=0
g.length = 4
h.length = 3
e.length = 3
e.length = 2
The array a is initially just a null handle, and the compiler prevents you from doing
anything with this handle until you’ve properly initialized it. The array b is initialized to
point to an array of Weeble handles, but no actual Weeble objects are ever placed in that
array. However, you can still ask what the size of the array is, since b is pointing to a
legitimate object. This brings up a slight drawback: you can’t find out how many elements
are actually in the array, since length tells you only how many elements can be placed in
the array; that is, the size of the array object, not the number of elements it actually
holds. However, when an array object is created its handles are automatically initialized to
null so you can see whether a particular array slot has an object in it by checking to see
whether it’s null. Similarly, an array of primitives is automatically initialized to zero for
numeric types, null for char and false for boolean.
Array c shows the creation of the array object followed by the assignment of Weeble
objects to all the slots in the array. Array d shows the “aggregate initialization” syntax
that causes the array object to be created (implicitly with new on the heap, just like for
Array c) and initialized with Weeble objects, all in one statement.
The expression
a = d;
shows how you can take a handle that’s attached to one array object and assign it to
another array object, just as you can do with any other type of object handle. Now both a
and d are pointing to the same array object on the heap.
Java 1.1 adds a new array initialization syntax, which could be thought of as a “dynamic
aggregate initialization.” The Java 1.0 aggregate initialization used by d must be used at
the point of d’s definition, but with the new Java 1.1 syntax you can create and initialize
an array object anywhere at all. For example, suppose hide( ) is a method that takes an
array of Weeble objects. You could call it by saying:
hide(d);
but in Java 1.1 you can also dynamically create the array you want to pass as the
argument:
hide(new Weeble[] { new Weeble(), new Weeble() });
This new syntax provides a more convenient way to write code in some situations.
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![The second part of the above example shows that primitive arrays work just like object
arrays except that primitive arrays hold the primitive values directly.
Collections of primitives
Collection classes can hold only handles to objects. An array, however, can be created to
hold primitives directly, as well as handles to objects. It is possible to use the “wrapper”
classes such as Integer, Double etc. to place primitive values inside a collection, but as
you’ll see later in this chapter in the WordCount.java example, the wrapper classes for
primitives are only somewhat useful anyway. Whether you put primitives in arrays or
wrap them in a class that’s placed in a collection is a question of efficiency: it’s much
more efficient to create and access an array of primitives than a collection of wrapped
primitives.
Of course, if you’re using a primitive type and you need the flexibility of a collection that
automatically expands itself when more space is needed, the array won’t work and you’re
forced to use a collection of wrapped primitives. You might think that there should be a
specialized type of Vector for each of the primitive data types, but Java doesn’t provide
this for you. Some sort of templatizing mechanism might someday provide a better way
for Java to handle this problem1 .
Returning an array
Suppose you’re writing a method and you don’t just want to return one thing, but instead
a whole bunch of things. Languages like C and C++ make this difficult because you can’t
just return an array, but only a pointer to an array. This introduces problems because it
becomes messy to control the lifetime of the array, which easily leads to memory leaks.
Java takes a similar approach, but you just “return an array.” Actually, of course, you’re
returning a handle to an array but with Java you never worry about responsibility for
that array – it will be around as long as you need it, and the garbage collector will clean it
up when you’re done.
As an example, consider returning an array of String:
//: IceCream.java
// Returning arrays from methods
public class IceCream {
static String flav[] = {
"Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie"
};
static String[] flavorSet(int n) {
// Force it to be positive & within bounds:
n = Math.abs(n) % (flav.length + 1);
String results[] = new String[n];
int picks[] = new int[n];
for(int i = 0; i < picks.length; i++)
picks[i] = -1;
1 This is one of the places where C++ is distinctly superior to Java, since C++ supports
parameterized types with the template keyword.
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![for(int i = 0; i < picks.length; i++) {
retry:
while(true) {
int t =
(int)(Math.random() * flav.length);
for(int j = 0; j < i; j++)
if(picks[j] == t) continue retry;
picks[i] = t;
results[i] = flav[t];
break;
}
}
return results;
}
public static void main(String args[]) {
for(int i = 0; i < 20; i++) {
System.out.println(
"flavorSet(" + i + ") = ");
String fl[] = flavorSet(flav.length);
for(int j = 0; j < fl.length; j++)
System.out.println("t" + fl[j]);
}
}
} ///:~
The method flavorSet( ) creates an array of String called results. The size of this array is
n, determined by the argument you pass into the method. Then it proceeds to randomly
choose flavors from the array flav and place them into results, which it finally returns.
Returning an array is just like returning any other object: it’s a handle. It’s not important
that the array was created within flavorSet( ), or that the array was created anyplace
else, for that matter. The garbage collector takes care of cleaning up the array when
you’re done with it, and the array will persist for as long as you need it.
As an aside, notice that when flavorSet( ) randomly chooses flavors, it ensures that a
random choice hasn’t been picked before. This is performed in a seemingly infinite while
loop that keeps making random choices until it finds one that’s not already in the picks
array (of course a String comparison could also have been performed to see if the random
choice was already in the results array but String comparisons are very inefficient). If it’s
successful it adds the entry and breaks out to go find the next one (i gets incremented).
But if t is a number that’s already in picks, then a labeled continue is used to jump back
two levels which forces a new t to be selected. It’s particularly convincing to watch this
happen with a debugger.
main( ) prints out 20 full sets of flavors, so you can see that flavorSet( ) chooses the
flavors in a random order each time. It’s easiest to see this if you redirect the output into a
file. And while you’re looking at the file, remember, you’re not really hungry (You just
want the ice cream, you don’t need it).
Collections
To summarize what we’ve seen so far: your first, most efficient choice to hold a group of
objects should be an array, and you’re forced into this choice if you want to hold a group
of primitives. In the remainder of the chapter we’ll look at the more general case, when
you don’t know at the time you’re writing the program how many objects you’re going to
need, or if you need a more sophisticated way to store your objects. Java provides four
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![void print() {
System.out.println("Dog #" + dogNumber);
}
}
public class CatsAndDogs {
public static void main(String args[]) {
Vector cats = new Vector();
for(int i = 0; i < 7; i++)
cats.addElement(new Cat(i));
// Not a problem to add a dog to cats:
cats.addElement(new Dog(7));
for(int i = 0; i < cats.size(); i++)
((Cat)cats.elementAt(i)).print();
// Dog is detected only at run-time
}
} ///:~
You can see that using a Vector is straightforward: create one, put objects in using
addElement( ) and later get them out with elementAt( ) (notice that Vector has a method
size( ) to let you know how many elements have been added so you don’t inadvertently
run off the end and cause an exception).
The classes Cat and Dog are distinct – they have nothing in common except that they are
Objects (if you don’t explicitly say what class you’re inheriting from, you automatically
inherit from Object). The Vector class, which comes from java.util, holds Objects, so not
only can I put Cat objects into this collection using the Vector method addElement( ), but
I can also add Dog objects without complaint at either compile-time or run-time. When I
go to fetch out what I think are Cat objects using the Vector method elementAt( ), I get
back a handle to an Object that I must cast to a Cat. Then I have to surround the entire
expression with parentheses to force the evaluation of the cast before calling the print( )
method for Cat, otherwise I’ll get a syntax error. Then, at run-time, when I try to cast the
Dog object to a Cat, I’ll get an exception.
This is more than just an annoyance. It’s something that can create some difficult-to-find
bugs. If one part (or several parts) of a program inserts objects into a collection, and you
discover only in a separate part of the program, through an exception, that a bad object
was placed in the collection, then you must find out where the bad insert occurred by code
inspection, which is about the worst debugging tool we have. On the upside, it’s very
convenient to start with some standardized collection classes for programming, despite the
scarcity and awkwardness.
Sometimes it works right anyway
It turns out that in some cases things seem to work correctly without casting back to your
original type. The first case is quite special: the String class has some extra help from the
compiler to make it work smoothly. Whenever the compiler expects a String object and it
hasn’t got one, it will automatically call the toString( ) method that’s defined in Object
and may be overridden by any Java class. This method produces the desired String object,
which is then used wherever it was wanted.
Thus, all you need to do to make objects of your class magically print out is to override
the toString( ) method, as shown in the following example:
//: WorksAnyway.java
// In special cases, things just seem
// to work correctly.
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![import java.util.*;
class Mouse {
private int mouseNumber;
Mouse(int i) {
mouseNumber = i;
}
// Magic method:
public String toString() {
return "This is Mouse #" + mouseNumber;
}
void print(String msg) {
if(msg != null) System.out.println(msg);
System.out.println(
"Mouse number " + mouseNumber);
}
}
class MouseTrap {
static void caughtYa(Object m) {
Mouse mouse = (Mouse)m; // Cast from Object
mouse.print("Caught one!");
}
}
public class WorksAnyway {
public static void main(String args[]) {
Vector mice = new Vector();
for(int i = 0; i < 3; i++)
mice.addElement(new Mouse(i));
for(int i = 0; i < mice.size(); i++) {
// No cast necessary, automatic call
// to Object.toString():
System.out.println(
"Free mouse: " + mice.elementAt(i));
MouseTrap.caughtYa(mice.elementAt(i));
}
}
} ///:~
You can see the redefinition of toString( ) in Mouse. In the second for loop in main( ) you
find the statement:
System.out.println("Free mouse: " + mice.elementAt(i));
After the ‘+’ sign the compiler is expecting to see a String object. elementAt( ) produces
an Object, so to get the desired String the compiler implicitly calls toString( ).
Unfortunately, you can only this kind of magic work with String; it isn’t available for any
other type.
A second approach to hiding the cast has been placed inside Mousetrap: the caughtYa( )
method accepts, not a Mouse, but an Object which it then casts to a Mouse. This is quite
presumptuous, of course, since by accepting an Object anything could be passed to the
method. However, if the cast is incorrect – if you passed the wrong type – you’ll get an
exception at run-time. This is not as good as compile-time checking but it’s still robust.
Notice that in the use of this method:
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![MouseTrap.caughtYa(mice.elementAt(i));
no cast is necessary.
Making a type-conscious Vector
You might not want to give up on this issue just yet. A more ironclad solution is to create
a new class using the Vector, such that it will accept only your type and produce only
your type:
//: GopherVector.java
// A type-conscious Vector
import java.util.*;
class Gopher {
private int gopherNumber;
Gopher(int i) {
gopherNumber = i;
}
void print(String msg) {
if(msg != null) System.out.println(msg);
System.out.println(
"Gopher number " + gopherNumber);
}
}
class GopherTrap {
static void caughtYa(Gopher g) {
g.print("Caught one!");
}
}
class GopherVector {
private Vector v = new Vector();
public void addElement(Gopher m) {
v.addElement(m);
}
public Gopher elementAt(int index) {
return (Gopher)v.elementAt(index);
}
public int size() { return v.size(); }
public static void main(String args[]) {
GopherVector gophers = new GopherVector();
for(int i = 0; i < 3; i++)
gophers.addElement(new Gopher(i));
for(int i = 0; i < gophers.size(); i++)
GopherTrap.caughtYa(gophers.elementAt(i));
}
} ///:~
This is similar to the previous example, except that the new GopherVector class has a
private member of type Vector (inheriting from Vector tends to be frustrating, for reasons
you’ll see later), and methods just like Vector. However, it doesn’t accept and produce
generic Objects, only Gopher objects.
Because a GopherVector will accept only a Gopher, if you were to say:
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![original version, the method elementAt( ) was used to select each element, but in the
following modified version an enumeration is used:
//: CatsAndDogs2.java
// Simple collection with Enumeration
import java.util.*;
class Cat2 {
private int catNumber;
Cat2(int i) {
catNumber = i;
}
void print() {
System.out.println("Cat number " +catNumber);
}
}
class Dog2 {
private int dogNumber;
Dog2(int i) {
dogNumber = i;
}
void print() {
System.out.println("Dog number " +dogNumber);
}
}
public class CatsAndDogs2 {
public static void main(String args[]) {
Vector cats = new Vector();
for(int i = 0; i < 7; i++)
cats.addElement(new Cat2(i));
// Not a problem to add a dog to cats:
cats.addElement(new Dog2(7));
Enumeration e = cats.elements();
while(e.hasMoreElements())
((Cat2)e.nextElement()).print();
// Dog is detected only at run-time
}
} ///:~
You can see that the only change is in the last few lines. Instead of:
for(int i = 0; i < cats.size(); i++)
((Cat)cats.elementAt(i)).print();
an Enumeration is used to step through the sequence:
while(e.hasMoreElements())
((Cat2)e.nextElement()).print();
With the Enumeration, you don’t have to worry about the number of elements in the
collection. That’s taken care of for you by hasMoreElements( ) and nextElement( ).
As another example, consider the creation of a general-purpose printing method:
//: HamsterMaze.java
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![// Using an Enumeration
import java.util.*;
class Hamster {
private int hamsterNumber;
Hamster(int i) {
hamsterNumber = i;
}
public String toString() {
return "This is Hamster #" + hamsterNumber;
}
}
class Printer {
static void printAll(Enumeration e) {
while(e.hasMoreElements())
System.out.println(
e.nextElement().toString());
}
}
public class HamsterMaze {
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 3; i++)
v.addElement(new Hamster(i));
Printer.printAll(v.elements());
}
} ///:~
Look closely at the printing method:
static void printAll(Enumeration e) {
while(e.hasMoreElements())
System.out.println(
e.nextElement().toString());
}
Notice that there’s no information about the type of sequence. All you have is an
Enumeration, and that’s all you need to know about the sequence: that you can get the
next object, and that you can know when you’re at the end. This idea of taking a collection
of objects and passing through it to perform an operation on each one is very powerful
and will be seen again and again throughout this book.
This particular example is even more generic, since it uses the ubiquitous toString( )
method (ubiquitous only because it’s part of the Object class). An alternative way to call
print (although probably slightly less efficient, if you could even notice the difference) is:
System.out.println("" + e.nextElement());
which uses the “automatic conversion to String” that’s wired into Java. When the
compiler sees a String, followed by a ‘+’, it expects another String to follow and calls
toString( ) automatically (in Java 1.1 the first String is unnecessary; any object will be
converted to a String). You can also perform a cast, which has the effect of calling
toString( ):
System.out.println((String)e.nextElement());
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![In general, however, you’ll want to do something more than call Object methods, so you’ll
run up against the type-casting issue again. That is, you’ll have to assume that you’ve
gotten an Enumeration to a sequence of the particular type you’re interested in, and cast
the resulting objects to that (getting a run-time exception if you’re wrong).
Types of collections
The standard Java 1.0 and 1.1 library comes with a bare minimum set of collection
classes, but they’re probably enough to get by with for many of your programming
projects (as you’ll see at the end of this chapter, Java 1.2 provides a radically-redesigned
and filled-out library of collections).
Vector
The Vector is quite simple to use, as you’ve seen so far. Although most of the time you’ll
just use addElement( ) to insert objects, elementAt( ) to get them out one at a time and
elements( ) to get an Enumeration to the sequence, there’s also a set of other methods
that can be useful. As usual with the Java libraries, we won’t use or talk about them all
here, but be sure to look them up in the electronic documentation to get a feel for what
they can do.
Crashing Java
The Java standard collections contain a toString( ) method so they can produce a String
representation of themselves, including the objects they hold. Inside of Vector, for
example, the toString( ) steps through the elements of the Vector and calls toString( ) for
each one. Suppose you’d like to print out the address of your class. It seems to make sense
to simply refer to this (in particular, C++ programmers are prone to this approach):
//: CrashJava.java
// One way to crash Java
import java.util.*;
public class CrashJava {
public String toString() {
return "CrashJava address: " + this + "n";
}
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 10; i++)
v.addElement(new CrashJava());
System.out.println(v);
}
} ///:~
It turns out that if you simply create a CrashJava object and print it out, you’ll get an
endless sequence of exceptions. However, if you place the CrashJava objects in a Vector
and print out that Vector as shown here, it can’t handle it and you don’t even get an
exception; Java just crashes (but at least it didn’t bring down my operating system). This
was tested with Java 1.1.
What’s happening is automatic type conversion for Strings. When you say:
"CrashJava address: " + this
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![The compiler sees a String followed by a ‘+’ and something that’s not a String, so it tries
to convert this to a String. It does this conversion by calling toString( ), which produces a
recursive call. When this occurs inside a Vector it appears that the stack overflows
without the exception-handling mechanism getting a chance to respond.
If you really do want to print the address of the object in this case, the solution is to call
the Object toString( ) method, which does just that. So instead of saying this, you’d say
super.toString( ).
BitSet
A BitSet is really a Vector of bits, and is used if you want to efficiently store a whole lot of
on-off information. It’s efficient only from the standpoint of size; if you’re looking for
efficient access it is slightly slower than using an array of some native type.
In addition, the minimum size of the BitSet is that of a long: 64 bits. This implies that if
you’re storing anything smaller, like 8 bits, a BitSet will be wasteful so you’re better off
creating your own class to hold your flags.
In a normal Vector, the collection will expand as you add more elements. The BitSet does
this as well – sort of. That is, sometimes it works and sometimes it doesn’t, which makes
it appear that the Java version 1.0 implementation of BitSet is just badly done (it is fixed
in Java 1.1). The following example shows how the BitSet works and demonstrates the
version 1.0 bug:
//: Bits.java
// Demonstration of BitSet
import java.util.*;
public class Bits {
public static void main(String args[]) {
Random rand = new Random();
// Take the LSB of nextInt():
byte bt = (byte)rand.nextInt();
BitSet bb = new BitSet();
for(int i = 7; i >=0; i--)
if(((1 << i) & bt) != 0)
bb.set(i);
else
bb.clear(i);
System.out.println("byte value: " + bt);
printBitSet(bb);
short st = (short)rand.nextInt();
BitSet bs = new BitSet();
for(int i = 15; i >=0; i--)
if(((1 << i) & st) != 0)
bs.set(i);
else
bs.clear(i);
System.out.println("short value: " + st);
printBitSet(bs);
int it = rand.nextInt();
BitSet bi = new BitSet();
for(int i = 31; i >=0; i--)
if(((1 << i) & it) != 0)
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![//: Stacks.java
// Demonstration of Stack Class
import java.util.*;
import java.io.*;
public class Stacks {
static String months[] = {
"January", "February", "March", "April",
"May", "June", "July", "August", "September",
"October", "November", "December" };
public static void main(String args[]) {
Stack stk = new Stack();
for(int i = 0; i < months.length; i++)
stk.push(months[i] + "n");
System.out.println("stk = " + stk);
// Treating a stack as a Vector:
stk.addElement("The last linen");
System.out.println(
"element 5 = " + stk.elementAt(5));
System.out.println("popping elements:");
while(!stk.empty())
System.out.print(stk.pop());
}
} ///:~
Each line in the months array is inserted into the Stack with push( ), and later fetched
from the top of the stack with a pop( ). To make a point, Vector operations are also
performed on the Stack object. This is possible because, by virtue of inheritance, a Stack is
a Vector. Thus all operations that can be performed on a Vector can also be performed on
a Stack, such as elementAt( ).
Hashtable
A Vector allows you to select from a sequence of objects using a number, so in a sense it
associates numbers to objects. But what if you’d like to select from a sequence of objects
using some other criterion? A Stack is an example: its selection criterion is “the last thing
pushed on the stack.” A very powerful twist on this idea of “selecting from a sequence” is
alternately termed a map, a dictionary or an associative array. Conceptually, it seems like a
vector, but instead of looking up objects using a number, you look them up using another
object! This is very often a key process in a program.
The concept shows up in Java as the abstract class Dictionary. The interface for this class
is straightforward: size( ) tells you how many elements are within, isEmpty( ) is true if
there are no elements, put(Object key, Object value) adds a value (the thing you’ll be
wanting) and associates it with a key (the thing you’ll be looking it up with). get(Object
key) produces the value given the corresponding key, and remove(Object key) removes
the key-value pair from the list. There are enumerations: keys( ) produces an
Enumeration of the keys, and elements( ) produces an Enumeration of all the values.
That’s all there is to a Dictionary.
A Dictionary isn’t terribly difficult to implement. Here’s a simple approach, which uses
two Vectors, one for keys and one for values:
//: AssocArray.java
// Simple version of a Dictionary
import java.util.*;
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![public class AssocArray extends Dictionary {
private Vector keys = new Vector();
private Vector values = new Vector();
public int size() { return keys.size(); }
public boolean isEmpty() {
return keys.isEmpty();
}
public Object put(Object key, Object value) {
keys.addElement(key);
values.addElement(value);
return key;
}
public Object get(Object key) {
int index = keys.indexOf(key);
// indexOf() Returns -1 if key not found:
if(index == -1) return null;
return values.elementAt(index);
}
public Object remove(Object key) {
int index = keys.indexOf(key);
if(index == -1) return null;
keys.removeElementAt(index);
Object returnval = values.elementAt(index);
values.removeElementAt(index);
return returnval;
}
public Enumeration keys() {
return keys.elements();
}
public Enumeration elements() {
return values.elements();
}
// Test it:
public static void main(String args[]) {
AssocArray aa = new AssocArray();
for(char c = 'a'; c <= 'z'; c++)
aa.put(String.valueOf(c),
String.valueOf(c)
.toUpperCase());
char[] ca = { 'a', 'e', 'i', 'o', 'u' };
for(int i = 0; i < ca.length; i++)
System.out.println("Uppercase: " +
aa.get(String.valueOf(ca[i])));
}
} ///:~
The first thing you see in the definition of AssocArray is that it extends Dictionary. This
means that AssocArray is a type of Dictionary, so you can make the same requests of it
that you can a Dictionary. If you make your own Dictionary, as is done here, all you
have to do is fill in all the methods that are in Dictionary (and you must override all the
methods because all of them – with the exception of the constructor – are abstract).
The Vectors keys and values are linked by a common index number. That is, if I call
put( ) with a key of “roof” and a value of “blue” (assuming I’m associating the various
parts of a house with the colors they are to be painted) and there are already 100 elements
in the AssocArray, then “roof” will be the 101 element of keys and “blue” will be the 101
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![element of values. And if you look at get( ), when you pass “roof” in as the key, it
produces the index number with keys.indexOf( ), and then uses that index number to
produce the value in the associated values vector.
The test in main( ) is very simple; it’s just a map of lowercase characters to uppercase
characters, which could obviously be done in a number of more efficient ways. But it
shows that AssocArray is functional.
The standard Java library contains only one embodiment of a Dictionary, called
Hashtable3. Java’s Hashtable has the same basic interface as AssocArray (since they
both inherit Dictionary), but it differs in one distinct way: efficiency. If you look at what
must be done for a get( ), it seems pretty slow to search through a Vector for the key.
This is where Hashtable speeds things up: instead of the tedious linear search for the key,
it uses a special value called a hash code. The hash code is a way to take some information
in the object in question and turn it into a “relatively unique” int for that object. All
objects have a hash code, and hashCode( ) is a method in the root class Object. A
Hashtable takes the hashCode( ) of the object and uses it to quickly hunt for the key.
This results in a dramatic performance improvement. The way that a Hashtable works is
beyond the scope of this book4 – all you need to know is that Hashtable is a fast
Dictionary, and that a Dictionary is a very useful tool.
As an example of the use of a Hashtable, consider a program to check the randomness of
Java’s Math.random( ) method. Ideally, it would produce a perfect distribution of random
numbers, but to test this we need to generate a bunch of random numbers and count the
ones that fall in the various ranges. A Hashtable is perfect for this, since it associates
objects with objects (in this case, the values produced by Math.random( ) with the
number of times those values appear:
//: Statistics.java
// Simple demonstration of Hashtable
import java.util.*;
class Counter {
int i = 1;
public String toString() {
return Integer.toString(i) + "n";
}
}
class Statistics {
public static void main(String args[]) {
Hashtable ht = new Hashtable();
for(int i = 0; i < 10000; i++) {
// Produce a number between 0 and 20:
Integer r =
new Integer((int)(Math.random() * 20));
if(ht.containsKey(r))
((Counter)ht.get(r)).i++;
3 If you’re planning on using RMI (described in Chapter 15) you should be aware that there’s a
problem when putting remote objects into a Hashtable. (See Core Java, by Cornell & Horstmann,
Prentice-Hall 1997).
4 The best reference I know of is Practical Algorithms for Programmers, by Andrew Binstock and
John Rex, Addison-Wesley 1995.
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![Creating “key” classes
In the previous example, a standard library class (Integer) was used as a key for the
Hashtable. It worked fine as a key, because it has all the necessary wiring to make it
work correctly as a key. But a common pitfall occurs when using Hashtables when you
create your own classes to be used as keys. For example, consider a weather predicting
system that matches Groundhog objects to Prediction objects. It seems fairly
straightforward: you create the two classes and use Groundhog as the key and Prediction
as the value:
//: SpringDetector.java
// Looks plausible, but doesn't work right.
import java.util.*;
class Groundhog {
int ghNumber;
Groundhog(int n) { ghNumber = n; }
}
class Prediction {
boolean shadow = Math.random() > 0.5;
public String toString() {
if(shadow)
return "Six more weeks of Winter!";
else
return "Early Spring!";
}
}
public class SpringDetector {
public static void main(String args[]) {
Hashtable ht = new Hashtable();
for(int i = 0; i < 10; i++)
ht.put(new Groundhog(i), new Prediction());
System.out.println("ht = " + ht + "n");
System.out.println(
"Looking up prediction for groundhog #3:");
Groundhog gh = new Groundhog(3);
if(ht.containsKey(gh))
System.out.println((Prediction)ht.get(gh));
}
} ///:~
Each Groundhog is given an identity number, so you can look up a Prediction in the
Hashtable by saying “give me the Prediction associated with Groundhog number 3.” The
Prediction class contains a boolean which is initialized using Math.random( ), and a
toString( ) that interprets the result for you. In main( ), a Hashtable is filled with
Groundhogs and their associated Predictions. The Hashtable is printed so you can see
that it has in fact been filled. Then a Groundhog with an identity number of 3 is used to
look up the prediction for Groundhog #3.
It seems simple enough, but it doesn’t work. The problem is that Groundhog is inherited
from the common root class Object (which is what happens if you don’t specify a base
class, thus all classes are ultimately inherited from Object). It is Object’s hashCode( )
method that is used to generate the hash code for each object, and by default it just uses
the address of its object. Thus the first instance of Groundhog(3) does not produce a hash
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![code equal to the hash code for the second instance of Groundhog(3) that we tried to use
as a lookup.
So you might think that all you need to do is write an appropriate override for
hashCode( ). But it still won’t work until you’ve done one more thing: overridden the
equals( ) that is also part of Object. This method is used by the Hashtable when trying to
determine if your key is equal to any of the keys in the table. Again, the default
Object.equals( ) simply compares object addresses, so one Groundhog(3) is not equal to
another Groundhog(3).
Thus, to use your own classes as keys in a Hashtable, you must override both
hashCode( ) and equals( ), as shown in the following solution to the above problem:
//: SpringDetector2.java
// If you create a class that's used as a key in
// a Hashtable, you must override hashCode()
// and equals().
import java.util.*;
class Groundhog2 {
int ghNumber;
Groundhog2(int n) { ghNumber = n; }
public int hashCode() {
return ghNumber;
}
public boolean equals(Object o) {
if ((o != null) && (o instanceof Groundhog2))
return
ghNumber == ((Groundhog2)o).ghNumber;
else return false;
}
}
public class SpringDetector2 {
public static void main(String args[]) {
Hashtable ht = new Hashtable();
for(int i = 0; i < 10; i++)
ht.put(new Groundhog2(i),new Prediction());
System.out.println("ht = " + ht + "n");
System.out.println(
"Looking up prediction for groundhog #3:");
Groundhog2 gh = new Groundhog2(3);
if(ht.containsKey(gh))
System.out.println((Prediction)ht.get(gh));
}
} ///:~
Note that this uses the Prediction class from the previous example, so
SpringDetector.java must be compiled first or you’ll get a compile-time error when you
try to compile SpringDetector2.java.
Groundhog2.hashCode( ) returns the ground hog number as an identifier (in this
example, the programmer is responsible for ensuring that no two ground hogs exist with
the same ID number). The hashCode( ) is not required to return a unique identifier, but
the equals( ) method must be able to strictly determine whether two objects are
equivalent.
266 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-266-320.jpg)
![The equals( ) method does two sanity checks: to see if the object is null, and if not,
whether it is an instance of Groundhog2 (using the instanceof keyword, which is fully
explained in Chapter 11). It should be a Groundhog2 to even continue executing equals( ).
The comparison, as you can see, is based on the actual ghNumbers. This time, when you
run the program, you’ll see it produces the correct output.
Properties: a type of Hashtable
In the very first example in the book, a type of Hashtable was used called Properties. In
that example, the lines:
Properties p = System.getProperties();
p.list(System.out);
called the static method getProperties( ) to get a special Properties object that described
the system characteristics. The method list( ) is a method of Properties that sends the
contents to any stream output that you choose. In addition there’s a save( ) method to
allow you to write your property list to a file in a way that it can be retrieved later with
the load( ) method.
Although the Properties class is inherited from Hashtable, it also contains a second
Hashtable that acts to hold the list of “default” properties. So if a property isn’t found in
the primary list, the defaults will be searched.
The Properties class is also available for use in your programs.
Enumerators revisited
We can now demonstrate the true power of the Enumeration: the ability to separate the
operation of traversing a sequence from the underlying structure of that sequence. In the
following example, the class PrintData uses an Enumeration to move through a sequence
and call the toString( ) method for every object. Two different types of collections are
created, a Vector and a Hashtable, and they are each filled with, respectively, Mouse and
Hamster objects (these classes were defined earlier in the chapter; note you must have
compiled HamsterMaze.java and WorksAnyway.java for the following program to
compile). Because an Enumeration hides the structure of the underlying collection,
PrintData doesn’t know or care what kind of collection the Enumeration comes from:
//: Enumerators2.java
// Revisiting Enumerations
import java.util.*;
class PrintData {
static void print(Enumeration e) {
while(e.hasMoreElements())
System.out.println(
e.nextElement().toString());
}
}
class Enumerators2 {
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 5; i++)
v.addElement(new Mouse(i));
Hashtable h = new Hashtable();
for(int i = 0; i < 5; i++)
Chapter 8: Holding Your Objects 267](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-267-320.jpg)
![((String)r).toLowerCase()) < 0;
}
public boolean
lessThanOrEqual(Object l, Object r) {
return ((String)l).toLowerCase().compareTo(
((String)r).toLowerCase()) <= 0;
}
}
public static void main(String args[]) {
SortVector sv =
new SortVector(new StringCompare());
sv.addElement("d");
sv.addElement("A");
sv.addElement("C");
sv.addElement("c");
sv.addElement("b");
sv.addElement("B");
sv.addElement("D");
sv.addElement("a");
sv.sort();
Enumeration e = sv.elements();
while(e.hasMoreElements())
System.out.println(e.nextElement());
}
} ///:~
The inner class is static because it is invoked inside a static method (main( )). You can see
how, once the framework is set up, it’s very easy to reuse a design like this – you simply
write the class that encapsulates “the things that change” and hand an object to the
SortVector.
The comparison forces the strings to lower case, so that the capital A’s end up next to the
small a’s, and not in some entirely different place. This example shows, however, a slight
deficiency in this approach, since the test code above puts the uppercase and lowercase
single letters of the same letter in the order that they appear: A a b B c C d D. This is not
usually much of a problem because you’re usually working with longer strings, and in
that situation the effect doesn’t show up (the Java 1.2 collections provide sorting
functionality that solves this problem).
Inheritance (extends) is used here to create a new type of Vector – that is, StrSortVector is
a Vector with some added functionality. The use of inheritance here is powerful but it
presents problems. It turns out that some methods are final (described in Chapter 7) so
you cannot override them. If you want to create a sorted Vector that accepts and produces
only String objects you run into a wall, since addElement( ) and elementAt( ) are final,
and these are precisely the methods you’d need to override so they accept and produce
only String objects. No luck there.
On the other hand, consider composition: the placing of an object inside a new class.
Rather than rewrite the above code to accomplish this, we can simply use a SortVector
inside the new class. In this case, the inner class will be created anonymously:
//: StrSortVector.java
// Automatically sorted Vector that
// accepts and produces only Strings
package c08;
import java.util.*;
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![public class StrSortVector {
private SortVector v = new SortVector(
// Anonymous inner class:
new Compare() {
public boolean
lessThan(Object l, Object r) {
return
((String)l).toLowerCase().compareTo(
((String)r).toLowerCase()) < 0;
}
public boolean
lessThanOrEqual(Object l, Object r) {
return
((String)l).toLowerCase().compareTo(
((String)r).toLowerCase()) <= 0;
}
}
);
private boolean sorted = false;
public void addElement(String s) {
v.addElement(s);
sorted = false;
}
public String elementAt(int index) {
if(!sorted) {
v.sort();
sorted = true;
}
return (String)v.elementAt(index);
}
public Enumeration elements() {
if(!sorted) {
v.sort();
sorted = true;
}
return v.elements();
}
// Test it:
public static void main(String args[]) {
StrSortVector sv = new StrSortVector();
sv.addElement("d");
sv.addElement("A");
sv.addElement("C");
sv.addElement("c");
sv.addElement("b");
sv.addElement("B");
sv.addElement("D");
sv.addElement("a");
Enumeration e = sv.elements();
while(e.hasMoreElements())
System.out.println(e.nextElement());
}
} ///:~
This quickly reuses the code from SortVector to create the desired functionality. However,
all the public methods from SortVector and Vector do not appear in StrSortVector – its
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![that exception, so you can actually start throwing the exception later without requiring
changes to existing code.
Catching any exception
It is possible to create a handler that catches any type of exception. You do this by
catching the base-class exception type Exception (there are other types of base exceptions,
but Exception is the base that’s pertinent to virtually all programming activities):
catch(Exception e) {
System.out.println("caught an exception");
}
This will catch any exception, so if you use it you’ll want to put it at the end of your list
of handlers to avoid pre-empting any exception handlers that follow it.
Since the Exception class is the base of all the exception classes that are important to the
programmer, you don’t get much specific information about the exception, but you can
call the methods that come from its base type Throwable:
String getMessage( )
Gets the detail message .
String toString( )
Returns a short description of the Throwable, including the detail message if there is one.
void printStackTrace( )
void printStackTrace(PrintStream)
Prints the Throwable and the Throwable’s call stack trace. The call stack shows the
sequence of method calls that brought you to the point where the exception was thrown.
The first version prints to standard error, the second prints to a stream of your choice. If
you’re working under Windows, you can’t redirect standard error so you might want to
use the second version and send the results to System.out; that way the output can be
redirected any way you want.
In addition, you get some other methods from Throwable’s base type Object (everybody’s
base type). The one that might come in handy for exceptions is getClass( ), which returns
an object representing the class of this object. You can in turn query this Class object for
its name with getName( ) or toString( ). You can also do more sophisticated things with
Class objects that aren’t necessary in exception handling. Class objects will be studied later
in the book.
Here’s an example that shows the use of the Exception methods (see page 80 if you have
trouble executing this program):
//: ExceptionMethods.java
// Demonstrating the Exception Methods
package c09;
public class ExceptionMethods {
public static void main(String args[]) {
try {
throw new Exception("Here's my Exception");
} catch(Exception e) {
System.out.println("Caught Exception");
System.out.println(
280 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-280-320.jpg)
!["Inside g(), e.printStackTrace()");
e.printStackTrace();
throw e; // 17
// throw e.fillInStackTrace(); // 18
}
}
public static void
main(String args[]) throws Throwable {
try {
g();
} catch(Exception e) {
System.out.println(
"Caught in main, e.printStackTrace()");
e.printStackTrace();
}
}
} ///:~
The important line numbers are marked inside of comments. With line 17 uncommented
(as shown), the output is:
originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:8)
at Rethrowing.g(Rethrowing.java:12)
at Rethrowing.main(Rethrowing.java:24)
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:8)
at Rethrowing.g(Rethrowing.java:12)
at Rethrowing.main(Rethrowing.java:24)
So the exception stack trace always remembers its true point of origin, no matter how
many times it gets rethrown.
With line 17 commented and line 18 uncommented, fillInStackTrace( ) is used instead,
and the result is:
originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.f(Rethrowing.java:8)
at Rethrowing.g(Rethrowing.java:12)
at Rethrowing.main(Rethrowing.java:24)
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
at Rethrowing.g(Rethrowing.java:18)
at Rethrowing.main(Rethrowing.java:24)
Because of fillInStackTrace( ), line 18 becomes the new point of origin of the exception.
The class Throwable must appear in the exception specification for g( ) and main( )
because fillInStackTrace( ) produces a handle to a Throwable object. Since Throwable is a
base class of Exception, it’s possible to get an object that’s a Throwable but not an
Exception, so the handler for Exception in main( ) might miss it. To make sure
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![everything is in order, the compiler forces an exception specification for Throwable. For
example, the exception in the following program is not caught in main( ):
//: ThrowOut.java
public class ThrowOut {
public static void
main(String args[]) throws Throwable {
try {
throw new Throwable();
} catch(Exception e) {
System.out.println("Caught in main()");
}
}
} ///:~
It’s also possible to rethrow a different exception than the one you caught. If you do this,
you get a similar effect as when using fillInStackTrace( ): the information about the
original site of the exception is lost, and what you’re left with is the information
pertaining to the new throw:
//: RethrowNew.java
// Rethrow a different object than you catch
public class RethrowNew {
public static void f() throws Exception {
System.out.println(
"originating the exception in f()");
throw new Exception("thrown from f()");
}
public static void
main(String args[]) {
try {
f();
} catch(Exception e) {
System.out.println(
"Caught in main, e.printStackTrace()");
e.printStackTrace();
throw new NullPointerException("from main");
}
}
} ///:~
The output is:
originating the exception in f()
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
at RethrowNew.f(RethrowNew.java:8)
at RethrowNew.main(RethrowNew.java:13)
java.lang.NullPointerException: from main
at RethrowNew.main(RethrowNew.java:18)
The final exception knows only that it came from main( ), and not from f( ). Notice that
Throwable isn’t necessary in any of the exception specifications.
You never have to worry about cleaning up the previous exception, or any exceptions for
that matter: they’re all heap-based objects created with new, so the garbage collector
automatically cleans them all up.
Chapter 9: Error Handling with Exceptions 283](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-283-320.jpg)
![percolate all the way out to your main( ) method without being caught. To see what
happens in this case, try the following example:
//: NeverCaught.java
// Ignoring RuntimeExceptions
public class NeverCaught {
static void f() {
throw new RuntimeException("From f()");
}
static void g() {
f();
}
public static void main(String args[]) {
g();
}
} ///:~
You can already see that a RuntimeException (or anything inherited from it) is a special
case, since the compiler doesn’t require an exception specification for these types.
The output is:
java.lang.RuntimeException: From f()
at NeverCaught.f(NeverCaught.java:9)
at NeverCaught.g(NeverCaught.java:12)
at NeverCaught.main(NeverCaught.java:15)
So the answer is: if a RuntimeException gets all the way out to main( ) without being
caught, printStackTrace( ) is called for that exception as the program exits.
Keep in mind that it’s possible to ignore only RuntimeExceptions in your coding, since all
other handling is carefully enforced by the compiler. The reasoning is that a
RuntimeException represents a programming error:
1. An error you cannot catch (receiving a null handle handed to your method by a client
programmer, for example)
2. An error that you, as a programmer, should have checked for in your code (such as
ArrayIndexOutOfBoundsException where you should have paid attention to the size
of the array).
You can see what a tremendous benefit it is to have exceptions in this case, since they
assist in the debugging process.
It’s interesting to note that you cannot classify Java exception handling as a single-
purpose tool. Yes, it is designed to handle those pesky run-time errors that will occur
because of forces outside the control of your code, but it’s also essential for certain types
of programming bugs that the compiler cannot detect.
Creating your own exceptions
You’re not stuck using the Java exceptions. This is important because you’ll often need to
create your own exceptions to denote a special error that your library is capable of
creating, but which was not foreseen when the Java hierarchy was created.
Chapter 9: Error Handling with Exceptions 285](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-285-320.jpg)
![To create your own exception class, you’re forced to inherit from an existing type of
exception, preferably one that is very close in meaning to your new exception. Inheriting
an exception is quite simple:
//: Inheriting.java
// Inheriting your own exceptions
class MyException extends Exception {
public MyException() {}
public MyException(String msg) {
super(msg);
}
}
public class Inheriting {
public static void f() throws MyException {
System.out.println(
"Throwing MyException from f()");
throw new MyException();
}
public static void g() throws MyException {
System.out.println(
"Throwing MyException from g()");
throw new MyException("Originated in g()");
}
public static void main(String args[]) {
try {
f();
} catch(MyException e) {
e.printStackTrace();
}
try {
g();
} catch(MyException e) {
e.printStackTrace();
}
}
} ///:~
The inheritance occurs in the creation of the new class:
class MyException extends Exception {
public MyException() {}
public MyException(String msg) {
super(msg);
}
}
The key phrase here is extends Exception, which says “it’s everything an Exception is,
and more.” The added code is very small – the addition of two constructors that define the
way MyException is created. Remember that the compiler automatically calls the base-
class default constructor if you don't explicitly call a base-class constructor, as in the
MyException( ) default constructor. In the second constructor, the base-class constructor
with a String argument is invoked by using the super keyword.
The output of the program is:
286 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-286-320.jpg)
![Throwing MyException from f()
MyException
at Inheriting.f(Inheriting.java:16)
at Inheriting.main(Inheriting.java:24)
Throwing MyException from g()
MyException: Originated in g()
at Inheriting.g(Inheriting.java:20)
at Inheriting.main(Inheriting.java:29)
You can see the absence of the detail message in the MyException thrown from f( ).
The process of creating your own exceptions can be taken further: you can add extra
constructors and members:
//: Inheriting2.java
// Inheriting your own exceptions
class MyException2 extends Exception {
public MyException2() {}
public MyException2(String msg) {
super(msg);
}
public MyException2(String msg, int x) {
super(msg);
i = x;
}
public int val() { return i; }
private int i;
}
public class Inheriting2 {
public static void f() throws MyException2 {
System.out.println(
"Throwing MyException2 from f()");
throw new MyException2();
}
public static void g() throws MyException2 {
System.out.println(
"Throwing MyException2 from g()");
throw new MyException2("Originated in g()");
}
public static void h() throws MyException2 {
System.out.println(
"Throwing MyException2 from h()");
throw new MyException2("Originated in h()", 47);
}
public static void main(String args[]) {
try {
f();
} catch(MyException2 e) {
e.printStackTrace();
}
try {
g();
} catch(MyException2 e) {
e.printStackTrace();
}
Chapter 9: Error Handling with Exceptions 287](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-287-320.jpg)
![class BaseballException extends Exception {}
class Foul extends BaseballException {}
class Strike extends BaseballException {}
abstract class Inning {
Inning() throws BaseballException {}
void event () throws BaseballException {
// Doesn't actually have to throw anything
}
abstract void atBat() throws Strike, Foul;
void walk() {} // Throws nothing
}
class StormException extends Exception {}
class RainedOut extends StormException {}
class PopFoul extends Foul {}
interface Storm {
void event() throws RainedOut;
void rainHard() throws RainedOut;
}
public class StormyInning extends Inning
implements Storm {
// OK to add new exceptions for constructors,
// but you must deal with the base constructor
// exceptions:
StormyInning() throws RainedOut,
BaseballException {}
StormyInning(String s) throws Foul,
BaseballException {}
// Regular methods must conform to base class:
//! void walk() throws PopFoul {} //Compile error
// Interface CANNOT add exceptions to existing
// methods from the base class:
//! public void event() throws RainedOut {}
// If the method doesn't already exist in the
// base class, the exception is OK:
public void rainHard() throws RainedOut {}
// You can choose not to throw any exceptions,
// even if base version does:
public void event() {}
// Overridden methods can throw
// inherited exceptions:
void atBat() throws PopFoul {}
public static void main(String args[]) {
try {
StormyInning si = new StormyInning();
si.atBat();
} catch(PopFoul e) {
} catch(RainedOut e) {
} catch(BaseballException e) {}
// Strike not thrown in derived version.
try {
// What happens if you upcast?
Chapter 9: Error Handling with Exceptions 289](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-289-320.jpg)
![forces you to catch the exceptions for the base type. All these constraints produce much
more robust exception-handling code3 .
It’s useful to realize that, although exception specifications are enforced by the compiler
during inheritance, the exception specifications are not part of the type of a method,
which is comprised of only the method name and argument types. Therefore, you cannot
overload methods based on exception specifications. In addition, because an exception
specification exists in a base-class version of a method doesn’t mean it must exist in the
derived-class version of the method, and this is quite different from inheriting the methods
themselves (that is, a method in the base class must also exist in the derived class). Put
another way, the “exception specification interface” for a particular method may narrow
during inheritance and overriding, but it may not widen – this is precisely the opposite of
the rule for the class interface during inheritance.
Performing cleanup
with finally
There’s often some piece of code that you may want executed whether or not an exception
occurs in a try block. This usually pertains to some operation other than memory
recovery (since that’s taken care of by the garbage collector). To achieve this effect, you use
a finally clause4 at the end of all the exception handlers. The full picture of an exception-
handling section is thus:
try {
// The guarded region:
// Dangerous stuff that may throw A, B, or C
} catch (A a1) {
// Handle A
} catch (B b1) {
// Handle B
} catch (C c1) {
// Handle C
} finally {
// Stuff that happens every time
}
To demonstrate to yourself that the finally clause always runs, try this program:
//: FinallyWorks.java
// The finally clause is always executed
public class FinallyWorks {
static int count = 0;
public static void main(String args[]) {
while(true) {
3 ANSI/ISO C++ added similar constraints that require derived-method exceptions to be the same
as, or derived from, the exceptions thrown by the base-class method. This is one case where C++ is
actually able to check exception specifications at compile time.
4 C++ exception handling does not have the finally clause because it relies on destructors to
accomplish this sort of cleanup.
Chapter 9: Error Handling with Exceptions 291](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-291-320.jpg)
![void on() { state = true; }
void off() { state = false; }
}
public class OnOffSwitch {
static Switch sw = new Switch();
public static void main(String args[]) {
try {
sw.on();
// code that may throw exceptions...
sw.off();
} catch(NullPointerException e) {
System.out.println("NullPointerException");
sw.off();
} catch(IllegalArgumentException e) {
System.out.println("IOException");
sw.off();
}
}
} ///:~
The goal here is to make sure that the switch is off when main( ) is completed, so
sw.off( ) is placed at the end of the try block and the end of each exception handler. But
it’s possible that an exception could be thrown that isn’t caught here, and so sw.off( )
would be missed. However, with finally you can place the closure code from a try block in
just one place:
//: WithFinally.java
// Finally Guarantees cleanup
class Switch2 {
boolean state = false;
boolean read() { return state; }
void on() { state = true; }
void off() { state = false; }
}
public class WithFinally {
static Switch2 sw = new Switch2();
public static void main(String args[]) {
try {
sw.on();
// code that may throw exceptions...
} catch(NullPointerException e) {
System.out.println("NullPointerException");
} catch(IllegalArgumentException e) {
System.out.println("IOException");
} finally {
sw.off();
}
}
} ///:~
Here the sw.off( ) has been moved to just one place, where it’s guaranteed to run no
matter what happens.
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![Even in cases where the exception is not caught in the current set of catch clauses, finally
will be executed before the exception-handling mechanism continues its search for a
handler at the next higher level:
//: AlwaysFinally.java
// Finally is always executed
class Ex extends Exception {}
public class AlwaysFinally {
public static void main(String args[]) {
System.out.println(
"Entering first try block");
try {
System.out.println(
"Entering second try block");
try {
throw new Ex();
} finally {
System.out.println(
"finally in 2nd try block");
}
} catch(Ex e) {
System.out.println(
"Caught Ex in first try block");
} finally {
System.out.println(
"finally in 1st try block");
}
}
} ///:~
The output for this program shows you what happens:
Entering first try block
Entering second try block
finally in 2nd try block
Caught Ex in first try block
finally in 1st try block
The finally statement will also be executed in situations where break and continue
statements are involved. Note that, along with the labeled break and labeled continue,
finally eliminates the need for a goto statement in Java.
Pitfall: the lost exception
In general Java’s exception implementation is quite outstanding, but unfortunately there’s
a flaw. Although exceptions are an indication of a crisis in your program and should never
be ignored, it’s possible for an exception to simply be lost. This happens with a particular
configuration using a finally clause:
//: LostMessage.java
// How an exception can be lost
class VeryImportantException extends Exception {
public String toString() {
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![return "A very important exception!";
}
}
class HoHumException extends Exception {
public String toString() {
return "A trivial exception";
}
}
public class LostMessage {
void f() throws VeryImportantException {
throw new VeryImportantException();
}
void dispose() throws HoHumException {
throw new HoHumException();
}
public static void main(String args[])
throws Exception {
LostMessage lm = new LostMessage();
try {
lm.f();
} finally {
lm.dispose();
}
}
} ///:~
The output is:
A trivial exception
at LostMessage.dispose(LostMessage.java:21)
at LostMessage.main(LostMessage.java:29)
You can see that there’s no evidence of the VeryImportantException, which is simply
replaced by the HoHumException in the finally clause. This is a rather serious pitfall,
since it means an exception can be completely lost, and in a far more subtle and difficult-
to-detect fashion than the example above. In contrast, C++ treats the situation where a
second exception is thrown before the first one is handled as a dire programming error.
Perhaps a future version of Java will repair the problem (the above results were produced
with Java 1.1).
Constructors
When writing code with exceptions, it’s particularly important that you always ask: “If
an exception occurs, will this be properly cleaned up?” Most of the time you’re fairly safe,
but in constructors there’s a problem. The constructor puts the object into a safe starting
state, but it may perform some operation – such as opening a file – that doesn’t get
cleaned up until the user is finished with the object and calls a special cleanup method. If
you throw an exception from inside a constructor, these cleanup behaviors may not occur
properly. This means you must be especially diligent while writing your constructor.
Since you’ve just learned about finally, you may think that’s the correct solution. But it’s
not quite that simple because finally performs the cleanup code every time, even in the
situations where we don’t want the cleanup code executed until the cleanup method runs.
Thus, if you do perform cleanup in finally, you must set some kind of flag when the
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![} catch(IOException e2) {
System.out.println(
"in.close() unsuccessful");
}
}
}
public class Cleanup {
public static void main(String args[]) {
try {
InputFile in = new InputFile("Cleanup.java");
String s;
int i = 1;
while((s = in.getLine()) != null)
System.out.println(""+ i++ + ": " + s);
in.cleanup();
} catch(Exception e) {
System.out.println(
"Caught in main, e.printStackTrace()");
e.printStackTrace();
}
}
} ///:~
This example uses Java 1.1 IO classes.
The constructor for InputFile takes a String argument, which is the name of the file you
want to open. Inside a try block, it creates a FileReader using the file name. A FileReader
isn’t particularly useful until you turn around and use it to create a BufferedReader that
you can actually talk to – notice that one of the benefits of InputFile is that it combines
these two actions.
If the FileReader constructor is unsuccessful it throws a FileNotFoundException, which
must be caught separately because that’s the one case where you don’t want to close the
file since it wasn’t successfully opened. Any other catch clauses must close the file because
it was opened by the time those catch clauses are entered (of course, this is trickier if more
than one method can throw a FileNotFoundException. In that case, you may want to
break things up into several try blocks). The close( ) method itself throws an exception
which is tried and caught even though it’s within the block of another catch clause – it’s
just another pair of curly braces to the Java compiler. After performing local operations,
the exception is re-thrown, which is appropriate because this constructor failed and you
wouldn’t want the calling method to assume that the object had been properly created and
was valid.
In this example, which doesn’t use the aforementioned flagging technique, the finally
clause is definitely not the place to close( ) the file, since that would close it every time the
constructor completed. Since we want the file to be open for the useful lifetime of the
InputFile object this would not be appropriate.
The getLine( ) method returns a String containing the next line in the file. The
readLine( ) method that it calls can throw an exception which is caught and dealt with so
that getLine( ) doesn’t throw any exceptions. One of the design issues with exceptions is
whether to handle an exception completely at this level, to handle it partially and pass the
same exception (or a different one) on, or whether to simply pass it on. Passing it on,
when appropriate, can certainly simplify coding. The getLine( ) method becomes:
String getLine() throws IOException {
return in.readLine();
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![}
But of course, the caller is now responsible for handling any IOException that might
arise.
The cleanup( ) method must be called by the user when they are finished using the
InputFile object, to release the system resources (such as file handles) that are used by the
BufferedReader and/or FileReader objects6 . You don’t want to do this until you’re
finished with the InputFile object, at the point you’re going to let it go. You might think of
putting such functionality into a finalize( ) method, but as mentioned in Chapter 4 you
can’t always be sure that finalize( ) will be called (even if you can be sure that it will be
called, you don’t know when). This is one of the downsides to Java – all cleanup other than
memory cleanup doesn’t happen automatically, so you must inform the client
programmer that they are responsible, and possibly guarantee that cleanup occurs using
finalize( ).
In Cleanup.java an InputFile is created to open the same source file that creates the
program, and this file is read in a line at a time, and line numbers are added. All
exceptions are caught generically in main( ), although you could choose greater
granularity.
One of the benefits of this example is to show you why exceptions are introduced at this
point in the book. Exceptions are so integral to programming in Java, especially because
the compiler enforces them, that you can accomplish only so much without knowing how
to work with them.
Exception matching
When an exception is thrown, the exception-handling system looks through the “nearest”
handlers in the order they are written. When it finds a match, the exception is considered
handled, and no further searching occurs.
Matching an exception doesn’t require a perfect match between the exception and its
handler. A derived-class object will match a handler for the base class, as shown in this
example:
//: Human.java
// Catching Exception Hierarchies
class Annoyance extends Exception {}
class Sneeze extends Annoyance {}
public class Human {
public static void main(String args[]) {
try {
throw new Sneeze();
} catch(Sneeze s) {
System.out.println("Caught Sneeze");
} catch(Annoyance a) {
System.out.println("Caught Annoyance");
}
}
6 In C++, a destructor would handle this for you.
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![size of the file. In addition, the constructors require a second argument (identical to
fopen( ) in C) indicating whether you are just randomly reading (“r”) or reading and
writing (“rw”). There’s no support for write-only files, which could suggest that
RandomAccessFile might have worked well if it were inherited from DataInputStream.
What’s even more frustrating is that you could easily imagine wanting to seek within
other types of streams, such as a ByteArrayInputStream, but the seeking methods are
available only in RandomAccessFile, which works for files only. BufferedInputStream
does allow you to mark( ) a position (whose value is held in a single internal variable) and
reset( ) to that position, but this is very limited and not too useful.
The File class
The File class has a deceiving name – you might think it refers to an actual file, but it
doesn’t. It can represent either the name of a particular file, or the names of a set of files
in a directory. If it’s a set of files, you can ask for the set with the list( ) method, and this
returns an array of String. It makes sense to return an array rather than one of the
flexible collection classes because the number of elements is fixed, and if you want a
different directory listing you just create a different File object.. In fact, “FilePath” would
have been a better name. This section shows a complete example of the use of this class,
including the associated FilenameFilter interface.
A directory lister
Suppose you’d like to see a directory listing. The File object can be listed in two ways. If
you call list( ) with no arguments, you’ll get the full list that the File object contains.
However, if you want a restricted list, say for example all the files with an extension of
.java, then you use a “directory filter” which is a class that tells how to select the File
objects for display.
Here’s the code for the example (see page 80 if you have trouble executing this program):
//: DirList.java
// Displays directory listing
package c10;
import java.io.*;
public class DirList {
public static void main(String args[]) {
try {
File path = new File(".");
String[] list;
if(args.length == 0)
list = path.list();
else
list = path.list(new DirFilter(args[0]));
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
} catch(Exception e) {
e.printStackTrace();
}
}
}
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![public class DirList2 {
public static FilenameFilter
filter(final String afn) {
// Creation of anonymous inner class:
return new FilenameFilter() {
String fn = afn;
public boolean accept(File dir, String n) {
// Strip path information:
String f = new File(n).getName();
return f.indexOf(fn) != -1;
}
}; // End of anonymous inner class
}
public static void main(String args[]) {
try {
File path = new File(".");
String[] list;
if(args.length == 0)
list = path.list();
else
list = path.list(filter(args[0]));
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
Notice that the argument to filter( ) must be final. This is required by the anonymous
inner class for it to utilize an object from outside its scope.
This design is an improvement because the FilenameFilter class is now tightly bound to
DirList2. However, you can take this approach one step further and define the anonymous
inner class as an argument to list( ), in which case it’s even smaller:
//: DirList3.java
// Building the anonymous inner class "in-place"
import java.io.*;
public class DirList3 {
public static void main(final String args[]) {
try {
File path = new File(".");
String[] list;
if(args.length == 0)
list = path.list();
else
list = path.list(
new FilenameFilter() {
public boolean accept(File dir, String n) {
String f = new File(n).getName();
return f.indexOf(args[0]) != -1;
}
});
310 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-310-320.jpg)
![for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
The argument to main( ) is now final, since the anonymous inner class uses args[0]
directly.
This shows you how anonymous inner classes allow the creation of quick-and-dirty
classes to solve problems. Since everything in Java revolves around classes, this can be a
useful coding technique. One benefit is that it keeps the code that solves a particular
problem isolated together in one spot. On the other hand, it is not always as easy to read
so you must use it judiciously.
A sorted directory listing
Ah, you say you want the file names sorted? Since there’s no support for sorting in Java
1.0 or Java 1.1 (although sorting is included in Java 1.2) it will have to be added in the
program directly, using the SortVector created in Chapter 8:
//: SortedDirList.java
// Displays sorted directory listing
import java.io.*;
import c08.*;
public class SortedDirList {
private File path;
private String[] list;
public SortedDirList(final String afn) {
path = new File(".");
if(afn == null)
list = path.list();
else
list = path.list(
new FilenameFilter() {
public boolean
accept(File dir, String n) {
String f = new File(n).getName();
return f.indexOf(afn) != -1;
}
});
sort();
}
void print() {
for(int i = 0; i < list.length; i++)
System.out.println(list[i]);
}
private void sort() {
StrSortVector sv = new StrSortVector();
for(int i = 0; i < list.length; i++)
sv.addElement(list[i]);
// The first time an element is pulled from
// the StrSortVector the list is sorted:
for(int i = 0; i < list.length; i++)
list[i] = sv.elementAt(i);
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![}
// Test it:
public static void main(String[] args) {
SortedDirList sd;
if(args.length == 0)
sd = new SortedDirList(null);
else
sd = new SortedDirList(args[0]);
sd.print();
}
} ///:~
A few other improvements have been made: instead of creating path and list as local
variables to main( ), they are members of the class so their values can be accessible for the
lifetime of the object. In fact, main( ) is now just a way to test the class. You can see that
the constructor of the class automatically sorts the list once that list has been created.
The sort is case-insensitive so you don’t end up with a list of all the words starting with
capital letters, followed by the rest of the words starting with all the lowercase letters.
However, you’ll notice that within a group of file names that begin with the same letter
the capitalized words are listed first, which is still not quite the desired behavior for the
sort. This problem will be fixed in Java 1.2.
Checking for and creating directories
The File class is more than just a representation for an existing directory path, file or
group of files. You can also use a File object to create a new directory or an entire
directory path if it doesn’t exist. You can also look at the characteristics of files (size, last
modification date, read/write), whether a File object represents a file or a directory, and
you can delete a file. This program shows the remaining methods available with the File
class:
//: MakeDirectories.java
// Demonstrates the use of the File class to
// create directories and manipulate files.
import java.io.*;
public class MakeDirectories {
private final static String usage =
"Usage:MakeDirectories path1 ...n" +
"Creates each pathn" +
"Usage:MakeDirectories -d path1 ...n" +
"Deletes each pathn" +
"Usage:MakeDirectories -r path1 path2n" +
"Renames from path1 to path2n";
private static void usage() {
System.err.println(usage);
System.exit(1);
}
private static void fileData(File f) {
System.out.println(
"Absolute path: " + f.getAbsolutePath() +
"n Can read: " + f.canRead() +
"n Can write: " + f.canWrite() +
"n getName: " + f.getName() +
"n getParent: " + f.getParent() +
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!["n getPath: " + f.getPath() +
"n length: " + f.length() +
"n lastModified: " + f.lastModified());
if(f.isFile())
System.out.println("it's a file");
else if(f.isDirectory())
System.out.println("it's a directory");
}
public static void main(String args[]) {
if(args.length < 1) usage();
if(args[0].equals("-r")) {
if(args.length != 3) usage();
File
old = new File(args[1]),
rname = new File(args[2]);
old.renameTo(rname);
fileData(old);
fileData(rname);
return; // Exit main
}
int count = 0;
boolean del = false;
if(args[0].equals("-d")) {
count++;
del = true;
}
for( ; count < args.length; count++) {
File f = new File(args[count]);
if(f.exists()) {
System.out.println(f + " exists");
if(del) {
System.out.println("deleting..." + f);
f.delete();
}
}
else { // Doesn't exist
if(!del) {
f.mkdirs();
System.out.println("created " + f);
}
}
fileData(f);
}
}
} ///:~
In fileData( ) you can see the various file investigation methods put to use to display
information about the file or directory path.
The first method that’s exercised by main( ) is renameTo( ), which allows you to rename
(or move) a file to an entirely new path represented by the argument which is another File
object. This also works with directories of any length.
If you experiment with the above program, you’ll find you can make a directory path of
any complexity because mkdirs( ) will do all the work for you. In Java 1.0, the -d flag
reports that the directory is deleted but it’s still there; in Java 1.1 the directory is actually
deleted.
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![Typical uses of IO streams
Although there are lots of IO stream classes in the library that can be combined in many
different ways, there are just a few ways that you’ll probably end up using them.
However, they require attention to get the right combinations. The following rather long
example shows the creation and use of typical IO configurations, so you can use it as a
reference when writing your own code. Notice that each configuration begins with a
commented number and title that corresponds to the heading for the appropriate
explanation that follows in the text.
//: IOStreamDemo.java
// Typical IO Stream Configurations
import java.io.*;
import com.eckelobjects.tools.*;
public class IOStreamDemo {
public static void main(String args[]) {
try {
// 1. Buffered input file
DataInputStream in =
new DataInputStream(
new BufferedInputStream(
new FileInputStream(args[0])));
String s, s2 = new String();
while((s = in.readLine())!= null)
s2 += s + "n";
in.close();
// 2. Input from memory
StringBufferInputStream in2 =
new StringBufferInputStream(s2);
int c;
while((c = in2.read()) != -1)
System.out.print((char)c);
// 3. Formatted memory input
try {
DataInputStream in3 =
new DataInputStream(
new StringBufferInputStream(s2));
while(true)
System.out.print((char)in3.readByte());
} catch(EOFException e) {
System.out.println(
"End of stream encountered");
}
// 4. Line numbering & file output
try {
LineNumberInputStream li =
new LineNumberInputStream(
new StringBufferInputStream(s2));
DataInputStream in4 =
new DataInputStream(li);
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![InFile in6 = new InFile(args[0]);
String s3 = new String();
System.out.println(
"First line in file: " +
in6.readLine());
in.close();
// 8. Formatted file output shorthand
PrintFile out3 = new PrintFile("Data2.txt");
out3.print("Test of PrintFile");
out3.close();
// 9. Data file output shorthand
OutFile out4 = new OutFile("Data3.txt");
out4.writeBytes("Test of outDataFilenr");
out4.writeChars("Test of outDataFilenr");
out4.close();
} catch(FileNotFoundException e) {
System.out.println(
"File Not Found:" + args[0]);
} catch(IOException e) {
System.out.println("IO Exception");
}
}
} ///:~
Input streams
Of course, one very common thing you’ll want to do is print formatted output to the
console, but that’s already been simplified in the package com.eckelobjects.tools created
in Chapter 5.
Parts 1-4 demonstrate the creation and use of input streams (although part 4 also shows
the simple use of an output stream as a testing tool).
1. Buffered input file
To open a file for input, you use a FileInputStream with a String or a File object as the
file name. For speed, you’ll want that file to be buffered so you give the resulting handle to
the constructor for a BufferedInputStream. To read input in a formatted fashion, you
give that resulting handle to the constructor for a DataInputStream, which is your final
object and the interface you read from.
In this example only the readLine( ) method is used, but of course any of the
DataInputStream methods are available. When you reach the end of the file, readLine( )
returns null, so that is used to break out of the while loop.
The String s2 is used to accumulate the entire contents of the file (including newlines
which must be added since readLine( ) strips them off). s2 is then used in the later
portions of this program. Finally, close( ) is called to close the file. Technically, close( )
will be called when finalize( ) is run, and this is supposed to happen (whether or not
garbage collection occurs) as the program exits. However, Java 1.0 has a rather important
bug so this doesn’t happen. In Java 1.1 you must explicitly call
System.runFinalizersOnExit(true) to guarantee that finalize( ) will be called for every
object in the system. The safest approach is to explicitly call close( ) for files.
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![2. Input from memory
This piece takes the String s2 that now contains the entire contents of the file and uses it
to create a StringBufferInputStream (a String, not a StringBuffer, is required as the
constructor argument). Then read( ) is used to read each character one at a time and send
it out to the console. Note that read( ) returns the next byte as an int and thus it must be
cast to a char to print properly.
3. Formatted memory input
The interface for StringBufferInputStream is very limited, so you usually enhance it by
wrapping it inside a DataInputStream. However, if you choose to read the characters out
a byte at a time using readByte( ), any value is valid so the return value cannot be used to
detect the end of input. Instead, you can use the available( ) method to find out how
many more characters are available. Here’s an example that shows how to read a file a
byte at a time:
//: TestEOF.java
// Testing for the end of file while reading
// a byte at a time.
import java.io.*;
public class TestEOF {
public static void main(String args[]) {
try {
DataInputStream in =
new DataInputStream(
new BufferedInputStream(
new FileInputStream("TestEof.java")));
while(in.available() != 0)
System.out.print((char)in.readByte());
} catch (IOException e) {
System.err.println("IOException");
}
}
} ///:~
Note that available( ) works differently depending on what sort of medium you’re reading
from – it’s literally “the number of bytes that can be read without blocking.” With a file
this means the whole file, but with a different kind of stream this may not be true, so use
it thoughtfully.
You could also detect the end of input in cases like these by catching an exception.
However, the use of exceptions for control flow is considered a misuse of that feature.
4. Line numbering & file output
This example shows the use of the LineNumberInputStream to keep track of the input
line numbers. Here, you cannot simply gang all the constructors together, since you have
to keep a handle to the LineNumberInputStream (note this is not an inheritance
situation, so you cannot simply cast in4 to a LineNumberInputStream). Thus, li holds
the handle to the LineNumberInputStream which is then used to create a
DataInputStream for easy reading.
LineNumberInputStream may be more useful as an example of how to add functionality
to a stream by inheriting a new FileInputStream (remember, you have access to the
source code for the Java libraries). It’s not particularly hard to keep track of line numbers
yourself while reading input lines.
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![// How to read from standard input
import java.io.*;
public class Echo {
public static void main(String args[]) {
DataInputStream in =
new DataInputStream(
new BufferedInputStream(System.in));
String s;
try {
while((s = in.readLine()).length() != 0)
System.out.println(s);
// An empty line terminates the program
} catch(IOException e) {
e.printStackTrace();
}
}
} ///:~
The reason for the try block is that readLine( ) can throw an IOException. Note that
System.in should also be buffered, as with most streams
It’s a bit inconvenient that you’re forced to wrap System.in in a DataInputStream in
each program, but perhaps it was designed this way to allow maximum flexibility.
Piped streams
The PipedInputStream and PipedOutputStream have been mentioned only briefly in this
chapter. This is not to suggest that they aren’t useful, but their value is not apparent until
you begin to understand about multithreading, since the piped streams are used to
communicate between threads. This is covered along with an example in Chapter 14.
StreamTokenizer
Although StreamTokenizer is not derived from InputStream or OutputStream, it works
only with InputStream objects so it rightfully belongs in the IO portion of the library.
The StreamTokenizer class is used to break any InputStream into a sequence of “tokens,”
which are bits of text delimited by whatever you choose. For example, your tokens could
be words, and then they would be delimited by white space and punctuation.
Consider a program to count the occurrence of words in a text file:
//: SortedWordCount.java
// Counts words in a file, outputs
// results in sorted form.
import java.io.*;
import java.util.*;
import c08.*; // Contains StrSortVector
class Counter {
private int i = 1;
int read() { return i; }
void increment() { i++; }
}
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![return counts.elements();
}
Enumeration keys() { return counts.keys(); }
Counter getCounter(String s) {
return (Counter)counts.get(s);
}
Enumeration sortedKeys() {
Enumeration e = counts.keys();
StrSortVector sv = new StrSortVector();
while(e.hasMoreElements())
sv.addElement((String)e.nextElement());
// This call forces a sort:
return sv.elements();
}
public static void main(String args[]) {
try {
SortedWordCount wc =
new SortedWordCount(args[0]);
wc.countWords();
Enumeration keys = wc.sortedKeys();
while(keys.hasMoreElements()) {
String key = (String)keys.nextElement();
System.out.println(key + ": "
+ wc.getCounter(key).read());
}
wc.cleanup();
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
It makes sense to present these in a sorted form, but since Java 1.0 and Java 1.1 don’t
have any sorting methods, that will have to be mixed in. This is easy enough to do with a
StrSortVector (which was created in Chapter 8, and is part of the package created in that
chapter – remember that the starting directory for all the subdirectories in this book must
be in your class path for the program to compile successfully).
To open the file, a FileInputStream is used, and to turn the file into words a
StreamTokenizer is created from the FileInputStream. In StreamTokenizer, there is a
default list of separators, and you can add more with a set of methods. Here,
ordinaryChar( ) is used to say: “this character has no significance that I’m interested in,”
so the parser doesn’t include it as part of any of the words it creates. You can find more
information in the on-line documentation that comes with Java.
In countWords( ), the tokens are pulled one at a time from the stream, and the ttype
information is used to determine what to do with each token, since a token can be an end-
of-line, a number, a string, or a single character.
Once a token is found, the Hashtable counts is queried to see if it already contains the
token as a key. If it does, the corresponding Counter object is incremented to indicate that
another instance of this word has been found. If not, a new Counter is created – since the
Counter constructor initializes its value to one, this also acts to count the word.
SortedWordCount is not a type of Hashtable, so it wasn’t inherited. It performs a
specific type of functionality, so even though the keys( ) and values( ) methods must be
re-exposed, that still doesn’t mean that inheritance should be used since there are a
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![number of Hashtable methods that are inappropriate here. In addition, other methods
getCounter( ), which get the Counter for a particular String, and sortedKeys( ), which
produces an Enumeration, finish the change in the shape of SortedWordCount’s
interface.
In main( ) you can see the use of a SortedWordCount to open and count the words in a
file – it just takes two lines of code. Then an enumeration to a sorted list of keys (words) is
extracted, and this is used to pull out each key and associated Count. Note that in this
case, cleanup( ) is not necessary, since upon exiting the program the file will be closed.
However, you can’t normally assume that the SortedWordCount object is being called
from main( ).
A second example using StreamTokenizer can be found in Chapter 17.
StringTokenizer
Although it isn’t part of the IO library, the StringTokenizer has sufficiently similar
functionality to StreamTokenizer that it will be described here.
The StringTokenizer returns, one at a time, the tokens within a string. These tokens are
consecutive characters delimited by tabs, spaces, and newlines. Thus the tokens of the
string “Where is my cat?” are “Where”, “is”, “my”, and “cat?” Unlike the
StreamTokenizer, however, you cannot tell the StringTokenizer to break up the input in
any way that you want. It’s very limited, and you can use only the rules that it has hard-
wired into it. Thus, if it’s a very simple tokenization you need, StringTokenizer is fine,
but if you need more sophistication you’ll have to use a StreamTokenizer.
You ask a StringTokenizer object for the next token in the string using the next( )
method, which either returns the token or an empty string to indicate that no tokens
remain.
As an example, the following program performs a very limited analysis of a sentence,
looking for key phrase sequences to indicate whether happiness or sadness is implied.
//: AnalyzeSentence.java
// Look for particular sequences
// within sentences.
import java.util.*;
public class AnalyzeSentence {
public static void main(String args[]) {
analyze("I am happy about this");
analyze("I am not happy about this");
analyze("I am not! I am happy");
analyze("I am sad about this");
analyze("I am not sad about this");
analyze("I am not! I am sad");
analyze("Are you happy about this?");
analyze("Are you sad about this?");
analyze("It's you! I am happy");
analyze("It's you! I am sad");
}
static StringTokenizer st;
static void analyze(String s) {
prt("nnew sentence >> " + s);
boolean sad = false;
324 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-324-320.jpg)
![The DataOutputStream, in particular, is used without change, so for storing and
retrieving data in a transportable format you’re forced to stay in the InputStream and
OutputStream hierarchies.
An example
To see the effect of the new classes, let’s look at the appropriate portion of the
IOStreamDemo.java example modified to use the Reader and Writer classes:
//: NewIODemo.java
// Java 1.1 IO typical usage
import java.io.*;
public class NewIODemo {
public static void main(String args[]) {
try {
// 1. Reading input by lines:
BufferedReader in =
new BufferedReader(
new FileReader(args[0]));
String s, s2 = new String();
while((s = in.readLine())!= null)
s2 += s + "n";
in.close();
// 1b. Reading standard input:
BufferedReader stdin =
new BufferedReader(
new InputStreamReader(System.in));
System.out.print("Enter a line:");
System.out.println(stdin.readLine());
// 2. Input from memory
StringReader in2 = new StringReader(s2);
int c;
while((c = in2.read()) != -1)
System.out.print((char)c);
// 3. Formatted memory input
try {
DataInputStream in3 =
new DataInputStream(
// Oops: must use deprecated class:
new StringBufferInputStream(s2));
while(true)
System.out.print((char)in3.readByte());
} catch(EOFException e) {
System.out.println("End of stream");
}
// 4. Line numbering & file output
try {
LineNumberReader li =
new LineNumberReader(
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![new StringReader(s2));
BufferedReader in4 =
new BufferedReader(li);
PrintWriter out1 =
new PrintWriter(
new BufferedWriter(
new FileWriter("IODemo.out")));
while((s = in4.readLine()) != null )
out1.println(
"Line " + li.getLineNumber() + s);
out1.close();
} catch(EOFException e) {
System.out.println("End of stream");
}
// 5. Storing & recovering data
try {
DataOutputStream out2 =
new DataOutputStream(
new BufferedOutputStream(
new FileOutputStream("Data.txt")));
out2.writeDouble(3.14159);
out2.writeBytes("That was pi");
out2.close();
DataInputStream in5 =
new DataInputStream(
new BufferedInputStream(
new FileInputStream("Data.txt")));
BufferedReader in5br =
new BufferedReader(
new InputStreamReader(in5));
// Must use DataInputStream for data:
System.out.println(in5.readDouble());
// Can now use the "proper" readLine():
System.out.println(in5br.readLine());
} catch(EOFException e) {
System.out.println("End of stream");
}
// 6. Reading and writing random access
// files is the same as before.
// (not repeated here)
} catch(FileNotFoundException e) {
System.out.println(
"File Not Found:" + args[1]);
} catch(IOException e) {
System.out.println("IO Exception");
}
}
} ///:~
In general, you’ll see that the conversion is fairly straightforward and the code looks quite
similar. There are some important differences, though. First of all, since random access
files have not changed, Section 6 is not repeated.
330 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-330-320.jpg)
![Section 1 actually shrinks a bit since, if all you’re doing is reading line input you need
only to wrap a BufferedReader around a FileReader. Section 1b shows the new way to
wrap System.in for reading console input, and this expands because System.in is a
DataInputStream and BufferedReader needs a Reader argument, so
InputStreamReader is brought in to perform the translation.
In section 2 you can see that if you have a String and want to read from it you just use a
StringReader instead of a StringBufferInputStream and the rest of the code is identical.
Section 3 shows a bug in the design of the new IO stream library. If you have a String and
you want to read from it, you’re not supposed to use a StringBufferInputStream any
more. When you compile code involving a StringBufferInputStream constructor, you get
a deprecation message telling you not to use it. Instead you’re supposed to use a
StringReader. However, if you want to do formatted memory input as in Section 3, you’re
forced to use a DataInputStream – there is no “DataReader” to replace it – and a
DataInputStream constructor requires an InputStream argument. Thus you have no
choice but to use the deprecated StringBufferInputStream class. The compiler will give
you a deprecation message but there’s nothing you can do about it.1
Section 4 is a reasonably straightforward translation from the old streams to the new,
with no surprises. In section 5, you’re forced to use all the old streams classes because
DataOutputStream and DataInputStream require them and there are no alternatives.
However, you don’t get any deprecation messages at compile time. If a stream is
deprecated, typically its constructor produces a deprecation message to prevent you from
using the entire class, but in the case of DataInputStream only the readLine( ) method is
deprecated since you’re supposed to use a BufferedReader for readLine( ) (but a
DataInputStream for all other formatted input).
If you compare section 5 with that section in IOStreamDemo.java, you’ll notice that in
this version, the data is written before the text. That’s because a bug was introduced in
Java 1.1, which is shown in the following code:
//: IOBug.java
// Java 1.1 (and higher?) IO Bug
import java.io.*;
public class IOBug {
public static void main(String args[]) throws Exception {
DataOutputStream out =
new DataOutputStream(
new BufferedOutputStream(
new FileOutputStream("Data.txt")));
out.writeDouble(3.14159);
out.writeBytes("That was the value of pin");
out.writeBytes("This is the value of pi/2:n");
out.writeDouble(3.14159/2);
out.close();
DataInputStream in =
new DataInputStream(
new BufferedInputStream(
new FileInputStream("Data.txt")));
BufferedReader inbr =
new BufferedReader(
1 Perhaps by the time you read this, the bug will be fixed.
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![new InputStreamReader(in));
// The doubles written BEFORE the line of text
// read back correctly:
System.out.println(in.readDouble());
// Read the lines of text:
System.out.println(inbr.readLine());
System.out.println(inbr.readLine());
// Trying to read the doubles after the line
// produces an end-of-file exception:
System.out.println(in.readDouble());
}
} ///:~
It appears that anything you write after a call to writeBytes( ) is not recoverable. This is a
rather limiting bug, and we can hope that it will be fixed by the time you read this. You
should run the above program to test it; if you don’t get an exception and the values print
correctly then you’re out of the woods.
Redirecting standard IO
Java 1.1 has added methods in class System that allow you to redirect the standard input,
output and error IO streams using simple static method calls:
setIn(InputStream)
setOut(PrintStream)
setErr(PrintStream)
Redirecting output is especially useful if you suddenly start creating a large amount of
output on your screen and it’s scrolling past faster than you can read it. Redirecting input
is valuable for a command-line program when you want to test a particular user-input
sequence again and again. Here’s a simple example that shows the use of these methods:
//: Redirecting.java
// Demonstrates the use of redirection for
// standard IO in Java 1.1
import java.io.*;
class Redirecting {
public static void main(String args[]) {
try {
BufferedInputStream in =
new BufferedInputStream(
new FileInputStream(
"Redirecting.java"));
// Produces deprecation message:
PrintStream out =
new PrintStream(
new BufferedOutputStream(
new FileOutputStream("test.out")));
System.setIn(in);
System.setOut(out);
System.setErr(out);
BufferedReader br =
new BufferedReader(
new InputStreamReader(System.in));
332 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-332-320.jpg)
![Java 1.1 Compression Function
class
ZipInputStream A DeflaterInputStream that Decompresses
data that has been stored in the Zip file
format
GZIPInputStream A DeflaterInputStream that Decompresses
data that has been stored in the GZIP file
format
Although there are many compression algorithms, these two (Zip and GZIP) are possibly
the most commonly used, and as a result there are many tools for reading and writing
these formats.
Simple compression with GZIP
The GZIP interface is very simple and thus is probably more appropriate when you have a
single stream of data you want to compress (rather than a collection of dissimilar pieces of
data). Here’s an example that compresses a single file:
//: GZIPcompress.java
// Uses Java 1.1 GZIP compression to compress
// a file whose name is passed on the command
// line.
import java.io.*;
import java.util.zip.*;
public class GZIPcompress {
public static void main(String args[]) {
try {
BufferedReader in =
new BufferedReader(
new FileReader(args[0]));
BufferedOutputStream out =
new BufferedOutputStream(
new GZIPOutputStream(
new FileOutputStream("test.gz")));
System.out.println("Writing file");
int c;
while((c = in.read()) != -1)
out.write(c);
in.close();
out.close();
System.out.println("Reading file");
BufferedReader in2 =
new BufferedReader(
new InputStreamReader(
new GZIPInputStream(
new FileInputStream("test.gz"))));
String s;
while((s = in2.readLine()) != null)
System.out.println(s);
} catch(Exception e) {
334 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-334-320.jpg)
![e.printStackTrace();
}
}
} ///:~
The use of the compression classes is straightforward – you simply wrap your output
stream in a GZIPOutputStream or ZipOutputStream and your input stream in a
GZIPInputStream or ZipInputStream. All else is ordinary IO reading and writing. This
is, however, a good example of where you’re forced to mix the old IO streams with the
new: in uses the Reader classes, whereas GZIPOutputStream’s constructor can accept
only an OutputStream object, not a Writer object.
Multi-file storage with zip
The Java 1.1 library supporting the Zip format is much more extensive. With it you can
easily store multiple files, and there’s even a separate class to make the process of reading
a Zip file very easy. The library uses the standard Zip format so it works seamlessly with
all the tools currently downloadable on the Internet. The following example has the same
form as the previous example, but it handles as many command-line arguments as you
wish. In addition, it shows the use of the Checksum classes to calculate and verify the
checksum for the file. There are two Checksum types: Adler32 (which is faster) and
CRC32 (which is slower but slightly more accurate).
//: ZipCompress.java
// Uses Java 1.1 Zip compression to compress
// any number of files whose names are passed
// on the command line.
import java.io.*;
import java.util.*;
import java.util.zip.*;
public class ZipCompress {
public static void main(String args[]) {
try {
FileOutputStream f =
new FileOutputStream("test.zip");
CheckedOutputStream csum =
new CheckedOutputStream(
f, new Adler32());
ZipOutputStream out =
new ZipOutputStream(
new BufferedOutputStream(csum));
out.setComment("A test of Java Zipping");
// Can't read the above comment, though
for(int i = 0; i < args.length; i++) {
System.out.println(
"Writing file " + args[i]);
BufferedReader in =
new BufferedReader(
new FileReader(args[i]));
out.putNextEntry(new ZipEntry(args[i]));
int c;
while((c = in.read()) != -1)
out.write(c);
in.close();
}
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![In order to read the checksum, you must somehow have access to the associated
Checksum object. Here, a handle to the CheckedOutputStream and CheckedInputStream
objects is retained, but you could also just hold on to a handle to the Checksum object
itself.
A baffling method in Zip streams is setComment( ). As shown above, you can set a
comment when you’re writing a file, but there’s no way to recover the comment in the
ZipInputStream. Comments appear to be fully supported on an entry-by-entry basis only
via ZipEntry.
Of course you are not limited to files when using the GZIP or Zip libraries – you can
compress anything, including data to be sent through a network connection.
The Java archive (jar) utility
The Zip format is also used in the Java 1.1 JAR (Java ARchive) file format, which is a way
to collect a group of files into a single compressed file, just like Zip. However, like
everything else in Java, JAR files are cross-platform so you don't need to worry about
platform issues. You can also include audio and image files as well as class files.
JAR files are particularly helpful when dealing with the internet. Before JAR files, your
Web browser would have to make repeated requests of a Web server in order to download
all the files that make up an applet. In addition, each of these files was uncompressed. By
combining all the files for a particular applet into a single JAR file, only one server request
is necessary and the transfer is faster because of compression. In addition, each entry in a
JAR file may be digitally signed for security.
There’s a utility called jar that comes with Sun’s JDK. This automatically compresses the
files of your choice.
A JAR file consists of a single file containing a collection of zipped files along with a
“manifest” that describes them (you can create your own manifest file; otherwise the jar
program will do it for you). You can find out more about JAR manifests in the online
documentation.
You use the jar utility by invoking in on the command line:
jar [options] [manifest] destination input-file [input-files]
The options are simply a collection of letters (no hyphen or any other indicator is
necessary). These are:
c Creates a new or empty archive.
t Lists the table of contents.
x Extracts all files
x file Extracts the named file
f Says: “I’m going to give you the name of the file.” If you don’t use this, jar
assumes that its input will come from standard input, or, if it is creating a
file, its output will go to standard output.
m Says that the first argument will be the name of the user-created manifest
file
v Generates verbose output describing what jar is doing
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![}
public class Worm implements Serializable {
// Generate a random int value:
private static int r() {
return (int)(Math.random() * 10);
}
private Data[] d = {
new Data(r()), new Data(r()), new Data(r())
};
private Worm next;
private char c;
// Value of i == number of segments
Worm(int i, char x) {
System.out.println(" Worm constructor: " + i);
c = x;
if(--i > 0)
next = new Worm(i, (char)(x + 1));
}
Worm() {
System.out.println("Default constructor");
}
public String toString() {
String s = ":" + c + "(";
for(int i = 0; i < d.length; i++)
s += d[i].toString();
s += ")";
if(next != null)
s += next.toString();
return s;
}
public static void main(String args[]) {
Worm w = new Worm(6, 'a');
System.out.println("w = " + w);
try {
ObjectOutputStream out =
new ObjectOutputStream(
new FileOutputStream("worm.out"));
out.writeObject("Worm storage");
out.writeObject(w);
out.close(); // Also flushes output
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream("worm.out"));
String s = (String)in.readObject();
Worm w2 = (Worm)in.readObject();
System.out.println(s + ", w2 = " + w2);
} catch(Exception e) {
e.printStackTrace();
}
try {
ByteArrayOutputStream bout =
new ByteArrayOutputStream();
ObjectOutputStream out =
new ObjectOutputStream(bout);
340 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-340-320.jpg)
![Finding the class
You may wonder “what’s necessary for an object to be recovered from its serialized state?”
For example, suppose you serialize an object and send it as a file or through a network to
another machine. Could a program on the other machine reconstruct the object using only
the contents of the file?
The best way to answer this question is (as usual) by performing an experiment. The
following file goes in the subdirectory for this chapter:
//: Alien.java
// A serializable class
import java.io.*;
public class Alien implements Serializable {
} ///:~
The file that creates and serializes an Alien object goes in the same directory:
//: FreezeAlien.java
// Create a serialized output file
import java.io.*;
public class FreezeAlien {
public static void main(String args[])
throws Exception {
ObjectOutput out =
new ObjectOutputStream(
new FileOutputStream("file.x"));
Alien zorcon = new Alien();
out.writeObject(zorcon);
}
} ///:~
Rather than catching and handling exceptions, this program takes the quick and dirty
approach of just passing the exceptions out of main( ), so they’ll be reported on the
command line.
Once the program is compiled and run, copy the resulting file.x to a subdirectory called
xfiles, where the following code goes:
//: ThawAlien.java
// Try to recover a serialized file without the
// class of object that's stored in that file.
package c10.xfiles;
import java.io.*;
public class ThawAlien {
public static void main(String args[])
throws Exception {
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream("file.x"));
Object mystery = in.readObject();
System.out.println(
mystery.getClass().toString());
}
342 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-342-320.jpg)
![public void readExternal(ObjectInput in)
throws IOException, ClassNotFoundException {
System.out.println("Blip2.readExternal");
}
}
public class Blips {
public static void main(String args[]) {
System.out.println("Constructing objects:");
Blip1 b1 = new Blip1();
Blip2 b2 = new Blip2();
try {
ObjectOutputStream o =
new ObjectOutputStream(
new FileOutputStream("Blips.out"));
System.out.println("Saving objects:");
o.writeObject(b1);
o.writeObject(b2);
o.close();
// Now get them back:
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream("Blips.out"));
System.out.println("Recovering b1:");
b1 = (Blip1)in.readObject();
// OOPS! Throws an exception:
//! System.out.println("Recovering b2:");
//! b2 = (Blip2)in.readObject();
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
The output for this program is:
Constructing objects:
Blip1 Constructor
Blip2 Constructor
Saving objects:
Blip1.writeExternal
Blip2.writeExternal
Recovering b1:
Blip1 Constructor
Blip1.readExternal
The reason that the Blip2 object is not recovered is that trying to do so causes an
exception. Can you see the difference between Blip1 and Blip2? The constructor for Blip1
is public, while the constructor for Blip2 is not, and that causes the exception upon
recovery. Try making Blip2’s constructor public and removing the //! comments to see the
correct results.
When b1 is recovered, the Blip1 default constructor is called. This is different from
recovering a Serializable object, where the object is entirely constructed from its stored
bits, with no constructor calls. With an Externalizable object, all the normal default
construction behavior occurs (including the initializations at the point of field definition),
344 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-344-320.jpg)
![and then readExternal( ) is called. You need to be aware of this – in particular the fact
that all the default construction always takes place – to produce the correct behavior in
your Externalizable objects.
Here’s an example that shows what you must do to fully store and retrieve an
Externalizable object:
//: Blip3.java
// Reconstructing an externalizable object
import java.io.*;
import java.util.*;
class Blip3 implements Externalizable {
int i;
String s; // No initialization
public Blip3() {
System.out.println("Blip3 Constructor");
// s, i not initialized
}
public Blip3(String x, int a) {
System.out.println("Blip3(String x, int a)");
s = x;
i = a;
// s & i initialized only in non-default
// constructor.
}
public String toString() { return s + i; }
public void writeExternal(ObjectOutput out)
throws IOException {
System.out.println("Blip3.writeExternal");
// You must do this:
out.writeObject(s); out.writeInt(i);
}
public void readExternal(ObjectInput in)
throws IOException, ClassNotFoundException {
System.out.println("Blip3.readExternal");
// You must do this:
s = (String)in.readObject();
i =in.readInt();
}
public static void main(String args[]) {
System.out.println("Constructing objects:");
Blip3 b3 = new Blip3("A String ", 47);
System.out.println(b3.toString());
try {
ObjectOutputStream o =
new ObjectOutputStream(
new FileOutputStream("Blip3.out"));
System.out.println("Saving object:");
o.writeObject(b3);
o.close();
// Now get it back:
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream("Blip3.out"));
System.out.println("Recovering b3:");
b3 = (Blip3)in.readObject();
Chapter 10: The Java IO System 345](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-345-320.jpg)
![password = pwd;
}
public String toString() {
String pwd =
(password == null) ? "(n/a)" : password;
return "logon info: n " +
"username: " + username +
"n date: " + date.toString() +
"n password: " + pwd;
}
public static void main(String args[]) {
Logon a = new Logon("Hulk", "myLittlePony");
System.out.println( "logon a = " + a);
try {
ObjectOutputStream o =
new ObjectOutputStream(
new FileOutputStream("Logon.out"));
o.writeObject(a);
o.close();
// Delay:
int seconds = 5;
long t = System.currentTimeMillis()
+ seconds * 1000;
while(System.currentTimeMillis() < t)
;
// Now get them back:
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream("Logon.out"));
System.out.println(
"Recovering object at " + new Date());
a = (Logon)in.readObject();
System.out.println( "logon a = " + a);
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
You can see that the date and username fields are ordinary (not transient), and thus are
automatically serialized. However, the password is transient, and so is not stored to disk;
also the serialization mechanism makes no attempt to recover it. The output is:
logon a = logon info:
username: Hulk
date: Sun Mar 23 18:25:53 PST 1997
password: myLittlePony
Recovering object at Sun Mar 23 18:25:59 PST 1997
logon a = logon info:
username: Hulk
date: Sun Mar 23 18:25:53 PST 1997
password: (n/a)
When the object is recovered, the password field is null. Notice that toString( ) must
check for a null value of password because if you simply try to assemble a String object
using the overloaded ‘+’ operator, and that operator encounters a null handle, you’ll get a
NullPointerException (newer versions of Java may contain code to avoid this problem).
Chapter 10: The Java IO System 347](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-347-320.jpg)
![String a;
transient String b;
public SerialCtl(String aa, String bb) {
a = "Not Transient: " + aa;
b = "Transient: " + bb;
}
public String toString() {
return a + "n" + b;
}
private void
writeObject(ObjectOutputStream stream)
throws IOException {
stream.defaultWriteObject();
stream.writeObject(b);
}
private void
readObject(ObjectInputStream stream)
throws IOException, ClassNotFoundException {
stream.defaultReadObject();
b = (String)stream.readObject();
}
public static void main(String args[]) {
SerialCtl sc =
new SerialCtl("Test1", "Test2");
System.out.println("Before:n" + sc);
ByteArrayOutputStream buf =
new ByteArrayOutputStream();
try {
ObjectOutputStream o =
new ObjectOutputStream(buf);
o.writeObject(sc);
// Now get it back:
ObjectInputStream in =
new ObjectInputStream(
new ByteArrayInputStream(
buf.toByteArray()));
SerialCtl sc2 = (SerialCtl)in.readObject();
System.out.println("After:n" + sc2);
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
In this example, one String field is ordinary and the other is transient, to prove that the
non-transient field is saved by the defaultWriteObject( ) method and the transient field
is saved and restored explicitly. The fields are initialized inside the constructor rather than
the point of definition to prove that they are not being initialized by some automatic
mechanism during deserialization.
If you are going to use the default mechanism to write the non-transient parts of your
object, you must call defaultWriteObject( ) as the first operation in writeObject( ), and
defaultReadObject( ) as the first operation in readObject( ). These are strange method
calls. It would appear, for example, that you are calling defaultWriteObject( ) for an
ObjectOutputStream and passing it no arguments, and yet it somehow turns around and
knows the handle to your object, and how to write all the non-transient parts. Spooky.
Chapter 10: The Java IO System 349](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-349-320.jpg)
![The storage and retrieval of the transient objects uses more familiar code. And yet, think
about what happens here. In main( ), a SerialCtl object is created, and then it’s serialized
to an ObjectOutputStream (notice in this case a buffer is used instead of a file – it’s all
the same to the ObjectOutputStream). The serialization occurs in the line:
o.writeObject(sc);
The writeObject( ) method must be examining sc to see if it has its own writeObject( )
method (not by checking the interface – there isn’t one – or the class type, but by actually
hunting for the method using reflection). If so, it uses that. A similar approach holds true
for readObject( ). Perhaps this was the only practical way they could solve the problem,
but it’s certainly strange.
Versioning
It’s possible you may want to change the version of a serializable class (objects of the
original class may be stored in a database, for example). This is supported but you’ll
probably do it only in special cases, and it requires an extra depth of understanding that
we shall not attempt to achieve here. The JDK1.1 HTML documents downloadable from
Sun (which may be part of your Java package’s online documents) cover this topic quite
thoroughly.
Using persistence
It’s quite appealing to utilize serialization technology to store some of the state of your
program so that you can later easily restore the program to the current state. But before
you can do this, some questions must be answered. What happens if you serialize two
objects that both have a handle to a third object? When you restore those two objects from
their serialized state, do you get only one of the third object? What if you serialize your
two objects to separate files and deserialize them in different parts of your code?
Here’s an example that shows the problem:
//: MyWorld.java
import java.io.*;
import java.util.*;
class House implements Serializable {}
class Animal implements Serializable {
String name;
House preferredHouse;
Animal(String nm, House h) {
name = nm;
preferredHouse = h;
}
public String toString() {
return name + "[" + super.toString() +
"], " + preferredHouse + "n";
}
}
public class MyWorld {
public static void main(String args[]) {
House house = new House();
Vector animals = new Vector();
animals.addElement(
350 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-350-320.jpg)
![new Animal("Bosco the dog", house));
animals.addElement(
new Animal("Ralph the hamster", house));
animals.addElement(
new Animal("Fronk the cat", house));
System.out.println("animals: " + animals);
try {
ByteArrayOutputStream buf1 =
new ByteArrayOutputStream();
ObjectOutputStream o1 =
new ObjectOutputStream(buf1);
o1.writeObject(animals);
o1.writeObject(animals); // Write a 2nd set
// Write to a different stream:
ByteArrayOutputStream buf2 =
new ByteArrayOutputStream();
ObjectOutputStream o2 =
new ObjectOutputStream(buf2);
o2.writeObject(animals);
// Now get them back:
ObjectInputStream in1 =
new ObjectInputStream(
new ByteArrayInputStream(
buf1.toByteArray()));
ObjectInputStream in2 =
new ObjectInputStream(
new ByteArrayInputStream(
buf2.toByteArray()));
Vector animals1 = (Vector)in1.readObject();
Vector animals2 = (Vector)in1.readObject();
Vector animals3 = (Vector)in2.readObject();
System.out.println("animals1: " + animals1);
System.out.println("animals2: " + animals2);
System.out.println("animals3: " + animals3);
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
One thing that’s rather interesting here is that it’s possible to use object serialization to
and from a byte array as a way of doing a “deep copy” of any object that’s Serializable (a
deep copy means you’re duplicating the entire web of objects, rather than just the basic
object and its handles). Copying is covered in depth in Chapter 12.
Animal objects contain fields of type House. In main( ), a Vector of these Animals is
created and it is serialized, twice to one stream and then again to a separate stream. When
these are deserialized and printed, you see the following results (for one run; the objects
will be in different memory locations each run):
animals: [Bosco the dog[Animal@1cc76c], House@1cc769
, Ralph the hamster[Animal@1cc76d], House@1cc769
, Fronk the cat[Animal@1cc76e], House@1cc769
]
animals1: [Bosco the dog[Animal@1cca0c], House@1cca16
, Ralph the hamster[Animal@1cca17], House@1cca16
Chapter 10: The Java IO System 351](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-351-320.jpg)
![, Fronk the cat[Animal@1cca1b], House@1cca16
]
animals2: [Bosco the dog[Animal@1cca0c], House@1cca16
, Ralph the hamster[Animal@1cca17], House@1cca16
, Fronk the cat[Animal@1cca1b], House@1cca16
]
animals3: [Bosco the dog[Animal@1cca52], House@1cca5c
, Ralph the hamster[Animal@1cca5d], House@1cca5c
, Fronk the cat[Animal@1cca61], House@1cca5c
]
Of course you expect that the deserialized objects have different addresses than their
originals. But notice that in animals1 and animals2 the same addresses appear, including
the references to the House object that both share. On the other hand, when animals3 is
recovered the system has no way of knowing that the objects in this other stream are
aliases of the objects in the first stream, so it makes a completely different web of objects.
This means that as long as you’re serializing everything to a single stream, you’ll be able
to recover the same web of objects that you wrote, with no accidental duplication of
objects. Of course you can change the state of your objects in between the time you write
the first of them and the last, but that’s your responsibility – the objects will be written in
whatever state they are in (and with whatever connections they have to other objects) at
the time you serialize them.
The safest thing to do if you want to save the state of a system is to serialize as an
“atomic” operation; that is, don’t serialize some things, do some other work, and serialize
some more, etc. The simplest way to accomplish this is to put all the objects that comprise
the state of your system in a single collection and simply write that collection out in one
operation. Then you can restore it with a single method call, as well.
The following example is a pretend computer-aided design (CAD) system that
demonstrates the approach. In addition, it throws in the issue of static fields – if you look
at the documentation you’ll see that Class is Serializable, so it should be easy to store the
static fields by simply serializing the Class object. That seems like a sensible approach,
anyway.
//: CADState.java
// Saving and restoring the state of a
// pretend CAD system.
import java.io.*;
import java.util.*;
abstract class Shape implements Serializable {
public static final int
RED = 1, BLUE = 2, GREEN = 3;
private int xPos, yPos, dimension;
private static Random r = new Random();
private static int counter = 0;
abstract public void setColor(int newColor);
abstract public int getColor();
public Shape(int xVal, int yVal, int dim) {
xPos = xVal;
yPos = yVal;
dimension = dim;
}
public String toString() {
return getClass().toString() +
352 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-352-320.jpg)
![" color[" + getColor() +
"] xPos[" + xPos +
"] yPos[" + yPos +
"] dim[" + dimension + "]n";
}
public static Shape randomFactory() {
int xVal = r.nextInt() % 100;
int yVal = r.nextInt() % 100;
int dim = r.nextInt() % 100;
switch(counter++ % 3) {
default:
case 0: return new Circle(xVal, yVal, dim);
case 1: return new Square(xVal, yVal, dim);
case 2: return new Line(xVal, yVal, dim);
}
}
}
class Circle extends Shape {
private static int color = RED;
public Circle(int xVal, int yVal, int dim) {
super(xVal, yVal, dim);
}
public void setColor(int newColor) {
color = newColor;
}
public int getColor() {
return color;
}
}
class Square extends Shape {
private static int color;
public Square(int xVal, int yVal, int dim) {
super(xVal, yVal, dim);
color = RED;
}
public void setColor(int newColor) {
color = newColor;
}
public int getColor() {
return color;
}
}
class Line extends Shape {
private static int color = RED;
public static void
serializeStaticState(ObjectOutputStream os)
throws IOException {
os.writeInt(color);
}
public static void
deserializeStaticState(ObjectInputStream os)
throws IOException {
color = os.readInt();
Chapter 10: The Java IO System 353](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-353-320.jpg)
![}
public Line(int xVal, int yVal, int dim) {
super(xVal, yVal, dim);
}
public void setColor(int newColor) {
color = newColor;
}
public int getColor() {
return color;
}
}
public class CADState {
public static void main(String args[])
throws Exception {
Vector shapeTypes, shapes;
if(args.length == 0) {
shapeTypes = new Vector();
shapes = new Vector();
// Add handles to the class objects:
shapeTypes.addElement(Circle.class);
shapeTypes.addElement(Square.class);
shapeTypes.addElement(Line.class);
// Make some shapes:
for(int i = 0; i < 10; i++)
shapes.addElement(Shape.randomFactory());
// Set all the static colors to GREEN:
for(int i = 0; i < 10; i++)
((Shape)shapes.elementAt(i))
.setColor(Shape.GREEN);
// Save the state vector:
ObjectOutputStream out =
new ObjectOutputStream(
new FileOutputStream("CADState.out"));
out.writeObject(shapeTypes);
Line.serializeStaticState(out);
out.writeObject(shapes);
} else { // There's a command-line argument
ObjectInputStream in =
new ObjectInputStream(
new FileInputStream(args[0]));
// Read in the same order they were written:
shapeTypes = (Vector)in.readObject();
Line.deserializeStaticState(in);
shapes = (Vector)in.readObject();
}
// Display the shapes:
System.out.println(shapes);
}
} ///:~
The Shape class implements Serializable, so anything that is inherited from Shape is
automatically Serializable as well. Each Shape contains data, and each derived Shape
class contains a static field that determines the color of all of those types of Shapes
(placing a static field in the base class would result in only one field, since static fields are
not duplicated in derived classes). Methods in the base class can be overridden to set the
354 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-354-320.jpg)
![color for the various types (static methods are not dynamically bound, so these are normal
methods). The randomFactory( ) method creates a different Shape each time you call it,
using random values for the Shape data.
Circle and Square are straightforward extensions of Shape; the only difference is that
Circle initializes color at the point of definition and Square initializes it in the
constructor. We’ll leave the discussion of Line for later.
In main( ), one Vector is used to hold the Class objects and the other to hold the shapes
themselves. If you don’t provide a command-line argument the shapeTypes Vector is
created and the Class objects are added, and then the shapes Vector is created and Shape
objects are added. Next, all the static color values are set to GREEN, and everything is
serialized to the file CADState.out.
If you provide a command-line argument (presumably CADState.out), that file is opened
and used to restore the state of the program. In both situations, the resulting Vector of
Shapes is printed out. The results from one run are:
>java CADState
[class Circle color[3] xPos[-51] yPos[-99] dim[38]
, class Square color[3] xPos[2] yPos[61] dim[-46]
, class Line color[3] xPos[51] yPos[73] dim[64]
, class Circle color[3] xPos[-70] yPos[1] dim[16]
, class Square color[3] xPos[3] yPos[94] dim[-36]
, class Line color[3] xPos[-84] yPos[-21] dim[-35]
, class Circle color[3] xPos[-75] yPos[-43] dim[22]
, class Square color[3] xPos[81] yPos[30] dim[-45]
, class Line color[3] xPos[-29] yPos[92] dim[17]
, class Circle color[3] xPos[17] yPos[90] dim[-76]
]
>java CADState CADState.out
[class Circle color[1] xPos[-51] yPos[-99] dim[38]
, class Square color[0] xPos[2] yPos[61] dim[-46]
, class Line color[3] xPos[51] yPos[73] dim[64]
, class Circle color[1] xPos[-70] yPos[1] dim[16]
, class Square color[0] xPos[3] yPos[94] dim[-36]
, class Line color[3] xPos[-84] yPos[-21] dim[-35]
, class Circle color[1] xPos[-75] yPos[-43] dim[22]
, class Square color[0] xPos[81] yPos[30] dim[-45]
, class Line color[3] xPos[-29] yPos[92] dim[17]
, class Circle color[1] xPos[17] yPos[90] dim[-76]
]
You can see that the values of xPos, yPos and dim were all stored and recovered
successfully, but there’s something wrong with the retrieval of the static information. It’s
all ‘3’ going in, but it doesn’t come out that way. Circles have a value of 1 (RED, which is
the definition), and Squares have a value of 0 (remember they are initialized in the
constructor). It’s as if the statics didn’t get serialized at all! That’s right – even though
class Class is Serializable, it doesn’t do what you expect. So if you want to serialize
statics, you must do it yourself.
This is what the serializeStaticState( ) and deserializeStaticState( ) static methods in
Line are for. You can see they are explicitly called as part of the storage and retrieval
process (notice that the order of writing to the serialize file and reading back from it must
be maintained).
Chapter 10: The Java IO System 355](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-355-320.jpg)
![}
public class Shapes {
public static void main(String args[]) {
Vector s = new Vector();
s.addElement(new Circle());
s.addElement(new Square());
s.addElement(new Triangle());
Enumeration e = s.elements();
while(e.hasMoreElements())
((Shape)e.nextElement()).draw();
}
} ///:~
The base class could be coded as an interface, an abstract class or an ordinary class. Since
Shape has no concrete members (that is, members with definitions) and it’s not intended
that you ever create a plain Shape object, the most appropriate and flexible representation
is an interface. It’s also cleaner because you don’t have all those abstract keywords lying
about.
Each of the derived classes overrides the base-class draw method so it behaves differently.
In main( ), specific types of Shape are created and then added to a Vector. This is the
point where the upcast occurs because the Vector holds only Objects. Since everything in
Java (with the exception of primitives) is an Object, a Vector can also hold Shape objects.
But during an upcast to Object, it also loses any specific information, including the fact
that the objects are shapes. To the Vector, they are just Objects.
At the point you fetch an element out of the Vector with nextElement( ), things get a
little busy. Since Vector holds only Objects, nextElement( ) naturally produces an Object
handle. But we know it’s actually a Shape handle, and we want to send Shape messages to
that object. So a cast to Shape is necessary using the traditional “(Shape)” cast. This is
actually the most basic form of RTTI, since in Java all casts are checked at run-time for
correctness. That’s exactly what RTTI means: at run-time, the type of an object is
identified.
In this case, the RTTI cast is only partial: the Object is cast to a Shape, and not all the way
to a Circle, Square or Triangle. That’s because the only thing we know at this point is that
the Vector is full of Shapes. At compile-time, this is enforced only by your own self-
imposed rules, but at run-time the cast ensures it.
Now polymorphism takes over and the exact method that’s called for the Shape is
determined by whether the handle is for a Circle, Square or Triangle. And this is how it
should be, in general: you want the bulk of your code to know as little as possible about
specific types of objects, and just deal with the general representation of a family of objects
(in this case, Shape). As a result, your code will be easier to write, read and maintain and
your designs will be easier to implement, understand and change. So polymorphism is the
general goal in object-oriented programming.
But what if you have a special programming problem that’s easiest to solve if you know
the exact type of a generic handle? For example, suppose you want to allow your users to
highlight all the shapes of any particular type by turning them purple. This way, they can
find all the triangles on the screen by highlighting them. This is what RTTI accomplishes:
you can ask a handle to a Shape exactly what type it’s referring to.
Chapter 11: Run-Time Type Identification 361](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-361-320.jpg)
![The Class object
To understand how RTTI works in Java, you must first know how type information is
represented at run time. This is accomplished through a special kind of object called the
Class object that contains information about the class itself (this is sometimes called a
meta-class). In fact, the Class object is used to create all the “regular” objects of your class.
There’s a Class object for each class that is part of your program. That is, each time you
write a new class, a single Class object is also created (and stored, appropriately enough,
in an identically-named .class file). At run time, when you want to make an object of that
class, the Java Virtual Machine (JVM) that’s executing your program first checks to see if
the Class object for that type is loaded. If not, the JVM loads it by finding the .class file
with that name. Thus a Java program isn’t completely loaded before it begins, which is
different than many traditional languages.
Once the Class object for that type is in memory, it is used to create all objects of that
type.
If this seems shadowy or you don’t really believe it, here’s a demonstration program to
prove it:
//: SweetShop.java
// Examination of the way the class loader works
class Candy {
static {
System.out.println("Loading Candy");
}
}
class Gum {
static {
System.out.println("Loading Gum");
}
}
class Cookie {
static {
System.out.println("Loading Cookie");
}
}
public class SweetShop {
public static void main(String args[]) {
System.out.println("inside main");
new Candy();
System.out.println("After creating Candy");
try {
Class.forName("Gum");
} catch(ClassNotFoundException e) {
e.printStackTrace();
}
System.out.println(
"After Class.forName("Gum")");
new Cookie();
System.out.println("After creating Cookie");
362 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-362-320.jpg)
![//: PetCount.java
// Using instanceof
package c11.petcount;
import java.util.*;
class Pet {}
class Dog extends Pet {}
class Pug extends Dog {}
class Cat extends Pet {}
class Rodent extends Pet {}
class Gerbil extends Rodent {}
class Hamster extends Rodent {}
class Counter { int i; }
public class PetCount {
static String[] typenames = {
"Pet", "Dog", "Pug", "Cat",
"Rodent", "Gerbil", "Hamster",
};
public static void main(String args[]) {
Vector pets = new Vector();
try {
Class[] petTypes = {
Class.forName("c11.petcount.Dog"),
Class.forName("c11.petcount.Pug"),
Class.forName("c11.petcount.Cat"),
Class.forName("c11.petcount.Rodent"),
Class.forName("c11.petcount.Gerbil"),
Class.forName("c11.petcount.Hamster"),
};
for(int i = 0; i < 15; i++)
pets.addElement(
petTypes[
(int)(Math.random()*petTypes.length)]
.newInstance());
} catch(InstantiationException e) {}
catch(IllegalAccessException e) {}
catch(ClassNotFoundException e) {}
Hashtable h = new Hashtable();
for(int i = 0; i < typenames.length; i++)
h.put(typenames[i], new Counter());
for(int i = 0; i < pets.size(); i++) {
Object o = pets.elementAt(i);
if(o instanceof Pet)
((Counter)h.get("Pet")).i++;
if(o instanceof Dog)
((Counter)h.get("Dog")).i++;
if(o instanceof Pug)
((Counter)h.get("Pug")).i++;
if(o instanceof Cat)
((Counter)h.get("Cat")).i++;
if(o instanceof Rodent)
((Counter)h.get("Rodent")).i++;
if(o instanceof Gerbil)
((Counter)h.get("Gerbil")).i++;
Chapter 11: Run-Time Type Identification 365](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-365-320.jpg)
![if(o instanceof Hamster)
((Counter)h.get("Hamster")).i++;
}
for(int i = 0; i < pets.size(); i++)
System.out.println(
pets.elementAt(i).getClass().toString());
for(int i = 0; i < typenames.length; i++)
System.out.println(
typenames[i] + " quantity: " +
((Counter)h.get(typenames[i])).i);
}
} ///:~
There’s a rather narrow restriction on instanceof in Java 1.0: you can compare it only to
a named type, and not to a Class object. In the above example you may feel like it’s tedious
to write out all those instanceof expressions, and you’re right. But in Java 1.0 there is no
way to cleverly automate it by creating a Vector of Class objects and comparing to those
instead. This isn’t as great a restriction as you might think, because you’ll eventually
understand that your design is probably flawed if you end up writing a lot of instanceof
expressions.
Of course this example is contrived – you’d probably put a static data member in each
type and increment it in the constructor to keep track of the counts. You would do
something like that if you had control of the source code for the class and could change it.
Since this is not always the case, RTTI can come in handy.
Using class literals
It’s interesting to see how the PetCount.java example can be rewritten using Java 1.1
class literals. The result is cleaner in many ways:
//: PetCount2.java
// Using Java 1.1 class literals
package c11.petcount2;
import java.util.*;
class Pet {}
class Dog extends Pet {}
class Pug extends Dog {}
class Cat extends Pet {}
class Rodent extends Pet {}
class Gerbil extends Rodent {}
class Hamster extends Rodent {}
class Counter { int i; }
public class PetCount2 {
public static void main(String args[]) {
Vector pets = new Vector();
Class[] petTypes = {
// Class literals work in Java 1.1 only:
Pet.class,
Dog.class,
Pug.class,
Cat.class,
Rodent.class,
Gerbil.class,
366 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-366-320.jpg)
![Hamster.class,
};
try {
for(int i = 0; i < 15; i++) {
// Offset by one to eliminate Pet.class:
int rnd = 1 + (int)(
Math.random() * (petTypes.length - 1));
pets.addElement(
petTypes[rnd].newInstance());
}
} catch(InstantiationException e) {}
catch(IllegalAccessException e) {}
Hashtable h = new Hashtable();
for(int i = 0; i < petTypes.length; i++)
h.put(petTypes[i].toString(),
new Counter());
for(int i = 0; i < pets.size(); i++) {
Object o = pets.elementAt(i);
if(o instanceof Pet)
((Counter)h.get(
"class c11.petcount2.Pet")).i++;
if(o instanceof Dog)
((Counter)h.get(
"class c11.petcount2.Dog")).i++;
if(o instanceof Pug)
((Counter)h.get(
"class c11.petcount2.Pug")).i++;
if(o instanceof Cat)
((Counter)h.get(
"class c11.petcount2.Cat")).i++;
if(o instanceof Rodent)
((Counter)h.get(
"class c11.petcount2.Rodent")).i++;
if(o instanceof Gerbil)
((Counter)h.get(
"class c11.petcount2.Gerbil")).i++;
if(o instanceof Hamster)
((Counter)h.get(
"class c11.petcount2.Hamster")).i++;
}
for(int i = 0; i < pets.size(); i++)
System.out.println(
pets.elementAt(i).getClass().toString());
Enumeration keys = h.keys();
while(keys.hasMoreElements()) {
String nm = (String)keys.nextElement();
Counter cnt = (Counter)h.get(nm);
System.out.println(
nm.substring(nm.lastIndexOf('.') + 1) +
" quantity: " + cnt.i);
}
}
} ///:~
Here, the typenames array has been removed in favor of getting the type name strings
from the Class object. Note the extra work for this: the class name is not, for example,
Chapter 11: Run-Time Type Identification 367](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-367-320.jpg)
![Gerbil but instead c11.petcount2.Gerbil since the package name is included. Note also
that the system can distinguish between classes and interfaces.
You can also see that the creation of petTypes does not need to be surrounded by a try
block, since it’s evaluated at compile time and thus won’t throw any exceptions, unlike
Class.forName( ).
When the Pet objects are dynamically created, you can see that the random number is
restricted so it is between 1 and petTypes.length and does not include zero. That’s because
zero refers to Pet.class, and presumably a generic Pet object is not interesting. However,
since Pet.class is part of petTypes the result is that all the pets get counted.
A dynamic instanceof
Java 1.1 has added the isInstance method to the class Class. This allows you to
dynamically call the instanceof operator which you could do only statically in Java 1.0
(as previously shown). Thus, all those tedious instanceof statements can be removed in
the PetCount example:
//: PetCount3.java
// Using Java 1.1 isInstance()
package c11.petcount3;
import java.util.*;
class Pet {}
class Dog extends Pet {}
class Pug extends Dog {}
class Cat extends Pet {}
class Rodent extends Pet {}
class Gerbil extends Rodent {}
class Hamster extends Rodent {}
class Counter { int i; }
public class PetCount3 {
public static void main(String args[]) {
Vector pets = new Vector();
Class[] petTypes = {
Pet.class,
Dog.class,
Pug.class,
Cat.class,
Rodent.class,
Gerbil.class,
Hamster.class,
};
try {
for(int i = 0; i < 15; i++) {
// Offset by one to eliminate Pet.class:
int rnd = 1 + (int)(
Math.random() * (petTypes.length - 1));
pets.addElement(
petTypes[rnd].newInstance());
}
} catch(InstantiationException e) {}
catch(IllegalAccessException e) {}
Hashtable h = new Hashtable();
368 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-368-320.jpg)
![for(int i = 0; i < petTypes.length; i++)
h.put(petTypes[i].toString(),
new Counter());
for(int i = 0; i < pets.size(); i++) {
Object o = pets.elementAt(i);
// Using isInstance to eliminate individual
// instanceof expressions:
for (int j = 0; j < petTypes.length; ++j)
if (petTypes[j].isInstance(o)) {
String key = petTypes[j].toString();
((Counter)h.get(key)).i++;
}
}
for(int i = 0; i < pets.size(); i++)
System.out.println(
pets.elementAt(i).getClass().toString());
Enumeration keys = h.keys();
while(keys.hasMoreElements()) {
String nm = (String)keys.nextElement();
Counter cnt = (Counter)h.get(nm);
System.out.println(
nm.substring(nm.lastIndexOf('.') + 1) +
" quantity: " + cnt.i);
}
}
} ///:~
You can see that the Java 1.1 isInstance( ) method has eliminated the need for the
instanceof expressions. In addition, this means that you can add new types of pets by
simply changing the petTypes array; the rest of the program does not need modification
(as it did when using the instanceof expressions).
RTTI syntax
Java performs its RTTI using the Class object, even if you’re doing something like a cast.
The class Class also has a number of other ways you can use RTTI.
First, you must get a handle to the appropriate Class object. One way to do this, as shown
in the previous example, is to use a string and the Class.forName( ) method. This is
convenient because you don’t need an object of that type in order to get the Class handle.
However, if you do already have an object of the type you’re interested in you can fetch
the Class handle by calling a method that’s part of the Object root class: getClass( ). This
returns the Class handle representing the actual type of the object. Class itself has several
interesting and sometimes useful methods, demonstrated in the following example:
//: ToyTest.java
// Testing class Class
interface HasBatteries {}
interface Waterproof {}
interface ShootsThings {}
class Toy {
// Comment out the following default
// constructor to see
// NoSuchMethodError from (*1*)
Chapter 11: Run-Time Type Identification 369](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-369-320.jpg)
![Toy() {}
Toy(int i) {}
}
class FancyToy extends Toy
implements HasBatteries,
Waterproof, ShootsThings {
FancyToy() { super(1); }
}
public class ToyTest {
public static void main(String args[]) {
Class c = null;
try {
c = Class.forName("FancyToy");
} catch(ClassNotFoundException e) {}
printInfo(c);
Class faces[] = c.getInterfaces();
for(int i = 0; i < faces.length; i++)
printInfo(faces[i]);
Class cy = c.getSuperclass();
Object o = null;
try {
// Requires default constructor:
o = cy.newInstance(); // (*1*)
} catch(InstantiationException e) {}
catch(IllegalAccessException e) {}
printInfo(o.getClass());
}
static void printInfo(Class cc) {
System.out.println(
"Class name: " + cc.getName() +
" is interface? [" +
cc.isInterface() + "]");
}
} ///:~
You can see that class FancyToy is quite complicated, since it inherits from Toy and
implements the interfaces of HasBatteries, Waterproof and ShootsThings. In main( ), a
Class handle is created and initialized to the FancyToy Class using forName( ) inside an
appropriate try block.
The Class.getInterfaces( ) method returns an array of Class objects representing the
interfaces that are contained in the Class object of interest.
If you have a Class object you can also ask it for its direct base class using
getSuperclass( ). This, of course, returns a Class handle which you can further query.
This means that, at run time, you can discover an object’s entire class hierarchy.
The newInstance( ) method of Class can at first seem like just another way to clone( ) an
object. However, you can create a new object with newInstance( ) without an existing
object, as seen here, because there is no Toy object, only cy which is a handle to y’s Class
object. This is a way to implement a “virtual constructor,” which allows you to say: “I
don’t know exactly what type you are, but create yourself properly anyway.” In the above
example, cy is just a Class handle with no further type information known at compile
time. And when you create a new instance, you get back an Object handle. But that handle
370 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-370-320.jpg)
![is actually pointing to a Toy object. Of course, before you can send any messages other
than those accepted by Object, you have to investigate it a bit and do some casting. In
addition, the class that’s being created with newInstance( ) must have a default
constructor. There’s no way to use newInstance( ) to create objects that have non-default
constructors, so this can be a bit limiting in Java 1. However, the reflection API in Java 1.1
(discussed in the next section) allows you to dynamically use any constructor in a class.
The final method in the listing is printInfo( ) which takes a Class handle and gets its
name with getName( ) and finds out whether it’s an interface with isInterface( ).
The output from this program is:
Class name: FancyToy is interface? [false]
Class name: HasBatteries is interface? [true]
Class name: Waterproof is interface? [true]
Class name: ShootsThings is interface? [true]
Class name: Toy is interface? [false]
Thus, with the Class object you can find out just about everything you want to know
about an object.
Reflection: run-time class
information
If you don’t know the precise type of an object, RTTI will tell you that type. However,
there’s a limitation: the type must be known at compile time in order for you to be able to
detect it using RTTI and do something useful with the information. Put another way, the
compiler must know about all the classes you’re working with for RTTI.
This doesn’t seem like that much of a limitation at first, but suppose you’re given a handle
to an object that’s not in your program space. In fact, the class of the object isn’t even
available to your program at compile time. For example, suppose you get a bunch of bytes
from a disk file or from a network connection, and you’re told those bytes represent a
class. Since the compiler can’t know about the class while it’s compiling the code, how can
you possibly use such a class?
In a traditional programming environment this seems like a far-fetched scenario. But as
we move into a larger programming world there are important cases where this happens.
The first is component-based programming where you build projects using Rapid-
Application-Development (RAD) in an application builder tool. This is a visual approach to
creating a program (which you see on the screen as a form) by moving icons that represent
components onto the form. These components are then configured by setting some of their
values at program time. This design-time configuration requires that any component be
instantiable and that it expose some part of itself and allow its values to be read and set. In
addition, components that handle GUI events must expose information about appropriate
methods so the RAD environment may assist the programmer in overriding these event-
handling methods. Reflection provides the mechanism to detect the available methods and
produce the method names. Java 1.1 provides a structure for component-based
programming through Java Beans (described in Chapter 13).
Another compelling motivation for discovering class information at run-time is to provide
the ability to create and execute objects on remote platforms across a network. This is
called Remote Method Invocation (RMI) and it allows a Java program (version 1.1 and higher)
to have objects distributed across many machines. This distribution may happen for a
number of reasons: perhaps you’re doing a computation-intensive task and you want to
Chapter 11: Run-Time Type Identification 371](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-371-320.jpg)
!["To show all methods in class or: n" +
"ShowMethods qualified.class.name wordn" +
"To search for methods involving 'word'";
public static void main(String args[]) {
if(args.length < 1) {
System.out.println(usage);
System.exit(0);
}
try {
Class c = Class.forName(args[0]);
Method[] m = c.getMethods();
Constructor[] ctor = c.getConstructors();
if(args.length == 1) {
for (int i = 0; i < m.length; i++)
System.out.println(m[i].toString());
for (int i = 0; i < ctor.length; i++)
System.out.println(ctor[i].toString());
}
else {
for (int i = 0; i < m.length; i++)
if(m[i].toString()
.indexOf(args[1])!= -1)
System.out.println(m[i].toString());
for (int i = 0; i < ctor.length; i++)
if(ctor[i].toString()
.indexOf(args[1])!= -1)
System.out.println(ctor[i].toString());
}
} catch (ClassNotFoundException e) {
System.out.println("No such class: " + e);
}
}
} ///:~
The Class methods getMethods( ) and getConstructors( ) return an array of Method and
Constructor, respectively. Each of these classes has further methods to dissect the names,
arguments and return values of the methods they represent. But you can also just use
toString( ), as is done here, to produce a String with the entire method signature. The rest
of the code is just for extracting command-line information, determining if a particular
signature matches with your target string (using indexOf( )) and printing the results.
This shows reflection in action, since the result produced by Class.forName( ) cannot be
known at compile-time, and therefore all the method signature information is being
extracted at run-time. If you investigate your online documentation on reflection, you’ll
see there is enough support to actually set up and make a method call on an object that’s
totally unknown at compile-time. Again, this is something you’ll probably never need to
do yourself – the support is there for Java itself and so that a programming environment
can manipulate Java Beans – but it’s interesting.
An interesting experiment is to run java ShowMethods ShowMethods. This produces a
listing that includes a public default constructor, even though you can see from the code
that no constructor was defined. The constructor you see is the one that’s automatically
synthesized by the compiler. If you then make ShowMethods a non-public class (that is,
friendly), the synthesized default constructor no longer shows up in the output. The
synthesized default constructor is automatically given the same access as the class itself.
Chapter 11: Run-Time Type Identification 373](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-373-320.jpg)
![The output for ShowMethods is still a little tedious. For example, here’s a portion of the
output produced by invoking java ShowMethods java.lang.String:
public boolean
java.lang.String.startsWith(java.lang.String,int)
public boolean
java.lang.String.startsWith(java.lang.String)
public boolean
java.lang.String.endsWith(java.lang.String)
It would be even nicer if the qualifiers like java.lang could be stripped off. The
StreamTokenizer class introduced in the previous chapter can help solve this problem:
//: ShowMethodsClean.java
// ShowMethods with the qualifiers stripped
// to make the results easier to read
import java.lang.reflect.*;
import java.io.*;
public class ShowMethodsClean {
static final String usage =
"usage: n" +
"ShowMethodsClean qualified.class.namen" +
"To show all methods in class or: n" +
"ShowMethodsClean qualified.class.name wordn" +
"To search for methods involving 'word'";
public static void main(String args[]) {
if(args.length < 1) {
System.out.println(usage);
System.exit(0);
}
try {
Class c = Class.forName(args[0]);
Method[] m = c.getMethods();
Constructor[] ctor = c.getConstructors();
// Convert to an array of cleaned Strings:
String n[] =
new String[m.length + ctor.length];
for(int i = 0; i < m.length; i++) {
String s = m[i].toString();
n[i] = StripQualifiers.strip(s);
}
for(int i = 0; i < ctor.length; i++) {
String s = ctor[i].toString();
n[i + m.length] =
StripQualifiers.strip(s);
}
if(args.length == 1)
for (int i = 0; i < n.length; i++)
System.out.println(n[i]);
else
for (int i = 0; i < n.length; i++)
if(n[i].indexOf(args[1])!= -1)
System.out.println(n[i]);
} catch (ClassNotFoundException e) {
System.out.println("No such class: " + e);
374 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-374-320.jpg)
![Passing handles around
When you pass a handle into a method, you’re still pointing to the same object. A simple
experiment demonstrates this (see page 80 if you have trouble executing this program):
//: PassHandles.java
// Passing handles around
package c12;
public class PassHandles {
static void f(PassHandles h) {
System.out.println("h inside f(): " + h);
}
public static void main(String args[]) {
PassHandles p = new PassHandles();
System.out.println("p inside main(): " + p);
f(p);
}
} ///:~
The method toString( ) is automatically invoked in the print statements, and PassHandles
inherits directly from Object with no redefinition of toString( ). Thus, Object’s version of
toString( ) is used, which prints out the class of the object followed by the address where
that object is located (not the handle, but the actual object storage). The output looks like
this:
p inside main(): PassHandles@1653748
h inside f(): PassHandles@1653748
You can see that both p and h refer to the same object. This is far more efficient than
duplicating a new PassHandles object just so you can send an argument to a method. But
it brings up an important issue.
Aliasing
Aliasing means that more than one handle is tied to the same object, as in the above
example. The problem with aliasing occurs when someone writes to that object. If the
owners of the other handles aren’t expecting that object to change, they’ll be surprised.
This can be demonstrated with a simple example:
//: Alias1.java
// Aliasing two handles to one object
public class Alias1 {
int i;
Alias1(int ii) { i = ii; }
public static void main(String args[]) {
Alias1 x = new Alias1(7);
Alias1 y = x; // Assign the handle
System.out.println("x: " + x.i);
System.out.println("y: " + y.i);
System.out.println("Incrementing x");
x.i++;
System.out.println("x: " + x.i);
System.out.println("y: " + y.i);
380 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-380-320.jpg)
![}
} ///:~
In the line:
Alias1 y = x; // Assign the handle
A new Alias1 handle is created, but instead of being assigned to a fresh object created with
new, it’s assigned to an existing handle. This means that the contents of handle x, which
is the address of the object x is pointing to, is assigned to y, and thus both x and y are
attached to the same object. So when x’s i is incremented in the statement:
x.i++;
y’s i will be affected as well. This can be seen in the output:
x: 7
y: 7
Incrementing x
x: 8
y: 8
One very good solution in this case is simply not to do it: don’t consciously alias more
than one handle to an object at the same scope. Your code will be much easier to
understand and debug. However, when you’re passing a handle in as an argument – which
is the way Java is supposed to work – you automatically alias because the local handle
that’s created can modify the “outside object” (the object that was created outside the scope
of the method). Here’s an example:
//: Alias2.java
// Method calls implicitly alias their
// arguments.
public class Alias2 {
int i;
Alias2(int ii) { i = ii; }
static void f(Alias2 handle) {
handle.i++;
}
public static void main(String args[]) {
Alias2 x = new Alias2(7);
System.out.println("x: " + x.i);
System.out.println("Calling f(x)");
f(x);
System.out.println("x: " + x.i);
}
} ///:~
The output is:
x: 7
Calling f(x)
x: 8
The method is changing its argument, the outside object. When this kind of situation
arises, you must decide whether it makes sense, whether the user expects it, and whether
it’s going to cause problems.
Chapter 12: Passing & Returning Objects 381](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-381-320.jpg)
![course, the assumption is that you’re always thinking (and caring) that handles
are being passed, but it seems like the Java design has gone a long way towards
allowing you to ignore (most of the time) that you’re working with a handle.
That is, it seems to allow you to think of the handle as “the object,” since it
implicitly dereferences it whenever you make a method call.
2. Java passes primitives by value (no argument there) but objects are passed by
reference. This is the world view that the handle is an alias for the object itself,
so you don’t think about passing handles, but instead say “I’m passing the
object.” Since you don’t get a local copy of the object when you pass it into a
method, objects are clearly not passed by value. There appears to be some
support for this view within Sun itself, since one of the “reserved but not
implemented” keywords is byvalue (there’s no knowing, however, whether that
keyword will ever see the light of day).
Having given both camps a good airing and after saying “it depends on how you think of a
handle,” I will attempt to sidestep the issue for the rest of the book. In the end, it isn’t that
important – what is important is that you understand that passing a handle allows the
caller’s object to be changed unexpectedly.
Cloning objects
The most likely reason for making a local copy of an object is if you’re going to modify
that object and you don’t want to modify the caller’s object. If you decide that you want to
make a local copy, you simply use the clone( ) method to perform the operation. This is a
method that’s defined as protected in the base class Object and which you must override
as public in any derived classes that you want to clone. For example, the standard library
class Vector overrides clone( ), so we can call clone( ) for Vector:
//: Cloning.java
// The clone() operation works for only a few
// items in the standard Java library.
import java.util.*;
class Int {
private int i;
public Int(int ii) { i = ii; }
public void increment() { i++; }
public String toString() {
return Integer.toString(i);
}
}
public class Cloning {
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 10; i++ )
v.addElement(new Int(i));
System.out.println("v: " + v);
Vector v2 = (Vector)v.clone();
// Increment all v2's elements:
for(Enumeration e = v2.elements();
e.hasMoreElements(); )
((Int)e.nextElement()).increment();
// See if it changed v's elements:
System.out.println("v: " + v);
Chapter 12: Passing & Returning Objects 383](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-383-320.jpg)
![}
} ///:~
The clone( ) method produces an Object, which must then be recast to the proper type.
This example shows how Vector’s clone( ) method does not automatically try to clone each
of the objects that the Vector contains – the old Vector and the cloned Vector are aliased to
the same objects. This is often called a shallow copy, since it’s copying only the “surface”
portion of an object. The actual object consists of this “surface” plus all the objects that
the handles are pointing to, plus all the objects those objects are pointing to, etc. This is
often referred to as the “web of objects.” Copying the entire mess is called a deep copy.
You can see the effect of the shallow copy in the output, where the actions performed on
v2 affect v:
v: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
v: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Not trying to clone( ) the objects contained in the Vector is probably a fair assumption
because there’s no guarantee that those objects are cloneable.
Adding cloneability to a class
Even though the clone method is defined in the base-of-all-classes Object, cloning is not
automatically available in every class. This would seem to be counterintuitive to the idea
that base-class methods are always available in derived classes. Cloning in Java goes
against this idea; if you want it to exist for a class, you must specifically add code to make
cloning work.
Using a trick with protected
To prevent default clonability in every class you create, the clone( ) method is protected in
the base class Object. Not only does this mean that it’s not available by default to the
client programmer who is simply using the class (not subclassing it), but it also means
that you cannot call clone( ) via a handle to the base class (although that might seem to be
useful in some situations: polymorphically clone a bunch of Objects). It is in effect a way
to give you, at compile time, the information that your object is not cloneable – and oddly
enough most classes in the standard Java library are not cloneable. Thus, if you say:
Integer x = new Integer(1);
x = x.clone();
You will get, at compile time, an error message that says clone( ) is not accessible (since
Integer doesn’t override it and it defaults to the protected version).
If, however, you’re in a class derived from Object (as all classes are) then you have
permission to call Object.clone( ) because it’s protected and you’re an inheritor. The base
class clone( ) has very useful functionality – it performs the actual bitwise duplication of
the derived-class object, thus acting as the common cloning operation. However, you then
need to make your clone operation public for it to be accessible. Thus two key issues when
you clone are: virtually always call super.clone( ) and make your clone public.
You’ll probably want to override clone( ) in any further derived classes, otherwise your
(now public) clone( ) will be used, and that may not do the right thing (although, since
Object.clone( ) makes a copy of the actual object, it may). Thus the protected trick works
only once, the first time you inherit from a class that has no clonability and you want to
make a class that’s cloneable. In any classes inherited from your class the clone( ) method
is available since it’s not possible in Java to reduce the access of a method during
384 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-384-320.jpg)
![v.i++;
return v;
}
static MyObject f(MyObject v) {
v = (MyObject)v.clone(); // Local copy
v.i++;
return v;
}
public static void main(String args[]) {
MyObject a = new MyObject(11);
MyObject b = g(a);
// Testing handle equivalence,
// not object equivalence:
if(a == b)
System.out.println("a == b");
else
System.out.println("a != b");
System.out.println("a = " + a);
System.out.println("b = " + b);
MyObject c = new MyObject(47);
MyObject d = f(c);
if(c == d)
System.out.println("c == d");
else
System.out.println("c != d");
System.out.println("c = " + c);
System.out.println("d = " + d);
}
} ///:~
First of all, clone( ) must be accessible so you’ll need to make it public. Second, for the
initial part of your clone( ) operation you should call the base-class version of clone( ).
The clone( ) that’s being called here is the one that’s pre-defined inside Object, and you
can call it because it’s protected and thereby accessible in derived classes.
Object.clone( ) figures out how big the actual object is, creates enough memory for a new
one, and copies all the bits from the old to the new. This is called a bitwise copy, and is
typically what you’d expect a clone( ) method to do. But before Object.clone( ) performs
its operations, it first checks to see if a class is Cloneable, that is, whether it implements
the Cloneable interface. If it doesn’t, Object.clone( ) throws a
CloneNotSupportedException to indicate that you can’t clone it. Thus, you’ve got to
surround your call to super.clone( ) with a try-catch block, to catch an exception that
should never happen (because you’ve implemented the Cloneable interface).
In LocalCopy, the two methods g( ) and f( ) demonstrate the difference between the two
approaches for argument passing. g( ) shows passing by reference where it modifies the
outside object and returns a reference to that same outside object, while f( ) clones the
argument, thereby decoupling it and leaving the original object alone. It can then proceed
to do whatever it wants, and even to return a handle to this new object without any ill
effects to the original. Notice the somewhat curious-looking statement:
v = (MyObject)v.clone();
This is where the local copy is created. To keep from being confused by such a statement,
you must remember that this rather strange coding idiom is perfectly feasible in Java
because everything that has a name is actually a handle. So the handle v is used to
clone( ) a copy of what it refers to, and this returns a handle to the base type Object
386 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-386-320.jpg)
![next.increment();
}
public String toString() {
String s = ":" + c;
if(next != null)
s += next.toString();
return s;
}
public Object clone() {
Object o = null;
try {
o = super.clone();
} catch (CloneNotSupportedException e) {}
return o;
}
public static void main(String args[]) {
Snake s = new Snake(5, 'a');
System.out.println("s = " + s);
Snake s2 = (Snake)s.clone();
System.out.println("s2 = " + s2);
s.increment();
System.out.println(
"after s.increment, s2 = " + s2);
}
} ///:~
A Snake is made up of a bunch of segments, each of type Snake. Thus it’s a singly-linked
list. The segments are created recursively, decrementing the first constructor argument for
each segment until zero is reached. To give each segment a unique tag, the second
argument, a char, is incremented for each recursive constructor call.
The increment( ) method recursively increments each tag so you can see the change, and
the toString( ) recursively prints each tag. The output is:
s = :a:b:c:d:e
s2 = :a:b:c:d:e
after s.increment, s2 = :a:c:d:e:f
This means that only the first segment is duplicated by Object.clone( ), thus it does a
shallow copy. If you want the whole snake to be duplicated – a deep copy – you must
perform the additional operations inside your overridden clone( ).
Therefore, you’ll typically call super.clone( ) in any class derived from a cloneable class,
to make sure that all the base-class operations (including Object.clone( )) take place. This
is followed by an explicit call to clone( ) for every handle in your object – otherwise those
handles will be aliased to those of the original object. It’s very analogous to the way
constructors are called – base-class constructor first, then the next-derived constructor
and so on to the most-derived constructor. The difference is that clone( ) is not a
constructor so there’s nothing to make it happen automatically. You must make sure to do
it yourself.
Cloning a composed object
What other operations should you perform after super.clone( ) is called? This depends on
your class, and whether you want to perform a shallow copy or a deep copy. If you want
to perform a shallow copy, then the operations performed by Object.clone( ) (bitwise
388 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-388-320.jpg)
![try {
o = (OceanReading)super.clone();
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
// Must clone handles:
o.depth = (DepthReading)o.depth.clone();
o.temperature =
(TemperatureReading)o.temperature.clone();
return o; // Upcasts back to Object
}
}
public class DeepCopy {
public static void main(String args[]) {
OceanReading reading =
new OceanReading(33.9, 100.5);
// Now clone it:
OceanReading r =
(OceanReading)reading.clone();
}
} ///:~
DepthReading and TemperatureReading are quite similar; they both contain only
primitives and therefore the clone( ) method can be quite simple: it calls only
super.clone( ) and returns the result. Note that the clone( ) code for both classes is
identical.
OceanReading is composed of DepthReading and TemperatureReading objects and so,
to produce a deep copy, its clone( ) must clone the handles inside OceanReading. To
accomplish this, the result of super.clone( ) must be cast to an OceanReading object (so
you can access the depth and temperature handles).
A deep copy with Vector
Let’s revisit the Vector example from earlier in this chapter. This time the Int2 class is
cloneable so the Vector can be deep copied:
//: AddingClone.java
// You must go through a few gyrations to
// add cloning to your own class.
import java.util.*;
class Int2 implements Cloneable {
private int i;
public Int2(int ii) { i = ii; }
public void increment() { i++; }
public String toString() {
return Integer.toString(i);
}
public Object clone() {
Object o = null;
try {
o = super.clone();
} catch (CloneNotSupportedException e) {
System.out.println("Int2 can't clone");
390 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-390-320.jpg)
![}
return o;
}
}
// Once it's cloneable, inheritance
// doesn't remove cloneability:
class Int3 extends Int2 {
private int j; // Automatically duplicated
public Int3(int i) { super(i); }
}
public class AddingClone {
public static void main(String args[]) {
Int2 x = new Int2(10);
Int2 x2 = (Int2)x.clone();
x2.increment();
System.out.println(
"x = " + x + ", x2 = " + x2);
// Anything inherited is also cloneable:
Int3 x3 = new Int3(7);
x3 = (Int3)x3.clone();
Vector v = new Vector();
for(int i = 0; i < 10; i++ )
v.addElement(new Int2(i));
System.out.println("v: " + v);
Vector v2 = (Vector)v.clone();
// Now clone each element:
for(int i = 0; i < v.size(); i++)
v2.setElementAt(
((Int2)v2.elementAt(i)).clone(), i);
// Increment all v2's elements:
for(Enumeration e = v2.elements();
e.hasMoreElements(); )
((Int2)e.nextElement()).increment();
// See if it changed v's elements:
System.out.println("v: " + v);
System.out.println("v2: " + v2);
}
} ///:~
Int3 is inherited from Int2 and a new primitive member int j is added. You might think
you’d need to override clone( ) again to make sure j is copied, but that’s not the case:
when Int2’s clone( ) is called as Int3’s clone( ), it calls Object.clone( ) which determines
that it’s working with an Int3 and duplicates all the bits in the Int3. Thus, as long as you
don’t add handles that need to be cloned, the one call to Object.clone( ) performs all the
necessary duplication, regardless of how far down in the hierarchy clone( ) is defined.
You can see what’s necessary in order to do a deep copy of a Vector: after the Vector itself
is cloned, you have to step through and clone each one of the objects pointed to by the
Vector. You’d have to do something similar to this to do a deep copy of a Hashtable.
The remainder of the example shows that the cloning did in fact happen by showing that,
once an object is cloned, you can change it and the original object is left untouched.
Chapter 12: Passing & Returning Objects 391](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-391-320.jpg)
![Deep copy via serialization
When you consider Java 1.1 object serialization (introduced in Chapter 10), you may
observe that an object that’s serialized and then deserialized is, in effect, cloned. Indeed,
the transient keyword has the same effect for both cloning and serialization (that field is
not cloned/serialized).
So why not use serialization to perform deep copying? Here’s an example that compares
the two approaches by timing them:
//: Compete.java
import java.io.*;
import java.util.*;
class Thing1 implements Serializable {}
class Thing2 implements Serializable {
Thing1 o1 = new Thing1();
}
class Thing3 implements Cloneable {
public Object clone() {
Object o = null;
try {
o = super.clone();
} catch (CloneNotSupportedException e) {
System.out.println("Thing3 can't clone");
}
return o;
}
}
class Thing4 implements Cloneable {
Thing3 o3 = new Thing3();
public Object clone() {
Thing4 o = null;
try {
o = (Thing4)super.clone();
} catch (CloneNotSupportedException e) {
System.out.println("Thing4 can't clone");
}
// Clone the field, too:
o.o3 = (Thing3)o3.clone();
return o;
}
}
public class Compete implements Serializable {
static final int SIZE = 5000;
public static void main(String args[]) {
Thing2 a[] = new Thing2[SIZE];
for(int i = 0; i < a.length; i++)
a[i] = new Thing2();
Thing4 b[] = new Thing4[SIZE];
for(int i = 0; i < b.length; i++)
b[i] = new Thing4();
try {
392 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-392-320.jpg)
![long t1 = System.currentTimeMillis();
ByteArrayOutputStream buf =
new ByteArrayOutputStream();
ObjectOutputStream o =
new ObjectOutputStream(buf);
for(int i = 0; i < a.length; i++)
o.writeObject(a[i]);
// Now get copies:
ObjectInputStream in =
new ObjectInputStream(
new ByteArrayInputStream(
buf.toByteArray()));
Thing2 c[] = new Thing2[SIZE];
for(int i = 0; i < c.length; i++)
c[i] = (Thing2)in.readObject();
long t2 = System.currentTimeMillis();
System.out.println(
"Duplication via serialization: " +
(t2 - t1) + " Milliseconds");
// Now try cloning:
t1 = System.currentTimeMillis();
Thing4 d[] = new Thing4[SIZE];
for(int i = 0; i < d.length; i++)
d[i] = (Thing4)b[i].clone();
t2 = System.currentTimeMillis();
System.out.println(
"Duplication via cloning: " +
(t2 - t1) + " Milliseconds");
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
So that there’s some deep copying going on, Thing2 and Thing4 contain member objects.
It’s interesting to note that while Serializable classes are easy to set up, there’s much
more work going on to duplicate them. Cloning involves a lot of work to set up the class
but the actual duplication of objects is relatively simple. But the results really tell the tale.
Here is the output from three different runs:
Duplication via serialization: 3400 Milliseconds
Duplication via cloning: 110 Milliseconds
Duplication via serialization: 3410 Milliseconds
Duplication via cloning: 110 Milliseconds
Duplication via serialization: 3520 Milliseconds
Duplication via cloning: 110 Milliseconds
Despite the obviously huge time difference between serialization and cloning, you’ll also
notice that the serialization technique seems to vary significantly in its duration, while
cloning takes the same amount of time every time.
Chapter 12: Passing & Returning Objects 393](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-393-320.jpg)
![Adding cloneability further down a hierarchy
If you create a new class, its base class defaults to Object which defaults to non-
clonability (as you’ll see in the next section). As long as you don’t explicitly add
clonability, you won’t get it. But you can add it in at any layer, like this:
//: HorrorFlick.java
// You can insert Cloneability at any
// level of inheritance.
import java.util.*;
class Person {}
class Hero extends Person {}
class Scientist extends Person
implements Cloneable {
public Object clone() {
try {
return super.clone();
} catch (CloneNotSupportedException e) {
// this should never happen:
// It's Cloneable already!
throw new InternalError();
}
}
}
class MadScientist extends Scientist {}
public class HorrorFlick {
public static void main(String args[]) {
Person p = new Person();
Hero h = new Hero();
Scientist s = new Scientist();
MadScientist m = new MadScientist();
// p = (Person)p.clone(); // Compile error
// h = (Hero)h.clone(); // Compile error
s = (Scientist)s.clone();
m = (MadScientist)m.clone();
}
} ///:~
Before clonability was added, the compiler stopped you from trying to clone things. When
clonability is added in Scientist, then Scientist and all its descendants are cloneable.
Why this strange design?
If all this seems to be a strange scheme, that’s because it is. You may wonder why it
worked out this way. What is the meaning behind this design? What follows is not a
substantiated story – probably because much of the marketing around Java makes it out
to be a perfectly-designed language – but it does go a long way towards explaining how
things ended up the way they did.
Originally, Java was designed as a language to control hardware boxes, and definitely not
with the Internet in mind. In a general-purpose language like this, it makes sense that the
programmer be able to clone any object. Thus clone( ) was placed in the root class Object,
394 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-394-320.jpg)
![// Somehow make a copy of b
// and return that copy. This is a dummy
// copy, just to make the point:
return new BackOn();
}
public Object clone() {
// Doesn't call NoMore.clone():
return duplicate(this);
}
}
// Can't inherit from this, so can't override
// the clone method like in BackOn:
final class ReallyNoMore extends NoMore {}
public class CheckCloneable {
static Ordinary tryToClone(Ordinary ord) {
String id = ord.getClass().getName();
Ordinary x = null;
if(ord instanceof Cloneable) {
try {
System.out.println("Attempting " + id);
x = (Ordinary)((IsCloneable)ord).clone();
System.out.println("Cloned " + id);
} catch(CloneNotSupportedException e) {
System.out.println(
"Could not clone " + id);
}
}
return x;
}
public static void main(String args[]) {
// Upcasting:
Ordinary ord[] = {
new IsCloneable(),
new WrongClone(),
new NoMore(),
new TryMore(),
new BackOn(),
new ReallyNoMore(),
};
Ordinary x = new Ordinary();
// This won't compile, since clone() is
// protected in Object:
//! x = (Ordinary)x.clone();
// tryToClone() checks first to see if
// a class implements Cloneable:
for(int i = 0; i < ord.length; i++)
tryToClone(ord[i]);
}
} ///:~
The first class, Ordinary, represents the kinds of classes we’ve been seeing throughout the
book: no support for cloning, but as it turns out, no prevention of cloning either. But if
you have a handle to an Ordinary object that may have been upcast from a more derived
class, you can’t tell if it can be cloned or not.
Chapter 12: Passing & Returning Objects 397](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-397-320.jpg)
![2. Override clone( ).
3. Call super.clone( ) inside your clone( ).
4. Capture exceptions inside your clone( ).
This will produce the most convenient effects.
The copy-constructor
Cloning may seem to be a complicated process to set up. It may seem like there should be
an alternative. One approach that might occur to you (especially if you’re a C++
programmer) is to make a special constructor whose job it is to duplicate an object. In
C++, this is called the copy constructor. At first, this seems like the obvious solution. Here’s
an example:
//: CopyConstructor.java
// A constructor for copying an object
// of the same type, as an attempt to create
// a local copy.
class FruitQualities {
private int weight;
private int color;
private int firmness;
private int ripeness;
private int smell;
// etc.
FruitQualities() { // Default constructor
// do something meaningful...
}
// Other constructors:
// ...
// Copy constructor:
FruitQualities(FruitQualities f) {
weight = f.weight;
color = f.color;
firmness = f.firmness;
ripeness = f.ripeness;
smell = f.smell;
// etc.
}
}
class Seed {
// Members...
Seed() { /* Default constructor */ }
Seed(Seed s) { /* Copy constructor */ }
}
class Fruit {
private FruitQualities fq;
private int seeds;
private Seed[] s;
Fruit(FruitQualities q, int seedCount) {
fq = q;
seeds = seedCount;
Chapter 12: Passing & Returning Objects 399](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-399-320.jpg)
![s = new Seed[seeds];
for(int i = 0; i < seeds; i++)
s[i] = new Seed();
}
// Other constructors:
// ...
// Copy constructor:
Fruit(Fruit f) {
fq = new FruitQualities(f.fq);
// Call all Seed copy-constructors:
for(int i = 0; i < seeds; i++)
s[i] = new Seed(f.s[i]);
// Other copy-construction activities...
}
// To allow derived constructors (or other
// methods) to put in different qualities:
protected void addQualities(FruitQualities q) {
fq = q;
}
protected FruitQualities getQualities() {
return fq;
}
}
class Tomato extends Fruit {
Tomato() {
super(new FruitQualities(), 100);
}
Tomato(Tomato t) { // Copy-constructor
super(t); // Upcast for base copy-constructor
// Other copy-construction activities...
}
}
class ZebraQualities extends FruitQualities {
private int stripedness;
ZebraQualities() { // Default constructor
// do something meaningful...
}
ZebraQualities(ZebraQualities z) {
super(z);
stripedness = z.stripedness;
}
}
class GreenZebra extends Tomato {
GreenZebra() {
addQualities(new ZebraQualities());
}
GreenZebra(GreenZebra g) {
super(g); // Calls Tomato(Tomato)
// Restore the right qualities:
addQualities(new ZebraQualities());
}
void evaluate() {
400 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-400-320.jpg)
![ZebraQualities zq =
(ZebraQualities)getQualities();
// Do something with the qualities
// ...
}
}
public class CopyConstructor {
public static void ripen(Tomato t) {
// Use the "copy constructor":
t = new Tomato(t);
System.out.println("In ripen, t is a " +
t.getClass().getName());
}
public static void slice(Fruit f) {
f = new Fruit(f); // Hmmm... will this work?
System.out.println("In slice, f is a " +
f.getClass().getName());
}
public static void main(String args[]) {
Tomato tomato = new Tomato();
ripen(tomato); // OK
slice(tomato); // OOPS!
GreenZebra g = new GreenZebra();
ripen(g); // OOPS!
slice(g); // OOPS!
g.evaluate();
}
} ///:~
This seems a bit strange at first. Sure, fruit has qualities, but why not just put data
members representing those qualities directly into the Fruit class? There are two potential
reasons. The first is that you may want to easily insert or change the qualities. Notice
that Fruit has a protected addQualities( ) method to allow derived classes to do this (you
might think the logical thing to do is to have a protected constructor in Fruit that takes a
FruitQualities argument, but constructors don’t inherit so it wouldn’t be available in
second or greater level classes). By making the fruit qualities into a separate class, you
have greater flexibility, including the ability to change the qualities midway through the
lifetime of a particular Fruit object.
The second reason for making FruitQualities a separate object is in case you want to add
new qualities or change the behavior, via inheritance and polymorphism. Notice that for
GreenZebra (which really is a type of tomato – I’ve grown them and they’re fabulous),
the constructor calls addQualities( ) and passes it a ZebraQualities object, which is
derived from FruitQualities and so can be attached to the FruitQualities handle in the
base class. Of course, when GreenZebra uses the FruitQualities it must downcast it to
the right type (as seen in evaluate( )), but it always knows that type is ZebraQualities.
You’ll also see that there’s a Seed class, and that Fruit (which by definition carries its own
seeds) contains an array of Seeds.
Finally, notice that each class has a copy constructor, and that each copy constructor must
take care to call the copy constructors for the base class and member objects so as to
produce a deep copy. The copy constructor is tested inside the class CopyConstructor. The
method ripen( ) takes a Tomato argument and performs copy-construction on it in order
to duplicate the object:
Chapter 12: Passing & Returning Objects 401](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-401-320.jpg)
![t = new Tomato(t);
while slice( ) takes a more generic Fruit object and also duplicates it:
f = new Fruit(f);
These are tested with different kinds of Fruit in main( ). Here’s the output:
In ripen, t is a Tomato
In slice, f is a Fruit
In ripen, t is a Tomato
In slice, f is a Fruit
This is where the problem shows up. After the copy-construction that happens to the
Tomato inside slice( ), the result is no longer a Tomato object, but just a Fruit. It has lost
all its tomato-ness. Further, when you take a GreenZebra, both ripen( ) and slice( ) turn
it into a Tomato and a Fruit, respectively. Thus, unfortunately, the copy constructor
scheme is no good to us in Java when attempting to make a local copy of an object.
Why does it work in C++ and not Java?
The copy constructor is a fundamental part of C++, since it automatically makes a local
copy of an object. Yet the above example proves it does not work for Java. Why is this?
The issue is that in Java everything that we manipulate is a handle, while in C++ you can
have handle-like entities but you can also pass around the objects directly. That’s what the
C++ copy constructor is for: when you want to take an actual object and pass it in by
value, thus duplicating the object. So it works fine in C++, but you should keep in mind
that this scheme fails in Java, so don’t use it.
Read-only classes
While the local copy produced by clone( ) gives the desired results in the appropriate
cases, it is an example of forcing the programmer (the author of the method) to be
responsible for preventing the ill effects of aliasing. What if you’re making a library that’s
so general-purpose and commonly used that you cannot make the assumption that it will
always be cloned in the proper places? Or more likely, what if you want to allow aliasing
for efficiency – to prevent the needless duplication of objects – but you don’t want the
negative side effects of aliasing?
One solution is to create immutable objects. You can define a class such that no methods in
the class cause changes to the internal state of the object. In such a class, aliasing has no
impact since you can read only the internal state, so if many pieces of code are reading the
same object there’s no problem.
As a simple example of immutable objects, Java’s standard library contains “wrapper”
classes for all the primitive types. You may have already discovered that, if you want to
store an int inside a collection like a Vector (which takes only Object handles), you can
wrap your int inside the standard library Integer class:
//: ImmutableInteger.java
// The Integer class cannot be changed
import java.util.*;
public class ImmutableInteger {
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 10; i++)
402 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-402-320.jpg)
![v.addElement(new Integer(i));
// But how do you change the int
// inside the Integer?
}
} ///:~
The Integer class (as well as all the primitive “wrapper” classes) implements immutability
in a very simple fashion: they have no methods that allow you to change the object.
This means that if you do need an object that holds a primitive type that can be modified,
you must create it yourself. Fortunately, this is trivial:
//: MutableInteger.java
// A changeable wrapper class
import java.util.*;
class IntValue {
int n;
IntValue(int x) { n = x; }
public String toString() {
return Integer.toString(n);
}
}
public class MutableInteger {
public static void main(String args[]) {
Vector v = new Vector();
for(int i = 0; i < 10; i++)
v.addElement(new IntValue(i));
System.out.println(v);
for(int i = 0; i < v.size(); i++)
((IntValue)v.elementAt(i)).n++;
System.out.println(v);
}
} ///:~
Note that n is friendly to simplify coding.
Actually, IntValue can be even simpler if the default initialization to zero is adequate
(then you don’t need the constructor) and you don’t care about printing it out (then you
don’t need the toString( )):
class IntValue { int n; }
Fetching the element out and casting it is a bit awkward, but that’s a feature of Vector and
not IntValue.
Creating read-only classes
It’s possible to create your own read-only class. Here’s an example:
//: Immutable1.java
// Objects that cannot be modified
// are immune to aliasing.
public class Immutable1 {
private int data;
public Immutable1(int initVal) {
Chapter 12: Passing & Returning Objects 403](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-403-320.jpg)
![data = initVal;
}
public int read() { return data; }
public boolean nonzero() { return data != 0; }
public Immutable1 quadruple() {
return new Immutable1(data * 4);
}
static void f(Immutable1 i1) {
Immutable1 quad = i1.quadruple();
System.out.println("i1 = " + i1.read());
System.out.println("quad = " + quad.read());
}
public static void main(String args[]) {
Immutable1 x = new Immutable1(47);
System.out.println("x = " + x.read());
f(x);
System.out.println("x = " + x.read());
}
} ///:~
All data is private, and you’ll see that none of the public methods modify that data.
Indeed, the method that does appear to modify an object is quadruple( ), but this actually
creates a new Immutable1 object and leaves the original one untouched.
The method f( ) takes an Immutable1 object and performs various operations on it, and
the output of main( ) demonstrates that there is no change to x. Thus, x’s object could be
aliased many times without harm, because the Immutable1 class is designed to guarantee
that objects cannot be changed.
The drawback to immutability
Creating an immutable class seems at first to provide an elegant solution. However,
whenever you do need a modified object of that new type you must suffer the overhead of
a new object creation, as well as potentially causing more frequent garbage collections. For
some classes this is not a problem, but for others (such as the String class) it is
prohibitively expensive.
The solution is to create a companion class that can be modified. Then when you’re doing a
lot of modifications, you can switch to using the modifiable companion class and then
switch back to the immutable class when you’re done.
The above example can be modified to show this:
//: Immutable2.java
// A companion class for making changes
// to immutable objects.
class Mutable {
private int data;
public Mutable(int initVal) {
data = initVal;
}
public Mutable add(int x) {
data += x;
return this;
}
404 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-404-320.jpg)
![public Mutable multiply(int x) {
data *= x;
return this;
}
public Immutable2 makeImmutable2() {
return new Immutable2(data);
}
}
public class Immutable2 {
private int data;
public Immutable2(int initVal) {
data = initVal;
}
public int read() { return data; }
public boolean nonzero() { return data != 0; }
public Immutable2 add(int x) {
return new Immutable2(data + x);
}
public Immutable2 multiply(int x) {
return new Immutable2(data * x);
}
public Mutable makeMutable() {
return new Mutable(data);
}
public static Immutable2 modify1(Immutable2 y){
Immutable2 val = y.add(12);
val = val.multiply(3);
val = val.add(11);
val = val.multiply(2);
return val;
}
// This produces the same result:
public static Immutable2 modify2(Immutable2 y){
Mutable m = y.makeMutable();
m.add(12).multiply(3).add(11).multiply(2);
return m.makeImmutable2();
}
public static void main(String args[]) {
Immutable2 i2 = new Immutable2(47);
Immutable2 r1 = modify1(i2);
Immutable2 r2 = modify2(i2);
System.out.println("i2 = " + i2.read());
System.out.println("r1 = " + r1.read());
System.out.println("r2 = " + r2.read());
}
} ///:~
Immutable2 contains methods that, as before, preserve the immutability of the objects by
producing new objects whenever a modification is desired. These are the add( ) and
multiply( ) methods. The companion class is called Mutable, and it also has add( ) and
multiply( ) methods, but these modify the Mutable object itself rather than making a new
one. In addition, Mutable has a method to use its data to produce an Immutable2 object,
and vice versa.
The two static methods modify1( ) and modify2( ) show two different approaches to
producing the same result. In modify1( ), everything is done within the Immutable2
Chapter 12: Passing & Returning Objects 405](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-405-320.jpg)
![class and you can see that four new Immutable2 objects are created in the process (and
each time val is reassigned, the previous object becomes garbage).
In the method modify2( ), you can see that the first action is to take the Immutable2 y
and produce a Mutable from it (this is just like calling clone( ) as you saw earlier, but
this time a different type of object is created). Then the Mutable object is used to perform
a lot of change operations without requiring the creation of many new objects. Finally it’s
turned back into an Immutable2. Here, two new objects are created (the Mutable and the
result Immutable2) instead of four.
This approach makes sense, then, when:
1. You need immutable objects and
2. You often need to make a lot of modifications or
3. It’s very expensive to create new immutable objects
Immutable String s
Consider the following code:
//: Stringer.java
public class Stringer {
static String upcase(String s) {
return s.toUpperCase();
}
public static void main(String[] args) {
String q = new String("howdy");
System.out.println(q); // howdy
String qq = upcase(q);
System.out.println(qq); // HOWDY
System.out.println(q); // howdy
}
} ///:~
When q is passed in to upcase( ) it’s actually a copy of the handle to q. The object this
handle is connected to stays put in a single physical location. The handles are copied as
they are passed around.
Looking at the definition for upcase( ), you can see that the handle that’s passed in has
the name s, and it exists for only as long as the body of upcase( ) is being executed. When
upcase( ) completes, the local handle s vanishes. upcase( ) returns the result which is the
original string with all the characters set to uppercase. Of course, it actually returns a
handle to the result. But it turns out that the handle that it returns is for a new object,
and the original q is left alone. How does this happen?
Implicit constants
If you say:
String s = "asdf";
String x = Stringer.upcase(s);
do you really want the upcase( ) method to change the argument? Generally you don’t,
because an argument usually looks to the reader of the code as a piece of information
406 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-406-320.jpg)
![//: ImmutableStrings.java
// Demonstrating StringBuffer
public class ImmutableStrings {
public static void main(String args[]) {
String foo = "foo";
String s = "abc" + foo +
"def" + Integer.toString(47);
System.out.println(s);
// The "equivalent" using StringBuffer:
StringBuffer sb =
new StringBuffer("abc"); // Creates String!
sb.append(foo);
sb.append("def"); // Creates String!
sb.append(Integer.toString(47));
System.out.println(sb);
}
} ///:~
In the creation of String s, the compiler is actually doing the rough equivalent of the
subsequent code that uses sb: a StringBuffer is created and append( ) is used to add new
characters directly into the StringBuffer object (rather than making new copies each
time). While this is more efficient, it’s worth noting that each time you create a quoted
character string like “abc” and “def”, the compiler actually turns those into String
objects. Thus, there may be more objects created than you expect, despite the efficiency
afforded through StringBuffer.
The String and StringBuffer classes
Here is an overview of the methods available for both String and StringBuffer, so you can
get a feel for the way they interact. These tables don’t contain every single method, but
rather the ones that are important to this discussion. Methods that are overloaded are
summarized in a single row.
First, the String class:
Method Arguments, Overloading Use
Constructor Overloaded: Default, String, Creating String objects.
StringBuffer, char arrays,
byte arrays.
length( ) Number of characters in
String.
charAt() int Index The char at a location in the
String.
getChars( ), getBytes( ) The beginning and end from Copy chars or bytes into an
which to copy, the array to external array.
copy into, an index into the
destination array.
toCharArray( ) Produces a char[] containing
the characters in the String.
408 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-408-320.jpg)
![Method Arguments, Overloading Use
changes need to be made.
valueOf( ) Overloaded: Object, char[], Returns a String containing
char[] and offset and count, a character representation
boolean, char, int, long, of the argument.
float, double.
intern( ) Produces one and only one
String handle for each
unique character sequence
You can see that every String method carefully returns a new String object when it’s
necessary to change the contents. Also notice that if the contents don’t need changing the
method will just return a handle to the original String. This saves storage and overhead.
Here’s the StringBuffer class:
Method Arguments, overloading Use
Constructor Overloaded: default, Create a new StringBuffer
length of buffer to create, object.
String to create from.
toString( ) Creates a String from this
StringBuffer.
length( ) Number of characters in the
StringBuffer.
capacity( ) Returns current number of
spaces allocated
ensureCapacity( ) Integer indicating desired Makes the StringBuffer hold at
capacity. least the desired number of
spaces.
setLength( ) Integer indicating new Truncates or expands the
length of character string previous character string. If
in buffer. expanding, pads with nulls.
charAt( ) Integer indicating the Returns the char at that location
location of the desired in the buffer.
element.
setCharAt( ) Integer indicating the Modifies the value at that
location of the desired location.
element, and the new
char value for the
element.
getChars( ) The beginning and end Copy chars into an external
from which to copy, the array. There’s no getBytes( ) as
array to copy into, an in String.
index into the destination
array.
410 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-410-320.jpg)
![Method Arguments, overloading Use
append( ) Overloaded: Object, The argument is converted to a
String, char[], char[] string and appended to the end
with offset and length, of the current buffer, increasing
boolean, char, int, long, the buffer if necessary.
float, double.
insert( ) Overloaded, each with a The second argument is
first argument of the converted to a string and
offset at which to start inserted into the current buffer
inserting: Object, String, beginning at the offset. The
char[], boolean, char, buffer is increased if necessary.
int, long, float, double.
reverse( ) The order of the characters in
the buffer is reversed.
The most commonly-used method is append( ), which is used by the compiler when
evaluating String expressions containing the ‘+’ and ‘+=‘ operators. The insert( ) method
has a similar form, and both methods perform significant manipulations to the buffer
itself rather than creating new objects.
String s are special
By now you’ve seen that the String class is not just another class in Java – there are a lot
of special cases in String, not the least of which is that it’s a built-in class and
fundamental to Java. Then there’s the fact that a quoted character string is converted to a
String by the compiler, and the special overloaded operators + and +=. In this chapter
you’ve seen the remaining special case: the carefully-built immutability using the
companion StringBuffer and some extra magic in the compiler.
Summary
Because everything is a handle in Java, and because every object is created on the heap and
garbage collected only when it is no longer used, the flavor of object manipulation
changes, especially when passing and returning objects. For example, in C or C++, if you
wanted to initialize some piece of storage in a method, you’d probably request that the
user pass the address of that piece of storage into the method. Otherwise you’d have to
worry about who was responsible for the destruction of that storage. Thus the interface
and understanding of such methods is more complicated. But in Java, you never have to
worry about responsibility or whether an object will still exist when it is needed, since
that is always taken care of for you. This means that your programs can create an object
at the point that it is needed, and no sooner, and never worry about the mechanics of
passing around responsibility for that object: you simply pass the handle. Sometimes the
simplification that this provides is unnoticed, other times it is staggering.
The down side to all this underlying magic is twofold:
1. You always take the efficiency hit for the extra memory management
(although this can be quite small), and there’s always a slight amount of
uncertainty about the time something can take to run (since the garbage
collector can be forced into action whenever you get low on memory). For most
applications, the benefits outweigh the drawbacks, and particularly time-
critical sections can be written using native methods (see Appendix A).
Chapter 12: Passing & Returning Objects 411](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-411-320.jpg)
![You can have any number of CheckboxGroups on a form.
Drop-down lists
Like a group of radio buttons, a drop-down list is a way to force the user to select only
one element from a group of possibilities. However, it’s a much more compact way to
accomplish this, and it’s easier to change the elements of the list without surprising the
user (you can change radio buttons dynamically, but that tends to be visibly jarring).
Java’s Choice box is not like the combo box in Windows, which lets you select from a list
or type in your own selection. With a Choice box you choose one and only one element
from the list. In the following example, the Choice box starts with a certain number of
entries, and then new entries are added to the box when a button is pressed. This allows
you to see some interesting behaviors in Choice boxes:
//: Choice1.java
// Using drop-down lists
import java.awt.*;
import java.applet.*;
public class Choice1 extends Applet {
String description[] = { "Ebullient", "Obtuse",
"Recalcitrant", "Brilliant", "Somnescent",
"Timorous", "Florid", "Putrescent" };
TextField t = new TextField(30);
Choice c = new Choice();
Button b = new Button("Add items");
int count = 0;
public void init() {
t.setEditable(false);
for(int i = 0; i < 4; i++)
c.addItem(description[count++]);
add(t);
add(c);
add(b);
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(c))
t.setText("index: " + c.getSelectedIndex() +
" " + (String)arg);
else if(evt.target.equals(b)) {
if(count < description.length)
c.addItem(description[count++]);
}
else
return super.action(evt, arg);
return true;
}
} ///:~
The TextField displays the “selected index” which is the sequence number of the currently
selected element, as well as the String representation of the second argument of action( ),
which is in this case the string that was selected.
428 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-428-320.jpg)
![When you run this applet, pay attention to the way in which the size of the Choice box is
determined: in Windows, the size is fixed from the first time you drop down the list. This
means that if you drop down the list, then add more elements to the list, the elements will
be there but the drop-down list won’t get any longer2 (you can scroll through the
elements). However, if you add all the elements before the first time the list is dropped
down, then it will be sized correctly. Of course, the user will expect to see the whole list
when it’s dropped down, and so this behavior puts some significant limitations on adding
elements to Choice boxes.
List boxes
List boxes are significantly different from Choice boxes, and not just in appearance. While
a Choice box drops down when you activate it, a List occupies some fixed number of lines
on a screen all the time, and doesn’t change. In addition, a List allows multiple selection: if
you click on more than one item the original item stays highlighted and you can select as
many as you want. If you want to see the items in a list, you simply call
getSelectedItems( ) which produces an array of String of the item that’s been selected. To
remove an item from a group you have to click it again.
A problem with a List is that the default action is double-clicking, not single clicking. A
single click adds or removes elements from the selected group, and a double click calls
action( ). One way around this is to re-educate your user, which is the assumption made
in the following program:
//: List1.java
// Using lists with action()
import java.awt.*;
import java.applet.*;
public class List1 extends Applet {
String flavors[] = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
// Show 6 items, allow multiple selection:
List lst = new List(6, true);
TextArea t = new TextArea(flavors.length, 30);
Button b = new Button("test");
int count = 0;
public void init() {
t.setEditable(false);
for(int i = 0; i < 4; i++)
lst.addItem(flavors[count++]);
add(t);
add(lst);
add(b);
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(lst)) {
t.setText("");
String[] items = lst.getSelectedItems();
2 This behavior is apparently a bug and will be fixed in a later version of Java.
Chapter 13: Creating Windows & Applets 429](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-429-320.jpg)
![for(int i = 0; i < items.length; i++)
t.appendText(items[i] + "n");
}
else if(evt.target.equals(b)) {
if(count < flavors.length)
lst.addItem(flavors[count++], 0);
}
else
return super.action(evt, arg);
return true;
}
} ///:~
When you press the button it adds items to the top of the list (because of the second
argument 0 to addItem( )). Adding elements to a List is more reasonable than the Choice
box because users expect to scroll a list box (for one thing, it has a built-in scroll bar) but
they don’t expect to have to figure out how to get a drop-down list to scroll, as in the
previous example.
However, the only way for action( ) to be called is through a double-click. If you need to
monitor other activities that the user is doing on your List (in particular, single clicks) you
must take an alternative approach.
handleEvent( )
So far we’ve been using action( ), but there’s another method that gets first crack at
everything: handleEvent( ). Any time an event happens, it happens “over” or “to” a
particular object. The handleEvent( ) method for that object is automatically called and
an Event object is created and passed to handleEvent( ). The default handleEvent( )
(which is defined in Component, the base class for virtually all the “controls” in the AWT)
will call either action( ) as we’ve been using, or other similar methods to indicate mouse
activity, keyboard activity or to indicate that the focus has moved. We’ll look at those in a
later section in this chapter.
What if these other methods – action( ) in particular – don’t satisfy your needs? In the
case of List, for example, when you want to catch single mouse clicks but action( )
responds to only double clicks? The solution is to override handleEvent( ) for your applet,
which after all is derived from Applet and can therefore override any non-final methods.
When you override handleEvent( ) for the applet you’re getting all the applet events
before they are routed, so you cannot just assume “it’s to do with my button so I can
assume it’s been pressed” since that’s true only for action( ). Inside handleEvent( ) it’s
possible that the button has the focus and someone is typing to it. Whether it makes sense
or not, those are events that you can detect and act upon in handleEvent( ).
To modify the List example so that it will react to single mouse-clicks, the button detection
will be left in action( ) but the code to handle the List will be moved into handleEvent( )
as follows:
//: List2.java
// Using lists with handleEvent()
import java.awt.*;
import java.applet.*;
public class List2 extends Applet {
String flavors[] = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
430 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-430-320.jpg)
!["Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
// Show 6 items, allow multiple selection:
List lst = new List(6, true);
TextArea t = new TextArea(flavors.length, 30);
Button b = new Button("test");
int count = 0;
public void init() {
t.setEditable(false);
for(int i = 0; i < 4; i++)
lst.addItem(flavors[count++]);
add(t);
add(lst);
add(b);
}
public boolean handleEvent(Event evt) {
if(evt.id == Event.LIST_SELECT ||
evt.id == Event.LIST_DESELECT) {
if(evt.target.equals(lst)) {
t.setText("");
String[] items = lst.getSelectedItems();
for(int i = 0; i < items.length; i++)
t.appendText(items[i] + "n");
}
}
else
return super.handleEvent(evt);
return true;
}
public boolean action(Event evt, Object arg) {
if(evt.target.equals(b)) {
if(count < flavors.length)
lst.addItem(flavors[count++], 0);
}
else
return super.action(evt, arg);
return true;
}
} ///:~
The example is the same as before except for the addition of handleEvent( ). Inside, a
check is made to see whether a list selection or deselection has occurred. Now remember
handleEvent( ) is being overridden for the applet, so that means this occurrence could be
anywhere on the form, so it could be happening to another list. Thus you must also check
to see what the target is (although in this case there’s only one list on the applet so we
could have made the assumption that all list events must be about that list – this is bad
practice since it’s going to be a problem as soon as another list is added). If the list
matches the one we’re interested in the same code as before will do the trick.
Notice that the form for handleEvent( ) is similar to action( ): if you deal with a
particular event you return true, but if you’re not interested in any of the other events
via handleEvent( ) you must return super.handleEvent(evt). This is very important
because if you don’t, none of the other event-handling code will get called. For example,
try commenting out the return super.handleEvent(evt) in the above code. You’ll discover
that action( ) never gets called, certainly not what you want. Thus, for both action( ) and
handleEvent( ) it’s important to follow the above format and always call the base-class
version of the method (fortunately, these kinds of bug-prone details are relegated to Java
Chapter 13: Creating Windows & Applets 431](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-431-320.jpg)
![TextField t =
(TextField)parent.h.get("mouseDown");
t.setText(evt.toString());
return true;
}
public boolean mouseDrag(Event evt,int x,int y){
TextField t =
(TextField)parent.h.get("mouseDrag");
t.setText(evt.toString());
return true;
}
public boolean mouseEnter(Event evt,int x,int y){
TextField t =
(TextField)parent.h.get("mouseEnter");
t.setText(evt.toString());
return true;
}
public boolean mouseExit(Event evt,int x,int y){
TextField t =
(TextField)parent.h.get("mouseExit");
t.setText(evt.toString());
return true;
}
public boolean mouseMove(Event evt,int x,int y){
TextField t =
(TextField)parent.h.get("mouseMove");
t.setText(evt.toString());
return true;
}
public boolean mouseUp(Event evt,int x,int y) {
TextField t =
(TextField)parent.h.get("mouseUp");
t.setText(evt.toString());
return true;
}
}
public class AutoEvent extends Applet {
Hashtable h = new Hashtable();
String event[] = {
"keyDown", "keyUp", "lostFocus",
"gotFocus", "mouseDown", "mouseUp",
"mouseMove", "mouseDrag", "mouseEnter",
"mouseExit"
};
MyButton
b1 = new MyButton(this, Color.blue, "test1"),
b2 = new MyButton(this, Color.red, "test2");
public void init() {
setLayout(new GridLayout(event.length+1,2));
for(int i = 0; i < event.length; i++) {
TextField t = new TextField();
t.setEditable(false);
add(new Label(event[i], Label.CENTER));
add(t);
h.put(event[i], t);
Chapter 13: Creating Windows & Applets 439](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-439-320.jpg)
![}
add(b1);
add(b2);
}
} ///:~
You can see the constructor uses the technique of using the same name for the argument
as what it’s assigned to, and differentiating between the two using this:
this.label = label;
The paint( ) method starts out simple: it fills a “round rectangle” with the button’s color,
and then draws a black line around it. Notice the use of size( ) to determine the width and
height of the component (in pixels, of course). After this, paint( ) seems quite complicated
because there’s a lot of calculation going on to figure out how to center the button’s label
inside the button using the “font metrics.” You can get a pretty good idea of what’s going
on by looking at the method call, and it turns out that this is pretty stock code, so you can
just cut and paste it when you want to center a label inside any component.
You can’t understand exactly how the keyDown( ), keyUp( ) etc. methods work until you
look down at the AutoEvent class. This contains a Hashtable to hold the strings
representing the type of event and the TextField where information about that event is
held. Of course, these could have been created statically rather than putting them in a
Hashtable, but I think you’ll agree that it’s a lot easier to use and change. In particular, if
you need to add or remove a new type of event in AutoEvent, you simply add or remove a
string in the event[] array – everything else happens automatically.
The place where you look up the strings is in the keyDown( ), keyUp( ) etc. methods back
in MyButton. Each of these methods uses the parent handle to reach back to the parent
window. Since that parent is an AutoEvent it contains the Hashtable h, and the get( )
method, when provided with the appropriate String, will produce a handle to an Object
which we happen to know is a TextField – so it is cast to that. Then the Event object is
converted to its String representation which is displayed in the TextField.
It turns out this example is rather fun to play with since you can really see what’s going
on with the events in your program.
Applet restrictions
For safety’s sake, applets are quite restricted and there are many things you can’t do. You
can generally answer the question of what an applet is able to do by looking at what it is
supposed to do: extend the functionality of a Web page in a browser. Since, as a net surfer,
you never really know if a Web page is from a friendly place or not, you want any code
that it runs to be very safe. So the biggest restrictions you’ll notice are probably:
1) An applet can’t touch the local disk. This means writing or reading, since you wouldn’t
want an applet to read and transmit important information about you across the Web.
Writing is prevented, of course, since that would be an open invitation to a virus.
2) An applet can’t have menus. This is probably less oriented towards safety and more
towards reducing confusion. You may have noticed that an applet looks like it blends right
in as part of a Web page; you often don’t see the boundaries of the applet. There’s no
frame or title bar to hang the menu from, other than the one belonging to the Web
browser. Perhaps the design could be changed to allow you to merge your applet menu
with the browser menu – that would not only be complicated but would probably get a bit
too close to the edge of safety by allowing the applet to affect its environment.
440 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-440-320.jpg)
![import java.awt.*;
public class Menu1 extends Frame {
String flavors[] = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
TextField t = new TextField("No flavor", 30);
MenuBar mb1 = new MenuBar();
Menu f = new Menu("File");
Menu m = new Menu("Flavors");
Menu s = new Menu("Safety");
// Alternative approach:
CheckboxMenuItem safety[] = {
new CheckboxMenuItem("Guard"),
new CheckboxMenuItem("Hide")
};
MenuItem file[] = {
new MenuItem("Open"),
new MenuItem("Exit")
};
// A second menu bar to swap to:
MenuBar mb2 = new MenuBar();
Menu fooBar = new Menu("fooBar");
MenuItem other[] = {
new MenuItem("Foo"),
new MenuItem("Bar"),
new MenuItem("Baz"),
};
Button b = new Button("Swap Menus");
public Menu1() {
for(int i = 0; i < flavors.length; i++) {
m.add(new MenuItem(flavors[i]));
// Add separators at intervals:
if((i+1) % 3 == 0)
m.addSeparator();
}
for(int i = 0; i < safety.length; i++)
s.add(safety[i]);
f.add(s);
for(int i = 0; i < file.length; i++)
f.add(file[i]);
mb1.add(f);
mb1.add(m);
setMenuBar(mb1);
t.setEditable(false);
add("Center", t);
// Set up the system for swapping menus:
add("North", b);
for(int i = 0; i < other.length; i++)
fooBar.add(other[i]);
mb2.add(fooBar);
}
public boolean handleEvent(Event evt) {
if(evt.id == Event.WINDOW_DESTROY)
System.exit(0);
Chapter 13: Creating Windows & Applets 443](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-443-320.jpg)
![else
return super.handleEvent(evt);
return true;
}
public boolean action(Event evt, Object arg) {
if(evt.target.equals(b)) {
MenuBar m = getMenuBar();
if(m == mb1) setMenuBar(mb2);
else if (m == mb2) setMenuBar(mb1);
}
else if(evt.target instanceof MenuItem) {
if(arg.equals("Open")) {
String s = t.getText();
boolean chosen = false;
for(int i = 0; i < flavors.length; i++)
if(s.equals(flavors[i])) chosen = true;
if(!chosen)
t.setText("Choose a flavor first!");
else
t.setText("Opening "+ s +". Mmm, mm!");
}
else if(evt.target.equals(file[1]))
System.exit(0);
// CheckboxMenuItems cannot use String
// matching; you must match the target:
else if(evt.target.equals(safety[0]))
t.setText("Guard the Ice Cream! " +
"Guarding is " + safety[0].getState());
else if(evt.target.equals(safety[1]))
t.setText("Hide the Ice Cream! " +
"Is it cold? " + safety[1].getState());
else
t.setText(arg.toString());
}
else
return super.action(evt, arg);
return true;
}
public static void main(String args[]) {
Menu1 f = new Menu1();
f.resize(300,200);
f.show();
}
} ///:~
In this program I avoided the typical long lists of add( ) calls for each menu, because that
seemed like a lot of unnecessary typing. Instead I placed the menu items into arrays, and
then simply stepped through each array calling add( ) in a for loop. This also means that
adding or subtracting a menu item is less tedious.
As an alternative approach (which I find less desirable since it requires more typing) the
CheckboxMenuItems are created in an array of handles called safety; this is true for the
arrays file and other as well.
This program creates not one but two MenuBars to demonstrate that menu bars can be
actively swapped while the program is running. You can see how a MenuBar is made up
444 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-444-320.jpg)
![else
return super.handleEvent(evt);
return true;
}
}
public class ToeTest extends Frame {
TextField rows = new TextField("3");
TextField cols = new TextField("3");
public ToeTest() {
setTitle("Toe Test");
Panel p = new Panel();
p.setLayout(new GridLayout(2,2));
p.add(new Label("Rows", Label.CENTER));
p.add(rows);
p.add(new Label("Columns", Label.CENTER));
p.add(cols);
add("North", p);
add("South", new Button("go"));
}
public boolean handleEvent(Event evt) {
if(evt.id == Event.WINDOW_DESTROY)
System.exit(0);
else
return super.handleEvent(evt);
return true;
}
public boolean action(Event evt, Object arg) {
if(arg.equals("go")) {
Dialog d = new ToeDialog(
this,
Integer.parseInt(rows.getText()),
Integer.parseInt(cols.getText()));
d.show();
}
else
return super.action(evt, arg);
return true;
}
public static void main(String args[]) {
Frame f = new ToeTest();
f.resize(200,100);
f.show();
}
} ///:~
The ToeButton class keeps a handle to its parent, which must be of type ToeDialog. As
before, this introduces high coupling because a ToeButton can be used only with a
ToeDialog but it solves a number of problems, and in truth it doesn’t seem like such a bad
solution because there’s no other kind of dialog that’s keeping track of whose turn it is. Of
course, you can take another approach which is to make ToeDialog.turn a static member
of ToeButton, which eliminates the coupling, but prevents you from having more than
one ToeDialog at a time (more than one that works properly, anyway).
The paint( ) method is concerned with the graphics: drawing the square around the button
and drawing the “x” or the “o.” This is full of tedious calculations, but it’s
straightforward.
Chapter 13: Creating Windows & Applets 447](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-447-320.jpg)
![filename.setEditable(false);
p = new Panel();
p.setLayout(new GridLayout(2,1));
p.add(filename);
p.add(directory);
add("North", p);
}
public boolean handleEvent(Event evt) {
if(evt.id == Event.WINDOW_DESTROY)
System.exit(0);
else
return super.handleEvent(evt);
return true;
}
public boolean action(Event evt, Object arg) {
if(evt.target.equals(open)) {
// Two arguments, defaults to open file:
FileDialog d = new FileDialog(this,
"What file do you want to open?");
d.setFile("*.java"); // Filename filter
d.setDirectory("."); // Current directory
d.show();
String openFile;
if((openFile = d.getFile()) != null) {
filename.setText(openFile);
directory.setText(d.getDirectory());
} else {
filename.setText("You pressed cancel");
directory.setText("");
}
}
else if(evt.target.equals(save)) {
FileDialog d = new FileDialog(this,
"What file do you want to save?",
FileDialog.SAVE);
d.setFile("*.java");
d.setDirectory(".");
d.show();
String saveFile;
if((saveFile = d.getFile()) != null) {
filename.setText(saveFile);
directory.setText(d.getDirectory());
} else {
filename.setText("You pressed cancel");
directory.setText("");
}
}
else
return super.action(evt, arg);
return true;
}
public static void main(String args[]) {
Frame f = new FileDialogTest();
f.resize(250,110);
f.show();
}
Chapter 13: Creating Windows & Applets 449](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-449-320.jpg)
![}
This doesn’t work, but it will drive you crazy trying to figure out why, since everything
will compile and run just fine – except that closing the window won’t exit the program.
Can you see the problem? It’s in the name of the method: WindowClosing( ) instead of
windowClosing( ). A simple slip in capitalization results in the addition of a completely
new method. However, this is not the method that’s called when the window is closing, so
you don’t get the desired results.
Making windows and applets
with the Java 1.1 AWT
Often you’ll want to be able to create a class that can be invoked as both a window and an
applet. To accomplish this, you simply add a main( ) to your applet that builds an
instance of the applet inside a Frame. As a simple example, let’s look at
Button2New.java modified to work as both an application and an applet:
//: Button2NewB.java
// An application and an applet
import java.awt.*;
import java.awt.event.*; // Must add this
import java.applet.*;
public class Button2NewB extends Applet {
Button
b1 = new Button("Button 1"),
b2 = new Button("Button 2");
TextField t = new TextField(20);
public void init() {
b1.addActionListener(new B1());
b2.addActionListener(new B2());
add(b1);
add(b2);
add(t);
}
class B1 implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Button 1");
}
}
class B2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Button 2");
}
}
// To close the application:
static class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
}
// A main() for the application:
public static void main(String args[]) {
Button2NewB applet = new Button2NewB();
Frame aFrame = new Frame("Button2NewB");
Chapter 13: Creating Windows & Applets 457](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-457-320.jpg)
![t1.getText().length());
// Stop regular character from appearing:
e.consume();
}
}
class B1 implements ActionListener {
public void actionPerformed(ActionEvent e) {
s = t1.getSelectedText();
if(s.length() == 0) s = t1.getText();
t1.setEditable(true);
}
}
class B2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
t1.setText("Inserted by Button 2: " + s);
t1.setEditable(false);
}
}
public static void main(String args[]) {
TextNew applet = new TextNew();
Frame aFrame = new Frame("TextNew");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
The TextField t3 is included as a place to report when the action listener for the TextField
t1 is fired. You’ll see that the action listener for a TextField is fired only when you press
the “enter” key.
The TextField t1 has several listeners attached to it. The T1 listener copies all text from t1
into t2, and the T1K listener forces all characters to upper case. You’ll notice that the two
work together, and if you add the T1K listener after you add the T1 listener, it doesn’t
matter: all characters will still be forced to upper case in both text fields. So it would seem
that keyboard events are always fired before TextComponent events, and if you want the
characters in t2 to retain the original case that was typed in, you’ll have to do some extra
work.
T1K has some other activities of interest. You must detect a backspace (since you’re
controlling everything now) and perform the deletion. The caret must be explicitly set to
the end of the field, otherwise it won’t behave as you expect. And finally, to prevent the
original character from being handled by the default mechanism, the event must be
“consumed” using the consume( ) method that exists for event objects. This tells the
system to stop firing the rest of the event handlers for this particular event.
This example also quietly demonstrates one of the benefits of the design of inner classes.
Notice that in the inner class:
Chapter 13: Creating Windows & Applets 461](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-461-320.jpg)
![}
class B1L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t5.append(t1.getText() + "n");
}
}
class B2L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t2.setText("Inserted by Button 2");
t2.append(": " + t1.getText());
t5.append(t2.getText() + "n");
}
}
class B3L implements ActionListener {
public void actionPerformed(ActionEvent e) {
String s = " Replacement ";
t2.replaceRange(s, 3, 3 + s.length());
}
}
class B4L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t2.insert(" Inserted ", 10);
}
}
public static void main(String args[]) {
TextAreaNew applet = new TextAreaNew();
Frame aFrame = new Frame("TextAreaNew");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,725);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
You’ll notice that you can control the scrollbars only at the time of construction of the
TextArea. Also, even if a TextArea doesn’t have a scrollbar, you can move the cursor such
that scrolling will be forced (you can see this behavior by playing with the example).
Check boxes & radio buttons
As noted previously, check boxes and radio buttons are both created with the same class,
Checkbox, but radio buttons are Checkboxes placed into a CheckboxGroup. In either
case, the interesting event is ItemEvent, for which you create an ItemListener.
When dealing with a group of check boxes or radio buttons, you have a choice: either
create a new inner class to handle the event for each different Checkbox, or create one
inner class that determines which Checkbox was clicked and register a single object of
that inner class with each Checkbox object. The following example shows both
approaches:
//: RadioCheckNew.java
Chapter 13: Creating Windows & Applets 463](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-463-320.jpg)
![// Radio buttons and Check Boxes in Java 1.1
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class RadioCheckNew extends Applet {
TextField t = new TextField(30);
Checkbox cb[] = {
new Checkbox("Check Box 1"),
new Checkbox("Check Box 2"),
new Checkbox("Check Box 3") };
CheckboxGroup g = new CheckboxGroup();
Checkbox
cb4 = new Checkbox("four", g, false),
cb5 = new Checkbox("five", g, true),
cb6 = new Checkbox("six", g, false);
public void init() {
t.setEditable(false);
add(t);
ILCheck il = new ILCheck();
for(int i = 0; i < cb.length; i++) {
cb[i].addItemListener(il);
add(cb[i]);
}
cb4.addItemListener(new IL4());
cb5.addItemListener(new IL5());
cb6.addItemListener(new IL6());
add(cb4); add(cb5); add(cb6);
}
// Checking the source:
class ILCheck implements ItemListener {
public void itemStateChanged(ItemEvent e) {
for(int i = 0; i < cb.length; i++) {
if(e.getSource().equals(cb[i])) {
t.setText("Check box " + (i + 1));
return;
}
}
}
}
// vs. an individual class for each item:
class IL4 implements ItemListener {
public void itemStateChanged(ItemEvent e) {
t.setText("Radio button four");
}
}
class IL5 implements ItemListener {
public void itemStateChanged(ItemEvent e) {
t.setText("Radio button five");
}
}
class IL6 implements ItemListener {
public void itemStateChanged(ItemEvent e) {
t.setText("Radio button six");
}
464 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-464-320.jpg)
![}
public static void main(String args[]) {
RadioCheckNew applet = new RadioCheckNew();
Frame aFrame = new Frame("RadioCheckNew");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
ILCheck has the advantage that it automatically adapts when you add or subtract
Checkboxes. Of course, you can use this with radio buttons as well. It should be used,
however, only when your logic is general enough to support this approach, otherwise
you’ll end up with a cascaded if statement, a sure sign you should revert to using
independent listener classes.
Drop-down lists
Drop-down lists (Choice) in Java 1.1 also use ItemListeners to notify you when a choice
has changed:
//: ChoiceNew.java
// Drop-down lists with Java 1.1
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class ChoiceNew extends Applet {
String description[] = { "Ebullient", "Obtuse",
"Recalcitrant", "Brilliant", "Somnescent",
"Timorous", "Florid", "Putrescent" };
TextField t = new TextField(100);
Choice c = new Choice();
Button b = new Button("Add items");
int count = 0;
public void init() {
t.setEditable(false);
for(int i = 0; i < 4; i++)
c.addItem(description[count++]);
add(t);
add(c);
add(b);
c.addItemListener(new CL());
b.addActionListener(new BL());
}
class CL implements ItemListener {
public void itemStateChanged(ItemEvent e) {
t.setText("index: " + c.getSelectedIndex()
+ " " + e.toString());
Chapter 13: Creating Windows & Applets 465](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-465-320.jpg)
![}
}
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(count < description.length)
c.addItem(description[count++]);
}
}
public static void main(String args[]) {
ChoiceNew applet = new ChoiceNew();
Frame aFrame = new Frame("ChoiceNew");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(750,100);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Nothing else here is particularly new (except that Java 1.1 has significantly fewer bugs in
the UI classes).
Lists
You’ll recall that one of the problems with the Java 1.0 List design is that it took extra
work to make it do what you’d expect: react to a single click on one of the list elements.
Java 1.1 has solved this problem:
//: ListNew.java
// Java 1.1 Lists are easier to use
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class ListNew extends Applet {
String flavors[] = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
// Show 6 items, allow multiple selection:
List lst = new List(6, true);
TextArea t = new TextArea(flavors.length, 30);
Button b = new Button("test");
int count = 0;
public void init() {
t.setEditable(false);
for(int i = 0; i < 4; i++)
lst.addItem(flavors[count++]);
add(t);
add(lst);
add(b);
466 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-466-320.jpg)
![lst.addItemListener(new LL());
b.addActionListener(new BL());
}
class LL implements ItemListener {
public void itemStateChanged(ItemEvent e) {
t.setText("");
String[] items = lst.getSelectedItems();
for(int i = 0; i < items.length; i++)
t.append(items[i] + "n");
}
}
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(count < flavors.length)
lst.addItem(flavors[count++], 0);
}
}
public static void main(String args[]) {
ListNew applet = new ListNew();
Frame aFrame = new Frame("ListNew");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
You can see that no extra logic is required to support a single click on a list item. You just
attach a listener like you do everywhere else.
Menus
The event handling for menus does seem to benefit from the new Java 1.1 event model, but
Java’s approach to menus is still very messy and requires a lot of hand-coding. The right
medium for a menu seems to be a resource rather than lots of code. Keep in mind that
program-building tools will generally handle the creation of menus for you, so that will
reduce the pain somewhat (as long as they will also handle the maintenance!).
In addition, you’ll find the events for menus are inconsistent and can lead to confusion:
MenuItems use ActionListeners, but CheckboxMenuItems use ItemListeners. The
Menu objects themselves can also support ActionListeners, but that’s not usually helpful.
Generally you’ll just attach listeners to each MenuItem or CheckboxMenuItem, but the
following example (revised from the earlier version) also shows ways to combine the
capture of multiple menu components into a single listener class. As you’ll see, it’s
probably not worth the hassle to do this.
//: MenuNew.java
// Menus in Java 1.1
import java.awt.*;
import java.awt.event.*;
Chapter 13: Creating Windows & Applets 467](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-467-320.jpg)
![public class MenuNew extends Frame {
String flavors[] = { "Chocolate", "Strawberry",
"Vanilla Fudge Swirl", "Mint Chip",
"Mocha Almond Fudge", "Rum Raisin",
"Praline Cream", "Mud Pie" };
TextField t = new TextField("No flavor", 30);
MenuBar mb1 = new MenuBar();
Menu f = new Menu("File");
Menu m = new Menu("Flavors");
Menu s = new Menu("Safety");
// Alternative approach:
CheckboxMenuItem safety[] = {
new CheckboxMenuItem("Guard"),
new CheckboxMenuItem("Hide")
};
MenuItem file[] = {
// No menu shortcut:
new MenuItem("Open"),
// Adding a menu shortcut is very simple:
new MenuItem("Exit",
new MenuShortcut(KeyEvent.VK_E))
};
// A second menu bar to swap to:
MenuBar mb2 = new MenuBar();
Menu fooBar = new Menu("fooBar");
MenuItem other[] = {
new MenuItem("Foo"),
new MenuItem("Bar"),
new MenuItem("Baz"),
};
// Initialization code:
{
ML ml = new ML();
CMI cmi = new CMI();
safety[0].setActionCommand("Guard");
safety[0].addItemListener(cmi);
safety[1].setActionCommand("Hide");
safety[1].addItemListener(cmi);
file[0].setActionCommand("Open");
file[0].addActionListener(ml);
file[1].setActionCommand("Exit");
file[1].addActionListener(ml);
other[0].addActionListener(new FooL());
other[1].addActionListener(new BarL());
other[2].addActionListener(new BazL());
}
Button b = new Button("Swap Menus");
public MenuNew() {
FL fl = new FL();
for(int i = 0; i < flavors.length; i++) {
MenuItem mi = new MenuItem(flavors[i]);
mi.addActionListener(fl);
m.add(mi);
// Add separators at intervals:
if((i+1) % 3 == 0)
468 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-468-320.jpg)
![m.addSeparator();
}
for(int i = 0; i < safety.length; i++)
s.add(safety[i]);
f.add(s);
for(int i = 0; i < file.length; i++)
f.add(file[i]);
mb1.add(f);
mb1.add(m);
setMenuBar(mb1);
t.setEditable(false);
add(t, BorderLayout.CENTER);
// Set up the system for swapping menus:
b.addActionListener(new BL());
add(b, BorderLayout.NORTH);
for(int i = 0; i < other.length; i++)
fooBar.add(other[i]);
mb2.add(fooBar);
}
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
MenuBar m = getMenuBar();
if(m == mb1) setMenuBar(mb2);
else if (m == mb2) setMenuBar(mb1);
}
}
class ML implements ActionListener {
public void actionPerformed(ActionEvent e) {
MenuItem target = (MenuItem)e.getSource();
String actionCommand =
target.getActionCommand();
if(actionCommand.equals("Open")) {
String s = t.getText();
boolean chosen = false;
for(int i = 0; i < flavors.length; i++)
if(s.equals(flavors[i])) chosen = true;
if(!chosen)
t.setText("Choose a flavor first!");
else
t.setText("Opening "+ s +". Mmm, mm!");
} else if(actionCommand.equals("Exit")) {
dispatchEvent(
new WindowEvent(MenuNew.this,
WindowEvent.WINDOW_CLOSING));
}
}
}
class FL implements ActionListener {
public void actionPerformed(ActionEvent e) {
MenuItem target = (MenuItem)e.getSource();
t.setText(target.getLabel());
}
}
// Alternatively, you can create a different
// class for each different MenuItem. Then you
// Don't have to figure out which one it is:
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![class FooL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Foo selected");
}
}
class BarL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Bar selected");
}
}
class BazL implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText("Baz selected");
}
}
class CMI implements ItemListener {
public void itemStateChanged(ItemEvent e) {
CheckboxMenuItem target =
(CheckboxMenuItem)e.getSource();
String actionCommand =
target.getActionCommand();
if(actionCommand.equals("Guard"))
t.setText("Guard the Ice Cream! " +
"Guarding is " + target.getState());
else if(actionCommand.equals("Hide"))
t.setText("Hide the Ice Cream! " +
"Is it cold? " + target.getState());
}
}
public static void main(String args[]) {
MenuNew f = new MenuNew();
f.setSize(300,200);
f.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
f.setVisible(true);
}
} ///:~
This code is very similar to the previous (Java 1.0) version, until you get to the
initialization section (marked by the opening brace right after the comment “Initialization
code:”). Here you can see the ItemListeners and ActionListeners attached to the various
menu components.
Java 1.1 supports “menu shortcuts,” so you can select a menu item using the keyboard
instead of the mouse. These are quite simple: you just use the overloaded MenuItem
constructor that takes as a second argument a MenuShortcut object. The constructor for
MenuShortcut takes the key of interest, which magically appears on the menu item when
it drops down. The above example adds Control-E to the “Exit” menu item.
You can also see the use of setActionCommand( ). This seems a bit strange because in
each case the “action command” is exactly the same as the label on the menu component.
Why not just use the label, instead of this alternative string? The problem is
470 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-470-320.jpg)
![else
state = (state == 1 ? 2 : 1);
repaint();
}
}
}
}
class BL implements ActionListener {
public void actionPerformed(ActionEvent e) {
Dialog d = new ToeDialog(
Integer.parseInt(rows.getText()),
Integer.parseInt(cols.getText()));
d.show();
}
}
static class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
}
public static void main(String args[]) {
Frame f = new ToeTestNew();
f.setSize(200,100);
f.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
f.setVisible(true);
}
} ///:~
There are some restrictions when using inner classes here. In particular, statics can be at
only the outer level of the class, so inner classes cannot have static data or static inner
classes.
File dialogs
Converting from FileDialogTest.java to the new event model is very straightforward:
//: FileDialogNew.java
// Demonstration of File dialog boxes
import java.awt.*;
import java.awt.event.*;
public class FileDialogNew extends Frame {
TextField filename = new TextField();
TextField directory = new TextField();
Button open = new Button("Open");
Button save = new Button("Save");
public FileDialogNew() {
setTitle("File Dialog Test");
Panel p = new Panel();
p.setLayout(new FlowLayout());
open.addActionListener(new OpenL());
p.add(open);
Chapter 13: Creating Windows & Applets 473](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-473-320.jpg)
![save.addActionListener(new SaveL());
p.add(save);
add(p, BorderLayout.SOUTH);
directory.setEditable(false);
filename.setEditable(false);
p = new Panel();
p.setLayout(new GridLayout(2,1));
p.add(filename);
p.add(directory);
add(p, BorderLayout.NORTH);
}
class OpenL implements ActionListener {
public void actionPerformed(ActionEvent e) {
// Two arguments, defaults to open file:
FileDialog d = new FileDialog(
FileDialogNew.this,
"What file do you want to open?");
d.setFile("*.java");
d.setDirectory("."); // Current directory
d.show();
String yourFile = "*.*";
if((yourFile = d.getFile()) != null) {
filename.setText(yourFile);
directory.setText(d.getDirectory());
} else {
filename.setText("You pressed cancel");
directory.setText("");
}
}
}
class SaveL implements ActionListener {
public void actionPerformed(ActionEvent e) {
FileDialog d = new FileDialog(
FileDialogNew.this,
"What file do you want to save?",
FileDialog.SAVE);
d.setFile("*.java");
d.setDirectory(".");
d.show();
String saveFile;
if((saveFile = d.getFile()) != null) {
filename.setText(saveFile);
directory.setText(d.getDirectory());
} else {
filename.setText("You pressed cancel");
directory.setText("");
}
}
}
public static void main(String args[]) {
Frame f = new FileDialogNew();
f.setSize(250,110);
f.setVisible(true);
f.addWindowListener(
new WindowAdapter() {
474 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-474-320.jpg)
![System.out.println("Button 1 pressed");
ActionListener a = new CountListener(i++);
v.addElement(a);
b2.addActionListener(a);
}
}
class B2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println("Button 2 pressed");
int end = v.size() -1;
if(end >= 0) {
b2.removeActionListener(
(ActionListener)v.elementAt(end));
v.removeElementAt(end);
}
}
}
class BWL extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.out.println("Window Closing");
System.exit(0);
}
}
public static void main(String args[]) {
Frame f = new DynamicEvents();
f.setSize(300,200);
f.show();
}
} ///:~
The new twists in this example are:
1. There is more than one listener attached to each Button. Usually, components
handle events as multicast, meaning you can register many listeners for a single
event. In the special components where an event is handled as unicast, you’ll get
a TooManyListenersException.
2. During the execution of the program, listeners are dynamically added and
removed from the Button b2. Adding is accomplished in the way you’ve seen
before, but each component also has a removeXXXListener( ) method to remove
each type of listener.
This kind of flexibility provides much greater power in your programming.
You should note that event listeners are not guaranteed to be called in the order they are
added (although most implementations do in fact work that way).
Separating business logic from ui logic
In general you’ll want to design your classes so that each one does “only one thing.” This
is particularly important where user-interface code is concerned, since it’s very easy to
wrap up “what you’re doing” with “how you’re displaying it.” This kind of coupling
prevents code reuse. It’s much more desirable to separate your “business logic” from the
GUI. This way, you can not only reuse the business logic more easily, it’s also easier to
reuse the GUI.
476 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-476-320.jpg)
![return 0;
}
}
class Calc1L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText(Integer.toString(
bl.calculation1(getValue(t))));
}
}
class Calc2L implements ActionListener {
public void actionPerformed(ActionEvent e) {
t.setText(Integer.toString(
bl.calculation2(getValue(t))));
}
}
class ModL implements TextListener {
public void textValueChanged(TextEvent e) {
bl.setModifier(getValue(mod));
}
}
public static void main(String args[]) {
Separation applet = new Separation();
Frame aFrame = new Frame("Separation");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(200,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
You can see that BusinessLogic is a very straightforward class that performs its
operations without even a hint that it may be used in a GUI environment. It just does its
job.
Separation keeps track of all the UI details, and it talks only to BusinessLogic through
its public interface. All the operations are centered around getting information back and
forth through the UI and the BusinessLogic object. So Separation, in turn, just does its
job. Since it’s talking only to a BusinessLogic object and doesn’t know much else about it,
this could be massaged into talking to other types of objects without much trouble.
Thinking in terms of separating UI from business logic also makes life easier when you’re
adapting legacy code to work with Java.
Recommended coding approaches
Inner classes, the new event model, and the fact that the old event model is still supported
along with new support for old-style programming has added a new element of confusion.
Now there are even more different ways for people to write unpleasant code.
Unfortunately, this kind of code is showing up in books and article examples, and even in
478 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-478-320.jpg)
![public static void main(String args[]) {
Frame f = new GoodIdea();
f.setSize(300,200);
f.setVisible(true);
}
} ///:~
This is fairly trivial: each button has its own listener which prints something out to the
console. But notice there isn’t an if statement in the entire program, or any statement
which says “I wonder what caused this event.” Each piece of code is concerned with doing,
not type-checking. This is the best way to write your code – not only is it easier to
conceptualize, but much easier to read and maintain. Cutting-and-pasting to create new
programs is also much easier.
Implementing the main class as a listener
The first bad idea is a very common and recommended approach. This makes the main
class (typically Applet or Frame, but it could be any class) implement the various
listeners. Here’s an example:
//: BadIdea1.java
// Some literature recommends this approach,
// but it's missing the point of the new event
// model in Java 1.1
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class BadIdea1 extends Frame
implements ActionListener, WindowListener {
Button
b1 = new Button("Button 1"),
b2 = new Button("Button 2");
public BadIdea1() {
setLayout(new FlowLayout());
addWindowListener(this);
b1.addActionListener(this);
b2.addActionListener(this);
add(b1);
add(b2);
}
public void actionPerformed(ActionEvent e) {
Object source = e.getSource();
if(source == b1)
System.out.println("Button 1 pressed");
else if(source == b2)
System.out.println("Button 2 pressed");
else
System.out.println("Something else");
}
public void windowClosing(WindowEvent e) {
System.out.println("Window Closing");
System.exit(0);
}
public void windowClosed(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
480 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-480-320.jpg)
![public void windowActivated(WindowEvent e) {}
public void windowDeactivated(WindowEvent e) {}
public void windowOpened(WindowEvent e) {}
public static void main(String args[]) {
Frame f = new BadIdea1();
f.setSize(300,200);
f.setVisible(true);
}
} ///:~
The use of this shows up in the three lines:
addWindowListener(this);
b1.addActionListener(this);
b2.addActionListener(this);
Since BadIdea1 implements ActionListener and WindowListener these lines are certainly
acceptable, and if you’re still stuck in the mode of trying to make fewer classes to reduce
server hits during applet loading, it seems to be a good idea. However:
1. Java 1.1 supports JAR files so all your files can be placed in a single compressed
JAR archive which requires only one server hit. You no longer need to reduce
class count for Internet efficiency.
2. The above code is much less modular so it’s harder to grab and paste. Notice
that you must not only implement the various interfaces for your main class,
but in actionPerformed( ) you’ve got to detect which action was performed
using a cascaded if statement. Not only is this going backwards, away from the
listener model, but you can’t easily reuse the actionPerformed( ) method since
it’s very specific to this particular application. Contrast this with
GoodIdea.java, where you can just grab one listener class and paste it in
anywhere else with minimal fuss. Plus you can register multiple listener classes
with a single event, allowing even more modularity in what each listener class
does.
Mixing the approaches
The second bad idea is to mix the two approaches: use inner listener classes, but also
implement one or more listener interfaces as part of the main class. This approach has
appeared without explanation in books and documentation, and I can assume only that the
authors thought they must use the different approaches for different purposes. But you
don’t – in your programming you can probably use inner listener classes exclusively.
//: BadIdea2.java
// An improvement over BadIdea1.java, since it
// uses the WindowAdapter as an inner class
// instead of implementing all the methods of
// WindowListener, but still misses the
// valuable modularity of inner classes
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class BadIdea2 extends Frame
implements ActionListener {
Button
b1 = new Button("Button 1"),
Chapter 13: Creating Windows & Applets 481](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-481-320.jpg)
![b2 = new Button("Button 2");
public BadIdea2() {
setLayout(new FlowLayout());
addWindowListener(new WL());
b1.addActionListener(this);
b2.addActionListener(this);
add(b1);
add(b2);
}
public void actionPerformed(ActionEvent e) {
Object source = e.getSource();
if(source == b1)
System.out.println("Button 1 pressed");
else if(source == b2)
System.out.println("Button 2 pressed");
else
System.out.println("Something else");
}
class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.out.println("Window Closing");
System.exit(0);
}
}
public static void main(String args[]) {
Frame f = new BadIdea2();
f.setSize(300,200);
f.setVisible(true);
}
} ///:~
Since actionPerformed( ) is still tightly coupled to the main class, it’s hard to reuse that
code. It’s also messier and less pleasant to read than the inner class approach.
There’s no reason you have to use any of the old thinking for events in Java 1.1 – so why
do it?
Inheriting a component
Another place where you’ll often see variations on the old way of doing things is when
creating a new type of component. Here’s a simple example showing that here, too, the
new way works:
//: GoodTechnique.java
// Your first choice when overriding components
// should be to install listeners. The code is
// much safer, more modular and maintainable.
import java.awt.*;
import java.awt.event.*;
class Display {
public static final int
EVENT = 0, COMPONENT = 1,
MOUSE = 2, MOUSE_MOVE = 3,
FOCUS = 4, KEY = 5, ACTION = 6,
LAST = 7;
482 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-482-320.jpg)
![public String evnt[];
Display() {
evnt = new String[LAST];
for(int i = 0; i < LAST; i++)
evnt[i] = new String();
}
public void show(Graphics g) {
for(int i = 0; i < LAST; i++)
g.drawString(evnt[i], 0, 10 * i + 10);
}
}
class EnabledPanel extends Panel {
Color c;
int id;
Display display = new Display();
public EnabledPanel(int i, Color mc) {
id = i;
c = mc;
setLayout(new BorderLayout());
add(new MyButton(), BorderLayout.SOUTH);
addComponentListener(new CL());
addFocusListener(new FL());
addKeyListener(new KL());
addMouseListener(new ML());
addMouseMotionListener(new MML());
}
// To eliminate flicker:
public void update(Graphics g) {
paint(g);
}
public void paint(Graphics g) {
g.setColor(c);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
g.setColor(Color.black);
display.show(g);
}
// Don't need to enable anything for this:
public void
processEvent(AWTEvent e) {
display.evnt[Display.EVENT]= e.toString();
repaint();
super.processEvent(e);
}
class CL implements ComponentListener {
public void componentMoved(ComponentEvent e) {
display.evnt[Display.COMPONENT] =
"Component moved";
repaint();
}
public void componentResized(ComponentEvent e) {
display.evnt[Display.COMPONENT] =
"Component resized";
repaint();
}
Chapter 13: Creating Windows & Applets 483](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-483-320.jpg)
![public void componentHidden(ComponentEvent e) {
display.evnt[Display.COMPONENT] =
"Component hidden";
repaint();
}
public void componentShown(ComponentEvent e) {
display.evnt[Display.COMPONENT] =
"Component shown";
repaint();
}
}
class FL implements FocusListener {
public void focusGained(FocusEvent e) {
display.evnt[Display.FOCUS] =
"FOCUS gained";
repaint();
}
public void focusLost(FocusEvent e) {
display.evnt[Display.FOCUS] =
"FOCUS lost";
repaint();
}
}
class KL implements KeyListener {
public void keyPressed(KeyEvent e) {
display.evnt[Display.KEY] =
"KEY pressed: ";
showCode(e);
}
public void keyReleased(KeyEvent e) {
display.evnt[Display.KEY] =
"KEY released: ";
showCode(e);
}
public void keyTyped(KeyEvent e) {
display.evnt[Display.KEY] =
"KEY typed: ";
showCode(e);
}
void showCode(KeyEvent e) {
int code = e.getKeyCode();
display.evnt[Display.KEY] +=
KeyEvent.getKeyText(code);
repaint();
}
}
class ML implements MouseListener {
public void mouseClicked(MouseEvent e) {
requestFocus(); // Get FOCUS on click
display.evnt[Display.MOUSE] =
"MOUSE clicked";
showMouse(e);
}
public void mousePressed(MouseEvent e) {
display.evnt[Display.MOUSE] =
484 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-484-320.jpg)
!["MOUSE pressed";
showMouse(e);
}
public void mouseReleased(MouseEvent e) {
display.evnt[Display.MOUSE] =
"MOUSE released";
showMouse(e);
}
public void mouseEntered(MouseEvent e) {
display.evnt[Display.MOUSE] =
"MOUSE entered";
showMouse(e);
}
public void mouseExited(MouseEvent e) {
display.evnt[Display.MOUSE] =
"MOUSE exited";
showMouse(e);
}
void showMouse(MouseEvent e) {
display.evnt[Display.MOUSE] +=
", x = " + e.getX() +
", y = " + e.getY();
repaint();
}
}
class MML implements MouseMotionListener {
public void mouseDragged(MouseEvent e) {
display.evnt[Display.MOUSE_MOVE] =
"MOUSE dragged";
showMouse(e);
}
public void mouseMoved(MouseEvent e) {
display.evnt[Display.MOUSE_MOVE] =
"MOUSE moved";
showMouse(e);
}
void showMouse(MouseEvent e) {
display.evnt[Display.MOUSE_MOVE] +=
", x = " + e.getX() +
", y = " + e.getY();
repaint();
}
}
}
class MyButton extends Button {
int clickCounter;
String label = "";
public MyButton() {
addActionListener(new AL());
}
public void paint(Graphics g) {
g.setColor(Color.green);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
g.setColor(Color.black);
Chapter 13: Creating Windows & Applets 485](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-485-320.jpg)
![g.drawRect(0, 0, s.width - 1, s.height - 1);
drawLabel(g);
}
private void drawLabel(Graphics g) {
FontMetrics fm = g.getFontMetrics();
int width = fm.stringWidth(label);
int height = fm.getHeight();
int ascent = fm.getAscent();
int leading = fm.getLeading();
int horizMargin =
(getSize().width - width)/2;
int verMargin =
(getSize().height - height)/2;
g.setColor(Color.red);
g.drawString(label, horizMargin,
verMargin + ascent + leading);
}
class AL implements ActionListener {
public void actionPerformed(ActionEvent e) {
clickCounter++;
label = "click #" + clickCounter +
" " + e.toString();
repaint();
}
}
}
public class GoodTechnique extends Frame {
GoodTechnique() {
setLayout(new GridLayout(2,2));
add(new EnabledPanel(1, Color.cyan));
add(new EnabledPanel(2, Color.lightGray));
add(new EnabledPanel(3, Color.yellow));
addWindowListener(new WL());
}
class WL extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.out.println(e);
System.out.println("Window Closing");
System.exit(0);
}
}
public static void main(String args[]) {
Frame f = new GoodTechnique();
f.setTitle("Good Technique");
f.setSize(700,700);
f.setVisible(true);
}
} ///:~
This example also demonstrates the various events that occur, and displays the
information about them. The class Display is a way to centralize that information display.
There’s an array of Strings to hold information about each type of event, and the method
show( ) takes a handle to whatever Graphics object you have and writes directly on that
surface. The scheme is intended to be somewhat reusable.
486 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-486-320.jpg)
![LAST = 7;
public String evnt[];
Display() {
evnt = new String[LAST];
for(int i = 0; i < LAST; i++)
evnt[i] = new String();
}
public void show(Graphics g) {
for(int i = 0; i < LAST; i++)
g.drawString(evnt[i], 0, 10 * i + 10);
}
}
class EnabledPanel extends Panel {
Color c;
int id;
Display display = new Display();
public EnabledPanel(int i, Color mc) {
id = i;
c = mc;
setLayout(new BorderLayout());
add(new MyButton(), BorderLayout.SOUTH);
// Type checking is lost. You can enable and
// process events that the component doesn't
// capture:
enableEvents(
// Panel doesn't handle these:
AWTEvent.ACTION_EVENT_MASK |
AWTEvent.COMPONENT_EVENT_MASK |
AWTEvent.ADJUSTMENT_EVENT_MASK |
AWTEvent.ITEM_EVENT_MASK |
AWTEvent.TEXT_EVENT_MASK |
AWTEvent.WINDOW_EVENT_MASK |
// Panel can handle these:
AWTEvent.FOCUS_EVENT_MASK |
AWTEvent.KEY_EVENT_MASK |
AWTEvent.MOUSE_EVENT_MASK |
AWTEvent.MOUSE_MOTION_EVENT_MASK |
AWTEvent.CONTAINER_EVENT_MASK);
// You can enable an event without
// overriding its process method.
}
// To eliminate flicker:
public void update(Graphics g) {
paint(g);
}
public void paint(Graphics g) {
g.setColor(c);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
g.setColor(Color.black);
display.show(g);
}
public void
processEvent(AWTEvent e) {
488 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-488-320.jpg)
![display.evnt[Display.EVENT]= e.toString();
repaint();
super.processEvent(e);
}
public void
processComponentEvent(ComponentEvent e) {
switch(e.getID()) {
case ComponentEvent.COMPONENT_MOVED:
display.evnt[Display.COMPONENT] =
"Component moved";
break;
case ComponentEvent.COMPONENT_RESIZED:
display.evnt[Display.COMPONENT] =
"Component resized";
break;
case ComponentEvent.COMPONENT_HIDDEN:
display.evnt[Display.COMPONENT] =
"Component hidden";
break;
case ComponentEvent.COMPONENT_SHOWN:
display.evnt[Display.COMPONENT] =
"Component shown";
break;
default:
}
repaint();
// Must always remember to call the "super"
// version of whatever you override:
super.processComponentEvent(e);
}
public void
processFocusEvent(FocusEvent e) {
switch(e.getID()) {
case FocusEvent.FOCUS_GAINED:
display.evnt[Display.FOCUS] =
"FOCUS gained";
break;
case FocusEvent.FOCUS_LOST:
display.evnt[Display.FOCUS] =
"FOCUS lost";
break;
default:
}
repaint();
super.processFocusEvent(e);
}
public void
processKeyEvent(KeyEvent e) {
switch(e.getID()) {
case KeyEvent.KEY_PRESSED:
display.evnt[Display.KEY] =
"KEY pressed: ";
break;
case KeyEvent.KEY_RELEASED:
display.evnt[Display.KEY] =
"KEY released: ";
Chapter 13: Creating Windows & Applets 489](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-489-320.jpg)
![break;
case KeyEvent.KEY_TYPED:
display.evnt[Display.KEY] =
"KEY typed: ";
break;
default:
}
int code = e.getKeyCode();
display.evnt[Display.KEY] +=
KeyEvent.getKeyText(code);
repaint();
super.processKeyEvent(e);
}
public void
processMouseEvent(MouseEvent e) {
switch(e.getID()) {
case MouseEvent.MOUSE_CLICKED:
requestFocus(); // Get FOCUS on click
display.evnt[Display.MOUSE] =
"MOUSE clicked";
break;
case MouseEvent.MOUSE_PRESSED:
display.evnt[Display.MOUSE] =
"MOUSE pressed";
break;
case MouseEvent.MOUSE_RELEASED:
display.evnt[Display.MOUSE] =
"MOUSE released";
break;
case MouseEvent.MOUSE_ENTERED:
display.evnt[Display.MOUSE] =
"MOUSE entered";
break;
case MouseEvent.MOUSE_EXITED:
display.evnt[Display.MOUSE] =
"MOUSE exited";
break;
default:
}
display.evnt[Display.MOUSE] +=
", x = " + e.getX() +
", y = " + e.getY();
repaint();
super.processMouseEvent(e);
}
public void
processMouseMotionEvent(MouseEvent e) {
switch(e.getID()) {
case MouseEvent.MOUSE_DRAGGED:
display.evnt[Display.MOUSE_MOVE] =
"MOUSE dragged";
break;
case MouseEvent.MOUSE_MOVED:
display.evnt[Display.MOUSE_MOVE] =
"MOUSE moved";
490 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-490-320.jpg)
![break;
default:
}
display.evnt[Display.MOUSE_MOVE] +=
", x = " + e.getX() +
", y = " + e.getY();
repaint();
super.processMouseMotionEvent(e);
}
}
class MyButton extends Button {
int clickCounter;
String label = "";
public MyButton() {
enableEvents(AWTEvent.ACTION_EVENT_MASK);
}
public void paint(Graphics g) {
g.setColor(Color.green);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
g.setColor(Color.black);
g.drawRect(0, 0, s.width - 1, s.height - 1);
drawLabel(g);
}
private void drawLabel(Graphics g) {
FontMetrics fm = g.getFontMetrics();
int width = fm.stringWidth(label);
int height = fm.getHeight();
int ascent = fm.getAscent();
int leading = fm.getLeading();
int horizMargin =
(getSize().width - width)/2;
int verMargin =
(getSize().height - height)/2;
g.setColor(Color.red);
g.drawString(label, horizMargin,
verMargin + ascent + leading);
}
public void
processActionEvent(ActionEvent e) {
clickCounter++;
label = "click #" + clickCounter +
" " + e.toString();
repaint();
super.processActionEvent(e);
}
}
public class BadTechnique extends Frame {
BadTechnique() {
setLayout(new GridLayout(2,2));
add(new EnabledPanel(1, Color.cyan));
add(new EnabledPanel(2, Color.lightGray));
add(new EnabledPanel(3, Color.yellow));
// You can also do it for Windows:
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![enableEvents(AWTEvent.WINDOW_EVENT_MASK);
}
public void processWindowEvent(WindowEvent e) {
System.out.println(e);
if(e.getID() == WindowEvent.WINDOW_CLOSING) {
System.out.println("Window Closing");
System.exit(0);
}
}
public static void main(String args[]) {
Frame f = new BadTechnique();
f.setTitle("Bad Technique");
f.setSize(700,700);
f.setVisible(true);
}
} ///:~
Sure, it works. But it’s ugly and hard to write, read, debug, maintain and reuse. So why
bother when you can use inner listener classes?
New Java 1.1 UI APIs
Java 1.1 has also added some important new functionality, including focus traversal,
desktop color access, printing “inside the sandbox” and the beginnings of clipboard
support.
Focus traversal is quite easy, since it’s transparently present in the AWT library
components and you don’t have to do anything to make it work. If you make your own
components and want them to handle focus traversal, you override isFocusTraversable( )
to return true. If you want to capture the keyboard focus on a mouse click, you catch the
mouse down event and call requestFocus( ).
Desktop colors
The desktop colors provide a way for you to know what the various color choices are on
the current user’s desktop. This way, you can use those colors in your program, if you
desire. The colors are automatically initialized and placed in static members of class
SystemColor, so all you need to do is read the member you’re interested in. The names are
intentionally self-explanatory: desktop, activeCaption, activeCaptionText,
activeCaptionBorder, inactiveCaption, inactiveCaptionText, inactiveCaptionBorder,
window, windowBorder, windowText, menu, menuText, text, textText, textHighlight,
textHighlightText, textInactiveText, control, controlText, controlHighlight,
controlLtHighlight, controlShadow, controlDkShadow, scrollbar, info (for help),
infoText (for help text).
Printing
Unfortunately, there isn’t much that’s automatic with printing; instead you must go
through a number of mechanical, non-OO steps in order to print. Printing a component
graphically can be slightly more automatic: by default, the print( ) method calls paint( )
to do its work. There are times when this is satisfactory, but if you want to do anything
more specialized you must know that you’re printing so you can in particular find out the
page dimensions.
492 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-492-320.jpg)
![The following example demonstrates the printing of both text and graphics, and the
different approaches you can use for printing graphics. In addition, it tests the printing
support:
//: PrintDemo.java
// Printing with Java 1.1
import java.awt.*;
import java.awt.event.*;
public class PrintDemo extends Frame {
Button
printText = new Button("Print Text"),
printGraphics = new Button("Print Graphics");
TextField ringNum = new TextField(3);
Choice faces = new Choice();
Graphics g = null;
Plot plot = new Plot3(); // Try different plots
Toolkit tk = Toolkit.getDefaultToolkit();
public PrintDemo() {
ringNum.setText("3");
ringNum.addTextListener(new RingL());
Panel p = new Panel();
p.setLayout(new FlowLayout());
printText.addActionListener(new TBL());
p.add(printText);
p.add(new Label("Font:"));
p.add(faces);
printGraphics.addActionListener(new GBL());
p.add(printGraphics);
p.add(new Label("Rings:"));
p.add(ringNum);
setLayout(new BorderLayout());
add(p, BorderLayout.NORTH);
add(plot, BorderLayout.CENTER);
String fontList[] = tk.getFontList();
for(int i = 0; i < fontList.length; i++)
faces.add(fontList[i]);
faces.select("Serif");
}
class PrintData {
public PrintJob pj;
public int pageWidth, pageHeight;
PrintData(String jobName) {
pj = getToolkit().getPrintJob(
PrintDemo.this, jobName, null);
if(pj != null) {
pageWidth = pj.getPageDimension().width;
pageHeight = pj.getPageDimension().height;
g = pj.getGraphics();
}
}
void end() { pj.end(); }
}
class ChangeFont {
private int stringHeight;
ChangeFont(String face, int style, int point) {
Chapter 13: Creating Windows & Applets 493](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-493-320.jpg)
![}
}
public static void main(String args[]) {
Frame pdemo = new PrintDemo();
pdemo.setTitle("Print Demo");
pdemo.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
pdemo.setSize(500, 500);
pdemo.setVisible(true);
}
}
class Plot extends Canvas {
public int rings = 3;
}
class Plot1 extends Plot {
// Default print() calls paint():
public void paint(Graphics g) {
int w = getSize().width;
int h = getSize().height;
int xc = w / 2;
int yc = w / 2;
int x = 0, y = 0;
for(int i = 0; i < rings; i++) {
if(x < xc && y < yc) {
g.drawOval(x, y, w, h);
x += 10; y += 10;
w -= 20; h -= 20;
}
}
}
}
class Plot2 extends Plot {
// To fit the picture to the page, you must
// know whether you're printing or painting:
public void paint(Graphics g) {
int w, h;
if(g instanceof PrintGraphics) {
PrintJob pj =
((PrintGraphics)g).getPrintJob();
w = pj.getPageDimension().width;
h = pj.getPageDimension().height;
}
else {
w = getSize().width;
h = getSize().height;
}
int xc = w / 2;
int yc = w / 2;
int x = 0, y = 0;
Chapter 13: Creating Windows & Applets 495](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-495-320.jpg)
![public void actionPerformed(ActionEvent e) {
String selection = text.getSelectedText();
StringSelection clipString =
new StringSelection(selection);
clipbd.setContents(clipString, clipString);
text.replaceRange("",
text.getSelectionStart(),
text.getSelectionEnd());
}
}
class PasteL implements ActionListener {
public void actionPerformed(ActionEvent e) {
Transferable clipData =
clipbd.getContents(CutAndPaste.this);
try {
String clipString =
(String)clipData.
getTransferData(
DataFlavor.stringFlavor);
text.replaceRange(clipString,
text.getSelectionStart(),
text.getSelectionEnd());
} catch(Exception ex) {
System.out.println("not String flavor");
}
}
}
public static void main(String args[]) {
CutAndPaste cp = new CutAndPaste();
cp.setSize(300,200);
cp.setVisible(true);
cp.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}
} ///:~
The creation and addition of the menu and TextArea should by now seem a pedestrian
activity. What’s different is the creation of the Clipboard field clipbd, which is done
through the Toolkit.
All the action takes place in the listeners. The CopyL and CutL listeners are the same
except for the last line of CutL, which erases the line that’s been copied. The special two
lines are the creation of a StringSelection object from the String, and the call to
setContents( ) with this StringSelection. That’s all there is to putting a String on the
clipboard.
In PasteL data is pulled off the clipboard using getContents( ). What comes back is a
fairly anonymous Transferable object, and you don’t really know what it contains. One
way to find out is to call getTransferDataFlavors( ), which returns an array of
DataFlavor objects indicating which flavors are supported by this particular object. You
can also ask it directly with isDataFlavorSupported( ), passing in the flavor you’re
interested in. Here, however, the bold approach is taken: getTransferData( ) is called
500 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-500-320.jpg)
![Bean (which it can do if there’s a default constructor) and then, without access to the
Bean’s source code, extract all the necessary information to create the property sheet and
event handlers.
Part of the solution is already evident from the end of Chapter 11: Java 1.1 reflection allows
all the methods of an anonymous class to be discovered. This is perfect for solving the
Bean problem without requiring you to use any extra language keywords like those
required in other visual programming languages. In fact, one of the prime reasons
reflection was added to Java 1.1 was to support Beans (although reflection also supports
object serialization and remote method invocation). So you might expect that the creator of
the application builder tool would have to reflect each Bean and hunt through its methods
to find the properties and events for that Bean.
This is certainly possible, but the Java designers wanted to provide a standard interface for
everyone to use, not only to make Beans simpler to use but also to provide a standard
gateway to the creation of more complex Beans. This interface is the Introspector class,
and the most important method in this class is the static getBeanInfo( ). You pass a Class
handle to this method, and it fully interrogates that class and returns a BeanInfo object
that you can then dissect to find properties, methods and events.
Normally you won’t care about any of this – you’ll probably get most of your Beans off
the shelf from vendors and you don’t need to know all the magic that’s going on
underneath. You’ll simply drag your Beans onto your form, then configure their
properties and write handlers for the events you’re interested in. However, it’s an
interesting and educational exercise to use the Introspector to display information about a
Bean, so here’s a tool that does it:
//: BeanDumper.java
// A method to introspect a Bean
import java.beans.*;
import java.lang.reflect.*;
public class BeanDumper {
public static void dump(Class bean){
BeanInfo bi = null;
try {
bi = Introspector.getBeanInfo(
bean, java.lang.Object.class);
} catch(IntrospectionException ex) {
System.out.println("Couldn't introspect " +
bean.getName());
System.exit(1);
}
PropertyDescriptor properties[] =
bi.getPropertyDescriptors();
for(int i = 0; i < properties.length; i++) {
Class p = properties[i].getPropertyType();
System.out.println(
"Property type:n " + p.getName());
System.out.println(
"Property name:n " +
properties[i].getName());
Method readMethod =
properties[i].getReadMethod();
if(readMethod != null)
System.out.println(
504 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-504-320.jpg)
!["Read method:n " +
readMethod.toString());
Method writeMethod =
properties[i].getWriteMethod();
if(writeMethod != null)
System.out.println(
"Write method:n " +
writeMethod.toString());
System.out.println("====================");
}
System.out.println("Public methods:");
MethodDescriptor methods[] =
bi.getMethodDescriptors();
for(int i = 0; i < methods.length; i++)
System.out.println(
methods[i].getMethod().toString());
System.out.println("======================");
System.out.println("Event support:");
EventSetDescriptor events[] =
bi.getEventSetDescriptors();
for(int i = 0; i < events.length; i++) {
System.out.println("Listener type:n " +
events[i].getListenerType().getName());
Method lm[] =
events[i].getListenerMethods();
for(int j = 0; j < lm.length; j++)
System.out.println(
"Listener method:n " +
lm[j].getName());
MethodDescriptor lmd[] =
events[i].getListenerMethodDescriptors();
for(int j = 0; j < lmd.length; j++)
System.out.println(
"Method descriptor:n " +
lmd[j].getMethod().toString());
Method addListener =
events[i].getAddListenerMethod();
System.out.println(
"Add Listener Method:n " +
addListener.toString());
Method removeListener =
events[i].getRemoveListenerMethod();
System.out.println(
"Remove Listener Method:n " +
removeListener.toString());
System.out.println("====================");
}
}
// Dump the class of your choice:
public static void main(String args[]) {
if(args.length < 1) {
System.err.println("usage: n" +
"BeanDumper fully.qualified.class");
System.exit(0);
}
Class c = null;
Chapter 13: Creating Windows & Applets 505](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-505-320.jpg)
![try {
c = Class.forName(args[0]);
} catch(ClassNotFoundException ex) {
System.err.println(
"Couldn't find " + args[0]);
System.exit(0);
}
dump(c);
}
} ///:~
BeanDumper.dump( ) is the method that does all the work. First it tries to create a
BeanInfo object, and if successful calls the methods of BeanInfo that produce
information about properties, methods and events. In Introspector.getBeanInfo( ), you’ll
see there is a second argument. This tells the Introspector where to stop in the inheritance
hierarchy. Here, it stops before it parses all the methods from Object, since we’re not
interested in seeing those.
For properties, getPropertyDescriptors( ) returns an array of PropertyDescriptors. For
each PropertyDescriptor you can call getPropertyType( ) to find the class of object that is
passed in and out via the property methods. Then for each property you can get its
pseudonym (extracted from the method names) with getName( ), the method for reading
with getReadMethod( ) and the method for writing with getWriteMethod( ). These last
two methods return a Method object which can actually be used to invoke the
corresponding method on the object (this is part of reflection).
For the public methods (including the property methods), getMethodDescriptors( )
returns an array of MethodDescriptors. For each one you can get the associated Method
object and print out its name.
For the events, getEventSetDescriptors( ) returns an array of (what else?)
EventSetDescriptors. Each of these can be queried to find out the class of the listener, the
methods of that listener class, and the add- and remove-listener methods. The
BeanDumper program prints out all of this information.
If you invoke BeanDumper on the Frog class like this:
java BeanDumper Frog
the output, after removing extra details that are unnecessary here, is:
class name: Frog
Property type:
Color
Property name:
color
Read method:
public Color getColor()
Write method:
public void setColor(Color)
====================
Property type:
Spots
Property name:
spots
Read method:
public Spots getSpots()
506 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-506-320.jpg)
![actionListener.actionPerformed(
new ActionEvent(BangBean.this,
ActionEvent.ACTION_PERFORMED, null));
}
}
class MM extends MouseMotionAdapter {
public void mouseMoved(MouseEvent e) {
xm = e.getX();
ym = e.getY();
repaint();
}
}
public Dimension getPreferredSize() {
return new Dimension(200, 200);
}
// Testing the BangBean:
public static void main(String args[]) {
BangBean bb = new BangBean();
try {
bb.addActionListener(new BBL());
} catch(TooManyListenersException e) {}
Frame aFrame = new Frame("BangBean Test");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(bb, BorderLayout.CENTER);
aFrame.setSize(300,300);
aFrame.setVisible(true);
}
// During testing, send action information
// to the console:
static class BBL implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println("BangBean action");
}
}
} ///:~
The first thing you’ll notice is that BangBean implements the Serializable interface. This
means that the application builder tool can “pickle” all the information for the BangBean
using serialization after the program designer has adjusted the values of the properties.
When the Bean is created as part of the running application, these “pickled” properties are
restored so you get exactly what you designed.
You can see that all the fields are private, which is what you’ll normally do with a Bean –
allow access only through methods, usually using the “property” scheme.
When you look at the signature for addActionListener( ), you’ll see that it may throw a
TooManyListenersException. This indicates that it is unicast, which means it notifies
only one listener when the event occurs. Normally, you’ll use multicast events, which
means that many listeners may be notified of an event. However, that runs into issues
that you won’t be ready for until the next chapter, so it will be revisited there (under the
heading “Java Beans revisited”). A unicast event sidesteps the problem.
510 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-510-320.jpg)
![Responsive user interfaces
As a starting point, consider a program that performs some CPU-intensive operation and
thus ends up ignoring user input and being unresponsive. This one, a combined
applet/application, will simply display the result of a running counter:
//: Counter1.java
// A non-responsive user interface
package c14;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class Counter1 extends Applet {
private int count = 0;
private Button
onOff = new Button("Toggle"),
start = new Button("Start");
private TextField t = new TextField(10);
private boolean runFlag = true;
public void init() {
add(t);
start.addActionListener(new StartL());
add(start);
onOff.addActionListener(new OnOffL());
add(onOff);
}
public void go() {
while (true) {
try {
Thread.currentThread().sleep(100);
} catch (InterruptedException e){}
if(runFlag)
t.setText(Integer.toString(count++));
}
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
go();
}
}
class OnOffL implements ActionListener {
public void actionPerformed(ActionEvent e) {
runFlag = !runFlag;
}
}
public static void main(String args[]) {
Counter1 applet = new Counter1();
Frame aFrame = new Frame("Counter1");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
516 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-516-320.jpg)
![run( ), which you must override to make the thread do your bidding. Thus, run( ) is the
code that will be executed “simultaneously” with the other threads in a program.
The following example creates any number of threads which it keeps track of by assigning
each thread a unique number, generated with a static variable. The Thread’s run( )
method is overridden to count down each time it passes through its loop, and to finish
when the count is zero (at the point when run( ) returns, the thread is terminated).
//: SimpleThread.java
// Very simple Threading example
public class SimpleThread extends Thread {
private int countDown = 5;
private int threadNumber;
private static int threadCount = 0;
public SimpleThread() {
threadNumber = ++threadCount;
System.out.println("Making " + threadNumber);
}
public void run() {
while(true) {
System.out.println("Thread " +
threadNumber + "(" + countDown + ")");
if(--countDown == 0) return;
}
}
public static void main(String args[]) {
for(int i = 0; i < 5; i++)
new SimpleThread().start();
System.out.println("All Threads Started");
}
} ///:~
A run( ) method virtually always has some kind of loop that continues until the thread is
no longer necessary, so you’ll have to establish the condition on which to break out of this
loop (or, in the above case, simply return from run( )). Very often, run( ) is cast in the
form of an infinite loop, which means that, barring some external call to stop( ) or
destroy( ) for that thread, it will run forever (until the program completes).
In main( ) you can see a number of threads being created and run. The special method
that comes with the Thread class is start( ), which performs special initialization for the
thread and then calls run( ). So the steps are: the constructor is called to build the object,
then start( ) configures the thread and calls run( ). If you don’t call start( ) (which you
can do in the constructor, if that’s appropriate) the thread will never be started.
The output for this program is:
Making 1
Making 2
Making 3
Making 4
Making 5
Thread 1(5)
Thread 1(4)
Thread 1(3)
Thread 1(2)
Thread 2(5)
518 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-518-320.jpg)
![start();
}
public void invertFlag() { runFlag = !runFlag;}
public void run() {
while (true) {
try {
sleep(100);
} catch (InterruptedException e){}
if(runFlag)
c2.t.setText(Integer.toString(count++));
}
}
}
public class Counter2 extends Applet {
TextField t = new TextField(10);
private SeparateSubTask sp = null;
private Button
onOff = new Button("Toggle"),
start = new Button("Start");
public void init() {
add(t);
start.addActionListener(new StartL());
add(start);
onOff.addActionListener(new OnOffL());
add(onOff);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(sp == null)
sp = new SeparateSubTask(Counter2.this);
}
}
class OnOffL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(sp != null)
sp.invertFlag();
}
}
public static void main(String args[]) {
Counter2 applet = new Counter2();
Frame aFrame = new Frame("Counter2");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
520 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-520-320.jpg)
![start.addActionListener(new StartL());
add(start);
onOff.addActionListener(new OnOffL());
add(onOff);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(sp == null)
sp = new SeparateSubTask();
}
}
class OnOffL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(sp != null)
sp.runFlag = !sp.runFlag; // invertFlag();
}
}
public static void main(String args[]) {
Counter2i applet = new Counter2i();
Frame aFrame = new Frame("Counter2i");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
This SeparateSubTask name will not collide with the SeparateSubTask in the previous
example even though they’re in the same directory, since it’s hidden as an inner class. You
can also see that the inner class is private, which means that its fields and methods can be
given default access (except for run( ), which must be public since it is public in the base
class). The private inner class is not accessible to anyone but Counter2i, and since the two
classes are tightly coupled it’s convenient to loosen the access restrictions between them.
In SeparateSubTask you can see that the invertFlag( ) method has been removed since
Counter2i can now directly access runFlag.
Also notice that SeparateSubTask’s constructor has been simplified – now it only starts
the thread. The handle to the Counter2i object is still being captured as in the previous
version, but instead of doing it by hand and referencing the outer object by hand, the inner
class mechanism takes care of it automatically. In run( ), you can see that t is simply
accessed, as if it were a field of SeparateSubTask. The t field in the parent class can now
be made private since SeparateSubTask can access it without getting any special
permission – and it’s always good to make fields “as private as possible” so they cannot be
accidentally changed by forces outside your class.
Anytime you notice classes that appear to have high coupling with each other, consider
the coding and maintenance improvements you might get by using inner classes.
522 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-522-320.jpg)
![}
}
class OnOffL implements ActionListener {
public void actionPerformed(ActionEvent e) {
runFlag = !runFlag;
}
}
public static void main(String args[]) {
Counter3 applet = new Counter3();
Frame aFrame = new Frame("Counter3");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,200);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Now the run( ) is inside the class, but it’s still dormant after init( ) completes. When you
press the start button, the thread is created (if it doesn’t already exist) in the somewhat
obscure expression:
new Thread(Counter3.this);
When something has a Runnable interface, it simply means it has a run( ) method, but
there’s nothing special about that – it doesn’t produce any innate threading abilities, like
those of a class inherited from Thread. So to produce a thread from a Runnable object,
you must actually create a thread separately and hand it the Runnable object; there’s a
special constructor for this that takes a Runnable as its argument. You can then call
start( ) for that thread:
selfThread.start();
This performs the usual initialization and then calls run( ).
The convenient thing about the Runnable interface is that everything belongs to the same
class. If you need to access something, you simply do it without going through a separate
object. The penalty for this convenience is strict, though – you can have only a single
thread running for that particular object (although you can create more objects of that
type, or create other threads in different classes).
Note that the Runnable interface is not what imposes this restriction. It’s the combination
of Runnable and your main class that does it, since you can have only one object of your
main class per application.
Making many threads
Consider the creation of many different threads. You can’t do this with the previous
example, so you’ll have to go back to having separate classes inherited from Thread to
encapsulate the run( ). But this is a more general solution and easier to understand, so
524 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-524-320.jpg)
![while the previous example shows a coding style you’ll often see, I can’t recommend it for
most cases because it’s just a little bit more confusing and less flexible.
The following example repeats the form of the above examples with counters and toggle
buttons. But now all the information for a particular counter, including the button and
text field, is inside its own object which is inherited from Thread. All the fields in Ticker
are private, which means that the Ticker implementation can be changed at will,
including the quantity and type of data components to acquire and display information.
When a Ticker object is created, the constructor requires a handle to an AWT Container
which Ticker fills with its visual components. This way, if you change the visual
components, the code that uses Ticker doesn’t need to be modified.
//: Counter4.java
// If you separate your thread from the main
// class, you can have as many threads as you
// want.
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
class Ticker extends Thread {
private Button b = new Button("Toggle");
private TextField t = new TextField(10);
private int count = 0;
private boolean runFlag = true;
public Ticker(Container c) {
b.addActionListener(new ToggleL());
Panel p = new Panel();
p.add(t);
p.add(b);
c.add(p);
}
class ToggleL implements ActionListener {
public void actionPerformed(ActionEvent e) {
runFlag = !runFlag;
}
}
public void run() {
while (true) {
if(runFlag)
t.setText(Integer.toString(count++));
try {
sleep(100);
} catch (InterruptedException e){}
}
}
}
public class Counter4 extends Applet {
private Button start = new Button("Start");
private boolean started = false;
private Ticker s[];
private boolean isApplet = true;
private int size;
public void init() {
// Get parameter "size" from Web page:
if(isApplet)
Chapter 14: Multiple Threads 525](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-525-320.jpg)
![size =
Integer.parseInt(getParameter("size"));
s = new Ticker[size];
for(int i = 0; i < s.length; i++)
s[i] = new Ticker(this);
start.addActionListener(new StartL());
add(start);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(!started) {
started = true;
for(int i = 0; i < s.length; i++)
s[i].start();
}
}
}
public static void main(String args[]) {
Counter4 applet = new Counter4();
// This isn't an applet, so set the flag and
// produce the parameter values from args[]:
applet.isApplet = false;
applet.size =
(args.length == 0 ? 5 :
Integer.parseInt(args[0]));
Frame aFrame = new Frame("Counter4");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(200, applet.size * 50);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Ticker contains not only its threading equipment but also the way to control and display
the thread. You can create as many threads as you want without explicitly creating the
windowing components.
In Counter4 there’s an array of Ticker objects called s. For maximum flexibility, the size
of this array is initialized by reaching out into the Web page itself, using applet
parameters. Here’s what the size parameter looks like on the page, embedded inside the
applet description:
<applet code=Counter4 width=600 height=600>
<param name=size value="20">
</applet>
The param, name and value are all Web-page keywords. name is what you’ll be
referring to in your program, and value can be any string, not just something that
resolves to a number.
526 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-526-320.jpg)
![You’ll notice that the determination of the size of the array s is done inside init( ), and not
as part of an inline definition of s. That is, you cannot say as part of the class definition
(outside of any methods):
int size = Integer.parseInt(getParameter("size"));
Ticker s[] = new Ticker[size];
Actually, you can compile this but you’ll get a strange null-pointer exception at run time.
It works fine if you move the getParameter( ) initialization inside of init( ). The applet
framework performs the necessary startup to grab the parameters before entering init( ).
In addition, this code is set up to be either an applet or an application. When it’s an
application the size argument is extracted from the command line (or a default value is
provided).
Once the size of the array is established, new Ticker objects are created; as part of the
Ticker constructor the button and text field for each Ticker is added to the applet.
Pressing the start button means looping through the whole array of Tickers and calling
start( ) for each one. Remember, start( ) performs necessary thread initialization and then
calls run( ) for that thread.
The ToggleL listener simply inverts the flag in Ticker, and when the associated thread
next takes note it can react accordingly.
One value of this example is that it allows you to easily create large sets of independent
subtasks and to monitor their behavior. In this case, you’ll see that as the number of
subtasks gets larger, your machine will probably show more and more divergence in the
displayed numbers because of the way that the threads are served.
You can also experiment to discover how important the sleep(100) is inside Ticker.run( ).
If you remove the sleep( ), things will work fine until you press a toggle button. Then that
particular thread has a false runFlag and the run( ) is just tied up in a very tight infinite
loop, which appears very difficult to break during multithreading, so the responsiveness
and speed of the program really bogs down.
Daemon threads
A “daemon” thread is one that is supposed to provide a general service in the background
as long as the program is running, but is not part of the essence of the program. Thus,
when all the non-daemon threads complete then the program is terminated. Conversely, if
there are any non-daemon threads still running the program doesn't terminate (there is,
for instance, a thread that runs main( )).
You can find out if a thread is a daemon by calling isDaemon( ), and you can turn the
daemonhood of a thread on and off with setDaemon( ). If a thread is a daemon, then any
threads it creates will automatically be daemons.
The following example demonstrates daemon threads:
//: Daemons.java
// Daemonic behavior
import java.io.*;
class Daemon extends Thread {
private static final int SIZE = 10;
private Thread t[] = new Thread[SIZE];
public Daemon() {
Chapter 14: Multiple Threads 527](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-527-320.jpg)
![setDaemon(true);
start();
}
public void run() {
for(int i = 0; i < SIZE; i++)
t[i] = new DaemonSpawn(i);
for(int i = 0; i < SIZE; i++)
System.out.println(
"t[" + i + "].isDaemon() = "
+ t[i].isDaemon());
while(true)
yield();
}
}
class DaemonSpawn extends Thread {
public DaemonSpawn(int i) {
System.out.println(
"DaemonSpawn " + i + " started");
start();
}
public void run() {
while(true)
yield();
}
}
public class Daemons {
public static void main(String args[]) {
Thread d = new Daemon();
System.out.println(
"d.isDaemon() = " + d.isDaemon());
// Allow the daemon threads to finish
// their startup processes:
BufferedReader stdin =
new BufferedReader(
new InputStreamReader(System.in));
System.out.println("Waiting for CR");
try {
stdin.readLine();
} catch(IOException e) {}
}
} ///:~
The Daemon thread sets its daemon flag to “true” and then spawns a bunch of other
threads to show that they are also daemons. Then it goes into an infinite loop which calls
yield( ) to give up control to the other processes. In an earlier version of this program, the
infinite loops would increment int counters, but this seemed to bring the whole program
to a stop. Using yield( ) makes the program quite peppy.
There’s nothing to keep the program from terminating once main( ) finishes its job since
there are nothing but daemon threads running. So you can see the results of starting all
the daemon threads, System.in is set up to read so the program waits for a carriage
return before terminating. Without this you see only some of the results from the creation
of the daemon threads (try replacing the readLine( ) code with sleep( ) calls of various
lengths to see this behavior).
528 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-528-320.jpg)
![try {
sleep(500);
} catch (InterruptedException e){}
}
}
public void synchTest() {
Sharing1.incrementAccess();
if(count1 != count2)
l.setText("Unsynched");
}
}
class Watcher extends Thread {
private Sharing1 p;
public Watcher(Sharing1 p) {
this.p = p;
start();
}
public void run() {
while(true) {
for(int i = 0; i < p.s.length; i++)
p.s[i].synchTest();
try {
sleep(500);
} catch (InterruptedException e){}
}
}
}
public class Sharing1 extends Applet {
TwoCounter s[];
private static int accessCount = 0;
private static TextField aCount =
new TextField("0", 10);
public static void incrementAccess() {
accessCount++;
aCount.setText(Integer.toString(accessCount));
}
private Button
start = new Button("Start"),
observer = new Button("Observe");
private boolean isApplet = true;
private int numCounters = 0;
private int numObservers = 0;
public void init() {
if(isApplet) {
numCounters =
Integer.parseInt(getParameter("size"));
numObservers =
Integer.parseInt(
getParameter("observers"));
}
s = new TwoCounter[numCounters];
for(int i = 0; i < s.length; i++)
s[i] = new TwoCounter(this);
530 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-530-320.jpg)
![Panel p = new Panel();
start.addActionListener(new StartL());
p.add(start);
observer.addActionListener(new ObserverL());
p.add(observer);
p.add(new Label("Access Count"));
p.add(aCount);
add(p);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
for(int i = 0; i < s.length; i++)
s[i].start();
}
}
class ObserverL implements ActionListener {
public void actionPerformed(ActionEvent e) {
for(int i = 0; i < numObservers; i++)
new Watcher(Sharing1.this);
}
}
public static void main(String args[]) {
Sharing1 applet = new Sharing1();
// This isn't an applet, so set the flag and
// produce the parameter values from args[]:
applet.isApplet = false;
applet.numCounters =
(args.length == 0 ? 5 :
Integer.parseInt(args[0]));
applet.numObservers =
(args.length < 2 ? 5 :
Integer.parseInt(args[1]));
Frame aFrame = new Frame("Sharing1");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(350, applet.numCounters *100);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
As before, each counter contains its own display components: two text fields and a label
that initially indicates the counts are equivalent. These components are added to the
Container in the TwoCounter constructor. Because this thread is started via a button
press by the user, it’s possible that start( ) may be called more than once. It’s illegal for
Thread.start( ) to be called more than once for a thread (an exception is thrown). You can
see the machinery to prevent this in the started flag and the overridden start( ) method.
In run( ), count1 and count2 are incremented and displayed in a manner that would seem
to keep them exactly identical. Then sleep( ) is called; without this call the program
becomes very balky because it becomes hard for the CPU to swap tasks.
Chapter 14: Multiple Threads 531](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-531-320.jpg)
![started = true;
super.start();
}
}
public synchronized void run() {
while (true) {
t1.setText(Integer.toString(count1++));
t2.setText(Integer.toString(count2++));
try {
sleep(500);
} catch (InterruptedException e){}
}
}
public synchronized void synchTest() {
Sharing2.incrementAccess();
if(count1 != count2)
l.setText("Unsynched");
}
}
class Watcher2 extends Thread {
private Sharing2 p;
public Watcher2(Sharing2 p) {
this.p = p;
start();
}
public void run() {
while(true) {
for(int i = 0; i < p.s.length; i++)
p.s[i].synchTest();
try {
sleep(500);
} catch (InterruptedException e){}
}
}
}
public class Sharing2 extends Applet {
TwoCounter2 s[];
private static int accessCount = 0;
private static TextField aCount =
new TextField("0", 10);
public static void incrementAccess() {
accessCount++;
aCount.setText(Integer.toString(accessCount));
}
private Button
start = new Button("Start"),
observer = new Button("Observe");
private boolean isApplet = true;
private int numCounters = 0;
private int numObservers = 0;
public void init() {
if(isApplet) {
numCounters =
534 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-534-320.jpg)
![Integer.parseInt(getParameter("size"));
numObservers =
Integer.parseInt(
getParameter("observers"));
}
s = new TwoCounter2[numCounters];
for(int i = 0; i < s.length; i++)
s[i] = new TwoCounter2(this);
Panel p = new Panel();
start.addActionListener(new StartL());
p.add(start);
observer.addActionListener(new ObserverL());
p.add(observer);
p.add(new Label("Access Count"));
p.add(aCount);
add(p);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
for(int i = 0; i < s.length; i++)
s[i].start();
}
}
class ObserverL implements ActionListener {
public void actionPerformed(ActionEvent e) {
for(int i = 0; i < numObservers; i++)
new Watcher2(Sharing2.this);
}
}
public static void main(String args[]) {
Sharing2 applet = new Sharing2();
// This isn't an applet, so set the flag and
// produce the parameter values from args[]:
applet.isApplet = false;
applet.numCounters =
(args.length == 0 ? 5 :
Integer.parseInt(args[0]));
applet.numObservers =
(args.length < 2 ? 5 :
Integer.parseInt(args[1]));
Frame aFrame = new Frame("Sharing2");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(350, applet.numCounters *100);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Chapter 14: Multiple Threads 535](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-535-320.jpg)
![al.actionPerformed(a);
}
}
class ML extends MouseAdapter {
public void mousePressed(MouseEvent e) {
Graphics g = getGraphics();
g.setColor(tColor);
g.setFont(
new Font(
"TimesRoman", Font.BOLD, fontSize));
int width =
g.getFontMetrics().stringWidth(text);
g.drawString(text,
(getSize().width - width) /2,
getSize().height/2);
g.dispose();
notifyListeners();
}
}
class MM extends MouseMotionAdapter {
public void mouseMoved(MouseEvent e) {
xm = e.getX();
ym = e.getY();
repaint();
}
}
// Testing the BangBean2:
public static void main(String args[]) {
BangBean2 bb = new BangBean2();
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("ActionEvent" + e);
}
});
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("BangBean2 action");
}
});
bb.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
System.out.println("More action");
}
});
Frame aFrame = new Frame("BangBean2 Test");
aFrame.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(bb, BorderLayout.CENTER);
aFrame.setSize(300,300);
aFrame.setVisible(true);
}
} ///:~
Chapter 14: Multiple Threads 539](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-539-320.jpg)
![The array of Blockable handles b[] is not initialized at its point of definition because the
piped streams cannot be set up before that definition takes place (the need for the try
block prevents this).
///:Continuing
/////////// Testing Everything ///////////
public class Blocking extends Applet {
private Button
start = new Button("Start"),
stopPeekers = new Button("Stop Peekers");
private boolean started = false;
private Blockable b[];
private PipedWriter out;
private PipedReader in;
public void init() {
out = new PipedWriter();
try {
in = new PipedReader(out);
} catch(IOException e) {}
b = new Blockable[] {
new Sleeper1(this),
new Sleeper2(this),
new SuspendResume1(this),
new SuspendResume2(this),
new WaitNotify1(this),
new WaitNotify2(this),
new Sender(this, out),
new Receiver(this, in)
};
start.addActionListener(new StartL());
add(start);
stopPeekers.addActionListener(
new StopPeekersL());
add(stopPeekers);
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(!started) {
started = true;
for(int i = 0; i < b.length; i++)
b[i].start();
}
}
}
class StopPeekersL implements ActionListener {
public void actionPerformed(ActionEvent e) {
// Demonstration of the preferred
// alternative to Thread.stop():
for(int i = 0; i < b.length; i++)
b[i].stopPeeker();
}
}
public static void main(String args[]) {
Blocking applet = new Blocking();
Frame aFrame = new Frame("Blocking");
aFrame.addWindowListener(
548 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-548-320.jpg)
![import java.awt.event.*;
import java.applet.*;
class Blocked extends Thread {
public synchronized void run() {
try {
wait(); // Blocks
} catch(InterruptedException e) {
System.out.println("InterruptedException");
}
System.out.println("Exiting run()");
}
}
public class Interrupt extends Applet {
private Button
interrupt = new Button("Interrupt");
private Blocked blocked = new Blocked();
public void init() {
add(interrupt);
interrupt.addActionListener(
new ActionListener() {
public
void actionPerformed(ActionEvent e) {
System.out.println("Button pressed");
if(blocked == null) return;
Thread remove = blocked;
blocked = null; // to release it
remove.interrupt();
}
});
blocked.start();
}
public static void main(String args[]) {
Interrupt applet = new Interrupt();
Frame aFrame = new Frame("Interrupt");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(200,100);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
The wait( ) inside Blocked.run( ) produces the blocked thread. When you press the
button, the blocked handle is set to null so the garbage collector will clean it up, and then
the object’s interrupt( ) method is called. The first time you press the button you’ll see
that the thread quits, but after that there’s no thread to kill so you just see that the button
has been pressed.
550 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-550-320.jpg)
![public
void actionPerformed(ActionEvent e) {
ss.fauxSuspend();
}
});
add(suspend);
resume.addActionListener(
new ActionListener() {
public
void actionPerformed(ActionEvent e) {
ss.fauxResume();
}
});
add(resume);
}
public static void main(String args[]) {
Suspend applet = new Suspend();
Frame aFrame = new Frame("Suspend");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300,100);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
The flag suspended inside Suspendable is used to turn suspension on and off. To suspend,
the flag is set to true by calling fauxSuspend( ) and this is detected inside run( ). The
wait( ), as described earlier in this chapter, must be synchronized so it has the object
lock. In fauxResume( ), the suspended flag is set to false and notify( ) is called – since
this wakes up wait( ) inside a synchronized clause the fauxResume( ) method must also
be synchronized so that it acquires the lock before calling notify( ) (thus the lock is
available for the wait( ) to wake up with). If you follow the style shown in this program
you can avoid using wait( ) and notify( ).
The destroy( ) method of Thread has never been implemented; it’s like a suspend( ) that
cannot resume and thus it has the same deadlock issues as suspend( ). However, this is
not a deprecated method and it may be implemented in a future version of Java (after 1.2)
for special situations where the risk of a deadlock is acceptable.
You may wonder why these methods, now deprecated, were included in Java in the first
place. It seems a clear admission of a rather significant mistake to simply remove them
outright (and pokes yet another hole in the arguments for Java’s exceptional design and
infallibility touted by Sun marketing people). The heartening thing about the change is
that it clearly indicates that the technical people and not the marketing people are running
the show – they discovered a problem and they are fixing it. I find this much more
promising and hopeful than leaving the problem in because fixing it would admit an error.
It means that Java will continue to improve, even if it means a little discomfort on the
part of Java programmers. I’d rather deal with the discomfort than watch the language
stagnate.
552 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-552-320.jpg)
![public void actionPerformed(ActionEvent e) {
int newPriority = getPriority() - 1;
if(newPriority < Thread.MIN_PRIORITY)
newPriority = Thread.MIN_PRIORITY;
setPriority(newPriority);
}
}
public void run() {
while (true) {
if(runFlag) {
t.setText(Integer.toString(count++));
pr.setText(
Integer.toString(getPriority()));
}
yield();
}
}
}
public class Counter5 extends Applet {
private Button
start = new Button("Start"),
upMax = new Button("Inc Max Priority"),
downMax = new Button("Dec Max Priority");
private boolean started = false;
private static final int SIZE = 10;
private Ticker2 s[] = new Ticker2[SIZE];
private TextField mp = new TextField(3);
public void init() {
for(int i = 0; i < s.length; i++)
s[i] = new Ticker2(this);
add(new Label("MAX_PRIORITY = "
+ Thread.MAX_PRIORITY));
add(new Label("MIN_PRIORITY = "
+ Thread.MIN_PRIORITY));
add(new Label("Thread Group Max Priority = "));
add(mp);
add(start);
add(upMax); add(downMax);
start.addActionListener(new StartL());
upMax.addActionListener(new UpMaxL());
downMax.addActionListener(new DownMaxL());
showMaxPriority();
// Recursively display parent thread groups:
ThreadGroup parent =
s[0].getThreadGroup().getParent();
while(parent != null) {
add(new Label(
"Parent threadgroup max priority = "
+ parent.getMaxPriority()));
parent = parent.getParent();
}
}
public void showMaxPriority() {
mp.setText(Integer.toString(
554 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-554-320.jpg)
![s[0].getThreadGroup().getMaxPriority()));
}
class StartL implements ActionListener {
public void actionPerformed(ActionEvent e) {
if(!started) {
started = true;
for(int i = 0; i < s.length; i++)
s[i].start();
}
}
}
class UpMaxL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int maxp =
s[0].getThreadGroup().getMaxPriority();
if(++maxp > Thread.MAX_PRIORITY)
maxp = Thread.MAX_PRIORITY;
s[0].getThreadGroup().setMaxPriority(maxp);
showMaxPriority();
}
}
class DownMaxL implements ActionListener {
public void actionPerformed(ActionEvent e) {
int maxp =
s[0].getThreadGroup().getMaxPriority();
if(--maxp < Thread.MIN_PRIORITY)
maxp = Thread.MIN_PRIORITY;
s[0].getThreadGroup().setMaxPriority(maxp);
showMaxPriority();
}
}
public static void main(String args[]) {
Counter5 applet = new Counter5();
Frame aFrame = new Frame("Counter5");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(300, 600);
applet.init();
applet.start();
aFrame.setVisible(true);
}
} ///:~
Ticker2 follows the form established earlier in this chapter, but there’s an extra TextField
for displaying the priority of the thread and two more buttons, for incrementing and
decrementing the priority.
Also notice the use of yield( ), which voluntarily hands control back to the scheduler.
Without this the multithreading mechanism still works, but you’ll notice it runs very
slowly (try removing the call to yield( )!). You could also call sleep( ), but then the rate of
counting would be controlled by the sleep( ) duration instead of the priority.
Chapter 14: Multiple Threads 555](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-555-320.jpg)
![// How threads can access other threads
// in a parent thread group
public class TestAccess {
public static void main(String args[]) {
ThreadGroup
x = new ThreadGroup("x"),
y = new ThreadGroup(x, "y"),
z = new ThreadGroup(y, "z");
Thread
one = new TestThread1(x, "one"),
two = new TestThread2(z, "two");
}
}
class TestThread1 extends Thread {
private int i;
TestThread1(ThreadGroup g, String name) {
super(g, name);
}
void f() {
i++; // modify this thread
System.out.println(getName() + " f()");
}
}
class TestThread2 extends TestThread1 {
TestThread2(ThreadGroup g, String name) {
super(g, name);
start();
}
public void run() {
ThreadGroup g =
getThreadGroup().getParent().getParent();
g.list();
Thread gAll[] = new Thread[g.activeCount()];
g.enumerate(gAll);
for(int i = 0; i < gAll.length; i++) {
gAll[i].setPriority(Thread.MIN_PRIORITY);
((TestThread1)gAll[i]).f();
}
g.list();
}
} ///:~
In main( ), several ThreadGroups are created, leafing off from each other: x has no
argument but its name (a String) and thus it is automatically placed in the “system”
thread group, while y is under x and z is under y. Notice that initialization happens in
textual order so this code is legal.
Two threads are created and placed in different thread groups. TestThread1 doesn’t have a
run( ) method, but it does have an f( ) that modifies the thread and prints something so
you can see it was indeed called. TestThread2 is a subclass of TestThread1 and it’s run( )
is fairly elaborate: it first gets the thread group of the current thread, and then moves up
the heritage tree by two levels using getParent( ) (this is contrived since I purposely place
the TestThread2 object two levels down in the hierarchy). At this point, an array of
handles to Threads is created, using the method activeCount( ) to ask how many threads
Chapter 14: Multiple Threads 557](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-557-320.jpg)
![are in this thread group and all the child thread groups. The enumerate( ) method places
handles to all these threads in the array gAll, and then I simply move through the entire
array calling the f( ) method for each thread, as well as modifying the priority. Thus, a
thread in a “leaf” thread group modifies threads in parent thread groups.
The debugging method list( ) prints all the information about a thread group to standard
output, and is very helpful when investigating thread group behavior. Here’s the output of
the program:
java.lang.ThreadGroup[name=x,maxpri=10]
Thread[one,5,x]
java.lang.ThreadGroup[name=y,maxpri=10]
java.lang.ThreadGroup[name=z,maxpri=10]
Thread[two,5,z]
one f()
two f()
java.lang.ThreadGroup[name=x,maxpri=10]
Thread[one,1,x]
java.lang.ThreadGroup[name=y,maxpri=10]
java.lang.ThreadGroup[name=z,maxpri=10]
Thread[two,1,z]
Not only does list( ) print the class name of ThreadGroup or Thread, but it also prints
the thread group name and its maximum priority. For threads, the thread name is printed,
followed by the thread priority and the group it belongs to. Note that list( ) indents the
threads and thread groups to indicate that they are children of the un-indented thread
group.
You can see that f( ) is called by the TestThread2 run( ) method, so it’s obvious that all
threads in a group are vulnerable. However, you can access only the threads that branch
off from your own system thread group tree, and perhaps this is what is meant by
“safety.” You cannot access anyone else’s system thread group tree.
Controlling thread groups
Putting aside the safety issue, one thing thread groups do seem to be useful for is control:
you can perform certain operations on an entire thread group with a single command. The
following example demonstrates this and the restrictions on priorities within thread
groups. The commented numbers in parentheses provide a reference to compare to the
output.
//: ThreadGroup1.java
// How thread groups control priorities
// of the threads inside them.
public class ThreadGroup1 {
public static void main(String[] args) {
// Get the system thread & print its Info:
ThreadGroup sys =
Thread.currentThread().getThreadGroup();
sys.list(); // (1)
// Reduce the system thread group priority:
sys.setMaxPriority(Thread.MAX_PRIORITY - 1);
// Increase the main thread priority:
Thread curr = Thread.currentThread();
curr.setPriority(curr.getPriority() + 1);
sys.list(); // (2)
558 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-558-320.jpg)
![// Attempt to set a new group to the max:
ThreadGroup g1 = new ThreadGroup("g1");
g1.setMaxPriority(Thread.MAX_PRIORITY);
// Attempt to set a new thread to the max:
Thread t = new Thread(g1, "A");
t.setPriority(Thread.MAX_PRIORITY);
g1.list(); // (3)
// Reduce g1's max priority, then attempt
// to increase it:
g1.setMaxPriority(Thread.MAX_PRIORITY - 2);
g1.setMaxPriority(Thread.MAX_PRIORITY);
g1.list(); // (4)
// Attempt to set a new thread to the max:
t = new Thread(g1, "B");
t.setPriority(Thread.MAX_PRIORITY);
g1.list(); // (5)
// Lower the max priority below the default
// thread priority:
g1.setMaxPriority(Thread.MIN_PRIORITY + 2);
// Look at a new thread's priority before
// and after changing it:
t = new Thread(g1, "C");
g1.list(); // (6)
t.setPriority(t.getPriority() -1);
g1.list(); // (7)
// Make g2 a child Threadgroup of g1 and
// try to increase its priority:
ThreadGroup g2 = new ThreadGroup(g1, "g2");
g2.list(); // (8)
g2.setMaxPriority(Thread.MAX_PRIORITY);
g2.list(); // (9)
// Add a bunch of new threads to g2:
for (int i = 0; i < 5; i++)
new Thread(g2, Integer.toString(i));
// Show information about all threadgroups
// and threads:
sys.list(); // (10)
System.out.println("Starting all threads:");
Thread[] all = new Thread[sys.activeCount()];
sys.enumerate(all);
for(int i = 0; i < all.length; i++)
if(!all[i].isAlive())
all[i].start();
// Suspends & Stops all threads in
// this group and its subgroups:
System.out.println("All threads started");
sys.suspend(); // Deprecated in Java 1.2
// Never gets here...
System.out.println("All threads suspended");
sys.stop(); // Deprecated in Java 1.2
System.out.println("All threads stopped");
}
} ///:~
Chapter 14: Multiple Threads 559](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-559-320.jpg)
![The output that follows has been edited to allow it to fit on the page (the java.lang. has
been removed) and to add numbers to correspond to the commented numbers in the listing
above.
(1) ThreadGroup[name=system,maxpri=10]
Thread[main,5,system]
(2) ThreadGroup[name=system,maxpri=9]
Thread[main,6,system]
(3) ThreadGroup[name=g1,maxpri=9]
Thread[A,9,g1]
(4) ThreadGroup[name=g1,maxpri=8]
Thread[A,9,g1]
(5) ThreadGroup[name=g1,maxpri=8]
Thread[A,9,g1]
Thread[B,8,g1]
(6) ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,6,g1]
(7) ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,3,g1]
(8) ThreadGroup[name=g2,maxpri=3]
(9) ThreadGroup[name=g2,maxpri=3]
(10)ThreadGroup[name=system,maxpri=9]
Thread[main,6,system]
ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,3,g1]
ThreadGroup[name=g2,maxpri=3]
Thread[0,6,g2]
Thread[1,6,g2]
Thread[2,6,g2]
Thread[3,6,g2]
Thread[4,6,g2]
Starting all threads:
All threads started
All programs have at least one thread running, and the first action in main( ) is to call the
static method of Thread called currentThread( ). From this thread, the thread group is
produced, and list( ) is called for the result. The output is:
(1) ThreadGroup[name=system,maxpri=10]
Thread[main,5,system]
You can see that the name of the main thread group is system, and the name of the main
thread is main, and it belongs to the system thread group.
The second exercise shows that the system group’s maximum priority can be reduced and
the main thread can have it’s priority increased:
(2) ThreadGroup[name=system,maxpri=9]
Thread[main,6,system]
560 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-560-320.jpg)
![The third exercise creates a new thread group g1, which automatically belongs to the
system thread group since it isn’t otherwise specified. A new thread A is placed in g1.
After attempting to set this group’s maximum priority to the highest level and A’s
priority to the highest level, the result is:
(3) ThreadGroup[name=g1,maxpri=9]
Thread[A,9,g1]
Thus it’s not possible to change the thread group’s maximum priority to be higher than
it’s parent thread group.
The fourth exercise reduces g1’s maximum priority by two and then tries to increase it up
to Thread.MAX_PRIORITY. The result is:
(4) ThreadGroup[name=g1,maxpri=8]
Thread[A,9,g1]
You can see that the increase in maximum priority didn’t work. You can only decrease a
thread group’s maximum priority, not increase it. Also, note that thread A’s priority
didn’t change, and now it is higher than the thread group’s maximum priority. Changing a
thread group’s maximum priority doesn’t affect existing threads.
The fifth exercise attempts to set a new thread to maximum priority:
(5) ThreadGroup[name=g1,maxpri=8]
Thread[A,9,g1]
Thread[B,8,g1]
The new thread cannot be changed to anything higher than the maximum thread group
priority.
The default thread priority for this program is 6; that’s the priority a new thread will be
created at and where it will stay if you don’t manipulate the priority. Exercise six lowers
the maximum thread group priority below the default thread priority to see what happens
when you create a new thread under this condition:
(6) ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,6,g1]
Even though the maximum priority of the thread group is 3, the new thread is still created
using the default priority of 6. Thus, maximum thread group priority does not affect
default priority (in fact, there appears to be no way to set the default priority for new
threads).
After changing the priority, attempting to decrement it by one, the result is:
(7) ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,3,g1]
Only when you attempt to change the priority is the thread group’s maximum priority
enforced.
A similar experiment is performed in (8) and (9), when a new thread group g2 is created
as a child of g1 and it’s maximum priority is changed. You can see that it’s impossible for
g2’s maximum to go higher than g1’s:
Chapter 14: Multiple Threads 561](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-561-320.jpg)
![(8) ThreadGroup[name=g2,maxpri=3]
(9) ThreadGroup[name=g2,maxpri=3]
Also note that g2 is automatically set to the thread group maximum priority of g1 as g2
is created.
After all these experiments, the entire system of thread groups and threads is printed out:
(10)ThreadGroup[name=system,maxpri=9]
Thread[main,6,system]
ThreadGroup[name=g1,maxpri=3]
Thread[A,9,g1]
Thread[B,8,g1]
Thread[C,3,g1]
ThreadGroup[name=g2,maxpri=3]
Thread[0,6,g2]
Thread[1,6,g2]
Thread[2,6,g2]
Thread[3,6,g2]
Thread[4,6,g2]
So because of the rules of thread groups, a child group must always have a maximum
priority that’s less than or equal to its parent’s maximum priority.
The last part of this program demonstrates methods for an entire group of threads. First
the program moves through the entire tree of threads and starts each one that hasn’t been
started. For drama, the system group is then suspended and finally stopped (although it’s
interesting to see that suspend( ) and stop( ) work on entire thread groups, you should
keep in mind that these methods are deprecated in Java 1.2). But when you suspend the
system group you also suspend the main thread and the whole program shuts down, so it
never gets to the point where the threads are stopped. Actually, if you do stop the main
thread it throws a ThreadDeath exception, so this is not a very typical thing to do. Since
ThreadGroup is inherited from Object which contains the wait( ) method, you can also
choose to suspend the program for any number of seconds by calling wait(seconds *
1000). This must acquire the lock inside a synchronized block, of course.
The ThreadGroup class also has suspend( ) and resume( ) methods so you can stop and
start an entire thread group and all its threads and subgroups with a single command.
(Again, suspend( ) and resume( ) are deprecated in Java 1.2)
Thread groups can seem a bit mysterious at first, but keep in mind you probably won’t be
using them directly very often.
Runnable revisited
Earlier in this chapter, I suggested that you think carefully before making an applet or
main Frame as an implementation of Runnable, because then you can make only one of
those threads in your program. This limits your flexibility if you decide you want to have
more than one thread of that type.
Of course, if you must inherit from a class and you want to add threading behavior to the
class, Runnable is the correct solution. The final example in this chapter exploits this by
making a Runnable Canvas class that paints different colors on itself. This application is
set up to take values from the command line to determine how big the grid of colors is and
how long to sleep( ) between color changes, and by playing with these values you’ll
discover some interesting and possibly inexplicable features of threads:
562 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-562-320.jpg)
![//: ColorBoxes.java
// Using the Runnable interface
import java.awt.*;
import java.awt.event.*;
class CBox extends Canvas implements Runnable {
private Thread t;
private int pause;
private static final Color colors[] = {
Color.black, Color.blue, Color.cyan,
Color.darkGray, Color.gray, Color.green,
Color.lightGray, Color.magenta,
Color.orange, Color.pink, Color.red,
Color.white, Color.yellow
};
private Color cColor = newColor();
private static final Color newColor() {
return colors[
(int)(Math.random() * colors.length)
];
}
public void paint(Graphics g) {
g.setColor(cColor);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
}
public CBox(int pause) {
this.pause = pause;
t = new Thread(this);
t.start();
}
public void run() {
while(true) {
cColor = newColor();
repaint();
try {
t.sleep(pause);
} catch(InterruptedException e) {}
}
}
}
public class ColorBoxes extends Frame {
public ColorBoxes(int pause, int grid) {
setTitle("ColorBoxes");
setLayout(new GridLayout(grid, grid));
for (int i = 0; i < grid * grid; i++)
add(new CBox(pause));
addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}
public static void main(String[] args) {
Chapter 14: Multiple Threads 563](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-563-320.jpg)
![int pause = 50;
int grid = 8;
if(args.length > 0)
pause = Integer.parseInt(args[0]);
if(args.length > 1)
grid = Integer.parseInt(args[1]);
Frame f = new ColorBoxes(pause, grid);
f.setSize(500, 400);
f.setVisible(true);
}
} ///:~
ColorBoxes is a typical application with a constructor that sets up the GUI. This
constructor takes an argument of int grid to set up the GridLayout so it has grid cells in
each dimension. Then it adds the appropriate number of CBox objects to fill the grid,
passing the pause value to each one. In main( ) you can see how pause and grid have
default values that can be changed if you pass in command-line arguments.
CBox is where all the work takes place. This is inherited from Canvas and it implements
the Runnable interface so each Canvas can also be a Thread. Remember that when you
implement Runnable, you don’t make a Thread object, just a class that has a run( )
method. Thus you must explicitly create a Thread object and hand the Runnable object to
the constructor, then call start( ) (this happens in the constructor). In CBox this thread is
called t.
Note the array colors, which is an enumeration of all the colors in class Color. This is
used in newColor( ) to produce a randomly-selected color. The current cell color is cColor.
paint( ) is quite simple – it just sets the color to cColor and fills the whole canvas with
that color.
In run( ), you see the infinite loop that sets the cColor to a new random color and then
calls repaint( ) to show it. Then the thread goes to sleep( ) for the amount of time
specified on the command line.
Precisely because this design is flexible and threading is tied to each Canvas element, you
can experiment by making as many as you want (in reality, there is a restriction imposed
by the number of threads your JVM can comfortably handle).
This program also makes an interesting benchmark, since it can show dramatic speed
differences between one JVM implementation and another.
Too many threads
At some point, you’ll find that ColorBoxes bogs down. On my machine, this occurred
somewhere after a 10 by 10 grid. Why does this happen? You’re naturally suspicious that
the AWT might have something to do with it, so here’s an example that tests the premise
by making fewer threads. The code is reorganized so a Vector implements Runnable and
that Vector holds a number of color blocks, and randomly chooses ones to update. Then a
number of these Vector objects are created, roughly depending on the grid dimension you
choose. As a result, you have far fewer threads than color blocks, so if there’s a speedup
we’ll know it was because there were too many threads in the previous example:
//: ColorBoxes2.java
// Balancing thread use
import java.awt.*;
import java.awt.event.*;
564 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-564-320.jpg)
![import java.util.*;
class CBox2 extends Canvas {
private static final Color colors[] = {
Color.black, Color.blue, Color.cyan,
Color.darkGray, Color.gray, Color.green,
Color.lightGray, Color.magenta,
Color.orange, Color.pink, Color.red,
Color.white, Color.yellow
};
private Color cColor = newColor();
private static final Color newColor() {
return colors[
(int)(Math.random() * colors.length)
];
}
void nextColor() {
cColor = newColor();
repaint();
}
public void paint(Graphics g) {
g.setColor(cColor);
Dimension s = getSize();
g.fillRect(0, 0, s.width, s.height);
}
}
class CBoxVector
extends Vector implements Runnable {
private Thread t;
private int pause;
public CBoxVector(int pause) {
this.pause = pause;
t = new Thread(this);
}
public void go() { t.start(); }
public void run() {
while(true) {
int i = (int)(Math.random() * size());
((CBox2)elementAt(i)).nextColor();
try {
t.sleep(pause);
} catch(InterruptedException e) {}
}
}
}
public class ColorBoxes2 extends Frame {
private CBoxVector v[];
public ColorBoxes2(int pause, int grid) {
setTitle("ColorBoxes2");
setLayout(new GridLayout(grid, grid));
v = new CBoxVector[grid];
for(int i = 0; i < grid; i++)
v[i] = new CBoxVector(pause);
for (int i = 0; i < grid * grid; i++) {
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![v[i % grid].addElement(new CBox2());
add((CBox2)v[i % grid].lastElement());
}
for(int i = 0; i < grid; i++)
v[i].go();
addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}
public static void main(String[] args) {
// Shorter default pause than ColorBoxes:
int pause = 5;
int grid = 8;
if(args.length > 0)
pause = Integer.parseInt(args[0]);
if(args.length > 1)
grid = Integer.parseInt(args[1]);
Frame f = new ColorBoxes2(pause, grid);
f.setSize(500, 400);
f.setVisible(true);
}
} ///:~
In ColorBoxes2 an array of CBoxVector is created and initialized to hold grid
CBoxVectors, each of which knows how long to sleep. An equal number of Cbox2 objects
are then added to each CBoxVector, and each vector is told to go( ), which starts its
thread.
CBox2 is very similar to CBox: it paints itself with a randomly-chosen color. But that’s all
a CBox2 does. All of the threading has been moved into CBoxVector.
The CBoxVector could also have inherited Thread and had a member object of type Vector.
That design has the advantage that the addElement( ) and elementAt( ) methods could
then be given specific argument and return value types rather than just generic Objects
(their names could also be changed to something shorter). However, the design used here
seemed at first glance to require less code. In addition it automatically retains all the other
behaviors of a Vector. With all the casting and parentheses necessary for elementAt( ),
this might not be the case as your body of code grows.
As before, when you implement Runnable you don’t get all the equipment that comes
with Thread, so you actually have to create a new Thread and hand yourself to it’s
constructor in order to have something to start( ), as you can see in the CBoxVector
constructor and in go( ). The run( ) method simply chooses a random element number
within the vector and calls nextColor( ) for that element to cause it to choose a new
randomly-selected color.
Upon running this program, you see that it does indeed run faster and respond more
quickly (for instance, when you interrupt it, it stops more quickly), and it doesn’t seem to
bog down as much at higher grid sizes. Thus a new factor is added into the threading
equation: you must watch to see that you don’t have “too many threads” (whatever that
turns out to mean for your particular program and platform). If so, you must try to use
techniques like the one above to “balance” the number of threads in your program. Thus,
if you see performance problems in a multithreaded program you now have a number of
issues to examine:
566 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-566-320.jpg)
![Identifying a machine
Of course, in order to tell one machine from another and to make sure you are connected
with exactly the machine you want, there must be some way of uniquely identifying
machines on a network. Early networks were satisfied to provide unique names for
machines within the local network. However, Java works within the Internet, which
requires a way to uniquely identify a machine from all the others in the world. This is
accomplished with the IP (Internet Protocol) address that can exist in two forms:
1. The familiar DNS (Domain Name Service) form. My domain name is
eckelobjects.com, so suppose I have a computer called Opus in my domain.
Its domain name would be Opus.eckelobjects.com. This is exactly the kind of
name that you use when you send email to people, and is often incorporated
into a World-Wide-Web address.
2. Alternatively, you can use the “dotted quad” form, which is four numbers
separated by dots, such as 123.255.28.120.
In both cases, the IP address is represented internally as a 32-bit number1 (so each of the
quad numbers cannot exceed 255), and you can get a special Java object to represent this
number from either of the above forms by using the static InetAddress.getByName( )
method that’s in java.net. The result is an object of type InetAddress that you can use to
build a “socket” as you shall see later.
As a simple example of using InetAddress.getByName( ), consider what happens if you
have a dial-up Internet Service Provider (ISP). Each time you dial up, you are assigned a
temporary IP address. But while you're connected, your IP address has the same validity as
any other IP address on the Internet. If someone connects to your machine using your IP
address then they can connect to a Web server or FTP server that you have running on
your machine. Of course, they need to know your IP address, and since it's assigned each
time you dial up, how can you find out what it is?
The following program uses InetAddress.getByName( ) to produce your IP address. To
use it, you must know the name of your computer. It has been tested only on Windows
95, but there you can go to “Settings,” “Control Panel,” “Network,” and then select the
“Identification” tab. “Computer name” is the name to put on the command line.
//: WhoAmI.java
// Finds out your network address when you're
// connected to the internet.
package c15;
import java.net.*;
public class WhoAmI {
public static void main(String args[])
throws Exception {
if(args.length != 1) {
System.err.println(
"Usage: WhoAmI MachineName");
System.exit(1);
1 This means a maximum of just over 4 billion numbers, which is rapidly running out. The new
standard for IP addresses will use a 128-bit number, which should produce enough unique IP
addresses for the foreseeable future.
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![}
InetAddress a =
InetAddress.getByName(args[0]);
System.out.println(a);
}
} ///:~
In my case, the machine is called “Colossus” (from the movie of the same name, because I
keep putting bigger disks on it). So, once I’ve connected to my ISP I run the program:
java WhoAmI Colossus
I get back a message like this (of course, the address is different each time):
Colossus/199.190.87.75
If I tell my friend this address, he can log onto my personal Web server by going to the
URL http://199.190.87.75 (only as long as I continue to stay connected during that
session). This can sometimes be a handy way to distribute information to someone else, or
to test out a Web site configuration before posting it to a “real” server.
Servers and clients
The whole point of a network is to allow two machines to connect and talk to each other.
Once the two machines have found each other they can have a nice, two-way
conversation. But how do they find each other? It’s like getting lost in an amusement
park: one machine has to stay in one place and listen while the other machine says “hey,
where are you?”
The machine that “stays in one place” is called the server, and the one that seeks is called
the client. This distinction is important only while the client is trying to connect to the
server. Once they’ve connected, it becomes a two-way communication process and it
doesn’t matter anymore that one happened to take the role of server and the other
happened to take the role of the client.
So the job of the server is to listen for a connection, and that’s performed by the special
server object that you create. The job of the client is to try to make a connection to a
server, and this is performed by the special client object you create. Once the connection is
made, you’ll see that at both server and client ends, the connection is just magically turned
into an IO stream object, and from then on you can treat the connection as if you were
reading from and writing to a file. Thus, after the connection is made you will just use the
familiar IO commands from Chapter 10. This is one of the very nice features of Java
networking.
Testing programs without a network
For many reasons, you may not have a client machine, a server machine, and a network
available to test your programs. You may be performing exercises in a classroom
situation, or you may be writing programs that aren’t yet stable enough to put onto the
network. How can you test your code?
The creators of the Internet Protocol were aware of this issue, and they created a special
address called localhost to be the “local loopback” IP address for testing without a
network. The generic way to produce this address in Java is:
InetAddress addr = InetAddress.getByName(null);
If you hand getByName( ) a null, it defaults to using the localhost. The InetAddress is
what you use to refer to the particular machine, and you must produce this before you
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![getInputStream( ) and getOutputStream( ) to produce the corresponding InputStream
and OutputStream objects from each Socket. These must be wrapped inside buffers and
formatting classes just like any other stream object described in Chapter 10.
The use of the term ServerSocket would seem to be another example of a confusing name
scheme in the Java libraries. You might think ServerSocket would be better named
“ServerConnector” or something without the word “Socket” in it. You might also think
that ServerSocket and Socket should both be inherited from some common base class.
Indeed, the two classes do have several methods in common but not enough to give them a
common base class. Instead, ServerSocket’s job is to wait until some other machine
connects to it, then to return an actual Socket. This is why ServerSocket seems to be a bit
misnamed, since its job isn’t really to be a socket but instead to make a Socket object
when someone else connects to it.
However, the ServerSocket does in fact create a physical “server” or listening socket on
the host machine. This socket listens for incoming connections and then returns an
“established” socket (with the local and remote endpoints defined) via the accept( )
method. The confusing part is that both these sockets (listening and established) are
associated with the same server socket. The listening socket can accept only new
connection requests and not data packets. So while ServerSocket doesn't make much sense
programmatically, it does “physically.”
When you create a ServerSocket, you give it only a port number. You don’t have to give it
an IP address because it’s already on the machine it represents. When you create a Socket,
however, you must give both the IP address and the port number where you’re trying to
connect (on the other hand, the Socket that comes back from ServerSocket.accept( )
already contains all this information).
A simple server and client
This example makes the simplest use of servers and clients using sockets. All the server
does is wait for a connection, then uses the Socket produced by that connection to create
an InputStream and OutputStream. After that, everything it reads from the
InputStream it echoes to the OutputStream until it receives the line END, at which time
it closes the connection.
The client makes the connection to the server, then creates an OutputStream. Lines of text
are sent through the OutputStream. The client also creates an InputStream to hear what
the server is saying (which, in this case, is just the words echoed back).
Both the server and client use the same port number and the client uses the local loopback
address to connect to the server on the same machine so you don’t have to test it over a
network (for some configurations, you may need to be connected to a network for the
programs to work, even if you aren’t actually communicating over that network).
Here is the server:
//: JabberServer.java
// Very simple server that just
// echoes whatever the client sends.
import java.io.*;
import java.net.*;
public class JabberServer {
// Choose a port outside of the range 1-1024:
public static final int PORT = 8080;
public static void main(String args[])
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![Both the ServerSocket and the Socket produced by accept( ) are printed to System.out.
This means their toString( ) methods are automatically called. These produce:
ServerSocket[addr=0.0.0.0,PORT=0,localport=8080]
Socket[addr=127.0.0.1,PORT=1077,localport=8080]
Shortly, you’ll see how these fit together with what the client is doing.
The next part of the program looks just like opening files for reading and writing except
that the InputStream and OutputStream are created from the Socket object. Both the
InputStream and OutputStream objects are converted to Java 1.1 Reader and Writer
objects using the “converter” classes InputStreamReader and OutputStreamWriter,
respectively. You could also have used the Java 1.0 InputStream and OutputStream
classes directly, but with output there’s a distinct advantage to using the Writer approach.
This appears with PrintWriter, which has an overloaded constructor that takes a second
argument, a boolean flag which indicates whether or not to automatically flush the
output at the end of each println( ) (but not print( )) statement. Every time you write to
out, its buffer must be flushed so the information actually goes out over the network.
Flushing is important for this particular example because the client and server each wait
for a line from the other party before proceeding. If flushing doesn’t occur, the
information will not be put onto the network until the buffer is full which causes lots of
problems in this example.
When writing network programs you need to be careful about using automatic flushing.
That’s because every time you flush the buffer a packet must be created and sent. In this
case, that’s exactly what we want, since if the packet containing the line isn’t sent then
the handshaking back and forth between server and client will stop. Put another way, the
end of a line is the end of a message. But in many cases messages aren’t delimited by lines
so it’s much more efficient not to use auto flushing and to let the built-in buffering decide
when to build and send a packet. This way, larger packets can be sent and the process will
be faster.
Notice that, like virtually all streams you open, these are buffered. There’s an exercise at
the end of the chapter to show you what happens if you don’t buffer the streams (things
get very slow).
The infinite while loop just reads lines from the BufferedReader in and writes
information to System.out and to the PrintWriter out. Notice that these could be any
streams at all, they just happen to be connected to the network.
When the client sends the line consisting of “END” the program breaks out of the loop and
closes the Socket.
Here’s the client:
//: JabberClient.java
// Very simple client that just sends
// lines to the server and reads lines
// that the server sends.
import java.net.*;
import java.io.*;
public class JabberClient {
public static void main(String args[])
throws IOException {
// Passing null to getByName() produces the
// special "Local Loopback" IP address, for
// testing on one machine w/o a network:
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![which also happens to be on the same machine (127.0.0.1) as the server. Now in order for
data to move between the client and server, each side has to know where to send it.
Therefore, during the process of connecting to the “known” server, the client sends a
“return address” so that the server knows where to send its data. This is what you see in
the example output for the server side:
Socket[addr=127.0.0.1,port=1077,localport=8080]
This means the server just accepted a connection from 127.0.0.1 on port 1077 while
listening on its local port (8080). On the client side:
Socket[addr=localhost/127.0.0.1,PORT=8080,localport=1077]
Which means the client made a connection to 127.0.0.1 on port 8080 using the local port
1077.
You’ll notice that every time you start up the client anew, the local port number is
incremented. It starts at 1025 (one past the reserved block of ports) and keeps going up
until you reboot the machine, at which point it starts back at 1025 again (on UNIX
machines, once the upper limit of the socket range is reached the numbers will wrap
around to the lowest available number again).
Once the Socket object has been created, the process of turning it into a BufferedReader
and PrintWriter is the same as in the server (again, in both cases you start with a Socket).
Here, the client initiates the conversation by sending the string “howdy” followed by a
number. Note that the buffer must again be flushed (which happens automatically via the
second argument to the PrintWriter constructor); if you don’t the whole conversation will
hang because the initial “howdy” will never get sent (the buffer isn’t full enough to cause
the send to happen automatically). Each line that is sent back from the server is written to
System.out to verify that everything is working correctly. To terminate the conversation,
the agreed-upon “END” is sent. If the client simply hangs up then the server throws an
exception.
You can see that the same care is taken here to ensure that the network resources
represented by the Socket are properly cleaned up, using a try-finally block.
Sockets produce a “dedicated” connection which persists until it is explicitly disconnected
(the dedicated connection can still be disconnected un-explicitly if one side, or an
intermediary link, of the connection crashes). This means the two parties are locked in
communication and the connection is constantly open. This seems like a logical approach
to networking, but it puts an extra load on the network. Later in the chapter you’ll see a
different approach to networking, where the connections are only temporary.
Serving multiple clients
The JabberServer works, but it can handle only one client at a time. In a typical server,
you’ll want to be able to deal with many clients at once. The answer is multithreading,
and in languages that don’t directly support multithreading this means all sorts of
complications. In Chapter 14 you saw that multithreading in Java is about as simple as
possible, considering that multithreading is a rather complex topic. Because threading in
Java is reasonably straightforward, making a server that handles multiple clients is
relatively easy.
The basic scheme is to make a single ServerSocket in the server, and call accept( ) to wait
for a new connection. When accept( ) returns, you take the resulting Socket and use it to
create a new thread whose job is to serve that particular client, then you call accept( )
again to wait for a new client.
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![In the following server code, you can see that it looks very similar to the
JabberServer.java example except that all the operations to serve a particular client have
been moved inside a separate thread class:
//: MultiJabberServer.java
// A server that uses multithreading to handle
// any number of clients.
import java.io.*;
import java.net.*;
class ServeOneJabber extends Thread {
private Socket socket;
private BufferedReader in;
private PrintWriter out;
public ServeOneJabber(Socket s)
throws IOException {
socket = s;
in =
new BufferedReader(
new InputStreamReader(
socket.getInputStream()));
// Enable auto-flush:
out =
new PrintWriter(
new BufferedWriter(
new OutputStreamWriter(
socket.getOutputStream())), true);
// If any of the above calls throw an
// exception, the caller is responsible for
// closing the socket. Otherwise the thread
// will close it.
start(); // Calls run()
}
public void run() {
try {
while (true) {
String str = in.readLine();
if (str.equals("END")) break;
System.out.println("Echoing: " + str);
out.println(str);
}
System.out.println("closing...");
} catch (IOException e) {
} finally {
try {
socket.close();
} catch(IOException e) {}
}
}
}
public class MultiJabberServer {
static final int PORT = 8080;
public static void main(String[] args)
throws IOException {
ServerSocket s = new ServerSocket(PORT);
580 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-580-320.jpg)
![}
}
}
public class MultiJabberClient {
static final int MAX_THREADS = 40;
public static void main(String args[])
throws IOException, InterruptedException {
InetAddress addr =
InetAddress.getByName(null);
while(true) {
if(JabberClientThread.threadCount()
< MAX_THREADS)
new JabberClientThread(addr);
Thread.currentThread().sleep(100);
}
}
} ///:~
The JabberClientThread constructor takes an InetAddress and uses it to open a Socket.
You’re probably starting to see the pattern – the Socket is always used to create some kind
of Reader and/or Writer (or InputStream and/or OutputStream) object, which is the
only way the Socket can be used (you can of course write a class or two to automate this
process rather than doing all the typing if it becomes painful). Again, start( ) performs
thread initialization and calls run( ). Here, messages are sent to the server and
information from the server is echoed to the screen. However, the thread has a limited
lifetime and eventually completes. Notice that the socket is cleaned up if the constructor
fails after the socket is created but before the constructor completes. Otherwise the
responsibility for calling close( ) for the socket is relegated to the run( ) method.
The threadcount keeps track of how many JabberClientThread objects currently exist. It
is incremented as part of the constructor, and decremented as run( ) exits (which means
the thread is terminating). In MultiJabberClient.main( ) you can see that the number of
threads is tested, and if there are too many, no more are created. Then the method sleeps.
This way, some threads will eventually terminate and more can be created. You can
experiment with MAX_THREADS to see where your particular system begins to have
trouble with too many connections.
Datagrams
The examples you’ve seen so far use the Transmission Control Protocol (TCP, also known as
stream-based sockets), which is designed for ultimate reliability and guarantees that the
data will get there. It allows retransmission of lost data, it provides multiple paths
through different routers in case one goes down, and bytes are delivered in the order they
are sent. All this control and reliability comes at a cost: TCP has a high overhead.
There’s a second protocol, called User Datagram Protocol (UDP), which doesn’t guarantee
that the packets will be delivered and doesn’t guarantee that they will arrive in the order
they were sent. It’s called an “unreliable protocol” (TCP is a “reliable protocol”), which
sounds bad but because it’s much faster it can be very useful. There are some applications,
such as an audio signal, where it isn’t so critical if a few packets are dropped here or there
but speed is very important. Or consider a time-of-day server, where it really doesn’t
matter if one of the messages is lost. Also, some applications may be able to fire off a UDP
message to a server and can then assume, if there is no response in a reasonable period of
time, that the message was lost.
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![To simplify the creation of a DatagramPacket from a String and vice-versa, the example
begins with a utility class, Dgram, to do the work for you:
//: Dgram.java
// A utility class to convert back and forth
// Between Strings and DataGramPackets.
import java.net.*;
public class Dgram {
public static DatagramPacket toDatagram(
String s, InetAddress destIA, int destPort) {
// Deprecated in Java 1.1, but it works:
byte buf[] = new byte[s.length() + 1];
s.getBytes(0, s.length(), buf, 0);
// The correct Java 1.1 approach, but it's
// Broken (it truncates the String):
// byte buf[] = s.getBytes();
return new DatagramPacket(buf, buf.length,
destIA, destPort);
}
public static String toString(DatagramPacket p){
// The Java 1.0 approach:
// return new String(p.getData(),
// 0, 0, p.getLength());
// The Java 1.1 approach:
return
new String(p.getData(), 0, p.getLength());
}
} ///:~
The first method of Dgram takes a String, an InetAddress and a port number and builds
a DatagramPacket by copying the contents of the String into a byte buffer and passing
the buffer into the DatagramPacket constructor. Note the “+1” in the buffer allocation –
this was necessary to prevent truncation. The getBytes( ) method of String is a special
operation that copies the chars of a String into a byte buffer. This method is now
deprecated; Java 1.1 has a “better” way to do this but it’s commented out here because it
truncates the String. So you’ll get a deprecation message when you compile it under Java
1.1, but the behavior will be correct (this bug may very well be fixed by the time you read
this).
The Dgram.toString( ) method shows both the Java 1.0 approach and the new Java 1.1
approach (which is different because there’s a new kind of String constructor).
Here is the server for the datagram demonstration:
//: ChatterServer.java
// A server that echoes datagrams
import java.net.*;
import java.io.*;
import java.util.*;
public class ChatterServer {
static final int INPORT = 1711;
private byte buf[] = new byte[1000];
private DatagramPacket dp =
new DatagramPacket(buf, buf.length);
// Can listen & send on the same socket:
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![private DatagramSocket socket;
public ChatterServer() {
try {
socket = new DatagramSocket(INPORT);
System.out.println("Server started");
while(true) {
// Block until a datagram appears:
socket.receive(dp);
String rcvd = Dgram.toString(dp) +
", from address: " + dp.getAddress() +
", port: " + dp.getPort();
System.out.println(rcvd);
String echoString =
"Echoed: " + rcvd;
// Extract the address and port from the
// received datagram to find out where to
// send it back:
DatagramPacket echo =
Dgram.toDatagram(echoString,
dp.getAddress(), dp.getPort());
socket.send(echo);
}
} catch(SocketException e) {
System.err.println("Can't open socket");
System.exit(1);
} catch(IOException e) {
System.err.println("Communication error");
e.printStackTrace();
}
}
public static void main(String[] args) {
new ChatterServer();
}
} ///:~
The ChatterServer contains a single DatagramSocket for receiving messages, instead of
creating one each time you’re ready to receive a new message. The single DatagramSocket
can be used over and over. This DatagramSocket has a port number because this is the
server and the client must have an exact address where it wants to send the datagram. It is
given a port number but not an Internet address because it resides on “this” machine so it
knows what its Internet address is (in this case, the default localhost). In the infinite while
loop, the socket is told to receive( ), whereupon it blocks until a datagram shows up, and
then sticks it into our designated receiver, the DatagramPacket dp. The packet is
converted to a String along with information about the Internet address and socket where
the packet came from. This information is displayed, and then an extra string is added to
indicate that it is being echoed back from the server.
Now there’s a bit of a quandary. As you shall see, there are potentially many different
Internet addresses and port numbers that the messages may come from – that is, the
clients may reside on any machine (in this demonstration they all reside on the localhost,
but the port number for each client is different). To send a message back to the client that
originated it, you need to know that client’s Internet address and port number.
Fortunately, this information is conveniently packaged inside the DatagramPacket that
sent the message, so all you have to do is pull it out using getAddress( ) and getPort( ),
which are used to build the DatagramPacket echo which is sent back through the same
586 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-586-320.jpg)
![socket that’s doing the receiving. In addition, when the socket sends the datagram, it
automatically adds the Internet address and port information of this machine, so that
when the client receives the message, it can use getAddress( ) and getPort( ) to find out
where the datagram came from. In fact, the only time that getAddress( ) and getPort( )
don’t tell you where the datagram came from is if you create a datagram to send, and you
call getAddress( ) and getPort( ) before you send the datagram (in which case it tells the
address and port of this machine, the one the datagram is being sent from). This is an
essential part of datagrams: you don’t need to keep track of where a message came from,
because it’s always stored inside the datagram. In fact, the most reliable way to program
is if you don’t try to keep track, but instead always extract the address and port from the
datagram in question (as is done here).
To test this server, here’s a program that makes a number of clients, all of which fire
datagram packets to the server and wait for the server to echo them back.
//: ChatterClient.java
// Tests the ChatterServer by starting up multiple
// clients, each of which sends datagrams.
import java.lang.Thread;
import java.net.*;
import java.io.*;
public class ChatterClient extends Thread {
// Can listen & send on the same socket:
private DatagramSocket s;
private InetAddress hostAddress;
private byte buf[] = new byte[1000];
private DatagramPacket dp =
new DatagramPacket(buf, buf.length);
private int id;
public ChatterClient(int identifier) {
id = identifier;
try {
// Auto-assign port number:
s = new DatagramSocket();
hostAddress =
InetAddress.getByName("localhost");
} catch(UnknownHostException e) {
System.err.println("Cannot find host");
System.exit(1);
} catch(SocketException e) {
System.err.println("Can't open socket");
e.printStackTrace();
System.exit(1);
}
System.out.println("ChatterClient starting");
}
public void run() {
try {
for(int i = 0; i < 25; i++) {
String outMessage = "Client #" +
id + ", message #" + i;
// Make and send a datagram:
s.send(Dgram.toDatagram(outMessage,
hostAddress,
ChatterServer.INPORT));
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![// Block until it echoes back:
s.receive(dp);
// Print out the echoed contents:
String rcvd = "Client #" + id +
", rcvd from " +
dp.getAddress() + ", " +
dp.getPort() + ": " +
Dgram.toString(dp);
System.out.println(rcvd);
}
} catch(IOException e) {
e.printStackTrace();
System.exit(1);
}
}
public static void main(String[] args) {
for(int i = 0; i < 10; i++)
new ChatterClient(i).start();
}
} ///:~
ChatterClient is created as a Thread so that multiple clients may be made to bother the
server. Here you can see the receiving DatagramPacket looks just like the one used for
ChatterServer. In the constructor, the DatagramSocket is created with no arguments
since it doesn’t need to advertise itself as being at a particular port number. The Internet
address used for this socket will be “this machine” (for the example, localhost) and the
port number will be automatically assigned as you will see from the output. This
DatagramSocket, like the one for the server, will be used both for sending and receiving.
The hostAddress is the Internet address of the host machine you want to talk to. The one
place in the program where you must know an exact Internet address and port number is
when you make the outgoing DatagramPacket. As is always the case, the host must be at
a known address and port number so that clients may originate conversations with the
host.
Each thread is given a unique identification number (although the port number
automatically assigned to the thread would also provide a unique identifier). In run( ), a
message String is created that contains the thread’s identification number and the message
number this thread is currently sending. This String is used to create a datagram which is
sent to the host at its address; the port number is taken directly from a constant in
ChatterServer. Once the message is sent, receive( ) blocks until the server replies with an
echoing message. All the information that’s shipped around with the message allows you
to see that what comes back to this particular thread is derived from the message that
originated from this thread. In this example, even though UDP is an “unreliable” protocol,
you’ll see that all the datagrams get where they’re supposed to (this will be true for
localhost and LAN situations, but you may begin to see some failures for non-local
connections).
When you run this program, you’ll see that each of the threads finishes, which means
that each of the datagram packets sent to the server is actually turned around and echoed
to the right recipient, otherwise one or more threads would hang, blocking until their
input shows up.
You might think that the only correct way to, for example, transfer a file from one
machine to another is through TCP sockets, since they’re “reliable.” However, because of
the speed of datagrams they can actually be a better solution. You simply break the file up
into packets and number each packet. The receiving machine takes the packets and
588 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-588-320.jpg)
![The server application
Now consider the server application, which will be called NameCollector. What happens
if more than one user at a time tries to submit their email address? If NameCollector uses
TCP/IP sockets, then it must use the multithreading approach shown earlier to handle
more than one client at a time. But all these threads will try to write to a single file where
all the email addresses will be kept. This would require a locking mechanism to make sure
that more than one thread doesn’t access the file at once. A semaphore will do the trick,
but perhaps there’s a simpler way.
If we use datagrams instead, multithreading is unnecessary. A single datagram socket will
listen for incoming datagrams, and when one appears the program will process the
message and send the reply as a datagram back to whomever sent the request. If the
datagram gets lost, then the user will notice that no reply comes and can then re-submit
the request.
When the server application receives a datagram and unpacks it, all it must do is extract
the email address and check the file to see if that address is there already (and if it isn’t,
add it). And now we run into another problem. It turns out that Java 1.0 doesn’t quite
have the horsepower to easily manipulate the file containing the email addresses (Java 1.1
does). However, the problem can be solved in C quite readily, and this will provide an
excuse to show you the easiest way to connect a non-Java program to a Java program. A
Runtime object for a program has a method called exec( ) which will start up a separate
program on the machine, and return a Process object. You can get an OutputStream that
connects to standard input for this separate program, and an InputStream that connects
to standard output. This means all you need to do is write a program using any language
that takes its input from standard input and writes the output to standard output. This is
a very convenient trick when you run into a problem that can’t be solved easily or quickly
enough in Java (or when you have legacy code you don’t want to rewrite). You can also use
Java’s native methods (see Appendix A) but those are much more involved.
The C program
The job of this non-Java application is to manage the list of email addresses. Standard
input will accept an email address and the program will look up the name in the list to see
if it’s already there. If not, it will add it and report success, but if the name is already
there it will report that. Don’t worry if you don’t completely understand what the
following code means; it’s just one example of how you can write a program in another
language and use it from Java, but the particular programming language doesn’t really
matter as long as it can read from standard input and write to standard output.
//: Listmgr.c
// Used by NameCollector.java to manage
// the email list file on the server
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BSIZE 250
int alreadyInList(FILE* list, char* name) {
char lbuf[BSIZE];
// Go to the beginning of the list:
fseek(list, 0, SEEK_SET);
// Read each line in the list:
while(fgets(lbuf, BSIZE, list)) {
// Strip off the newline:
char * newline = strchr(lbuf, 'n');
590 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-590-320.jpg)
![if(newline != 0)
*newline = '0';
if(strcmp(lbuf, name) == 0)
return 1;
}
return 0;
}
int main() {
char buf[BSIZE];
FILE* list = fopen("emlist.txt", "a+t");
if(list == 0) {
perror("could not open emlist.txt");
exit(1);
}
while(1) {
gets(buf); /* From stdin */
if(alreadyInList(list, buf)) {
printf("Already in list: %s", buf);
fflush(stdout);
}
else {
fseek(list, 0, SEEK_END);
fprintf(list, "%sn", buf);
fflush(list);
printf("%s added to list", buf);
fflush(stdout);
}
}
} ///:~
This assumes that the C compiler accepts ‘//’ style comments (many do, and you can also
compile this program with a C++ compiler). If yours doesn’t, simply delete those
comments.
The first function in the file checks to see whether the name you hand it as a second
argument (a pointer to a char) is in the file. Here, the file is passed as a FILE pointer to an
already-opened file (the file is opened inside main( )). The function fseek( ) moves around
in the file; here it is used to move to the top of the file. fgets( ) reads a line from the file
list into the buffer lbuf, not exceeding the buffer size BSIZE. This is inside a while loop so
that each line in the file is read. Next, strchr( ) is used to locate the newline character so
that it can be stripped off. Finally, strcmp( ) is used to compare the name you’ve passed
into the function to the current line int the file. strcmp( ) returns zero if it finds a match.
In this case the function exits and a one is returned to indicate that yes, the name was
already in the list (notice the function returns as soon as it discovers the match, so it
doesn’t waste time looking at the rest of the list). If you get all the way through the list
without a match, the function returns zero.
In main( ), the file is opened using fopen( ). The first argument is the file name and the
second is the way to open the file; a+ means “append, and open (or create if the file does
not exist) for update at the end of the file.” The fopen( ) function returns a FILE pointer
which, if it’s zero, means the open was unsuccessful. This is dealt with by printing an
error message with perror( ) and terminating the program with exit( ).
Assuming the file was opened successfully, the program enters an infinite loop. The
function call gets(buf) gets a line from standard input (which will be connected to the
Java program, remember) and places it in the buffer buf. This is simply passed to the
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![alreadyInList( ) function, and if it’s already in the list, printf( ) sends that message to
standard output (where the Java program is listening). fflush( ) is a way to flush the
output buffer.
If the name is not already in the list, fseek( ) is used to move to the end of the list and
fprintf( ) “prints” the name to the end of the list. Then printf( ) is used to indicate that the
name was added to the list (again flushing standard output) and the infinite loop goes back
to waiting for a new name.
Remember that you normally cannot compile this program on your computer and just
load it onto the Web server machine, since that machine may use a different processor and
operating system. For example, my Web server runs on an Intel processor but it uses
Linux, so I must download the source code and compile using remote commands (via
telnet) with the C compiler that comes with the Linux distribution.
The Java program
This program will first start the above C program and make the necessary connections to
talk to it. Then it will create a datagram socket which will be used to listen for datagram
packets from the applet.
//: NameCollector.java
// Extracts email names from datagrams and stores
// them inside a file, using Java 1.02.
import java.net.*;
import java.io.*;
import java.util.*;
public class NameCollector {
final static int COLLECTOR_PORT = 8080;
final static int BUFFER_SIZE = 1000;
byte buf[] = new byte[BUFFER_SIZE];
DatagramPacket dp =
new DatagramPacket(buf, buf.length);
// Can listen & send on the same socket:
DatagramSocket socket;
Process listmgr;
PrintStream nameList;
DataInputStream addResult;
public NameCollector() {
try {
listmgr =
Runtime.getRuntime().exec("listmgr.exe");
nameList = new PrintStream(
new BufferedOutputStream(
listmgr.getOutputStream()));
addResult = new DataInputStream(
new BufferedInputStream(
listmgr.getInputStream()));
} catch(IOException e) {
System.err.println(
"Cannot start listmgr.exe");
System.exit(1);
}
try {
socket =
592 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-592-320.jpg)
![new DatagramSocket(COLLECTOR_PORT);
System.out.println(
"NameCollector Server started");
while(true) {
// Block until a datagram appears:
socket.receive(dp);
String rcvd = new String(dp.getData(),
0, 0, dp.getLength());
// Send to listmgr.exe standard input:
nameList.println(rcvd.trim());
nameList.flush();
byte resultBuf[] = new byte[BUFFER_SIZE];
int byteCount =
addResult.read(resultBuf);
if(byteCount != -1) {
String result =
new String(resultBuf, 0).trim();
// Extract the address and port from
// the received datagram to find out
// where to send the reply:
InetAddress senderAddress =
dp.getAddress();
int senderPort = dp.getPort();
byte echoBuf[] = new byte[BUFFER_SIZE];
result.getBytes(
0, byteCount, echoBuf, 0);
DatagramPacket echo =
new DatagramPacket(
echoBuf, echoBuf.length,
senderAddress, senderPort);
socket.send(echo);
}
else
System.out.println(
"Unexpected lack of result from " +
"listmgr.exe");
}
} catch(SocketException e) {
System.err.println("Can't open socket");
System.exit(1);
} catch(IOException e) {
System.err.println("Communication error");
e.printStackTrace();
}
}
public static void main(String[] args) {
new NameCollector();
}
} ///:~
The first definitions in NameCollector should look familiar: the port is chosen, a
datagram packet is created, and there’s a handle to a DatagramSocket. The next three
definitions concern the connection to the C program: a Process object is what comes back
when the C program is fired up by the Java program, and that Process object produces the
InputStream and OutputStream objects representing, respectively, the standard output
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![private String str = new String();
private Label
l = new Label(), l2 = new Label();
private DatagramSocket s;
private InetAddress hostAddress;
private byte buf[] =
new byte[NameCollector.BUFFER_SIZE];
private DatagramPacket dp =
new DatagramPacket(buf, buf.length);
private int vcount = 0;
public void init() {
setLayout(new BorderLayout());
Panel p = new Panel();
p.setLayout(new GridLayout(2, 1));
p.add(t);
p.add(send);
add("North", p);
Panel labels = new Panel();
labels.setLayout(new GridLayout(2, 1));
labels.add(l);
labels.add(l2);
add("Center", labels);
try {
// Auto-assign port number:
s = new DatagramSocket();
hostAddress = InetAddress.getByName(
getCodeBase().getHost());
} catch(UnknownHostException e) {
l.setText("Cannot find host");
} catch(SocketException e) {
l.setText("Can't open socket");
}
l.setText("Ready to send your email address");
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(send)) {
if(pl != null) {
// pl.stop(); Deprecated in Java 1.2
Thread remove = pl;
pl = null;
remove.interrupt();
}
l2.setText("");
// Check for errors in email name:
str = t.getText().toLowerCase().trim();
if(str.indexOf(' ') != -1) {
l.setText("Spaces not allowed in name");
return true;
}
if(str.indexOf(',') != -1) {
l.setText("Commas not allowed in name");
return true;
}
if(str.indexOf('@') == -1) {
l.setText("Name must include '@'");
l2.setText("");
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![return true;
}
if(str.indexOf('@') == 0) {
l.setText("Name must preceed '@'");
l2.setText("");
return true;
}
String end =
str.substring(str.indexOf('@'));
if(end.indexOf('.') == -1) {
l.setText("Portion after '@' must " +
"have an extension, such as '.com'");
l2.setText("");
return true;
}
// Everything's OK, so send the name. Get a
// fresh buffer, so it's zeroed. For some
// reason you must use a fixed size rather
// than calculating the size dynamically:
byte sbuf[] =
new byte[NameCollector.BUFFER_SIZE];
str.getBytes(0, str.length(), sbuf, 0);
DatagramPacket toSend =
new DatagramPacket(
sbuf, 100, hostAddress,
NameCollector.COLLECTOR_PORT);
try {
s.send(toSend);
} catch(Exception e) {
l.setText("Couldn't send datagram");
return true;
}
l.setText("Sent: " + str);
send.setLabel("Re-send");
pl = new Thread(this);
pl.start();
l2.setText(
"Waiting for verification " + ++vcount);
}
else return super.action(evt, arg);
return true;
}
// The thread portion of the applet watches for
// the reply to come back from the server:
public void run() {
try {
s.receive(dp);
} catch(Exception e) {
l2.setText("Couldn't receive datagram");
return;
}
l2.setText(new String(dp.getData(),
0, 0, dp.getLength()));
}
} ///:~
596 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-596-320.jpg)
![executable program that resides in the directory that’s typically called “cgi-bin” on your
Web server3 (if the named program is not in the cgi-bin directory, you won’t see any
results at all). If you fill out this form and press the “submit” button, you will see in the
URL address window of the browser something like:
http://www.myhome.com/cgi-bin/Listmgr2.exe?
name=First+Last&email=email@domain.com&submit=Submit
(Without the line break, of course). Here you see a little bit of the way that data is encoded
to send to CGI. For one thing, there are no spaces allowed (since spaces typically separate
command-line arguments). Spaces are replaced by ‘+’ signs. In addition, each field
contains the field name (which is determined by the HTML page) followed by an ‘=’ and
the field data, and terminated by a ‘&’.
At this point you may wonder about the ‘+’, ‘=’ and ‘&’. What if those are used in the
field, as in “John & Marsha Smith”? This is encoded to:
John+%26+Marsha+Smith
That is, the special character is turned into a ‘%’ followed by its ASCII value in hex.
Fortunately, Java has a tool to perform this encoding for you. It’s a static method of the
class URLEncoder called encode( ). You can experiment with this method using the
following program:
//: EncodeDemo.java
// Demonstration of URLEncoder.encode()
import java.net.*;
public class EncodeDemo {
public static void main(String args[]) {
String s = "";
for(int i = 0; i < args.length; i++)
s += args[i] + " ";
s = URLEncoder.encode(s.trim());
System.out.println(s);
}
} ///:~
This takes the command-line arguments and combines them into a string of words
separated by spaces (the final space is removed using String.trim( )). These are then
encoded and printed.
To invoke a CGI program, then, all the applet needs to do is collect the data from its fields
(or wherever it needs to collect the data from), URL-encode each piece of data, and then
assemble it into a single string, placing the name of each field followed by an ‘=’, followed
by the data, followed by an ‘&’. To form the entire CGI command, this string is placed after
the URL of the CGI program and a ‘?’. That’s all it takes to invoke any CGI program, and
as you’ll see you can easily do it within an applet.
3 You can test this under Windows32 using the Microsoft Personal Web Server that comes with
Microsoft Office 97 and some of their other products. This is a nice way to experiment since you can
perform local tests (and it's also very fast). If you're on a different platform or you don't have
Office 97 you may be able to find a freeware Web server for testing by searching the Internet.
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![else return super.action(evt, arg);
return true;
}
} ///:~
The beauty of the URL class is how much it shields you from. You can connect to Web
servers without knowing much at all about what’s going on under the covers.
The CGI program in C++
At this point you could follow the previous example and write the CGI program for the
server using ANSI C. One argument for doing this is that ANSI C can be found virtually
everywhere. However, C++ has become quite ubiquitous, especially in the form of the
GNU C++ Compiler4 (g++) which can be downloaded free from the Internet for virtually
any platform (and often comes pre-installed with operating systems such as Linux). As
you will see, this means you can get the benefit of object-oriented programming in a CGI
program.
To avoid throwing too many new concepts at you all at once, this program will not be a
“pure” C++ program; some code will be written in plain C even though C++ alternatives
exist. This isn’t a significant issue because the biggest benefit in using C++ for this
program is the ability to create classes. Since what we’re concerned with when parsing the
CGI information is the field name-value pairs, one class (Pair) will be used to represent a
single name-value pair, and a second class (CGI_vector) will automatically parse the CGI
string into Pair objects which it will hold (as a vector) so you can fetch each Pair out at
your leisure.
This program is also interesting because it demonstrates some of the pluses and minuses
of C++ in contrast with Java. You’ll see some similarities; for example the class keyword.
Access control has identical keywords public and private, but they’re used differently:
they control a block instead of a single method or field (that is, if you say private: each
following definition is private until you say public:). Also, when you create a class, all the
definitions automatically default to private.
One of the reasons for using C++ here is the convenience of the C++ Standard Template
Library (STL). Among other things, the STL contains a vector class. This is a C++
template, which means it will be configured at compile time so it will hold objects of only
a particular type (in this case, Pair objects). Unlike the Java Vector, which will accept
anything, the C++ vector template will cause a compile-time error message if you try to
put anything but a Pair object into the vector, and when you get something out of the
vector it will automatically be a Pair object, without casting. Thus, the checking happens
at compile time and produces a more robust program. In addition, the program can run
faster since you don’t have to perform run-time casts. The vector also overloads the
operator[] so you have a convenient syntax for extracting Pair objects. The vector
template will be used in the creation of CGI_vector, which you’ll see is a fairly short
definition considering how powerful it is.
On the down side, look at the complexity of the definition of Pair in the following code.
Pair has more method definitions than you’re used to seeing in Java code, because the
C++ programmer must know how to control copying with the copy-constructor and
4 GNU stands for “Gnu’s Not Unix.” The project, created by the Free Software Foundation, was
originally intended to replace the Unix operating system with a free version of that OS. Linux
appears to have replaced this initiative, but the GNU tools have played an integral part in the
development of Linux which comes packaged with many GNU components.
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![assignment with the overloaded operator=. As described in Chapter 12, occasionally you
need to concern yourself with similar things in Java, but in C++ you must be aware of
them almost constantly.
The project will start with a reusable portion, which consists of Pair and CGI_vector in a
C++ header file. Technically, you shouldn’t cram this much into a header file, but for
these examples it doesn’t hurt anything and it will also look more Java-like, so it will be
easier for you to read:
//: CGITools.h
// Automatically extracts and decodes data
// from CGI GETs and POSTs. Tested with GNU C++
// (available for most server machines).
#include <string.h>
#include <vector> // STL vector
using namespace std;
// A class to hold a single name-value pair from
// a CGI query. CGI_vector holds Pair objects and
// returns them from its operator[].
class Pair {
char* nm;
char* val;
public:
Pair() { nm = val = 0; }
Pair(char* name, char* value) {
// Creates new memory:
nm = decodeURLString(name);
val = decodeURLString(value);
}
const char* name() const { return nm; }
const char* value() const { return val; }
// Test for "emptiness"
bool empty() const {
return (nm == 0) || (val == 0);
}
// Automatic type conversion for boolean test:
operator bool() const {
return (nm != 0) && (val != 0);
}
// The following constructors & destructor are
// necessary for bookkeeping in C++.
// Copy-constructor:
Pair(const Pair& p) {
if(p.nm == 0 || p.val == 0) {
nm = val = 0;
} else {
// Create storage & copy rhs values:
nm = new char[strlen(p.nm) + 1];
strcpy(nm, p.nm);
val = new char[strlen(p.val) + 1];
strcpy(val, p.val);
}
}
// Assignment operator:
Pair& operator=(const Pair& p) {
606 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-606-320.jpg)
![// Clean up old lvalues:
delete nm;
delete val;
if(p.nm == 0 || p.val == 0) {
nm = val = 0;
} else {
// Create storage & copy rhs values:
nm = new char[strlen(p.nm) + 1];
strcpy(nm, p.nm);
val = new char[strlen(p.val) + 1];
strcpy(val, p.val);
}
return *this;
}
~Pair() { // Destructor
delete nm; // 0 value OK
delete val;
}
// If you use this method outide this class,
// you're responsible for calling 'delete' on
// the pointer that's returned:
static char*
decodeURLString(const char* URLstr) {
int len = strlen(URLstr);
char* result = new char[len + 1];
memset(result, len + 1, 0);
for(int i = 0, j = 0; i <= len; i++, j++) {
if(URLstr[i] == '+')
result[j] = ' ';
else if(URLstr[i] == '%') {
result[j] =
translateHex(URLstr[i + 1]) * 16 +
translateHex(URLstr[i + 2]);
i += 2; // Move past hex code
} else // An ordinary character
result[j] = URLstr[i];
}
return result;
}
// Translate a single hex character; used by
// decodeURLString():
static char translateHex(char hex) {
if(hex >= 'A')
return (hex & 0xdf) - 'A' + 10;
else
return hex - '0';
}
};
// Parses any CGI query and turns it
// into an STL vector of Pair objects:
class CGI_vector : public vector<Pair> {
char* qry;
const char* start; // Save starting position
// Prevent assignment and copy-construction:
void operator=(CGI_vector&);
Chapter 15: Network Programming 607](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-607-320.jpg)
![CGI_vector(CGI_vector&);
public:
// const fields must be initialized in the C++
// "Constructor initializer list":
CGI_vector(char* query) :
start(new char[strlen(query) + 1]) {
qry = (char*)start; // Cast to non-const
strcpy(qry, query);
Pair p;
while((p = nextPair()) != 0)
push_back(p);
}
// Destructor:
~CGI_vector() { delete start; }
private:
// Produces name-value pairs from the query
// string. Returns an empty Pair when there's
// no more query string left:
Pair nextPair() {
char* name = qry;
if(name == 0 || *name == '0')
return Pair(); // End, return null Pair
char* value = strchr(name, '=');
if(value == 0)
return Pair(); // Error, return null Pair
// Null-terminate name, move value to start
// of its set of characters:
*value = '0';
value++;
// Look for end of value, marked by '&':
qry = strchr(value, '&');
if(qry == 0) qry = ""; // Last pair found
else {
*qry = '0'; // Terminate value string
qry++; // Move to next pair
}
return Pair(name, value);
}
}; ///:~
After the #include statements, you see a line that says
using namespace std;
Namespaces in C++ solve one of the problems taken care of by the package scheme in
Java: hiding library names. The std namespace refers to the Standard C++ library, and
vector is in this library so the line is required.
The Pair class starts out looking pretty simple: it just holds two (private) character
pointers, one for the name and one for the value. The default constructor simply sets these
pointers to zero, since in C++ an object’s memory isn’t automatically zeroed. The second
constructor calls the method decodeURLString( ) which produces a decoded string in
newly-allocated heap memory. This memory must be managed and destroyed by the
object, as you shall see in the destructor. The name( ) and value( ) methods produce
read-only pointers to the respective fields. The empty( ) method is a way for you to ask
the Pair object whether either of its fields are empty; it returns bool which is C++’s
608 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-608-320.jpg)
![// Automatic type conversion for boolean test:
operator bool() const {
return (nm.length() != 0) && (val.length() != 0);
}
(Also, for this case decodeURLString( ) returns a string instead of a char*). You don’t
need to define a copy-constructor, operator= or destructor because the compiler does that
for you, and does it correctly. But even if it sometimes works automatically, C++
programmers must still know the details of copy-construction and assignment.
The remainder of the Pair class consists of the two methods decodeURLString( ) and a
helper method translateHex( ) which is used by decodeURLString( ). After allocating
adequate storage (which must be released by the destructor), decodeURLString( ) moves
through and replaces each ‘+’ with a space, and each hex code (beginning with a ‘%’) with
the appropriate character.
CGI_vector parses and holds an entire CGI GET command. It is inherited from the STL
vector which is instantiated to hold Pairs. Inheritance in C++ is denoted by using a colon
where you’d say extends in Java. In addition, inheritance defaults to private so you’ll
almost always need to use the public keyword as was done here. You can also see that
CGI_vector has a copy-constructor and an operator=, but they’re both declared as
private. This is to prevent the compiler from synthesizing the two functions (which it will
do if you don’t declare them yourself), but it also prevents the client programmer from
passing a CGI_vector by value or from using assignment.
CGI_vector’s job is to take the QUERY_STRING and parse it into name-value pairs, which
it will do with the aid of Pair. First it copies the string into locally-allocated memory and
keeps track of the starting address with the constant pointer start (this is later used in the
destructor to release the memory). Then it uses its method nextPair( ) to parse the string
into raw name-value pairs, delimited by ‘=’ and ‘&’ signs. These are handed by
nextPair( ) to the Pair constructor so nextPair( ) can return the Pair object which is then
added to the vector with push_back( ). When nextPair( ) runs out of QUERY_STRING, it
returns zero.
Now that the basic tools are defined, they can easily be used in a CGI program, like this:
//: Listmgr2.cpp
// CGI version of Listmgr.c in C++, which
// extracts its input via the GET submission
// from the associated applet. Also works as
// an ordinary CGI program with HTML forms.
#include <stdio.h>
#include "CGITools.h"
const char* dataFile = "list2.txt";
const char* notify = "Bruce@EckelObjects.com";
#undef DEBUG
// Similar code as before, except that it looks
// for the email name inside of '<>':
int inList(FILE* list, const char* emailName) {
const int BSIZE = 255;
char lbuf[BSIZE];
char emname[BSIZE];
// Put the email name in '<>' so there's no
// possibility of a match within another name:
sprintf(emname, "<%s>", emailName);
// Go to the beginning of the list:
610 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-610-320.jpg)
![fseek(list, 0, SEEK_SET);
// Read each line in the list:
while(fgets(lbuf, BSIZE, list)) {
// Strip off the newline:
char * newline = strchr(lbuf, 'n');
if(newline != 0)
*newline = '0';
if(strstr(lbuf, emname) != 0)
return 1;
}
return 0;
}
void main() {
// You MUST print this out, otherwise the
// server will not send the response:
printf("Content-type: text/plainnn");
FILE* list = fopen(dataFile, "a+t");
if(list == 0) {
printf("error: could not open database. ");
printf("Notify %s", notify);
return;
}
// For a CGI "GET," the server puts the data
// in the environment variable QUERY_STRING:
CGI_vector query(getenv("QUERY_STRING"));
#if defined(DEBUG)
// Test: dump all names and values
for(int i = 0; i < query.size(); i++) {
printf("query[%d].name() = [%s], ",
i, query[i].name());
printf("query[%d].value() = [%s]n",
i, query[i].value());
}
#endif(DEBUG)
Pair name = query[0];
Pair email = query[1];
if(name.empty() || email.empty()) {
printf("error: null name or email");
return;
}
if(inList(list, email.value())) {
printf("Already in list: %s", email.value());
return;
}
// It's not in the list, add it:
fseek(list, 0, SEEK_END);
fprintf(list, "%s <%s>;n",
name.value(), email.value());
fflush(list);
fclose(list);
printf("%s <%s> added to listn",
name.value(), email.value());
} ///:~
Chapter 15: Network Programming 611](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-611-320.jpg)
![The alreadyInList( ) function is almost identical to the previous version, except that it
assumes all email names are inside ‘<>’.
When you use the GET approach (which is normally done in the HTML METHOD tag of
the FORM directive, but which is controlled here by the way the data is sent), the Web
server grabs everything after the ‘?’ and puts in into the environment variable
QUERY_STRING. So to read that information all you have to do is get the value of
QUERY_STRING, which you do using the standard C library function getenv( ). In
main( ), notice how simple the act of parsing the QUERY_STRING is: you just hand it to
the constructor for the CGI_vector object called query and all the work is done for you.
From then on you can pull out the names and values from query as if it were an array
(this is because the operator[] is overloaded in vector). You can see how this works in the
debug code, which is surrounded by the preprocessor directives #if defined(DEBUG) and
#endif(DEBUG).
Now it’s important to understand something about CGI. A CGI program is handed its input
in one of two ways: through QUERY_STRING during a GET (as in this case) or through
standard input during a POST. But a CGI program sends its output through standard
output, typically using printf( ) in a C program. Where does this output go? Back to the
web server, which decides what to do with it. The server makes this decision based on the
content-type header, which means that if the content-type header isn’t the first thing it
sees, it won’t know what to do with the data. Thus it’s essential that you start the output
of all CGI programs with the content-type header.
In this case, we want the server to feed all the information directly back to the client
program (which is our applet, waiting for its reply). The information should be
unchanged, so the content-type is text/plain. Once the server sees this, it will echo all
strings right back to the client. Thus each of the strings you see, three for error conditions
and one for a successful add, will end up right back at the applet.
Adding the email name uses the same code. In the case of the CGI script, however, there
isn’t an infinite loop – the program just responds and then terminates. Each time a CGI
request comes in, the program is started in response to that request, and then it shuts
down. Thus there is no possibility of CPU hogging, and the only performance issue
concerns starting the program up and opening the file, which are dwarfed by the overhead
of the Web server as it handles the CGI request.
One of the advantages of this design is that, now that Pair and CGI_vector are defined,
most of the work is done for you so you can easily create your own CGI program simply
by modifying main( ). Eventually, Servlet servers will probably be ubiquitous but in the
meantime C++ is still pretty handy for creating fast CGI programs.
What about POST?
Using a GET is fine for many applications. However, GET passes its data to the CGI
program through an environment variable, and some Web servers can run out of
environment space with long GET strings (you should start worrying at about 200
characters). CGI provides a solution for this: POST. With POST, the data is encoded and
concatenated the same way as a GET, but POST uses standard input to pass the encoded
query string to the CGI program. All you have to do is determine the length of the query
string, and this length is stored in the environment variable CONTENT_LENGTH. Once you
know the length, you can allocate storage and read the precise number of bytes from
standard input.
The Pair and CGI_vector from CGITools.h can be used as is for a CGI program that
handles POSTs. The following listing shows how simple it is to write such a CGI program.
612 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-612-320.jpg)
![In this example, “pure” C++ will be used so the stdio.h library will be dropped in favor of
iostreams. With iostreams, two pre-defined objects are available: cin which connects to
standard input, and cout which connects to standard output. There are several ways to
read from cin and write to cout, but the following program take the common approach of
using ‘<<’ to send information to cout, and the use of a member function (in this case,
read( )) to read from cin.
//: POSTtest.cpp
// CGI_vector works as easily with POST as it
// does with GET. Written in "pure" C++.
#include <iostream.h>
#include "CGITools.h"
void main() {
cout << "Content-type: text/plainn" << endl;
// For a CGI "POST," the server puts the length
// of the content string in the environment
// variable CONTENT_LENGTH:
char* clen = getenv("CONTENT_LENGTH");
if(clen == 0) {
cout << "Zero CONTENT_LENGTH" << endl;
return;
}
int len = atoi(clen);
char* query_str = new char[len + 1];
cin.read(query_str, len);
query_str[len] = '0';
CGI_vector query(query_str);
// Test: dump all names and values
for(int i = 0; i < query.size(); i++)
cout << "query[" << i << "].name() = [" <<
query[i].name() << "], " <<
"query[" << i << "].value() = [" <<
query[i].value() << "]" << endl;
delete query_str; // Release storage
} ///:~
The getenv( ) function returns a pointer to a character string representing the content
length. If this pointer is zero, the CONTENT_LENGTH environment variable has not been
set, so something is wrong. Otherwise, the character string must be converted to an
integer using the ANSI C library function atoi( ). The length is used with new to allocate
enough storage to hold the query string (plus its null terminator), and then read( ) is
called for cin. The read( ) function takes a pointer to the destination buffer and the
number of bytes to read. The query_str is then null-terminated to indicate the end of the
character string.
At this point the query string is no different from a GET query string, so it is handed to
the constructor for CGI_vector. The different fields in the vector are then available just as
in the previous example.
To test this program, you must compile it in the cgi-bin directory of your host Web server.
Then you can perform a simple test by writing an HTML page like this:
<HTML>
<HEAD>
<META CONTENT="text/html">
<TITLE>A test of standard HTML POST</TITLE>
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![</HEAD>
Test, uses standard html POST
<Form method="POST" ACTION="/cgi-bin/POSTtest">
<P>Field1: <INPUT TYPE = "text" NAME = "Field1"
VALUE = "" size = "40"></p>
<P>Field2: <INPUT TYPE = "text" NAME = "Field2"
VALUE = "" size = "40"></p>
<P>Field3: <INPUT TYPE = "text" NAME = "Field3"
VALUE = "" size = "40"></p>
<P>Field4: <INPUT TYPE = "text" NAME = "Field4"
VALUE = "" size = "40"></p>
<P>Field5: <INPUT TYPE = "text" NAME = "Field5"
VALUE = "" size = "40"></p>
<P>Field6: <INPUT TYPE = "text" NAME = "Field6"
VALUE = "" size = "40"></p>
<p><input type = "submit" name = "submit" > </p>
</Form>
</HTML>
When you fill this out and submit it, you’ll get back a simple text page containing the
parsed results, so you can see that the CGI program works correctly.
Of course, it’s a little more interesting to submit the data using an applet. Submitting
POST data is a different process, however. After you invoke the CGI program in the usual
way, you must make a direct connection to the server so you can feed it the query string.
The server then turns around and feeds the query string to the CGI program via standard
input.
To make a direct connection to the server, you must take the URL you’ve created and call
openConnection( ) to produce a URLConnection. Then, because a URLConnection
doesn’t normally allow you to send data to it, you must call the magic function
setDoOutput(true), along with setDoInput(true) and setAllowUserInteraction(false)6 .
Finally, you can call getOutputStream( ) to produce an OutputStream, which you wrap
inside a DataOutputStream so you can talk to it conveniently. Here’s an applet that does
just that, after collecting data from its various fields:
//: POSTtest.java
// An applet that sends its data via a CGI POST
import java.awt.*;
import java.applet.*;
import java.net.*;
import java.io.*;
public class POSTtest extends Applet {
final static int SIZE = 10;
Button submit = new Button("Submit");
TextField t[] = new TextField[SIZE];
String query = "";
Label l = new Label();
TextArea ta = new TextArea(15, 60);
6 I can’t say I really understand what’s going on here, but I managed to get it working by
studying Java Network Programming by Elliotte Rusty Harold (O’Reilly 1997). He alludes to a
number of confusing bugs in the Java networking libraries, so this is an area where you can’t just
write code and have it working right away. Be warned.
614 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-614-320.jpg)
![public void init() {
Panel p = new Panel();
p.setLayout(new GridLayout(t.length + 2, 2));
for(int i = 0; i < t.length; i++) {
p.add(new Label(
"Field " + i + " ", Label.RIGHT));
p.add(t[i] = new TextField(30));
}
p.add(l);
p.add(submit);
add("North", p);
add("South", ta);
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(submit)) {
query = "";
ta.setText("");
// Encode the query from the field data:
for(int i = 0; i < t.length; i++)
query += "Field" + i + "=" +
URLEncoder.encode(
t[i].getText().trim()) +
"&";
query += "submit=Submit";
// Send the name using CGI's POST process:
try {
URL u = new URL(
getDocumentBase(), "cgi-bin/POSTtest");
URLConnection urlc = u.openConnection();
urlc.setDoOutput(true);
urlc.setDoInput(true);
urlc.setAllowUserInteraction(false);
DataOutputStream server =
new DataOutputStream(
urlc.getOutputStream());
// Send the data
server.writeBytes(query);
server.close();
// Read and display the response. You
// cannot use
// getAppletContext().showDocument(u);
// to display the results as a Web page!
DataInputStream in =
new DataInputStream(
urlc.getInputStream());
String s;
while((s = in.readLine()) != null) {
ta.appendText(s + "n");
}
in.close();
}
catch (Exception e) {
l.setText(e.toString());
}
}
else return super.action(evt, arg);
Chapter 15: Network Programming 615](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-615-320.jpg)
![the name using your database administration software (the process of
registration varies from platform to platform).
All this information is combined together into one string, the “database URL.” For
example, to connect through the ODBC subprotocol to a database identified as “people,”
the database URL could be:
String dbUrl = "jdbc:odbc:people";
If you’re connecting across a network, the database URL will also contain the information
identifying the remote machine.
When you’re ready to connect to the database, you call the static method
DriverManager.getConnection( ), passing it the database URL, the user name and a
password to get into the database. You get back a Connection object which you can then
use to query and manipulate the database.
The following example opens a database of contact information and looks for a person’s
last name as given on the command line. It selects only the names of people that have
email addresses, then prints out all the ones that match the given last name:
//: Lookup.java
// Looks up email addresses in a
// local database using JDBC
import java.sql.*;
public class Lookup {
public static void main(String args[]) {
String dbUrl = "jdbc:odbc:people";
String user = "";
String password = "";
try {
// Load the driver (registers itself)
Class.forName(
"sun.jdbc.odbc.JdbcOdbcDriver");
Connection c = DriverManager.getConnection(
dbUrl, user, password);
Statement s = c.createStatement();
// SQL code:
ResultSet r =
s.executeQuery(
"SELECT FIRST, LAST, EMAIL " +
"FROM people.csv people " +
"WHERE " +
"(LAST='" + args[0] + "') " +
" AND (EMAIL Is Not Null) " +
"ORDER BY FIRST");
while(r.next()) {
// Capitalization doesn't matter:
System.out.println(
r.getString("Last") + ", "
+ r.getString("fIRST")
+ ": " + r.getString("EMAIL") );
}
s.close(); // Also closes ResultSet
} catch(Exception e) {
e.printStackTrace();
}
Chapter 15: Network Programming 617](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-617-320.jpg)
!["FROM people.csv people " +
"WHERE " +
"(LAST='" + args[0] + "') " +
" AND (EMAIL Is Not Null) " +
"ORDER BY FIRST");
SQL has another way to insert names into a query called stored procedures, which is used
for speed. But for much of your database experimentation and for your first cut, building
your own query strings in Java is fine.
You can see from this example that by using the tools currently available – in particular
the query-building tool – database programming with SQL and JDBC can be quite
straightforward.
A GUI version of the lookup program
It’s more useful to leave the lookup program running all the time and simply switch to it
and type in a name whenever you want to look someone up. The following program
creates the lookup program as an application/applet, and it also adds name completion so
the data will show up without forcing you to type the entire last name:
//: VLookup.java
// GUI version of Lookup.java
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.sql.*;
public class VLookup extends Applet {
String dbUrl = "jdbc:odbc:people";
String user = "";
String password = "";
Statement s;
TextField searchFor = new TextField(20);
Label completion =
new Label(" ");
TextArea results = new TextArea(40, 20);
public void init() {
searchFor.addTextListener(new SearchForL());
Panel p = new Panel();
p.add(new Label("Last name to search for:"));
p.add(searchFor);
p.add(completion);
setLayout(new BorderLayout());
add(p, BorderLayout.NORTH);
add(results, BorderLayout.CENTER);
try {
// Load the driver (registers itself)
Class.forName(
"sun.jdbc.odbc.JdbcOdbcDriver");
Connection c = DriverManager.getConnection(
dbUrl, user, password);
s = c.createStatement();
} catch(Exception e) {
results.setText(e.getMessage());
}
Chapter 15: Network Programming 621](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-621-320.jpg)
![}
class SearchForL implements TextListener {
public void textValueChanged(TextEvent te) {
ResultSet r;
if(searchFor.getText().length() == 0) {
completion.setText("");
results.setText("");
return;
}
try {
// Name completion:
r = s.executeQuery(
"SELECT LAST FROM people.csv people " +
"WHERE (LAST Like '" +
searchFor.getText() +
"%') ORDER BY LAST");
if(r.next())
completion.setText(
r.getString("last"));
r = s.executeQuery(
"SELECT FIRST, LAST, EMAIL " +
"FROM people.csv people " +
"WHERE (LAST='" +
completion.getText() +
"') AND (EMAIL Is Not Null) " +
"ORDER BY FIRST");
} catch(Exception e) {
results.setText(
searchFor.getText() + "n");
results.append(e.getMessage());
return;
}
results.setText("");
try {
while(r.next()) {
results.append(
r.getString("Last") + ", "
+ r.getString("fIRST") +
": " + r.getString("EMAIL") + "n");
}
} catch(Exception e) {
results.setText(e.getMessage());
}
}
}
public static void main(String args[]) {
VLookup applet = new VLookup();
Frame aFrame = new Frame("Email lookup");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
622 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-622-320.jpg)
![import java.rmi.registry.*;
import java.net.*;
public class PerfectTime
extends UnicastRemoteObject
implements PerfectTimeI {
// Implementation of the interface:
public long getPerfectTime()
throws RemoteException {
return System.currentTimeMillis();
}
// Must implement constructor to throw
// RemoteException:
public PerfectTime() throws RemoteException {
// super(); // Called automatically
}
// Registration for RMI serving:
public static void main(String args[]) {
System.setSecurityManager(
new RMISecurityManager());
try {
PerfectTime pt = new PerfectTime();
Naming.bind(
"//colossus:2005/PerfectTime", pt);
System.out.println("Ready to do time");
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
Here, main( ) handles all the details of setting up the server. When you’re serving RMI
objects, at some point in your program you must:
1. Create and install a security manager that supports RMI. The only one available
for RMI as part of the Java distribution is RMISecurityManager.
2. Create one or more instances of a remote object. Here, you can see the creation
of the PerfectTime object.
3. Register at least one of the remote objects with the RMI remote object registry,
for bootstrapping purposes. One remote object may have methods that produce
handles to other remote objects. This allows you to set it up so the client must
go to the registry only once, to get the first remote object.
Setting up the registry
Here, you see a call to the static method Naming.bind( ). However, this call requires that
the registry be running as a separate process on the computer. The name of the registry
server is rmiregistry, and under 32-bit Windows you say:
start rmiregistry
to start it in the background. On Unix, it is
rmiregistry &
Like many network programs, the rmiregistry is located at the IP address of whatever
machine started it up, but it must also be listening at a port. If you invoke the rmiregistry
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![import java.rmi.registry.*;
public class DisplayPerfectTime {
public static void main(String args[]) {
System.setSecurityManager(
new RMISecurityManager());
try {
PerfectTimeI t =
(PerfectTimeI)Naming.lookup(
"//colossus:2005/PerfectTime");
for(int i = 0; i < 10; i++)
System.out.println("Perfect time = " +
t.getPerfectTime());
} catch(Exception e) {
e.printStackTrace();
}
}
} ///:~
The ID string is the same as the one used to register the object with Naming, and the first
part represents the URL and port number. Since you’re using a URL, you can also specify a
machine on the Internet.
What comes back from Naming.lookup( ) must be cast to the remote interface, not the
class itself. If you use the class instead, you’ll get an exception.
You can see in the method call
t.getPerfectTime( )
that once you have a handle to the remote object, programming with it is indistinguishable
from programming with a local object (with one difference: remote methods throw
RemoteException).
Alternatives to RMI
RMI is just one way to create objects that can be distributed across a network. It has the
advantage of being a “pure Java” solution, but if you have a lot of code written in some
other language, it may not meet your needs. The two most compelling alternatives are
Microsoft’s DCOM (which, according to Microsoft’s plan, will eventually be hosted on
platforms other than Windows) and CORBA, which is supported in Java 1.1 and was
designed from the start to be cross-platform. You can get an in-depth treatment of
distributed objects in Java (albeit with a clear bias towards CORBA) in Client/Server
Programming with Java and CORBA by Orfali & Harkey (John Wiley & Sons, 1997).
Summary
There’s actually a lot more to networking than can be covered in this introductory
treatment. Java networking also provides fairly extensive support for URLs, including
protocol handlers for different types of content that can be discovered at an Internet site.
In addition, an up-and-coming technology is the Servlet Server which is an Internet server
that uses Java to handle requests instead of the slow and rather awkward CGI (Common
Gateway Interface) protocol. This means that to provide services on the server side you’ll
be able to write in Java instead of using some other language you may not know as well.
628 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-628-320.jpg)
![private Singleton(int x) { i = x; }
public static Singleton getHandle() {
return s;
}
public int getValue() { return i; }
public void setValue(int x) { i = x; }
}
public class SingletonPattern {
public static void main(String[] args) {
Singleton s = Singleton.getHandle();
System.out.println(s.getValue());
Singleton s2 = Singleton.getHandle();
s2.setValue(9);
System.out.println(s.getValue());
try {
// Can't do this: compile-time error.
// Singleton s3 = (Singleton)s2.clone();
} catch(Exception e) {}
}
} ///:~
The key to creating a singleton is to prevent the client programmer from having any way
to create an object except the ways you provide. Thus, you must make all constructors
private, and you must create at least one constructor to prevent the compiler from
synthesizing a default constructor for you (which it will create as “friendly”).
At this point, you decide how you’re going to create your object. Here, it’s created
statically, but you can also wait until the client programmer asks for one and create it on
demand. In any case, the object should be stored privately. You provide access through
public methods. Here, getHandle( ) produces the handle to the Singleton object. The rest
of the interface (getValue( ) and setValue( )) is the normal class interface.
Java also allows the creation of objects through cloning. In this example, making the class
final prevents cloning. Since Singleton is inherited directly from Object, the clone( )
method remains protected so it cannot be used (doing so produces a compile-time error).
However, If you’re inheriting from a class hierarchy that has already overridden clone( )
as public and implemented Cloneable, the way to prevent cloning is to override clone( )
and throw a CloneNotSupportedException as described in Chapter 12. (You could also
override clone( ) and simply return this, but that would be deceiving since the client
programmer would think they were cloning the object but would instead still be dealing
with the original).
Note that you aren’t restricted to creating only one object. This is also a technique to
create a limited pool of objects. In that situation, however, you may be confronted with the
problem of sharing objects in the pool. If this is an issue, you can create a solution
involving a check-out and check-in of the shared objects.
Classifying patterns
In Design Patterns, there are 23 different patterns classified under three purposes (all of
which revolve around the particular aspect that can vary). The three purposes are:
1. Creational: how an object can be created. This often involves isolating the
details of object creation so your code isn’t dependent on what types of objects
there are and thus doesn’t have to be changed when you add a new type of
Chapter 16: Design Patterns 633](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-633-320.jpg)
![// You must inherit a new type of Observable:
class BoxObservable extends Observable {
public void notifyObservers(Object b) {
// Otherwise it won't propagate changes:
setChanged();
super.notifyObservers(b);
}
}
public class BoxObserver extends Frame {
Observable notifier = new BoxObservable();
public BoxObserver(int grid) {
setTitle("Demonstrates Observer pattern");
setLayout(new GridLayout(grid, grid));
for(int x = 0; x < grid; x++)
for(int y = 0; y < grid; y++)
add(new OCBox(x, y, notifier));
}
public static void main(String[] args) {
int grid = 8;
if(args.length > 0)
grid = Integer.parseInt(args[0]);
Frame f = new BoxObserver(grid);
f.setSize(500, 400);
f.setVisible(true);
f.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}
}
class OCBox extends Canvas implements Observer {
Observable notifier;
int x, y; // Locations in grid
Color cColor = newColor();
static final Color colors[] = {
Color.black, Color.blue, Color.cyan,
Color.darkGray, Color.gray, Color.green,
Color.lightGray, Color.magenta,
Color.orange, Color.pink, Color.red,
Color.white, Color.yellow
};
static final Color newColor() {
return colors[
(int)(Math.random() * colors.length)
];
}
OCBox(int x, int y, Observable notifier) {
this.x = x;
this.y = y;
notifier.addObserver(this);
this.notifier = notifier;
Chapter 16: Design Patterns 635](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-635-320.jpg)
![class Glass extends Trash {
static double val = 0.23f;
Glass(double wt) { super(wt); }
double value() { return val; }
static void value(double newval) {
val = newval;
}
}
public class RecycleA {
public static void main(String args[]) {
Vector bin = new Vector();
// Fill up the Trash bin:
for(int i = 0; i < 30; i++)
switch((int)(Math.random() * 3)) {
case 0 :
bin.addElement(new
Aluminum(Math.random() * 100));
break;
case 1 :
bin.addElement(new
Paper(Math.random() * 100));
break;
case 2 :
bin.addElement(new
Glass(Math.random() * 100));
}
Vector
glassBin = new Vector(),
paperBin = new Vector(),
alBin = new Vector();
Enumeration sorter = bin.elements();
// Sort the Trash:
while(sorter.hasMoreElements()) {
Object t = sorter.nextElement();
// RTTI to show class membership:
if(t instanceof Aluminum)
alBin.addElement(t);
if(t instanceof Paper)
paperBin.addElement(t);
if(t instanceof Glass)
glassBin.addElement(t);
}
Trash.sumValue(alBin);
Trash.sumValue(paperBin);
Trash.sumValue(glassBin);
Trash.sumValue(bin);
}
} ///:~
The first thing you’ll notice is the package statement:
package c16.recyclea;
638 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-638-320.jpg)
![Enumeration e = bin.elements();
double val = 0.0f;
while(e.hasMoreElements()) {
// One kind of RTTI:
// A dynamically-checked cast
Trash t = (Trash)e.nextElement();
val += t.weight() * t.value();
System.out.println(
"weight of " +
// Using RTTI to get type
// information about the class:
t.getClass().getName() +
" = " + t.weight());
}
System.out.println("Total value = " + val);
}
// Remainder of class provides support for
// prototyping:
public static class PrototypeNotFoundException
extends Exception {}
public static class CannotCreateTrashException
extends Exception {}
private static Vector trashTypes =
new Vector();
public static Trash factory(Info info)
throws PrototypeNotFoundException,
CannotCreateTrashException {
for(int i = 0; i < trashTypes.size(); i++) {
// Somehow determine the new type
// to create, and create one:
Class tc =
(Class)trashTypes.elementAt(i);
if (tc.getName().indexOf(info.id) != -1) {
try {
// Get the dynamic constructor method
// that takes a double argument:
Constructor ctor =
tc.getConstructor(
new Class[] {double.class});
// Call the constructor to create a
// new object:
return (Trash)ctor.newInstance(
new Object[]{new Double(info.data)});
} catch(Exception ex) {
ex.printStackTrace();
throw new CannotCreateTrashException();
}
}
}
// Class was not in the list. Try to load it,
// but it must be in your class path!
try {
System.out.println("Loading " + info.id);
trashTypes.addElement(
Class.forName(info.id));
} catch(Exception e) {
Chapter 16: Design Patterns 643](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-643-320.jpg)
![e.printStackTrace();
throw new PrototypeNotFoundException();
}
// Loaded successfully. Recursive call
// should work this time:
return factory(info);
}
public static class Info {
public String id;
public double data;
public Info(String name, double data) {
id = name;
this.data = data;
}
}
} ///:~
The basic Trash class and sumValue( ) are the same as before. The rest of the class
supports the prototyping pattern. You can first see two inner classes (which are static, so
they are inner classes only for code organization) describing exceptions that can occur.
This is followed by a Vector trashTypes which is used to hold the Class handles.
In Trash.factory( ), the String inside the Info object id (a different version of the Info
class than in the prior discussion) contains the type name of the Trash to be created; this
String is compared to the Class names in the list. If there’s a match, then that’s the object
to create. Of course, there are many ways to determine what object you want to make.
This one is used so that information read in from a file can be turned into objects.
Once you’ve discovered which kind of Trash to create, then the reflection methods come
into play. The getConstructor( ) method takes an argument that is an array of Class
handles. This array represents the arguments, in their proper order, for the constructor
that you’re looking for. Here, the array is dynamically created using the Java 1.1 array-
creation syntax:
new Class[] {double.class}
This code assumes that every Trash type has a constructor that takes a double (and note
that double.class is distinct from Double.class). It’s also possible, for a more flexible
solution, to call getConstructors( ) which returns an array of the possible constructors.
What comes back from getConstructor( ) is a handle to a Constructor object (part of
java.lang.reflect). You call the constructor dynamically with the method newInstance( ),
which takes an array of Object containing the actual arguments. This array is again
created using the new Java 1.1 syntax:
new Object[]{new Double(info.data)}
In this case, however, the double must be placed inside a wrapper class so it can be part of
this array of objects. The process of calling newInstance( ) extracts the double, but you
can see it is a bit confusing – an argument might be a double or a Double, but when you
make the call you must always pass in a Double. Fortunately this issue exists only for the
primitive types.
Once you understand how to do it, the process of creating a new object given only a Class
handle is remarkably simple. Reflection also allows you to call methods in this same
dynamic fashion.
644 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-644-320.jpg)
![// Any object that can be filled with Trash
package c16.trash;
public interface Fillable {
void addTrash(Trash t);
} ///:~
Anything that supports this interface can be used with fillBin. Of course, Vector doesn’t
implement Fillable, so it won’t work. Since Vector is used in most of the examples, it
makes sense to add a second overloaded fillBin( ) method that takes a Vector. To allow the
Vector to be used as a Fillable object, it must be adapted. This is easily accomplished using
a class:
//: FillableVector.java
// Adapter that makes a Vector Fillable
package c16.trash;
import java.util.*;
public class FillableVector implements Fillable {
private Vector v;
public FillableVector(Vector vv) { v = vv; }
public void addTrash(Trash t) {
v.addElement(t);
}
} ///:~
You can see that the only job of this class is to connect Fillable’s addTrash( ) method to
Vector’s addElement( ). With this class in hand, the overloaded fillBin( ) method can be
used with a Vector in ParseTrash.java:
public static void
fillBin(String filename, Vector bin) {
fillBin(filename, new FillableVector(bin));
}
This approach works for any collection class that’s used frequently. Alternatively, the
collection class can provide its own adapter that implements Fillable (you’ll see this later,
in DynaTrash.java).
Recycling with prototyping
Now you can see the revised version of RecycleA.java using the prototyping technique:
//: RecycleAP.java
// Recycling with RTTI and Prototypes
package c16.recycleap;
import c16.trash.*;
import java.util.*;
public class RecycleAP {
public static void main(String args[]) {
Vector bin = new Vector();
// Fill up the Trash bin:
ParseTrash.fillBin("Trash.dat", bin);
Vector
glassBin = new Vector(),
paperBin = new Vector(),
alBin = new Vector();
648 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-648-320.jpg)
![method certainly behaves differently for a Vector that holds Paper than one that holds
Glass. But what it does is strictly dependent on the type, and nothing else. This could be
interpreted as a different state, and since Java has a class to represent type (Class) this can
be used to determine the type of Trash a particular Tbin will hold.
The constructor for this Tbin requires that you hand it the Class of your choice. This tells
the Vector what type it is supposed to hold. Then the grab( ) method uses Class BinType
and RTTI to see if the Trash object you’ve handed it matches the type it’s supposed to grab.
Here is the whole program. The commented numbers (e.g. (*1*) ) mark sections that will
be described following the code.
//: RecycleB.java
// Adding more objects to the recycling problem
package c16.recycleb;
import c16.trash.*;
import java.util.*;
// A vector that admits only the right type:
class Tbin extends Vector {
Class binType;
Tbin(Class binType) {
this.binType = binType;
}
boolean grab(Trash t) {
// Comparing class types:
if(t.getClass().equals(binType)) {
addElement(t);
return true; // Object grabbed
}
return false; // Object not grabbed
}
}
class TbinList extends Vector { //(*1*)
boolean sort(Trash t) {
Enumeration e = elements();
while(e.hasMoreElements()) {
Tbin bin = (Tbin)e.nextElement();
if(bin.grab(t)) return true;
}
return false; // bin not found for t
}
void sortBin(Tbin bin) { // (*2*)
Enumeration e = bin.elements();
while(e.hasMoreElements())
if(!sort((Trash)e.nextElement()))
System.out.println("Bin not found");
}
}
public class RecycleB {
static Tbin bin = new Tbin(Trash.class);
public static void main(String args[]) {
// Fill up the Trash bin:
ParseTrash.fillBin("Trash.dat", bin);
Chapter 16: Design Patterns 651](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-651-320.jpg)
![and think. The whole idea of polymorphism (dynamically-bound method calls) is to handle
type-specific information for you. So why are you hunting for types?
The answer is something you probably don’t think about: Java performs only single
dispatching. That is, if you are performing an operation on more than one object whose
type is unknown, Java will invoke the dynamic binding mechanism on only one of those
types. This doesn’t solve the problem, so you end up detecting some types manually and
effectively producing your own dynamic binding behavior.
The solution is called multiple dispatching, which means setting up a configuration such
that a single method call produces more than one dynamic method call, and thus
determines more than one type in the process. To get this effect, then, you need to work
with more than one type hierarchy: you’ll need a type hierarchy for each dispatch. The
following example works with two hierarchies: the existing Trash family and a hierarchy
of the types of trash bins that the trash will be placed into. This second hierarchy isn’t
always obvious and in this case it needed to be created in order to produce multiple
dispatching (in this case there will be only two dispatches, which is referred to as double
dispatching).
Implementing the double dispatch
Remember that polymorphism can occur only via method calls, so if you want double
dispatching to occur there must be two method calls, one to determine the type in each
hierarchy. In the Trash hierarchy there will be a new method called addToBin( ), which
takes an argument of an array of TypedBin. It uses this array to step through and try to
add itself to the appropriate bin, and this is where you’ll see the double dispatch.
Trash
addToBin(TypedBin[])
Aluminum Paper Glass Cardboard
addToBin() addToBin() addToBin() addToBin()
The new hierarchy is TypedBin, and it contains its own method called add( ) which is also
used polymorphically. But here’s an additional twist: add( ) is overloaded to take
arguments of the different types of trash. So an essential part of the double dispatching
scheme also involves overloading.
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![TypedBin
add(Aluminum)
add(Paper)
add(Glass)
add(Cardboard)
AluminumBin PaperBin GlassBin CardboardBin
add(Aluminum) add(Paper) add(Glass) add(Cardboard)
Redesigning the program encounters a dilemma: it’s now necessary that the base class
Trash contain a method addToBin( ). One approach to this is to copy all the code and
change the base class. Another approach, which is one you can take when you don’t have
control of the source code, is to put the addToBin( ) method in an interface, leave Trash
alone, and inherit new specific types of Aluminum, Paper, Glass and Cardboard. This is
the approach taken here.
Most of the classes in this design must be public so they are placed in their own files.
Here’s the interface
//: TypedBinMember.java
// An interface for adding the double dispatching
// method to the trash hierarchy without
// modifying the original hierarchy.
package c16.doubledispatch;
interface TypedBinMember {
// The new method:
boolean addToBin(TypedBin[] tb);
} ///:~
In each particular subtype of Aluminum, Paper, Glass and Cardboard, the addToBin( )
method in the interface TypedBinMember is implemented,, but it looks like the code is
exactly the same in each case:
//: DDAluminum.java
// Aluminum for double dispatching
package c16.doubledispatch;
import c16.trash.*;
public class DDAluminum extends Aluminum
implements TypedBinMember {
public DDAluminum(double wt) { super(wt); }
public boolean addToBin(TypedBin[] tb) {
for(int i = 0; i < tb.length; i++)
if(tb[i].add(this))
return true;
return false;
}
} ///:~
//: DDPaper.java
654 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-654-320.jpg)
![// Paper for double dispatching
package c16.doubledispatch;
import c16.trash.*;
public class DDPaper extends Paper
implements TypedBinMember {
public DDPaper(double wt) { super(wt); }
public boolean addToBin(TypedBin[] tb) {
for(int i = 0; i < tb.length; i++)
if(tb[i].add(this))
return true;
return false;
}
} ///:~
//: DDGlass.java
// Glass for double dispatching
package c16.doubledispatch;
import c16.trash.*;
public class DDGlass extends Glass
implements TypedBinMember {
public DDGlass(double wt) { super(wt); }
public boolean addToBin(TypedBin[] tb) {
for(int i = 0; i < tb.length; i++)
if(tb[i].add(this))
return true;
return false;
}
} ///:~
//: DDCardboard.java
// Cardboard for double dispatching
package c16.doubledispatch;
import c16.trash.*;
public class DDCardboard extends Cardboard
implements TypedBinMember {
public DDCardboard(double wt) { super(wt); }
public boolean addToBin(TypedBin[] tb) {
for(int i = 0; i < tb.length; i++)
if(tb[i].add(this))
return true;
return false;
}
} ///:~
The code in each addToBin( ) calls add( ) for each TypedBin object in the array. But notice
the argument: this. The type of this is different for each subclass of Trash, so the code is
in fact different (although this code will benefit if a parameterized type mechanism is ever
added to Java). So this is the first part of the double dispatch, because once you’re inside
this method you know you’re Aluminum, or Paper, etc. During the call to add( ), this
information is passed via the type of this. The compiler resolves the call to the proper
overloaded version of add( ). But since tb[i] produces a handle to the base type TypedBin,
this call will end up calling a different method depending on the actual type of TypedBin
that’s currently selected. This is the second dispatch.
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![}
}
class TrashBinSet {
private TypedBin[] binSet = {
new AluminumBin(),
new PaperBin(),
new GlassBin(),
new CardboardBin()
};
public void sortIntoBins(Vector bin) {
Enumeration e = bin.elements();
while(e.hasMoreElements()) {
TypedBinMember t =
(TypedBinMember)e.nextElement();
if(!t.addToBin(binSet))
System.err.println("Couldn't add " + t);
}
}
public TypedBin[] binSet() { return binSet; }
}
public class DoubleDispatch {
public static void main(String args[]) {
Vector bin = new Vector();
TrashBinSet bins = new TrashBinSet();
// ParseTrash still works, without changes:
ParseTrash.fillBin("DDTrash.dat", bin);
// Sort from the master bin into the
// individually-typed bins:
bins.sortIntoBins(bin);
TypedBin[] tb = bins.binSet();
// Perform sumValue for each bin...
for(int i = 0; i < tb.length; i++)
Trash.sumValue(tb[i].v);
// ... and for the master bin
Trash.sumValue(bin);
}
} ///:~
TrashBinSet encapsulates all the different types of TypedBins, along with the
sortIntoBins( ) method which is where all the double dispatching takes place. You can see
that once the structure is set up, sorting into the various TypedBins is remarkably easy.
In addition, the efficiency of two dynamic method calls is probably better than any other
way you could sort.
Note the ease of use of this system inside main( ), as well as the complete independence of
any specific type information within main( ). All other methods that talk only to the
Trash base-class interface will be equally invulnerable to changes in Trash types.
The code changes necessary to add a new type are relatively isolated: you inherit the new
type of Trash with its addToBin( ) method, then you inherit a new TypedBin (this is
really just a copy and simple edit) and finally you add a new type in the aggregate
initialization for TrashBinSet.
658 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-658-320.jpg)
![public class TrashVisitor {
public static void main(String args[]) {
Vector bin = new Vector();
// ParseTrash still works, without changes:
ParseTrash.fillBin("VTrash.dat", bin);
// You could even iterate through
// a list of visitors!
PriceVisitor pv = new PriceVisitor();
WeightVisitor wv = new WeightVisitor();
Enumeration it = bin.elements();
while(it.hasMoreElements()) {
Visitable v = (Visitable)it.nextElement();
v.accept(pv);
v.accept(wv);
}
pv.total();
wv.total();
}
} ///:~
Notice that the shape of main( ) has changed again. Now there’s only a single Trash bin.
The two Visitor objects are accepted into every element in the sequence, and they perform
their operations. The visitors keep their own internal data to tally the total weights and
prices.
Finally, there’s no run-time type identification other than the inevitable cast to Trash
when pulling things out of the sequence. This, too, could be eliminated with the
implementation of parameterized types in Java.
One way you can distinguish this solution from the double dispatching solution described
previously is to note that in the double dispatching solution only one of the overloaded
methods add( ) was overridden when each subclass was created, while here each one of the
overloaded visit( ) methods is overridden in every subclass of Visitor.
More coupling?
There’s a lot more code here, and there’s definite coupling between the Trash hierarchy
and the Visitor hierarchy. However there’s also very high cohesion within the respective
sets of classes: they each do only one thing (Trash describes Trash, while Visitor describes
actions performed on Trash), which is an indicator of a good design. Of course, in this case
it works well only if you’re adding new Visitors, but it gets in the way when you add new
types of Trash.
Low coupling between classes and high cohesion within a class is definitely an important
design goal. Applied mindlessly, though, it can prevent you from achieving a more elegant
design. In particular it seems that some classes inevitably have a certain intimacy with
each other. These often occur in pairs that could perhaps be called couplets, for example
collections and iterators (Enumerations). The above Trash-Visitor pair appears to be
another such couplet.
RTTI considered harmful?
Various designs in this chapter attempt to remove RTTI, and this could give you the
impression that it’s “considered harmful” (the condemnation used for poor, ill-fated goto
which was thus never put into Java). This isn’t true; the misuse of RTTI is the problem.
The reason these designs removed RTTI is because the misapplication of that feature
664 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-664-320.jpg)
![prevented extensibility, and the stated goal was to be able to add a new type to the system
with as little impact on surrounding code as possible. Since RTTI is often misused to look
for every single type in your system, it causes code to be non-extensible: when you add a
new type, you have to go hunting for all the code where RTTI is used, and if you miss any
you won’t get help from the compiler.
However, using RTTI doesn’t automatically create non-extensible code. Let’s revisit the
trash recycler once more. This time, a new tool will be introduced: a Hashtable that holds
Vectors. The keys for this Hashtable, which I shall call a TypeMap, are the types of trash
in the associated Vector. The beauty of this design (suggested by Larry O’Brien) is that the
TypeMap dynamically adds a new pair whenever it encounters a new type, so whenever
you add a new type to the system (even if you add the new type at run-time) it adapts.
The example will again build on the structure of the Trash types in package c16.Trash
(and the Trash.dat file used there can be utilized here without change):
//: DynaTrash.java
// Using a Hashtable of Vectors and RTTI
// to automatically sort trash into
// vectors. This solution, despite the
// use of RTTI, is extensible.
package c16.dynatrash;
import c16.trash.*;
import java.util.*;
// Generic TypeMap works in any situation:
class TypeMap {
private Hashtable t = new Hashtable();
public void add(Object o) {
Class type = o.getClass();
if(t.containsKey(type))
((Vector)t.get(type)).addElement(o);
else {
Vector v = new Vector();
v.addElement(o);
t.put(type,v);
}
}
public Vector get(Class type) {
return (Vector)t.get(type);
}
public Enumeration keys() { return t.keys(); }
// Returns handle to adapter class to allow
// callbacks from ParseTrash.fillBin():
public Fillable filler() {
// Anonymous inner class:
return new Fillable() {
public void addTrash(Trash t) { add(t); }
};
}
}
public class DynaTrash {
public static void main(String args[]) {
TypeMap bin = new TypeMap();
ParseTrash.fillBin("Trash.dat",bin.filler());
Enumeration keys = bin.keys();
Chapter 16: Design Patterns 665](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-665-320.jpg)
![SourceCodeFile f =
(SourceCodeFile)v.elementAt(i);
f.writeFile(rootpath);
}
// Add file indicating file quantity
// written to this directory as a check:
IO.close(IO.dosOpen(
new File(new File(rootpath, dir),
Integer.toString(v.size())+".files")));
}
}
}
public class CodePackager {
private static final String usageString =
"usage: java CodePackager packedFileName" +
"nExtracts source code files from packed n" +
"version of Tjava.doc sources into " +
"directories off current directoryn" +
"java CodePackager packedFileName newDirn" +
"Extracts into directories off newDirn" +
"java CodePackager -p source.txt packedFile" +
"nCreates packed version of source files" +
"nfrom text version of Tjava.doc";
private static void usage() {
System.err.println(usageString);
System.exit(1);
}
public static void main(String args[]) {
if(args.length == 0) usage();
if(args[0].equals("-p")) {
if(args.length != 3)
usage();
createPackedFile(args);
}
else {
if(args.length > 2)
usage();
extractPackedFile(args);
}
}
private static String currentLine;
private static BufferedReader in;
private static DirMap dm;
private static void
createPackedFile(String args[]) {
dm = new DirMap();
in = IO.disOpen(args[1]);
try {
while((currentLine = in.readLine())
!= null) {
if(currentLine.startsWith(
SourceCodeFile.startMarker)) {
dm.add(new SourceCodeFile(
currentLine, in));
676 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-676-320.jpg)
![}
else if(currentLine.startsWith(
SourceCodeFile.endMarker))
Pr.error("file has no start marker");
// Else ignore the input line
}
} catch(IOException e) {
Pr.error("Error reading " + args[1]);
}
IO.close(in);
dm.writePackedFile(args[2]);
}
private static void
extractPackedFile(String args[]) {
if(args.length == 2) // Alternate directory
dm = new DirMap(args[1]);
else // Current directory
dm = new DirMap();
in = IO.disOpen(args[0]);
String s = null;
try {
s = in.readLine();
} catch(IOException e) {
Pr.error("Cannot read from " + in);
}
// Capture the separator used in the system
// that packed the file:
if(s.indexOf("###Old Separator:") != -1 ) {
String oldsep = s.substring(
"###Old Separator:".length());
oldsep = oldsep.substring(
0, oldsep. indexOf('#'));
SourceCodeFile.oldsep = oldsep;
}
SourceCodeFile sf = new SourceCodeFile(in);
while(sf.hasFile()) {
dm.add(sf);
sf = new SourceCodeFile(in);
}
dm.write();
}
} ///:~
You’ll first notice the package statement which is commented out. Since this is the first
program in the chapter, the package statement is necessary to tell CodePackager that the
chapter has changed, but actually putting it in a package would be a problem. When you
create a package, you tie the resulting program to a particular directory structure, which
is fine for most of the examples in this book. Here, however, the CodePackager program
must be compiled and run from an arbitrary directory, so the package statement is
commented out. It will still look like an ordinary package statement to CodePackager,
though, since the program isn’t sophisticated enough to detect multi-line comments (it has
no need for such sophistication, a fact which comes in handy here).
The first two classes are support/utility classes designed to make the rest of the program
more consistent to write and more easily readable. The first, Pr, is very similar to the
ANSI C library perror, since it prints an error message (but also exits the program). The
second class encapsulates the creation of files, a process that was shown in Chapter 10 as
Chapter 17: Projects 677](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-677-320.jpg)
![be changed. This is a little annoying, but it’s still much easier than trying to find all these
cases by staring at your code.
Here’s the listing, which will be explained immediately following the code:
//: ClassScanner.java
// Scans all files in directory for classes
// and identifiers, to check capitalization.
// Assumes properly compiling code listings.
// Doesn't do everything right, but is a very
// useful aid.
import java.io.*;
import java.util.*;
class MultiStringMap extends Hashtable {
public void add(String key, String value) {
if(!containsKey(key))
put(key, new Vector());
((Vector)get(key)).addElement(value);
}
public Vector getVector(String key) {
if(!containsKey(key)) {
System.err.println(
"ERROR: cannot find key: " + key);
System.exit(1);
}
return (Vector)get(key);
}
public void printValues(PrintStream p) {
Enumeration k = keys();
while(k.hasMoreElements()) {
String oneKey = (String)k.nextElement();
Vector val = getVector(oneKey);
for(int i = 0; i < val.size(); i++)
p.println((String)val.elementAt(i));
}
}
}
public class ClassScanner {
private File path;
private String[] fileList;
private Properties classes = new Properties();
private MultiStringMap
classMap = new MultiStringMap(),
identMap = new MultiStringMap();
private StreamTokenizer in;
public ClassScanner() {
path = new File(".");
fileList = path.list(new JavaFilter());
for(int i = 0; i < fileList.length; i++) {
System.out.println(fileList[i]);
scanListing(fileList[i]);
}
}
void scanListing(String fname) {
682 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-682-320.jpg)
![if(in.ttype == '/')
discardLine();
else if(in.ttype != '*')
in.pushBack();
else
while(true) {
if(in.nextToken() ==
StreamTokenizer.TT_EOF)
break;
if(in.ttype == '*')
if(in.nextToken() !=
StreamTokenizer.TT_EOF
&& in.ttype == '/')
break;
}
}
} catch(IOException e) {
e.printStackTrace();
}
}
public String[] classNames() {
String[] result = new String[classes.size()];
Enumeration e = classes.keys();
int i = 0;
while(e.hasMoreElements())
result[i++] = (String)e.nextElement();
return result;
}
public void checkClassNames() {
Enumeration files = classMap.keys();
while(files.hasMoreElements()) {
String file = (String)files.nextElement();
Vector cls = classMap.getVector(file);
for(int i = 0; i < cls.size(); i++) {
String className =
(String)cls.elementAt(i);
if(Character.isLowerCase(
className.charAt(0)))
System.out.println(
"class capitalization error, file: "
+ file + ", class: "
+ className);
}
}
}
public void checkIdentNames() {
Enumeration files = identMap.keys();
Vector reportSet = new Vector();
while(files.hasMoreElements()) {
String file = (String)files.nextElement();
Vector ids = identMap.getVector(file);
for(int i = 0; i < ids.size(); i++) {
String id =
(String)ids.elementAt(i);
if(!classes.contains(id)) {
684 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-684-320.jpg)
![// Ignore identifiers of length 3 or
// longer that are all uppercase
// (probably static final values):
if(id.length() >= 3 &&
id.equals(
id.toUpperCase()))
continue;
// Check to see if first char is upper:
if(Character.isUpperCase(id.charAt(0))){
if(reportSet.indexOf(file + id)
== -1){ // Not reported yet
reportSet.addElement(file + id);
System.out.println(
"Ident capitalization error in:"
+ file + ", ident: " + id);
}
}
}
}
}
}
static final String usage =
"Usage: n" +
"ClassScanner classnames -an" +
"tAdds all the class names in this n" +
"tdirectory to the repository file n" +
"tcalled 'classnames'n" +
"ClassScanner classnamesn" +
"tChecks all the java files in this n" +
"tdirectory for capitalization errors, n" +
"tusing the repository file 'classnames'";
private static void usage() {
System.err.println(usage);
System.exit(1);
}
public static void main(String[] args) {
if(args.length < 1 || args.length > 2)
usage();
ClassScanner c = new ClassScanner();
File old = new File(args[0]);
if(old.exists()) {
try {
// Try to open an existing
// properties file:
InputStream oldlist =
new BufferedInputStream(
new FileInputStream(old));
c.classes.load(oldlist);
oldlist.close();
} catch(IOException e) {
System.err.println("Could not open "
+ old + " for reading");
System.exit(1);
}
}
if(args.length == 1) {
Chapter 17: Projects 685](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-685-320.jpg)
![c.checkClassNames();
c.checkIdentNames();
}
// Write the class names to a repository:
if(args.length == 2) {
if(!args[1].equals("-a"))
usage();
try {
BufferedOutputStream out =
new BufferedOutputStream(
new FileOutputStream(args[0]));
c.classes.save(out,
"Classes found by ClassScanner.java");
out.close();
} catch(IOException e) {
System.err.println(
"Could not write " + args[0]);
System.exit(1);
}
}
}
}
class JavaFilter implements FilenameFilter {
public boolean accept(File dir, String name) {
// Strip path information:
String f = new File(name).getName();
return f.trim().endsWith(".java");
}
} ///:~
The class MultiStringMap is a tool that allows you to map a group of strings onto each
key entry. As in the previous example, it uses a Hashtable (this time with inheritance)
with the key as the single string that’s mapped onto the Vector value. The add( ) method
simply checks to see if there’s a key already in the Hashtable, and if not it puts one there.
The getVector( ) method produces a Vector for a particular key, and printValues( ), which
is primarily useful for debugging, prints out all the values, Vector by Vector.
To keep life simple, the class names from the standard Java libraries are all put into a
Properties object (from the standard Java library). Remember that a Properties object is a
Hashtable that holds only String objects for both the key and value entries. However, it
can be saved to disk and restored from disk in one method call, so it’s ideal for the
repository of names. Actually, we need only a list of names, and a Hashtable cannot
accept null for either its key or value entry. So the same object will be used for both the
key and the value.
For the classes and identifiers that are discovered for the files in a particular directory, two
MultiStringMaps are used: classMap and identMap. In addition, when the program
starts up it loads the standard class name repository into the Properties object called
classes, and when a new class name is found in the local directory that is also added to
classes as well as to classMap. This way, classMap can be used to step through all the
classes in the local directory, and classes can be used to see if the current token is a class
name (which indicates a definition of an object or method is beginning, so grab the next
tokens – until a semicolon – and put them into identMap).
686 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-686-320.jpg)
![value like TT_EOF. Of course, this is not a perfect algorithm but it assumes you’ll
eventually notice any all-uppercase identifiers that are out of place.
Rather than reporting every identifier that starts with an uppercase character, this
method keeps track of which ones have already been reported in a Vector called
reportSet( ). This treats the Vector as a “set” which tells you whether an item is already
in the set. The item is produced by concatenating the file name and identifier. If the
element isn’t in the set, it is added and then the report is made.
The rest of the listing is comprised of main( ), which busies itself by handling the
command line arguments and figuring out whether you’re building a repository of class
names from the standard Java library or checking the validity of code you’ve written. In
both cases it makes a ClassScanner object.
Whether you’re building a repository or using one, you must try to open the existing
repository. By making a File object and testing for existence, you can decide whether to
open the file and load( ) the Properties list classes inside ClassScanner (the classes from
the repository add to rather than overwrite the classes found by the ClassScanner
constructor). If you provide only one command-line argument it means you just want to
perform a check of the class names and identifier names, but if you provide two
arguments (the second being “-a”) you’re building a class name repository. In this case an
output file is opened and the method Properties.save( ) is used to write the list to a file
along with a string which provides header file information.
A method lookup tool
Chapter 11 introduced the Java 1.1 concept of reflection and used that feature to look up
methods for a particular class – either the entire list of methods, or a subset that matches
a keyword you provide. The magic of this is that it automatically shows you all the
methods for a class without forcing you to walk up the inheritance hierarchy examining
the base classes at each level. Thus it provides a valuable timesaving tool for
programming: since most Java methods are verbose and descriptive, you can look for the
methods that contain a word and when you find what you think you’re looking for, check
the online documentation.
However, you hadn’t seen the AWT by Chapter 11 so that tool was developed as a
command-line application. Here is the more useful GUI version, which dynamically
updates the output as you type and also allows you to cut and paste from the output:
//: DisplayMethods.java
// Display the methods of any class inside
// a window. Dynamically narrows your search.
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.lang.reflect.*;
import java.io.*;
public class DisplayMethods extends Applet {
Class cl;
Method m[];
Constructor ctor[];
String n[] = new String[0];
TextField
name = new TextField(40),
688 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-688-320.jpg)
![searchFor = new TextField(30);
Checkbox strip =
new Checkbox("Strip Qualifiers");
TextArea results = new TextArea(40, 65);
public void init() {
strip.setState(true);
name.addTextListener(new NameL());
searchFor.addTextListener(new SearchForL());
strip.addItemListener(new StripL());
Panel
top = new Panel(),
lower = new Panel(),
p = new Panel();
top.add(new Label("Qualified class name:"));
top.add(name);
lower.add(
new Label("String to search for:"));
lower.add(searchFor);
lower.add(strip);
p.setLayout(new BorderLayout());
p.add(top, BorderLayout.NORTH);
p.add(lower, BorderLayout.SOUTH);
setLayout(new BorderLayout());
add(p, BorderLayout.NORTH);
add(results, BorderLayout.CENTER);
}
class NameL implements TextListener {
public void textValueChanged(TextEvent e) {
String nm = name.getText().trim();
if(nm.length() == 0) {
results.setText("No match");
n = new String[0];
return;
}
try {
cl = Class.forName(nm);
} catch (ClassNotFoundException ex) {
results.setText("No match");
return;
}
m = cl.getMethods();
ctor = cl.getConstructors();
// Convert to an array of Strings:
n = new String[m.length + ctor.length];
for(int i = 0; i < m.length; i++)
n[i] = m[i].toString();
for(int i = 0; i < ctor.length; i++)
n[i + m.length] = ctor[i].toString();
reDisplay();
}
}
void reDisplay() {
// Create the result set:
String rs[] = new String[n.length];
String find = searchFor.getText();
int j = 0;
Chapter 17: Projects 689](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-689-320.jpg)
![// Select from the list if find exists:
for (int i = 0; i < n.length; i++) {
if(find == null)
rs[j++] = n[i];
else if(n[i].indexOf(find) != -1)
rs[j++] = n[i];
}
results.setText("");
if(strip.getState() == true)
for (int i = 0; i < j; i++)
results.append(
StripQualifiers.strip(rs[i]) + "n");
else // Leave qualifiers on
for (int i = 0; i < j; i++)
results.append(rs[i] + "n");
}
class StripL implements ItemListener {
public void itemStateChanged(ItemEvent e) {
reDisplay();
}
}
class SearchForL implements TextListener {
public void textValueChanged(TextEvent e) {
reDisplay();
}
}
public static void main(String args[]) {
DisplayMethods applet = new DisplayMethods();
Frame aFrame = new Frame("Display Methods");
aFrame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
aFrame.add(applet, BorderLayout.CENTER);
aFrame.setSize(500,750);
applet.init();
applet.start();
aFrame.setVisible(true);
}
}
class StripQualifiers {
private StreamTokenizer st;
public StripQualifiers(String qualified) {
st = new StreamTokenizer(
new StringReader(qualified));
st.ordinaryChar(' ');
}
public String getNext() {
String s = null;
try {
if(st.nextToken() !=
StreamTokenizer.TT_EOF) {
690 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-690-320.jpg)
![Beast aBeast = (Beast)beasts.elementAt(0);
int counter = 0;
while(e.hasMoreElements()) {
aBeast = (Beast) e.nextElement();
if(aBeast != viewer) {
Point p = aBeast.position();
Point v = viewer.position();
float bearing =
aBeast.bearingFromPointAlongAxis(
v.x, v.y, viewer.currentDirection);
if(Math.abs(bearing) < fieldOfView / 2)
output.addElement(aBeast);
}
}
return output;
}
public void paint(Graphics g) {
Enumeration e = beasts.elements();
while(e.hasMoreElements()) {
((Beast)e.nextElement()).draw(g);
}
}
public static void main(String args[]) {
FieldOBeasts field = new FieldOBeasts();
field.xExtent = 640;
field.yExtent = 480;
Frame frame = new Frame("Field 'O Beasts");
// Optionally use a command-line argument
// for the sleep time:
if(args.length >= 1)
field.delay = Integer.parseInt(args[0]);
frame.addWindowListener(
new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
frame.add(field, BorderLayout.CENTER);
frame.setSize(640,480);
field.init();
field.start();
frame.setVisible(true);
}
} ///:~
Although this isn’t a perfect reproduction of the behavior in Craig Reynold’s “Boids”
example, it exhibits its own fascinating characteristics, which you can modify by
adjusting the numbers. You can find out more about the modeling of flocking behavior and
see a spectacular 3-D version of Boids at Craig Reynold’s page
http://www.hmt.com/cwr/boids.html.
To run this program as an applet, put the following applet tag in an HTML file:
<applet
code=FieldOBeasts
width=640
height=480>
Chapter 17: Projects 697](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-697-320.jpg)
![H
Index
Please note that some names will be duplicated in capitalized form.
Following Java style, the capitalized names refer to Java classes, while
lowercase names refer to a general concept.
-, 84 Abstract Window Toolkit (AWT), 413
!=, 85; operator, 387 abstraction, 30; in program design, 632
&, 88 accept( ), 574
&&, 86 access: class, 164; control, 33, 151, 166; inner
&=, 89 classes & access rights, 221; package access and
[ ]: indexing operator [ ], 142 friendly, 159; specifiers, 33, 151, 159; within a
^, 89 directory, via the unnamed package, 160
^=, 89 action( ), 420, 430, 436, 597; cannot combine with
|, 88 listeners, 479
||, 86 ActionEvent, 452, 454, 471, 511
|=, 89 ActionListener, 452
‘+’: operator + for String, 407 actionPerformed( ), 455
+, 84 adapters: listener adapters, 456
<, 85 add( ), 432
<<, 89 addActionListener( ), 452, 508, 540
<<=, 90 addAdjustmentListener( ), 452
<=, 85 addComponentListener( ), 452
==, 85; operator, 387; vs. equals( ), 326 addContainerListener( ), 452
>, 85 addElement( ), Vector, 252, 255
>=, 85 addFocusListener( ), 452
>>, 89 addItemListener( ), 452
>>=, 90 addition, 82
abstract: class, 203; inheriting from an abstract addKeyListener( ), 452
class, 204; vs. interface, 210 addListener, 451
abstract keyword, 204 addMouseListener( ), 452
720 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-720-320.jpg)
![HTML, 589, 599; name, 526; param, 526; value, instance: instance initialization in Java 1.1, 220;
526 non-static instance initialization, 141
if-else statement, 92, 105 instanceof: dynamic instanceof, 368; keyword, 364
IllegalMonitorStateException, 545 Integer: parseInt( ), 448
immutable objects, 402 interface: and implementation, separation, 163; and
implementation: and interface, 181, 206; and inheritance, 211; base-class interface, 199;
interface, separation, 33, 163; hiding, 163, 214 Cloneable interface used as a flag, 385; common
implements keyword, 206 interface, 203; graphical user interface (GUI), 227,
import keyword, 152 413; initializing fields in interfaces, 212; keyword,
increment operator, 84 206; Runnable, 524; separation of interface and
incremental development, 56, 182 implementation, 33; upcasting to an interface,
209; vs. abstract, 210; vs. implemenation, 181
indexed property, 512
internationalization, in IO library, 326
indexing operator [ ], 142
Internet: Internet Protocol, 572; Internet Service
indexOf( ), 647, 679; String, 309, 373, 597
Provider (ISP), 417
InetAddress, 597
interrupt( ), 549
InflaterInputStream, 333
InterruptedException, 517
inheritance, 162, 169, 172, 193; and cloning, 394;
Intranet, 442
and design patterns, 632; and final, 189; and
finalize( ), 234; and synchronized, 540; choosing Introspector, 504
composition vs. inheritance, 181; class inheritance IO: and threads, blocking, 541; available( ), 317;
diagrams, 184; combining composition & blocking on IO, 546; blocking, and available( ),
inheritance, 176; designing with inheritance, 239; 317; BufferedInputStream, 306, 316;
extending interfaces with inheritance, 211; from BufferedOutputStream, 307, 318; BufferedReader,
an abstract class, 204; from inner classes, 224; 296, 328, 679; BufferedWriter, 328;
inheritance and method overloading vs. ByteArrayInputStream, 302;
overriding, 180; initialization with inheritance, ByteArrayOutputStream, 303; characteristics of
190; multiple inheritance in C++ and Java, 209; files, 312; CharArrayReader, 327;
pure inheritance vs. extension, 240; specialization, CharArrayWriter, 327; CheckedInputStream, 333;
182; vs composition, 323; vs. composition, 184 CheckedOutputStream, 333; close( ), 316;
init( ), 416, 418 connecting to a non-Java program, 590; console
input, 331; controlling the process of
initialization: and class loading, 190; array
serialization, 343; data file output shorthand,
initialization, 142; base class, 174; class member,
320; DataInput, 307; DataInputStream, 305,
171; constructor initialization during inheritance
316, 317, 318, 328, 670; DataOutput, 307;
and composition, 176; initializing class members
DataOutputStream, 307, 318, 328, 670, 680;
at point of definition, 136; initializing with the
DeflaterOutputStream, 333; deprecated, 301;
constructor, 119; instance initialization, 141;
directory lister, 308; directory, creating directories
instance initialization in Java 1.1, 220; member
and paths, 312; exec( ), 590; Externalizable, 343;
initializers, 234; non-static instance initialization,
File, 303, 328, 680, 688; File class, 308; file
141; of class data members, 135; of method
input shorthand, 319; File.list( ), 308;
variables, 135; order of initialization, 137, 238;
FileDescriptor, 303; FileInputStream, 303, 316;
static, 191; with inheritance, 190
FilenameFilter, 308, 687; FileOutputStream, 304,
inline method calls, 188
318; FileReader, 296, 327; FileWriter, 327;
inner class, 213, 461, 479, 521, 644; access rights,
FilterInputStream, 303; FilterOutputStream, 304;
221; and super, 224; and overriding, 225; and
FilterReader, 328; FilterWriter, 328; finalize( ),
control frameworks, 227; and the AWT, 451; and
318; formatted file output shorthand, 319; from
upcasting, 214; anonymous, 666; anonymous
standard input, 320; GZIPInputStream, 334;
inner class, 309, 551; anonymous inner class and
GZIPOutputStream, 333; InflaterInputStream,
constructors, 219; hidden reference to the object
333; input, 302; InputStream, 302, 577, 590;
of the enclosing class, 222; identifiers and .class
InputStreamReader, 326, 327, 577;
files, 226; in methods & scopes, 216; inheriting
internationalization, 326; Java 1.1 compression
from inner classes, 224; nesting within any
library, 333; library, 301; lightweight persistence,
arbitrary scope, 217; private, 522; private inner
339; LineNumberInputStream, 306, 317;
classes, 229; referring to the outer class object,
LineNumberReader, 328; mark( ), 308; mkdirs( ),
223; static, 222; static inner classes, 222
313, 680; nextToken( ), 687;
input: console input, 331; file input shorthand, 319 ObjectOutputStream, 339; output, 302;
InputStream, 302, 577, 590 OutputStream, 302, 303, 577, 590;
InputStreamReader, 326, 327, 577 OutputStreamWriter, 326, 327, 577; pipe, 302;
piped stream, 546; PipedInputStream, 303, 321;
Index 727](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-727-320.jpg)
![multithreading, 579; openConnection( ), 614; openConnection( ), 614
port, 574; POST using an applet, 614; openStream( ), URL, 604
QUERY_STRING, 612; receive( ), 586, 597; operator, 79; + and += overloading for String,
reliable protocol, 583; server, 573; ServerSocket, 173; +, for String, 407; == and !=, 387; binary,
574; serving multiple clients, 579; Servlet Server, 89; bitwise, 88; casting, 94; comma, 93; comma
628; servlet servers, 598; operator, 108; common pitfalls, 93; indexing
setAllowUserInteraction(false), 614; operator [ ], 142; logical, 86; logical operators
setDoInput(true), 614; setDoOutput(true), 614; and short-circuiting, 87; ones-complement, 89;
showDocument( ), 604, 616; Socket, 574, 578; operator overloading for String, 407;
stream-based sockets, 583; testing programs overloading, 93; precedence, 80; precedence
without a network, 573; Transmission Control mnemonic, 96; relational, 85; shift, 89; ternary,
Protocol (TCP), 583; unreliable protocol, 583; 92; unary, 84, 89
URL, 603, 605; URL openStream( ), 604; operator overloading: in C++, 609
URLConnection, 614; URLEncoder.encode( ), 600; OR, 94; (||), 86
User Datagram Protocol (UDP), 583; Web order: of constructor calls with inheritance, 233; of
application, 589 finalization of objects, 237; of initialization, 137,
new operator, 130; and primitives, array, 144 190, 238
newInstance( ): reflection, 370 organization, code, 159
newInstance( ), reflection, 644 output: data file output shorthand, 320; formatted
nextElement( ), Enumeration, 256 file output shorthand, 319
nextToken( ), 687 OutputStream, 302, 303, 577, 590
no-arg: constructors, 123 OutputStreamWriter, 326, 327, 577
non-Java: connecting to a non-Java program, 590 overflow: and primitive types, 104; stack overflow,
North, 433 259
not equivalent (!=), 85 overloading: and constructors, 121; distinguishing
notify( ), 541 overloaded methods, 123; lack of name hiding
notifyAll( ), 541 during inheritance, 180; method overloading,
notifyListeners( ), 540 121; on return values, 127; operator + and +=
notifyObservers( ), 634, 636 overloading for String, 173; operator
null, 248 overloading, 93; operator overloading for String,
NullPointerException, 284 407; overloading vs. overriding, 180; vs.
numbers, binary, 95 overriding, 202
object, 30; aliasing, 81; arrays are first-class objects, overriding: and inner classes, 225; overloading vs.
246; assigning objects by copying handles, 80; overriding, 180; vs. overloading, 202
assignment, 80; business object/logic, 477; Class package, 152, 638; access, and friendly, 159; and
object, 352, 362, 533; creation, 120; equals( ) applets, 418; and directory structure, 158;
method, 85; equivalence, 85; equivalence vs creating unique package names, 154; names,
handle equivalence, 85; final, 185; handle capitalization, 69; naming convention, 154;
equivalence vs. object equivalence, 387; statement and book chapter subdirectories, 80;
immutable objects, 402; lock, for unnamed package, 160; visibility, friendly, 214
multithreading, 533; object-oriented paint( ), 416, 437, 447, 492, 497
programming, 360; order of finalization of Panel, 434
objects, 237; process of creation, 140; param, HTML keyword, 526
serialization, 338; web of objects, 339, 384 parameter, applet, 417
Object, 246, 263, 639; clone( ), 384, 387; parameterized type, 255, 272, 655
getClass( ), 369 parseInt( ), 448
object-oriented: analysis & design, 52 pass: pass by value, 382; passing a handle into a
object-oriented programming, 29 method, 380
ObjectOutputStream, 339 patterns, design patterns, 166, 631
ObjectSpace, 272 performance, 713; and final, 189
Observable, 634 persistence, 350; lightweight persistence, 339
Observer, 634 pipe, 302
observer design pattern, 634 piped stream, 546
Octal, 95 PipedInputStream, 303, 321
ODBC, 616 PipedOutputStream, 303, 304, 321
ones complement operator, 89 PipedReader, 327
OOP, 163; protocol, 206 PipedWriter, 327
730 Thinking in Java Bruce Eckel - www.eckelobjects.com](https://image.slidesharecdn.com/tjava10a-100729223753-phpapp02/85/Tjava10a-730-320.jpg)
Tjava10a
- 1. About “Thinking in C++”: Best Book! Winner of the Software Development Magazine Jolt Award! “This book is a tremendous achievement. You owe it to yourself to have a copy on your shelf. The chapter on iostreams is the most comprehensive and understandable treatment of that subject I’ve seen to date.” Al Stevens Contributing Editor, Doctor Dobbs Journal “Eckel’s book is the only one to so clearly explain how to rethink program construction for object orientation. That the book is also an excellent tutorial on the ins and outs of C++ is an added bonus.” Andrew Binstock Editor, Unix Review “Bruce continues to amaze me with his insight into C++, and Thinking in C++ is his best collection of ideas yet. If you want clear answers to difficult questions about C++, buy this outstanding book.” Gary Entsminger Author, The Tao of Objects “Thinking in C++ patiently and methodically explores the issues of when and how to use inlines, references, operator overloading, inheritance and dynamic objects, as well as advanced topics such as the proper use of templates, exceptions and multiple inheritance. The entire effort is woven in a fabric that includes Eckel’s own philosophy of object and program design. A must for every C++ developer’s bookshelf, Thinking in C++ is the one C++ book you must have if you’re doing serious development with C++.” Richard Hale Shaw Contributing Editor, PC Magazine
- 3. Thinking in Java Bruce Eckel President, MindView Inc. ©1997 All Rights Reserved Revision 10a, November 14, 1997 This file may be mirrored along with the site by obtaining permission from the author. The electronic version of the book is available free; you can get an updated copy along with source code at http://www.EckelObjects.com. Corrections are greatly appreciated; please send them to Bruce@EckelObjects.com Contact the author if you would like to include an electronic version of this book on your product CD
- 5. Check http://www.EckelObjects.com for in-depth details and the date and location of the next Hands-On Java Seminar • Based on this book • Taught personally by Bruce Eckel • Includes in-class programming exercises • Also: Intermediate/Advanced seminars The electronic version of the book will continue to be free. You have permission to print a copy of the book for your own use. If you like the book, please: 1. Recommend it to your friends 2. Create links to the above site from your web page 3. Report any errors or problems you find 4. When the book is printed consider buying a copy (see the web site) – it's probably cheaper than the toner cartridge you'll use up. However, if it works well enough for you as an electronic document, great! 5. Consider coming to one of my public Hand-On Java Seminars
- 6. Bruce Eckel’s Hands-On Java Seminar Multimedia CD It’s like coming to the seminar! Available at http://www.EckelObjects.com (Available December 1997) Contains: • Overhead slides and Audio for all the lectures: just play it to see and hear the lectures! • Entire set of lectures is keyword indexed so you can rapidly locate the discussion of the subject you’re interested in • Electronic version of “Thinking in Java” book on CD with automatic Internet update system • Source code from the book
- 7. Dedication To the person who, even now, is creating the next great computer language
- 9. Overview Overview 9 What’s Inside... 11 Preface 17 1: Introduction to objects 29 2: Everything is an object 59 3: Controlling program flow 79 4: Initialization & cleanup 119 5: Hiding the implementation 151 6: Reusing classes 169 7: Polymorphism 193 8: Holding your objects 245 9: Error handling with exceptions 275 10: The Java IO system 301 11: Run-time type identification 359 12: Passing and returning objects 379 13: Creating windows & applets 413 14: Multiple threads 515 15: Network programming 571 16: Design patterns 631 17: Projects 669 A: Using non-Java code 699 B: Comparing C++ and Java 701 C: Java programming guidelines 709 D: A bit about garbage collection 713 E: Recommended reading 717 Index 720
- 11. @ What’s Inside... The singly-rooted hierarchy....40 Overview 9 Collection libraries and support for easy collection use .............................41 What’s Inside... 11 The housekeeping dilemma: who should clean up?....................................42 Preface 17 Exception handling: dealing with errors Prerequisites ....................... 17 ...........................................43 Learning Java...................... 18 Multithreading ....................44 Goals .................................. 18 Persistence...........................45 Chapters ............................. 19 Java and the Internet ...........45 Exercises ............................. 23 What is the Web?......................45 Source code ......................... 23 Client-side programming..........47 Coding standards...................... 24 Server-side programming.........51 Java versions ...................... 25 A separate arena: applications.51 Seminars & Mentoring ........ 25 Online documentation..........51 Errors ................................. 26 Analysis & Design ...............52 Acknowledgements.............. 26 Staying on course .....................52 Phase 0: Let’s make a plan .......53 1: Introduction to objects 29 Phase 1: What are we making?53 Phase 2: How will we build it? 54 The progress of abstraction . 30 Phase 3: Let’s build it!...............55 An object has an interface ... 31 Phase 4: Iteration......................55 The hidden implementation . 32 Plans pay off..............................56 Reusing the implementation33 Java vs. C++? .....................56 Inheritance: reusing the interface ........................................... 34 2: Everything is an object 59 Overriding base-class functionality You manipulate objects through handles ................................................... 35 ...........................................60 Is-a vs. is-like-a relationships.. 35 You must create all the objects60 Interchangeable objects with Where storage lives...................61 polymorphism .................... 36 Special case: primitive types.....62 Dynamic binding ...................... 37 Arrays in Java...........................63 The abstract base class............. 37 You never have to destroy an object Object landscapes and lifetimes ...........................................63 ........................................... 38 Scoping.......................................64 Collections and iterators........... 39
- 12. Scope of objects ......................... 64 Distinguishing overloaded methods Creating new data types: class65 ..................................................123 Fields and methods ................... 65 Overloading with primitives...123 Methods, arguments and return values Overloading on return values 127 ........................................... 67 Default constructors...............127 The argument list ..................... 67 The this keyword ...................128 Building a Java program ..... 68 Cleanup: finalization & garbage Name visibility .......................... 68 collection...........................130 Using other components.......... 69 What is finalize( ) for?..........131 The static keyword.................. 69 You must perform cleanup.....132 Your first Java program ...... 71 Member initialization ........135 Comments & embedded documentation Specifying initialization..........136 ........................................... 73 Constructor initialization.......137 Comment documentation......... 73 Array initialization............142 Syntax ....................................... 74 Mulitdimensional arrays........146 Embedded HTML....................... 74 Summary ..........................148 @see: referring to other classes75 Exercises............................149 Class documentation tags ........ 75 Variable documentation tags... 76 5: Hiding the implementation 151 Method documentation tags.... 76 Package: the library unit....152 Documentation example........... 77 Creating unique package names154 Coding style ........................ 77 A custom tool library..............156 Summary............................ 78 Package caveat.........................158 Exercises ............................. 78 Java access specifiers.........159 “Friendly” ................................159 3: Controlling program flow 79 public: interface access..........159 Using Java operators........... 79 private: you can’t touch that!161 Precedence ................................. 80 protected: “sort of friendly”.162 Assignment ............................... 80 Interface & implementation163 Mathematical operators........... 82 Class access .......................164 Auto increment and decrement84 Summary ..........................166 Relational operators.................. 85 Exercises............................167 Logical operators....................... 86 Bitwise operators...................... 88 6: Reusing classes 169 Shift operators .......................... 89 Composition syntax...........169 Ternary if-else operator........... 92 Inheritance syntax.............172 The comma operator ................ 93 Initializing the base class........174 String operator +..................... 93 Combining composition & inheritance Common pitfalls when using operators .........................................176 ................................................... 93 Guaranteeing proper cleanup.177 Casting operators...................... 94 Name hiding ............................180 Java has no “sizeof” ................. 96 Choosing composition vs. inheritance Precedence revisited .................. 96 .........................................181 A compendium of operators..... 97 protected ..........................182 Execution control .............. 105 Incremental development ...182 True and false .........................105 If-else.......................................105 Upcasting..........................183 Iteration...................................106 Why “upcasting”?...................184 Do-while..................................107 The final keyword.............185 For............................................107 Final data.................................185 Break and continue.................108 Final methods..........................188 Switch......................................113 Final classes.............................188 Summary.......................... 116 Final caution............................189 Exercises ........................... 117 Initialization & class loading190 Initialization with inheritance190 4: Initialization & cleanup 119 Summary ..........................191 Guaranteed initialization with the Exercises............................192 constructor ....................... 119 Method overloading........... 121
- 13. 7: Polymorphism 193 Catching an exception........277 The try block...........................278 Upcasting ......................... 193 Exception handlers..................278 Why upcast?...........................194 The exception specification.....279 The twist .......................... 196 Catching any exception ..........280 Method call binding................196 Rethrowing an exception........281 Producing the right behavior.197 Standard java exceptions....284 Extensibility.............................199 The special case of RuntimeException Overriding vs. overloading. 202 ..................................................284 Abstract classes & methods203 Creating your own exceptions285 Interfaces.......................... 206 Exception restrictions ........288 “Multiple inheritance” in Java209 Performing cleanup with finally Extending an interface with inheritance .........................................291 .................................................211 What’s finally for? ................292 Grouping constants................212 Pitfall: the lost exception.........294 Initializing fields in interfaces212 Constructors......................295 Inner classes ..................... 213 Exception matching ...........298 Inner classes and upcasting...214 Exception guidelines................299 Inner classes in methods & scopes 216 The link to the outer class......220 Summary ..........................300 Inheriting from inner classes.224 Exercises............................300 Can inner classes be overridden?225 Inner class identifiers .............226 10: The Java IO system 301 Why inner classes: control frameworks Input and output ...............302 .................................................227 Types of InputStream...........302 Constructors & polymorphism232 Types of OutputStream........303 Order of constructor calls ......233 Adding attributes & useful interfaces Inheritance and finalize( )....234 .........................................304 Behavior of polymorphic methods inside Reading from an InputStream with constructors............................237 FilterInputStream ................305 Designing with inheritance 239 Writing to an OutputStream with Pure inheritance vs. extension240 FilterOutputStream .............306 Downcasting & run-time type identification Off by itself: RandomAccessFile .................................................242 .........................................307 Summary.......................... 243 The File class.....................308 Exercises ........................... 244 A directory lister .....................308 Checking for and creating directories 8: Holding your objects 245 ..................................................312 Arrays .............................. 245 Typical uses of IO streams .314 Arrays are first-class objects.246 Input streams..........................316 Returning an array ................249 Output streams .......................318 Collections ........................ 250 Shorthand for file manipulation319 Disadvantage: unknown type251 Reading from standard input.320 Enumerators (iterators) ..... 255 Piped streams...........................321 Types of collections ........... 258 StreamTokenizer ..............321 Vector .....................................258 StringTokenizer....................324 BitSet ......................................259 Java 1.1 IO streams...........326 Stack .......................................260 Sources & sinks of data...........327 Hashtable ..............................261 Modifying stream behavior....327 Enumerators revisited ............267 Unchanged classes..................328 Sorting.............................. 268 An example..............................329 The generic collection library272 Redirecting standard IO..........332 Summary.......................... 272 Compression......................333 Exercises ........................... 273 Simple compression with GZIP334 Multi-file storage with zip .....335 9: Error handling with The Java archive (jar) utility..337 Object serialization............338 exceptions 275 Finding the class......................342 Basic exceptions ................ 276 Controlling serialization.........343 Exception arguments..............277 Using persistence ....................350
- 14. Summary.......................... 356 Radio buttons ....................427 Exercises ........................... 357 Drop-down lists ................428 List boxes ..........................429 11: Run-time handleEvent( ).......................430 type identification 359 Controlling layout .............432 FlowLayout............................432 The need for RTTI .............. 359 BorderLayout ........................433 The Class object......................362 GridLayout.............................433 Checking before a cast............364 CardLayout............................434 RTTI syntax....................... 369 GridBagLayout .....................435 Reflection: run-time class information Alternatives to action() .....436 ......................................... 371 Applet restrictions.............440 A class method extractor........372 Applet advantages...................441 Summary.......................... 376 Windowed applications......442 Exercises ........................... 376 Menus ......................................442 Dialog boxes.............................445 12: Passing and The new AWT....................450 returning objects 379 The new event model..............451 Passing handles around ..... 380 Event and listener types..........452 Aliasing....................................380 Making windows and applets with the Java Making local copies........... 382 1.1 AWT ..................................457 Pass by value...........................382 Revisiting the earlier examples459 Cloning objects........................383 Binding events dynamically...475 Adding cloneability to a class.384 Separating business logic from ui logic Successful cloning...................385 ..................................................476 The effect of Object.clone( )..387 Recommended coding approaches478 Cloning a composed object .....388 New Java 1.1 UI APIs.........492 A deep copy with Vector .......390 Desktop colors .........................492 Deep copy via serialization.....392 Printing....................................492 Adding cloneability further down a hierarchy The clipboard...........................499 .................................................394 Visual programming & Beans501 Why this strange design? ......394 What is a Bean? ......................502 Controlling cloneability ..... 395 Extracting BeanInfo with the Introspector The copy-constructor.............399 ..................................................503 Read-only classes .............. 402 A more sophisticated Bean .....508 Creating read-only classes.....403 Packaging a Bean ....................511 The drawback to immutability404 More complex Bean support...512 Immutable Strings .................406 More to Beans .........................513 The String and StringBuffer classes Summary ..........................513 .................................................408 Exercises............................513 Strings are special..................411 Summary.......................... 411 14: Multiple threads 515 Exercises ........................... 412 Responsive user interfaces .516 Inheriting from Thread .........517 13: Creating windows Threading for a responsive interface ..................................................519 & applets 413 Combining the thread with the main class Why use the AWT? ........... 414 ..................................................523 The basic applet ................ 415 Making many threads............524 Testing applets ........................417 Daemon threads......................527 A more graphical example......418 Sharing limited resources...529 Demonstrating the framework methods Improperly accessing resources529 .................................................418 How Java shares resources....533 Making a button ............... 419 Java Beans revisited................537 Capturing an event............ 420 Blocking ............................540 Text fields.......................... 422 Becoming blocked....................541 Text areas.......................... 423 Deadlock...................................549 Labels ............................... 424 Priorities ...........................553 Check boxes ...................... 426 Thread groups.........................556
- 15. Runnable revisited ........... 562 16: Design patterns 631 Too many threads ..................564 The pattern concept...........631 Summary.......................... 567 The singleton...........................632 Exercises ........................... 568 Classifying patterns................633 The observer pattern..........634 15: Network programming 571 Simulating the trash recycler636 Identifying a machine........ 572 Improving the design .........639 Servers and clients..................573 “Make more objects”...............640 Port: a unique place within the machine A pattern for prototyping creation .................................................574 ..................................................642 Sockets ............................. 574 Abstracting usage ..............649 A simple server and client ......575 Multiple dispatching..........652 Serving multiple clients..... 579 Implementing the double dispatch 653 Datagrams........................ 583 The “visitor” pattern..........659 A Web application............. 589 RTTI considered harmful?...664 The server application ............590 Summary ..........................666 The NameSender applet........594 Exercises............................667 Problems with this approach.598 Connecting Java to CGI...... 599 Encoding data for CGI ............599 17: Projects 669 The applet................................601 Text processing ..................669 The CGI program in C++ ......605 Extracting code listings...........669 What about POST? .................612 Checking capitalization style..681 Connecting to databases with JDBC A method lookup tool ........688 ......................................... 616 Complexity theory.............692 Getting the example to work..618 Summary ..........................698 A GUI version of the lookup program Exercises............................698 .................................................621 Why the JDBC API seems so complex A: Using non-Java code 699 .................................................623 Native methods..................699 Remote methods................ 623 CORBA ..............................699 Remote interfaces....................624 Implementing the remote interface B: Comparing C++ and Java 701 .................................................624 Creating stubs and skeletons .627 C: Java programming guidelines Using the remote object..........627 Alternatives to RMI ................628 D: A bit about garbage collection Summary.......................... 628 Exercises ........................... 629 E: Recommended reading 717 Index 720
- 17. ( Preface Like any human language, Java provides a way to express concepts. If successful, this medium of expression will be significantly easier and more flexible than the alternatives as problems grow larger and more complex. You can’t look at Java as just a collection of features; some of the features make no sense in isolation. You can use the sum of the parts only if you are thinking about design, not simply coding. And to understand Java in this way, you must understand the problems with Java and with programming in general. This book discusses programming problems, why they are problems, and the approach Java has taken to solve such problems. Thus, the set of features I explain in each chapter will be based on the way I see a particular type of problem being solved with the language. In this way I hope to move you, a little at a time, to the point where the Java mindset becomes your native tongue. Throughout, I’ll be taking the attitude that you want to build a model in your head that allows you to develop a deep understanding of the language; if you encounter a puzzle you’ll be able to feed it to your model and deduce the answer. Prerequisites This book assumes you have some programming familiarity, so you understand that a program is a collection of statements, the idea of a subroutine/function/macro, control statements like “if” and looping constructs like “while,” etc. However, you might have learned this in many places, such as programming with a macro language or a tool like Perl. Just so long as you’ve programmed to the point where you feel comfortable with the basic ideas of programming, you’ll be able to work through this book. Of course, the book will be easier for the C programmers and more so for the C++ programmers, but don’t count yourself out if you’re not experienced with those languages (but come willing to 17
- 18. work hard). I’ll be introducing the concepts of object-oriented programming and Java’s basic control mechanisms, so you’ll be exposed to those, and the first exercises will involve the basic control-flow statements. Although references will often be made to C and C++ language features these are not intended to be insider comments, but instead to help all programmers put Java in perspective with those languages which, after all, Java is descended from. I will attempt to make these references simple and to explain anything I think a non- C/C++ programmer would not be familiar with. Learning Java At about the same time that my first book Using C++ (Osborne/McGraw-Hill 1989) came out, I began teaching the language. Teaching programming languages has become my profession; I’ve seen nodding heads, blank faces, and puzzled expressions in audiences all over the world since 1989. As I began giving in-house training with smaller groups of people, I discovered something during the exercises. Even those people who were smiling and nodding were confused about many issues. I found out, by chairing the C++ track at the Software Development Conference for the last few years (and now also the Java track), that I and other speakers tended to give the typical audience too many topics, too fast. So eventually, through both variety in the audience level and the way that I presented the material, I would end up losing some portion of the audience. Maybe it’s asking too much, but because I am one of those people resistant to traditional lecturing (and for most people, I believe, such resistance results from boredom), I wanted to try to keep everyone up to speed. For a time, I was creating a number of different presentations in fairly short order. Thus, I ended up learning by experiment and iteration (a technique that also works well in Java program design). Eventually I developed a course using everything I had learned from my teaching experience, one I would be happy giving for a long time. It tackles the learning problem in discrete, easy-to-digest steps and for a hands-on seminar (the ideal learning situation), there are exercises following each of the short lessons. I now give this course in public Java seminars which you can find out about at http://www.EckelObjects.com. The feedback that I get from each seminar helps me change and refocus the material until I feel it works well as a teaching medium. But this book isn’t just a seminar handout – I tried to pack as much information as I could within these pages, and structure it to draw you through, onto the next subject. More than anything, the book is designed to serve the solitary reader, struggling with a new programming language. Goals Like my previous book Thinking in C++, this book has come to be structured around the process of teaching the language. In particular, my motivation is to create something that provides me a way to teach the language in my own seminars. Thus, when I think of a chapter in the book, I think in terms of what makes a good lesson during a seminar. My goal is to get bite-sized pieces that can be taught in a reasonable amount of time, followed by exercises that are feasible to accomplish in a classroom situation. My goals in this book are to: 1. Present the material a simple step at a time, so the reader can easily digest each concept before moving on. 18 Thinking in Java Bruce Eckel
- 19. 2. Use examples that are as simple and short as possible. This sometimes prevents me from tackling “real-world” problems, but I’ve found that beginners are usually happier when they can understand every detail of an example rather than being impressed by the scope of the problem it solves. Also, there’s a severe limit to the amount of code that can be absorbed in a classroom situation. For this I will no doubt receive criticism for using “toy examples,” but I’m willing to accept that in favor of producing something pedagogically useful. 3. Carefully sequence the presentation of features so that you aren’t seeing something you haven’t been exposed to. Of course, this isn’t always possible; in those situations, a brief introductory description will be given. 4. Give you what I think is important for you to understand about the language, rather than everything I know. I believe there is an “information importance hierarchy,” and there are some facts that 95% of programmers will never need to know, but would just confuse people and add to their perception of the complexity of the language. To take an example from C, if you memorize the operator precedence table (I never did) you can write clever code. But if you have to think about it, it will also confuse the reader/maintainer of that code. So forget about precedence, and use parentheses when things aren’t clear. 5. Keep each section focused enough so the lecture time – and the time between exercise periods – is small. Not only does this keep the audience’ minds more active and involved during a hands-on seminar, but it gives the reader a greater sense of accomplishment. 6. Provide the reader with a solid foundation so they can understand the issues well enough to move on to more difficult coursework and books. Chapters This course was designed with one thing in mind: the way people learn the Java language. Audience feedback helped me understand which parts were difficult and needed extra illumination. In the areas where I got ambitious and included too many features all at once, I came to know – through the process of presenting the material – that if you include a lot of new features, you have to explain them all, and the student’s confusion is easily compounded. As a result, I’ve taken a great deal of trouble to introduce the features as few at a time as possible. The goal, then, is for each chapter to teach a single feature, or a small group of associated features, in such a way that no additional features are relied upon. That way you can digest each piece in the context of your current knowledge before moving on. Here is a brief description of the chapters contained in the book, which correspond to lectures and exercise periods in my hands-on seminars. Chapter 1: Introduction to Objects This chapter is an overview of what object-oriented programming is all about, including the answer to the basic question “what’s an object?”, interface vs. implementation, abstraction and encapsulation, messages and functions, inheritance and composition, and the all-important polymorphism. Then you’ll be introduced to issues of object creation like constructors, where the objects live, where to put them once they’re created (answer: in collections), and the magical garbage collector that Preface 19
- 20. cleans up the objects that are no longer needed. Other issues will be introduced, like error handling with exceptions and multithreading for responsive user interfaces. You’ll also learn about what makes Java special and why it’s been so successful. Chapter 2: Everything is an Object This chapter moves you to the point where you can write your first Java program, so it must give an overview of the essentials, including: the concept of a “handle” to an object; how to create an object; an introduction to primitive types and arrays; scoping and the way objects are destroyed by the garbage collector; how everything in Java is a new data type (class) and how to create your own classes; functions, arguments, and return values; name visibility and using components from other libraries; the static keyword; comments and embedded documentation. Chapter 3: Controlling Program Flow This chapter begins with all the operators that come to Java from C and C++. In addition, you’ll discover common operator pitfalls, casting, promotion and precedence. This is followed by the basic control-flow and selection operations that you get with virtually any programming language: choice with if-else; looping with for and while; quitting a loop with break and continue as well as Java’s labeled break and labeled continue (which account for the “missing goto” in Java); and selection using switch. Although much of this material has common threads with C and C++ code, there are some differences. In addition, all the examples will be full Java examples so you’ll be getting more comfortable with what Java looks like. Chapter 4: Initialization & Cleanup This chapter begins by introducing the constructor, which guarantees proper initialization. The definition of the constructor leads into the concept of function overloading (since you might want several constructors). This is followed by a discussion of the process of cleanup, which is not always as simple as it seems. Normally you just drop an object when you’re done with it and the garbage collector eventually comes along and releases the memory. This portion explores the garbage collector and some of its idiosyncrasies. The chapter concludes with a closer look at how things are initialized: automatic member initialization, specifying member initialization, the order of initialization, static initialization, and array initialization. Chapter 5: Hiding The Implementation This chapter covers the way that code is packaged together, and how some parts of a library are exposed and other parts are hidden. It begins by looking at the package and import keywords, which perform file-level packaging and allow you to build libraries of classes. The subject of directory paths and file names is examined. The remainder of the chapter looks at the public, private, and protected keywords and the concept of “friendly” access, and what the different levels of access control mean when used in various contexts. Chapter 6: Reusing Classes The concept of inheritance is standard in virtually all OOP languages. It’s a way to take an existing class and add to its functionality (as well as change it, the subject of Chapter 7), so inheritance is often a way to re-use code by leaving the “base class” the same, and just patching things here and there to produce what you want. However, inheritance isn’t the only way to make new classes from existing ones; you can also embed an object inside your new class with composition. In this chapter you’ll learn about these two ways to reuse in Java and how to apply them. Chapter 7: Polymorphism On your own, you might take nine months to discover and understand this cornerstone of OOP. Through small, simple examples you’ll see how to create a family of types with inheritance and manipulate objects in that family through their 20 Thinking in Java Bruce Eckel
- 21. common base class. Java’s polymorphism allows you to treat all objects in this family generically, which means the bulk of your code doesn’t rely on specific type information. This makes your programs extensible, so building programs and code maintenance is easier and cheaper. In addition, Java provides a third way to set up a reuse relationship: through the interface, which is a pure abstraction of the interface of an object. Once you’ve seen polymorphism, the interface can be clearly understood. This chapter also introduces Java 1.1 inner classes. Chapter 8: Holding Your Objects It’s a fairly simple program that has only a fixed quantity of objects with known lifetimes. In general your programs will always be creating new objects at a variety of times that will be known only while the program is running. In addition, you won’t know until run-time the quantity or even the exact type of the objects you need. To solve the general programming problem, you need to create any number of objects, anytime, anywhere. This chapter explores in depth the tools that Java supplies to hold objects while you’re working with them: the simple arrays and more sophisticated collections (data structures) like Vector and Hashtable. Chapter 9: Error Handling With Exceptions The basic philosophy of Java is that “badly-formed code will not be run.” As much as possible, the compiler catches problems, but sometimes the problems – either programmer error or a natural error condition that occurs as part of the normal execution of the program – can be detected and dealt with only at run-time. Java has exception handling to deal with any problems that arise while the program is running. This chapter examines how the keywords try, catch, throw, throws, and finally work in Java, when you should throw exceptions, and what to do when you catch them. In addition, you’ll see Java’s standard exceptions, how to create your own, what happens with exceptions in constructors, and how exception handlers are located. Chapter 10: The Java IO System Theoretically you can divide any program into 3 parts: input, process, and output. This implies that IO (input/output) is a pretty important part of the equation. In this chapter you’ll learn about the different classes that Java provides for reading and writing files, blocks of memory, and the console. The distinction between “old” IO and “new” Java 1.1 IO will be shown. In addition, this section examines the process of taking an object, “streaming” it (so that it can be placed on disk or sent across a network) and reconstructing it, which is handled for you in Java version 1.1. Also, Java 1.1’s compression libraries, which are used in the Java ARchive file format (JAR), are examined. Chapter 11: Run-time type identification Java run-time type identification (RTTI) lets you find the exact type of an object when you have a handle to only the base type. Normally, you’ll want to intentionally ignore the exact type of an object and let Java’s dynamic binding mechanism (polymorphism) implement the correct behavior for that type. But occasionally it is very helpful to know the exact type of an object for which you have only a base handle; often this information allows you to perform a special-case operation more efficiently. This chapter explains what RTTI is for, how to use it, and how to get rid of it when it doesn’t belong there. In addition, the Java 1.1 reflection feature is introduced. Chapter 12: Passing & Returning Objects Since the only way you talk to objects in Java is through “handles,” the concepts of passing an object into a function, and returning an object from a function, have some interesting consequences. This explains what you need to know to manage objects Preface 21
- 22. when you’re moving in and out of functions, and also shows the String class, which uses a different approach to the problem. Chapter 13: Creating Windows and Applets Java comes with the Abstract Window Toolkit (AWT), which is a set of classes that handle windowing in a portable fashion; these windowing programs can either be “applets” or stand-alone applications. This chapter is an introduction to the AWT and the creation of World-Wide-Web applets. We’ll also look at pros and cons of the AWT, and the GUI improvements introduced in Java 1.1. Finally, the very important “Java Beans” technology is introduced, which is fundamental for the creation of Rapid- Application Development (RAD) program-building tools. Chapter 14: Multiple Threads Java provides a built-in facility to support multiple concurrent subtasks, called threads, running within a single program (unless you have multiple processors on your machine, this is only the appearance of multiple subtasks). Although these can be used anywhere, they are most powerful when trying to create a responsive user interface so, for example, a user isn’t prevented from pressing a button or entering data while some processing is going on. This chapter looks at the syntax and semantics of multithreading in Java. Chapter 15: Network Programming All the Java features and libraries seem to really come together when you start writing programs to work across networks. This chapter explores communication across the Internet, and the classes that Java provides to make this easier. It also shows you how to create a Java applet that talks to a common gateway interface (CGI) program, shows you how to write CGI programs in C++, and covers Java 1.1’s Java DataBase Connectivity (JDBC) and Remote Method Invocation (RMI). Chapter 16: Design patterns This chapter introduces the very important and yet non-traditional “patterns” approach to program design. An example of the design evolution process will be studied, starting with an initial solution and moving through the logic and process of evolving the solution to more appropriate designs. You’ll see one way that a design can materialize over time. Chapter 17: Projects This chapter includes a set of projects that build on the material presented in this book, or otherwise didn’t fit in earlier chapters. These projects are significantly more complex than the examples in the rest of the book, and they often demonstrate new techniques and uses of class libraries. In addition, there are subjects that didn’t seem to fit within the core of the book, and yet I find that I discuss them during seminars. These are placed in the appendices: Appendix A: Using non-Java Code A totally portable Java program has serious drawbacks: speed and the inability to access platform-specific services. When you know the platform that you’re running on it’s possible to dramatically speed up certain operations by making them native methods, which are functions that are written in another programming language (currently, only C/C++ is supported). There are other ways that Java supports non- Java code, including CORBA. This appendix contains pointers to other resources for connecting Java to non-Java code. Appendix B: Comparing C++ and Java If you’re a C++ programmer you already have the basic idea of object-oriented programming, and the syntax of Java no doubt looks very familiar to you. This 22 Thinking in Java Bruce Eckel
- 23. makes sense since Java was derived from C++. However, there are a surprising number of differences between C++ and Java. These differences are intended to be significant improvements, and if you understand the differences you’ll see why Java is such a beneficial programming language. This appendix takes you through the important features that make Java distinct from C++. Appendix C: Java programming guidelines This appendix contains suggestions to help guide you while performing low-level program design and also while writing code. Appendix D: A bit about garbage collection Describes the operation and approaches that are used to implement garbage collection. Appendix E: Recommended reading There are a lot of Java books out there, and a lot of them simply take the online documentation downloadable from Sun and format those docs into a book, with some hasty prose added. They’re not all like that, however, and these are some of the Java books I’ve found particularly useful. Exercises I’ve discovered that simple exercises are exceptionally useful during a seminar to complete a student’s understanding, so you’ll find a set at the end of each chapter, which are those that I give in my own seminar. These are designed to be easy enough that they can be finished in a reasonable amount of time in a classroom situation while the instructor observes, making sure all the students are absorbing the material. Some exercises are more advanced to prevent boredom on the part of experienced students. They’re all designed to be solved in a short time and are there only to test and polish your knowledge rather than present major challenges (presumably, you’ll find those on your own – or more likely they’ll find you). Source code All the source code for this book is available as copyrighted freeware, distributed as a single package, by visiting the Web site http://www.EckelObjects.com. To make sure that you get the most current version, this is the official site for distribution of the code and the electronic version of the book. You can find mirrored versions of the electronic book and the code on other sites (some of these sites will be found at http://www.EckelObjects.com), but you should check the official site to ensure that the mirrored version is actually the most recent edition. You may distribute the code in classroom and other educational situations. The primary goal of the copyright is to ensure that the source of the code is properly cited, and to prevent you from republishing the code in print media without permission (as long as the source is cited, using examples from the book in most media is generally not a problem). In each source-code file you will find the following copyright notice: ////////////////////////////////////////////////// // Copyright (c) Bruce Eckel, 1997 // Source code file from the book "Thinking in Java" Preface 23
- 24. // All rights reserved EXCEPT as allowed by the // following statements: You can freely use this file // for your own work (personal or commercial), // including modifications and distribution in // executable form only. Permission is granted to use // this file in classroom situations, including its // use in presentation materials, as long as the book // "Thinking in Java" is cited as the source. // Except in classroom situations, you cannot copy // and distribute this code; instead, the sole // distribution point is http://www.EckelObjects.com // (and official mirror sites) where it is // freely available. You cannot remove this // copyright and notice. You cannot distribute // modified versions of the source code in this // package. You cannot use this file in printed // media without the express permission of the // author. Bruce Eckel makes no representation about // the suitability of this software for any purpose. // It is provided "as is" without express or implied // warranty of any kind, including any implied // warranty of merchantability, fitness for a // particular purpose or non-infringement. The entire // risk as to the quality and performance of the // software is with you. Bruce Eckel and the // publisher shall not be liable for any damages // suffered by you or any third party as a result of // using or distributing software. In no event will // Bruce Eckel or the publisher be liable for any // lost revenue, profit or data, or for direct, // indirect, special, consequential, incidental or // punitive damages, however caused and regardless of // the theory of liability, arising out of the use of // or inability to use software, even if Bruce Eckel // and the publisher have been advised of the // possibility of such damages. Should the software // prove defective, you assume the cost of all // necessary servicing, repair, or correction. If you // think you've found an error, please email all // modified files with clearly commented changes to: // Bruce@EckelObjects.com. (please use the same // address for non-code errors found in the book). ////////////////////////////////////////////////// You may use the code in your projects and in the classroom (including your presentation materials) as long as the copyright notice that appears in each source file is retained. Coding standards In the text of this book, identifiers (function, variable, and class names) will be set in bold. Most keywords will also be set in bold, except for those keywords which are used so much that the bolding can become tedious, such as “class.” I use a particular coding style for the examples in this book. This style seems to be supported by most Java development environments. It was developed over a number of years, and was inspired by Bjarne Stroustrup’s style in his original The C++ Programming 24 Thinking in Java Bruce Eckel
- 25. Language (Addison-Wesley, 1991; 2nd ed.). The subject of formatting style is good for hours of hot debate, so I’ll just say I’m not trying to dictate correct style via my examples; I have my own motivation for using the style that I do. Because Java is a free-form programming language, you can continue to use whatever style you’re comfortable with. The programs in this book are files that are included by the word processor in the text, directly from compiled files. Thus, the code files printed in the book should all work without compiler errors. The errors that should cause compile-time error messages are commented out with the comment //! so they can be easily discovered and tested using automatic means. Errors discovered and reported to the author will appear first in the distributed source code and later in updates of the book (which will also appear on the Web site http://www.EckelObjects.com) Java versions Although I test the code in this book with several different vendor implementations of Java, I generally rely on the Sun implementation as a reference when determining whether behavior is correct. By the time you read this, Sun will have released three major versions of Java: 1.0, about a year later version 1.1, and then roughly nine months later version 1.2 (Sun says it will make a major release about every 9 months!). Version 1.1 represents a very significant change to the language and should probably have been labeled 2.0 (and if 1.1 is such a big change from 1.0, I shudder to think what will justify the number 2.0). However, it's version 1.2 that seems to finally bring Java into the prime time, in particular where user interface tools are concerned. This book covers versions 1.0, 1.1 and 1.2, although in places where the new approach is clearly superior to the old, I definitely favor the new version, often choosing to teach the better approach and completely ignore the 1.0 approach (there are plenty of other books that teach 1.0). However, there are some cases where its unavoidable to teach the old approach before the new – in particular with the AWT, since not only is there a lot of old 1.0 code out there, but some platforms still support only 1.0. I will try to be scrupulous about pointing out which features belong to which version. One thing you’ll notice is that I don’t use the sub-revision numbers. At this writing, the released version of 1.0 from Sun was 1.02 and the released version of 1.1 was 1.1.3. In this book I will refer only to Java 1.0, Java 1.1 and Java 1.2, to guard against typographical errors produced by further sub-revisioning of these products. Seminars & Mentoring My company provides five-day, hands-on, public & in-house training seminars based on the material in this book. Selected material from each chapter represents a lesson, which is followed by a monitored exercise period so each student receives personal attention. The lectures and slides for the introductory seminar is also captured on CD-ROM to provide at least some of the experience of the seminar without the travel and expense. For more information, go to http://www.EckelObjects.com or email: Bruce@EckelObjects.com Preface 25
- 26. My company also provides consulting services to help guide your project through its development cycle, especially your company’s first Java project. Errors No matter how many tricks a writer uses to detect errors, some always creep in and these often leap off the page for a fresh reader. If you discover anything you believe to be an error, please send the original source file (which you can find at http://www.EckelObjects.com) with a clearly-commented error and suggested correction via electronic mail to Bruce@EckelObjects.com so it may be fixed in the electronic version (on the Web site) and the next printing of the book. Also, suggestions for additional exercises or requests to cover specific topics in the next edition are welcome. Your help is appreciated. Acknowledgements First of all, thanks to the Doyle Street Cohousing Community for putting up with me for the 2 years that it took me to write this book (and for putting up with me at all). Thanks very much to Kevin & Sonda Donovan for subletting their great place in gorgeous Crested Butte Colorado for the summer while I worked on the book. Also thanks to the friendly residents of Crested Butte and the Rocky Mountain Biological Laboratory who made me feel so welcome. I’m especially indebted to Gen Kiyooka and his company Digigami, who have graciously provided my Web server, and to Scott Callaway who has maintained it. This has been an invaluable aid while I was learning about the Web. Thanks to Cay Horstmann (co-author of Core Java, Prentice Hall 1996), D’Arcy Smith (Symantec), and Paul Tyma (co-author of Java Primer Plus, The Waite Group 1996), for helping me clarify concepts in the language. Thanks to people who have spoken in my Java track at the Software Development Conference, and students in my seminars, who ask the questions I need to hear in order to make the material clearer. Special thanks to Larry and Tina O’Brien, who turned this book and my seminar into a teaching CD ROM (you can find out more at http://www.EckelObjects.com). Lots of people sent in corrections and I am indebted to them all, but particular thanks go to: Kevin Raulerson (tons of great bugs found), Bob Resendes (simply incredible), John Pinto, Dr. Robert Stephenson, Franklin Chen, Zev Griner, David Karr, Joe Sharp, Leander A. Stroschein, Joe Dante, Steve Clark, Charles A. Lee, Austin Maher, Dennis P. Roth, Roque Oliveira, Douglas Dunn, Dejan Ristic, Neil Galarneau, David B. Malkovsky, Steve Wilkinson, and others. Prof. Ir. Marc Meurrens put in a great deal of effort to publicize and make the book available in Europe. There have been a spate of smart technical people in my life who have become friends and have also been both influential and unusual in that they’re vegetarians, do Yoga and practice other forms of spiritual enhancement, which I find quite inspirational and instructional: Kraig Brockshmidt, Gen Kiyooka, Andrea Provaglio (who helps in the understanding of Java and programming in general in Italy). 26 Thinking in Java Bruce Eckel
- 27. It’s not that much of a surprise to me that understanding Delphi helped me understand Java, since there are many concepts and language design decisions in common. My Delphi friends provided assistance by helping me gain insight into that marvelous programming environment: Marco Cantu (another Italian – perhaps being steeped in Latin gives one aptitude for programming languages?), Neil Rubenking (who used to do the Yoga/Vegetarian/Zen thing but discovered computers) and of course Zack Urlocker, long- time pal whom I’ve traveled the world with. My friend Richard Hale Shaw’s insights and support have been very helpful (and Kim’s, too). Richard and I spent many months giving seminars together and trying to work out the perfect learning experience for the attendees. Thanks also to KoAnn Vikoren, Eric Faurot, Deborah Sommers, Julie Shaw, Nicole Freeman, Cindy Blair, Barbara Hanscome, Regina Ridley, Alex Dunne, and the rest of the cast and crew at MFI. The book design, cover design, and cover photo were created by my friend Daniel Will- Harris, noted author and designer (http://www.Will-Harris.com), who used to play with rub-on letters in junior high school while he awaited the invention of computers and desktop publishing, and complained of me mumbling over my algebra problems. However, I produced the camera-ready pages myself, so the typesetting errors are mine. Microsoft® Word for Windows 97 was used to write the book and to create camera-ready pages. The body typeface is Bitstream Carmina and the headlines are in Bitstream Calligraph 421 (www.bitstream.com). The symbols at the start of each chapter are Leonardo Extras from P22 (http://www.p22.com). The cover typeface is ITC Rennie Mackintosh. Thanks to the vendors who supplied me with compilers: Borland, Microsoft, Symantec, Sybase/Powersoft/Watcom, and of course Sun. A special thanks to all my teachers, and all my students (who are my teachers as well). The most fun writing teacher was Gabrielle Rico (author of Writing the Natural Way, Putnam 1983). I’ll always treasure the terrific week at Esalen. The supporting cast of friends includes, but is not limited to: Andrew Binstock, Steve Sinofsky, JD Hildebrandt, Tom Keffer, Brian McElhinney, Brinkley Barr, Bill Gates at Midnight Engineering Magazine, Larry Constantine & Lucy Lockwood, Greg Perry, Dan Putterman, Christi Westphal, Gene Wang, Dave Mayer, David Intersimone, Andrea Rosenfield, Claire Sawyers, more Italians (Laura Fallai, Corrado, Ilsa and Cristina Giustozzi), Chris & Laura Strand, The Almquists, Brad Jerbic, Marilyn Cvitanic, The Mabrys, The Haflingers, The Pollocks, Peter Vinci, The Robbins Families, The Moelter Families (& the McMillans), Michael Wilk, Dave Stoner, Laurie Adams, The Cranstons, Larry Fogg, Mike & Karen Sequeira, Gary Entsminger & Allison Brody, Kevin Donovan & Sonda Eastlack, Chester & Shannon Andersen, Joe Lordi, Dave & Brenda Bartlett, David Lee, The Rentschlers, The Sudeks, Dick, Patty, and Lee Eckel, Lynn & Todd, and their families. And of course, Mom & Dad. Preface 27
- 29. ABC 1: Introduction to objects Why has object-oriented programming had such a sweeping impact on the software development community? Object-oriented programming appeals at multiple levels. For managers it promises faster and cheaper development and maintenance. For analysts and designers the modeling process becomes simpler and produces a clear, manageable design. For programmers the elegance and clarity of the object model and the power of object-oriented tools and libraries makes programming a much more pleasant task, and programmers experience an increase in productivity. Everybody wins, it would seem. If there’s a downside it is the expense of the learning curve. Thinking in objects is a dramatic departure from thinking procedurally, and the process of designing objects is much more challenging than procedural design, especially if you’re trying to create reusable objects. In the past, a novice practitioner of object-oriented programming was faced with a choice of daunting tasks: 1. Choose a language like Smalltalk where you had to learn a large library before becoming productive. 2. Choose C++ with virtually no libraries at all1 , and struggle through the depths of the language in order to write your own libraries of objects. It is, in fact, difficult to design objects well – for that matter, it’s hard to design anything well. But the intent is that a relatively few experts design the best objects for others to 1 Fortunately, this has changed significantly with the advent of third-party libraries and the Standard C++ library. 29
- 30. consume. Successful OOP languages incorporate not just language syntax and a compiler, but an entire development environment including a significant library of well-designed, easy to use objects. Thus, the primary job of most programmers is to utilize existing objects to solve their application problems. The goal of this chapter is to show you what object-oriented programming is and how simple it can be. This chapter will introduce many of the ideas of Java and object-oriented programming on a conceptual level, but keep in mind that you’re not expected to be able to write full- fledged Java programs after reading this chapter. All the detailed descriptions and examples will follow throughout the course of this book. The progress of abstraction All programming languages provide abstractions. It can be argued that the complexity of the problems you can solve is directly related to the kind and quality of abstraction. By “kind” I mean: what is it you are abstracting? Assembly language is a small abstraction of the underlying machine. Many so-called “imperative” languages that followed (like FORTRAN, BASIC, and C) were abstractions of assembly language. These languages are big improvements over assembly language, but their primary abstraction still requires you to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer is required to establish the association between the machine model (in the “solution space”) and the model of the problem that is actually being solved (in the “problem space”). The effort required to perform this mapping, and the fact that it is extrinsic to the programming language, produces programs that are difficult to write and expensive to maintain, and as a side effect created the entire “programming methods” industry. The alternative to modeling the machine is to model the problem you’re trying to solve. Early languages like LISP and APL chose particular views of the world (“all problems are ultimately lists” or “all problems are mathematical”). PROLOG casts all problems into chains of decisions. Languages have been created for constraint-based programming and for programming exclusively by manipulating graphical symbols (the latter proved to be too restrictive). Each of these approaches is a good solution to the particular class of problem they’re designed to solve, but when you step outside of that domain they become awkward. The object-oriented approach takes a further step by providing tools for the programmer to represent elements in the problem space. This representation is general enough that the programmer is not constrained to any particular type of problem. We refer to the elements in the problem space and their representations in the solution space as “objects” (of course, you will also need other objects that don’t have problem-space analogs). The idea is that the program is allowed to adapt itself to the lingo of the problem by adding new types of objects, so when you read the code describing the solution, you’re reading words that also express the problem. This is a more flexible and powerful language abstraction than what we’ve had before. Thus, OOP allows you to describe the problem in the terms of the problem, rather than the terms of the solution. There’s still a connection back to the computer, though. Each object looks quite a bit like a little computer: it has a state, and it has operations you can ask it to perform. However, this doesn’t seem like such a bad analogy to objects in the real world: they all have characteristics and behaviors. 30 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 31. Alan Kay summarized five basic characteristics of Smalltalk, the first successful object- oriented language and one of the languages upon which Java is based. This represents a pure approach to object-oriented programming: 1. Everything is an object. Think of an object as a fancy variable: it stores data, but you can also ask it to perform operations on itself by making requests. In theory, you can take any conceptual component in the problem you’re trying to solve (dogs, buildings, services, etc.) and represent it as an object in your program. 2. A program is a bunch of objects telling each other what to do by sending messages. To make a request of an object, you “send a message” to that object. More concretely, you can think of a message as a request to call a function for a particular object. 3. Each object has its own memory made up of other objects. Or, you make a new kind of object by making a package containing existing objects. Thus, you can build up complexity in a program while hiding it behind the simplicity of objects. 4. Every object has a type. Using the parlance, each object is an instance of a class, where “class” is synonymous with “type.” The most important distinguishing characteristic of a class is “what messages can you send to it?” 5. All objects of a particular type can receive the same messages. This is actually a very loaded statement, as you will see later: because an object of type circle is also an object of type shape, a circle is guaranteed to receive shape messages. This means you can write code that talks to shapes, and automatically handle anything that fits the description of a shape. This substitutability is one of the most powerful concepts in OOP. Some language designers have decided that object-oriented programming itself is not adequate to easily solve all programming problems, and advocate the combination of various approaches into multiparadigm programming languages2 . An object has an interface Aristotle was probably the first to begin a careful study of the concept of type. He was known to speak of “the class of fishes and the class of birds.” The concept that all objects, while being unique, are also part of a set of objects that have characteristics and behaviors in common was directly used in the first object-oriented language, Simula-67, with its fundamental keyword class that introduces a new type into a program (thus class and type are often used synonymously3 ). Simula, as its name implies, was created for the purpose of developing simulations such as the classic “bank-teller problem.” In this, you have a bunch of tellers, customers, accounts, transactions, etc. The members of each class share some commonality: every account has a balance, every teller can accept a deposit, etc. At the same time, each member has its own state: each account has a different balance, each teller has a name. Thus the tellers, customers, accounts, transactions, etc. can each be represented with a unique entity in the computer program. This entity is the object, and each object belongs to a particular class that defines its characteristics and behaviors. 2 See Multiparadigm Programming in Leda by Timothy Budd (Addison-Wesley 1995). 3 Some people make a distinction, stating that type determines the interface while class is a particular implementation of that interface. Chapter 1: Introduction to Objects 31
- 32. So, although what we really do in object-oriented programming is to create new data types, virtually all object-oriented programming languages use the “class” keyword. When you see the word “type,” think “class” and vice versa. Once a type is established, you can make as many objects of that type as you like, and then manipulate those objects as if they were themselves the elements that exist in the problem you are trying to solve. Indeed, one of the challenges of object-oriented programming is to create a one-to-one mapping between the elements in the problem space (the place where the problem actually exists) and the solution space (the place where you’re modeling that problem, e.g. the computer). But how do you get an object to do useful work for you? There must be some way of making a request of that object so that it will do something (complete a transaction, draw something on the screen, turn on a switch, etc.). In addition, each object can satisfy only certain requests. The requests you can make of an object are defined by its interface, and the type is what determines the interface. The idea of type being equivalent to interface is fundamental in object-oriented programming. A simple example might be a representation of a light bulb: Type Name Light on( ) off( ) Interface brighten( ) dim( ) Light lt = new Light(); lt.on(); The name of the type/class is Light, and the requests that you can make of a Light object are to turn it on, turn it off, make it brighter or make it dimmer. You create a “handle” for a Light simply by declaring a name (lt) for that identifier, and you make an object of type Light with the new keyword, assigning it to the handle with the = sign. To send a message to the object, you state the handle name and connect it to the message name with a period (dot). From the standpoint of the user of a pre-defined class, that’s pretty much all there is to programming with objects. The hidden implementation It is helpful to break up the playing field into class creators (those who create new data types) and client programmers4 (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only what’s necessary to the client programmer, and keeps everything else hidden. Why? Because if it’s hidden, the client programmer can’t use it, which means that the 4 I’m indebted to my friend Scott Meyers for this term. 32 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 33. class creator can change the hidden portion at will, without worrying about the impact to anyone else. The interface establishes what requests you can make for a particular object. However, there must be code somewhere to satisfy that request. This, along with the hidden data, comprises the implementation. From a procedural programming standpoint, it’s not that complicated. A type has a function associated with each possible request, and when you make a particular request to an object, that function is called. This process is often summarized by saying that you “send a message” (make a request) to an object, and the object figures out what to do with that message (it executes code). In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is another programmer, but one putting together an application or using your library to build a bigger library. If all the members of a class are available to everyone, then the client programmer can do anything they want with that class and there’s no way to force any particular behaviors. Even though you might really prefer that the client programmer not directly manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the world. There are two reasons for controlling access to members. The first is to keep client programmers’ hands off portions they shouldn’t touch, parts that are necessary for the internal machinations of the data type, but not part of the interface that users need to solve their particular problems. This is actually a service to users because they can easily see what’s important to them and what they can ignore. The second reason for access control is to allow the library designer to change the internal workings of the structure without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion, for ease of development, and then later decide you need to rewrite it to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this and require only a relink by the user. Java uses three explicit keywords and one “implied keyword” to set the boundaries in a class: public, private, protected and (the implied keyword) “friendly,” which is what you get if you don’t specify one of the other keywords. Their use and meaning are remarkably straightforward. These access specifiers determine who can use the definition that follows. public means the following definition is available to everyone. The private keyword, on the other hand, means no one can access that definition except you, the creator of the type, inside function members of that type. private is a brick wall between you and the client programmer. If someone tries to access a private member, they’ll get a compile-time error. “Friendly” has to do with something called a “package,” which is Java’s way of making libraries. If something is “friendly” it’s available within the package, but not outside the package (thus this access level is sometimes referred to as “package access”). protected acts just like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be discussed shortly. Reusing the implementation Once a class has been created and tested, it should (ideally) represent a very useful unit of code. It turns out that this reusability is not nearly so easy to achieve as many would hope – it takes experience and insight to achieve a good design. But once you have such a design, Chapter 1: Introduction to Objects 33
- 34. it begs to be reused. Code reuse is arguably the greatest leverage that object-oriented programming languages provide. The simplest way to reuse a class is to place an object of that class inside a new class: we call this “creating a member object.” Your new class can be made up of any number and type of other objects, whatever is necessary to achieve the functionality desired in your new class. This concept is called composition, since you are composing a new class from existing classes. Sometimes composition is referred to as a “has-a” relationship, as in “a car has a trunk.” Composition comes with a great deal of flexibility. The member objects of your new class are usually private, making them inaccessible to client programmers using the class. Thus you can change those members without disturbing existing client code. You can also change the member objects at run time, which provides great flexibility. Inheritance, which is described next, does not have this flexibility since the compiler must place restrictions on classes created with inheritance. Because inheritance is so important in object-oriented programming it is often very highly emphasized, and the new programmer can get the idea that inheritance should be used everywhere. This can result in awkward and overcomplicated designs. Instead, you should first look to composition when creating new classes, since it is simpler and more flexible. If you take this approach, your designs will stay cleaner. When you need inheritance, it will be reasonably obvious. Inheritance: reusing the interface By itself, the concept of an object is a very convenient tool, since it allows you to package data and functionality together by concept, so you can represent an appropriate problem- space idea rather than being forced to use the idioms of the underlying machine. In addition, these concepts are expressed in the primary idea of the programming language: as a data type (using the class keyword). However, it seems a pity to go to all the trouble to create a data type and then be forced to create a brand new one that might have very similar functionality. It would be nicer if we could take the existing data type, clone it and make additions and modifications to the clone. This is effectively what you get with inheritance, with the exception that if the original class (called the base or super or parent class) is changed, the modified “clone” (called the derived or inherited or sub or child class) also reflects the appropriate changes. When you inherit you create a new type, and a key factor is that the new type contains not only all the members of the existing type (although the private ones are hidden away and inaccessible), but more importantly it duplicates the interface of the base class. That is, all the messages you can send to objects of the base class, you can also send to objects of the derived class. Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class. This type equivalence via inheritance is one of the fundamental gateways in understanding the meaning of object-oriented programming. Since both the base class and derived class have the same interface, there must be some implementation to go along with that interface. That is, there must be a method to execute when an object receives a particular message. If you simply inherit a class and don’t do anything else, the methods from the base-class interface come right along into the derived 34 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 35. class. That means objects of the derived class have not only the same type, they also have the same behavior, which doesn’t seem particularly interesting. You have two ways to differentiate your new derived class from the original base class it inherits from. The first is quite straightforward: you simply add brand new functions to the derived class. These new functions are not part of the base class interface. This means that the base class simply didn’t do as much as you wanted it to, so you add more functions. This very simple and primitive use for inheritance is, at times, the perfect solution to your problem. However, you should look closely for the possibility that your base class might need these additional functions. The second way, discussed in the following section, is to change the behavior of an existing base-class function by overriding it. Overriding base-class functionality Inheritance is implemented in Java with the extends keyword: you make a new class and you say that it extends an existing class. Although this implies that you are going to add new functions to the interface, that’s not necessarily true. You might also want to change the behavior of an existing interface function: this is referred to as overriding that function. To override a function, you simply create a new definition for the function in the derived class. You’re saying: “I’m using the same interface function here, but I want it to do something different for my new type.” Is-a vs. is-like-a relationships There’s a certain debate that can occur about inheritance: should inheritance override only base-class functions? This means that the derived type is exactly the same type as the base class since it has exactly the same interface. As a result, you can exactly substitute an object of the derived class for an object of the base-class. This can be thought of as pure substitution. In a sense, this is the ideal way to treat inheritance. We often refer to the relationship between the base class and derived classes in this case as an is-a relationship, because you can say “a circle is a shape.” A test for inheritance is whether you can state the is-a relationship about the classes and have it make sense. However, there are times when you must add new interface elements to a derived type, thus extending the interface and creating a new type. The new type can still be substituted for the base type, but the substitution isn’t perfect in a sense, since your new functions are not accessible from the base type. This can be described as an is-like-a relationship: the new type has the interface of the old type but it also contains other functions so you can’t really say it’s exactly the same. For example, consider an air conditioner. Suppose your house is wired with all the controls for cooling – that is, it has an interface that allows you to control cooling. Now the air conditioner breaks down and you replace it with a heat pump, which can both heat and cool. The heat pump is-like-an air conditioner, but it can do more. Because your house is only wired to control cooling, it can only communicate with the cooling part of the new object. The interface of the new object has been extended, and the existing system doesn’t know about anything except the original interface. When you see the substitution principle it’s very easy to feel like that’s the only way to do things, and in fact it is very nice if your design works out that way. But you’ll find that there are times when it’s equally clear that you must add new functions to the interface of a derived class. With inspection both cases should be reasonably obvious. Chapter 1: Introduction to Objects 35
- 36. Interchangeable objects with polymorphism Inheritance usually ends up creating a family of classes, all based on the same uniform interface. We express this with an inverted tree diagram5 : Shape draw() erase() Circle Square Line draw() draw() draw() erase() erase() erase() One of the most important things you do with such a family of classes is to treat an object of a derived class as an object of the base class. Why is this important? It means we can write a single piece of code that ignores the specific details of type, and talks just to the base class. That code is then decoupled from type-specific information, and thus is simpler to write and easier to understand. In addition, if a new type is added through inheritance, say a Triangle, the code you write will work just as well for the new type of Shape as it did on the existing types. Thus the program is extensible. Consider the above example. If you write a function in Java: void doStuff(Shape s) { s.erase(); // ... s.draw(); } This function is independent of the specific type of object it’s drawing and erasing. If in some other program we use the doStuff( ) function: Circle c = new Circle(); Triangle t = new Triangle(); Line l = new Line(); doStuff(c); doStuff(t); doStuff(l); 5 This uses the Unified Notation, which will primarily be used in this book. 36 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 37. The calls to doStuff( ) work just right regardless of the exact type of the object. This is actually a pretty amazing trick. Consider the line: doStuff(c); What’s happening here is that a Circle handle is being passed into a function that’s expecting a Shape handle. Since a Circle is a Shape it can be treated as one by doStuff( ). That is, any message that doStuff( ) can send to a Shape, a Circle can accept. Thus it is a completely safe and logical thing to do. We call this process of treating a derived type as though it were its base type upcasting. The name cast is used in the sense of “casting into a mold” and the “up” comes from the way the inheritance diagram is typically arranged, with the base type at the top and the derived classes fanning out downward. Thus, casting to a base type is moving up the inheritance diagram: upcasting. An object-oriented program contains some upcasting somewhere, because that’s how you decouple yourself from knowing about the exact type you’re working with. Look at the code in doStuff( ): s.erase(); // ... s.draw(); Not “if you’re a Circle, do this, if you’re a Square, do that, etc.” Just “you’re a shape, I know you can erase( ) yourself, do it and take care of the details correctly.” If you had to write code that checked for all the possible types a Shape could actually be, it would be messy and you’d have to change it every time you added a new kind of Shape. Dynamic binding What’s amazing about the code in doStuff( ) is that somehow the right thing happens. Drawing a Circle causes different code to be executed than drawing a Square or a Line, but when the draw( ) message is sent to an anonymous Shape, the correct behavior occurs based on the actual type that Shape handle happens to be connected to. This is amazing because when the Java compiler is compiling the code for doStuff( ), it cannot know what exact types it is dealing with. So ordinarily, you’d expect it to end up calling the version of erase( ) for Shape, and draw( ) for Shape, and not for the specific Circle, Square or Line. And yet the right thing happens. How can this be? When you send a message to an object even though you don’t know what specific type it is, and the right thing happens, that’s called polymorphism. The process used by object- oriented programming languages to implement polymorphism is called dynamic binding. The compiler and run-time system handle the details; all you need to know is that it happens and more importantly how to design with it. Some languages require that you use a special keyword to enable dynamic binding. In C++ this keyword is virtual. In Java, you never have to remember to add a keyword, since all functions are automatically dynamically bound. So you can always expect that, when you send a message to an object, the object will do the right thing, even when upcasting is involved. The abstract base class Very often in a design, you want the base class to present only an interface for its derived classes. That is, you don’t want anyone to actually create an object of the base class, only Chapter 1: Introduction to Objects 37
- 38. to upcast to it so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents them. Thus this is a tool for design, to enforce a particular design. You can also use the abstract keyword to describe a method that hasn’t been implemented yet, as a stub saying “here is an interface function for all types inherited from this class, but at this point I don’t have any implementation for it.” An abstract method may only be created inside an abstract class. When the class is inherited, that method must be implemented, or the inherited class becomes abstract as well. Creating an abstract method allows you to put a method in an interface without being forced to provide a (possibly meaningless) body of code for that method. Object landscapes and lifetimes Technically, OOP is just about abstract data typing, inheritance and polymorphism, but other issues can be at least as important. The remainder of this section will discuss these issues. One of the most important factors concerns the way objects are created and destroyed: where is the data for an object and how is the lifetime of the object controlled? There are different philosophies at work here. C++ takes the approach that control of efficiency is the most important issue, so the programmer has a choice. For maximum run-time speed, the storage and lifetime can be determined while the program is being written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, the control of which can be very valuable in some situations. However, you sacrifice flexibility: you must know the exact quantity, lifetime and type of objects while you’re writing the program. If you are trying to solve a more general problem like computer- aided design, package management or air-traffic control, this is too restrictive. The second approach is to create objects dynamically, in a pool of memory called the heap. In this approach you don’t know until run time how many objects you need, what their lifetime is or what their exact type is. All that is determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run time, the amount of time required to allocate storage on the heap is significantly longer than creating storage on the stack (which is often a single assembly instruction to move the stack pointer down, and another to move it back up). The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact on the creation of an object. In addition the greater flexibility is essential to solve the general programming problem. C++ allows you to determine whether the objects are created while you write the program or at run time to allow the control of efficiency. You’d normally think that since it’s more flexible, you’d always want to create objects on the heap rather than the stack. There’s another issue, however, and that’s the lifetime of an object. If you create an object on the stack or in static storage, the compiler determines how long the object lasts and can automatically destroy it. However, if you create it on the heap the compiler has no knowledge of its lifetime. How does the object get destroyed? This produces two more options: the programmer can determine programmatically when to destroy the object, or 38 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 39. the environment can provide a process called a garbage collector that automatically discovers when an object is no longer in use and destroys it. Of course, a garbage collector is much more convenient, but it requires that all systems have some kind of multithreading support and that all applications be able to tolerate the existence of the garbage collector and the other overhead for garbage collection. This does not meet the design requirements of the C++ language and so it was not included. Some languages, like Object Pascal (as seen in Delphi), Java and Smalltalk require that all objects be created on the heap, so there is no option for the optimization allowed in C++. These languages have narrower scopes of problems they can solve, but they provide an easier way to solve those problems. In addition, Java and Smalltalk have built-in garbage collectors (Delphi has the necessary wiring to easily add garbage collection, so it might happen sometime after this writing). The rest of this section looks at additional factors concerning object lifetimes and landscapes. Collections and iterators If you don’t know how many objects you’re going to need to solve a particular problem, or how long they will last, you also don’t know how to store those objects. How can you know how much space to create for those objects? You can’t, since that information isn’t known until run time. The solution to most problems in object-oriented design seems flippant: you create another type of object. The job of this object is to hold handles to other objects. Of course, you could do this with the array, which is available in most languages. But there’s more: this new object, generally called a collection (also called a container, but that term is used by the AWT so this book will use “collection”), will expand itself whenever necessary to accommodate everything you place inside it. Thus you don’t need to know how many objects you’re going to hold in a collection. Just create a collection object and let it take care of the details. Fortunately, a good OOP language comes with a set of collections as part of the package. In C++, it’s the Standard Template Library (STL). Object Pascal has collections in its VCL. Java also has collections in its standard library. In some libraries, a generic collection is considered good enough for all needs, and in others (C++ in particular) the library has different types of collections for different needs: a vector for consistent access to all elements, and a linked list for consistent insertion at all elements, for example, so you can choose the particular type that fits your needs. These may include sets, queues, hash tables, trees, stacks, etc. All collections have in common some way to put things in and get things out. The way you place something into a collection is fairly obvious: there’s a function called “push” or “add” or a similar name. Fetching things out of a collection is not always as apparent: if it’s an array-like entity such as a vector, you may be able to use an indexing operator or function. But in many situations this doesn’t make sense. In addition, a single-selection function is restrictive: what if you want to manipulate or compare a set of elements in the collection instead of just one? The solution is called an iterator, which is an object whose job is to select the elements within a collection and present them to the user of the iterator. However, there’s more to an iterator: as a class, it also provides a level of abstraction. This abstraction can be used to separate the details of the collection from the code that’s accessing that collection. The collection, via the iterator, is abstracted to be simply a sequence. The iterator allows you to traverse that sequence without worrying about the underlying structure – that is, whether it’s a vector, a linked list, a stack, or something else. This gives you the flexibility Chapter 1: Introduction to Objects 39
- 40. to easily change the underlying data structure without disturbing the code in your program. Java provides a standard iterator (called Enumeration) for all its collection classes. From the design standpoint, all you really want is a sequence that can be manipulated to solve your problem, and if a single type of sequence satisfied all your needs, there’d be no reason to have different kinds. There are two reasons that you need a choice of collections. First, collections provide different types of interfaces and external behavior. A stack has a different interface and behavior than a queue, which is different than a set or a list. One of these might provide a more flexible solution to your problem than another. Second, different collections have different efficiencies for certain operations. The best example is a vector and a list. Both are simple sequences which can have identical interfaces and external behaviors. But certain operations can have radically different costs. Randomly accessing elements in a vector is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a linked list it is expensive to move through the list to randomly select an element, and it takes longer to find an element if it is further down in the list. On the other hand, if you want to insert an element in the middle of a sequence, it’s much cheaper in a list than in a vector. These and other operations have different efficiencies depending upon the underlying structure of the sequence. In the design phase, you might start with a list and, when tuning for performance, change to a vector. Because of the abstraction via iterators, you can change from one to the other with minimal impact on your code. But in the end, remember that a collection is only a storage cabinet to put objects in. If that cabinet solves all your needs it doesn’t really matter how it is implemented (a basic concept with most types of objects). If you’re working in a programming environment that has built-in overhead due to other factors (running under Windows, for example, or the cost of a garbage collector), then the cost difference between a vector and a linked list might not matter, so you may need only one type of sequence (the standard Java library makes this assumption: it provides only a vector). You could even imagine the “perfect” collection abstraction, which could automatically change its underlying implementation according to the way it was used. The singly-rooted hierarchy One of the issues in OOP that has become especially prominent since the introduction of C++ is: should all classes be ultimately inherited from a single base class? In Java the answer is “yes” and the name of this ultimate base class is simply Object. It turns out that the benefits of the singly-rooted hierarchy are many. All objects in a singly-rooted hierarchy have an interface in common, so they are all ultimately the same type. The alternative (provided by C++) is that you don’t know that everything is the same fundamental type. From a backwards-compatibility standpoint this fits the model of C better and can be thought of as “less restrictive” but when you want to do full-on object-oriented programming you must then build your own hierarchy to provide the same convenience that’s built into other OOP languages. In addition, in any new class library you acquire, some other incompatible interface will be used, and it requires effort (and possibly multiple inheritance) to work the new interface into your design. Is the extra “flexibility” of C++ worth it? If you need it, it’s very valuable: if you have a large investment in C. If you’re starting from scratch, other alternatives such as Java can often be more productive. All objects in a singly-rooted hierarchy (such as Java provides) can be guaranteed to have certain functionality. Thus you’re guaranteed that you can perform certain basic operations on every object in your system. 40 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 41. It’s possible to make all objects have the same size by forcing them to be created on the heap and passing them around as handles, instead of copying the object. This is the way Java works, and it greatly simplifies argument passing (one of the more complex topics in C++). A singly-rooted hierarchy allows the implementation of a garbage collector. The necessary support can be installed in the base class, and the garbage collector can thus send the appropriate messages to every object in the system. Without a singly-rooted hierarchy and a system to manipulate an object via a handle, it is very difficult to implement a garbage collector. Since run-time type information is guaranteed to be in all objects, you’ll never end up with an object whose type you cannot determine. This is especially important with system level operations like exception handling, and to allow greater flexibility in programming. So, if the use of a singly-rooted hierarchy is so beneficial, why isn’t it in C++? It’s the old bugaboo of efficiency and control. A singly-rooted hierarchy puts constraints on your program designs, and in particular it was perceived to put constraints on the use of existing C code. These constraints cause problems only in certain situations, but for maximum flexibility there is no requirement for a singly-rooted hierarchy in C++. In Java, which started from scratch and has no backward-compatibility issues with any existing language, it was a logical choice to use the singly-rooted hierarchy in common with most other object-oriented programming languages. Collection libraries and support for easy collection use Since a collection is a tool that you’ll use on a very frequent basis, it makes sense to have a library of collections that are built in a reusable fashion, so you can take one off the shelf and plug it into your program. Java provides such a library, although it is fairly limited. And yet, it might satisfy most of your needs. More extensive libraries have been appearing on the Internet. Downcasting vs. templates/generics To make these collections reusable, they contain the one universal type in Java that was previously mentioned: Object. Since the singly-rooted hierarchy means that everything is an Object, a collection that holds Objects can hold anything. Thus it’s easy to reuse. To use such a collection, you simply add object handles to it, and later ask for them back. But, since the collection holds only Objects, when you add your object handle into the collection it is upcast to Object, thus losing its identity. When you fetch it back out, you get an Object handle, and not a handle to the type that you put in. So how do you turn it back into something that has the useful interface of the object that you put into the collection? Here, the cast is used again, but this time you’re not casting up the inheritance hierarchy to a more general type, but instead down the hierarchy to a more specific type. Therefore this manner of casting is called downcasting. But with upcasting, you know for example that a Circle is a type of Shape so it’s safe to upcast, but you don’t know that an Object is necessarily a Circle or a Shape so it’s hardly safe to downcast unless you know that’s what you’re dealing with. It’s not completely dangerous, however, since if you downcast to the wrong thing you’ll get a run-time error called an exception that will be described shortly. When you fetch object handles from a collection, though, you must have some way to remember exactly what they are so you can perform a proper downcast. Chapter 1: Introduction to Objects 41
- 42. Downcasting and the run-time checks require extra time for the running program, and extra effort on the part of the programmer. Wouldn’t it make sense to somehow create the collection so that it knows the types that it holds, thus eliminating the need for the downcast and possible mistake? The solution is parameterized types, which are classes that the compiler can automatically customize to work with particular types. For example, with a parameterized collection, the compiler could customize that collection so it would accept only Shapes and fetch only Shapes. Parameterized types are an important part of C++ because C++ has no singly-rooted hierarchy. In C++, the keyword that implements parameterized types is template. Java currently has no parameterized types, since it is possible for it to get by – however awkwardly – using the singly-rooted hierarchy. At one point the word generic (the keyword used by Ada for its templates) was on a list of keywords that were “reserved for future implementation.” Some of these seemed to have mysteriously slipped into a kind of “keyword Bermuda Triangle” and it’s quite difficult to know what might eventually happen. The housekeeping dilemma: who should clean up? Each object requires resources in order to exist, most notably memory. When an object is no longer needed it must be cleaned up in order that these resources are released so they can be reused. In simple programming situations the question of how an object is cleaned up doesn’t seem too challenging: you create the object, use it for as long as it’s needed, and then it should be destroyed. However, it’s not too hard to encounter situations where the situation is more complex. Suppose, for example, you are designing a system to manage air traffic for an airport (although the same model might work for managing packages, or a video rental system, or a kennel for boarding pets). At first it seems simple: make a collection to hold airplanes, then create a new airplane and place it in the collection for each airplane that enters the air-traffic-control zone. For cleanup, simply delete the appropriate airplane object when a plane leaves the zone. But what if you have some other system which is recording data about the planes; perhaps data that doesn’t require such immediate attention as the main controller function. Perhaps it’s a record of the flight plans of all the small planes that leave the airport. So you have a second collection of small planes, and whenever you create a plane object you also put it in this collection if it’s a small plane. Then some background process performs operations on the objects in this collection during idle moments. Now the problem is more difficult: how can you possibly know when to destroy the objects? When you’re done with the object, some other part of the system might not be. This same problem can arise in a number of other situations, and in programming systems (like C++) where you must explicitly delete an object when you’re done with it this can become quite complex6 . With Java, the garbage collector is designed to take care of the problem of releasing the memory (although this doesn’t include other aspects of cleaning up an object). The garbage 6 Note that this is true only for objects that are created on the heap, with new. However, the problem described, and indeed any general programming problem, requires objects to be created on the heap. 42 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 43. collector “knows” when an object is no longer in use, and it then automatically releases the memory for that object. This, combined with the fact that all objects are inherited from the single root class Object and that you can only create objects one way, on the heap, makes the process of programming in Java much simpler than programming in C++, since you have far fewer decisions to make and hurdles to overcome. Garbage collectors vs. efficiency and flexibility If all this is such a good idea, why didn’t they do the same thing in C++? Well of course there’s a price you pay for all this programming convenience, and that price is run-time overhead. As mentioned before, in C++ you can create objects on the stack, and in this case they’re automatically cleaned up (but you don’t have the flexibility of creating as many as you want at run-time). Creating objects on the stack is the most efficient way to allocate storage for objects, and also to free that storage. Creating objects on the heap is much more expensive. Always inheriting from a base class, and making all function calls polymorphic also exacts a toll. But the garbage collector is a particular problem, because you never quite know when it’s going to start up nor how long it will take. This means that there’s an inconsistency in the rate of execution of a Java program, so you can’t use it in certain situations: where the rate of execution of a program is uniformly critical (these are generally called real time programs, although not all real-time programming problems are this stringent). The designers of the C++ language, trying as they were to woo C programmers (and most successfully, at that), did not want to add any features to the language that would impact the speed or the use of C++ in any situation where C might be used. This goal was realized, but at the price of greater complexity when programming in C++. Java is simpler than C++, but the tradeoff is in efficiency and applicability. For a significant portion of programming problems, however, Java will often be the superior choice. Exception handling: dealing with errors Since the beginning of programming languages, error handling has been one of the most difficult issues. Because it’s so hard to design a good error-handling scheme, many languages simply ignore the issue, passing the problem on to library designers who come up with halfway measures that can work in many situations but can easily be circumvented, generally by just ignoring them. A major problem with most error-handling schemes is that they rely on programmer vigilance in following an agreed-upon convention that is not enforced by the language. If the programmer is not vigilant – very often, if they are simply in a hurry – these schemes can be ignored. Exception handling wires error handling directly into the programming language itself (and sometimes even the operating system). An exception is an object that is “thrown” from the site of the error, and can be “caught” by an appropriate exception handler that is designed to handle that particular type of error. It’s as if exception handling is a different, parallel path of execution that may be taken when things go wrong. And because it uses a separate execution path, it doesn’t need to interfere with your normally-executing code, which makes that code simpler to write (since you aren’t constantly forced to check for errors). In addition, a thrown exception is unlike an error value that’s returned from a function, or a flag that’s set by a function to indicate an error condition – these can be ignored. An exception cannot be ignored, thus it’s guaranteed to be dealt with at some point. Finally, exceptions provide a way to reliably recover from a bad situation, so instead of just exiting Chapter 1: Introduction to Objects 43
- 44. you are often able to set things right and restore the execution of a program, which produces much more robust programs. Java’s exception handling stands out among programming languages, because in Java, exception-handling was wired in from the beginning and you’re forced to use it. If you don’t write your code to properly handle exceptions, you’ll get a compile-time error message. This guaranteed consistency makes error-handling a much easier issue to deal with. It’s worth noting that exception handling isn’t an object-oriented feature, although in object-oriented languages the exception is normally represented with an object. Exception handling existed before object-oriented languages. Multithreading A fundamental concept in computer programming is the idea of handling more than one task at a time. Many programming problems require that the program be able to stop what it’s doing, deal with some other problem, and return to the main process. The solution has been approached in many ways: initially, programmers with low-level knowledge of the machine wrote interrupt service routines, and the suspension of the main process was initiated through a hardware interrupt. Although this worked well, it was difficult and very non-portable, so it made moving a program to a new machine slow and expensive. Sometimes interrupts are necessary for handling time-critical tasks, but there’s a large class of problems where you’re simply trying to partition the problem into separately- running pieces so the whole program can be more responsive. Within a program, these separately-running pieces are called threads and the general concept is called multithreading. A common example of multithreading is the user interface: by using threads, when a user presses a button they can get a quick response, rather than being forced to wait until the program finishes its current task. Normally threads are just a way to allocate the time of a single processor, but if the operating system supports multiple processors, each thread can be assigned to a different processor and they can truly run in parallel. One of the very convenient features of multithreading at the language level is that the programmer doesn’t need to worry about whether there are many processors or just one – the program is logically divided into threads, and if the machine has more than one processor then the program runs faster, without any special adjustments. All this makes threading sound pretty simple. However, there’s a catch: shared resources. If you have more than one thread running that’s expecting to access the same resource you have a problem. For example, two processes can’t simultaneously send information to a printer. To solve the problem, resources that can be shared (like the printer) must be locked while they are being used. So a thread locks a resource, completes its task, then releases the lock so someone else can use the resource. Java’s threading is built into the language, which makes a complicated subject much simpler. The threading is supported on an object level, so one thread of execution is represented by one object. Java also provides limited resource locking: it can lock the memory of any object (which is, after all, one kind of shared resource) so that only one thread can use it at a time. This is accomplished with the synchronized keyword. Other types of resources must be locked explicitly by the programmer, typically by creating an object to represent the lock that all threads must check before accessing that resource. 44 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 45. Persistence When you create an object, it exists for as long as you need it, but under no circumstances does it exist when the program terminates. While this makes sense at first, there are situations where it would be incredibly useful if an object were to exist and hold its information even while the program wasn’t running. Then the next time you started the program up, the object would be there and it would have the same information it had the previous time the program was running. Of course you can get a similar effect now by writing the information to a file or a database, but in the spirit of making everything an object it would be quite convenient to be able to declare an object persistent and have all the details taken care of for you. Java 1.1 provides support for “lightweight persistence,” which means you can easily store objects on disk and later retrieve them. The reason it’s “lightweight” is that you’re still forced to make explicit calls to do the storage and retrieval. In some future release more complete support for persistence may appear. Java and the Internet If Java is, in fact, yet another computer programming language, why is it so important and why is it being promoted as a revolutionary step in computer programming? The answer isn’t immediately obvious if you’re coming from a traditional programming perspective. Although Java will solve traditional stand-alone programming problems, the reason it is important is that it will also solve programming problems on the world-wide web (“the Web”). What is the Web? The Web can seem a bit of a mystery at first, with all this talk of “surfing” and “presence” and “home pages.” There has even been a growing reaction against “Internet-mania,” questioning the economic value and outcome of such a sweeping movement. It’s helpful to step back and see what it really is, but to do this you must understand client/server systems (another aspect of computing that’s full of confusing issues). Client/Server computing The primary idea of a client/server system is that you have a central repository of information – some kind of data, typically in a database – that you want to distribute on demand to some set of people or machines. A key to the client/server concept is that the repository of information is centrally located so that it can be changed and so those changes will propagate out to the information consumers. Taken together, the information repository, the software that distributes the information and the machine(s) where the information and software reside is called the server. The software that resides on the remote machine, and that communicates with the server, fetches the information and that processes and displays it on the remote machine is called the client. The basic concept of client/server computing, then, is not so complicated. The problems arise because you have a single server trying to serve many clients at once. Generally a database management system is involved that allows the designer to “balance” the layout of data into tables for optimal use. In addition, systems often allow a client to insert new information into a server, and so you have the issue of making sure that one client’s new data doesn’t walk over another client’s new data, or that data isn’t lost in the process of adding it to the database (this is called transaction processing). As client software changes, it must be built, debugged and installed on the client machines, which turns out to be more Chapter 1: Introduction to Objects 45
- 46. complicated and expensive than you might think; it’s especially problematic to support multiple types of computers and operating systems. Finally there’s the all-important performance issue: you might have hundreds of clients making requests of your server at any one time, and so any small delay is crucial. To minimize latency, programmers work hard to offload processing tasks, often to the client machine but sometimes to other machines at the server site using so-called middleware. So the simple idea of distributing information to people has so many layers of complexity in the process of implementing it that the whole problem can seem hopelessly enigmatic. And yet it’s crucial: client/server computing accounts for roughly half of all programming activities. It’s responsible for everything from order-taking and credit-card transactions to the distribution of any kind of data: stock market, scientific, government – you name it. What we’ve come up with in the past is individual solutions to individual problems, inventing a new solution each time. These were hard to create and hard to use and the user had to learn a new interface for each one. The entire client/server problem needs to be solved in a big way. The Web as a giant server The Web is actually one giant client-server system. It’s a bit worse than that, since you have all the servers and clients coexisting on a single network all at once. But you don’t need to know that, since all you care about is connecting to and interacting with one server at a time (even though you might be hopping around the world in your search for the right server). Initially it was a very simple one-way process: you made a request of a server and it handed you a file, which your machine’s browser software (i.e. the client) would interpret by formatting onto your local machine. But in short order people began wanting to do more than just deliver pages from a server; they wanted full client/server capability so that the client could feed information back to the server, for example to do database lookups on the server, to add new information to the server or to place an order (which required more security than the original systems offered). These are the changes we’ve been seeing in the development of the Web. The Web browser was a big step forward: the concept that one piece of information could be displayed on any type of computer without change. However, browsers were still rather primitive and rapidly bogged down by the demands placed on them. They weren’t particularly interactive and tended to clog up both the server and the Internet because any time you needed to do something that required programming you had to send information back to the server to be processed. It could take many seconds or minutes to find out you had misspelled something in your request. Since the browser was just a viewer it couldn’t perform even the simplest computing tasks (on the other hand, it was safe, since it couldn’t execute any programs on your local machine that contained bugs or viruses). To solve this problem, some different approaches have been taken. For one thing, graphics standards have been enhanced to allow better animation and video within browsers. However, the remainder of the problem can be solved only by incorporating the ability to run programs on the client end, under the browser. This is called client-side programming. 46 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 47. Client-side programming 7 The Web’s initial server-browser design provided for interactive content, but the interactivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HTML contains very simple mechanisms for data gathering: text-entry boxes, check boxes, radio boxes, lists and drop-down lists, as well as a button which can be programmed to do only two things: reset the data on the form or “submit” the data on the form back to the server. This submission passes through the Common Gateway Interface (CGI) provided on all Web servers. The text within the submission tells CGI what to do with it; the most common action is to run a program located on the server in a directory that’s typically called “cgi- bin” (if you watch the address window at the top of your browser when you push a button on a Web page, you can sometimes see “cgi-bin” within all the gobbledygook there). These programs can be written in most languages, but Perl is a common choice because it is designed for text manipulation and is interpreted, and so can be installed on any server regardless of processor or operating system. Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with it. The problem is response time. The response of a CGI program depends on how much data must be sent as well as the load on both the server and the Internet (on top of this, starting a CGI program tends to be slow). The initial designers of the Web did not foresee how rapidly this bandwidth would be exhausted for the kinds of applications people developed. For example, any sort of dynamic graphing is nearly impossible to perform with consistency, since a GIF file must be created and moved from the server to the client for each version of the graph. And you’ve no doubt had direct experience with something as simple as validating the data on an input form: you press the submit button on a page, the data is shipped back to the server which starts a CGI program that discovers an error, formats an HTML page informing you of the error and sends the page back to you, at which point you must back up a page and try again. Not only is this slow, it’s inelegant. The solution is client-side programming. Most machines that are running Web browsers are powerful engines capable of doing vast work, and with the original static HTML approach they are sitting there, just idly waiting for the server to dish up the next page. Client-side programming means that the Web browser is harnessed to do whatever work it can, and the result for the user is a much speedier and more interactive experience at your Web site. The problem with discussions of client-side programming is that they aren’t much different than discussions of programming in general. The parameters are almost the same, but the platform is different: a Web browser is like a very limited operating system. In the end, it’s still programming and this accounts for the dizzying array of problems and solutions produced by client-side programming. The rest of this section provides an overview of the issues and approaches in client-side programming. Plug-ins One of the most significant steps forward in client-side programming is the development of the plug-in. This is a way for a programmer to add new functionality to the browser by downloading a piece of code that plugs itself into the appropriate spot in the browser. It tells the browser: “from now on you can perform this new activity” (you need to download the plug-in only once). Some very fast and powerful behavior is added to browsers via plug-ins, but writing a plug-in is not a trivial task and isn’t something you’d want to do 7 The material in this section is adapted from an article by the author that originally appeared on Mainspring, at www.mainspring.com. Used with permission. Chapter 1: Introduction to Objects 47
- 48. as part of the process of building a particular site. The value of the plug-in is that it allows an expert programmer to develop a new language for client-side programming and add that language to a browser without the permission of the browser manufacturer. Thus, plug-ins provide the back door that allows the creation of new client-side programming languages (although not all languages are implemented as plug-ins). Scripting languages Plug-ins resulted in an explosion of scripting languages. With a scripting language you embed the source code for your client-side program directly into the HTML page, and the plug-in that interprets that language is automatically activated while the HTML page is being displayed. Scripting languages tend to be reasonably simple to understand, and because they are simply text that is part of an HTML page they load very quickly, as part of the single server hit required to procure that page. The trade-off is that your code is exposed for everyone to see (and steal) but generally you aren’t doing amazingly sophisticated things with scripting languages so it’s not too much of a hardship. This points out that scripting languages are really intended to solve specific types of problems, primarily the creation of richer and more interactive graphical user interfaces (GUIs). However, a scripting language might solve 80% of the kinds of problems encountered in client-side programming. Your problems may very well fit completely within that 80%, and since scripting languages tend to be easier and faster to develop you should probably consider a scripting language before looking at a more involved solution such as Java or ActiveX programming. The most commonly-discussed scripting languages are JavaScript (nothing to do with Java; it’s named that way just to grab some of Java’s marketing momentum), VBscript (which looks like Visual Basic) and Tcl/Tk which comes from the popular cross-platform GUI-building language. There are others out there and no doubt more in development. JavaScript is probably the most commonly supported; it comes built into both Netscape Navigator and the Microsoft Internet Explorer (IE). In addition, there are probably more JavaScript books out than for the others, and some tools automatically create pages using JavaScript. However, if you’re already fluent in Visual Basic or Tcl/Tk, you’ll be more productive using those scripting languages rather than learning a new one (you’ll have your hands full dealing with the Web issues already). Java If a scripting language can solve 80% of the client-side programming problems, what about the other 20%, the “really hard” stuff? The most popular solution today is Java. Not only is it a very powerful programming language built to be secure, cross-platform and international, but Java is being continuously extended to provide language features and libraries that elegantly handle problems that are difficult in traditional programming languages, such as multithreading, database access, network programming and distributed computing. Java allows client-side programming via the applet. An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page (just as, for example, a graphic is automatically downloaded) and when the applet is activated it executes a program. This is part of its beauty – it provides you with a way to automatically distribute the client software from the server, at the time the user needs the client software, and no sooner (so they get the latest version of the client software without fail, and without difficult re- installation). In addition, because of the way Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters (this safely includes the vast majority of machines). Since Java is a full-fledged programming language you can do as much work as possible on the client before and after making requests of the server. For example, you 48 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 49. won’t have to send a request form across the Internet to discover that you’ve gotten a date or some other parameter wrong, and your client computer can quickly do the work of plotting data instead of waiting for the server to make a plot and ship a graphic image back to you. Not only do you get the immediate win of speed and responsiveness, but the general network traffic and load upon servers can be reduced, thereby preventing the entire Internet from slowing down. One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client. On the other hand, a Java applet can be decompiled without too much trouble, and hiding your code is often not an important issue anyway. Two other factors can be important: as you will see later in the book, a compiled Java applet can comprise many modules and take multiple server “hits” (accesses) to download (In Java 1.1 this is minimized by Java archives, called JAR files, that allow all the required modules to be packaged together for a single download). A scripted program will just be integrated into the Web page as part of its text (and will generally be smaller as well as not requiring any extra server hits). This may or may not be important to the responsiveness of your Web site. Finally, there’s the all-important learning curve. Regardless of what you’ve heard, Java is not a trivial language to learn. If you’re a Visual Basic programmer, moving to VBscript will be your fastest solution and since it will probably solve most typical client/server problems you might be hard pressed to justify learning Java. If you’re experienced with a scripting language you will certainly benefit from looking at JavaScript or VBscript before committing to Java, since they may fit your needs handily and you’ll be more productive sooner. ActiveX In effect, the competitor to Java is Microsoft’s ActiveX, although it takes a completely different approach. ActiveX is originally a Windows-only solution, although it is now being developed via an independent consortium to become cross-platform. Effectively, ActiveX says “if your program connects to its environment just so, it can be dropped into a Web page and run under a browser that supports ActiveX” (IE directly supports ActiveX and Netscape does so using a plug-in). Thus, ActiveX does not constrain you to a particular language. If, for example, you’re already an experienced Windows programmer using a language like C++, Visual Basic or Borland’s Delphi, you can create ActiveX components with almost no changes to your programming knowledge. ActiveX also provides a path for the use of legacy code in your Web pages. Security Automatically downloading and running programs across the Internet can sound like a virus-builder’s dream. ActiveX especially brings up the thorny issue of security in client- side programming. If you click on a Web site, you might automatically download any number of things along with the HTML page: GIF files, script code, compiled Java code, and ActiveX components. Some of these are benign: GIF files can’t do any harm, and scripting languages are generally very limited in what they can do. Java was also designed to run its applets within a “sandbox” of safety, which prevents it from writing to disk or accessing memory outside the sandbox. ActiveX is at the opposite end of the spectrum. Programming with ActiveX is like programming Windows – you can do anything you want. So if you click on a page which downloads an ActiveX component, that component might cause damage to the files on your disk. Of course, any program that you load onto your computer using non-Internet means can do the same thing and viruses downloaded from BBSs have long been a problem, but the speed of the Internet amplifies the difficulty. The solution seems to be “digital signatures,” whereby code is verified to show who the author is. This is based on the idea that a virus works because its creator can be anonymous, so if you remove the anonymity individuals will be forced to be responsible Chapter 1: Introduction to Objects 49
- 50. for their actions. This seems like a good plan because it allows programs to be much more functional, and I suspect it will in fact eliminate malicious mischief. However, if a program has an unintentional bug that’s destructive it will still cause problems. The Java approach is to prevent any of these problems from occurring via the sandbox. The Java interpreter that lives on your local Web browser examines the applet for any untoward instructions as the applet is being loaded. In particular, the applet cannot write files to disk nor erase files (one of the mainstays of the virus). Applets are generally considered to be very safe, and since this is essential for reliable client-server systems, any bugs that allow viruses are rapidly repaired (it’s worth noting that the browser software actually enforces these security restrictions, and some browsers allow you to select different security levels to provide varying degrees of access to your system). You might be skeptical of this rather draconian restriction against writing files to your local disk. What if you want to build a local database or save any other kind of data for later use, offline? The initial vision seemed to be that eventually everyone would be online to do anything important, but that was soon seen to be impractical (although low-cost “Internet appliances” might someday satisfy the needs of a significant segment of users). The solution is the “signed applet” which uses public-key encryption to verify that an applet does indeed come from where it claims it does. A signed applet can then go ahead and trash your disk, but the theory is that since you can now hold the applet creator accountable they won’t do vicious things. Java 1.1 provides a framework for digital signatures so you will eventually be able to allow an applet to step outside the sandbox if necessary. I think digital signatures have missed an important issue, which is the speed that people move around on the Internet. If you do in fact download a buggy program and it does something untoward, how long will it be before you discover the damage? It could be days or even weeks. And by then, how will you track down the program that’s done it (and what good will it do at that point?). Internet vs. Intranet Since the Web is the most general solution to the client/server problem, it makes sense that you can use the same technology to solve a subset of the problem, in particular the classic client/server problem within a company. With traditional client/server approaches you have the problem of multiple different types of client computers, as well as the difficulty of installing new client software, both of which are handily solved with Web browsers and client-side programming. When Web technology is used this way, it is referred to as an Intranet. Intranets provide much greater security than the Internet, since you can physically control access to the servers within your company. In terms of training, it seems that once people understand the general concept of a browser it’s much easier for them to deal with differences in the way pages and applets look, so the learning curve for new kinds of systems would seem to be reduced. The security problem brings us to one of the divisions that seems to be automatically forming in the world of client-side programming. If your program is running on the Internet, you don’t know what platform it will be working under and you want to be extra careful that you don’t disseminate buggy code. Thus, you need something cross-platform and very secure, like a scripting language or Java. If you’re running on an Intranet you might have a different set of constraints. It’s not uncommon that all your machines could be Wintel platforms. On an Intranet, you’re responsible for the quality of your own code, and can repair bugs when they’re discovered. In addition, you might already have a body of legacy code that you’ve been using in a more traditional client/server approach, whereby you must physically install client programs every time you do an upgrade. The time wasted in this last activity is the most compelling 50 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 51. reason to move to browsers since upgrades are invisible and automatic. If you are involved in such an Intranet, the most sensible approach to take is ActiveX rather than trying to re-code your programs in a new language. When faced with this bewildering array of solutions to the client-side programming problem, the best plan of attack is a cost-benefit analysis: what are the constraints of your problem, and what is the fastest way to get to your solution? Since client-side programming is still programming, it’s always a good idea to take the fastest development approach for your particular situation. This is an aggressive stance to prepare for inevitable encounters with the problems of program development. Server-side programming This whole discussion has ignored the issue of server-side programming. What happens when you make a request of a server? Most of the time the request is simply “send me this file.” Your browser then interprets the file in some appropriate fashion: as an HTML page, a graphic image, a Java applet, a script program, etc. A more complicated request to a server generally involves a database transaction. A common scenario involves a request for a complex database search, which the server then formats into an HTML page and sends to you as the result (of course, if the client has more intelligence via Java or a scripting language, the raw data can be sent and formatted at the client end, which will be faster and less load on the server). Or you might want to register your name in a database when joining a group, or place an order, which will involve changes to that database. These database requests must be processed via some code on the server side, which is generally referred to as server-side programming. Traditionally server-side programming has been performed using Perl and CGI scripts, but more sophisticated systems have been appearing, including Java-based Web servers that allow you to perform all your server- side programming in Java. A separate arena: applications Most of the brouhaha over Java has been about applets. But Java is actually a general- purpose programming language that can solve any type of problem, at least in theory. And as pointed out previously, there might be more effective ways to solve most client/server problems. When you move out of the applet arena (and simultaneously release the restrictions, such as the one against writing to disk) you enter the world of general-purpose applications that run standalone, without a Web browser, just like any ordinary program does. Here, Java’s strength is not only in its portability, but also its programmability. As you’ll see throughout this book, Java has many features that allow you to create robust programs in a shorter period than with previous programming languages. Be aware this is a mixed blessing, though. You pay for the improvements through slower execution speed (although there is significant work going on in this area). Like any language, Java has built-in limitations that might make it inappropriate to solve certain types of programming problems. Java is a rapidly-evolving language, however, and as each new release comes out it becomes more and more attractive for solving larger sets of problems. Online documentation The Java language and libraries from Sun Microsystems (a free download) come with documentation in electronic form, readable using a Web browser, and virtually every 3rd party implementation of Java has this or an equivalent documentation system. Almost all Chapter 1: Introduction to Objects 51
- 52. the books published on Java have duplicated this documentation. So you either already have it or you can download it, and unless necessary, this book will not repeat that documentation because (although the Sun documentation at this writing could only be described as “weak”) you’ll generally find it more useful to find the class descriptions with your Web browser than you will to look them up in a book (plus it will be up-to-date). Thus this book will provide extra descriptions of the classes only when it’s necessary to supplement the documentation so you can understand a particular example. Analysis & Design The object-oriented paradigm is a new and different way of thinking about programming and many folks have trouble at first knowing how to approach a project. Now that everything is supposed to be an object, how do you go about creating a “good” design, one that will take advantage of all the benefits that OOP has to offer? Books on OOP analysis and design are coming out of the woodwork. I find most of these books to be filled lots of long words, awkward prose and important-sounding pronouncements.8 I come away thinking the book would be better as a chapter or at the most a very short book, feeling annoyed that this process couldn’t be described simply and directly (it disturbs me that people who purport to specialize in managing complexity have such trouble writing clear and simple books). After all, the whole point of OOP is to make the process of software development easier, and I know it would seem to threaten the livelihood of those of us who consult because things are complex, but why not make it simple? So, hoping I’ve built a healthy skepticism within you, I shall endeavor to give you my own perspective on analysis and design in as few paragraphs as possible. Staying on course While you’re going through the development process, the most important issue is this: don’t get lost. It’s easy to do. Most of these methodologies are designed to solve the very largest of problems (which makes sense: these are the especially difficult projects that justify calling in that author as consultant, and justify the author’s large fees). Remember that most projects don’t fit into that category, so you can usually do just fine in your analysis and design with a relatively small subset of what the author is recommending. But some sort of process, no matter how limited, will generally get you on your way in a much better fashion than simply beginning to code. That said, if you’re looking at a methodology that contains tremendous detail and suggests many steps and documents, it’s still difficult to know when to stop. Keep in mind what you’re trying to discover: 1. What are the objects (how do you partition your project up into its component parts)? 2. What are their interfaces (what messages do you need to be able to send to each object)? If you come up with nothing more than the objects and their interfaces then you can write a program. For various reasons you might need more descriptions and documents than this, but you can’t really get away with any less. 8 The best introduction is still Grady Booch’s Object-Oriented Design with Applications, 2nd edition, Wiley & Sons 1996. His insights are clear and his prose is straightforward, although his notations are needlessly complex for most designs (you can easily get by with a subset). 52 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 53. The process can be undertaken in four phases, and a phase 0 which is just the initial commitment to using some kind of structure. Phase 0: Let’s make a plan The first step is to decide what steps you’re going to have in your process. It sounds simple (in fact, all of this sounds simple) and yet, very often, people don’t even get around to phase one before they start coding. If your plan is “let’s jump in and start coding,” fine (sometimes that’s appropriate, when you have a well-understood problem). At least agree that this is the plan. You might also decide at this phase that some additional process structure is necessary but not the whole nine yards. Understandably enough, some programmers like to work in “vacation mode” where no structure is imposed on the process of developing their work: “it will be done when it’s done.” This can be appealing for awhile, but I’ve found that having a few milestones along the way helps to focus and galvanize your efforts around those milestones instead of being stuck with a single big one: “finish the project.” In addition, it divides the project into more bite-sized pieces and make it seem less threatening. When I began to study story structure (so that I will someday write a novel) I was initially resistant to the idea, feeling that when I wrote I simply let it flow onto the page. What I found was that yes, when I wrote about computers the structure was simple enough so I didn’t have to think much about it, but I was still structuring my work, albeit only semi- consciously in my head. So even if you think that your plan is to just start coding, you still go through the following phases while asking and answering certain questions. Phase 1: What are we making? In the previous generation of program design (procedural design), this would be called “creating the requirements analysis and system specification.” These, of course, were places to get lost: intimidatingly-named documents that could become big projects in their own right. Their intention was good, however: the requirements analysis says “make a list of the guidelines we will we use to know when the job is done and the customer is satisfied.” The system specification says: “here’s a description of what the program will do (not how) to satisfy the requirements.” The requirements analysis is really a contract between you and the customer (even if the customer works within your company or is actually some other object or system), and the system specification is a top-level exploration into the problem and in some sense a discovery of whether it can be done and how long it will take. Since both of these will require consensus among people, I think it’s best to keep them as bare as possible – ideally, lists and basic diagrams – to save time. You might have other constraints that require you to elaborate them into bigger documents. It’s necessary to stay focused on the heart of what you’re trying to accomplish in this phase: determine what the system is really supposed to do. The most valuable tool for this is a collection of what are called “use cases.” These are essentially descriptive answers to questions that start with “what does the system do if …” For example: “What does the auto-teller do if a customer has just deposited a check within 24 hours and there’s not enough in the account without the check to provide the desired withdrawal?” The use-case then describes what the auto-teller does in that case. You try to discover all the possible use-cases for your system, and once you’ve done that you’ve got the core of what the system is supposed to do. The nice thing about focusing on use-cases is that they always bring you back to the essentials and keep you from drifting off into issues that aren’t critical for getting the job done. That is, if you have a full set of use cases you can describe your system and move onto the next phase. You probably won’t Chapter 1: Introduction to Objects 53
- 54. get it all figured out perfectly at this phase, but that’s OK: everything will reveal itself in the fullness of time, and if you demand a perfect system specification at this point you’ll get stuck. It helps to kick-start this phase by describing the system in a few paragraphs and then looking for nouns and verbs: the nouns become the objects and the verbs become the methods in the object interfaces. You’ll be surprised at how useful a tool this can be; sometimes it will accomplish the lion’s share of the work for you. Although it’s a black art, at this point some kind of scheduling can be very useful. You now have an overview of what you’re building so you’ll probably be able to get some idea of how long it will take. Lots of factors come into play here – if you estimate a long schedule then the company might not decide to build it, or a manager might have already decided how long the project should take and will try to influence your estimate. But it’s best to have an honest schedule from the beginning and deal with the tough decisions early. There have been lots of attempts to come up with accurate scheduling techniques (like techniques to predict the stock market) but probably the best approach is to rely on your experience and intuition: get a gut feeling for how long it will really take, then double that and add 10%. Your gut feeling is probably right: you can get something working in that time. The “doubling” will turn that into something decent, and the 10% will deal with final polishing and details. However you want to explain it, and regardless of the moans and manipulations that happen when you reveal such a schedule, it just seems to work out that way. Phase 2: How will we build it? In this phase you must come up with a design which describes what the classes look like and how they will interact. A useful diagramming tool that has evolved over time is the Unified Modeling Language (UML). You can get the specification for UML at www.rational.com. UML can also be very helpful as a descriptive tool during Phase 1, and some of the diagrams you create there will probably show up unmodified in phase 2. You don’t have to use UML, but it can be helpful, especially if you want to put a diagram up on the wall for everyone to ponder (a good idea). An alternative to UML is a textual description of the objects and their interfaces (as I described in “Thinking in C++”) but this can be more limiting in its descriptive abilities. The most successful consulting experiences I’ve had when coming up with an initial design involves standing in front of the team, who hadn’t built an OOP project before, and drawing objects on a whiteboard. We talked about how the objects should communicate with each other, and erased some of them and replaced them with other objects. The team (who knew what the project was supposed to do) were the ones who actually created the design – so they “owned” the design rather than having it given to them. All I was doing was guiding the process by asking the right questions, trying out the assumptions and taking the feedback from the team to modify those assumptions. The true beauty of the process was that the team learned how to do object-oriented design not by reviewing abstract examples, but by working on the one design that was most interesting to them at that moment: theirs. You’ll know you’re done when you have described the objects and their interfaces. Well, most of them, anyway – there are usually a few that slip through the cracks and don’t make themselves known until Phase 3. But that’s OK. All you are concerned with is that you eventually discover all your objects. It’s nice to discover them early in the process but OOP provides enough structure so that it’s not so bad if you discover them later. 54 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 55. Phase 3: Let’s build it! Well, if you’re reading this book you’re probably a programmer so this is the part you’ve been trying to get to. By following a plan – no matter how simple and brief – and coming up with design structure before coding, you’ll discover that things fall together far more easily than if you dive in and start hacking, and this in itself provides a great deal of satisfaction. Getting code to run and do what you want is fulfilling, probably like some kind of drug if you look at the obsessive behavior of some programmers. But it’s my experience that coming up with an elegant solution is deeply satisfying at an entirely different level – it feels closer to art than technology. And elegance always pays off; it’s not a frivolous pursuit. Not only does it give you a program that’s easier to build and debug, but also to understand and maintain, and that’s where the financial value lies. After you build the system and get it running, it’s important to do a reality check, and here’s where the requirements analysis and system specification comes in: you go through your program and make sure that all the requirements are checked off, and that all the use cases work they way they’re described. Now you’re done… or are you? Phase 4: Iteration This is the point in the development cycle that has traditionally been called “maintenance,” a catch-all term that can mean everything from “getting it to work the way it was really supposed to in the first place” to “adding features that the customer forgot to mention before” to the more traditional “fixing the bugs that show up” and “adding new features as the need arises.” So many misconceptions have been applied to the term “maintenance” that it has taken on a slightly deceiving quality, partly because it suggests that you’ve actually built a pristine program and that all you need to do is change parts, oil it and keep it from rusting. Perhaps there’s a better term to describe what’s going on. The term is iteration. That is: “you won’t get it right the first time, so give yourself the latitude to learn and to go back and make changes.” You might need to make a lot of changes as you learn and understand the problem more deeply. But the elegance you’ll produce if you iterate until you’ve got it right will pay off, both in the short and the long run. What it means to “get it right” isn’t just that the program works according to the requirements and the use cases. It also means that the internal structure of the code makes sense to you, and feels like it fits together well, with no awkward syntax, oversized objects or ungainly exposed bits of code. In addition, you must have some sense that the program structure will survive the changes that it will inevitably go through during its lifetime, and that those changes can be made easily and cleanly. This is no small feat: you must not only understand what you’re building, but also how the program will evolve (what I call the vector of change). Fortunately, object-oriented programming languages are particularly adept at supporting this kind of continuing modification – the boundaries created by the objects are what tend to keep the structure from breaking down, and also what allow you to make changes that would seem drastic in a procedural program without causing earthquakes throughout your code. In fact, this might be the most important benefit of OOP. With iteration, you create something that at least approximates what you think you’re building, and then you kick the tires, compare it to your requirements, and see where it falls short. Now you can go back and fix it by redesigning and re-implementing the Chapter 1: Introduction to Objects 55
- 56. portions of the program that didn’t work right9 . You might actually need to solve the problem, or an aspect of the problem, several times before you hit on the right solution (a study of Design Patterns by Gamma et. al., Addison-Wesley 1995, is usually helpful here). Iteration also occurs when you build a system, see that it matches your requirements, and then discover it wasn’t actually what you wanted – now that you see what it is, you realize you want to solve a different problem. If you think this kind of iteration is going to happen, then you owe it to yourself to build your first version as quickly as possible so you can find out if it’s what you want. Iteration is closely tied to incremental development. Incremental development means you start with the core of your system and implement it as a framework upon which to build the rest of the system piece-by-piece. Then you start adding features one at a time. The trick to this is in designing a framework that will accommodate all the features you plan to add to it (see the design patterns chapter for more insight into this issue). The advantage is that once you get the core framework working, each feature you add is like a small project in itself rather than part of a big project. Also, new features that are incorporated later in the development or maintenance phases can be added more easily. OOP supports incremental development because if your program is designed well, your increments will turn out to be discreet objects or groups of objects. Plans pay off Of course you wouldn’t build a house without a lot of carefully-drawn plans. If you build a deck or a doghouse your plans won’t be so elaborate but you’ll still probably start with some kind of sketches to guide you on your way. Software development has gone to extremes: for a long time, people didn’t have much structure in their development, but then big projects began failing. In reaction, we ended up with methodologies that had an intimidating amount of structure and detail. These were too scary to use – it looked like you’d spend all your time writing documents, and no time programming (this was often the case). I hope that what I’ve shown you here suggests a middle path, a sliding scale: use an approach that fits your needs (and your personality). But, no matter how minimal you choose to make it, some kind of plan will make a big improvement in your project over no plan at all. Remember that, by some estimates, over %50 of projects fail. Java vs. C++? Should you use Java instead of C++ for your project? Other than Web applets, there are two issues to consider. First, if you want to use a lot of existing libraries (and you’ll certainly get a lot of productivity gains there) or you have an existing C or C++ code base, then Java will probably slow you down rather than speed you up. If you’re developing all your code primarily from scratch, then the simplicity of Java over C++ will shorten your development time. 9 This is something like “rapid prototyping,” where you were supposed to build a quick- and-dirty version so you could learn about the system, and then throw your prototype and build it right. The trouble with that approach is that people didn’t throw away the prototype, but instead built upon it. Combined with the lack of structure in procedural programming, this often lead to messy, expensive-to-maintain systems. 56 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 57. The biggest issue is speed. Interpreted Java can be very slow, even on the order of 20-50 times slower than C in the original Java interpreters. This has improved quite a bit over time, but it will still remain an important number. Computers are about speed; if it wasn’t significantly faster to do something on a computer then you’d do it by hand. Thus the key to making Java feasible for most non-Web development projects is the appearance of speed improvements like so-called “Just-In Time” (JIT) compilers and possibly even native code compilers (two of which already exist at this writing). Of course, these will eliminate the touted cross-platform execution of the compiled programs, but they will also bring the speed of the executable closer to that of C and C++. In addition, cross-compiling programs in Java should be a lot easier than doing so in C or C++ (in theory, you just recompile, but that promise has been made before, for other languages). You can find comparisons of Java and C++, observations about Java realities and practicality, and coding guidelines in the appendices. Chapter 1: Introduction to Objects 57
- 59. b 2: Everything is an object Although it is based on C++, Java is more of a “pure” object-oriented language. Both C++ and Java are hybrid languages, but in Java the designers felt that the hybridization was not so important as it was in C++. A hybrid language allows multiple programming styles; the reason C++ is hybrid is to support backwards compatibility with the C language. Because C++ is a superset of the C language, it includes many of that language’s undesirable features. The resulting language then becomes overly complicated and rife with impenetrable details. The Java language assumes you want to do only object-oriented programming. This means that before you can begin you must shift your mindset into an object-oriented world (unless it’s already there). The benefit for this initial effort is the ability to program in a language that is simple to learn and to use. In this chapter we’ll see the basic components of a Java program, and we’ll learn that everything in Java is an object, even a Java program. 59
- 60. You manipulate objects through handles Each programming language has its own means of manipulating data. Sometimes the programmer must constantly be aware of what type of manipulation is going on: are you manipulating the object itself, directly, or are you dealing with some kind of indirect representation (a pointer in C or C++) that must be treated with a special syntax? All this is simplified in Java: you treat everything as an object, so there is a single consistent syntax that you use everywhere. Although you treat everything as an object, the identifier you manipulate is actually a “handle” (you might see this called a reference or even a pointer in other discussions of Java) to an object. You might imagine this scene as a television (the object) with your remote control as a handle. As long as you’re holding this handle, you have a connection to the television, but when someone says “change the channel” or “lower the volume” what you’re manipulating is the handle, which in turn modifies the object. If you want to move around the room and still control the television, you take the handle with you, not the whole television. Also, you can have the remote control, but no television. That is, just because you have a handle doesn’t mean there’s necessarily an object connected to it. So if you want to hold a word or sentence, you create a String handle: String s; But here, you’ve created only the handle, not an object. If you decided to send a message to s at this point, you’ll get an error (at run-time) because s isn’t actually attached to anything (there’s no television). A safer practice, then, is always to initialize a handle when you create it: String s = "asdf"; However, this uses a special case: strings can be initialized with quoted text. Normally you must use a more general type of initialization for objects. You must create all the objects When you create a handle, you want to connect it with a new object. You do so, in general, with the new keyword. new says “make me a new one of these objects.” So in the above example, you can say: String s = new String("asdf"); Not only does this say “make me a new string,” but it also gives information about how to make the string by supplying an initial character string. Of course, String is not the only type that exists: Java comes with a plethora of ready- made types. But what’s more important is that you can create your own types. In fact, that’s the fundamental activity in Java programming, and it’s what you’ll be learning about in the rest of the book. 60 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 61. Where storage lives It’s useful to be able to visualize some aspects of the way things are laid out while the program is running, in particular how memory is arranged. There are 6 different places to store data: 1. Registers. This is the fastest of all storage because it exists in a different place than the other storage: inside the processor itself. However, the number of registers is severely limited and so registers are allocated by the compiler according to its needs and you don’t have direct control, nor do you see any evidence in your programs that registers even exist. 2. The stack. This lives in the general RAM (Random-access memory) area, but has direct support from the processor via its stack pointer. The stack pointer is moved down to create new memory and moved up to release that memory. This is an extremely fast and efficient way to allocate storage, slower only than registers. The Java compiler must know, while it is creating the program, the exact size and lifetime of all the data that is stored on the stack, because it must generate the code to move the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack – in particular, object handles – Java objects are not placed on the stack. 3. The heap. This is a general-purpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesn’t need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus there’s a great deal of flexibility in using storage on the heap. Whenever you need to create an object, you simply write the code to create it using new and the storage is allocated on the heap when that code is executed. And of course there’s a price you pay for this flexibility: it takes more time to allocate heap storage. 4. Static storage. “Static” is used here in the sense of “in a fixed location” (although it’s also in RAM). Static storage contains data that is available for the entire time a program is running. You can use the static keyword to specify that a particular element of an object is static, but Java objects themselves are never placed in static storage. 5. Constant storage. Constant values are often placed directly in the program code itself, which is safe since they can never change. Sometimes constants are cordoned off by themselves so they can be optionally placed in ROM – read-only memory. 6. Non-RAM storage. If data lives completely outside a program it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects where objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects where the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAM-based object when necessary. Java 1.1 provides support for lightweight persistence, and future versions of Java might provide more complete solutions for persistence. Chapter 2: Everything is an Object 61
- 62. Special case: primitive types There is a group of types that gets special treatment; you can think of these as “primitive” types that you use quite often in your programming. The reason for the special treatment is that to create an object with new, especially a small, simple variable, isn’t very efficient because new places objects on the heap. For these types Java falls back on the approach taken by C and C++: instead of creating the variable using new, an “automatic” variable is created which is not a handle. The variable holds the value itself, and it’s placed on the stack so it’s much more efficient. Java determines the size of each primitive type. These sizes don’t change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are so portable. Primitiv Size Minimum Maximum Wrapper e type type boolean 1-bit – – Boolean char 16-bit Unicode 0 Unicode 216- Character 1 byte 8-bit -128 +127 Byte1 short 16-bit -215 +215 – 1 Short1 int 32-bit -231 +231 – 1 Integer long 64-bit -263 +263 – 1 Long float 32-bit IEEE754 IEEE754 Float double 64-bit IEEE754 IEEE754 Double void – – – Void1 All numeric types are signed, so don’t go looking for unsigned types. The primitive data types also have “wrapper” classes for them. That means if you want to make a non-primitive object on the heap to represent that primitive type, you use the associated wrapper. For example: char c = 'x'; Character C = new Character(c); or you could also say: Character C = new Character('x'); The reasons for doing this will be shown in a later chapter. 1 In Java version 1.1 only, not in 1.0. 62 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 63. High-precision numbers Java 1.1 has added two classes for performing high-precision arithmetic: BigInteger and BigDecimal. Although these approximately fit into the same category as the above “wrapper” classes, neither one has a primitive analogue. Both classes have methods that provide analogues for the operations that you perform on primitive types. That is, you can do anything with a BigInteger or BigDecimal that you can with an int or float, it’s just that you must use method calls instead of operators. Also, since there’s more involved the operations will be slower; you’re exchanging speed for accuracy. BigInteger supports arbitrary-precision integers. This means you can accurately represent integral values of any size without losing any information during operations. BigDecimal is for arbitrary-precision fixed-point numbers; you can use these for accurate monetary calculations, for example. Consult your on-line documentation for details about the constructors and methods you can call for these two classes. Arrays in Java Virtually all programming languages support arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory, and if a program accesses the array outside of its memory block or uses the memory before initialization (common programming errors) there will be unpredictable results.2 One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot be accessed outside of its range. The range checking comes at the price of having a small amount of memory overhead on each array as well as verifying the index at run time, but the assumption is that the safety and increased productivity is worth the expense. When you create an array of objects, you are really creating an array of handles, and each of those handles is automatically initialized to null. You must assign an object to each handle before you use it, and if you try to use a handle that’s still null the problem will be reported at run-time. Thus, typical array errors are prevented in Java. You can also create an array of primitives. Again, the compiler guarantees initialization because it zeroes the memory for that array. Arrays will be covered in detail in later chapters. You never have to destroy an object In most programming languages, the concept of the lifetime of a variable occupies a significant portion of the programming effort. How long does the variable last? If you are supposed to destroy it, when should you? Confusion over variable lifetimes can lead to lots of bugs, and this section shows how Java greatly simplifies the issue by doing all the cleanup work for you. 2 In C++ you should often use the safer containers in the Standard Template Library as an alternative to arrays. Chapter 2: Everything is an Object 63
- 64. Scoping Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++ and Java, scope is determined by the placement of curly braces {}. So for example: { int x = 12; /* only x available */ { int q = 96; /* both x & q available */ } /* only x available */ /* q “out of scope” */ } A variable defined within a scope is available only to the end of that scope. Indentation makes Java code easier to read. Since Java is a “free form” language, the extra spaces, tabs and carriage returns do not affect the resulting program. Note that you cannot do the following, even though it is legal in C and C++: { int x = 12; { int x = 96; /* illegal */ } } The compiler will announce that the variable x has already been defined. Thus the C/C++ ability to “hide” a variable in a larger scope is disallowed because the Java designers felt it led to confusing programs. Scope of objects Java objects do not have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Thus if you say: { String s = new String("a string"); } /* end of scope */ the handle s vanishes at the end of the scope. However, the String object that s was pointing to is still occupying memory. In this bit of code, there is no way to access the object because the only handle to it is out of scope. In later chapters you’ll see how the handle to the object may be passed around and duplicated during the course of a program. It turns out that because objects created with new stay around for as long as you want them, a whole slew of programming problems simply vanish (in C++ and Java). The hardest problems seem to occur in C++ because you don’t get any help from the language in making sure the objects are available when they’re needed. And more importantly, in C++ you must make sure that you destroy the objects when you’re done with them. That brings up an interesting question. If Java leaves the objects lying around, what keeps them from filling up memory and halting your program? This is exactly the kind of 64 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 65. problem that would occur in C++. This is where a bit of magic happens: Java has a garbage collector, which is a process running in the background (with a low priority, so it doesn’t much interfere with the execution of your program). The garbage collector looks at all the objects that were created with new and figures out which ones are not being referenced anymore. Then it releases the memory for those objects, so the memory can be used for new objects. Thus, you never have to worry about reclaiming memory yourself. You simply create objects, and when you no longer need them they will go away by themselves. This eliminates a certain class of programming problem: the so-called “memory leak,” where the programmer forgets to release memory. Creating new data types: class If everything is an object, what determines how a particular class of object looks and behaves? Put another way, what establishes the type of an object? You might expect there to be a keyword called “type” and that certainly would have made sense. Historically, however, most object-oriented languages have used the keyword class to say: “I’m about to tell you what a new type of object looks like.” The class keyword (which is so common that it will not be emboldened throughout the book) is followed by the name of the new type, like this: class ATypeName { /* class body goes here */ } This introduces a new type , so you can now create an object of this type using new: ATypeName a = new ATypeName(); In ATypeName, the class body consists only of a comment (the stars and slashes and what is inside, which will be discussed later in this chapter) so there is not too much you can do with it. In fact, you cannot tell it to do much of anything (that is, you cannot send it any interesting messages) until you define some methods for it. Fields and methods When you define a class (and all you do in Java is define classes, make objects of those classes, and send messages to those objects) you can put two types of elements in your class: data members (sometimes called fields) and member functions (typically called methods). A data member is an object (that you communicate with via its handle) of any type, or it can be one of the primitive types (which isn’t a handle). If it is a handle to an object, you must initialize that handle to connect it to an actual object (using new, as seen earlier) in a special function called a constructor (described fully in Chapter 4). If it is a primitive type you can initialize it directly at the point of definition in the class. (As you’ll see later, handles may also be initialized at the point of definition). Each object keeps its own storage for its data members; the data members are not shared among objects. Here is an example of a class with some data members: class DataOnly { int i; float f; boolean b; } This class doesn’t do anything, but you can create an object: DataOnly d = new DataOnly(); Chapter 2: Everything is an Object 65
- 66. You can assign values to the data members, but you must first know how to refer to a member of an object. This is accomplished by stating the name of the object handle, followed by a period (dot), followed by the name of the member inside the object (objectHandle.member). For example: d.i = 47; d.f = 1.1f; d.b = false; It is also possible that your object might contain other objects which contain data you’d like to modify. For this, you just keep “connecting the dots.” For example: myPlane.leftTank.capacity = 100; The DataOnly class cannot do much of anything except hold data, because it has no member functions (methods). To understand how those work, you must first understand arguments and return values, which will be described shortly. Default values for primitive members When a primitive data type is a member of a class, it is guaranteed to get a default value if you do not initialize it: Primitiv Default e type boolean false char ‘u0000’ (null) byte (byte)0 short (short)0 int 0 long 0L float 0.0f double 0.0d Note carefully that the default values are what Java guarantees when the variable is used as a member of a class. This ensures that member variables of primitive types will always be initialized (something C++ doesn’t do), reducing a source of bugs. However, this guarantee doesn’t apply to “local” variables – those that are not fields of a class. Thus, if within a function definition you have: int x; Then (as in C and C++) x will get some random value; it will not automatically be initialized to zero. You are responsible for assigning an appropriate value before you use x. What happens if you forget? Here, Java definitely improves on C++: you get a compile-time error telling you the variable might not have been initialized. (Many C++ compilers will warn you about uninitialized variables, but in Java these are errors). 66 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 67. Methods, arguments and return values Up until now, the term function has been used to describe a named subroutine. However, the term that is more commonly used in Java is method as in “a way to do something.” If you want, you can go on thinking in terms of functions. It’s really only a syntactic difference, but from now on “method” will be used in this book rather than “function.” Methods in Java determine the messages an object can receive. In this section you will learn how simple it is to define a method. The fundamental parts of a method are the name, the arguments, the return type, and the body. Here is the basic form: returnType methodName( /* argument list */ ) { /* Method body */ } The return type is the type of the value that pops out of the method after you call it. The method name, as you might imagine, identifies the method. The argument list gives the types and names for the information you want to pass into the method. Methods in Java can be created only as part of a class. A method can be called only for an object3 , and that object must be able to perform that method call. If you try to call the wrong method for an object, you’ll get an error message at compile time. You call a method for an object by naming the object followed by a period (dot), followed by the name of the method and its argument list, like this: objectName.methodName(arg1, arg2, arg3). For example, suppose you have a method f( ) that takes no arguments and returns a value of type int. Then, if you have an object called a for which f( ) can be called, you can say this: int x = a.f(); The type of the return value must be compatible with the type of x. This act of calling a method is commonly referred to as sending a message to an object. In the above example, the message is f( ) and the object is a. Object-oriented programming is often summarized as simply “sending messages to objects.” The argument list The method argument list specifies what information you pass into the method. As you might guess, this information – like everything else in Java – takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing handles4 . The type of the handle must be correct, however: if the argument is supposed to be a String, what you pass in must be a string. 3 static methods, which you’ll learn about soon, can be called for the class, without an object. 4 With the usual exception of the aforementioned “special” data types boolean, char, byte, short, int, long, float, and double. In general, though, you pass objects, which really means you pass handles to objects. Chapter 2: Everything is an Object 67
- 68. Consider a method which takes a string as its argument. Here is the definition, which must be placed within a class definition for it to compile: int storage(String s) { return s.length() * 2; } This method tells you how many bytes are required to hold the information in a particular String (each char in a String is 16 bits long to support Unicode characters). The argument is of type String and is called s. Once s is passed into the method, you can treat it just like any other object (you can send messages to it). Here, the length( ) method is called, which is one of the methods for strings – it returns the number of characters in a string. You can also see the use of the return keyword, which does two things. First, it says “leave the method, I’m done.” Second, if the method produces a value, that value is placed right after the return statement. In this case, the return value is produced by evaluating the expression s.length( ) * 2. You can return any type you want, but if you don’t want to return anything at all, you do so by indicating that the method returns void. Here are some examples: boolean flag() { return true; } float euler() { return 2.718; } void nothing() { return; } void nothing2() {} When the return type is void, then the return keyword is used only to exit the method, and is therefore unnecessary when you reach the end of the method. You can return from a method at any point, but if you've given a non-void return type then the compiler will ensure that you return the appropriate type of value regardless of where you return. At this point, it can look like a program is just a bunch of objects with methods that take other objects as arguments, and send messages to those other objects. That is indeed much of what goes on, but in the following chapter you’ll learn how to do the detailed low-level work by making decisions within a method. But for this chapter, sending messages will suffice. Building a Java program There are several other issues you must understand before seeing your first Java program. Name visibility A problem in any programming language is the control of names. If you use a name in one module of the program, and another programmer uses the same name in another module, how do you distinguish one name from another and prevent the two names from “clashing”? In C this is a particular problem because a program is often an unmanageable sea of names. C++ classes (on which Java classes are based) nest functions within classes, so they cannot clash with function names nested within other classes. However, C++ still allowed global data and global functions so clashing was still possible. To solve this problem, C++ introduced namespaces using additional keywords. Java was able to avoid all this by taking a fresh approach. To produce an unambiguous name for a library, the specifier used is not unlike an Internet domain name; in fact, the Java creators want you to use your Internet domain name in reverse since those are 68 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 69. guaranteed to be unique. Since my domain name is EckelObjects.com, then my utility library of foibles would be named com.eckelobjects.utility.foibles. After your reversed domain name, the dots are intended to represent subdirectories. In Java 1.0 and Java 1.1 the domain extension com, edu, org, net, etc., was capitalized by convention, so the library would appear: COM.eckelobjects.utility.foibles. Partway through the development of Java 1.2, however, it was discovered that this caused problems and so now the entire package name is lowercase. This mechanism in Java means that all your files automatically live in their own namespaces, and each class within a file automatically has a unique identifier (class names within a file must be unique, of course). Thus you do not need to learn special language features to solve this problem – the language takes care of it for you. Using other components Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source code file that it’s being called from. In that case, you simply use the class – even if the class doesn’t get defined until later in the file. Java eliminates the “forward referencing” problem so you don’t have to think about it. What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. What if you want to use a class of a particular name, but the definition for that class exists in more than one file? Or worse, you’re writing a program and as you’re building it you add a new class to your library which conflicts with the name of an existing class. To solve this problem, all potential ambiguities must be eliminated. This is accomplished by telling the Java compiler exactly what classes you want using the import keyword. import tells the compiler to bring in a package, which is a library of classes (in other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class). Much of the time you’ll be using components from the standard Java libraries that come with your compiler. With these, you don’t need to worry about long reversed domain names; you just say, for example: import java.util.Vector; to tell the compiler that you want to use Java’s Vector class. However, util contains a number of classes and you might want to use several of them without declaring them all explicitly. This is easily accomplished by using ‘*’ to indicate a wildcard: import java.util.*; It is more common to import a collection of classes in this manner than to import classes individually. The static keyword Normally, when you create a class you are describing how objects of that class look and how they will behave. You don’t actually get anything until you create an object of that class with new, and at that point data storage is created and methods become available. But there are two situations where this approach is not sufficient. What if you want to have only one piece of storage for a particular piece of data, regardless of how many objects are created, or even if no objects at all are created? And similarly, what if you need Chapter 2: Everything is an Object 69
- 70. a method that isn’t associated with any particular object of this class? That is, a method that you can call even if no objects are created. Both these effects are achieved with the static keyword. When you say something is static, it means that data or method is not tied to any particular object instance of that class. Thus, even if you’ve never created an object of that class you can call a static method or access a piece of static data. With ordinary, non-static data and methods you must create an object, and use that object, to access the data or method since non-static data and methods must know the particular object they are working with. Of course, since static methods don’t need any objects to be created before they are used, they cannot directly access non-static members or methods by simply calling those other members without referring to a named object (since non- static members and methods must be tied to a particular object). Some object-oriented languages use the terms class data and class methods, meaning that the data and methods exist only for the class as a whole, and not for any particular objects of the class. Sometimes the Java literature uses these terms also. To make a data member or method static, you simply place the keyword before the definition. For example, this produces a static data member and initializes it: class StaticTest { static int i = 47; } Now even if you make two StaticTest objects, there will still be only one piece of storage for StaticTest.i – both objects will share the same i. Consider: StaticTest st1 = new StaticTest(); StaticTest st2 = new StaticTest(); At this point, both st1.i and st2.i have the same value of 47 since they refer to the same piece of memory. There are two ways to refer to a static variable. As indicated above, you can name it via an object, by saying, for example, st2.i. But you can also refer to it directly through its class name, something you cannot do with a non-static member (and the preferred way to refer to a static variable, since it emphasizes that variable’s static nature): StaticTest.i++; The ++ operator increments the variable. At this point, both st1.i and st2.i will have the value 48. Similar logic applies to static methods. You can refer to a static method either through an object as you can with any method, or with the special additional syntax classname.method( ). You define a static method in a similar way: class StaticFun { static void incr() { StaticTest.i++; } } You can see that the StaticFun method incr( ) increments the static data i. You can call incr( ) in the typical way, through an object: StaticFun sf = new StaticFun(); sf.incr(); Or, because incr( ) is a static method, you can call it directly through its class: StaticFun.incr(); 70 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 71. While static, when applied to a data member, definitely changes the way the data is created (only one vs. the non-static one for each object), when applied to a method it’s not so dramatic. An important use of static for methods is to allow you to call that method without creating an object. This is essential, as we shall see, in defining the main( ) method which is the entry point for running an application. Like any method, a static method may create or use named objects of its type, so a static method is often used as a “sheperd” for a flock of instances of its own type. Your first Java program Finally, here’s the program5 . It prints out information about the system that it’s running on using various methods of the System object from the Java standard library. Note that an additional style of comment is introduced here: the ‘//’ which is a comment until the end of the line: // Property.java import java.util.*; public class Property { public static void main(String args[]) { System.out.println(new Date()); Properties p = System.getProperties(); p.list(System.out); System.out.println("--- Memory Usage:"); Runtime rt = Runtime.getRuntime(); System.out.println("Total Memory = " + rt.totalMemory() + " Free Memory = " + rt.freeMemory()); } } At the beginning of each program file, you must place the import statement to bring in any extra classes you’ll need for the code in that file. Notice that I said “extra.” That’s because there’s a certain library of classes that are automatically brought into every java file: java.lang. Start up your web browser and look at the documentation from Sun (if you haven’t downloaded it from java.sun.com or otherwise installed the Java documentation, do so now). If you look at the packages.html file, you’ll see a list of all the different class libraries that come with Java. Select java.lang. Under “Class Index” you’ll see a list of all the classes that are part of that library. Since java.lang is implicitly included in every Java code file, these classes are automatically available. In the list, you’ll 5 Some programming environments will flash programs up on the screen and close them before you've had a chance to see the results. You can put in the following bit of code at the end of main( ) to pause the output: try { Thread.currentThread().sleep(5 * 1000); } catch(InterruptedException e) {} } This will pause for 5 seconds. This code involves concepts that will not be introduced until much later in the book, so you won't understand it until then, but it will do the trick. Chapter 2: Everything is an Object 71
- 72. see System and Runtime, which are used in Property.java. However, there’s no Date class listed in java.lang, which means you’ll need to import another library to use that. If you don’t know the library where a particular class is (or if you want to see all the classes) you can select “Class Hierarchy” in the Java documentation. In a web browser, this takes a while to construct, but you can find every single class that comes with Java. Then you can use the browser’s “find” function to find Date, and when you do you’ll see it listed as java.util.Date, which tells you it’s in the util library and that you must import java.util.* in order to use Date. Again looking at the documentation starting from the packages.html file (which I’ve set in my web browser as the default starting page), if you select java.lang and then System, you’ll see that the System class has several fields, and if you select out you’ll discover that it’s a static PrintStream object. Since it’s static you don’t have to create anything, the out object is always there and you can just use it. But what can you do with this out object? That is determined by what type it is – it’s a PrintStream. Conveniently, PrintStream is shown in the description as a hyperlink, so if you click on that you’ll see a list of all the methods you can call for PrintStream. There are quite a few and these will be covered later in the book, but for now all we’re interested in is println( ), which in effect means “print out what I’m giving you to the console, and end with a new line). Thus in any Java program you write you can say System.out.println(“things”) whenever you want to print things to the console. The name of the class is the same as the name of the file. When you’re creating a stand- alone program like this one, one of the classes in the file must have the same name as the file (the compiler complains if you don’t do this) and that class must contain a method called main( ) with the signature shown: public static void main(String args[]) { The public keyword means the method is available to the outside world (described in detail in Chapter 5). The argument to main( ) is an array of String objects. The args won’t be used in this program, but they have to be there because they hold the arguments invoked on the command line. The first line of the program is quite interesting: System.out.println(new Date()); Look at the argument: a Date object is being created just to send its value to println( ). As soon as this statement is finished, that Date is unnecessary, and the garbage collector can come along and get it anytime. We don’t have to worry about cleaning it up. The second line calls System.getProperties( ). Again consulting the on-line documentation using your web browser, you’ll see that getProperties( ) is a static method of class System. Since it’s static, you don’t have to create any objects in order to call the method; a static method is always available whether an object of its class exists or not. When you call getProperties( ), it produces the system properties as an object of class Properties. The handle that comes back is stored in a Properties handle called p. In Line three, you can see that the Properties object has a method called list( ) that sends its entire contents to a PrintStream object that you pass as an argument. The fourth and sixth lines in main( ) are typical print statements. Notice that to print multiple String values, we simply separate them with ‘+’ signs. However, there’s something strange going on here: the ‘+’ sign doesn’t mean “addition” when it’s used with String objects. Normally you wouldn’t ascribe any meaning at all to ‘+’ when you think of strings. However, the Java String class is blessed with something called “operator overloading.” That is, the ‘+’ sign, only when used with String objects, behaves differently than it does with everything else. For Strings, it means: “concatenate these two strings.” 72 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 73. But that’s not all. If you look at the statement: System.out.println("Total Memory = " + rt.totalMemory() + " Free Memory = " + rt.freeMemory()); totalMemory( ) and freeMemory( ) return numerical values, and not String objects. What happens when you “add” a numerical value to a String? Well, the compiler sees the problem and magically calls a method that turns that numerical value (int, float, etc.) into a String, which can then be “added” with the plus sign. This automatic type conversion also falls into the category of operator overloading. Much of the Java literature states vehemently that operator overloading (a feature in C++) is bad, and yet here it is! However, this is wired into the compiler, only for String objects, and you can’t overload operators for any of the code you write. The fifth line in main( ) creates a Runtime object by calling the static method getRuntime( ) for the class Runtime. What’s returned is a handle to a Runtime object; whether this is a static object or one created with new doesn’t need to concern you, since you can use the objects without worrying about who’s responsible for cleaning them up. As shown, the Runtime object can tell you information about memory usage. Comments & embedded documentation There are two types of comments in Java. The first is the traditional C-style comment that was inherited by C++. These comments begin with a /* and continue, possibly across many lines, until a */. Note that many programmers will begin each line of a continued comment with a *, so you’ll often see: /* This is * A comment that continues * Across lines */ Remember, however, that everything inside the /* and */ is ignored so it’s no different to say: /* This is a comment that continues across lines */ The second form of comment comes from C++. It is the single-line comment, which starts at a // and continues until the end of the line. This type of comment is convenient and commonly used because it’s easy: you don’t have to hunt on the keyboard to find / and then * (you just press the same key twice) and you don’t have to close the comment. So you will often see: // this is a one-line comment Comment documentation One of the thoughtful parts of the Java language is that the designers didn’t only consider writing code, they also thought about documenting it. Possibly the biggest problem with documenting code has been maintaining that documentation. If the documentation and the Chapter 2: Everything is an Object 73
- 74. code are separate, it becomes a hassle to change the documentation every time you change the code. The solution seems simple: link the code to the documentation. The easiest way to do this is to put everything in the same file. To complete the picture, however, you need a special comment syntax to mark special documentation and a tool to extract those comments and put them in a useful form. This is what Java has done. The tool to extract the comments is called javadoc – it uses some of the technology from the Java compiler to look for special comment tags you put in your programs. It not only extracts the information marked by these tags, but it also pulls out the class name or method name that is adjoining the comment. This way you can get away with the minimal amount of work to generate decent program documentation. The output of javadoc is an HTML file that you can view with your Web browser. This tool allows you to create and maintain a single source file and automatically generate useful documentation. Because of javadoc we have a standard for creating documentation, and it’s easy enough that we can expect or even demand documentation with all Java libraries. Syntax All of the javadoc commands occur only within /** comments. The comments end with */ as usual. There are two primary ways to use javadoc: embed HTML, or use “doc tags.” Doc tags are commands that start with a ‘@’ and are placed at the beginning of a comment line (a leading ‘*’, however, is ignored). There are three “types” of comment documentation, which correspond to the element the comment precedes: class, variable, or method. That is, a class comment appears right before the definition of a class, a variable comment appears right in front of the definition of a variable, and a method comment appears right in front of the definition of a method. As a simple example: /** A class comment */ public class docTest { /** A variable comment */ public int i; /** A method comment */ public void f() {} } Note that javadoc will process comment documentation for only public and protected members. Comments for private and “friendly” (see Chapter 5) members are ignored and you’ll see no output. This makes sense, since only public and protected members are available outside the file, which is the client programmer’s perspective. However, all class comments are included in the output. The output for the above code is an HTML file that has the same standard format as all the rest of the Java documentation, so users will be comfortable with the format and can easily navigate your classes. It’s worth entering the above code, sending it through javadoc, and viewing the resulting HTML file to see the results. Embedded HTML Javadoc passes HTML commands through to the generated HTML document. This allows you full use of HTML; however, the primary motive is to let you format code, such as: /** * <pre> 74 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 75. * System.out.println(new Date()); * </pre> */ You can also use HTML just as you would in any other Web document, to format the regular text in your descriptions: /** * You can <em>even</em> insert a list: * <ol> * <li> Item one * <li> Item two * <li> Item three * </ol> */ Note that within the documentation comment, asterisks at the beginning of a line are thrown away by javadoc, along with leading spaces. javadoc reformats everything so it conforms to the standard documentation appearance. Don’t use headings such as <h1> or <hr> as embedded HTML since javadoc inserts its own headings and yours will interfere with them. All types of comment documentation: class, variable and method, can support embedded HTML. @see : referring to other classes All three types of comment documentation can contain @see tags, which allow you to refer to the documentation in other classes. javadoc will generate HTML with the @see tags hyperlinked to the other documentation. The forms are: @see classname @see fully-qualified-classname @see fully-qualified-classname#method-name Each one adds a hyperlinked “See Also” entry to the generated documentation. javadoc will not check the hyperlinks you give it to make sure they are valid. Class documentation tags Along with embedded HTML and @see references, class documentation can also include tags for version information and the author’s name. Class documentation can also be used for interfaces (described later in the book). @version This is of the form: @version version-information where version-information is any significant information you see fit to include. When the -version flag is placed on the javadoc command-line, the version information will be called out specially in the generated HTML documentation. @author This is of the form: @author author-information Chapter 2: Everything is an Object 75
- 76. Where author-information is, presumably, your name, but could also include your email address or any other appropriate information. When the -author flag is placed on the javadoc command line, the author information will be called out specially in the generated HTML documentation. You can have multiple author tags for a list of authors, but they must be placed consecutively. All the author information will be lumped together into a single paragraph in the generated HTML. Variable documentation tags Variable documentation can include only embedded HTML and @see references. Method documentation tags As well as embedded documentation and @see references, methods allow documentation tags for parameters, return values, and exceptions: @param This is of the form: @param parameter-name description where parameter-name is the identifier in the parameter list, and description is text that can continue on subsequent lines; the description is considered finished when a new documentation tag is encountered. You can have any number of these, presumably one for each parameter. @return This is of the form: @return description where description tells you the meaning of the return value. It can continue on subsequent lines. @exception Exceptions will be described in Chapter 9, but briefly they are objects that can be “thrown” out of a method if that method fails. Although only one exception object can emerge when you call a method, a particular method might produce any number of different types of exceptions, all of which need descriptions. So the form for the exception tag is: @exception fully-qualified-class-name description where fully-qualified-class-name gives an unambiguous name of an exception class that’s defined somewhere, and description (which can continue on subsequent lines) tells you why this particular type of exception can emerge from the method call. @deprecated This is new in Java 1.1. It is used to tag features which have been superseded by an improved feature. The deprecated tag is a suggestion that you no longer use this particular feature, since sometime in the future it is likely to be removed from the language. 76 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 77. Documentation example Here is the first Java program again, this time with documentation comments added: //: Property.java import java.util.*; /** The first example program in "Thinking in Java." * Lists system information on current machine. * @author Bruce Eckel * @author http://www.EckelObjects.com * @version 1.0 */ public class Property { /** Sole entry point to class & application * @param args Array of string arguments * @return No return value * @exception exceptions No exceptions thrown */ public static void main(String args[]) { System.out.println(new Date()); Properties p = System.getProperties(); p.list(System.out); System.out.println("--- Memory Usage:"); Runtime rt = Runtime.getRuntime(); System.out.println("Total Memory = " + rt.totalMemory() + " Free Memory = " + rt.freeMemory()); } } ///:~ The first line: //: Property.java uses my own technique of putting a ‘:’ as a special marker for the comment line containing the source file name. The last line also finishes with a comment, and this one indicates the end of the source-code listing, which allows it to be automatically extracted from the text of the book and checked with a compiler. This is described in detail in Chapter 17. Coding style The unofficial standard in Java is to capitalize the first letter of a class name. If the class name consists of several words, they are run together (that is, you don’t use underscores to separate the names) and the first letter of each embedded word is capitalized, such as: class AllTheColorsOfTheRainbow { // ... For almost everything else: methods, fields (member variables) and object handle names, the accepted style is just as it is for classes except that the first letter of the identifier is lower case. For example: class AllTheColorsOfTheRainbow { int anIntegerRepresentingColors; Chapter 2: Everything is an Object 77
- 78. void changeTheHueOfTheColor(int newHue) { // ... } // ... } Of course, you should remember that the user must also type all these long names, and be merciful. Summary In this chapter you have seen enough of Java programming to understand how to write a simple program, and you have gotten an overview of the language and some of its basic ideas. However, the examples so far have all been of the form “do this, then do that, then do something else.” What if you want the program to make choices, such as “if the result of doing this is red, do that, otherwise do something else?” The support in Java for this fundamental programming activity will be covered in the next chapter. Exercises 1. Following the first example in this chapter, create a “Hello, World” program that simply prints out that statement. You need to create only a single method in your class (the “main” one that gets executed when the program starts). Remember to make it static and to put the argument list in, even though you don’t use the argument list. Compile the program with javac and run it using java. 2. Write a program that prints three arguments taken from the command line. 3. Find the code for the second version of Property.java, which is the simple comment documentation example. Execute javadoc on the file and view the results with your Web browser. 4. Take the program in Exercise 1 and add comment documentation to it. Extract this comment documentation into an HTML file using javadoc and view it with your Web browser. 78 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 79. c 3: Controlling program flow Like a sentient creature, a program must manipulate its world and make choices during execution. In Java you manipulate objects and data using operators, and you make choices with execution control statements. As Java was inherited from C++, most of these statements and operators will be familiar to C and C++ programmers. Java has also added some improvements and simplifications. Using Java operators An operator takes one or more arguments and produces a new value. The arguments are in a different form than ordinary method calls, but the effect is the same. You should be reasonably comfortable with the general concept of operators from your previous programming experience. Addition (+), subtraction and unary minus (-), multiplication (*), division (/) and assignment (=) all work much the same in any programming language. All operators produce a value from their operands. Additionally, an operator can change the value of an operand; this is called a side effect. The most common use for operators that modify their operands is to generate the side effect, but you should keep in mind that the value produced is available for your use just as in operators without side effects. Almost all operators work only with primitives. The exceptions are ‘=’, ‘==’ and ‘!=’, which work with all objects (and are a point of confusion for objects). In addition, the String class supports ‘+’ and ‘+=’. 79
- 80. Precedence Operator precedence defines how an expression evaluates when several operators are present. Java has specific rules that determine the order of evaluation. The easiest to remember is that multiplication and division happen before addition and subtraction. The other precedence rules are often forgotten by programmers, so you should use parentheses to make the order of evaluation explicit. For example: A = X + Y - 2/2 + Z; has a very different meaning from the same statement with a particular grouping of parentheses: A = X + (Y - 2)/(2 + Z); Assignment Assignment is performed with the operator =. It means “take the value of the right-hand side (often called the rvalue) and copy it into the left-hand side (often called the lvalue). An rvalue is any constant, variable, or expression that can produce a value, but an lvalue must be a distinct, named variable (that is, there must be a physical space to store a value). For instance, you can assign a constant value to a variable (A = 4;), but you cannot assign anything to constant value – it cannot be an lvalue (you can’t say 4 = A;). Assignment of primitives is quite straightforward. Since the primitive holds the actual value and not a handle to an object, when you assign primitives you copy the contents from one place to another. That is, if you say A = B for primitives then the contents of B is copied into A. If you then go on to modify A, B is naturally unaffected by this modification. This is what you’ve come to expect as a programmer for most situations. When you assign objects, however, things change. Whenever you manipulate an object what you’re manipulating is the handle, so when you assign “from one object to another” you’re actually copying a handle from one place to another. This means if you say C = D for objects, what you end up with is both C and D pointing to the object that, originally, only D was pointing to. The following example will demonstrate this. As an aside, the first thing you see is a package statement for package c03, indicating this book’s Chapter 3. The first code listing of each chapter will contain a package statement like this, to establish the chapter number for the remaining code listings in that chapter. In Chapter 17, you’ll see that as a result, all the listings in this chapter (except those that have different package names) will be automatically placed in a subdirectory called c03, Chapter four’s listings will be in c04, etc. All this will happen via the CodePackager.java program shown in Chapter 17, and in Chapter 5 the concept of packages will be fully explained, but all you need to recognize at this point is that, for this book, lines of code of the form package c03 are used just to establish the chapter subdirectory for the listings in the chapter. In order to run the program, you must ensure that the classpath contains the root directory where you installed the source code for this book (from this directory, you’ll see the subdirectories c02, c03, c04, etc.). In addition, for later versions of Java (1.1.4 and on) you might need to give the full package name before the program name in order to run the program. In this case, the command line would be: java c03.Assignment 80 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 81. Keep this in mind any time you’re running a program that’s in a package. Here’s the example: //: Assignment.java // Assignment with objects is a bit tricky package c03; class Number { int i; } public class Assignment { public static void main(String args[]) { Number n1 = new Number(); Number n2 = new Number(); n1.i = 9; n2.i = 47; System.out.println("1: n1.i: " + n1.i + ", n2.i: " + n2.i); n1 = n2; System.out.println("2: n1.i: " + n1.i + ", n2.i: " + n2.i); n1.i = 27; System.out.println("3: n1.i: " + n1.i + ", n2.i: " + n2.i); } } ///:~ The Number class is very simple, and two instances of it (n1 and n2) are created within main( ). The i value within each Number is given a different value, and then n2 is assigned to n1, and n1 is changed. In many programming languages you would expect n1 and n2 to be independent at all times, but because you’ve actually assigned a handle here’s the output you’ll see: 1: n1.i: 9, n2.i: 47 2: n1.i: 47, n2.i: 47 3: n1.i: 27, n2.i: 27 Changing the n1 object appears to change the n2 object as well! This is because both n1 and n2 contain the same handle, which is pointing to the same actual object (the original handle that was in n1 that pointed to the object holding a value of 9 was overwritten during the assignment and, effectively, lost – it’s object will be cleaned up by the garbage collector). This phenomenon is often called aliasing and it’s a fundamental way that Java works with objects. But what if you don’t want aliasing to occur in this case? You could forego the assignment and say : n1.i = n2.i; This retains the two separate objects instead of tossing one and tying n1 and n2 to the same object, but you’ll soon realize that manipulating the fields within objects is pretty messy and goes against good object-oriented design principles. Because this is a non-trivial topic, it is left for Chapter 12, which is devoted to the issue of aliasing. In the meantime, you should keep in mind that assignment for objects can add surprises. Chapter 3: Controlling Program Flow 81
- 82. Aliasing during method calls Aliasing will also occur when you pass an object into a method: //: PassObject.java // Passing objects to methods can be a bit tricky class Letter { char c; } public class PassObject { static void f(Letter y) { y.c = 'z'; } public static void main(String args[]) { Letter x = new Letter(); x.c = 'a'; System.out.println("1: x.c: " + x.c); f(x); System.out.println("2: x.c: " + x.c); } } ///:~ In many programming languages, the method f( ) would appear to be making a copy of its argument Letter y inside the scope of the method. But once again a handle is being passed so the line y.c = 'z'; is actually changing the object outside of f( ). The output shows this: 1: x.c: a 2: x.c: z Aliasing and its solution is a complex issue and, although you must wait until Chapter 12 for all the answers, you should be aware of it at this point so you can watch for pitfalls. Mathematical operators The basic mathematical operators are the same as the ones available in most programming languages: addition (+), subtraction (-), division (/), multiplication (*) and modulus (%, produces the remainder from integer division). Integer division truncates, rather than rounds, the result. Java also uses a shorthand notation to perform an operation and an assignment at the same time. This is denoted by an operator followed by an equal sign, and is consistent with all the operators in the language (whenever it makes sense). For example, to add 4 to the variable x and assign the result to x, you say: x += 4;. This example shows the use of the mathematical operators: //: MathOps.java // Demonstrates the mathematical operators import java.util.*; public class MathOps { // Create a shorthand to save typing: 82 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 83. static void prt(String s) { System.out.println(s); } // shorthand to print a string and an int: static void pInt(String s, int i) { prt(s + " = " + i); } // shorthand to print a string and a float: static void pFlt(String s, float f) { prt(s + " = " + f); } public static void main(String args[]) { // Create a random number generator, // seeds with current time by default: Random rand = new Random(); int i, j, k; // '%' limits maximum value to 99: j = rand.nextInt() % 100; k = rand.nextInt() % 100; pInt("j",j); pInt("k",k); i = j + k; pInt("j + k", i); i = j - k; pInt("j - k", i); i = k / j; pInt("k / j", i); i = k * j; pInt("k * j", i); i = k % j; pInt("k % j", i); j %= k; pInt("j %= k", j); // Floating-point number tests: float u,v,w; // applies to doubles, too v = rand.nextFloat(); w = rand.nextFloat(); pFlt("v", v); pFlt("w", w); u = v + w; pFlt("v + w", u); u = v - w; pFlt("v - w", u); u = v * w; pFlt("v * w", u); u = v / w; pFlt("v / w", u); // the following also works for // char, byte, short, int, long, // and double: u += v; pFlt("u += v", u); u -= v; pFlt("u -= v", u); u *= v; pFlt("u *= v", u); u /= v; pFlt("u /= v", u); } } ///:~ The first thing you will see are some shorthand methods for printing: the prt( ) method prints a String, the pInt( ) prints a String followed by an int, and the pFlt( ) prints a String followed by a float. Of course, they all ultimately end up using System.out.println( ). To generate numbers the program first creates a Random object. Because no arguments are passed during creation, Java uses the current time as a seed for the random number generator . The program generates a number of different types of random numbers with the Random object simply by calling different methods: nextInt( ), nextLong( ), nextFloat( ) or nextDouble( ). Chapter 3: Controlling Program Flow 83
- 84. The modulus operator, when used with the result of the random number generator, limits the result to an upper bound of the operand minus one (99, in this case). Unary minus and plus operators The unary minus (-) and unary plus (+) are the same operators as binary minus and plus – the compiler figures out which usage is intended by the way you write the expression. For instance, the statement x = -a; has an obvious meaning. The compiler is able to figure out: x = a * -b; but the reader might get confused, so it is clearer to say: x = a * (-b); The unary minus produces the negative of the value. Unary plus provides symmetry with unary minus, although it doesn’t do much. Auto increment and decrement Java, like C, is full of shortcuts. Shortcuts can make code much easier to type, and either easier or harder to read. Two of the nicer shortcuts are the increment and decrement operators (often referred to as the auto-increment and auto-decrement operators). The decrement operator is -- and means “decrease by one unit.” The increment operator is ++ and means “increase by one unit.” If A is an int, for example, the expression ++A is equivalent to (A = A + 1). Increment and decrement operators produce the value of the variable as a result. There are two versions of each type of operator, often called the prefix and postfix versions. Pre-increment means the ++ operator appears before the variable or expression, and post-increment means the ++ operator appears after the variable or expression. Similarly, pre-decrement means the -- operator appears before the variable or expression, and post-decrement means the -- operator appears after the variable or expression. For pre-increment and pre-decrement, (i.e., ++A or --A), the operation is performed and the value is produced. For post-increment and post-decrement (i.e. A++ or A--), the value is produced, then the operation is performed. As an example: //: AutoInc.java // Demonstrates the ++ and -- operators public class AutoInc { public static void main(String args[]) { int i = 1; prt("i : " + i); prt("++i : " + ++i); // Pre-increment prt("i++ : " + i++); // Post-increment prt("i : " + i); prt("--i : " + --i); // Pre-decrement prt("i-- : " + i--); // Post-decrement prt("i : " + i); } static void prt(String s) { System.out.println(s); 84 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 85. } } ///:~ The output for this program is: i : 1 ++i : 2 i++ : 2 i : 3 --i : 2 i-- : 2 i : 1 You can see that for the prefix form you get the value after the operation has been performed, but with the postfix form you get the value before the operation is performed. These are the only operators (other than those involving assignment) that have side effects (that is, they change the operand rather than using just its value). The increment operator is one explanation for the name C++, implying “one step beyond C.” In an early Java speech, Bill Joy (one of the creators), said that "Java=C++--" (C plus plus minus minus), suggesting that Java is C++ with the unnecessary hard parts removed and therefore a much simpler language. As you progress in this book you'll see that many parts are simpler, and yet Java isn't that much easier than C++. Relational operators Relational operators generate a boolean result. They evaluate the relationship between the values of the operands. A relational expression produces true if the relationship is true, and false if the relationship is untrue. The relational operators are less than (<), greater than (>), less than or equal to (<=), greater than or equal to (>=), equivalent (==) and not equivalent (!=). Equivalence and nonequivalence works with all built-in data types, but the other comparisons won’t work with type boolean. Testing object equivalence The relational operators == and != also work with all objects, but their meaning often confuses the first-time Java programmer. Here’s an example: //: Equivalence.java public class Equivalence { public static void main(String args[]) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1 == n2); System.out.println(n1 != n2); } } ///:~ The expression System.out.println(n1 == n2) will print out the result of the boolean comparison within. Surely the output should be true and then false, since both Integer objects are the same. But while the contents of the objects are the same, the handles are not the same and the operators == and != compare object handles. So the output is actually false and then true. Naturally, this surprises people at first. What if you want to compare the actual contents of an object for equivalence? You must use the special method equals( ) that exists for all objects (not primitives, which work fine with == and !=). Here’s how it’s used: Chapter 3: Controlling Program Flow 85
- 86. //: EqualsMethod.java public class EqualsMethod { public static void main(String args[]) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1.equals(n2)); } } ///:~ The result will be true, as you would expect. Ah, but it’s not so simple as that. If you create your own class, like this: //: EqualsMethod2.java class Value { int i; } public class EqualsMethod2 { public static void main(String args[]) { Value v1 = new Value(); Value v2 = new Value(); v1.i = v2.i = 100; System.out.println(v1.equals(v2)); } } ///:~ You’re back to square one: the result is false. This is because the default behavior of equals( ) is to compare handles, so unless you override equals( ) in your new class, you won’t get the desired behavior. Alas, you won’t learn about overriding until Chapter 7, but being aware of the way equals( ) behaves might save you some grief in the meantime. Most of the Java library classes implement equals( ) so that it compares the contents of objects, rather than their handles. Logical operators The logical operators AND (&&), OR (||) and NOT (!) produce a boolean value of true or false based on the logical relationship of its arguments. This example uses the relational and logical operators: //: Bool.java // Relational and logical operators import java.util.*; public class Bool { public static void main(String args[]) { Random rand = new Random(); int i = rand.nextInt() % 100; int j = rand.nextInt() % 100; prt("i = " + i); prt("j = " + j); prt("i > j is " + (i > j)); prt("i < j is " + (i < j)); prt("i >= j is " + (i >= j)); 86 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 87. prt("i <= j is " + (i <= j)); prt("i == j is " + (i == j)); prt("i != j is " + (i != j)); // Treating an int as a boolean is // not legal Java //! prt("i && j is " + (i && j)); //! prt("i || j is " + (i || j)); //! prt("!i is " + !i); prt("(i < 10) && (j < 10) is " + ((i < 10) && (j < 10)) ); prt("(i < 10) || (j < 10) is " + ((i < 10) || (j < 10)) ); } static void prt(String s) { System.out.println(s); } } ///:~ You can apply AND, OR, or NOT to boolean values only. You can’t use a non-boolean as if it were a boolean in a logical expression as you can in C and C++. You can see the failed attempts at doing this commented out with a //! comment marker. The subsequent expressions, however, produce boolean values using relational comparisons, then use logical operations on the results. One output listing looked like this: i = 85 j = 4 i > j is true i < j is false i >= j is true i <= j is false i == j is false i != j is true (i < 10) && (j < 10) is false (i < 10) || (j < 10) is true Notice that a boolean value is automatically converted to an appropriate text form if it's used where a String is expected. You can replace the definition for int in the above program with any other primitive data type except boolean. Be aware, however, that the comparison of floating-point numbers is very strict: a number that is the tiniest fraction different from another number is still “not equal.” A number that is the tiniest bit above zero is still nonzero. Short-circuiting When dealing with logical operators you run into a phenomenon called “short circuiting.” This means that the expression will be evaluated only until the truth or falsehood of the entire expression can be unambiguously determined. As a result, all the parts of a logical expression might not be evaluated. Here’s an example that demonstrates short-circuiting: //: ShortCircuit.java // Demonstrates short-circuiting behavior // with logical operators. Chapter 3: Controlling Program Flow 87
- 88. public class ShortCircuit { static boolean test1(int val) { System.out.println("test1(" + val + ")"); System.out.println("result: " + (val < 1)); return val < 1; } static boolean test2(int val) { System.out.println("test2(" + val + ")"); System.out.println("result: " + (val < 2)); return val < 2; } static boolean test3(int val) { System.out.println("test3(" + val + ")"); System.out.println("result: " + (val < 3)); return val < 3; } public static void main(String args[]) { if(test1(0) && test2(2) && test3(2)) System.out.println("expression is true"); else System.out.println("expression is false"); } } ///:~ Each test performs a comparison against the argument and returns true or false. It also prints information to show you that it’s being called. The tests are used in the expression: if(test1(0) && test2(2) && test3(2)) You might naturally think that all three tests would be executed, but the output shows otherwise: test1(0) result: true test2(2) result: false expression is false The first test produced a true result, so the expression evaluation continues. However, the second test produced a false result. Since this means that the whole expression must be false, why continue evaluating the rest of the expression? It could be expensive. The reason for short-circuiting, in fact, is precisely that: you can get a potential performance increase if all the parts of a logical expression do not need to be evaluated. Bitwise operators The bitwise operators allow you to manipulate individual bits in an integral primitive data type. Bitwise operators perform boolean algebra on the corresponding bits in the two arguments to produce the result. The bitwise operators come from C’s low-level orientation; you were often manipulating hardware directly and had to set the bits in hardware registers. Java was originally designed to be embedded in TV set-top boxes and so this low-level orientation still made sense. However, you probably won’t use the bitwise operators that much. The bitwise AND operator (&) produces a one in the output bit if both input bits are one; otherwise it produces a zero. The bitwise OR operator (|) produces a one in the output bit 88 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 89. if either input bit is a one and produces a zero only if both input bits are zero. The bitwise, EXCLUSIVE OR, or XOR (^) produces a one in the output bit if one or the other input bit is a one, but not both. The bitwise NOT (~, also called the ones complement operator) is a unary operator – it takes only one argument (all other bitwise operators are binary operators). Bitwise NOT produces the opposite of the input bit – a one if the input bit is zero, a zero if the input bit is one. Since the bitwise operators and logical operators use the same characters, it is helpful to have a mnemonic device to help you remember the meanings: since bits are “small,” there is only one character in the bitwise operators. Bitwise operators can be combined with the = sign to unite the operation and assignment: &=, |= and ^= are all legitimate (since ~ is a unary operator it cannot be combined with the = sign). The boolean type is treated as a one-bit value so it is somewhat different. You can perform a bitwise AND, OR and XOR, but you can’t perform a bitwise NOT (presumably to prevent confusion with the logical NOT). For booleans the bitwise operators have the same effect as the logical operators except that they do not short circuit. Also, the bitwise operators on booleans gives you a XOR logical operator not included under the list of "logical" operators. You’re prevented from using booleans in shift expressions (described next). Shift operators The shift operators also manipulate bits. They can be used only on primitive, integral types. The left-shift operator (<<) produces the operand to the left of the operator shifted to the left by the number of bits specified after the operator (inserting zeroes at the lower- order bits). The signed right-shift operator (>>) produces the operand to the left of the operator shifted to the right by the number of bits specified after the operator. The signed right shift >> uses sign extension: if the value is positive, zeroes are inserted at the higher- order bits, if the value is negative, ones are inserted at the higher-order bits. Java has also added the unsigned right shift >>> which uses zero extension: regardless of the sign, zeroes are inserted at the higher-order bits. This operator does not exist in C or C++. If you shift a char, byte, or short, it will be promoted to int before the shift takes place, and the result will be an int. Only the five low-order bits of the right-hand side will be used. This prevents you from shifting more than the number of bits in an int. If you’re operating on a long, long will be the result. Only the six low-order bits of the right-hand side will be used so you can’t shift more than the number of bits in a long. There is a problem, however, with the unsigned right shift: if you use it with byte or short you might not get the right results (it’s broken in Java 1.0 and Java 1.1). These are promoted to int and right shifted, but the zero extension does not occur, so you get -1 in those cases. The following example can be used to test your implementation: //: URShift.java // Test of unsigned right shift public class URShift { public static void main(String args[]) { int i = -1; i >>>= 10; System.out.println(i); long l = -1; l >>>= 10; System.out.println(l); short s = -1; Chapter 3: Controlling Program Flow 89
- 90. s >>>= 10; System.out.println(s); byte b = -1; b >>>= 10; System.out.println(b); } } ///:~ Shifts can be combined with the equal sign (<<= or >>= or >>>=). The lvalue is replaced by the lvalue shifted by the rvalue. Here’s an example that demonstrates the use of all the operators involving bits: //: BitManipulation.java // Using the bitwise operators import java.util.*; public class BitManipulation { public static void main(String args[]) { Random rand = new Random(); int i = rand.nextInt(); int j = rand.nextInt(); pBinInt("-1", -1); pBinInt("+1", +1); int maxpos = 2147483647; pBinInt("maxpos", maxpos); int maxneg = -2147483648; pBinInt("maxneg", maxneg); pBinInt("i", i); pBinInt("~i", ~i); pBinInt("-i", -i); pBinInt("j", j); pBinInt("i & j", i & j); pBinInt("i | j", i | j); pBinInt("i ^ j", i ^ j); pBinInt("i << 5", i << 5); pBinInt("i >> 5", i >> 5); pBinInt("(~i) >> 5", (~i) >> 5); pBinInt("i >>> 5", i >>> 5); pBinInt("(~i) >>> 5", (~i) >>> 5); long l = rand.nextLong(); long m = rand.nextLong(); pBinLong("-1L", -1L); pBinLong("+1L", +1L); long ll = 9223372036854775807L; pBinLong("maxpos", ll); long lln = -9223372036854775808L; pBinLong("maxneg", lln); pBinLong("l", l); pBinLong("~l", ~l); pBinLong("-l", -l); pBinLong("m", m); pBinLong("l & m", l & m); pBinLong("l | m", l | m); pBinLong("l ^ m", l ^ m); 90 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 91. pBinLong("l << 5", l << 5); pBinLong("l >> 5", l >> 5); pBinLong("(~l) >> 5", (~l) >> 5); pBinLong("l >>> 5", l >>> 5); pBinLong("(~l) >>> 5", (~l) >>> 5); } static void pBinInt(String s, int i) { System.out.println( s + ", int: " + i + ", binary: "); System.out.print(" "); for(int j = 31; j >=0; j--) if(((1 << j) & i) != 0) System.out.print("1"); else System.out.print("0"); System.out.println(); } static void pBinLong(String s, long l) { System.out.println( s + ", long: " + l + ", binary: "); System.out.print(" "); for(int i = 63; i >=0; i--) if(((1L << i) & l) != 0) System.out.print("1"); else System.out.print("0"); System.out.println(); } } ///:~ The two methods at the end, pBinInt( ) and pBinLong( ) take an int or a long, respectively, and print it out in binary format along with a descriptive string. You can ignore the implementation of these for now. You’ll notice the use of System.out.print( ) instead of System.out.println( ). The print( ) method does not put out a newline, so it allows you to output a line in pieces. As well as demonstrating the effect of all the bitwise operators for int and long, this example also shows the minimum, maximum, +1 and -1 values for int and long so you can see what they look like. Note that the high bit represents the sign: 0 means positive and 1 means negative. The output for the int portion looks like this: -1, int: -1, binary: 11111111111111111111111111111111 +1, int: 1, binary: 00000000000000000000000000000001 maxpos, int: 2147483647, binary: 01111111111111111111111111111111 maxneg, int: -2147483648, binary: 10000000000000000000000000000000 i, int: 59081716, binary: 00000011100001011000001111110100 ~i, int: -59081717, binary: 11111100011110100111110000001011 -i, int: -59081716, binary: 11111100011110100111110000001100 j, int: 198850956, binary: Chapter 3: Controlling Program Flow 91
- 92. 00001011110110100011100110001100 i & j, int: 58720644, binary: 00000011100000000000000110000100 i | j, int: 199212028, binary: 00001011110111111011101111111100 i ^ j, int: 140491384, binary: 00001000010111111011101001111000 i << 5, int: 1890614912, binary: 01110000101100000111111010000000 i >> 5, int: 1846303, binary: 00000000000111000010110000011111 (~i) >> 5, int: -1846304, binary: 11111111111000111101001111100000 i >>> 5, int: 1846303, binary: 00000000000111000010110000011111 (~i) >>> 5, int: 132371424, binary: 00000111111000111101001111100000 The binary representation of the numbers is referred to as signed two’s complement. Ternary if-else operator This operator is unusual because it has three operands. It is truly an operator because it produces a value, unlike the ordinary if-else statement which you’ll see in the next section of this chapter. The expression is of the form boolean-exp ? value0 : value1 If boolean-exp evaluates to true, value0 is evaluated and its result becomes the value produced by the operator. If boolean-exp is false, value1 is evaluated and its result becomes the value produced by the operator. Of course, you could use an ordinary if-else statement (described later) but the ternary operator is much terser. Although C prides itself on being a terse language and the ternary operator might have been introduced partly for efficiency, you should be somewhat wary of using it on an everyday basis – it’s easy to produce unreadable code. The conditional operator can be used for its side effects or for the value it produces, but generally you want the value since that’s what makes the operator distinct from the if- else. Here’s an example: static int ternary(int i) { return i < 10 ? i * 100 : i * 10; } You can see that this code is more compact than what you’d have to write without the ternary operator: static int alternative(int i) { if (i < 10) return i * 100; return i * 10; } The second form is easier to understand, and doesn’t require a lot more typing. Thus, ponder your reasons when choosing the ternary operator. 92 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 93. The comma operator The comma is used in C and C++ not only as a separator in function argument lists, but also as an operator for sequential evaluation. The only place that the comma operator is used in Java is in for loops, which will be described later in this chapter. String operator + There’s one special usage of an operator in Java: the + operator can be used to concatenate strings, as you’ve already seen. It seems a very natural use of the + even though it doesn’t fit with the traditional way that + is used. This capability seemed like a good idea in C++ and so operator overloading was added to C++, to allow the C++ programmer to add meanings to almost any operator. Unfortunately, operator overloading combined with some of the other restrictions in C++ turns out to be a fairly complicated feature for programmers to design into their classes. Although operator overloading would have been much simpler to implement in Java than it was in C++, this feature was still considered too complex and thus Java programmers cannot implement their own overloaded operators as you can in C++. The use of the String + has some interesting behavior: if an expression begins with a String, then all operands that follow must be Strings: int x = 0, y = 1, z = 2; String sString = "x, y, z "; System.out.println(sString + x + y + z); Here, the Java compiler will convert x, y, and z into their String representations instead of adding them together first. However, if you say: System.out.println(x + sString); earlier versions of Java will signal an error (later versions, however, will turn x into a String). So if you’re putting together a String with addition, make sure the first element is a String (or a quoted sequence of characters, which the compiler recognizes as a String). Common pitfalls when using operators One of the pitfalls when using operators is trying to get away without parentheses when you are even the least bit uncertain about how an expression will evaluate. This is still true in Java. An extremely common error in C and C++ looks like this: while(x = y) { // .... } The programmer was trying to test for equivalence (==) rather than do an assignment. In C and C++ the result of this assignment will always be true if y is nonzero, and you’ll probably get an infinite loop. In Java, the result of this expression is not a boolean and the compiler expects a boolean and won’t convert from an int, so it will conveniently give you a compile-time error and catch the problem before you ever try to run the program. So the pitfall never happens in Java (the only time you won’t get a compile-time error is when x and y are boolean, in which case x = y is a legal expression, and in the above case, probably an error). Chapter 3: Controlling Program Flow 93
- 94. A similar problem in C/C++ is using bitwise AND and OR instead of logical. Bitwise AND and OR use one of the characters (& or |) while logical AND and OR use two (&& and ||). Just as with = and ==, it’s easy to type just one character instead of two. In Java, this is again prevented by the compiler because it prevents you from cavalierly using one type where it doesn’t belong. Casting operators The word cast is used in the sense of “casting into a mold.” Java will automatically change one type of data into another when appropriate. For instance, if you assign an integral value to a floating-point variable, the compiler will automatically convert the int to a float. Casting allows you to make this type conversion explicit, or to force it when it wouldn’t normally happen. To perform a cast, put the desired data type (including all modifiers) inside parentheses to the left of any value. Here’s an example: void casts() { int i = 200; long l = (long)i; long l2 = (long)200; } As you can see, it’s possible to perform a cast on a numeric value as well as a variable. In both casts shown here, however, the cast is superfluous, since the compiler will automatically promote an int value to a long when necessary. You can still put a cast in to make a point or to make your code clearer. In other situations, a cast is essential just to get the code to compile. In C and C++, casting can cause some headaches. In Java casting is safe, with the exception that when you perform a so-called narrowing conversion (that is, when you go from a data type that can hold more information to one that doesn’t hold as much) you run the risk of losing information. Here the compiler forces you to do a cast, in effect saying: “this can be a dangerous thing to do – if you want me to do it anyway you must make the cast explicit.” With a widening conversion an explicit cast is not needed because the new type will more than hold the information from the old type and thus no information is ever lost. Java allows you to cast any primitive type to any other primitive type, except for boolean which doesn’t allow any casting at all. Class types do not allow casting; to convert one to the other there must be special methods (String is a special case, and you’ll find out later in the book that objects can be cast within a family of types: an Oak can be cast to a Tree and vice-versa, but not to a foreign type like a Rock). Literals Ordinarily when you insert a literal value into a program the compiler knows exactly what type to make it. Sometimes, however, the type is ambiguous. When this happens you must guide the compiler by adding some extra information in the form of characters associated with the literal value. The following code shows these characters: //: Literals.java class Literals { char c = 0xffff; // max char hex value byte b = 0x7f; // max byte hex value short s = 0x7fff; // max short hex value 94 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 95. int i1 = 0x2f; // Hexadecimal (lowercase) int i2 = 0X2F; // Hexadecimal (uppercase) int i3 = 0177; // Octal (leading zero) // Hex and Oct also work with long. long n1 = 200L; // long suffix long n2 = 200l; // long suffix long n3 = 200; //! long l6(200); // not allowed float f1 = 1; float f2 = 1F; // float suffix float f3 = 1f; // float suffix float f4 = 1e-45f; // 10 to the power float f5 = 1e+9f; // float suffix double d1 = 1d; // double suffix double d2 = 1D; // double suffix double d3 = 47e47d; // 10 to the power } ///:~ Hexadecimal (base 16), which works with all the integral data types, is denoted by a leading 0x or 0X followed by 0–9 and a–f either in upper or lower case. If you try to initialize a variable with a value bigger than it can hold (regardless of the numerical form of the value) the compiler will give you an error message. Notice in the above code the maximum possible hexadecimal values for char, byte, and short. If you exceed these the compiler will automatically make the value an int and tell you that you need a narrowing cast for the assignment. You’ll know you’ve stepped over the line. Octal (base 8) is denoted by a leading zero in the number and digits from 0-7. There is no literal representation for binary numbers in C, C++ or Java. A trailing character after a literal value establishes its type. Upper or lowercase L means long, upper or lowercase F means float, and upper or lowercase D means double. Exponents use a notation that I’ve always found rather dismaying: 1.39 e-47f. In science and engineering, ‘e’ refers to Euler's constant which is the base of natural logarithms, approximately 2.718 (a more precise double value is available in Java as Math.E). This is used in exponentiation expressions such as 1.39 x e-47, which means 1.39 x 2.719-47. However, when FORTRAN was invented they decided that e would naturally mean “ten to the power,” which is an odd decision since FORTRAN was designed for science and engineering and one would think its designers would be sensitive about introducing such an ambiguity1 . At any rate, this custom was followed in C, C++ and now Java. So if you’re used to thinking in terms of Euler’s constant, you must do a mental translation when you see an expression like 1.39 e-47f in Java: it means 1.39 x 10-47. 1 John Kirkham writes: “I started computing in 1962 using FORTRAN II on an IBM 1620. At that time and throughout the 1960s and into the 1970s FORTRAN was an all uppercase language. This probably started because many of the early input devices were old teletype units which used 5 bit Baudot code which had no lowercase capability. The ‘E’ in the exponential notation was also always upper case and was never confused with the natural logarithm base ‘e’ which is always lower case. The ‘E’ simply stood for exponential which was for the base of the number system used – usually 10. At the time octal was also widely used by programmers. Although I never saw it used, if I had seen an octal number in exponential notation I would have considered it to be base 8. The first time I remember seeing an exponential using a lower case ‘e’ was in the late 1970's and I had the same confusion you described. The problem arose as lowercase crept into FORTRAN, not at it's beginning. We actually had functions to use if you really wanted to use the natural logarithm base, but they were all uppercase.” Chapter 3: Controlling Program Flow 95
- 96. Notice that you don’t have to use the trailing character when the compiler can figure out the appropriate type. With long n3 = 200; there’s no ambiguity, so an L after the 200 would be superfluous. However, with float f4 = 1e-47f; // 10 to the power the compiler normally takes exponential numbers as doubles, so without the trailing f it will give you an error telling you that you must use a cast to convert double to float. Promotion You’ll discover that if you perform any mathematical or bitwise operations on primitive data types that are smaller than an int (that is, char, byte, or short), those values will be promoted to int before performing the operations, and the resulting value will be of type int. So if you want to assign back into the smaller type, you must use a cast (and, since you’re assigning back into a smaller type, you might be losing information). In general, the largest data type in an expression is the one that determines the size of the result of that expression; if you multiply a float and a double, the result will be double; if you add an int and a long, the result will be long. Java has no “sizeof” In C and C++, the sizeof( ) operator satisfies a specific need: it tells you the number of bytes allocated for data items. The most compelling need for sizeof( ) in C and C++ is for portability. Different data types might be different sizes on different machines, so the programmer must find out how big those types are when performing operations that may be sensitive to size. For example, one computer might store integers in 32 bits, whereas another might store integers as 16 bits, and therefore programs could store larger values in integers on the first machine. As you might imagine, portability is a huge headache for C and C++ programmers. Java does not need a sizeof( ) operator for this purpose because all the data types are the same size on all machines. You do not have to think about portability on this level – it is designed into the language. Precedence revisited Upon hearing me complain about the complexity of remembering operator precedence during one of my seminars, a student suggested a mnemonic that is simultaneously a commentary: “Ulcer Addicts Really Like C A lot.” Mnemonic Operator type Operators Ulcer Unary + - ++ – [[ rest…]] Addicts Arithmetic (and shift) * / % + - << >> Really Relational > < >= <= == != Like Logical (and bitwise) && || & | ^ C Conditional (ternary) A>B?X:Y A Lot Assignment = (and compound assignment like *=) 96 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 97. Of course, with the shift and bitwise operators distributed around the table it is not a perfect mnemonic, but for non-bit operations it works. A compendium of operators The following example shows which primitive data types can be used with particular operators. Basically, it is the same example repeated over and over, but using different primitive data types. The file will compile without error because the lines that would cause errors are commented out with a //!. //: AllOps.java // Tests all the operators on all the // primitive data types to show which // ones are accepted by the Java compiler. class AllOps { // To accept the results of a boolean test: void f(boolean b) {} void boolTest(boolean x, boolean y) { // Arithmetic operators: //! x = x * y; //! x = x / y; //! x = x % y; //! x = x + y; //! x = x - y; //! x++; //! x--; //! x = +y; //! x = -y; // Relational and logical: //! f(x > y); //! f(x >= y); //! f(x < y); //! f(x <= y); f(x == y); f(x != y); f(!y); x = x && y; x = x || y; // Bitwise operators: //! x = ~y; x = x & y; x = x | y; x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: //! x += y; //! x -= y; //! x *= y; //! x /= y; //! x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; Chapter 3: Controlling Program Flow 97
- 98. x &= y; x ^= y; x |= y; // Casting: //! char c = (char)x; //! byte B = (byte)x; //! short s = (short)x; //! int i = (int)x; //! long l = (long)x; //! float f = (float)x; //! double d = (double)x; } void charTest(char x, char y) { // Arithmetic operators: x = (char)(x * y); x = (char)(x / y); x = (char)(x % y); x = (char)(x + y); x = (char)(x - y); x++; x--; x = (char)+y; x = (char)-y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x= (char)~y; x = (char)(x & y); x = (char)(x | y); x = (char)(x ^ y); x = (char)(x << 1); x = (char)(x >> 1); x = (char)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean b = (boolean)x; 98 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 99. byte B = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; } void byteTest(byte x, byte y) { // Arithmetic operators: x = (byte)(x* y); x = (byte)(x / y); x = (byte)(x % y); x = (byte)(x + y); x = (byte)(x - y); x++; x--; x = (byte)+ y; x = (byte)- y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = (byte)~y; x = (byte)(x & y); x = (byte)(x | y); x = (byte)(x ^ y); x = (byte)(x << 1); x = (byte)(x >> 1); x = (byte)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; Chapter 3: Controlling Program Flow 99
- 100. } void shortTest(short x, short y) { // Arithmetic operators: x = (short)(x * y); x = (short)(x / y); x = (short)(x % y); x = (short)(x + y); x = (short)(x - y); x++; x--; x = (short)+y; x = (short)-y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = (short)~y; x = (short)(x & y); x = (short)(x | y); x = (short)(x ^ y); x = (short)(x << 1); x = (short)(x >> 1); x = (short)(x >>> 1); // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; byte B = (byte)x; int i = (int)x; long l = (long)x; float f = (float)x; double d = (double)x; } void intTest(int x, int y) { // Arithmetic operators: x = x * y; x = x / y; 100 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 101. x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = ~y; x = x & y; x = x | y; x = x ^ y; x = x << 1; x = x >> 1; x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; byte B = (byte)x; short s = (short)x; long l = (long)x; float f = (float)x; double d = (double)x; } void longTest(long x, long y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; Chapter 3: Controlling Program Flow 101
- 102. x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: x = ~y; x = x & y; x = x | y; x = x ^ y; x = x << 1; x = x >> 1; x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; x <<= 1; x >>= 1; x >>>= 1; x &= y; x ^= y; x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; byte B = (byte)x; short s = (short)x; int i = (int)x; float f = (float)x; double d = (double)x; } void floatTest(float x, float y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); 102 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 103. f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); // Bitwise operators: //! x = ~y; //! x = x & y; //! x = x | y; //! x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; //! x &= y; //! x ^= y; //! x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; byte B = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; double d = (double)x; } void doubleTest(double x, double y) { // Arithmetic operators: x = x * y; x = x / y; x = x % y; x = x + y; x = x - y; x++; x--; x = +y; x = -y; // Relational and logical: f(x > y); f(x >= y); f(x < y); f(x <= y); f(x == y); f(x != y); //! f(!x); //! f(x && y); //! f(x || y); Chapter 3: Controlling Program Flow 103
- 104. // Bitwise operators: //! x = ~y; //! x = x & y; //! x = x | y; //! x = x ^ y; //! x = x << 1; //! x = x >> 1; //! x = x >>> 1; // Compound assignment: x += y; x -= y; x *= y; x /= y; x %= y; //! x <<= 1; //! x >>= 1; //! x >>>= 1; //! x &= y; //! x ^= y; //! x |= y; // Casting: //! boolean b = (boolean)x; char c = (char)x; byte B = (byte)x; short s = (short)x; int i = (int)x; long l = (long)x; float f = (float)x; } } ///:~ Note that boolean is very limited: you can assign to it the values true and false, and you can test it for truth or falsehood. However, you cannot add booleans or perform any other type of operation on them. In char, byte, and short you can see the effect of promotion with the arithmetic operators: each arithmetic operation on any of those types results in an int result, which must be explicitly cast back to the original type (a narrowing conversion which might lose information) to assign back to that type. With int values, however, you do not need to cast, because everything is already an int. Don’t be lulled into thinking everything is safe, though: if you multiply two ints that are big enough, you’ll overflow the result. The following example demonstrates this: //: Overflow.java // Surprise! Java lets you overflow. public class Overflow { public static void main(String args[]) { int big = 0x7fffffff; // max int value prt("big = " + big); int bigger = big * 4; prt("bigger = " + bigger); } static void prt(String s) { System.out.println(s); } 104 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 105. } ///:~ The output of this is: big = 2147483647 bigger = -4 and you get no errors or warnings from the compiler, and no exceptions at run-time. Java is good, but it’s not that good. Compound assignments do not require casts for char, byte, or short, even though they are performing promotions that have the same results as the direct arithmetic operations. On the other hand, the lack of the cast certainly simplifies the code. You can see that, with the exception of boolean, any primitive type can be cast to any other primitive type. Again, you must be aware of the effect of a narrowing conversion when casting to a smaller type, otherwise you might unknowingly lose information during the cast. Execution control Java uses all C’s execution control statements, so if you’ve programmed with C or C++ then most of what you see will be familiar. Most procedural programming languages have some kind of control statements, and there is often overlap among languages. In Java, the keywords include if-else, while, do-while, for, and a selection statement called switch. Java does not, however, support the much-maligned goto (which can still be the most expedient way to solve certain types of problems). You can still do a goto-like jump but it is much more constrained than a typical goto. True and false All conditional statements use the truth or falsehood of a conditional expression to determine the execution path. An example of a conditional expression is A == B. This uses the conditional operator == to see if the value of A is equivalent to the value of B. The expression returns true or false. Any of the relational operators you’ve seen earlier in this chapter can be used to produce a conditional statement. Note that Java doesn’t allow you to use a number as a boolean, even though it’s allowed in C and C++ (where truth is nonzero and falsehood is zero). If you want to use a non-boolean in a boolean test, such as if(a), you must first convert it to a boolean value using a conditional expression, such as if(a != 0). If-else The if-else statement is probably the most basic way to control program flow. The else is optional, so you can use if in two forms: if(Boolean-expression) statement or if(Boolean-expression) statement else statement Chapter 3: Controlling Program Flow 105
- 106. The conditional must produce a Boolean result. The statement means either a simple statement terminated by a semicolon or a compound statement, which is a group of simple statements enclosed in braces. Anytime the word “statement” is used, it always implies that the statement can be simple or compound. As an example of if-else, here is a test( ) method that will tell you whether a guess is above, below, or equivalent to a target number: static int test(int testval) { int result = 0; if(testval > target) result = -1; else if(testval < target) result = +1; else result = 0; // match return result; } It is conventional to indent the body of a control flow statement so the reader might easily determine where it begins and ends. return The return keyword has two purposes: it specifies what value a method will return (if it doesn’t have a void return value) and it also causes that value to be returned immediately. The test( ) method above can be rewritten to take advantage of this: static int test2(int testval) { if(testval > target) return -1; if(testval < target) return +1; return 0; // match } There’s no need for else because the method will not continue after executing a return. Iteration while, do-while and for control looping, and are sometimes classified as iteration statements. A statement repeats until the controlling Boolean-expression evaluates to false. The form for a while loop is while(Boolean-expression) statement The Boolean-expression is evaluated once at the beginning of the loop, and again before each further iteration of the statement. Here’s a simple example that generates random numbers until a particular condition is met: //: WhileTest.java // Demonstrates the while loop public class WhileTest { public static void main(String args[]) { 106 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 107. double r = 0; while(r < 0.99d) { r = Math.random(); System.out.println(r); } } } ///:~ This uses the static method random( ) in the Math library, which generates a double value between 0 and 1 (it includes 0, but not 1). The conditional expression for the while says “keep doing this loop until the number is 0.99 or greater.” Each time you run this program you’ll get a different-sized list of numbers. Do-while The form for do-while is do statement while(Boolean-expression); The only difference between while and do-while is that the statement of the do-while always executes at least once, even if the expression evaluates to false the first time. In a while, if the conditional is false the first time the statement never executes. In practice, do-while is less common than while. For A for loop performs initialization before the first iteration. Then it performs conditional testing and, at the end of each iteration, some form of “stepping.” The form of the for loop is: for(initialization; Boolean-expression; step) statement Any of the expressions initialization, Boolean-expression, or step may be empty. The expression is tested before each iteration, and as soon as it evaluates to false execution will continue at the line following the for statement. At the end of each loop, the step executes. for loops are usually used for “counting” tasks: //: ListCharacters.java // Demonstrates "for" loop by listing // all the ASCII characters. public class ListCharacters { public static void main(String args[]) { for( char c = 0; c < 128; c++) if (c != 26 ) // ANSI Clear screen System.out.println( "value: " + (int)c + " character: " + c); } } ///:~ Chapter 3: Controlling Program Flow 107
- 108. Notice that the variable c is defined at the point where it is used, inside the control expression of the for loop, rather than the beginning of the block denoted by the open curly brace. The scope of c is the expression controlled by the for. Traditional procedural languages like C require that all variables be defined at the beginning of a block so when the compiler creates a block it can allocate space for those variables. In Java and C++ you can spread your variable declarations throughout the block, defining them at the point that you need them. This allows a more natural coding style and makes code easier to understand. You can define multiple variables within a for statement, but they must be of the same type: for(int i = 0, j = 1; i < 10 && j != 11; i++, j++) /* body of for loop */; The ability to define variables in the control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements. The comma operator Earlier in this chapter I stated that the comma operator (not the comma separator, which is used to separate function arguments) has only one use in Java: in the control expression of a for loop. In both the initialization and step portions of the control expression, you can have a number of statements separated by commas, and those statements will be evaluated sequentially. The previous bit of code uses this ability. Here’s another example: //: CommaOperator.java public class CommaOperator { public static void main(String args[]) { for(int i = 1, j = i + 10; i < 5; i++, j = i * 2) { System.out.println("i= " + i + " j= " + j); } } } ///:~ Here’s the output: i= 1 j= 11 i= 2 j= 4 i= 3 j= 6 i= 4 j= 8 You can see that in both the initialization and step portions the statements are evaluated in sequential order. Also, the initialization portion can have any number of definitions of one type. Break and continue Inside the body of any of the iteration statements you can also control the flow of the loop using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin a new iteration. 108 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 109. This program shows examples of break and continue within for and while loops: //: BreakAndContinue.java // Demonstrates break and continue keywords public class BreakAndContinue { public static void main(String args[]) { for(int i = 0; i < 100; i++) { if(i == 74) break; // Out of for loop if(i % 9 != 0) continue; // Next iteration System.out.println(i); } int i = 0; // An "infinite loop": while(true) { i++; int j = i * 27; if(j == 1269) break; // Out of loop if(i % 10 != 0) continue; // Top of loop System.out.println(i); } } } ///:~ In the for loop the value of i never gets to 100 because the break statement breaks out of the loop when i is 74. Normally you’d use a break like this only if you didn’t know when the terminating condition was going to occur. The continue statement causes execution to go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly divisible by 9. When it is, the value is printed. The second portion shows an “infinite loop” that would, in theory, continue forever. However, inside the loop there is a break statement that will break out of the loop. In addition, you’ll see that the continue moves back to the top of the loop without completing the remainder (thus printing happens only when the value of i is divisible by 10). The output is: 0 9 18 27 36 45 54 63 72 10 20 30 40 The value 0 is printed because 0 % 9 produces 0. A second form of the infinite loop is for(;;). The compiler treats both while(true) and for(;;) in the same way so which one you use is a matter of programming taste. Chapter 3: Controlling Program Flow 109
- 110. The infamous “goto” The goto keyword has been present in programming languages from the beginning; indeed, goto was the genesis of program control in assembly language: “if condition A, then jump here, otherwise jump there.” If you read the assembly code that is ultimately generated by virtually any compiler, you’ll see that program control contains many jumps. However, goto jumps at the source-code level, and that’s what brought it into disrepute: if a program will always jump from one point to another, isn’t there some way to reorganize the code so the flow of control is not so jumpy? goto fell into true disfavor with the publication of the famous “Goto considered harmful” paper by Edsger Dijkstra, and since then goto-bashing has been a popular sport, with advocates of the cast-out keyword scurrying for cover. As is typical in situations like this, the middle ground is the most fruitful. The problem is not the use of goto but the overuse of goto, and in rare situations goto is the best way to structure control flow. Although goto is a reserved word in Java, it is not used in the language; Java has no goto. However, it does have something that looks a bit like a jump tied in with the break and continue keywords. It’s not a jump but rather a way to break out of an iteration statement. The reason it’s often thrown in with discussions of goto is because it uses the same mechanism: a label. A label is an identifier followed by a colon, like this: label1: The only place a label is useful in Java is right before an iteration statement. And that means right before – it does no good to put any other statement between the label and the iteration. And the only reason to put a label before an iteration is if you’re going to nest another iteration or a switch inside it. That’s because the break and continue keywords will normally interrupt the current loop only, but when used with a label they’ll interrupt the loops up to where the label exists: label1: outer-iteration { inner-iteration { //… break; // 1 //… continue; // 2 //… continue label1; // 3 //… break label1; // 4 } } In case 1, the break breaks out of the inner iteration and you end up in the outer iteration. In case 2, the continue moves back to the beginning of the inner iteration. But in case 3, the continue label1 breaks out of the inner iteration and the outer iteration, all the way back to label1. Then it does in fact continue the iteration, but starting at the outer iteration. In case 4, the break label1 also breaks all the way out to label1, but it does not re-enter the iteration. It actually does break out of both iterations. Here is an example using for loops: //: LabeledFor.java 110 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 111. // Java’s "labeled for loop" public class LabeledFor { public static void main(String args[]) { int i = 0; outer: // Can't have statements here for(; true ;) { // infinite loop inner: // Can't have statements here for(; i < 10; i++) { prt("i = " + i); if(i == 2) { prt("continue"); continue; } if(i == 3) { prt("break"); i++; // Otherwise i never // gets incremented. break; } if(i == 7) { prt("continue outer"); i++; // Otherwise i never // gets incremented. continue outer; } if(i == 8) { prt("break outer"); break outer; } for(int k = 0; k < 5; k++) { if(k == 3) { prt("continue inner"); continue inner; } } } } // Can't break or continue // to labels here } static void prt(String s) { System.out.println(s); } } ///:~ This uses the prt( ) method that has been defined in the other examples. Notice that break breaks out of the for loop, and that the increment-expression doesn’t occur until the end of the pass through the for loop. Since break skips the increment expression, the increment is performed by hand in the case of i == 3. The continue outer statement in the case of I == 7 also goes to the top of the loop and also skips the increment, so it too is incremented by hand. Here is the output: i = 0 Chapter 3: Controlling Program Flow 111
- 112. continue inner i = 1 continue inner i = 2 continue i = 3 break i = 4 continue inner i = 5 continue inner i = 6 continue inner i = 7 continue outer i = 8 break outer If not for the break outer statement, there would be no way to get out of the outer loop from within an inner loop, since break by itself can break out of only the innermost loop (the same is true for continue). Here is a demonstration of labeled break and continue statements with while loops: //: LabeledWhile.java // Java's "labeled while" loop public class LabeledWhile { public static void main(String args[]) { int i = 0; outer: while(true) { prt("Outer while loop"); while(true) { i++; prt("i = " + i); if(i == 1) { prt("continue"); continue; } if(i == 3) { prt("continue outer"); continue outer; } if(i == 5) { prt("break"); break; } if(i == 7) { prt("break outer"); break outer; } } } } static void prt(String s) { 112 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 113. System.out.println(s); } } ///:~ The same rules hold true for while: 1. A plain continue goes to the top of the innermost loop and continues 2. A labeled continue goes to the label and re-enters the loop right after that label 3. A break “drops out of the bottom” of the loop 4. A labeled break drops out of the bottom of the end of the loop denoted by the label. The output of this method makes it clear: Outer while loop i = 1 continue i = 2 i = 3 continue outer Outer while loop i = 4 i = 5 break Outer while loop i = 6 i = 7 break outer It’s important to remember that the only reason to use labels in Java is if you have nested loops, and you want to break or continue through more than one nested level. To revisit Dijkstra’s “goto considered harmful” paper, what he specifically objected to was the labels rather than the goto itself. His observation was that the number of bugs seems to increase with the number of labels in a program. Labels and gotos make programs difficult to analyze statically, since it introduces cycles in the program execution graph. Note that Java labels don't suffer from this problem, since they are constrained as to their placement and can't be used to transfer control in an ad hoc manner. It’s also interesting to note that this is a case where a language feature is made more useful by restricting the power of the statement. Switch The switch is sometimes classified as a selection statement. The switch statement selects from among pieces of code based on the value of an integral expression. Its form is: switch(integral-selector) { case integral-value1 : statement; break; case integral-value2 : statement; break; case integral-value3 : statement; break; case integral-value4 : statement; break; case integral-value5 : statement; break; // … Chapter 3: Controlling Program Flow 113
- 114. default: statement; } Integral-selector is an expression that produces an integral value. The switch compares the result of integral-selector to each integral-value. If it finds a match, the corresponding statement (simple or compound) executes. If no match occurs, the default statement executes. You will notice in the above definition that each case ends with a break, which causes execution to jump to the end of the switch body. This is the conventional way to build a switch statement, but the break is optional. If it is missing, the code for the following case statements execute until a break is encountered. Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer. Notice the last statement, for the default, doesn’t have a break because the execution just falls through to where the break would have taken it anyway. You could put a break at the end of the default statement with no harm, if you considered it important for style’s sake. The switch statement is a very clean way to implement multi-way selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value such as int or char. If you want to use, for example, a string or a floating-point number as a selector, it won’t work in a switch statement. For non- integral types, you must use a series of if statements. Here’s an example that randomly creates letters and determines whether they’re vowels or consonants: //: VowelsAndConsonants.java // Demonstrates the switch statement public class VowelsAndConsonants { public static void main(String args[]) { for(int i = 0; i < 100; i++) { char c = (char)(Math.random() * 26 + 'a'); System.out.print(c + ": "); switch(c) { case 'a': case 'e': case 'i': case 'o': case 'u': System.out.println("vowel"); break; case 'y': case 'w': System.out.println( "Sometimes a vowel"); break; default: System.out.println("consonant"); } } } } ///:~ Since Math.random( ) generates a value between 0 and 1, you need only to multiply it by the upper bound of the range of numbers you want to produce (26 for the letters in the alphabet) and add an offset to establish the lower bound. 114 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 115. Although it appears you’re switching on a character here, the switch statement is actually using the integral value of the character. The singly-quoted characters in the case statements also produce integral values which are used for comparison. Notice how the cases can be “stacked” on top of each other to provide multiple matches for a particular piece of code. You should also be aware that it’s essential to put the break statement at the end of a particular case, otherwise control will simply drop through and continue processing on the next case. Calculation details The statement: char c = (char)(Math.random() * 26 + 'a'); deserves a closer look. Math.random( ) produces a double, so the value 26 is converted to a double to perform the multiplication, which also produces a double. This means that ‘a’ must be converted to a double to perform the addition. The double result is turned back into a char with a cast. First, what does the cast to char do? That is, if you have the value 29.7 and you cast it to a char, is the resulting value 30 or 29? The answer to this can be seen with an example: //: CastingNumbers.java // What happens when you cast a float or double // to an integral value? public class CastingNumbers { public static void main(String args[]) { double above = 0.7, below = 0.4; System.out.println("above: " + above); System.out.println("below: " + below); System.out.println( "(int)above: " + (int)above); System.out.println( "(int)below: " + (int)below); System.out.println( "(char)('a' + above): " + (char)('a' + above)); System.out.println( "(char)('a' + below): " + (char)('a' + below)); } } ///:~ The output is: above: 0.7 below: 0.4 (int)above: 0 (int)below: 0 (char)('a' + above): a (char)('a' + below): a So the answer is that casting from a float or double to an integral value always truncates. Chapter 3: Controlling Program Flow 115
- 116. The second question has to do with Math.random( ). Does it produce a value from zero to one, inclusive or exclusive of the value ‘1’? In math lingo, is it (0,1) or [0,1] or (0,1] or [0,1)? (The square bracket means “includes” whereas the parenthesis means “doesn’t include”). Again, a test program provides the answer: //: RandomBounds.java // Does Math.random() produce 0.0 and 1.0? public class RandomBounds { static void usage() { System.err.println("Usage: nt" + "RandomBounds lowernt" + "RandomBounds upper"); System.exit(1); } public static void main(String args[]) { if(args.length != 1) usage(); if(args[0].equals("lower")) { while(Math.random() != 0.0) ; // Keep trying System.out.println("Produced 0.0!"); } else if(args[0].equals("upper")) { while(Math.random() != 1.0) ; // Keep trying System.out.println("Produced 1.0!"); } else usage(); } } ///:~ To run the program, you type a command line of either: java RandomBounds lower or java RandomBounds upper In both cases, you are forced to break out of the program manually, so it would appear that Math.random( ) never produces either 0.0 or 1.0. But this is where such an experiment can be deceiving. If you consider that there are 2128 different double fractions between 0 and 1, the likelihood of reaching any one value experimentally might exceed the lifetime of one computer, or even one experimenter. It turns out that 0.0 is included in the output of Math.random( ). Or, in math lingo, it is [0,1). Summary This chapter concludes the study of fundamental features that appear in most programming languages: calculation, operator precedence, type casting, and selection and iteration. Now you’re ready to begin taking steps that move you closer to the world of object-oriented programming. In the next chapter, the important issues of initialization and cleanup of objects will be covered, followed in the subsequent chapter by the essential concept of implementation hiding. 116 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 117. Exercises 1. Write a program that prints values from one to 100. 2. Modify Exercise 1 so the program exits by using the break keyword at value 47. Try using return instead. 3. Create a switch statement that prints a message for each case, and put the switch inside a for loop that tries each case. Put a break after each case and test it, then remove the breaks and see what happens. Chapter 3: Controlling Program Flow 117
- 119. o 4: Initialization & cleanup As the computer revolution progresses, “unsafe” programming has become one of the major culprits that makes programming expensive. Two of these safety issues are initialization and cleanup. Many C bugs occur when the programmer forgets to initialize a variable. This is especially true with libraries, when users don’t know how to initialize a library component, or even that they must. Cleanup is a special problem because it’s easy to forget about an element when you’re done with it, since it no longer concerns you. Thus, the resources used by that element are still retained, and you can easily end up running out of resources (most notably memory). C++ introduced the concept of a constructor, a special method automatically called when an object is created. Java also adopted the constructor, and in addition has a garbage collector that automatically releases memory resources when they’re no longer being used. This chapter examines the issues of initialization and cleanup and their support in Java. Guaranteed initialization with the constructor You can imagine creating a method called initialize( ) for every class you write. The name is a hint that it should be called before using the object. Unfortunately, this means the user must remember to call the method. In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a 119
- 120. constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed. The next challenge is what to name this method. There are two issues. The first is that any name you use could clash with a name you might like to use as a member in the class. The second is that because the compiler is responsible for calling the constructor, it must always know which method to call. The C++ solution seems the easiest and most logical, so it’s also used in Java: The name of the constructor is the same as the name of the class. It makes sense that such a method will be called automatically on initialization. Here’s a simple class with a constructor (see page 80 if you have trouble executing this program): //: SimpleConstructor.java // Demonstration of a simple constructor package c04; class Rock { Rock() { // This is the constructor System.out.println("Creating Rock"); } } public class SimpleConstructor { public static void main(String args[]) { for(int i = 0; i < 10; i++) new Rock(); } } ///:~ Now, when an object is created: new Rock(); storage is allocated and the constructor is called. It is guaranteed that the object will be properly initialized before you can get your hands on it. Note that the coding style of making the first letter of all methods lower case does not apply to constructors, since the name of the constructor must match the name of the class exactly. Like any method, the constructor can have arguments to allow you to specify how an object is created. The above example can easily be changed so the constructor takes an argument: class Rock { Rock(int i) { System.out.println( "Creating Rock number " + i); } } public class SimpleConstructor { public static void main(String args[]) { for(int i = 0; i < 10; i++) new Rock(i); } } 120 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 121. Constructor arguments provide you with a way to provide parameters for the initialization of an object. For example, if the class Tree has a constructor that takes a single integer argument denoting the height of the tree, you would create a Tree object like this: Tree t = new Tree(12); // 12-foot tree If Tree(int) is your only constructor, then the compiler won’t let you create a Tree object any other way. Constructors eliminate a large class of problems and make the code easier to read. In the preceding code fragment, for example, you don’t see an explicit call to some initialize( ) method that is conceptually separate from definition. In Java, definition and initialization are unified concepts – you can’t have one without the other. The constructor is a very unusual type of method: it has no return value. This is distinctly different from a void return value, where the method returns nothing but you still have the option to make it return something else. Constructors return nothing and you don’t have an option. If there were a return value, and if you could select your own, the compiler would somehow have to know what to do with that return value. Method overloading One of the important features in any programming language is the use of names. When you create an object, you give a name to a region of storage. A method is a name for an action. By using names to describe your system, you create a program that is easier for people to understand and change. It’s a lot like writing prose – the goal is to communicate with your readers. You refer to all objects and methods by using names. Well-chosen names make it easier for you and others to understand your code. A problem arises when mapping the concept of nuance in human language onto a programming language. Often, the same word expresses a number of different meanings – it’s overloaded. This is very useful, especially when it comes to trivial differences. You say “wash the shirt,” “wash the car,” and “wash the dog.” It would be silly to be forced to say, “shirtWash the shirt,” “carWash the car,” and “dogWash the dog” just so the listener doesn’t have to make any distinction about the action performed. Most human languages are redundant, so even if you miss a few words, you can still determine the meaning. We don’t need unique identifiers – we can deduce meaning from context. Most programming languages (C in particular) require that you have a unique identifier for each function. Thus you could not have one function called print( ) for printing integers and another called print( ) for printing floats – each function requires a unique name. In Java, another factor forces the overloading of method names: the constructor. Because the constructor’s name is predetermined by the name of the class, there can be only one constructor name. But what if you want to create an object in more than one way? For example, suppose you build a class that can initialize itself in a standard way and also by reading information from a file. You need two constructors, one that takes no arguments (the default constructor) and one that takes a String as an argument, which is the name of the file from which to initialize the object. Both are constructors, so they must have the same name – the name of the class. Thus method overloading is essential to allow the same method name to be used with different argument types. And although method overloading is a must for constructors, it’s a general convenience and can be used with any method. Chapter 4: Initialization & Cleanup 121
- 122. Here’s an example that shows both overloaded constructors and overloaded ordinary methods: //: Overloading.java // Demonstration of both constructor // and ordinary method overloading. import java.util.*; class Tree { int height; Tree() { prt("Planting a seedling"); height = 0; } Tree(int i) { prt("Creating new Tree that is " + i + " feet tall"); height = i; } void info() { prt("Tree is " + height + " feet tall"); } void info(String s) { prt(s + ": Tree is " + height + " feet tall"); } static void prt(String s) { System.out.println(s); } } public class Overloading { static Random rand = new Random(); static int pRand(int mod) { return Math.abs(rand.nextInt()) % mod; } public static void main(String args[]) { for(int i = 0; i < 5; i++) { Tree t = new Tree(i); t.info(); t.info("overloaded method"); } // Overloaded constructor: new Tree(); } } ///:~ A Tree object may be created either as a seedling, with no argument, or as grown in a nursery, with an existing height. To support this, there are two constructors, one that 122 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 123. takes no arguments (we call constructors that take no arguments default constructors1) and one that takes the existing height. You may also want to call the info( ) method in more than one way: with a String argument if you have an extra message you want printed, and without if you have nothing more to say. It would seem strange to have to give two separate names to what is obviously the same concept. Fortunately, method overloading allows you to use the same name for both. Distinguishing overloaded methods If the methods have the same name how can Java know which method you mean? There’s a very simple rule: Each overloaded method must take a unique list of argument types. If you think about this for a second, it makes sense: how else could you as a programmer tell the difference between two methods that have the same name, other than by the types of their arguments? Even differences in the ordering of arguments is sufficient to distinguish two methods: //: OverloadingOrder.java // Overloading based on the order of // the arguments. public class OverloadingOrder { static void print(String s, int i) { System.out.println( "String: " + s + ", int: " + i); } static void print(int i, String s) { System.out.println( "int: " + i + ", String: " + s); } public static void main(String args[]) { print("String first", 11); print(99, "Int first"); } } ///:~ The two print( ) methods have identical arguments, but the order is different, and that’s what makes them distinct. Overloading with primitives Primitives can be automatically promoted from a smaller type to a larger one and this can be slightly confusing in combination with overloading. The following example demonstrates what happens when a primitive is handed to an overloaded method: //: PrimitiveOverloading.java 1 In some of the Java literature from Sun they instead refer to these with the clumsy but descriptive name “no-arg constructors.” The term “default constructor” has been in use for many years and so I shall use that. Chapter 4: Initialization & Cleanup 123
- 124. // Promotion of primitives and overloading public class PrimitiveOverloading { // boolean can't be automatically converted static void prt(String s) { System.out.println(s); } void f1(char x) { prt("f1(char)"); } void f1(byte x) { prt("f1(byte)"); } void f1(short x) { prt("f1(short)"); } void f1(int x) { prt("f1(int)"); } void f1(long x) { prt("f1(long)"); } void f1(float x) { prt("f1(float)"); } void f1(double x) { prt("f1(double)"); } void f2(byte x) { prt("f2(byte)"); } void f2(short x) { prt("f2(short)"); } void f2(int x) { prt("f2(int)"); } void f2(long x) { prt("f2(long)"); } void f2(float x) { prt("f2(float)"); } void f2(double x) { prt("f2(double)"); } void f3(short x) { prt("f3(short)"); } void f3(int x) { prt("f3(int)"); } void f3(long x) { prt("f3(long)"); } void f3(float x) { prt("f3(float)"); } void f3(double x) { prt("f3(double)"); } void f4(int x) { prt("f4(int)"); } void f4(long x) { prt("f4(long)"); } void f4(float x) { prt("f4(float)"); } void f4(double x) { prt("f4(double)"); } void f5(long x) { prt("f5(long)"); } void f5(float x) { prt("f5(float)"); } void f5(double x) { prt("f5(double)"); } void f6(float x) { prt("f6(float)"); } void f6(double x) { prt("f6(double)"); } void f7(double x) { prt("f7(double)"); } void testConstVal() { prt("Testing with 5"); f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5); } void testChar() { char x = 'x'; prt("char argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testByte() { byte x = 0; prt("byte argument:"); 124 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 125. f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testShort() { short x = 0; prt("short argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testInt() { int x = 0; prt("int argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testLong() { long x = 0; prt("long argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testFloat() { float x = 0; prt("float argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } void testDouble() { double x = 0; prt("double argument:"); f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x); } public static void main(String args[]) { PrimitiveOverloading p = new PrimitiveOverloading(); p.testConstVal(); p.testChar(); p.testByte(); p.testShort(); p.testInt(); p.testLong(); p.testFloat(); p.testDouble(); } } ///:~ If you view the output of this program, you’ll see that the constant value 5 is treated as an int, so if an overloaded method is available that takes an int it is used. In all other cases, if you have a data type that is smaller than the argument in the method, that data type is promoted. char produces a slightly different effect, since if it doesn’t find an exact char match, it is promoted to int. What happens if your argument is bigger than the argument expected by the overloaded method? A modification of the above program gives the answer: //: Demotion.java // Demotion of primitives and overloading public class Demotion { static void prt(String s) { System.out.println(s); } Chapter 4: Initialization & Cleanup 125
- 126. void f1(char x) { prt("f1(char)"); } void f1(byte x) { prt("f1(byte)"); } void f1(short x) { prt("f1(short)"); } void f1(int x) { prt("f1(int)"); } void f1(long x) { prt("f1(long)"); } void f1(float x) { prt("f1(float)"); } void f1(double x) { prt("f1(double)"); } void f2(char x) { prt("f2(char)"); } void f2(byte x) { prt("f2(byte)"); } void f2(short x) { prt("f2(short)"); } void f2(int x) { prt("f2(int)"); } void f2(long x) { prt("f2(long)"); } void f2(float x) { prt("f2(float)"); } void f3(char x) { prt("f3(char)"); } void f3(byte x) { prt("f3(byte)"); } void f3(short x) { prt("f3(short)"); } void f3(int x) { prt("f3(int)"); } void f3(long x) { prt("f3(long)"); } void f4(char x) { prt("f4(char)"); } void f4(byte x) { prt("f4(byte)"); } void f4(short x) { prt("f4(short)"); } void f4(int x) { prt("f4(int)"); } void f5(char x) { prt("f5(char)"); } void f5(byte x) { prt("f5(byte)"); } void f5(short x) { prt("f5(short)"); } void f6(char x) { prt("f6(char)"); } void f6(byte x) { prt("f6(byte)"); } void f7(char x) { prt("f7(char)"); } void testDouble() { double x = 0; prt("double argument:"); f1(x);f2((float)x);f3((long)x);f4((int)x); f5((short)x);f6((byte)x);f7((char)x); } public static void main(String args[]) { Demotion p = new Demotion(); p.testDouble(); } } ///:~ Here, the methods take narrower primitive values. If your argument is wider then you must cast to the necessary type using the type name in parentheses. If you don’t do this the compiler will issue an error message. You should be aware that this is a narrowing conversion which means that you might lose information during the cast. This is why the compiler forces you to do it – to flag the narrowing conversion. 126 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 127. Overloading on return values It is common to wonder “Why only class names and method argument lists? Why not distinguish between methods based on their return values?” For example, these two methods, which have the same name and arguments, are easily distinguished from each other: void f() {} int f() {} This works fine when the compiler can unequivocally determine the meaning from the context, as in int x = f( ). However, you can call a method and ignore the return value; this is often referred to as calling a method for its side effect since you don’t care about the return value but instead want the other effects of the method call. So if you call the method this way: f(); how can Java determine which f() should be called? And how could someone reading the code see it? Because of this sort of problem, you cannot use return value types to distinguish overloaded methods. Default constructors As mentioned previously, a default constructor is one without arguments, used to create a “vanilla object.” If you create a class that has no constructors, the compiler will automatically create a default constructor for you. For example: //: DefaultConstructor.java class Bird { int i; } public class DefaultConstructor { public static void main(String args[]) { Bird nc = new Bird(); // default! } } ///:~ The line new Bird(); Creates a new object and calls the default constructor, even though one was not explicitly defined. Without it we would have no method to call to build our object. However, if you define any constructors (with or without arguments), the compiler will not synthesize one for you: class Bush { Bush(int i) {} Bush(double d) {} } Now if you say: new Bush(); Chapter 4: Initialization & Cleanup 127
- 128. The compiler will complain that it cannot find a constructor that matches. It’s as if when you don’t put in any constructors, it says: “you are bound to need some constructor, so let me make one for you.” But if you write a constructor, it says “you’ve written a constructor so you know what you’re doing; if you didn’t put in a default it’s because you meant to leave it out.” The this keyword If you have two objects of the same type called a and b, you might wonder how it is you can call a method f( ) for both those objects: class Banana { void f(int i) { /* ... */ } } Banana a = new Banana(), b = new Banana(); a.f(1); b.f(2); If there’s only one method called f( ), how can that method know whether it’s being called for the object a or b? To allow you to write the code in a convenient object-oriented syntax where you’re “sending a message to an object,” the compiler does some work for you under the covers. There’s a secret first argument passed to the method f( ), and that argument is the handle to the object that’s being manipulated. So the two method calls above become something like: Banana.f(a,1); Banana.f(b,2); This is internal and you can’t write these expressions and get the compiler to accept them, but it gives you an idea of what’s happening. Now suppose you’re inside a method and you’d like to get the handle to the current object. Since that handle is passed secretly by the compiler, there’s no identifier for it. However, for this purpose there’s a keyword: this. The this keyword – which can be used only inside a method – produces the handle to the object the method has been called for. You can treat this handle just like any other object handle. Keep in mind that if you’re calling a method of your class from within another method of your class, you don’t need to use this – you simply call the method. The current this handle is automatically used for the other method. Thus you can say: class Apricot { void pick() { /* ... */ } void pit() { pick(); /* ... */ } } Inside pit( ), you could say this.pick( ) but there’s no need to – the compiler does it for you automatically. Thus the this keyword is used only for special cases when you need to explicitly use the handle to the current object. For example, it’s often used in return statements when you want to return the handle to the current object: //: Leaf.java // Simple use of the "this" keyword public class Leaf { private int i = 0; Leaf increment() { i++; 128 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 129. return this; } void print() { System.out.println("i = " + i); } public static void main(String args[]) { Leaf x = new Leaf(); x.increment().increment().increment().print(); } } ///:~ Because increment( ) returns the handle to the current object via the this keyword, multiple operations can easily be performed on the same object. Calling constructors from constructors When you write several constructors for a class, there are times when you’d like to call one constructor from another, to avoid duplicating code. You can do this using the this keyword. Normally, when you say this, it is in the sense of “this object” or “the current object,” and by itself it produces the handle to the current object. In a constructor, the this keyword takes on a different meaning when you give it an argument list: it makes an explicit call to the constructor that matches that argument list. Thus you have a straightforward way to call other constructors: //: Flower.java // Calling constructors with "this" public class Flower { private int petalCount = 0; private String s = new String("null"); Flower(int petals) { petalCount = petals; System.out.println( "Constructor w/ int arg only, petalCount= " + petalCount); } Flower(String ss) { System.out.println( "Constructor w/ String arg only, s=" + ss); s = ss; } Flower(String s, int petals) { this(petals); //! this(s); // Can't call two! this.s = s; // Another use of "this" System.out.println("String & int args"); } Flower() { this("hi", 47); System.out.println( "default constructor (no args)"); } void print() { //! this(11); // Not inside non-constructor! System.out.println( "petalCount = " + petalCount + " s = "+ s); Chapter 4: Initialization & Cleanup 129
- 130. } public static void main(String args[]) { Flower x = new Flower(); x.print(); } } ///:~ The constructor Flower(String s, int petals) shows that, while you can call one constructor using this, you cannot call two. In addition, the constructor call must be the first thing you do, otherwise you’ll get a compiler error message. This example also shows another way you’ll see this used. Since the name of the argument s and the name of the member data s are the same, there’s an ambiguity. You can resolve it by saying this.s to refer to the member data. You’ll often see this form used in Java code, and it’s used in numerous places in this book. In print( ) you can see that the compiler won’t let you call a constructor from inside any method other than a constructor. The meaning of static With the this keyword in mind, you can more fully understand what it means to make a method static. It means there is no this for that particular method. Thus you cannot call non-static methods from inside static methods2 (although the reverse is possible), and it’s possible to call a static method for the class itself, without any object. In fact, that’s primarily what a static method is for. It’s as if you’re creating the equivalent of a global function (from C). Except that global functions are not permitted in Java, and putting the static method inside a class allows it access to other static methods and to static fields. Some people argue that static methods are not object-oriented, since they do have the semantics of a global function – with a static method you don’t send a message to an object, since there’s no this. This is probably a fair argument, and if you find yourself using a lot of static methods you should probably consider rethinking your strategy. However, statics are pragmatic and there are times you genuinely need them so whether or not they are “proper OOP” should be left to the theoreticians. Indeed, even Smalltalk has the equivalent in its “class methods.” Cleanup: finalization & garbage collection Programmers know about the importance of initialization, but often forget the importance of cleanup. After all, who needs to clean up an int? But with libraries, simply “letting go” of an object once you’re done with it is not always safe. Of course, Java has the garbage collector to reclaim the memory of objects that are no longer used. Now consider a very special and unusual case. Suppose your object allocates “special” memory without using new. The garbage collector knows only how to release memory allocated with new, so it won’t know how to release the object’s “special” memory. To handle this case, Java 2 The one case where this is possible occurs if you pass a handle to an object into the static method. Then, via the handle (which is now effectively this), you can call non-static methods and access non-static fields. But typically if you want to do something like this you’ll just make an ordinary, non-static method. 130 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 131. provides a method called finalize( ) that you can define for your class. Here’s how it’s supposed to work: when the garbage collector is ready to release the storage used for your object, it will first call finalize( ), and only on the next garbage-collection pass will it reclaim the object’s memory. So if you choose to use finalize( ), it gives you the ability to perform some important cleanup at the time of garbage collection. This is a potential programming pitfall, because some programmers, especially C++ programmers, might initially mistake finalize( ) for the destructor in C++, which is a function that is always called when an object is destroyed. But it is very important to distinguish between C++ and Java here, because in C++, objects always get destroyed (in a bug-free program), whereas in Java objects do not always get garbage-collected. Or, put another way: Garbage collection is not destruction. If you remember this, you will stay out of trouble. What it means is that if there is some activity that must be performed before you no longer need an object, you must perform that activity yourself. Java has no destructor or similar concept, so you must create an ordinary method to perform this cleanup. For example, suppose in the process of creating your object it draws itself on the screen. If you don’t explicitly erase its image from the screen, it might never get cleaned up. If you put some kind of erasing functionality inside finalize( ), then if an object is garbage-collected, the image will first be removed from the screen, but if it isn’t the image will remain. So a second point to remember is: Your objects might not get garbage collected. You may find that the storage for an object never gets released because your program never nears the point of running out of storage. If your program completes and the garbage collector never gets around to releasing the storage for any of your objects, that storage will be returned to the operating system en masse as the program exits. This is a good thing, because garbage collection has some overhead, and if you never do it you never incur that expense. What is finalize( ) for? You might believe at this point that you should not use finalize( ) as a general-purpose cleanup method. What good is it? A third point to remember is: Garbage collection is only about memory. That is, the sole reason for the existence of the garbage collector is to recover memory that is no longer being used by your program. So any activity that is associated with garbage collection, most notably your finalize( ) method, must also be only about memory. Does this mean that if your object contains other objects finalize( ) should explicitly release those objects? Well, no – the garbage collector takes care of the release of all object memory, regardless of how the object is created. It turns out that the need for finalize( ) is limited to special cases, where your object may allocate some storage in some way other than creating an object. But, you may observe, everything in Java is an object so how can this be? It would seem that finalize( ) is in place because of the possibility that you’ll do something C-like by allocating memory using a mechanism other than the normal one in Java. The way this can happen is primarily through native methods, which are a way to call non-Java code from Java (native methods are discussed in Appendix A). C and C++ are Chapter 4: Initialization & Cleanup 131
- 132. the only languages currently supported by native methods, but since they in turn can call subprograms in other languages, you can effectively call anything. Inside the non-Java code, C’s malloc( ) family of functions might be called to allocate storage, and unless you call free( ) that storage will not be released, causing a memory leak. Of course, free( ) is itself a C/C++ function, so you’d have to call that inside a native method inside your finalize( ). After reading this, you probably get the idea that you won’t use finalize( ) very much. You’re right – it is not the appropriate place for normal cleanup to occur. So where should normal cleanup be performed? You must perform cleanup The answer is this: To clean up an object, the user of that object must call a cleanup method at the point the cleanup is desired. This sounds pretty straightforward, but it collides a bit with the C++ concept of the destructor. In C++, all objects are destroyed. Or rather, all objects should be destroyed. If the C++ object is created as a local (not possible in Java) then the destruction happens at the closing curly brace of the scope where the object was created. If the object was created using new (like in Java) the destructor is called when the programmer calls the C++ operator delete (which doesn’t exist in Java). If the programmer forgets, the destructor is never called and you have a memory leak, plus the other parts of the object never get cleaned up. In contrast, Java doesn’t allow you to create local objects – you must always use new. But in Java, there’s no “delete” to call for releasing the object since the garbage collector releases the storage for you. So from a simplistic standpoint you could say that because of garbage collection, Java has no destructor. You’ll see as the book progresses, however, that the presence of a garbage collector does not remove the need or utility of destructors (and you should never call finalize( ) directly, so that’s not an appropriate avenue for a solution). If you want some kind of cleanup performed other than storage release you must still call a method in Java, which is the equivalent of a C++ destructor without the convenience. One of the things finalize( ) can be useful for is observing the process of garbage collection. The following example shows you what’s going on and summarizes the previous descriptions of garbage collection: //: Garbage.java // Demonstration of the garbage // collector and finalization class Chair { static boolean gcrun = false; static boolean f = false; static int created = 0; static int finalized = 0; int i; Chair() { i = created++; if(created == 47) System.out.println("Created 47"); } protected void finalize() { if(!gcrun) { gcrun = true; System.out.println( 132 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 133. "Beginning to finalize after " + created + " Chairs have been created"); } if(i == 47) { System.out.println( "Finalizing Chair #47, " + "Setting flag to stop Chair creation"); f = true; } finalized++; if(finalized >= created) System.out.println( "All " + finalized + " finalized"); } } public class Garbage { public static void main(String args[]) { if(args.length == 0) { System.err.println("Usage: n" + "java Garbage beforen or:n" + "java Garbage after"); return; } while(!Chair.f) { new Chair(); new String("To take up space"); } System.out.println( "After all Chairs have been created:n" + "total created = " + Chair.created + ", total finalized = " + Chair.finalized); if(args[0].equals("before")) { System.out.println("gc():"); System.gc(); System.out.println("runFinalization():"); System.runFinalization(); } System.out.println("bye!"); if(args[0].equals("after")) System.runFinalizersOnExit(true); } } ///:~ The above program creates many Chair objects, and at some point after the garbage collector begins running, the program stops creating Chairs. Since the garbage collector can be run at any time, you don’t know exactly when it will start up, so there’s a flag called gcrun to indicate whether the garbage collector has started running yet. A second flag f is a way for Chair to tell the main( ) loop that it should stop making objects. Both of these flags are set within finalize( ), which is called during garbage collection. Two other static variables, created and finalized, keep track of the number of objs created vs. the number that get finalized by the garbage collector. Finally, each Chair has its own (i.e.: non-static) int i so it can keep track of what number it is. When Chair number 47 is finalized, the flag is set to true to bring the process of Chair creation to a stop. All this happens in main( ), in the loop Chapter 4: Initialization & Cleanup 133
- 134. while(!Chair.f) { new Chair(); new String("To take up space"); } Normally you’d wonder how this loop could ever finish, since there’s nothing inside that changes the value of Chair.f. However, the finalize( ) process will, eventually, when it finalizes number 47. The creation of a String object during each iteration is simply extra garbage being created to encourage the garbage collector to kick in, which it will do when it starts to get nervous about the amount of memory available. When you run the program, you provide a command-line argument of “before” or “after.” The “before” argument will call the System.gc( ) method (to force execution of the garbage collector) along with the System.runFinalization( ) method to run the finalizers. These methods were available in Java 1.0, but the runFinalizersOnExit( ) method that is invoked by using the “after” argument is available only in Java 1.13 and beyond. The above program shows that, in Java 1.1, the promise that finalizers will always be run holds true, but only if you explicitly force it to happen yourself. If you use an argument that isn’t “before” or “after” (such as “none”) then neither finalization process will occur, and you’ll get an output like this: Created 47 Beginning to finalize after 8694 Chairs have been created Finalizing Chair #47, Setting flag to stop Chair creation After all Chairs have been created: total created = 9834, total finalized = 108 bye! Thus, not all finalizers get called by the time the program completes. To force finalization to happen, you can call System.gc( ) followed by System.runFinalization( ). This will destroy all the objects that are no longer in use up to that point. The odd thing about this is that you call gc( ) before you call runFinalization( ), which seems to contradict the Sun documentation which claims that finalizers are run first, and then the storage is released. However, if you call runFinalization( ) first, and then gc( ), the finalizers will not be executed. One reason that Java 1.1 may default to skipping finalization for all objects is because it seems to be expensive. When you use either of the approaches that force garbage collection you might notice longer delays than without the extra finalization. 3 Unfortunately, the implementations of the garbage collector in Java 1.0 would never call finalize( ) correctly. As a result, finalize( ) methods that were essential (such as those to close a file) often didn’t get called. The documentation claimed that all finalizers would be called at the exit of a program, even if the garbage collector hadn’t been run on those objects by the time the program terminated. This wasn’t true, so as a result you couldn’t reliably expect finalize( ) to be called for all objects. Effectively, finalize( ) was useless in Java 1.0. 134 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 135. Member initialization Java goes out of its way to guarantee that any variable is properly initialized before it is used. In the case of variables that are defined locally to a method, this guarantee comes in the form of a compile-time error. So if you say: void f() { int i; i++; } You’ll get an error message that says that i may not have been initialized. Of course, the compiler could have given i a default value, but it’s more likely that this is a programmer error and a default value would have covered that up. Forcing the programmer to provide an initialization value is more likely to catch a bug. If a primitive is a data member of a class, however, things are a bit different. Since any method may initialize or use that data, it might not be practical to force the user to initialize it to its appropriate value before the data is used. However, it’s unsafe to leave it with a garbage value, so each primitive data member of a class is guaranteed to get an initial value. Those values can be seen here: //: InitialValues.java // Shows default initial values class Measurement { boolean t; char c; byte b; short s; int i; long l; float f; double d; void print() { System.out.println( "Data type Inital valuen" + "boolean " + t + "n" + "char " + c + "n" + "byte " + b + "n" + "short " + s + "n" + "int " + i + "n" + "long " + l + "n" + "float " + f + "n" + "double " + d); } } public class InitialValues { public static void main(String args[]) { Measurement d = new Measurement(); d.print(); /* In this case you could also say: new Measurement().print(); */ } Chapter 4: Initialization & Cleanup 135
- 136. } ///:~ The output of this program is: Data type Inital value boolean false char byte 0 short 0 int 0 long 0 float 0.0 double 0.0 The char value is a null, which doesn’t print. You’ll see later that when you define an object handle inside a class without initializing it to a new object, that handle is given a value of null. You can see that even though the values are not specified, they automatically get initialized. So at least there’s no threat of working with uninitialized variables. Specifying initialization But what happens if you want to give an initial value? One very direct way to do this is simply assign the value at the point you define the variable in the class (note you cannot do this in C++, although C++ novices always try). Here the field definitions in class Measurement are changed to provide initial values: class Measurement { boolean b = true; char c = 'x'; byte B = 47; short s = 0xff; int i = 999; long l = 1; float f = 3.14f; double d = 3.14159; //. . . You can also initialize non-primitive objects in this same way. If Depth is a class, you can insert a variable and initialize it like so: class Measurement { Depth o = new Depth(); boolean b = true; // . . . If you haven’t given o an initial value and you go ahead and try to use it anyway, you’ll get an exception at run-time. You can even call a method to provide an initialization value: class CInit { int i = f(); //... } 136 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 137. This method may have arguments, of course, but those arguments cannot be other class members that haven’t been initialized yet. Thus, you can do this: class CInit { int i = f(); int j = g(i); //... } But you cannot do this: class CInit { int j = g(i); int i = f(); //... } This is one place where the compiler, appropriately, does complain about forward referencing, since this has to do with the order of initialization and not the way the program is compiled. This approach to initialization is simple and straightforward. It has the limitation that every object of type Measurement will get these same initialization values. Sometimes this is exactly what you need, but at other times you need more flexibility. Constructor initialization The constructor can be used to perform initialization, and this gives you greater flexibility in your programming, since you may call methods and perform actions at run time to determine the initial values. There’s one thing to keep in mind, however: you aren’t precluding the automatic initialization, which happens before the constructor is entered. So, for example, if you say: class Counter { int i; Counter() { i = 7; } // . . . then i will first be initialized to zero, then to 7. This is true with all the primitive types and with object handles, including those that are given explicit initialization at the point of definition. For this reason, the compiler doesn’t try to force you to initialize elements in the constructor at any particular place, or before they are used – initialization is already guaranteed.4 Order of initialization Within a class, the order of initialization is determined by the order that the variables are defined within the class. Even if the variable definitions are scattered throughout in between method definitions, the variables are initialized before any methods can be called, even the constructor. For example: //: OrderOfInitialization.java // Demonstrates initialization order. 4 In contrast, C++ has the constructor initializer list that causes initialization to occur before entering the constructor body, and is enforced for objects. See Thinking in C++. Chapter 4: Initialization & Cleanup 137
- 138. // When the constructor is called, to create a // Tag object, you'll see a message: class Tag { Tag(int marker) { System.out.println("Tag(" + marker + ")"); } } class Card { Tag t1 = new Tag(1); // Before constructor Card() { // Indicate we're in the constructor: System.out.println("Card()"); t3 = new Tag(33); // Re-initialize t3 } Tag t2 = new Tag(2); // After constructor void f() { System.out.println("f()"); } Tag t3 = new Tag(3); // At end } public class OrderOfInitialization { public static void main(String args[]) { Card t = new Card(); t.f(); // Shows that construction is done } } ///:~ In Card, the definitions of the Tag objects are intentionally scattered about to prove that they’ll all get initialized before the constructor is entered or anything else can happen. In addition, t3 is re-initialized inside the constructor. The output is: Tag(1) Tag(2) Tag(3) Card() Tag(33) f() Thus, the t3 handle gets initialized twice, once before and once during the constructor call (the first object is dropped, so it may be garbage-collected later). This might not seem very efficient at first, but it guarantees proper initialization – what would happen if an overloaded constructor were defined that did not initialize t3 and there wasn’t a “default” initialization for t3 in its definition? Static data initialization What happens when the data is static? Exactly the same thing: if it’s a primitive and you don’t initialize it, it gets the standard primitive initial values. If it’s a handle to an object, it’s null unless you create a new object and attach your handle to it. If you want to place initialization at the point of definition, it looks the same as for non- statics. But since there’s only one piece of storage for a static, regardless of how many objects are created, a question comes up: when does that storage get initialized? An example makes this question clear: 138 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 139. //: StaticInitialization.java // Specifying initial values in a // class definition. class Bowl { Bowl(int marker) { System.out.println("Bowl(" + marker + ")"); } void f(int marker) { System.out.println("f(" + marker + ")"); } } class Table { static Bowl b1 = new Bowl(1); Table() { System.out.println("Table()"); b2.f(1); } void f2(int marker) { System.out.println("f2(" + marker + ")"); } static Bowl b2 = new Bowl(2); } class Cupboard { Bowl b3 = new Bowl(3); static Bowl b4 = new Bowl(4); Cupboard() { System.out.println("Cupboard()"); b4.f(2); } void f3(int marker) { System.out.println("f3(" + marker + ")"); } static Bowl b5 = new Bowl(5); } public class StaticInitialization { public static void main(String args[]) { System.out.println( "Creating new Cupboard() in main"); new Cupboard(); System.out.println( "Creating new Cupboard() in main"); new Cupboard(); t2.f2(1); t3.f3(1); } static Table t2 = new Table(); static Cupboard t3 = new Cupboard(); } ///:~ Bowl allows you to view the creation of a class, and Table and Cupboard create static members of Bowl scattered through their class definitions. Notice that Cupboard creates a non-static Bowl b3 prior to the static definitions. The output shows what happens: Chapter 4: Initialization & Cleanup 139
- 140. Bowl(1) Bowl(2) Table() f(1) Bowl(4) Bowl(5) Bowl(3) Cupboard() f(2) Creating new Cupboard() in main Bowl(3) Cupboard() f(2) Creating new Cupboard() in main Bowl(3) Cupboard() f(2) f2(1) f3(1) The static initialization occurs only if it’s necessary – if you don’t create a Table object and you never refer to Table.b1 or Table.b2, the static Bowl b1 and b2 will never be created. However, they are created only when the first Table object is created (or the first static access occurs). After that, the static object is not re-initialized. The order of initialization is: statics first, if they haven’t already been initialized by a previous object creation, and then the non-static objects. You can see the evidence of this in the output. It's helpful to summarize the process of creating an object. Consider a class called Dog: 1. The first time an object of type Dog is created, or the first time a static method or static field of class Dog is accessed, the Java interpreter must locate Dog.class, which it does by searching through the classpath. 2. As Dog.class is loaded (this creates a Class object, which you'll learn about later), all its static initializers are run. Thus, static initialization takes place only once, as the Class object is loaded for the first time. 3. When you create a new Dog( ), the construction process for a Dog object first allocates enough storage for a Dog object on the heap. 4. This storage is wiped to zero, automatically setting all the primitives in Dog to their default values (zero for numbers and the equivalent for boolean and char). 5. Any initializations that occur at the point of field definition are executed. 6. Constructors are executed. As you shall see in Chapter 6, this might actually involve a fair amount of activity, especially when inheritance is involved. Explicit static initialization Java allows you to group other static initializations inside a special “static construction clause” (sometimes called a static block) in a class. It looks like this: class Spoon { static int i; static { i = 47; 140 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 141. } // . . . So it looks like a method, but it’s just the static keyword followed by a method body. This code, like the other static initialization, is executed only once, the first time you make an object of that class or you access a static member of that class (even if you never make an object of that class). For example: //: ExplicitStatic.java // Explicit static initialization // with the "static" clause. class Cup { Cup(int marker) { System.out.println("Cup(" + marker + ")"); } void f(int marker) { System.out.println("f(" + marker + ")"); } } class Cups { static Cup c1; static Cup c2; static { c1 = new Cup(1); c2 = new Cup(2); } Cups() { System.out.println("Cups()"); } } public class ExplicitStatic { public static void main(String args[]) { System.out.println("Inside main()"); Cups.c1.f(99); // (1) } static Cups x = new Cups(); // (2) static Cups y = new Cups(); // (2) } ///:~ The static initializers for Cups will be run when either the access of the static object c1 occurs on the line marked (1), or if line (1) is commented out and the lines marked (2) are un-commented. If both (1) and (2) are commented out, the static initialization for Cups never occurs. Non-static instance initialization non-static instance initialization Java 1.1 provides a similar syntax for initializing non- static variables for each object. Here’s an example: //: Mugs.java // Java 1.1 "Instance Initialization" class Mug { Mug(int marker) { System.out.println("Mug(" + marker + ")"); Chapter 4: Initialization & Cleanup 141
- 142. } void f(int marker) { System.out.println("f(" + marker + ")"); } } public class Mugs { Mug c1; Mug c2; { c1 = new Mug(1); c2 = new Mug(2); System.out.println("c1 & c2 initialized"); } Mugs() { System.out.println("Mugs()"); } public static void main(String args[]) { System.out.println("Inside main()"); Mugs x = new Mugs(); } } ///:~ You can see that the instance initialization clause: { c1 = new Mug(1); c2 = new Mug(2); System.out.println("c1 & c2 initialized"); } looks exactly like the static initialization clause except for the missing static keyword. This syntax is necessary to support the initialization of anonymous inner classes (see Chapter 7). Array initialization Initializing arrays in C is error-prone and tedious. C++ uses aggregate initialization to make it much safer5 . Java has no “aggregates” like C++, since everything is an object in Java. However it does have arrays, and these are supported with array initialization. An array is simply a sequence of objects, all the same type and packaged together under one identifier name. Arrays are defined and used with the square-brackets indexing operator [ ]. To define an array you simply follow your identifier with empty square brackets: int a1[]; However, you can also put the square brackets after the type name to produce exactly the same meaning: int[] a1; 5 See Thinking in C++ for a complete description of aggregate initialization. 142 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 143. This might be considered a more sensible syntax, since it says that the type is “an int array.” But the former style of definition conforms to expectations from C and C++ programmers. The compiler doesn’t allow you to tell it how big the array is. But we’re back to that issue of “handles”: all you have at this point is a handle to an array, and there’s been no space allocated for the array itself. To create storage for the array you must write an initialization expression. For arrays, initialization can appear anywhere in your code, but you can also use a special kind of initialization expression that must occur at the point the array is created. This special initialization is a set of values surrounded by curly braces. The storage allocation (the equivalent of using new) will be taken care of by the compiler in this case. For example: int a1[] = { 1, 2, 3, 4, 5 }; So why would you ever define an array handle without an array? int a2[]; Well, it’s possible to assign one array to another in Java, so you can say: a2 = a1; What you’re really doing is copying a handle, as demonstrated here: //: Arrays.java // Arrays of primitives. public class Arrays { public static void main(String args[]) { int a1[] = { 1, 2, 3, 4, 5 }; int a2[]; a2 = a1; for(int i = 0; i < a2.length; i++) a2[i]++; for(int i = 0; i < a1.length; i++) prt("a1[" + i + "] = " + a1[i]); } static void prt(String s) { System.out.println(s); } } ///:~ You can see that a1 is given an initialization value while a2 is not; a2 is assigned later – in this case, to another array. There’s something new here: all arrays have an intrinsic member (whether they’re arrays of objects or arrays of primitives) that you can query – but not change – to tell you how many elements there are in the array: length. Since arrays in Java, like C and C++, start counting from element zero the largest element you can index is length - 1. If you go out of bounds C and C++ quietly accept this and allow you to stomp all over your memory, the source of many infamous bugs. However, Java protects you against such problems by causing a run-time error (called an exception, the subject of Chapter 9) if you step out of bounds. Of course, checking every array access costs time and code and there’s no way to turn it off, which means that array accesses may be a source of inefficiency in your program if they occur at a critical juncture. For Internet security and programmer productivity, the Java designers felt this was a worthwhile tradeoff. Chapter 4: Initialization & Cleanup 143
- 144. What if you don’t know how many elements you’re going to need in your array while you’re writing the program? You simply use new to create the elements in the array. Here, new works even though it’s creating an array of primitives (new won’t create a non-array primitive): //: ArrayNew.java // Creating Arrays with new. import java.util.*; public class ArrayNew { static Random rand = new Random(); static int pRand(int mod) { return Math.abs(rand.nextInt()) % mod; } public static void main(String args[]) { int a[]; a = new int[pRand(20)]; prt("length of a = " + a.length); for(int i = 0; i < a.length; i++) prt("a[" + i + "] = " + a[i]); } static void prt(String s) { System.out.println(s); } } ///:~ Since the size of the array is chosen at random (using the pRand( ) method defined earlier), it’s clear that array creation is actually happening at run-time. In addition, you’ll see from the output of this program that array elements of primitive types are automatically initialized to ”empty” values (if they’re numeric, this is zero, if they are char, it’s null). Of course, the array could also have been defined and initialized in the same statement: int a[] = new int[pRand(20)]; If you’re dealing with an array of non-primitive objects, you must always use new. Here, the handle issue comes up again because what you create is an array of handles. Consider the wrapper type Integer which is a class and not a primitive: //: ArrayClassObj.java // Creating an array of non-primitive objects. import java.util.*; public class ArrayClassObj { static Random rand = new Random(); static int pRand(int mod) { return Math.abs(rand.nextInt()) % mod; } public static void main(String args[]) { Integer a[] = new Integer[pRand(20)]; prt("length of a = " + a.length); for(int i = 0; i < a.length; i++) { a[i] = new Integer(pRand(500)); prt("a[" + i + "] = " + a[i]); } } 144 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 145. static void prt(String s) { System.out.println(s); } } ///:~ Here, even after new is called to create the array: Integer a[] = new Integer[pRand(20)]; it’s only an array of handles, and not until the handle itself is initialized by creating a new Integer object is the initialization complete: a[i] = new Integer(pRand(500)); If you forget to create the object, however, you’ll get an exception at run-time when you try to read the empty array location. Take a look at the formation of the String object inside the print statements. You can see that the handle to the Integer object is automatically converted to produce a String representing the value inside the object. It’s also possible to initialize arrays of objects using the curly-brace-enclosed list. There are two forms, the first of which is the only one allowed in Java 1.0. The second (equivalent) form is allowed in Java 1.1: //: ArrayInit.java // Array initialization public class ArrayInit { public static void main(String args[]) { Integer[] a = { new Integer(1), new Integer(2), new Integer(3), }; // Java 1.1 only: Integer[] b = new Integer[] { new Integer(1), new Integer(2), new Integer(3), }; } } ///:~ This is useful at times, but it’s more limited since the size of the array is determined at compile time. The last comma in the list of initializers is optional (this feature makes for easier maintenance of long lists). The second form of array initialization, added in Java 1.1, provides a convenient syntax to create and call methods that take variable argument lists, including, if you choose, unknown size as well as unknown type. Since all classes are ultimately inherited from the common root class Object, you can create a method that take an array of Object and call it like this: //: VarArgs.java // Using the Java 1.1 array syntax to create // variable argument lists Chapter 4: Initialization & Cleanup 145
- 146. class A { int i; } public class VarArgs { static void f(Object[] x) { for(int i = 0; i < x.length; i++) System.out.println(x[i]); } public static void main(String args[]) { f(new Object[] { new Integer(47), new VarArgs(), new Float(3.14), new Double(11.11) }); f(new Object[] {"one", "two", "three" }); f(new Object[] {new A(), new A(), new A()}); } } ///:~ At this point, there’s not much you can do with these unknown objects, and this program uses the automatic String conversion to do something useful with each Object. In Chapter 11 (Run-Time Type Identification, or RTTI) you’ll learn how to discover the exact type of such objects so you can do something more interesting with them. Mulitdimensional arrays Java allows you to easily create multidimensional arrays: //: MultiDimArray.java // Creating multi-dimensional arrays. import java.util.*; public class MultiDimArray { static Random rand = new Random(); static int pRand(int mod) { return Math.abs(rand.nextInt()) % mod; } public static void main(String args[]) { int a1[][] = { { 1, 2, 3, }, { 4, 5, 6, }, }; for(int i = 0; i < a1.length; i++) for(int j = 0; j < a1[i].length; j++) prt("a1[" + i + "][" + j + "] = " + a1[i][j]); // 3-D array with fixed length: int a2[][][] = new int[2][2][4]; for(int i = 0; i < a2.length; i++) for(int j = 0; j < a2[i].length; j++) for(int k = 0; k < a2[i][j].length; k++) prt("a2[" + i + "][" + j + "][" + k + "] = " + a2[i][j][k]); // 3-D array with varied-length vectors: int a3[][][] = new int[pRand(7)][][]; for(int i = 0; i < a3.length; i++) { 146 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 147. a3[i] = new int[pRand(5)][]; for(int j = 0; j < a3[i].length; j++) a3[i][j] = new int[pRand(5)]; } for(int i = 0; i < a3.length; i++) for(int j = 0; j < a3[i].length; j++) for(int k = 0; k < a3[i][j].length; k++) prt("a3[" + i + "][" + j + "][" + k + "] = " + a3[i][j][k]); // Array of non-primitive objects: Integer[][] a4 = { { new Integer(1), new Integer(2)}, { new Integer(3), new Integer(4)}, { new Integer(5), new Integer(6)}, }; for(int i = 0; i < a4.length; i++) for(int j = 0; j < a4[i].length; j++) prt("a4[" + i + "][" + j + "] = " + a4[i][j]); Integer[][] a5; a5 = new Integer[3][]; for(int i = 0; i < a5.length; i++) { a5[i] = new Integer[3]; for(int j = 0; j < a5[i].length; j++) a5[i][j] = new Integer(i*j); } for(int i = 0; i < a5.length; i++) for(int j = 0; j < a5[i].length; j++) prt("a5[" + i + "][" + j + "] = " + a5[i][j]); } static void prt(String s) { System.out.println(s); } } ///:~ The code used for printing uses length so that it doesn’t depend on fixed array sizes. The first example shows a multi-dimensional array of primitives. You delimit each vector in the array with curly braces: int a1[][] = { { 1, 2, 3, }, { 4, 5, 6, }, }; Each set of square brackets moves you into the next level of the array. The second example shows a three-dimensional array allocated with new. Here, the whole array is allocated all at once: int a2[][][] = new int[2][2][4]; But the third example shows that each vector in the arrays that make up the matrix can be of any length: Chapter 4: Initialization & Cleanup 147
- 148. int a3[][][] = new int[pRand(7)][][]; for(int i = 0; i < a3.length; i++) { a3[i] = new int[pRand(5)][]; for(int j = 0; j < a3[i].length; j++) a3[i][j] = new int[pRand(5)]; } The first new creates an array with a random-length first element and the rest undetermined. The second new inside the for loop fills out the elements but leaves the third index undetermined until you hit the third new. You will see from the output that array values are automatically initialized to zero if you don’t give them an explicit initialization value. You can deal with arrays of non-primitive objects in a similar fashion, which is shown in the fourth example, demonstrating the ability to collect many new expressions with curly braces: Integer[][] a4 = { { new Integer(1), new Integer(2)}, { new Integer(3), new Integer(4)}, { new Integer(5), new Integer(6)}, }; The fifth example shows how an array of non-primitive objects can be built up piece by piece: Integer[][] a5; a5 = new Integer[3][]; for(int i = 0; i < a5.length; i++) { a5[i] = new Integer[3]; for(int j = 0; j < a5[i].length; j++) a5[i][j] = new Integer(i*j); } The i*j is just to put an interesting value into the Integer. Summary The seemingly elaborate mechanism for initialization – the constructor – provided by Java should give you a strong hint about the critical importance placed on initialization in the language. As Stroustrup was designing C++, one of the first observations he made about productivity in C was that a very significant portion of programming problems are caused by improper initialization of variables. These kinds of bugs are very hard to find, and similar issues apply to improper cleanup. Because constructors allow you to guarantee proper initialization and cleanup (the compiler will not allow an object to be created without the proper constructor calls), you get complete control and safety. In C++, destruction is quite important because objects created with new must be explicitly destroyed. In Java, memory for all objects is automatically released by the garbage collector, so the equivalent cleanup method in Java isn’t necessary much of the time. Thus (in cases where you don’t need destructor-like behavior) Java’s garbage collector greatly simplifies programming, and adds much-needed safety in managing memory. Some garbage collectors are even cleaning up other resources like graphics and file handles. However, the garbage collector does add a run-time cost, the expense of which is difficult to put into perspective because of the overall slowness of Java interpreters at 148 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 149. this writing. As this changes, we’ll be able to discover if the overhead of the garbage collector will preclude the use of Java for certain types of programs (one of the issues is the unpredictability of the garbage collector). Because of the guarantee that all objects will be constructed, there’s actually more to the constructor than what is shown here. In particular, when you create new classes using either composition or inheritance the guarantee of construction also holds, and some additional syntax is necessary to support this. You’ll learn about composition, inheritance and how they affect constructors in future chapters. Exercises 1. Create a class with a default constructor (one that takes no arguments) that prints a message. Create an object of this class. 2. Add an overloaded constructor to exercise 1 that takes a String argument and prints it along with your message. 3. Create an array of object handles of the class you created in exercise two, but don’t actually create objects to assign into the array. When you run the program, notice whether the initialization messages from the constructor calls are printed. 4. Complete exercise 3 by creating objects to attach to the array of handles. 5. Experiment with Garbage.java by running the program using the arguments “before,” “after” and “none.” Repeat the process and see if you detect any patterns in the output. Change the code so that System.RunFinalization( ) is called before System.gc( ) and observe the results. Chapter 4: Initialization & Cleanup 149
- 151. d 5: Hiding the implementation A primary consideration in object-oriented design is “separating the things that change from the things that stay the same” This is particularly important for libraries: the user of that library (also called the client programmer) must be able to rely on the part they are using, and know that they won’t have to rewrite code if a new version of the library comes out. And on the flip side, the library creator must have the freedom to make modifications and improvements with the certainty that the client programmer’s code won’t be affected by those changes. This can be achieved through convention. For example, The library programmer must agree not to remove existing methods when modifying a class in the library, since that would break the client programmer’s code. The reverse situation is thornier, however. In the case of a data member, how can the library creator know which data members have been accessed by client programmers? This is also true with methods that are only part of the implementation of a class, and not meant to be used directly by the client programmer. But what if the library creator wants to rip out an old implementation and put in a new one? Changing any of those members might break a client programmer’s code. Thus the library creator is in a straight jacket and can’t change anything. To solve this problem, Java provides access specifiers to allow the library creator to say: this is available to the client programmer, this is not. The levels of access control from “most access” to “least access” are public, “friendly” (which has no keyword), protected, and private. From the previous paragraph you might think that, as a library designer, you’ll want to keep everything as “private” as possible, and expose only the methods that you want the client programmer to use. This is exactly right, even though it’s often counterintuitive for people who program in other languages (especially C) and are used to 151
- 152. accessing everything without restriction. By the end of this chapter you should be convinced of the value of access control in Java. The concept of a library of components and the control over who can access the components of that library is not complete, however. There’s still the question of how the components are bundled together into a cohesive library unit. This is controlled with the package keyword in Java, and the access specifiers are affected by whether a class is in the same package or in a separate package. So to begin this chapter, you’ll learn how library components are placed into packages. Then you’ll be able to understand the complete meaning of the access specifiers. Package: the library unit A package is what you get when you use the import keyword to bring in an entire library, such as import java.util.*; This brings in the entire utility library that’s part of the standard Java distribution. Since Vector is in java.util, you can now either specify the full name java.util.Vector (which you can do without the import statement) or you can simply say Vector (because of the import). If you want to bring in a single class, you can name that class in the import statement import java.util.Vector; Now you can use Vector with no qualification. However, none of the other classes in java.util are available. The reason for all this importing is to provide a mechanism to manage “name spaces.” The names of all your class members are insulated from each other: a method f( ) inside a class A will not clash with an f( ) that has the same signature (argument list) in class B. But what about the class names themselves? Suppose you create a stack class which is installed on a machine that already has a stack class that’s written by someone else? With Java on the Internet, this can happen without the user knowing it since classes can get downloaded automatically in the process of running a Java program. This potential clashing of names is why it’s important to have complete control over the name spaces in Java, and to be able to create a completely unique name regardless of the constraints of the Internet. So far, the examples in this book have existed in a single file and have been designed for local use, and haven’t bothered with package names (in this case the class name is placed in the “default package”). This is certainly an option, and for simplicity’s sake this approach will be used whenever possible throughout the rest of the book. However, if you’re planning to create a program that is “Internet friendly” you’ll need to think about preventing class name clashes. When you create a source-code file for Java, it’s commonly called a compilation unit (sometimes a translation unit). Each compilation unit must have a name followed by .java, and inside the compilation unit there can be a public class that must have the same name as the file (including capitalization, but excluding the .java filename extension). If you don’t do this, the compiler will complain. There can be only one public class in each compilation unit (or the compiler will complain). The rest of the classes in that 152 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 153. compilation unit, if there are any, are hidden from the world outside that package because they’re not public, and they comprise “support” classes for the main public class. When you compile a .java file you get an output file with exactly the same name but an extension of .class for each class in the .java file. Thus you can end up with quite a few .class files from a small number of .java files. If you’ve programmed with a compiled language, you might be used to the compiler spitting out an intermediate form (usually an “Obj” file) that is then packaged together with others of its kind using a linker (to create an executable file) or a librarian (to create a library). That’s not how Java works. A working program is a bunch of .class files, which may be packaged and compressed into a JAR file (using the jar utility in Java 1.1). The Java interpreter is responsible for finding, loading and interpreting these files1 . A library is also a bunch of these class files – each file has one class that is public (you’re not forced to have a public class, but it’s typical), so there’s one component for each file. But how do you say that all these components (that are in their own separate .java and .class files) belong together? That’s where the package keyword comes in. When you say: package mypackage; at the beginning of a file (and the package statement must appear as the first non- comment in the file), you’re stating that this compilation unit is part of a library named mypackage. Or, put another way, you’re saying that the public class name within this compilation unit is under the umbrella of the name mypackage, and if anyone wants to use the name they’ll either have to fully specify the name or use the import keyword in combination with mypackage (using the choices given previously). Note that the convention for Java packages is to use all lowercase letters, even for intermediate words. For example, suppose the name of the file is MyClass.java. This means there can be one and only one public class in that file, and the name of that class must be MyClass (including the capitalization): package mypackage; public class MyClass { // . . . Now, if someone wants to use MyClass or, for that matter, any of the other public classes in mypackage, they must use the import keyword to make the name or names in mypackage available. The alternative is to give the fully-qualified name: mypackage.MyClass m = new mypackage.MyClass(); The import keyword can make this much cleaner: import mypackage.*; // . . . MyClass m = new MyClass(); It’s worth keeping in mind that what the package and import keywords allow you to do, as a library designer, is to divide up the single global name space so you won’t have 1 There’s nothing in Java that forces the use of an interpreter. There exist native-code Java compilers that generate a single executable file. Chapter 5: Hiding the Implementation 153
- 154. clashing names, no matter how many people get on the Internet and start writing classes in Java. Creating unique package names You may observe that, since a package never really gets “packaged” into a single file, a package could be made up of many .class files, and things could get a bit cluttered. To prevent this, a logical thing to do is to place all the .class files for a particular package into a single directory; that is, to use the hierarchical file structure of the operating system to advantage. This is how Java handles the problem of clutter. It also solves two other problems: creating unique package names, and finding those classes that might be buried in a directory structure someplace. This is accomplished, as was introduced in Chapter 2, by encoding the path of the location of the .class file into the name of the package. The compiler enforces this, but in addition, by convention, the first part of the package name is the Internet domain name of the creator of the class, reversed. Since Internet domain names are guaranteed to be unique (by InterNIC2 , who controls their assignment) if you follow this convention it’s guaranteed that your package name will be unique and thus you’ll never have a name clash (that is, until you lose the domain name to someone else who starts writing Java code with the same path names as you did). Of course, if you don’t have your own domain name then you’ll need to fabricate an unlikely combination (such as your first and last name) to create unique package names. However, if you’ve decided to start publishing Java code it’s worth the relatively small effort to get a domain name. The second part of this trick is resolving the package name into a directory on your machine, so when the Java program is running and it needs to load the .class file (which it may do dynamically, at the point in the program where it needs to create an object of that particular class, or the first time you access a static member of the class), it can locate the directory where the .class file resides. The Java interpreter proceeds as follows: first, it finds the environment variable CLASSPATH (set via the operating system when Java, or a tool like a Java-enabled browser, is installed on a machine). CLASSPATH contains one or more directories that may be used as roots for a search for .class files. Starting at that root, the interpreter will take the package name and replace each dot with a slash to generate a path name from the CLASSPATH root (so package foo.bar.baz becomes foobarbaz or foo/bar/baz depending on your OS). This is then concatenated to the various entries in the CLASSPATH. That’s where it looks for the .class file with the name corresponding to the class you’re trying to create. To understand this, you’ll need to study an example. Consider my domain name, which is eckelobjects.com. By reversing this, com.eckelobjects (the com, edu, org, etc. extension was formerly capitalized in Java packages, but this was changed in Java 1.2 so the entire package name is lowercase) establishes my unique global name for my classes. I can further subdivide this by deciding I want to create a library named util, so I’ll end up with a package name: package com.eckelobjects.util; Now this package name can be used as an umbrella name space for the following two files: //: Vector.java 2 ftp://ftp.internic.net 154 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 155. // Creating a package package com.eckelobjects.util; public class Vector { public Vector() { System.out.println( "com.eckelobjects.util.Vector"); } } ///:~ When you create your own packages, you’ll discover that the package statement must be the first non-comment code in the file. The second file looks much the same: //: List.java // Creating a package package com.eckelobjects.util; public class List { public List() { System.out.println( "com.eckelobjects.util.List"); } } ///:~ Both of these files are placed in the subdirectory on my system: C:DOCJavaTcomeckelobjectsutil If you walk back through this, you can see the package name com.eckelobjects.util, but what about the first portion of the path? That’s taken care of in the CLASSPATH environment variable, which is, on my machine: CLASSPATH=.;D:JAVALIB;C:DOCJavaT You can see that the CLASSPATH can contain a number of alternative search paths. Now, the following file can be placed in any directory (see page 80 if you have trouble executing this program): //: LibTest.java // Uses the library package c05; import com.eckelobjects.util.*; public class LibTest { public static void main(String args[]) { Vector v = new Vector(); List l = new List(); } } ///:~ When the compiler encounters the import statement, it begins searching at the directories specified by CLASSPATH, looking for a subdirectory comeckelobjectsutil, then seeking the compiled files of the appropriate names (Vector.class for Vector and List.class for List). Note that both the classes and the desired methods in Vector and List must be public. Chapter 5: Hiding the Implementation 155
- 156. Automatic compilation The first time you create an object of an imported class (or you access a static member of a class), the compiler will go hunting for the .class file of the same name (so if you’re creating an object of class X, it looks for X.class) in the appropriate directory. If it finds only X.class, that’s what it must use. However, if it also finds an X.java in the same directory, the compiler will first compare the date stamp on the two files, and if X.java is more recent than X.class, it will automatically recompile X.java to generate an up-to-date X.class. If a class is not in a .java file of the same name as that class, this behavior will not occur for that class. Collisions What happens if two libraries are imported via * and they include the same names? For example, suppose a program does this: import com.eckelobjects.util.*; import java.util.*; Since java.util.* also contains a Vector class, this causes a potential collision. However, as long as the collision doesn’t actually occur, everything is OK – which is good because otherwise you might end up doing a lot of typing to prevent collisions that would never happen. The collision does occur if you now try to make a Vector: Vector v = new Vector(); Which Vector class does this refer to? The compiler can’t know, and the reader can’t know either. So the compiler complains and forces you to be explicit. If I want the standard Java Vector, for example, I must say: java.util.Vector v = new java.util.Vector(); Since this (along with the CLASSPATH) completely specifies the location of that Vector, there’s no need for the import java.util.* statement unless I’m using something else from java.util. A custom tool library With this knowledge in hand, you can now create your own libraries of tools to reduce or eliminate duplicate code. Consider, for example, creating an alias for System.out.println( ) to reduce typing. This can be part of a package called tools: //: P.java // The P.rint & P.rintln shorthand package com.eckelobjects.tools; public class P { public static void rint(Object obj) { System.out.print(obj); } public static void rint(String s) { System.out.print(s); } public static void rint(char s[]) { 156 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 157. System.out.print(s); } public static void rint(char c) { System.out.print(c); } public static void rint(int i) { System.out.print(i); } public static void rint(long l) { System.out.print(l); } public static void rint(float f) { System.out.print(f); } public static void rint(double d) { System.out.print(d); } public static void rint(boolean b) { System.out.print(b); } public static void rintln() { System.out.println(); } public static void rintln(Object obj) { System.out.println(obj); } public static void rintln(String s) { System.out.println(s); } public static void rintln(char s[]) { System.out.println(s); } public static void rintln(char c) { System.out.println(c); } public static void rintln(int i) { System.out.println(i); } public static void rintln(long l) { System.out.println(l); } public static void rintln(float f) { System.out.println(f); } public static void rintln(double d) { System.out.println(d); } public static void rintln(boolean b) { System.out.println(b); } } ///:~ All the different data types can now be printed out either with a newline (P.rintln( )) or without a newline (P.rint( )). Chapter 5: Hiding the Implementation 157
- 158. You can guess that the location of this file must be in a directory that starts at one of the CLASSPATH locations, then continues com/eckelobjects/tools. After compiling, the P.class file can be used anywhere on your system with an import statement: //: ToolTest.java // Uses the tools library import com.eckelobjects.tools.*; public class ToolTest { public static void main(String args[]) { P.rintln("Available from now on!"); } } ///:~ So from now on, whenever you come up with a useful new utility, you can add it to the tools directory (or to your own personal util or tools directory). Classpath pitfall The P.java file brought up an interesting pitfall. Especially with early implementations of Java, setting the classpath correctly is generally quite a headache. During the development of this book, the P.java file was introduced and seemed to work fine, but at some point it began breaking. For a long time I was certain this was the fault of one implementation of Java or another, but finally I discovered that at one point I had introduced a program (CodePackager.java, shown in Chapter 17) that also used a different class P. Because it was used as a tool, it was sometimes placed in the classpath, and other times it wasn’t. When it was, the P in CodePackager.java was found first by Java when executing a program where it was looking for the class in com.eckelobjects.tools, and the compiler would say that a particular method couldn’t be found. This was very frustrating, because you can see the method in the above class P, and no further diagnostics were reported to give you a clue that it was finding a completely different class (that wasn’t even public). At first this could seem like a compiler bug, but if you look at the import statement it only says “here’s where you might find P.” However, the compiler is supposed to look anywhere in its classpath so if it finds a P there it will use it, and if it finds the “wrong” one first during a search then it will stop looking. This is slightly different than the case described on page 156 because there the offending classes were both in packages, and here there was a P that was not in a package, but could still be found during a normal classpath search. If you’re having an experience like this, check to make sure there’s only one class of each name anywhere in your classpath. Package caveat It’s worth remembering that anytime you create a package, you implicitly specify a directory structure when you give the package a name. The package must live in the directory indicated by its name, which must be a directory that is searchable starting from the CLASSPATH. This means that experimenting with the package keyword can be a bit frustrating at first because unless you adhere to the package-name to directory-path rule, you’ll get a lot of mysterious run-time messages about not being able to find a particular class, even if that class is sitting there in the same directory. If you get such a message, try commenting out the package statement, and if it runs you’ll know where the problem lies. 158 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 159. Java access specifiers The Java access specifiers public, protected and private are placed in front of each definition for each member in your class, whether it’s a data member or a method. Each access specifier controls the access for only that particular definition. This is a distinct contrast with C++, where the access specifier controls all the definitions following it, until another access specifier comes along. One way or another everything has some kind of access specified for it. In the following sections, you’ll learn all about the various types of access, starting with the default access. “Friendly” What if you give no access specifier at all, as in all the examples before this chapter? The default access has no keyword, but it is commonly referred to as “friendly.” It means that all the other classes in the current package have access to the friendly member, but to all the classes outside of this package the member appears to be private. Since a compilation unit – a file – can belong only to a single package, all the classes within a single compilation unit are automatically friendly with each other. Thus, friendly elements are also said to have package access. Friendly access allows you to group related classes together in a package so they can easily interact with each other. When you put classes together in a package (thus granting mutual access to their friendly members; e.g. making them “friends”) you “own” the code in that package. It makes sense that only code that you own should have friendly access to other code that you own. You could say that friendly access gives a meaning or a reason for grouping classes together in a package. In many languages the way you organize your definitions in files can be willy-nilly, but in Java you’re compelled to organize them in a sensible fashion. In addition, you’ll probably want to exclude classes that shouldn’t have access to the classes being defined in the current package. A very important rule in any relationship is “who can access my private implementation?” The class controls which code has access to its members. There’s no magic way to “break in”; someone in another package can’t declare a new class and say “hi, I’m a friend of Bob!” and expect to see the protected, friendly, and private members of Bob. Thus, the only way to grant access to a member is to 1. Make the member public. Then everybody, everywhere, can access it. 2. Make the member friendly by leaving off any access specifier, and put the other classes in the same package. Then the other classes can access the member. 3. As you’ll see in a later chapter where inheritance is introduced, an inherited class can access a protected member as well as public member (but not private members). It can access friendly members only if the two classes are in the same package. But don’t worry about that now. 4. Provide “accessor/mutator” methods (a.k.a. “get/set” methods) that read and change the value. This is the most civilized approach in terms of OOP, and it is fundamental to Java Beans, as you’ll see in Chapter 13. public : interface access When you use the public keyword, it means that the member declaration that immediately follows public is available to everyone, in particular to the client Chapter 5: Hiding the Implementation 159
- 160. programmer who is using the library. Suppose you define a package dessert containing the following compilation unit (see page 80 if you have trouble executing this program): //: Cookie.java // Creates a library package c05.dessert; public class Cookie { public Cookie() { System.out.println("Cookie constructor"); } void foo() { System.out.println("foo"); } } ///:~ Remember, Cookie.java must reside in a subdirectory called dessert, in a directory under C05 (indicating Chapter 5 of this book) which itself must be under one of the CLASSPATH directories. Don’t make the mistake of thinking that Java will always look at the current directory as one of the starting points for searching: if you don’t have a ‘.’ as one of the paths in your CLASSPATH, Java won’t look there. Now if you create a program that uses Cookie: //: Dinner.java // Uses the library import c05.dessert.*; public class Dinner { public Dinner() { System.out.println("Dinner constructor"); } public static void main(String args[]) { Cookie x = new Cookie(); //! x.foo(); // Can't access } } ///:~ You can create a Cookie object since its constructor is public and the class itself is public (we’ll look more at the concept of a public class later). However, the foo( ) member is inaccessible inside Dinner.java since foo( ) is friendly only within package dessert. The unnamed package You might be surprised to discover that the following code compiles, even though it would appear that it breaks the rules: //: Cake.java // Accesses a class in a separate // compilation unit. class Cake { public static void main(String args[]) { Pie x = new Pie(); x.f(); } } ///:~ In a second file, in the same directory: 160 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 161. //: Pie.java // The other class class Pie { void f() { System.out.println("Pie.f()"); } } ///:~ You might initially view these as completely foreign files, and yet Cake is able to create a Pie object and call its f( ) method! You’d normally think that Pie and f( ) are friendly and therefore not available to Cake. They are friendly – that part is correct. The reason they are available in Cake.java is because they are in the same directory and have no explicit package name. Java treats files like this as implicitly part of the “unnamed package” for that directory, and therefore friendly to all the other files in that directory. private : you can’t touch that! The private keyword means no one can access that member except that particular class, inside methods of that class. Other classes in the same package cannot access private members, so it’s as if you’re even insulating the class against yourself. On the other hand, it’s not unlikely that a package might be created by several people collaborating together, so private allows you to freely change that member without concern that it will affect another class in the same package. The default “friendly” package access is often an adequate amount of hiding – remember, a “friendly” member is inaccessible to the user of the package. This is nice, since the default access is the one that you normally use. Thus, you’ll typically think about access primarily for the members that you explicitly want to make public for the client programmer, and as a result you might not initially think you’ll use the private keyword very often since it’s tolerable to get away without it (this is a distinct contrast with C++). However, it turns out that the consistent use of private is very important, especially where multithreading is concerned (as you’ll see in Chapter 14). Here’s an example of the use of private: //: IceCream.java // Demonstrates "private" keyword class Sundae { private Sundae() {} static Sundae makeASundae() { return new Sundae(); } } public class IceCream { public static void main(String args[]) { //! Sundae x = new Sundae(); Sundae x = Sundae.makeASundae(); } } ///:~ This shows an example where private comes in handy: you might want to control how an object is created, and prevent anyone from directly accessing a particular constructor (or Chapter 5: Hiding the Implementation 161
- 162. all of them). In the above example, you cannot create a Sundae object via its constructor – instead you must call the makeASundae( ) method to do it for you.3 Any method that you’re certain is only a “helper” method for that class can be made private to ensure that you don’t accidentally use it elsewhere in the package and thus prohibit you from changing or removing the method. Making a method private guarantees that you retain this option. protected : “sort of friendly” The protected access specifier requires a jump ahead to understand. So first, you should be aware that you don’t need to understand this section to continue through the book up through the inheritance chapter. But for completeness an example using protected will be briefly described. The protected keyword deals with a concept called inheritance, which takes an existing class and adds new members to that class without touching the existing class, which we refer to as the base class. You can also change the behavior of existing members of the class. To inherit from an existing class, you say that your new class extends an existing class, like this: class Foo extends Bar { The rest of the class definition looks the same. If you create a new package and you inherit from a class in another package, the only members you have access to are the public members of the original package (of course, if you perform the inheritance in the same package you have the normal package access to all the “friendly” members). Sometimes the creator of the base class would like to take a particular member and grant access to derived classes but not the world in general. That’s what protected does. If you refer back to the file Cookie.java on page 159, the following class cannot access the “friendly” member: //: ChocolateChip.java // Can't access friendly member // in another class import c05.dessert.*; public class ChocolateChip extends Cookie { public ChocolateChip() { System.out.println( "ChocolateChip constructor"); } public static void main(String args[]) { ChocolateChip x = new ChocolateChip(); //! x.foo(); // Can't access foo } } ///:~ One of the interesting things about inheritance is that if a method foo( ) exists in class Cookie, then it also exists in any class inherited from Cookie. But since foo( ) is “friendly” in a foreign package, it’s unavailable to us in this one. Of course, you could make it 3 There’s another effect in this case: since the default constructor is the only one defined, and it’s private, it will prevent inheritance of this class (a subject that will be introduced in Chapter 6). 162 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 163. public, but then everyone would have access and maybe that’s not what you want. If we change the class Cookie as follows: public class Cookie { public Cookie() { System.out.println("Cookie constructor"); } protected void foo() { System.out.println("foo"); } } Then foo( ) still has “friendly” access within package dessert, but it is also accessible to anyone inheriting from Cookie. However, it is not public. Interface & implementation Access control is often referred to as implementation hiding. Wrapping data and methods within classes (combined with implementation hiding this is often called encapsulation) produces a data type with characteristics and behaviors, but access control puts boundaries within that data type, for two important reasons. The first is to establish what the client programmers can and can’t use: you can build your internal mechanisms into the structure without worrying that the client programmers will think it’s part of the interface they should be using. This feeds directly into the second reason, which is to separate the interface from the implementation. If the structure is used in a set of programs, but users can’t do anything but send messages to the public interface, then you can change anything that’s not public (e.g. “friendly,” protected or private) without requiring modifications to their code. We’re now in the world of object-oriented programming, where a class is actually describing “a class of objects,” as you would describe a class of fishes or a class of birds. Any object belonging to this class will share these characteristics and behaviors. The class is a description of the way all objects of this type will look and act. In the original OOP language, Simula-67, the keyword class was used to describe a new data type. The same keyword has been used for most object-oriented languages. This is the focal point of the whole language: the creation of new data types that are more than just boxes containing data and methods. The class is the fundamental OOP concept in Java. It is one of the keywords that will not be set in bold in this book – it becomes annoying with a word repeated as often as “class.” For clarity, you might prefer a style of creating classes that places the public members at the beginning, followed by the protected, friendly, and private members. The advantage of this is that the user of the class can then read down from the top and see first what’s important to them (the public members, because they can be accessed outside the file) and stop reading when they encounter the non-public members, which are part of the internal implementation. However, with the comment-documentation supported by javadoc (described in Chapter 2) the issue of code readability by the client programmer becomes less important. public class X { public void pub1( ) { /* . . . */ } public void pub2( ) { /* . . . */ } public void pub3( ) { /* . . . */ } private void priv1( ) { /* . . . */ } Chapter 5: Hiding the Implementation 163
- 164. private void priv2( ) { /* . . . */ } private void priv3( ) { /* . . . */ } private int i; // . . . } However, this will make it only partially easier to read because the interface and implementation are still mixed together. That is, you still see the source code – the implementation – because it’s right there in the class. Displaying the interface to the consumer of a class is really the job of the class browser, a tool whose job it is to look at all the available classes and show you what you can do with them (what members are available) in a useful fashion. By the time you read this, good browsers should be an expected part of any good Java development tool. Class access In Java, the access specifiers can also be used to determine which classes within a library will be available to the users of that library. If you want a class to be available to a client programmer, you place the public keyword somewhere before the opening brace of the class body. This controls whether the client programmer can even create an object of the class. To control the access of a class, the specifier must appear before the keyword class. Thus you can say: public class Widget { That is, if the name of your library is mylib any client programmer can access Widget by saying import mylib.Widget; or import mylib.*; However, there’s an extra pair of constraints: 1. There can be only one public class per compilation unit (file). The idea is that each compilation unit has a single public interface represented by that public class. It can have as many supporting “friendly” classes as you want. If you have more than one public class inside a compilation unit, the compiler will give you an error message. 2. The name of the public class must exactly match the name of the file containing the compilation unit, including capitalization. So for Widget, the name of the file must be Widget.java, not widget.java or WIDGET.java. Again, you’ll get a compile-time error if they don’t agree. What if you’ve got a class inside mylib that you’re just using to accomplish the tasks performed by Widget or some other public class in mylib? You don’t want to go to the bother of creating documentation for the client programmer, and you think that sometime later you might want to completely change things and rip out your class altogether, substituting a different one. To give you this flexibility, you need to ensure that no client programmers become dependent on your particular implementation details hidden inside mylib. To accomplish this, you just leave the public keyword off the class, in which case it becomes friendly (that class can be used only within that package). 164 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 165. Note that a class cannot be private (that would make it accessible to no one but the class itself), or protected4. So you have only two choices for class access: “friendly” or public. If you don’t want anyone else to have access to that class, you can make all the constructors private, thereby preventing anyone but you, inside a static member of the class, from creating an object of that class5 . Here’s an example: //: Lunch.java // Demonstrates class access specifiers. // Make a class effectively private // with private constructors: class Soup { private Soup() {} // (1) Allow creation via static method: public static Soup makeSoup() { return new Soup(); } // (2) Create a static object and // return a reference upon request. // (The "Singleton" pattern): private static Soup ps1 = new Soup(); public static Soup access() { return ps1; } public void f() {} } class Sandwich { void f() { new Lunch(); } } // Only one public class allowed per file: public class Lunch { void test() { // Can't do this! Private constructor: //! Soup priv1 = new Soup(); Soup priv2 = Soup.makeSoup(); Sandwich f1 = new Sandwich(); Soup.access().f(); } } ///:~ Up to now, most of the methods have been returning either void or a primitive type so the definition: public static Soup access() { return ps1; } 4 Actually, a Java 1.1 inner class can be private or protected, but that’s a special case. These will be introduced in Chapter 7. 5 Or inheriting (Chapter 6) from that class. Chapter 5: Hiding the Implementation 165
- 166. might look a little confusing at first. The word before the method name (access) tells what the method returns. So far this has most often been void which means it returns nothing, but you can also return a handle to an object which is what happens here. This method returns a handle to an object of class Soup. The class Soup shows how to prevent direct creation of a class by making all the constructors private. Remember that if you don’t explicitly create at least one constructor, the default constructor (a constructor with no arguments) will be created for you. By writing the default constructor, it won’t be created automatically. By making it private, no one can create an object of that class. But now how does anyone use this class? The above example shows two options. First, a static method is created that creates a new Soup and returns a handle to it. This could be useful if you want to do some extra operations on the Soup before returning it, or if you want to keep count of how many Soup objects to create (perhaps to restrict their population). The second option uses what’s called a design pattern, which will be discussed later in this book. This particular pattern is called a “singleton” because it allows only a single object to be created. The object of class Soup is created as a static private member of Soup, so there’s one and only one, and you can’t get at it except through the public method access( ). As previously mentioned, if you don’t put an access specifier for class access, it defaults to “friendly.” This means an object of that class can be created by any other class in the package, but not outside the package (remember, all the files within the same directory that don’t have explicit package declarations are implicitly part of the unnamed package for that directory). However, if a static member of that class is public, the client programmer can still access that static member, even though they cannot create an object of that class. Summary In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the user of that library – the client programmer – who is another programmer, but one putting together an application or using your library to build a bigger library. Without rules, client programmers can do anything they want with all the members of a class, even if you might really prefer they don’t directly manipulate some of the members. Everything’s naked to the world. This chapter looked at how classes are built to form libraries; first the way a group of classes is packaged within a library, and second the way the class itself controls access to its members. It is estimated that a C programming project begins to break down somewhere between 50K - 100K lines of code because C has a single “name space” so names begin to collide, causing an extra management overhead. In Java, the package keyword, the package naming scheme and the import keyword give you complete control over names, so the issue of name collision is easily avoided. There are two reasons for controlling access to members. The first is to keep users’ hands off tools they shouldn’t touch, tools that are necessary for the internal machinations of the data type, but not part of the interface that users need to solve their particular problems. So making methods and fields private is actually a service to users because they can easily see what’s important to them and what they can ignore. It simplifies their understanding of the class. 166 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 167. The second and most important reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. You might build a class one way at first, and then discover that restructuring your code will provide much greater speed. If the interface and implementation are clearly separated and protected, you can accomplish this without forcing the user to rewrite their code. Access specifiers in Java give valuable control to the creator of a class. The users of the class can clearly see exactly what they can use and what to ignore. More important, though, is the ability to ensure that no user becomes dependent on any part of the underlying implementation of a class. If you know this as the creator of the class, you can change the underlying implementation with the knowledge that no client programmer will be affected by the changes because they can’t access that part of the class. When you have the ability to change the underlying implementation, you can not only improve your design at some later time, but you also have the freedom to make mistakes. No matter how carefully you plan and design, you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn faster, and you’ll finish your project sooner. The public interface to a class is what the user does see, so that is the most important part of the class to get “right” during analysis and design. But even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more methods, as long as you don’t remove any that client programmers have already used in their code. Exercises 1. Create a class with public, private, protected, and “friendly” data members and method members. Create an object of this class and see what kind of compiler messages you get when you try to access all the class members. Be aware that classes in the same directory are part of the “default” package. 2. Create a class with protected data. Create a second class in the same file with a method that manipulates the protected data in the first class. 3. Create a new directory and edit your CLASSPATH to include that new directory. Copy the P.class file to your new directory and then change the names of the file, the P class inside and the method names. (you might also want to add additional output to watch how it works). Create another program in a different directory that uses your new class. 4. Create the following file in the c05 directory (presumably in your CLASSPATH): //: PackagedClass.java package c05; class PackagedClass { public PackagedClass() { System.out.println("Creating a packaged class"); } } ///:~ Then create the following file in a directory other than c05: //: Foreign.java Chapter 5: Hiding the Implementation 167
- 168. package c05.foreign; import c05.*; public class Foreign { public static void main (String args[]) { PackagedClass pc = new PackagedClass(); } } ///:~ Explain why the compiler generates an error. Would making the Foreign class part of the c05 package change anything? 168 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 169. k 6: Reusing classes One of the most compelling features about Java is code reuse. But to be revolutionary, you’ve got to be able to do a lot more than copy code and change it. That’s the approach used in procedural languages like C, and it hasn’t worked very well. As with everything in Java, the solution revolves around the class. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone has already built and debugged. The trick is to use the classes without soiling the existing code. In this chapter you’ll see two ways to accomplish this. The first is quite straightforward: You simply create objects of your existing class inside the new class. This is called composition because the new class is composed of objects of existing classes. Here, you’re simply reusing the functionality of the code, not its form. The second approach is more subtle. It creates a new class as a type of an existing class. You literally take the form of the existing class and add code to it, without modifying the existing class. This magical act is called inheritance, and the compiler does most of the work. Inheritance is one of the cornerstones of object-oriented programming and has additional implications that will be explored in the next chapter. It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense; they are both ways of making new types from existing types). In this chapter, you’ll learn about these code reuse mechanisms. Composition syntax Up till now composition has been used quite frequently. You simply place object handles inside new classes. For example, suppose you’d like an object that holds several String 169
- 170. objects, a couple of primitives and an object of another class. For the non-primitive objects, just put handles inside your new class, and for the primitives just define them inside your class (see page 80 if you have trouble executing this program): //: SprinklerSystem.java // Composition for code reuse package c06; class WaterSource { private String s; WaterSource() { System.out.println("WaterSource()"); s = new String("Constructed"); } public String toString() { return s; } } public class SprinklerSystem { private String valve1, valve2, valve3, valve4; WaterSource source; int i; float f; void print() { System.out.println("valve1 = " + valve1); System.out.println("valve2 = " + valve2); System.out.println("valve3 = " + valve3); System.out.println("valve4 = " + valve4); System.out.println("i = " + i); System.out.println("f = " + f); System.out.println("source = " + source); } public static void main(String args[]) { SprinklerSystem x = new SprinklerSystem(); x.print(); } } ///:~ One of the methods defined in WaterSource is special: toString( ). You will learn later that every non-primitive object has a toString( ) method, and it’s called in special situations when the compiler wants a string but its got one of these objects. So in the expression: System.out.println("source = " + source); The compiler sees you trying to add a String object (“source = “) to a WaterSource. This doesn’t make sense to it, because you can “add” a String only to another String, so it says “I’ll turn source into a String by calling toString( )!” After doing this it can combine the two Strings and pass the resulting String to System.out.println( ). Any time you want to allow this behavior with a class you create you need only to write a toString( ) method. At first glance, you might assume – Java being as safe and careful as it is – that the compiler would automatically construct objects for each of the handles in the above code, for example calling the default constructor for WaterSource to initialize source. The output of the print statement is in fact: valve1 = null valve2 = null 170 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 171. valve3 = null valve4 = null i = 0 f = 0.0 source = null Primitives that are fields in a class are automatically initialized to zero, as noted in Chapter 2. But the object handles are initialized to null, and if you try to call methods for any of them you’ll get an exception. It’s actually pretty good (and useful) that you can still print them out without throwing an exception. It makes sense that the compiler doesn’t just create a default object for every handle because that would incur unnecessary overhead in many cases. If you want the handles initialized, you can do it: 1. At the point the objects are defined. This means they’ll always be initialized before the constructor is called. 2. In the constructor for that class 3. Right before you actually need to use the object. This may reduce overhead, if there are situations where the object doesn’t need to be created. All three approaches are shown here: //: Bath.java // Constructor initialization with composition class Soap { private String s; Soap() { System.out.println("Soap()"); s = new String("Constructed"); } public String toString() { return s; } } public class Bath { private String // Initializing at point of definition: s1 = new String("Happy"), s2 = "Happy", s3, s4; Soap castille; int i; float toy; Bath() { System.out.println("Inside Bath()"); s3 = new String("Joy"); i = 47; toy = 3.14f; castille = new Soap(); } void print() { // Delayed initialization: if(s4 == null) s4 = new String("Joy"); System.out.println("s1 = " + s1); Chapter 6: Reusing Code & Classes 171
- 172. System.out.println("s2 = " + s2); System.out.println("s3 = " + s3); System.out.println("s4 = " + s4); System.out.println("i = " + i); System.out.println("toy = " + toy); System.out.println("castille = " + castille); } public static void main(String args[]) { Bath b = new Bath(); b.print(); } } ///:~ Notice that in the Bath constructor a statement is executed before any of the initializations take place. When you don’t initialize at the point of definition, there’s still no guarantee that you’ll perform any initialization before you send a message to an object handle – except for the inevitable run-time exception. Here’s the output for the program: Inside Bath() Soap() s1 = Happy s2 = Happy s3 = Joy s4 = Joy i = 47 toy = 3.14 castille = Constructed When print( ) is called it fills in s4 so all the fields are properly initialized by the time they are used. Inheritance syntax Inheritance is such an integral part of Java (and OOP languages in general) that it was introduced in Chapter 1 and has been used occasionally in chapters before this one, since certain situations required it. In addition, you’re always doing inheritance when you create a class, because if you don’t say otherwise you inherit from Java’s standard root class Object. The syntax for composition is obvious, but to perform inheritance there’s a distinctly different form. When you inherit, you are saying, “This new class is like that old class.” You state this in code by giving the name of the class, as usual, but before the opening brace of the class body, you put the keyword extends followed by the name of the base class. When you do this, you automatically get all the data members and methods in the base class. Here’s an example: //: Detergent.java // Inheritance syntax & properties class Cleanser { private String s = new String("Cleanser"); public void append(String a) { s += a; } public void dilute() { append(" dilute()"); } 172 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 173. public void apply() { append(" apply()"); } public void scrub() { append(" scrub()"); } public void print() { System.out.println(s); } public static void main(String args[]) { Cleanser x = new Cleanser(); x.dilute(); x.apply(); x.scrub(); x.print(); } } public class Detergent extends Cleanser { // Change a method: public void scrub() { append(" Detergent.scrub()"); super.scrub(); // Call base-class version } // Add methods to the interface: public void foam() { append(" foam()"); } // Test the new class: public static void main(String args[]) { Detergent x = new Detergent(); x.dilute(); x.apply(); x.scrub(); x.foam(); x.print(); System.out.println("Testing base class:"); Cleanser.main(args); } } ///:~ This demonstrates a number of features. First, in the Cleanser append( ) method, Strings are concatenated to s using the += operator, which is one of the operators (along with ‘+’) that the Java designers “overloaded” to work with Strings. Second, both Cleanser and Detergent contain a main( ) method. You can create a main( ) for each one of your classes, and it’s often recommended to code this way so that your test code is wrapped in with the class. Even if you have lots of classes in a program, only the main( ) for the public class invoked on the command line will be called (and you can have only one public class per file). So in this case, when you say java Detergent, Detergent.main( ) will be called. But you can also say java Cleanser to invoke Cleanser.main( ), even though Cleanser is not a public class. This technique of putting a main( ) in each class allows easy unit testing for each class. In addition, you don’t have to remove the main( ) when you’re finished testing; you can leave it in for later testing. Here, you can see that Detergent.main( ) calls Cleanser.main( ) explicitly. It’s important that all the methods in Cleanser are public. Remember that if you leave off any access specifier the member defaults to “friendly,” which allows access only to package members. Thus, within this package, anyone could use those methods if there were no access specifier. Detergent would have no trouble, for example. However if a class from some other package were to inherit Cleanser it could access only public members. So to plan for inheritance, as a general rule make all fields private and all methods public (protected members also allow access by derived classes; you’ll learn about this later). Of course, in particular cases you’ll need to make adjustments, but this is a useful guideline. Chapter 6: Reusing Code & Classes 173
- 174. Note that Cleanser has a set of methods in its interface: append( ), dilute( ), apply( ), scrub( ) and print( ). Because Detergent is derived from Cleanser (via the extends keyword) it automatically gets all these methods in its interface, even though you don’t see them all explicitly defined in Detergent. You can think of inheritance, then, as reusing the interface (the implementation comes along for free, but that part isn’t the primary point). As seen in scrub( ), it’s possible to take a method that’s been defined in the base class and modify it. In this case, you might want to call the method from the base class inside the new version. But inside scrub( ) you cannot simply call scrub( ), since that would produce a recursive call which isn’t what you want. To solve this problem Java has a keyword super which refers to the “superclass” that the current class has been inherited from. Thus the expression super.scrub( ) calls the base-class version of the method scrub( ). When inheriting you’re not restricted to using the methods of the base class. You can also add new methods to the derived class, exactly the way you put any method in a class: just define it. The extends keyword actually suggests that you are going to add new methods to the base-class interface, and the method foam( ) is an example of this. In Detergent.main( ) you can see that for a Detergent object you can call all the methods that are available in Cleanser as well as in Detergent (i.e. foam( )). Initializing the base class Since there are now two classes involved – the base class and the derived class – instead of just one, it can be a little bit confusing to try to imagine the resulting object produced by a derived class. From the outside, it looks like the new class has the same interface as the base class, and maybe some additional methods and fields. But inheritance doesn't just copy the interface of the base class. When you create an object of the derived class, it contains within it a subobject of the base class. This subobject is the same as if you had created an object of the base class by itself. It's just that, from the outside, the subobject of the base class is wrapped within the derived-class object. Of course, it’s essential that the base-class subobject be initialized correctly and there’s only one way to guarantee it: perform the initialization in the constructor, by calling the base-class constructor, which has all the appropriate knowledge and privileges to perform the base-class initialization. Java automatically inserts calls to the base-class constructor in the derived-class constructor. The following example shows this working with three levels of inheritance: //: Cartoon.java // Constructor calls during inheritance class Art { Art() { System.out.println("Art constructor"); } } class Drawing extends Art { Drawing() { System.out.println("Drawing constructor"); } } public class Cartoon extends Drawing { 174 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 175. Cartoon() { System.out.println("Cartoon constructor"); } public static void main(String args[]) { Cartoon x = new Cartoon(); } } ///:~ The output for this program shows the automatic calls: Art constructor Drawing constructor Cartoon constructor You can see that the construction happens from the base “outward,” so the base class is initialized before the derived-class constructors can access it. Even if you don’t create a constructor for Cartoon( ), the compiler will synthesize a default constructor for you that calls the base class constructor. Constructors with arguments The above example has default constructors – that is, they don’t have any arguments. It’s easy for the compiler to call these because there’s no question about what arguments to pass. But what if your class doesn’t have default arguments or you want to call a base- class constructor that has an argument? You must explicitly write the calls to the base- class constructor using the super keyword and the appropriate argument list: //: Chess.java // Inheritance, constructors and arguments class Game { Game(int i) { System.out.println("Game constructor"); } } class BoardGame extends Game { BoardGame(int i) { super(i); System.out.println("BoardGame constructor"); } } public class Chess extends BoardGame { Chess() { super(11); System.out.println("Chess constructor"); } public static void main(String args[]) { Chess x = new Chess(); } } ///:~ If you don’t call the base-class constructor in BoardGame( ), the compiler will complain that it can’t find a constructor of the form Game( ). In addition, the call to the base-class constructor must be the first thing you do in the derived-class constructor (the compiler will remind you if you get it wrong). Chapter 6: Reusing Code & Classes 175
- 176. Catching base constructor exceptions As just noted, the compiler forces you to place the base-class constructor call first in the body of the derived-class constructor. This means nothing else can appear before it. As you’ll see in Chapter 9, this also prevents a derived-class constructor from catching any exceptions that come from a base class. This can be inconvenient at times. Combining composition & inheritance Of course, you can use the two together. The following example shows the creation of a more complex class, using both inheritance and composition, along with the necessary constructor initialization: //: PlaceSetting.java // Combining composition & inheritance class Plate { Plate(int i) { System.out.println("Plate constructor"); } } class DinnerPlate extends Plate { DinnerPlate(int i) { super(i); System.out.println( "DinnerPlate constructor"); } } class Utensil { Utensil(int i) { System.out.println("Utensil constructor"); } } class Spoon extends Utensil { Spoon(int i) { super(i); System.out.println("Spoon constructor"); } } class Fork extends Utensil { Fork(int i) { super(i); System.out.println("Fork constructor"); } } class Knife extends Utensil { Knife(int i) { 176 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 177. super(i); System.out.println("Knife constructor"); } } // A cultural way of doing something: class Custom { Custom(int i) { System.out.println("Custom constructor"); } } public class PlaceSetting extends Custom { Spoon sp; Fork frk; Knife kn; DinnerPlate pl; PlaceSetting(int i) { super(i + 1); sp = new Spoon(i + 2); frk = new Fork(i + 3); kn = new Knife(i + 4); pl = new DinnerPlate(i + 5); System.out.println( "PlaceSetting constructor"); } public static void main(String args[]) { PlaceSetting x = new PlaceSetting(9); } } ///:~ While the compiler forces you to initialize the base classes, and requires that you do it right at the beginning of the constructor, it doesn’t watch over you to make sure you initialize the member objects, so you must remember to pay attention to that. Guaranteeing proper cleanup Java doesn’t have the C++ concept of a destructor, a method that is automatically called when an object is destroyed. The reason is probably that in Java the practice is simply to forget about objects rather than destroying them, allowing the garbage collector to reclaim the memory as necessary. Often this is fine, but there are times when your class may perform some activities during its lifetime that require cleanup. As mentioned in Chapter 4, you can’t rely on when the garbage collector will be called, or if it will ever be called. Thus, if you want something cleaned up for a class, you must write a special method to do it explicitly, and make sure that the client programmer knows they must call this method. On top of this, as described in Chapter 9 (exception handling), you must guard against an exception by putting such cleanup in a finally clause. Consider an example of a computer-aided-design system that draws pictures on the screen: //: CADSystem.java // Ensuring proper cleanup import java.util.*; Chapter 6: Reusing Code & Classes 177
- 178. class Shape { Shape(int i) { System.out.println("Shape constructor"); } void cleanup() { System.out.println("Shape cleanup"); } } class Circle extends Shape { Circle(int i) { super(i); System.out.println("Drawing a Circle"); } void cleanup() { System.out.println("Erasing a Circle"); super.cleanup(); } } class Triangle extends Shape { Triangle(int i) { super(i); System.out.println("Drawing a Triangle"); } void cleanup() { System.out.println("Erasing a Triangle"); super.cleanup(); } } class Line extends Shape { private int start, end; Line(int start, int end) { super(start); this.start = start; this.end = end; System.out.println("Drawing a Line: " + start + ", " + end); } void cleanup() { System.out.println("Erasing a Line: " + start + ", " + end); super.cleanup(); } } public class CADSystem extends Shape { private Circle c; private Triangle t; private Line[] lines = new Line[10]; CADSystem(int i) { super(i + 1); for(int j = 0; j < 10; j++) lines[j] = new Line(j, j*j); 178 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 179. c = new Circle(1); t = new Triangle(1); System.out.println("Combined constructor"); } void cleanup() { System.out.println("CADSystem.cleanup()"); t.cleanup(); c.cleanup(); for(int i = 0; i < lines.length; i++) lines[i].cleanup(); super.cleanup(); } public static void main(String args[]) { CADSystem x = new CADSystem(47); try { // Code and exception handling... } finally { x.cleanup(); } } } ///:~ Everything in this system is some kind of Shape (which is itself a kind of Object since it’s implicitly inherited from the root class). Each class redefines Shape’s cleanup( ) method in addition to calling the base-class version of that method using super. The specific Shape classes Circle, Triangle and Line all have constructors that “draw,” although any method called during the lifetime of the object could be responsible for doing something that needs cleanup. Each class has its own cleanup( ) method to restore non-memory things back to the way they were before the object existed. In main( ), you can see two keywords that are new, and won’t officially be introduced until Chapter 9: try and finally. The try keyword indicates that the block that follows (delimited by curly braces) is a guarded region, which means that it is given special treatment. One of these special treatments is that the code in the finally clause following this guarded region is always executed, no matter how the try block exits (with exception handling, it’s possible to leave a try block in a number of non-ordinary ways). Here, the finally clause is saying “always call cleanup( ) for x, no matter what happens.” These keywords will be explained thoroughly in Chapter 9. Note that in your cleanup method you must also pay attention to the order in which the base-class and member-object cleanup methods get called, in case one subobject may depend on another. In general you should follow the same form that is imposed by a C++ compiler on its destructors: first perform all the work specific to your class (which may require that base-class elements still be viable) then lastly call the base-class cleanup method, as demonstrated here. There may be many cases where the cleanup issue is not a problem; you just let the garbage collector do the work. But when you must do it explicitly, diligence and attention is required. Order of garbage collection There’s not much you can rely on when it comes to garbage collection. The garbage collector might never be called. If it is, it can reclaim objects in any order it wants. In addition, implementations of the garbage collector in Java 1.0 often don’t call the finalize( ) methods. It’s best not to rely on garbage collection for anything but memory reclamation, and if you want cleanup to take place, make your own cleanup methods and Chapter 6: Reusing Code & Classes 179
- 180. don’t rely on finalize( ) (as mentioned earlier, Java 1.1 can be forced to call all the finalizers). Name hiding Only C++ programmers might be surprised by this, since it works differently in that language. If a Java base class has a method name that’s overloaded several times, redefining that method name in the derived class will not hide any of the base-class versions. Thus overloading works regardless of whether the method was defined at this level or in a base class: //: Hide.java // Overloading a base-class method name // in a derived class does not hide the // base-class versions class Homer { char doh(char c) { System.out.println("doh(char)"); return 'd'; } float doh(float f) { System.out.println("doh(float)"); return 1.0f; } } class Milhouse {} class Bart extends Homer { void doh(Milhouse m) {} } class Hide { public static void main(String args[]) { Bart b = new Bart(); b.doh(1); // doh(float) used b.doh('x'); b.doh(1.0f); b.doh(new Milhouse()); } } ///:~ As you’ll see in the next chapter, it’s far more common to override methods of the same name using exactly the same signature and return type as in the base class. It can be confusing otherwise (which is why C++ disallows it, to prevent you from making what is probably a mistake). 180 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 181. Choosing composition vs. inheritance Both composition and inheritance allow you to place subobjects inside your new class. You may now be wondering what the difference is between the two, and when to choose one over the other. Composition is generally used when you want the features of an existing class inside your new class, but not its interface. That is, you embed an object so that you can use that object to implement features of your new class, but the user of your new class sees the interface you’ve defined rather than the interface from the embedded object. For this effect, you embed private objects of existing classes inside your new class. Sometimes it makes sense to allow the class user to directly access the composition of your new class, that is, to make the member objects public. The member objects use implementation hiding themselves, so this is a safe thing to do and when the user knows you’re assembling a bunch of parts, it makes the interface easier to understand. A car object is a good example: //: Car.java // Composition with public objects class Engine { public void start() {} public void rev() {} public void stop() {} } class Wheel { public void inflate(int psi) {} } class Window { public void rollup() {} public void rolldown() {} } class Door { public Window window = new Window(); public void open() {} public void close() {} } public class Car { public Engine engine = new Engine(); public Wheel wheel[] = new Wheel[4]; public Door left = new Door(), right = new Door(); // 2-door Car() { for(int i = 0; i < 4; i++) wheel[i] = new Wheel(); } public static void main(String args[]) { Car car = new Car(); Chapter 6: Reusing Code & Classes 181
- 182. car.left.window.rollup(); car.wheel[0].inflate(72); } } ///:~ Because the composition of a car is part of the analysis of the problem (and not simply part of the underlying design), making the members public assists the client programmer’s understanding of how to use the class and requires less code complexity for the creator of the class. When you inherit, you take an existing class and make a special version of it. Generally, this means you’re taking a general-purpose class and specializing it for a particular need. With a little thought, you’ll see that it would make no sense to compose a car using a vehicle object – a car doesn’t contain a vehicle, it is a vehicle. The is-a relationship is expressed with inheritance, and the has-a relationship is expressed with composition. protected Now that you’ve been introduced to inheritance, the keyword protected finally has meaning. In an ideal world, private members would always be hard-and-fast private, but in real projects there are times when you want to make something hidden from the world at large and yet allow access for members of derived classes. The protected keyword is a nod to pragmatism; it says, “This is private as far as the class user is concerned, but available to anyone who inherits from this class or anyone else in the same package.” That is, protected in Java is automatically “friendly.” The best tack to take is to leave the data members private – you should always preserve your right to change the underlying implementation. You can then allow controlled access to inheritors of your class through protected methods: //: Orc.java // The protected keyword import java.util.*; class Villain { private int i; protected int read() { return i; } protected void set(int ii) { i = ii; } public Villain(int ii) { i = ii; } public int value(int m) { return m*i; } } public class Orc extends Villain { private int j; public Orc(int jj) { super(jj); j = jj; } public void change(int x) { set(x); } } ///:~ You can see that change( ) has access to set( ) because it’s protected. Incremental development One of the advantages of inheritance is that it supports incremental development by allowing you to introduce new code without causing bugs in existing code. This also isolates new 182 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 183. bugs to the new code. By inheriting from an existing, functional class and adding data members and methods (and redefining existing methods) you leave the existing code – that someone else may still be using – untouched and unbugged. If a bug happens, you know it’s in your new code, which is much shorter and easier to read than if you had modified the body of existing code. It’s rather amazing how cleanly the classes are separated. You don’t even need the source code for the methods in order to reuse the code. At most, you just import a package. (This is true for both inheritance and composition.) It’s important to realize that program development is an incremental process, just like human learning. You can do as much analysis as you want, but you still won’t know all the answers when you set out on a project. You’ll have much more success – and more immediate feedback – if you start out to “grow” your project as an organic, evolutionary creature, rather than constructing it all at once like a glass-box skyscraper. Although inheritance for experimentation can be a useful technique, at some point after things stabilize you need to take a new look at your class hierarchy with an eye to collapsing it into a sensible structure. Remember that underneath it all, inheritance is meant to express a relationship that says, “This new class is a type of that old class.” Your program should not be concerned with pushing bits around, but instead with creating and manipulating objects of various types to express a model in the terms that come from the problem space. Upcasting The most important aspect of inheritance is not that it provides methods for the new class. It’s the relationship expressed between the new class and the base class. This relationship can be summarized by saying, “The new class is a type of the existing class.” This description is not just a fanciful way of explaining inheritance – it’s supported directly by the language. As an example, consider a base class called Instrument that represents musical instruments and a derived class called Wind. Because inheritance means that all the methods in the base class are also available in the derived class, any message you can send to the base class can also be sent to the derived class. If the Instrument class has a play( ) method, so will Wind instruments. This means we can accurately say that a Wind object is also a type of Instrument. The following example shows how the compiler supports this notion: //: Wind.java // Inheritance & upcasting import java.util.*; class Instrument { public void play() {} static void tune(Instrument i) { // ... i.play(); } } // Wind objects are instruments // because they have the same interface: class Wind extends Instrument { public static void main(String args[]) { Wind flute = new Wind(); Chapter 6: Reusing Code & Classes 183
- 184. Instrument.tune(flute); // Upcasting } } ///:~ What’s interesting in this example is the tune( ) method, which accepts an Instrument handle. However, in Wind.main( ) the tune( ) method is called by giving it a Wind handle. Given that Java is very particular about type checking, it seems strange that a method that accepts one type will readily accept another type, until you realize that a Wind object is also an Instrument object, and there’s no method that tune( ) could call for an Instrument that isn’t also in Wind. Inside tune( ), the code works for Instrument instrument wind and anything derived from Instrument, and the act of converting a Wind handle into an Instrument handle is called upcasting. Why “upcasting”? The reason for the term is historical and is based on the way class inheritance diagrams have traditionally been drawn: with the root at the top of the page, growing downward. (Of course, you can draw your diagrams any way you find helpful.) The inheritance diagram for Wind.java is then: Casting from derived to base moves up on the inheritance diagram, so it’s commonly referred to as upcasting. Upcasting is always safe because you’re going from a more specific type to a more general type. That is, the derived class is a superset of the base class – it may contain more methods than the base class, but it must contain at least the methods in the base class – so the only thing that can occur to the class interface during the upcast is that it can lose methods, not gain them. This is why the compiler allows upcasting without any explicit casts or other special notation. You can also perform the reverse of upcasting, called downcasting, but this involves a dilemma that is the subject of Chapter 11. Composition vs. inheritance revisited In object-oriented programming, the most likely way that you’ll create and use code is by simply packaging data and methods together into a class, and using objects of that class. Less often, you’ll use existing classes to build new classes with composition. Even less often than that you’ll use inheritance. Thus, although inheritance gets a lot of emphasis while learning OOP it doesn’t mean you should use it everywhere you possibly can; on the contrary you should use it sparingly, only when it’s clear that inheritance is useful. One of the clearest ways to determine whether you should be using composition or inheritance is by asking whether you’ll ever need to upcast from your new class to the base class. If you must upcast, then inheritance is necessary, but if you don’t need to upcast, then you should look closely at whether you need inheritance. The next chapter (polymorphism) provides one of the most compelling reasons for upcasting, but if you remember to ask: “do I need to upcast?” you’ll have a good tool for deciding between composition and inheritance. 184 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 185. The final keyword The final keyword has slightly different meanings depending on the context in which it is used, but in general it says “this cannot be changed.” You may want to prevent changes for two reasons: design or efficiency. Because these two reasons are quite different, it’s possible to misuse the final keyword. The following sections discuss the three places where final can be used: for data, methods and for a class itself. Final data Many programming languages have a way to tell the compiler that a piece of data is “constant.” A constant is useful for two reasons: 1. It may be a compile-time constant that cannot change 2. It may be a value initialized at run-time that you don’t want changed In the case of a compile-time constant the compiler may “fold” the constant value into any calculations where it’s used; that is, the calculation may be performed at compile time, thus eliminating some run-time overhead. In Java, these sorts of constants must be primitives and are expressed using the final keyword. A value must be given at the time of definition of such a constant. A field that is both static and final has only one piece of storage that cannot be changed. When using final with object handles rather than primitives, the meaning gets a bit confusing. With a primitive, final makes the value a constant, but with an object handle, final makes the handle itself a constant. The handle must be initialized to an object at the point of declaration, and the handle can never be changed to point to another object. However, the object itself may be modified; Java does not provide a way to make any arbitrary object a constant (you can, however, write your class so that objects have the effect of being constant). This restriction includes arrays, which are also objects. Here’s an example that demonstrates final fields: //: FinalData.java // The effect of final on fields class Value { int i = 1; } public class FinalData { // Can be compile-time constants final int i1 = 9; static final int I2 = 99; // Typical public constant: public static final int I3 = 39; // Cannot be compile-time constants: final int i4 = (int)(Math.random()*20); static final int i5 = (int)(Math.random()*20); Value v1 = new Value(); final Value v2 = new Value(); Chapter 6: Reusing Code & Classes 185
- 186. static final Value v3 = new Value(); //! final Value v4; // Pre-Java 1.1 Error: // no initializer // Arrays: final int a[] = { 1, 2, 3, 4, 5, 6 }; public void print(String id) { System.out.println( id + ": " + "i4 = " + i4 + ", i5 = " + i5); } public static void main(String[] args) { FinalData fd1 = new FinalData(); //! fd1.i1++; // Error: can't change value fd1.v2.i++; // Object isn't constant! fd1.v1 = new Value(); // OK -- not final for(int i = 0; i < fd1.a.length; i++) fd1.a[i]++; // Object isn't constant! //! fd1.v2 = new Value(); // Error: Can't //! fd1.v3 = new Value(); // change handle //! fd1.a = new int[3]; fd1.print("fd1"); System.out.println("Creating new FinalData"); FinalData fd2 = new FinalData(); fd1.print("fd1"); fd2.print("fd2"); } } ///:~ Since i1 and I2 are final primitives with compile-time values, they can both be used as compile-time constants and are not different in any important way. However, I3 is the more typical way you’ll see such constants defined: public so they’re usable outside the package, static to emphasize that there’s only one, and final to say it’s a constant. Notice that final static primitives with constant initial values (that is, compile-time constants) are named with all capitals by convention. Note that i5 cannot be known at compile time so it is not capitalized. Just because something is final doesn’t mean its value is known at compile-time. This is demonstrated by initializing i4 and i5 at run-time using randomly generated numbers. This portion of the example also shows the difference between making a final value static or non-static. This difference shows up only when the values are initialized at run-time, since the compile-time values are treated the same by the compiler (and presumably optimized out of existence). The difference is shown in the output from one run: fd1: i4 = 15, i5 = 9 Creating new FinalData fd1: i4 = 15, i5 = 9 fd2: i4 = 10, i5 = 9 Note that the values of i4 for fd1 and fd2 are unique, but the value for i5 is not changed by creating the second FinalData object. That’s because it’s static and is initialized once upon loading and not each time a new object is created. The variables v1 through v4 demonstrate the meaning of a final handle. As you can see in main( ), just because v2 is final doesn’t mean you can’t change its value. However, you cannot re-bind v2 to a new object, precisely because it’s final. That’s what final means 186 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 187. for a handle. You can also see the same meaning holds true for an array, which is just another kind of handle. Making handles final seems much less useful than making primitives final. Blank finals Java 1.1 allows the creation of blank finals, which are fields that are declared as final but which are not given an initialization value. In all cases, the blank final must be initialized before it is used, and the compiler ensures this. However, blank finals provide much more flexibility in the use of the final keyword since, for example, a final field inside a class can now be different for each object and yet still retains its immutable quality. Here’s an example: //: BlankFinal.java // "Blank" final data members class Poppet { } class BlankFinal { final int i = 0; // Initialized final final int j; // Blank final final Poppet p; // Blank final handle // Blank finals MUST be initialized // in the constructor: BlankFinal() { j = 1; // Initialize blank final p = new Poppet(); } BlankFinal(int x) { j = x; // Initialize blank final p = new Poppet(); } public static void main(String args[]) { BlankFinal bf = new BlankFinal(); } } ///:~ You’re forced to perform assignments to finals either with an expression at the point of definition of the field, or in every constructor. This way it’s guaranteed that the final field is always initialized before use. Final arguments Java 1.1 allows you to make arguments final by declaring them as such in the argument list. All this means is that inside the method you cannot change what the argument handle points to: //: FinalArguments.java // Using "final" with method arguments class Gizmo { public void spin() {} } public class FinalArguments { void with(final Gizmo g) { //! g = new Gizmo(); // Illegal -- g is final g.spin(); Chapter 6: Reusing Code & Classes 187
- 188. } void without(Gizmo g) { g = new Gizmo(); // OK -- g not final g.spin(); } public static void main(String args[]) { FinalArguments bf = new FinalArguments(); bf.without(null); bf.with(null); } } ///:~ Note that you can still assign a null handle to an argument that’s final without the compiler catching it, just like you can with a non-final argument. Final methods There are two reasons for final methods. The first is to put a “lock” on the method to prevent any inheriting class from changing its meaning. This is done for design reasons, when you want to make sure that the behavior of a method is retained during inheritance and cannot be overridden. The second reason for final methods is efficiency. If you make a method final, you are allowing the compiler to turn any calls to that method into inline calls. That is, when the compiler sees a final method call it may (at its discretion) skip the normal approach of inserting code to perform the method call mechanism (push arguments on the stack, hop over to the method code and execute it, hop back and clean off the stack arguments, deal with the return value) and instead replace the method call with a copy of the actual code in the method body. This eliminates the overhead of the method call. Of course, if a method is big then your code begins to bloat, and then you probably won’t see any performance gains from inlining since any improvements will be dwarfed by the amount of time spent inside the method. It is implied that the Java compiler is able to detect these situations and choose wisely whether to actually inline a final method. However, it’s better not to trust that the compiler is able to do this and only make a method final if it’s quite small or if you want to explicitly prevent overriding. Any private methods in a class are implicitly final. Because you can’t access a private method, you can’t override it. You can add the final specifier to a private method but it doesn’t give that method any extra meaning. Final classes When you say that an entire class is final (by preceding its definition with the final keyword) you state that you don’t want to inherit from this class or allow anyone else to do so. For some reason the design of your class is such that there is never a need to make any changes, or for safety or security reasons you don’t want to allow subclassing. Alternatively you might be dealing with an efficiency issue and you want to make sure that any activity involved with objects of this class is as efficient as possible. //: Jurassic.java // Making an entire class final class SmallBrain {} final class Dinosaur { 188 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 189. int i = 7; int j = 1; SmallBrain x = new SmallBrain(); void f() {} } //! class Further extends Dinosaur {} // error: Cannot extend final class 'Dinosaur' public class Jurassic { public static void main(String args[]) { Dinosaur n = new Dinosaur(); n.f(); n.i = 40; n.j++; } } ///:~ Note that the data members can be final or not, as you choose. The same rules apply to final for data members regardless of whether the class is defined as final. Defining the class as final simply prevents inheritance, no more. However, because it prevents inheritance this means that all methods in a final class are implicitly final, since there’s no way to override them. Thus the compiler has the same efficiency options as it does if you explicitly declare a method final. You can add the final specifier to a method in a final class, but it doesn’t add any meaning. Final caution It can seem to be very sensible to make a method final while you’re designing a class. You might feel that efficiency is very important when using your class and that no one could possibly want to override your methods anyway. Sometimes this is true. But be very careful with your assumptions here. In general, it’s very hard to anticipate how a class can be reused, especially a general-purpose class. If you define a method as final you might prevent the possibility of reusing your class through inheritance in some other programmer’s project simply because you couldn’t imagine it being used that way. The standard Java library is a good example of this. In particular, the Vector class is commonly used and might be even more useful if, in the name of efficiency, all the methods hadn’t been made final. It’s easily conceivable that you might want to inherit and override with such a fundamentally useful class, but the designers somehow decided this wasn’t appropriate. This is ironic in two places. First, Stack is inherited from Vector, which says that a Stack is a Vector, which isn’t really true. Second, many of the most important methods of Vector, such as addElement( ) and elementAt( ) are synchronized, which as you shall see in Chapter 14 incurs a significant performance overhead that probably wipes out any gains provided by final. This lends credence to the theory that programmers are consistently bad at guessing where optimizations should occur. It’s just too bad that such a clumsy design made it into the standard library, where we all have to cope with it. It’s also interesting to note that Hashtable, another important standard library class, does not have any final methods. As mentioned elsewhere in this book, it’s quite obvious that some classes were designed by completely different people than others (note the brevity of the method names in Hashtable compared to Vector). This is precisely the sort of thing that should not be obvious to consumers of a class library – when things are inconsistent it Chapter 6: Reusing Code & Classes 189
- 190. just makes more work for the user. Yet another paean to the value of design and code walkthroughs. Initialization & class loading In many more traditional languages, programs are loaded all at once as part of the startup process. This is followed by initialization, and then the program begins. The process of initialization in these languages must be carefully controlled so that the order of initialization of statics doesn’t cause trouble. C++, for example, has problems if one static is expecting another static to be valid before the second one has been initialized. Java doesn’t have this problem because it takes a different approach to loading. Because everything in Java is an object many activities become easier, and this is one of them. As you will learn in the next chapter, the code for each object exists in a separate file. That file isn’t loaded until the code is needed. In general, you can say that until an object of that class is constructed, the class code doesn’t get loaded. Since there can be some subtleties with static methods, you can also say “class code is loaded at the point of first use.” The point of first use is also where the static initialization takes place: all the static objects and the static code block will be initialized in textual order (that is, the order that you write them down in the class definition) at the point of loading. The statics, of course, are initialized only once. Initialization with inheritance It’s helpful to look at the whole initialization process including inheritance to get a full picture of what happens. Consider the following code: //: Beetle.java // The full process of initialization. class Insect { int i = 9; int j; Insect() { prt("i = " + i + ", j = " + j); j = 39; } static int x1 = prt("static Insect.x1 initialized"); static int prt(String s) { System.out.println(s); return 47; } } public class Beetle extends Insect { int k = prt("Beetle.k initialized"); Beetle() { prt("k = " + k); prt("j = " + j); } static int x2 = prt("static Beetle.x2 initialized"); 190 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 191. static int prt(String s) { System.out.println(s); return 63; } public static void main(String args[]) { prt("Beetle constructor"); Beetle b = new Beetle(); } } ///:~ The output for this program is: static Insect.x initialized static Beetle.x initialized Beetle constructor i = 9, j = 0 Beetle.k initialized k = 63 j = 39 The first thing that happens when you run Java on Beetle is that the loader goes out and finds that class. In the process of loading it, the loader notices that it has a base class (that’s what the extends keyword says), which it then loads. This will happen whether or not you’re actually going to make an object of that base class (try commenting out the object creation to prove it to yourself). If the base class itself has a base class, that second base class would then be loaded, and so on. Next, the static initialization in the root base class (in this case, Insect) is performed, then the next derived class and so on. This is important because the derived-class static initialization might depend on the base class member being initialized properly. At this point, the necessary classes have all been loaded so the object can be created. First, all the primitives in this object are set to their default values and the object handles are set to null. Then the base-class constructor will be called; in this case the call is automatic but you may also specify the constructor call (as the first operation in the Beetle( ) constructor) using super. The base class construction goes through the same process in the same order as the derived-class constructor. After the base-class constructor completes, the instance variables are initialized, in textual order. Finally, the rest of the body of the constructor is executed. Summary Both inheritance and composition allow you to create a new type from existing types. Typically, however, you use composition to reuse existing types as part of the underlying implementation of the new type and inheritance when you want to reuse the interface. Since the derived class has the base-class interface, it can be upcast to the base, which is critical for polymorphism as you’ll see in the next chapter. Despite the strong emphasis on inheritance in object-oriented programming, when starting a design, you should generally prefer composition during the first cut, and use inheritance only when it is clearly necessary (as you’ll see in the next chapter). Composition tends to be more generally flexible. In addition, by using the added artifice of inheritance with your member type, you can change the exact type, and thus the behavior, of those member objects at run-time, therefore you can change the behavior of the composed object at run-time. Chapter 6: Reusing Code & Classes 191
- 192. Although code reuse through composition and inheritance is very helpful for rapid project development, you’ll generally want to redesign your class hierarchy before allowing other programmers to become dependent on it. Your goal is a hierarchy where each class has a specific use and is neither too big (encompassing so much functionality that it’s unwieldy to reuse) nor annoyingly small (you can’t use it by itself or without adding functionality). Your finished classes should themselves be easily reused. Exercises 1. Create two classes, A and B, with default constructors (empty argument lists) that announce themselves. Inherit a new class called C from A, and create a member B inside C. Do not create a constructor for C. Create an object of class C and observe the results. 2. Modify exercise one so A and B have constructors with arguments instead of default constructors. Write a constructor for C and perform all initialization within C’s constructor. 3. Take the file Cartoon.java and comment out the constructor for the Cartoon class. Explain what happens. 4. Take the file Chess.java and comment out the constructor for the Chess class. Explain what happens. 192 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 193. 3 7: Polymorphism Polymorphism is the third essential feature of an object-oriented programming language, after data abstraction and inheritance. It provides another dimension of separation of interface from implementation, to decouple what from how. Polymorphism allows improved code organization and readability as well as the creation of extensible programs that can be “grown,” not only during the original creation of the project, but also when new features are desired. Encapsulation creates new data types by combining characteristics and behaviors. Implementation hiding separates the interface from the implementation by making the details private. This sort of mechanical organization makes ready sense to someone with a procedural programming background. But polymorphism deals with decoupling in terms of types. In the last chapter, you saw how inheritance allows the treatment of an object as its own type or its base type. This ability is critical because it allows many types (derived from the same base type) to be treated as if they were one type, and a single piece of code to work on all those different types equally. The polymorphic method call allows one type to express its distinction from another, similar type, as long as they’re both derived from the same base type. This distinction is expressed through differences in behavior of the methods you can call through the base class. In this chapter, you’ll learn about polymorphism (also called dynamic binding or late binding or run-time binding) starting from the very basics, with simple examples that strip away everything but the polymorphic behavior of the program. Upcasting In the last chapter you saw how an object can be used as its own type or as an object of its base type. Taking an object handle and treating it as the handle of the base type is called upcasting because of the way inheritance trees are drawn with the base class at the top. 193
- 194. You also saw a problem arise, which is embodied in the following (see page 80 if you have trouble executing this program): //: Music.java // Inheritance & upcasting package c07; class Note { private int value; private Note(int val) { value = val; } public static final Note middleC = new Note(0), cSharp = new Note(1), cFlat = new Note(2); } // Etc. class Instrument { public void play(Note n) { System.out.println("Instrument.play()"); } } // Wind objects are instruments // because they have the same interface: class Wind extends Instrument { // Redefine interface method: public void play(Note n) { System.out.println("Wind.play()"); } } public class Music { public static void tune(Instrument i) { // ... i.play(Note.middleC); } public static void main(String[] args) { Wind flute = new Wind(); tune(flute); // Upcasting } } ///:~ The method Music.tune( ) accepts an Instrument handle, but also without complaint anything derived from Instrument. In main( ), you can see this happening as a Wind handle is passed to tune( ), with no cast necessary. This is acceptable; the interface in Instrument must exist in Wind, because Wind is inherited from Instrument. Upcasting from Wind to Instrument may “narrow” that interface, but it cannot make it any less than the full interface to Instrument. Why upcast? This program might seem strange to you. Why should anyone intentionally forget the type of an object? This is what happens when you upcast, and it seems like it could be much more straightforward if tune( ) simply takes a Wind handle as its argument. This brings up an essential point: if you did that, you’d have to write a new tune( ) for every type of 194 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 195. Instrument in your system. Suppose we follow this reasoning and add Stringed and Brass instruments: //: Music2.java // Overloading instead of upcasting class Note2 { private int value; private Note2(int val) { value = val; } public static final Note2 middleC = new Note2(0), cSharp = new Note2(1), cFlat = new Note2(2); } // Etc. class Instrument2 { public void play(Note2 n) { System.out.println("Instrument2.play()"); } } class Wind2 extends Instrument2 { public void play(Note2 n) { System.out.println("Wind2.play()"); } } class Stringed2 extends Instrument2 { public void play(Note2 n) { System.out.println("Stringed2.play()"); } } class Brass2 extends Instrument2 { public void play(Note2 n) { System.out.println("Brass2.play()"); } } public class Music2 { public static void tune(Wind2 i) { i.play(Note2.middleC); } public static void tune(Stringed2 i) { i.play(Note2.middleC); } public static void tune(Brass2 i) { i.play(Note2.middleC); } public static void main(String[] args) { Wind2 flute = new Wind2(); Stringed2 violin = new Stringed2(); Brass2 frenchHorn = new Brass2(); tune(flute); // No upcasting tune(violin); tune(frenchHorn); Chapter 7: Polymorphism 195
- 196. } } ///:~ This works, but there’s a major drawback: you must write type-specific methods for each new Instrument2 class you add. This means more programming in the first place, but it also means that if you want to add a new method like tune( ) or a new type of Instrument you’ve got a lot of work to do. Add to that the fact that the compiler won’t give you any error messages if you forget to overload one of your methods and the whole process of working with types becomes unmanageable. Wouldn’t it be much nicer if you could just write a single method that takes the base class as its argument, and not any of the specific derived classes. That is, wouldn’t it be nice if you could forget that there are derived classes, and only write your code to talk to the base class? That’s exactly what polymorphism allows you to do. However, most programmers (who come from a procedural programming background) have a bit of trouble with the way polymorphism works. The twist The difficulty with Music.java can be seen by running the program. The output is Wind.play( ). This is clearly the desired output, but it doesn’t seem to make sense that it would work that way. Look at the tune( ) method: public static void tune(Instrument i) { // ... i.play(Note.middleC); } It receives an Instrument handle. So how can the compiler possibly know that this Instrument handle happens to be pointing to a Wind in this case and not a Brass or Stringed? Actually, the compiler can’t. To get a deeper understanding of the issue, it’s useful to examine the subject of binding. Method call binding Connecting a method call to a method body is called binding. When binding is performed before the program is run (by the compiler and linker, if there is one), it’s called early binding. You might not have heard the term before because it has never been an option with procedural languages: C compilers have only one kind of method call, and that’s early binding. The confusing part of the above program revolves around early binding because the compiler cannot know the correct method to call when it has only an Instrument handle. The solution is called late binding, which means the binding occurs at run-time, based on the type of the object. Late binding is also called dynamic binding or run-time binding. When a language implements late binding, there must be some mechanism to determine the type of the object at run-time and call the appropriate method. That is, the compiler still doesn’t know the actual object type, but the method-call mechanism finds out and calls the correct method body. The late-binding mechanism varies from language to language, but you can imagine that some sort of type information must be installed in the objects themselves. 196 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 197. All method binding in Java uses late binding, unless a method has been declared final. This means you ordinarily don’t have to make any decisions about whether late binding will occur – it just happens automatically. Why would you declare a method final? As noted in the last chapter, it prevents anyone from overriding that method. Perhaps more importantly, it effectively “turns off” dynamic binding, or rather it tells the compiler that dynamic binding isn’t necessary. This allows the compiler to generate more efficient code for final method calls. Producing the right behavior Once you know that all method binding in Java happens polymorphically via late binding, you can always write your code to talk to the base-class and know that all the derived- class cases will work correctly using the same code. Or to put it another way, you “send a message to an object and let the object figure out the right thing to do.” The classic example in OOP is the “shape” example. This is very commonly used because it is easy to visualize, but unfortunately it can confuse novice programmers into thinking that OOP is just for graphics programming, which is of course not the case. The shape example has a base class called Shape and various derived types: Circle, Shape Cast "up" the draw() inheritance diagram erase() Circle Square Line Handle of draw() draw() draw() Circle object erase() erase() erase() Square, Triangle, etc. The reason the example works so well is that it’s very easy to say “a circle is a type of shape” and be understood. The inheritance diagram shows the relationships: The upcast could occur in a statement as simple as: Shape s = new Circle(); Here, a Circle object is created and the resulting handle is immediately assigned to a Shape, which would seem to be an error (assigning one type to another) and yet it’s fine because a Circle is a Shape by inheritance. So the compiler agrees with the statement and doesn’t issue an error message. When you call one of the base class methods (that have been overridden in the derived classes): s.draw(); Chapter 7: Polymorphism 197
- 198. again, you might expect that Shape’s draw( ) is called because this is, after all, a Shape handle so how could the compiler know to do anything else? And yet the proper Circle.draw( ) is called because of late binding (polymorphism). The following example puts it a slightly different way: //: Shapes.java // Polymorphism in Java class Shape { void draw() {} void erase() {} } class Circle extends Shape { void draw() { System.out.println("Circle.draw()"); } void erase() { System.out.println("Circle.erase()"); } } class Square extends Shape { void draw() { System.out.println("Square.draw()"); } void erase() { System.out.println("Square.erase()"); } } class Triangle extends Shape { void draw() { System.out.println("Triangle.draw()"); } void erase() { System.out.println("Triangle.erase()"); } } public class Shapes { public static Shape randShape() { switch((int)(Math.random() * 3)) { default: // To quiet the compiler case 0: return new Circle(); case 1: return new Square(); case 2: return new Triangle(); } } public static void main(String args[]) { Shape s[] = new Shape[9]; // Fill up the array with shapes: for(int i = 0; i < s.length; i++) s[i] = randShape(); // Make polymorphic method calls: 198 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 199. for(int i = 0; i < s.length; i++) s[i].draw(); } } ///:~ The base class Shape establishes the common interface to anything inherited from Shape – that is, all shapes can be drawn and erased. The derived classes override these definitions to provide unique behavior for each specific type of shape. The main class Shapes contains a static method randShape( ) that produces a handle to a randomly-selected Shape object each time you call it. Notice that the upcasting is happening in each of the return statements, which take a handle to a Circle, Square or Triangle and send it out of the method as the return type, Shape. Thus when you call this method you never get a chance to see what specific type it is, since you always get back a plain Shape handle. main( ) contains an array of Shape handles which is filled through calls to randShape( ). At this point you know you have Shapes, but you don’t know anything more specific than that (and neither does the compiler). However, when you step through this array and call draw( ) for each one, the correct type-specific behavior magically occurs, as you can see from one output example: Circle.draw() Triangle.draw() Circle.draw() Circle.draw() Circle.draw() Square.draw() Triangle.draw() Square.draw() Square.draw() Of course, since the shapes are all chosen randomly each time, your runs will have different results. The point of choosing the shapes randomly is to drive home the understanding that the compiler can have no special knowledge that allows it to make the correct calls at compile time. All the calls to draw( ) are made through dynamic binding. Extensibility Now return to the musical instrument example. Because of polymorphism you can add as many new types as you want to the system without changing the tune( ) method. In a well-designed OOP program, most or all of your methods will follow the model of tune( ) and communicate only with the base-class interface. Such a program is extensible because you can add new functionality by inheriting new data types from the common base class. The methods that manipulate the base-class interface will not need to be changed at all to accommodate the new classes. Consider what happens if you take the instrument example and add more methods in the base class and a number of new classes. Here’s the diagram: Chapter 7: Polymorphism 199
- 200. Instrument void play() String what() void adjust() Wind Percussion Stringed void play() void play() void play() String what() String what() String what() void adjust() void adjust() void adjust() Woodwind Brass void play() void play() String what() void adjust() All these brand new classes work correctly with the old, unchanged tune( ) method. Even if tune( ) is in a separate file and new methods are added to the interface of Instrument, tune( ) works correctly without recompilation. Here is the implementation of the above diagram: //: Music3.java // An extensible program import java.util.*; class Instrument3 { public void play() { System.out.println("Instrument3.play()"); } public String what() { return "Instrument3"; } public void adjust() {} } class Wind3 extends Instrument3 { public void play() { System.out.println("Wind3.play()"); } public String what() { return "Wind3"; } public void adjust() {} } class Percussion3 extends Instrument3 { public void play() { System.out.println("Percussion3.play()"); } 200 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 201. public String what() { return "Percussion3"; } public void adjust() {} } class Stringed3 extends Instrument3 { public void play() { System.out.println("Stringed3.play()"); } public String what() { return "Stringed3"; } public void adjust() {} } class Brass3 extends Wind3 { public void play() { System.out.println("Brass3.play()"); } public void adjust() { System.out.println("Brass3.adjust()"); } } class Woodwind3 extends Wind3 { public void play() { System.out.println("Woodwind3.play()"); } public String what() { return "Woodwind3"; } } public class Music3 { // Doesn't care about type, so new types // added to the system still work right: static void tune(Instrument3 i) { // ... i.play(); } static void tuneAll(Instrument3[] e) { for(int i = 0; i < e.length; i++) tune(e[i]); } public static void main(String args[]) { Instrument3[] orchestra = new Instrument3[5]; int i = 0; // Upcasting during addition to the array: orchestra[i++] = new Wind3(); orchestra[i++] = new Percussion3(); orchestra[i++] = new Stringed3(); orchestra[i++] = new Brass3(); orchestra[i++] = new Woodwind3(); tuneAll(orchestra); } } ///:~ The new methods are what( ), which returns a String handle with a description of the class, and adjust( ), which provides some way to adjust each instrument. In main( ), when you place something inside the Instrument3 array you automatically upcast to Instrument3. Chapter 7: Polymorphism 201
- 202. You can see the tune( ) method is blissfully ignorant of all the code changes that have happened around it, and yet it works correctly. This is exactly what polymorphism is supposed to provide: your code changes don’t cause damage to parts of the program that should not be affected. Put another way, polymorphism is one of the most important techniques that allow the programmer to “separate the things that change from the things that stay the same.” Overriding vs. overloading Let’s take a different look at the first example in this chapter. In the following program, the interface of the method play( ) is changed in the process of overriding it, which means you haven’t actually overridden the method, but instead overloaded it. The compiler allows you to overload methods so it gives no complaint. But the behavior is probably not what you want. Here’s the example: //: WindError.java // Accidentally changing the interface class NoteX { public static final int MIDDLE_C = 0, C_SHARP = 1, C_FLAT = 2; } class InstrumentX { public void play(int NoteX) { System.out.println("InstrumentX.play()"); } } class WindX extends InstrumentX { // OOPS! Changes the method interface: public void play(NoteX n) { System.out.println("WindX.play(NoteX n)"); } } public class WindError { public static void tune(InstrumentX i) { // ... i.play(NoteX.MIDDLE_C); } public static void main(String[] args) { WindX flute = new WindX(); tune(flute); // Not the desired behavior! } } ///:~ There’s another confusing aspect thrown in here. In InstrumentX, the play( ) method takes an int which has the identifier NoteX. That is, even though NoteX is a class name it can also be used as an identifier without complaint. But in WindX, play( ) takes a NoteX handle that has an identifier n (although you could even say play(NoteX NoteX) without an error). Thus it appears the programmer intended to override play( ) but just mistyped the method a little bit. The compiler, however, assumed that an overload and not an 202 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 203. override was intended. Notice that if you follow the standard Java naming convention, the argument identifier would be noteX which would distinguish it from the class name. In tune, the InstrumentX i is sent the play( ) message, with one of NoteX’s members (MIDDLE_C) as an argument. Since NoteX contains int definitions, this means that the int version of the now-overloaded play( ) method is called, and since that has not been overridden the base-class version is used. The output is: InstrumentX.play() Which certainly doesn’t appear to be a polymorphic method call. Once you understand what’s happening you can fix the problem fairly easily, but imagine how difficult it might be to find the bug if it’s buried in a program of significant size. Abstract classes & methods In all the instrument examples, the methods in the base class Instrument were always “dummy” methods. If these methods are ever called, you’ve done something wrong. That’s because the intent of Instrument is to create a common interface for all the classes derived from it. The only reason to establish this common interface is so it can be expressed differently for each different subtype. It establishes a basic form, so you can say what’s in common with all the derived classes. Another way of saying this is to call Instrument an abstract base class (or simply an abstract class). You create an abstract class when you want to manipulate a set of classes through this common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism (however, as seen in the last section, if the method’s name is the same as the base class but the arguments are different, you’ve got overloading which probably isn’t what you want). If you have an abstract class like Instrument, objects of that class almost always have no meaning. That is, Instrument is meant to express only the interface, and not a particular implementation, so creating an Instrument object makes no sense, and you’ll probably want to prevent the user from doing it. This can be accomplished by making all the methods in Instrument print error messages, but this delays the information until run- time and requires reliable exhaustive testing on the part of the user. It’s always better to catch problems at compile time. Java provides a mechanism for doing this called the abstract method. This is a method that is incomplete; it has only a declaration and no method body. Here is the syntax for an abstract method declaration: abstract void X(); A class containing abstract methods is called an abstract class. If a class contains one or more abstract methods, the class itself must be qualified as abstract (otherwise the compiler gives you an error message). If an abstract class is incomplete, what is the compiler supposed to do when someone tries to make an object of that class? It cannot safely create an object of an abstract class, so you get an error message from the compiler. Thus, the compiler ensures the purity of the abstract class, and you don’t have to worry about misusing it. Chapter 7: Polymorphism 203
- 204. If you inherit from an abstract class and you want to make objects of the new type, you must provide method definitions for all the abstract methods in the base class. If you don’t (and you may choose not to) then the derived class is also abstract and the compiler will force you to qualify that class with the abstract keyword. It’s possible to declare a class as abstract without including any abstract methods. This is useful when you’ve got a class where it doesn’t make sense to have any abstract methods, and yet you want to prevent any instances of that class. The Instrument class can easily be turned into an abstract class. Only some of the methods will be abstract, since making a class abstract doesn’t force you to make all the methods abstract. Here’s what it looks like: abstract Instrument abstract void play(); String what() {/*…*/} abstract void adjust(); extends extends extends Wind Percussion Stringed void play() void play() void play() String what() String what() String what() void adjust() void adjust() void adjust() extends extends Woodwind Brass void play() void play() String what() void adjust() Here’s the orchestra example modified to use abstract classes and methods: //: Music4.java // Abstract classes and methods import java.util.*; abstract class Instrument4 { int i; // storage allocated for each public abstract void play(); public String what() { return "Instrument4"; } public abstract void adjust(); } class Wind4 extends Instrument4 { public void play() { 204 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 205. System.out.println("Wind4.play()"); } public String what() { return "Wind4"; } public void adjust() {} } class Percussion4 extends Instrument4 { public void play() { System.out.println("Percussion4.play()"); } public String what() { return "Percussion4"; } public void adjust() {} } class Stringed4 extends Instrument4 { public void play() { System.out.println("Stringed4.play()"); } public String what() { return "Stringed4"; } public void adjust() {} } class Brass4 extends Wind4 { public void play() { System.out.println("Brass4.play()"); } public void adjust() { System.out.println("Brass4.adjust()"); } } class Woodwind4 extends Wind4 { public void play() { System.out.println("Woodwind4.play()"); } public String what() { return "Woodwind4"; } } public class Music4 { // Doesn't care about type, so new types // added to the system still work right: static void tune(Instrument4 i) { // ... i.play(); } static void tuneAll(Instrument4[] e) { for(int i = 0; i < e.length; i++) tune(e[i]); } public static void main(String args[]) { Instrument4[] orchestra = new Instrument4[5]; int i = 0; // Upcasting during addition to the array: orchestra[i++] = new Wind4(); orchestra[i++] = new Percussion4(); orchestra[i++] = new Stringed4(); Chapter 7: Polymorphism 205
- 206. orchestra[i++] = new Brass4(); orchestra[i++] = new Woodwind4(); tuneAll(orchestra); } } ///:~ You can see that there’s really no change except in the base class. It’s helpful to create abstract classes and methods because they make the abstractness of a class explicit and tell both the user and the compiler how it was intended to be used. Interfaces The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists and return types, but no method bodies. An interface can also contain data members of primitive types, but these are implicitly static and final. An interface provides only a form, but no implementation. An interface says: “this is what all classes that implement this particular interface shall look like.” Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes (some object-oriented programming languages have a keyword called protocol to do the same thing). To create an interface, you use the interface keyword instead of the class keyword. Like a class, you can add the public keyword before the interface keyword (but only if that interface is defined in a file of the same name) or leave it off to give “friendly” status. To make a class that conforms to a particular interface (or group of interfaces) you use the implements keyword. You’re saying “the interface is what it looks like, here’s how it works.” Other than that, it bears a strong resemblance to inheritance. The diagram for the instrument example shows this: 206 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 207. interface Instrument void play(); String what(); void adjust(); implements implements implements Wind Percussion Stringed void play() void play() void play() String what() String what() String what() void adjust() void adjust() void adjust() extends extends Woodwind Brass void play() void play() String what() void adjust() Once you’ve implemented an interface, that implementation becomes an ordinary class which may be extended in the regular way. You can choose to explicitly declare the method declarations in an interface as public. However, they are public even if you don’t say it. This means that when you implement an interface, the methods from the interface must be defined as public. Otherwise they would default to “friendly” and you’d be restricting the accessibility of a method during inheritance, which is not allowed by the Java compiler. You can see this in the modified version of the Instrument example. Notice that every method in the interface is strictly a declaration, which is the only thing the compiler will allow. In addition, none of the methods in Instrument5 are declared as public, but they’re automatically public anyway: //: Music5.java // Interfaces import java.util.*; interface Instrument5 { // Compile-time constant: int i = 5; // static & final // Cannot have method definitions: void play(); // Automatically public String what(); void adjust(); } class Wind5 implements Instrument5 { public void play() { System.out.println("Wind5.play()"); } Chapter 7: Polymorphism 207
- 208. public String what() { return "Wind5"; } public void adjust() {} } class Percussion5 implements Instrument5 { public void play() { System.out.println("Percussion5.play()"); } public String what() { return "Percussion5"; } public void adjust() {} } class Stringed5 implements Instrument5 { public void play() { System.out.println("Stringed5.play()"); } public String what() { return "Stringed5"; } public void adjust() {} } class Brass5 extends Wind5 { public void play() { System.out.println("Brass5.play()"); } public void adjust() { System.out.println("Brass5.adjust()"); } } class Woodwind5 extends Wind5 { public void play() { System.out.println("Woodwind5.play()"); } public String what() { return "Woodwind5"; } } public class Music5 { // Doesn't care about type, so new types // added to the system still work right: static void tune(Instrument5 i) { // ... i.play(); } static void tuneAll(Instrument5[] e) { for(int i = 0; i < e.length; i++) tune(e[i]); } public static void main(String args[]) { Instrument5[] orchestra = new Instrument5[5]; int i = 0; // Upcasting during addition to the array: orchestra[i++] = new Wind5(); orchestra[i++] = new Percussion5(); orchestra[i++] = new Stringed5(); orchestra[i++] = new Brass5(); 208 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 209. orchestra[i++] = new Woodwind5(); tuneAll(orchestra); } } ///:~ The rest of the code works the same. That is, it doesn’t matter if you are upcasting to a “regular” class called Instrument5, an abstract class called Instrument5, or an interface called Instrument5. The behavior is the same. In fact, you can see in the tune( ) method that there isn’t even any evidence about whether Instrument5 is a “regular” class, an abstract class or an interface. This is the intent: each approach gives the programmer different control over the way objects are created and used. “Multiple inheritance” in Java The interface isn’t simply a “more pure” form of abstract class. It has a higher purpose than that. Because an interface has no implementation at all – That is, there is no storage associated with an interface – there’s nothing to prevent many interfaces from being combined. This is valuable because there are times when you need to say: “an x is an a and a b and a c.” In C++, this act of combining multiple class interfaces is called multiple inheritance, and it carries with it some rather sticky baggage because each class can have an implementation. In Java, you can perform the same act but only one of the classes can have an implementation, so the problems seen in C++ do not occur with Java when combining multiple interfaces: Abstract or concrete interface 1 base class interface 1 interface 1 base class functions interface 1 interface 2 .… interface n Although you aren’t forced to have an abstract or “concrete” (one with no abstract methods) base class, if you do you can have a maximum of only one. All the rest of the base elements must be interfaces. You place all the interface names after the implements keyword and separate them with commas. You can have as many interfaces as you want, and each one becomes an independent type that you can upcast to. The following example shows a concrete class combined with several interfaces to produce a new class: //: Adventure.java // Multiple interfaces import java.util.*; interface CanFight { void fight(); } interface CanSwim { void swim(); } Chapter 7: Polymorphism 209
- 210. interface CanFly { void fly(); } class ActionCharacter { public void fight() {} } class Hero extends ActionCharacter implements CanFight, CanSwim, CanFly { public void swim() {} public void fly() {} } public class Adventure { static void t(CanFight x) { x.fight(); } static void u(CanSwim x) { x.swim(); } static void v(CanFly x) { x.fly(); } static void w(ActionCharacter x) { x.fight(); } public static void main(String args[]) { Hero i = new Hero(); t(i); // Treat it as a CanFight u(i); // Treat it as a CanSwim v(i); // Treat it as a CanFly w(i); // Treat it as an ActionCharacter } } ///:~ You can see that Hero combines the concrete class ActionCharacter with the interfaces CanFight, CanSwim and CanFly. When you combine a concrete class with interfaces this way, the concrete class must come first, then the interfaces (the compiler gives an error otherwise). Notice that the signature for fight( ) is the same in the interface CanFight and the class ActionCharacter, and that fight( ) is not provided with a definition in Hero. The rule for an interface is that you can inherit from it (as you shall see shortly) but then you’ve got another interface. If you want to create an object of the new type, it must be a class with all definitions provided. But even though Hero does not explicitly provide a definition for fight( ), the definition comes along with ActionCharacter so it is automatically provided and it’s possible to create objects of Hero. In class Adventure, you can see there are four methods which take as arguments the various interfaces and the concrete class. When a Hero object is created, it can be passed to any of these methods, which means it is being upcast to each interface in turn. Because of the way interfaces are designed in Java, this works without a hitch and without any particular effort on the part of the programmer. Keep in mind that the core reason for interfaces is shown in the above example: to be able to upcast to more than one base type. However, a second reason for using interfaces is the same as using an abstract base class: to prevent the client programmer from making an object of this class and to establish that it is only an interface. This brings up a question: should you use an interface or an abstract class? Well, an interface gives you the benefits of an abstract class and the benefits of an interface, so if it’s possible to create your base class without any method definitions or member variables you should always prefer interfaces to abstract classes. In fact, if you know something is going to be a base class, 210 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 211. your first choice should be to make it an interface, and only if you’re forced to have method definitions or member variables should you change to an abstract class. Extending an interface with inheritance You can easily add new method declarations to an interface using inheritance, and you can also combine several interfaces into a new interface with inheritance. In both cases you get a new interface, as seen in this example: //: HorrorShow.java // Extending an interface with inheritance interface Monster { void menace(); } interface DangerousMonster extends Monster { void destroy(); } interface Lethal { void kill(); } class DragonZilla implements DangerousMonster { public void menace() {} public void destroy() {} } interface Vampire extends DangerousMonster, Lethal { void drinkBlood(); } class HorrorShow { static void u(Monster b) { b.menace(); } static void v(DangerousMonster d) { d.menace(); d.destroy(); } public static void main(String[] args) { DragonZilla if2 = new DragonZilla(); u(if2); v(if2); } } ///:~ DangerousMonster is a simple extension to Monster which produces a new interface. This is implemented in DragonZilla. The syntax used in Vampire works only when inheriting interfaces. Normally, you can use extends with only a single class, but since an interface can be made from multiple other interfaces, extends can refer to multiple base interfaces when building a new interface. As you can see, the interface names are simply separated with commas. Chapter 7: Polymorphism 211
- 212. Grouping constants Because any fields you put into an interface are automatically static and final, the interface is a convenient tool for creating groups of constant values, much as you would with an enum in C or C++. For example: //: Months.java // Using interfaces to create groups of constants package c07; public interface Months { int JANUARY = 1, FEBRUARY = 2, MARCH = 3, APRIL = 4, MAY = 5, JUNE = 6, JULY = 7, AUGUST = 8, SEPTEMBER = 9, OCTOBER = 10, NOVEMBER = 11, DECEMBER = 12; } ///:~ Note the Java style of using all uppercase letters (with underscores to separate multiple words in a single identifier) for static final primitives that have constant initializers – that is, for compile-time constants. The fields in an interface are automatically public, so it’s unnecessary to specify that. Now you can use the constants from outside the package by importing c07.* or c07.Months just as you would with any other package, and referencing the values with expressions like Months.JANUARY. Of course, what you get is just an int so there isn’t the extra type safety that C++’s enum has, but this (commonly-used) technique is certainly an improvement over hard-coding numbers into your programs (this is often referred to as using “magic numbers” and it produces very difficult-to-maintain code). Initializing fields in interfaces Fields defined in interfaces are automatically static and final. These cannot be “blank finals” but the can be initialized with non-constant expressions. For example: //: RandVals.java // Initializing interface fields with // non-constant initializers import java.util.*; public interface RandVals { int rint = (int)(Math.random() * 10); long rlong = (long)(Math.random() * 10); float rfloat = (float)(Math.random() * 10); double rdouble = Math.random() * 10; } ///:~ Since the fields are static, they are initialized when the class is first loaded, upon first access of any of the fields. Here’s a simple test: //: TestRandVals.java public class TestRandVals { public static void main(String args[]) { System.out.println(RandVals.rint); 212 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 213. System.out.println(RandVals.rlong); System.out.println(RandVals.rfloat); System.out.println(RandVals.rdouble); } } ///:~ The fields, of course, are not part of the interface itself but instead are stored in the static storage area for that interface. Inner classes In Java 1.1 it’s possible to place a class definition within another class definition. This is called an inner class. The inner class is a useful feature because it allows you to group classes that logically belong together and to control the visibility of one within the other. However, it’s important to understand that inner classes are distinctly different from composition. Often, the need for inner classes isn’t immediately obvious as you’re learning about them. At the end of this section, after all the syntax and semantics of inner classes have been described, you’ll find an example that should make clear the benefits of inner classes. You create an inner class just as you’d expect: by placing the class definition inside a surrounding class (see page 80 if you have trouble executing this program): //: Parcel1.java // Creating inner classes package c07.parcel1; public class Parcel1 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } // Using inner classes looks just like // using any other class, within Parcel1: public void ship(String dest) { Contents c = new Contents(); Destination d = new Destination(dest); } public static void main(String args[]) { Parcel1 p = new Parcel1(); p.ship("Tanzania"); } } ///:~ The inner classes, when used inside ship( ), look just like the use of any other classes. Here, the only practical difference is that the names are nested within Parcel1. You’ll see in a while that this isn’t the only difference. Chapter 7: Polymorphism 213
- 214. More typically, an outer class will have a method that returns a handle to an inner class, like this: //: Parcel2.java // Returning a handle to an inner class package c07.parcel2; public class Parcel2 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } public Destination to(String s) { return new Destination(s); } public Contents cont() { return new Contents(); } public void ship(String dest) { Contents c = cont(); Destination d = to(dest); } public static void main(String args[]) { Parcel2 p = new Parcel2(); p.ship("Tanzania"); Parcel2 q = new Parcel2(); // Defining handles to inner classes: Parcel2.Contents c = q.cont(); Parcel2.Destination d = q.to("Borneo"); } } ///:~ If you want to make an object of the inner class anywhere but inside a non-static method of the outer class, you must specify the type of that object as OuterClassName.InnerClassName, as seen in main( ). Inner classes and upcasting So far, inner classes don’t seem that dramatic. After all, if it’s hiding you’re after, Java already has a perfectly good hiding mechanism – just allow the class to be “friendly” (visible only within a package) rather than creating it as an inner class. However, inner classes really come into their own when you start upcasting to a base class, and in particular an interface (the effect of producing an interface handle from an object that implements it is essentially the same as upcasting to a base class). That’s because the inner class can then be completely unseen and unavailable to anyone; all you get back is a handle to the base class or the interface and it’s possible that you can’t even find out the exact type, as shown here: //: Parcel3.java // Returning a handle to an inner class 214 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 215. package c07.parcel3; abstract class Contents { abstract public int value(); } interface Destination { String readLabel(); } public class Parcel3 { private class PContents extends Contents { private int i = 11; public int value() { return i; } } protected class PDestination implements Destination { private String label; private PDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } } public Destination dest(String s) { return new PDestination(s); } public Contents cont() { return new PContents(); } } class Test { public static void main(String args[]) { Parcel3 p = new Parcel3(); Contents c = p.cont(); Destination d = p.dest("Tanzania"); // Illegal -- can't access private class: //! Parcel3.PContents c = p.new PContents(); } } ///:~ Now Contents and Destination represent interfaces available to the client programmer (the interface, remember, automatically makes all its members public). For convenience, these are placed inside a single file, but ordinarily Contents and Destination would each be public in their own files. In Parcel3, something new has been added: the inner class PContents is private so no one but Parcel3 can access it. PDestination is protected, so no one but Parcel3, classes in the Parcel3 package (since protected also gives package access; that is, protected is also “friendly”), and the inheritors of Parcel3 can access PDestination. This means that the client programmer has restricted knowledge and access to these members. In fact, you can’t even downcast to a private inner class (or a protected inner class unless you’re an inheritor), because you can’t access the name, as you can see in class Test. Thus, the private inner class provides a way for the class designer to completely prevent any type- coding dependencies and to completely hide details about implementation. In addition, extension of an interface is useless from the client programmer’s perspective since the client programmer cannot access any additional methods that aren’t part of the public Chapter 7: Polymorphism 215
- 216. interface class. This also provides an opportunity for the Java compiler to generate more efficient code. Normal (non-inner) classes cannot be made private or protected – only public or “friendly.” Note that Contents is an abstract class but it doesn’t have to be. You could use an ordinary class here as well, but the most typical starting point for such a design is an interface. Inner classes in methods & scopes What you’ve seen so far encompasses the typical use for inner classes. Generally, the code that you’ll write and read involving inner classes will be “plain” inner classes that are simple and easy to understand. However, the design for inner classes is quite complete and there are a number of other, more obscure, ways you can use them if you choose: inner classes may be created within a method or even an arbitrary scope. There are two reasons for doing this: 1. As shown previously, you’re implementing an interface of some kind so you can create and return a handle. 2. You’re solving a complicated problem and you want to create a class to aid in your solution, but you don’t want it to be publicly used. In the following examples, the previous code will be modified to use: 1. A class defined within a method 2. A class defined within a scope inside a method 3. An anonymous class implementing an interface 4. An anonymous class extending a class that has a non-default constructor. 5. An anonymous class that performs field initialization 6. An anonymous class that performs construction using instance initialization (anonymous inner classes cannot have constructors) This will all take place within the package innerscopes. First, the common interfaces from the previous code will be defined in their own files, so they can be used in all the examples: //: Destination.java package c07.innerscopes; interface Destination { String readLabel(); } ///:~ The point has been made that Contents could be an abstract class, so here it will be in a more natural form, as an interface: //: Contents.java package c07.innerscopes; interface Contents { int value(); 216 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 217. } ///:~ Although it’s an ordinary class with an implementation, Wrapping is also being used as a common “interface” to its derived classes: //: Wrapping.java package c07.innerscopes; public class Wrapping { private int i; public Wrapping(int x) { i = x; } public int value() { return i; } } ///:~ You’ll note that Wrapping has a constructor that requires an argument, to make things interesting. The first example shows the creation of an entire class within the scope of a method (instead of the scope of another class): //: Parcel4.java // Nesting a class within a method package c07.innerscopes; public class Parcel4 { public Destination dest(String s) { class PDestination implements Destination { private String label; private PDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } } return new PDestination(s); } public static void main(String args[]) { Parcel4 p = new Parcel4(); Destination d = p.dest("Tanzania"); } } ///:~ The class PDestination is part of dest( ) rather than being part of Parcel4 (also notice that you could use the class identifier PDestination for an inner class inside each class in the same subdirectory without a name clash). Therefore PDestination cannot be accessed outside of dest( ). Notice the upcasting that occurs during the return statement – nothing comes out of dest( ) except a handle to the base class Destination. Of course, the fact that the name of the class PDestination is placed inside dest( ) doesn’t mean that PDestination is not a valid object once dest( ) returns. The next example shows how you can nest an inner class within any arbitrary scope: //: Parcel5.java // Nesting a class within a scope package c07.innerscopes; public class Parcel5 { private void internalTracking(boolean b) { Chapter 7: Polymorphism 217
- 218. if(b) { class TrackingSlip { private String id; TrackingSlip(String s) { id = s; } String getSlip() { return id; } } TrackingSlip ts = new TrackingSlip("slip"); String s = ts.getSlip(); } // Can't use it here! Out of scope: //! TrackingSlip ts = new TrackingSlip("x"); } public void track() { internalTracking(true); } public static void main(String args[]) { Parcel5 p = new Parcel5(); p.track(); } } ///:~ The class TrackingSlip is nested inside the scope of an if statement. This does not mean that the class is conditionally created – it gets compiled along with everything else. However, it’s not available outside the scope in which it is defined. Other than that, it looks just like an ordinary class. The next example looks a little strange: //: Parcel6.java // A method that returns an anonymous inner class package c07.innerscopes; public class Parcel6 { public Contents cont() { return new Contents() { private int i = 11; public int value() { return i; } }; // Semicolon required in this case } public static void main(String args[]) { Parcel6 p = new Parcel6(); Contents c = p.cont(); } } ///:~ The cont( ) method combines the creation of the return value with the definition of the class that represents that return value! In addition, the class is anonymous – it has no name. To make matters a bit worse, it looks like you’re starting out to create a Contents object: return new Contents() but then, before you get to the semicolon you say “but wait, I think I’ll slip in a class definition”: return new Contents() { private int i = 11; 218 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 219. public int value() { return i; } }; What this strange syntax means is “create an object of an anonymous class that’s inherited from Contents.” The handle returned by the new expression is automatically upcast to a Contents handle. In the above example, Contents is created using a default constructor. The following code shows what to do if your base class needs a constructor with an argument: //: Parcel7.java // An anonymous inner class that calls the // base-class constructor package c07.innerscopes; public class Parcel7 { public Wrapping wrap(int x) { // Base constructor call: return new Wrapping(x) { public int value() { return super.value() * 47; } }; // Semicolon required } public static void main(String args[]) { Parcel7 p = new Parcel7(); Wrapping w = p.wrap(10); } } ///:~ That is, you just pass the appropriate argument to the base-class constructor, seen here as the x passed in new Wrapping(x). An anonymous class cannot have a constructor where you would normally call super( ). In both the previous examples, the semicolon doesn’t mark the end of the class body (as it does in C++). Instead, it marks the end of the expression that happens to contain the anonymous class. Thus it’s identical to the use of the semicolon everywhere else. What happens if you need to perform some kind of initialization for an object of an anonymous inner class? Since it’s anonymous, there’s no name to give the constructor so you can’t have a constructor. You can, however, perform initialization at the point of definition of your fields: //: Parcel8.java // An anonymous inner class that performs // initialization. A briefer version // of Parcel5.java. package c07.innerscopes; public class Parcel8 { // Argument must be final to use inside // anonymous inner class: public Destination dest(final String dest) { return new Destination() { private String label = dest; public String readLabel() { return label; } }; } Chapter 7: Polymorphism 219
- 220. public static void main(String args[]) { Parcel8 p = new Parcel8(); Destination d = p.dest("Tanzania"); } } ///:~ If you’re defining an anonymous inner class and want to use an object that’s defined outside the anonymous inner class, the compiler requires that the outside object be final. This is why the argument to dest( ) is final. If you forget, you’ll get a compile-time error message. As long as you’re simply assigning a field the above approach is fine, but what if you need to perform some constructor-like activity? With Java 1.1 instance initialization, you can effectively create a constructor for an anonymous inner class: //: Parcel9.java // Using "instance initialization" to perform // construction on an anonymous inner class package c07.innerscopes; public class Parcel9 { public Destination dest(final String dest, final float price) { return new Destination() { private int cost; // Instance initialization for each object: { cost = Math.round(price); if(cost > 100) System.out.println("Over budget!"); } private String label = dest; public String readLabel() { return label; } }; } public static void main(String args[]) { Parcel9 p = new Parcel9(); Destination d = p.dest("Tanzania", 101.395F); } } ///:~ Inside the instance initializer, you can see code that couldn’t be executed as part of a field initializer (that is, the if statement). So in effect, an instance initializer is the constructor for an anonymous inner class. Of course, it’s limited: you can’t overload instance initializers so you can have only one of these constructors. The link to the outer class So far, it appears that inner classes are just a name-hiding and code-organization scheme, which is helpful but not totally compelling. However, there’s another twist. When you create an inner class, objects of that inner class have a link to the enclosing object that made them, and so can access the members of that enclosing object – without any special 220 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 221. qualifications. In addition, inner classes have access rights to all the elements in the enclosing class1 . The following example demonstrates this: //: Sequence.java // Holds a sequence of Objects interface Selector { boolean end(); Object current(); void next(); } public class Sequence { private Object[] o; private int next = 0; public Sequence(int size) { o = new Object[size]; } public void add(Object x) { if(next < o.length) { o[next] = x; next++; } } private class SSelector implements Selector { int i = 0; public boolean end() { return i == o.length; } public Object current() { return o[i]; } public void next() { if(i < o.length) i++; } } public Selector getSelector() { return new SSelector(); } public static void main(String args[]) { Sequence s = new Sequence(10); for(int i = 0; i < 10; i++) s.add(Integer.toString(i)); Selector sl = s.getSelector(); while(!sl.end()) { System.out.println((String)sl.current()); sl.next(); } } } ///:~ 1 This is very different from the design of nested classes in C++, which is simply a name-hiding mechanism. There is no link to an enclosing object and no implied permissions in C++. Chapter 7: Polymorphism 221
- 222. The Sequence is simply a fixed-sized array of Object with a class wrapped around it. You call add( ) to add a new Object to the end of the sequence (if there’s room left). To fetch each of the objects in a Sequence, there’s an interface called Selector, which allows you to see if you’re at the end( ), to look at the current( ) Object, and to move to the next( ) Object in the Sequence. Because Selector is an interface, many other classes may implement the interface in their own ways, and many methods may take the interface as an argument, in order to create generic code. Here, the SSelector is a private class that provides Selector functionality. In main( ), you can see the creation of a Sequence, followed by the addition of a number of String objects. Then, a Selector is produced with a call to getSelector( ) and this is used to move through the Sequence and select each item. At first, the creation of SSelector looks like just another inner class. But examine it closely. Notice that each of the methods end( ), current( ) and next( ) refer to o, which is a handle that isn’t part of SSelector, but is instead a private field in the enclosing class. However, the inner class can access methods and fields from the enclosing class as if they owned them. This turns out to be very convenient, as you can see in the above example. So an inner class has access to the members of the enclosing class. But how can this happen? The inner class must keep a reference to the particular object of the enclosing class that was responsible for creating it. Then when you refer to a member of the enclosing class, that (hidden) reference is used to select that member. Fortunately, the compiler takes care of all these details for you, but you can also understand now that an object of an inner class can be created only in association with an object of the enclosing class. The process of construction requires the initialization of the handle to the object of the enclosing class, and the compiler will complain if it cannot access the handle. Most of the time this occurs without any intervention on the part of the programmer. Static inner classes To understand the meaning of static when applied to inner classes, you must remember that the object of the inner class implicitly keeps a handle to the object of the enclosing class that created it. This is not true, however, when you say an inner class is static. A static inner class means: 1. You don’t need an outer-class object in order to create an object of a static inner class. 2. You can’t access an outer-class object from an object of a static inner class. If you don’t need to create an object of the outer class in order to create an object of the inner class, you can make everything static. But for this to work, you must also make the inner classes themselves static: //: Parcel10.java // Static inner classes package c07.parcel10; abstract class Contents { abstract public int value(); } interface Destination { String readLabel(); } public class Parcel10 { private static class PContents extends Contents { 222 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 223. private int i = 11; public int value() { return i; } } protected static class PDestination implements Destination { private String label; private PDestination(String whereTo) { label = whereTo; } public String readLabel() { return label; } } public static Destination dest(String s) { return new PDestination(s); } public static Contents cont() { return new PContents(); } public static void main(String args[]) { Contents c = cont(); Destination d = dest("Tanzania"); } } ///:~ In main( ), no object of Parcel10 is necessary; instead you use the normal syntax for selecting a static member to call the methods that return handles to Contents and Destination. Referring to the outer class object If you need to produce the handle to the outer class object, you name the outer class followed by a dot and this. For example, in the class Sequence.SSelector, any of its methods can produce the stored handle to the outer class Sequence by saying Sequence.this. The resulting handle is automatically the correct type (this is known and checked at compile time, so there is no run-time overhead). Sometimes you want to tell some other object to create an object of one of its inner classes. To do this you must provide a handle to the other outer class object in the new expression, like this: //: Parcel11.java // Creating inner classes package c07.parcel11; public class Parcel11 { class Contents { private int i = 11; public int value() { return i; } } class Destination { private String label; Destination(String whereTo) { label = whereTo; } String readLabel() { return label; } } public static void main(String args[]) { Parcel11 p = new Parcel11(); // Must use instance of outer class Chapter 7: Polymorphism 223
- 224. // to create an instances of the inner class: Parcel11.Contents c = p.new Contents(); Parcel11.Destination d = p.new Destination("Tanzania"); } } ///:~ To create an object of the inner class directly, you don’t follow the same form and refer to the outer class name Parcel11 as you might expect, but instead you must use an object of the outer class to make an object of the inner class: Parcel11.Contents c = p.new Contents(); Thus it’s not possible to create an object of the inner class unless you already have an object of the outer class. This is because the object of the inner class is quietly connected to the object of the outer class that it was made from. If you make a static inner class, then it doesn’t need a handle to the outer class object. Inheriting from inner classes Because the inner class constructor must attach to a handle of the enclosing class object, things are slightly complicated when you inherit from an inner class. The problem is that the “secret” handle to the enclosing class object must be initialized, and yet in the derived class there’s no longer a default object to attach to. The answer is to use a syntax provided to make the association explicit: //: InheritInner.java // Inheriting an inner class class WithInner { class Inner {} } public class InheritInner extends WithInner.Inner { //! InheritInner() {} // Won't compile InheritInner(WithInner wi) { wi.super(); } public static void main(String args[]) { WithInner wi = new WithInner(); InheritInner ii = new InheritInner(wi); } } ///:~ You can see that InheritInner is extending only the inner class, not the outer one. But when it comes time to create a constructor, the default one is no good and you can’t just pass a handle to an enclosing object. In addition, you must use the syntax enclosingClassHandle.super(); inside the constructor. This provides the necessary handle and the program will then compile. 224 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 225. Can inner classes be overridden? What happens when you create an inner class, then inherit from the enclosing class and redefine the inner class? That is, is it possible to override an inner class? This seems like it would be a powerful concept, but “overriding” an inner class as if it were another method of the outer class doesn’t really do anything: //: BigEgg.java // An inner class cannot be overriden // like a method class Egg { protected class Yolk { public Yolk() { System.out.println("Egg.Yolk()"); } } private Yolk y; public Egg() { System.out.println("New Egg()"); y = new Yolk(); } } public class BigEgg extends Egg { public class Yolk { public Yolk() { System.out.println("BigEgg.Yolk()"); } } public static void main(String args[]) { new BigEgg(); } } ///:~ The default constructor is synthesized automatically by the compiler, and this calls the base-class default constructor. You might think that since a BigEgg is being created the “overridden” version of Yolk would be used, but this is not the case. The output is: New Egg() Egg.Yolk() The above example simply shows that there isn’t any extra inner class magic going on when you inherit from the outer class. However, it’s still possible to explicitly inherit from the inner class: //: BigEgg2.java // Proper inheritance of an inner class class Egg2 { protected class Yolk { public Yolk() { System.out.println("Egg2.Yolk()"); } public void f() { System.out.println("Egg2.Yolk.f()"); } Chapter 7: Polymorphism 225
- 226. } private Yolk y = new Yolk(); public Egg2() { System.out.println("New Egg2()"); } public void insertYolk(Yolk yy) { y = yy; } public void g() { y.f(); } } public class BigEgg2 extends Egg2 { public class Yolk extends Egg2.Yolk { public Yolk() { System.out.println("BigEgg2.Yolk()"); } public void f() { System.out.println("BigEgg2.Yolk.f()"); } } public BigEgg2() { insertYolk(new Yolk()); } public static void main(String args[]) { Egg2 e2 = new BigEgg2(); e2.g(); } } ///:~ Now BiggEgg2.Yolk explicitly extends Egg2.Yolk and overrides its methods. The method insertYolk( ) allows BigEgg2 to upcast one of its own Yolk objects into the y handle in Egg2, so when g( ) calls y.f( ) the overridden version of f( ) is used. The output is: Egg2.Yolk() New Egg2() Egg2.Yolk() BigEgg2.Yolk() BigEgg2.Yolk.f() The second call to Egg2.Yolk( ) is the base-class constructor call of the BigEgg2.Yolk constructor. You can see that the overridden version of f( ) is used when g( ) is called. Inner class identifiers Since every class produces a .class file which holds all the information about how to create objects of this type (this information produces a meta-class called the Class object), you might guess that inner classes must also produce .class files to contain the information for their Class objects. The names of these files/classes have a strict formula: the name of the enclosing class, followed by a ‘$’ followed by the name of the inner class. For example, the .class files created by InheritInner.java include: InheritInner.class WithInner$Inner.class WithInner.class If inner classes are unnamed, the compiler simply starts generating numbers as inner class identifiers. If inner classes are nested within inner classes, their names are simply appended after a ‘$’ and the outer class identifier(s). 226 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 227. Although this scheme of generating internal names is simple and straightforward, it’s also robust and handles most situations2 . Since it is the standard naming scheme for Java, the generated files are automatically platform-independent. Why inner classes: control frameworks At this point you’ve seen a whole lot of syntax and semantics describing the way inner classes work, but this doesn’t answer the question: why do they exist? Why did Sun go to so much trouble to add such a fundamental language feature in Java 1.1? The answer is something that I will refer to here as a control framework. An application framework is a class or set of classes that’s designed to solve a particular type of problem. To apply an application framework, you inherit from one or more classes and override some of the methods. The code you write in the overridden methods customizes the general solution provided by that application framework to solve your specific problem. The control framework is a particular type of application framework which is dominated by the need to respond to events; a system that primarily responds to events is called an event-driven system. One of the most important problems in application programming is the graphical user interface (GUI) which is almost entirely event-driven. As you will see in Chapter 13, the Java 1.1 AWT is a control framework that very elegantly solves the GUI problem using inner classes. To see how inner classes allow the simple creation and use of control frameworks, consider a control framework whose job is to execute events whenever those events are “ready.” Although “ready” could mean anything, in this case the default will be based on clock time. What follows is a control framework that contains no specific information about what it’s controlling. First, here is the interface that describes any control event. It’s an abstract class instead of an actual interface because the default behavior is control based on time, so some of the implementation can be included here: //: Event.java // The common methods for any control event package c07.controller; abstract public class Event { private long evtTime; public Event(long eventTime) { evtTime = eventTime; } public boolean ready() { return System.currentTimeMillis() >= evtTime; } abstract public void action(); abstract public String description(); } ///:~ The constructor simply captures the time at which you want the Event to run, while ready( ) tells you when it’s time to run it. Of course, ready( ) could be overridden in a derived class to base the Event on something other than time. 2 On the other hand, ‘$’ is a meta-character to the Unix shell and so you’ll sometimes have trouble when listing the .class files. This is a bit strange coming from Sun, a Unix-based company. My guess is that they weren’t considering this issue, but instead thought you’d naturally focus on the source-code files. Chapter 7: Polymorphism 227
- 228. action( ) is the method that’s called when the Event is ready( ), and description( ) gives textual information about the Event. The next file contains the actual control framework that manages and fires events. The first class is really just a “helper” class whose job is to hold Event objects. You could replace it with any appropriate collection, and in Chapter 8 you’ll discover other collections which will do the trick without requiring you to write this extra code: //: Controller.java // Along with Event, the generic // framework for all control systems: package c07.controller; // This is just a way to hold Event objects. class EventSet { private Event events[] = new Event[100]; private int index = 0; private int next = 0; public void add(Event e) { if(index >= events.length) return; // (Should throw exception) events[index++] = e; } public Event getNext() { boolean looped = false; int start = next; do { next = (next + 1) % events.length; // See if it has looped to the beginning: if(start == next) looped = true; // If it loops past start, the list // is empty: if((next == (start + 1) % events.length) && looped) return null; } while(events[next] == null); return events[next]; } public void removeCurrent() { events[next] = null; } } public class Controller { private EventSet es = new EventSet(); public void addEvent(Event c) { es.add(c); } public void run() { Event e; while((e = es.getNext()) != null) { if(e.ready()) { e.action(); System.out.println(e.description()); es.removeCurrent(); } } } 228 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 229. } ///:~ EventSet arbitrarily holds 100 Events (if a “real” collection from Chapter 8 is used here, you don’t have to worry about its maximum size, since it will resize itself). The index is used to keep track of the next available space, and next is used when you’re looking for the next Event in the list, to see whether you’ve looped around. This is important because Event objects are removed from the list (using removeCurrent( )) once they’re run, so getNext( ) will encounter holes in the list as it moves through it. Notice that removeCurrent( ) doesn’t just set some flag indicating the object is no longer in use. Instead, it sets the handle to null. This is important because if the garbage collector sees a handle that’s still in use then it can’t clean up the object. If you think your handles might hang around (as they would here) then it’s a good idea to set them to null to give the garbage collector permission to clean them up. Controller is where the actual work goes on. It uses an EventSet to hold its Event objects, and addEvent( ) allows you to add new events to this list. But the important method is run( ). This method loops through the EventSet, hunting for an Event object that’s ready( ) to run. For each one it finds ready( ), it calls the action( ) method, prints out the description( ) and then removes the Event from the list. Notice that so far in this design you know nothing about exactly what an Event does. And this is the crux of the design, how it “separates the things that change from the things that stay the same.” Or, to use my term, the “vector of change” is the different actions of the various kinds of Event objects, and you express different actions by creating different Event subclasses. This is where inner classes come into play. They allow two things: 1. To express the entire implementation of a control-framework application in a single class, thereby encapsulating everything that’s unique about that implementation. Inner classes are used to express the many different kinds of action( ) necessary to solve the problem. In addition, the following example uses private inner classes so the implementation is completely hidden and may be changed with impunity. 2. Inner classes keep this implementation from becoming awkward, since you’re able to easily access any of the members in the outer class. Without this ability the code might become unpleasant enough that you’d end up seeking an alternative. Consider a particular implementation of the control framework which is designed to control greenhouse functions3 . Each action is entirely different: turning lights, water and thermostats on and off, ringing bells and restarting the system. But the control framework is designed to easily isolate this different code. For each type of action you inherit a new Event inner class, and write the control code inside of action( ). As is typical with an application framework, the class GreenhouseControls is inherited from Controller: //: GreenhouseControls.java // This produces a specific application of the // control system, all in a single class. Inner // classes allow you to encapsulate different // functionality for each type of event. package c07.controller; 3 For some reason this has always been a pleasing problem for me to solve; it came from C++ Inside & Out, but Java allows a much more elegant solution. Chapter 7: Polymorphism 229
- 230. public class GreenhouseControls extends Controller { private boolean light = false; private boolean water = false; private String thermostat = "Day"; private class LightOn extends Event { public LightOn(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here to // physically turn on the light. light = true; } public String description() { return "Light is on"; } } private class LightOff extends Event { public LightOff(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here to // physically turn off the light. light = false; } public String description() { return "Light is off"; } } private class WaterOn extends Event { public WaterOn(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here water = true; } public String description() { return "Greenhouse water is on"; } } private class WaterOff extends Event { public WaterOff(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here water = false; } public String description() { return "Greenhouse water is off"; } 230 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 231. } private class ThermostatNight extends Event { public ThermostatNight(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here thermostat = "Night"; } public String description() { return "Thermostat on night setting"; } } private class ThermostatDay extends Event { public ThermostatDay(long eventTime) { super(eventTime); } public void action() { // Put hardware control code here thermostat = "Day"; } public String description() { return "Thermostat on day setting"; } } // An example of an action() which inserts a // new one of itself into the event list: private int rings; private class Bell extends Event { public Bell(long eventTime) { super(eventTime); } public void action() { // Ring bell every 2 seconds, rings times: System.out.println("Bing!"); if(--rings > 0) addEvent(new Bell( System.currentTimeMillis() + 2000)); } public String description() { return "Ring bell"; } } private class Restart extends Event { public Restart(long eventTime) { super(eventTime); } public void action() { long tm = System.currentTimeMillis(); // Instead of hard-wiring, you could parse // configuration information from a text // file here: rings = 5; addEvent(new ThermostatNight(tm)); addEvent(new LightOn(tm + 1000)); addEvent(new LightOff(tm + 2000)); Chapter 7: Polymorphism 231
- 232. addEvent(new WaterOn(tm + 3000)); addEvent(new WaterOff(tm + 8000)); addEvent(new Bell(tm + 9000)); addEvent(new ThermostatDay(tm + 10000)); // Can even add a Restart object! addEvent(new Restart(tm + 20000)); } public String description() { return "Restarting system"; } } public static void main(String args[]) { GreenhouseControls gc = new GreenhouseControls(); long tm = System.currentTimeMillis(); gc.addEvent(gc.new Restart(tm)); gc.run(); } } ///:~ Notice that light, water, thermostat and rings all belong to the outer class GreenhouseControls, and yet the inner classes have no problem accessing those fields. Also, most of the action( ) methods also involve some sort of hardware control, which would most likely involve calls to non-Java code. Most of the Event classes look very similar, but Bell and Restart are special. Bell rings, and if it hasn’t yet rung enough times it adds a new Bell object to the event list, so it will ring again later. Notice how inner classes almost look like multiple inheritance: Bell has all the methods of Event and it also appears to have all the methods of the outer class GreenhouseControls. Restart is responsible for initializing the system, so it adds all the appropriate events. Of course, a more flexible way to accomplish this is to avoid hard-coding the events and instead read them from a file (an exercise in Chapter 10 asks you to modify this example to do just that). Since Restart( ) is just another Event object, you can also add a Restart object within Restart.action( ) so that the system regularly restarts itself. And all you need to do in main( ) is create a GreenhouseControls object and add a Restart object to get it going. This example should move you a long way towards appreciating the value of inner classes, especially when used within a control framework. However, in the latter half of Chapter 13 you’ll see how elegantly inner classes are used to describe the actions of a graphical user interface. By the time you finish that section you should be fully convinced. Constructors & polymorphism As usual, constructors are different from other kinds of methods. This is also true when polymorphism is involved. Even though constructors themselves are not polymorphic (although you can have a kind of “virtual constructor” as you will see in Chapter 11), it’s important to understand the way constructors work in complex hierarchies and with polymorphism. This understanding will help you avoid unpleasant entanglements. 232 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 233. Order of constructor calls The order in which constructors are called was briefly discussed in Chapter 4, but that was before inheritance and polymorphism were introduced. A constructor for the base class is always called in the constructor for a derived class, chaining upward so that a constructor for every base class is called. This makes sense because the constructor has a special job: to see that the object is built properly. A derived class has access to its own members only, and not to those of the base class (whose members are typically private). Only the base-class constructor has the proper knowledge and access to initialize its own elements. Therefore it’s essential that all constructors get called, otherwise the entire object wouldn’t be constructed properly. That’s why the compiler enforces a constructor call for every portion of a derived class. It will silently call the default constructor if you don’t explicitly call a base-class constructor in the derived- class constructor body. If there is no default constructor, the compiler will complain. (In the case where a class has no constructors the compiler will automatically synthesize a default constructor.) Let’s take a look at an example that shows the effects of composition, inheritance and polymorphism on the order of construction: //: Sandwich.java // Order of constructor calls class Meal { Meal() { System.out.println("Meal()"); } } class Bread { Bread() { System.out.println("Bread()"); } } class Cheese { Cheese() { System.out.println("Cheese()"); } } class Lettuce { Lettuce() { System.out.println("Lettuce()"); } } class Lunch extends Meal { Lunch() { System.out.println("Lunch()");} } class PortableLunch extends Lunch { PortableLunch() { System.out.println("PortableLunch()"); } } class Sandwich extends PortableLunch { Bread b = new Bread(); Cheese c = new Cheese(); Lettuce l = new Lettuce(); Sandwich() { System.out.println("Sandwich()"); } Chapter 7: Polymorphism 233
- 234. public static void main(String[] args) { new Sandwich(); } } ///:~ This example creates a complex class out of other classes, and each class has a constructor that announces itself. The important class is Sandwich, which reflects three levels of inheritance (four, if you count the implicit inheritance from Object) and three member objects. When a Sandwich object is created in main( ), the output is: Meal() Lunch() PortableLunch() Bread() Cheese() Lettuce() Sandwich() This means that the order of constructor calls for a complex object is as follows: 1. The base-class constructor is called. This step is repeated recursively such that the very root of the hierarchy is constructed first, followed by the next-derived class, etc., until the most-derived class is reached. 2. Member initializers are called in the order of declaration. 3. The body of the derived-class constructor is called. The order of the constructor calls is important. When you inherit, you know all about the base class and can access any public and protected members of the base class. This means you must be able to assume that all the members of the base class are valid when you’re in the derived class. In a normal method, construction has already taken place, so all the members of all parts of the object have been built. Inside the constructor, however, you must be able to assume that all members that you use have been built. The only way to guarantee this is for the base-class constructor to be called first. Then when you’re in the derived-class constructor, all the members you can access in the base class have been initialized. “Knowing all members are valid” inside the constructor is also the reason that, whenever possible, you should initialize all member objects (that is, objects placed in the class using composition) at their point of definition in the class (e.g.: b, c and l in the example above). If you follow this practice, you will help ensure that all base class members and member objects of the current object have been initialized. Unfortunately, this doesn’t handle every case, as you will see in the next section. Inheritance and finalize( ) When you use composition to create a new class, you never worry about finalizing the member objects of that class. Each member is an independent object and thus is garbage collected and finalized regardless of whether it happens to be a member of your class. With inheritance, however, you must override finalize( ) in the derived class if you have any special cleanup that must happen as part of garbage collection. When you override finalize( ) in an inherited class, it’s important to remember to call the base-class version of finalize( ), since otherwise the base-class finalization will not happen. The following example proves this: //: Frog.java // Testing finalize with inheritance 234 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 235. class DoBaseFinalization { public static boolean flag = false; } class Characteristic { String s; Characteristic(String c) { s = c; System.out.println( "Creating Characteristic " + s); } protected void finalize() { System.out.println( "finalizing Characteristic " + s); } } class LivingCreature { Characteristic p = new Characteristic("is alive"); LivingCreature() { System.out.println("LivingCreature()"); } protected void finalize() { System.out.println( "LivingCreature finalize"); // Call base-class version LAST! if(DoBaseFinalization.flag) try { super.finalize(); } catch(Throwable t) {} } } class Animal extends LivingCreature { Characteristic p = new Characteristic("has heart"); Animal() { System.out.println("Animal()"); } protected void finalize() { System.out.println("Animal finalize"); if(DoBaseFinalization.flag) try { super.finalize(); } catch(Throwable t) {} } } class Amphibian extends Animal { Characteristic p = new Characteristic("can live in water"); Amphibian() { System.out.println("Amphibian()"); } protected void finalize() { Chapter 7: Polymorphism 235
- 236. System.out.println("Amphibian finalize"); if(DoBaseFinalization.flag) try { super.finalize(); } catch(Throwable t) {} } } public class Frog extends Amphibian { Frog() { System.out.println("Frog()"); } protected void finalize() { System.out.println("Frog finalize"); if(DoBaseFinalization.flag) try { super.finalize(); } catch(Throwable t) {} } public static void main(String args[]) { if(args.length != 0 && args[0].equals("finalize")) DoBaseFinalization.flag = true; else System.out.println("not finalizing bases"); new Frog(); // Instantly becomes garbage System.out.println("bye!"); // Must do this to guarantee that all // finalizers will be called: System.runFinalizersOnExit(true); } } ///:~ The class DoBaseFinalization simply holds a flag that indicates to each class in the hierarchy whether to call super.finalize( ). This flag is set based on a command-line argument, so you can view the behavior with and without base-class finalization. Each class in the hierarchy also contains a member object of class Characteristic. You will see that regardless of whether the base class finalizers are called, the Characteristic member objects are always finalized. Each overridden finalize( ) must have access of at least protected since the finalize( ) method in class Object is protected and the compiler will not allow you to reduce the access during inheritance (“friendly” is less accessible than protected). In Frog.main( ) the DoBaseFinalization flag is configured, and a single Frog object is created. Remember that garbage collection and in particular finalization might not happen for any particular object so to enforce this, System.runFinalizersOnExit(true) adds the extra overhead to guarantee that finalization takes place. Without base-class finalization, the output is: not finalizing bases Creating Characteristic is alive LivingCreature() Creating Characteristic has heart Animal() Creating Characteristic can live in water 236 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 237. Amphibian() Frog() bye! Frog finalize finalizing Characteristic is alive finalizing Characteristic has heart finalizing Characteristic can live in water You can see that indeed, no finalizers are called for the base classes of Frog. But if you add the “finalize” argument on the command line, you get: Creating Characteristic is alive LivingCreature() Creating Characteristic has heart Animal() Creating Characteristic can live in water Amphibian() Frog() bye! Frog finalize Amphibian finalize Animal finalize LivingCreature finalize finalizing Characteristic is alive finalizing Characteristic has heart finalizing Characteristic can live in water Although the order in which the member objects are finalized is the same order in which they are created, technically the order of finalization of objects is unspecified. With base classes, however, you have control over the order of finalization. The best order to use is the one that’s shown here, which is the reverse of the order of initialization. Following the form that’s used in C++ for destructors, you should perform the derived-class finalization first, then the base-class finalization. That’s because the derived-class finalization could call some methods in the base class that require that the base-class components are still alive, so you must not destroy them prematurely. Behavior of polymorphic methods inside constructors The hierarchy of constructor calls brings up an interesting dilemma. What happens if you’re inside a constructor and you call a dynamically-bound method? Inside an ordinary method you can imagine what will happen – the dynamically-bound call is resolved at run-time because the object cannot know whether it belongs to the class the method is in, or some class derived from it. For consistency, you might think this is what should happen inside constructors. This is not exactly the case. If you call a dynamically-bound method inside a constructor, the overridden definition for that method is in fact used. However, the effect can be rather unexpected, and can conceal some very difficult-to-find bugs. Conceptually, the constructor’s job is to bring the object into existence (which is hardly an ordinary feat). Inside any constructor, the entire object might only be partially formed – you can know only that the base-class objects have been initialized, but you cannot know which classes are inherited from you. A dynamically-bound method call, however, reaches “forward” or “outward” into the inheritance hierarchy. It calls a method in a derived class. Chapter 7: Polymorphism 237
- 238. If you do this inside a constructor, you call a method that might manipulate members that haven’t been initialized yet: a sure recipe for disaster. You can see the problem in the following example: //: PolyConstructors.java // Constructors and polymorphism // don't produce what you might expect. abstract class Glyph { abstract void draw(); Glyph() { System.out.println("Glyph() before draw()"); draw(); System.out.println("Glyph() after draw()"); } } class RoundGlyph extends Glyph { int radius = 1; RoundGlyph(int r) { radius = r; System.out.println( "RoundGlyph.RoundGlyph(), radius = " + radius); } void draw() { System.out.println( "RoundGlyph.draw(), radius = " + radius); } } public class PolyConstructors { public static void main(String[] args) { new RoundGlyph(5); } } ///:~ In Glyph, the draw( ) method is abstract, so it is designed to be overridden. Indeed, you are forced to override it in RoundGlyph. But the Glyph constructor calls this method, and the call ends up in RoundGlyph.draw( ), which would seem to be the intent. But look at the output: Glyph() before draw() RoundGlyph.draw(), radius = 0 Glyph() after draw() RoundGlyph.RoundGlyph(), radius = 5 When Glyph’s constructor calls draw( ), the value of radius isn’t even the default initial value 1. It’s zero. This would probably result in either a dot or nothing at all being drawn on the screen, and you’d be staring, trying to figure out why the program won’t work. The order of initialization described in the previous section isn’t quite complete, and that’s the key to solving the mystery. The actual process of initialization is: 1. The storage allocated for the object is initialized to binary zero before anything else happens. 238 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 239. 2. The base-class constructors are called as described previously. At this point, the overridden draw( ) method is called, (yes, before the RoundGlyph constructor is called) which discovers a radius value of zero, due to step one. 3. Member initializers are called in the order of declaration. 4. The body of the derived-class constructor is called. There’s an upside to this, which is that everything is at least initialized to zero (or whatever zero means for that particular data type) and not just left as garbage. This includes object handles that are embedded inside a class via composition, and so if you forget to initialize that handle you’ll get an exception at run time. Everything else gets zero, which is usually a telltale value when looking at output. On the other hand, you should be pretty horrified at the outcome of this program. You’ve done a perfectly logical thing and yet the behavior is mysteriously wrong, with no complaints from the compiler (C++ produces more rational behavior in this situation). Bugs like this could easily be buried and take a long time to discover. As a result, a good guideline for constructors is “do as little as possible to set the object into a good state, and if you can possibly avoid it, don’t call any methods.” The only safe methods to call inside a constructor are those that are final in the base class (this also applies to private methods, which are automatically final). These cannot be overridden and thus cannot produce this kind of surprise. Designing with inheritance Once you learn about polymorphism, it can seem that everything ought to be inherited because polymorphism is such a clever tool. This can burden your designs; in fact if you choose inheritance first when you’re using an existing class to make a new class things can become needlessly complicated. A better approach is to choose composition first, when it’s not obvious which one you should use. Composition does not force a design into an inheritance hierarchy. But composition is also more flexible since it’s possible to dynamically choose a type (and thus behavior) when using composition, whereas inheritance requires an exact type to be known at compile time. The following example illustrates this: //: Transmogrify.java // Dynamically changing the behavior of // an object via composition. interface Actor { void act(); } class HappyActor implements Actor { public void act() { System.out.println("HappyActor"); } } class SadActor implements Actor { public void act() { System.out.println("SadActor"); } Chapter 7: Polymorphism 239
- 240. } class Stage { Actor a = new HappyActor(); void change() { a = new SadActor(); } void go() { a.act(); } } public class Transmogrify { public static void main(String[] args) { Stage s = new Stage(); s.go(); // Prints "HappyActor" s.change(); s.go(); // Prints "SadActor" } } ///:~ A Stage object contains a handle to an Actor, which is initialized to a HappyActor object. This means go( ) produces a particular behavior. But since a handle can be re-bound to a different object at run time, a handle for a SadActor object can be substituted in a and then the behavior produced by go( ) changes. Thus you gain dynamic flexibility at run time. In contrast, you can’t decide to inherit differently at run time; that must be completely determined at compile time. A general guideline is “use inheritance to express differences in behavior, and member variables to express variations in state.” In the above example, both are used: two different classes are inherited to express the difference in the act( ) method, and Stage uses composition to allow its state to be changed. In this case, that change in state happens to produce a change in behavior. Pure inheritance vs. extension When studying inheritance, it would seem that the cleanest way to create an inheritance hierarchy is to take the “pure” approach. That is, only methods that have been established in the base class or interface are to be overridden in the derived class, as seen in this diagram: Shape draw() “is a” erase() Circle Square Line draw() draw() draw() erase() erase() erase() 240 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 241. This can be termed a pure “is-a” relationship, because the interface of a class establishes what it is. Inheritance guarantees that any derived class will have the interface of the base class and nothing less. If you follow the above diagram, derived classes will also have no more than the base class interface. This can be thought of as pure substitution, because it means that derived class objects can be perfectly substituted for the base class, and you never need to know any extra information about the subclasses when you’re using them: talks to Circle, Square, Shape message Line, or new type of Shape That is, the base class can receive any message you can send to the derived class because the two have exactly the same interface. This means that all you have to do is upcast from the derived class and never look back to see what exact type of object you’re dealing with. Everything is handled through polymorphism. When you see it this way, it seems like a pure “is-a” relationship is the only sensible way to do things, and any other design indicates muddled thinking and is by definition broken. This too is a trap. As soon as you start thinking this way, you’ll turn around and discover that extending the interface (which, unfortunately, the keyword extends seems to promote) is the perfect solution to a particular problem. This could be termed an “is-like- a” relationship because the derived class is like the base class – it has the same fundamental interface – but it has other features that require additional methods to implement: Useful void f() void g() } Assume this represents a big interface MoreUseful “is like a” void f() void g() } void u() void v() Extending the void w() interface While this is also a useful and sensible approach (depending on the situation) it has a drawback. The extended part of the interface in the derived class is not available from the base class, so once you upcast you can’t call the new methods: Chapter 7: Polymorphism 241
- 242. talks to Useful part Useful message object MoreUseful part You might not be upcasting in this case, so it may not bother you, but very often you’ll get into a situation where you need to rediscover the exact type of the object so you can access the extended methods of that type. Downcasting & run-time type identification Since you lose the specific type information via an upcast (moving up the inheritance hierarchy), it makes sense that to retrieve the type information – that is, to move back down the inheritance hierarchy – you use a downcast. However, you know an upcast is always safe: the base class cannot have a bigger interface than the derived class, therefore every message you send through the base class interface is guaranteed to be accepted. But with a downcast, you don’t really know that a shape (for example) is actually a circle. It could instead be a triangle or square or some other type. Useful void f() void g() upcast: downcast: always MoreUseful must be safe void f() checked void g() void u() void v() void w() To solve this problem there must be some way to guarantee that a downcast is correct, so you won’t accidentally cast to the wrong type and then send a message that the object can’t accept. This would be quite unsafe. In some languages (like C++) you must perform a special operation in order to get a type- safe downcast, but in Java every cast is checked! So even though it looks like you’re just performing an ordinary parenthesized cast, at run time this cast is checked to ensure that it is in fact the type you think it is. If it isn’t, you get a ClassCastException. This act of checking types at run time is called run-time type identification (RTTI). The following example demonstrates the behavior of RTTI: //: RTTI.java // Downcasting & Run-Time Type // Identification (RTTI) import java.util.*; 242 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 243. class Useful { public void f() {} public void g() {} } class MoreUseful extends Useful { public void f() {} public void g() {} public void u() {} public void v() {} public void w() {} } public class RTTI { public static void main(String args[]) { Useful x[] = { new Useful(), new MoreUseful() }; x[0].f(); x[1].g(); // Compile-time: method not found in Useful: //! x[1].u(); ((MoreUseful)x[1]).u(); // Downcast/RTTI ((MoreUseful)x[0]).u(); // Exception thrown } } ///:~ As in the diagram, MoreUseful extends the interface of Useful. But since it’s inherited, it can also be upcast to a Useful. You can see this happening in the initialization of the array x in main( ). Since both objects in the array are of class Useful, you can send the f( ) and g( ) methods to both, and if you try to call u( ) (which exists only in MoreUseful) you’ll get a compile-time error message. If you want to access the extended interface of a MoreUseful object, you can try to downcast. If it’s the right type, it will be successful. Otherwise, you’ll get a ClassCastException. You don’t have to write any special code for this exception, since it indicates a programmer error that could happen anywhere in a program. There’s more to RTTI than a simple cast. For example, there’s a way to see what type you’re dealing with before you try to downcast it. All of Chapter 11 is devoted to the study of different aspects of Java run-time type identification. Summary Polymorphism means “different forms.” In object-oriented programming, you have the same face (the common interface in the base class) and different forms using that face: the different versions of the dynamically-bound methods. You’ve seen in this chapter that it’s impossible to understand, or even create, an example of polymorphism without using data abstraction and inheritance. Polymorphism is a feature that cannot be viewed in isolation (like a switch statement, for example), but instead works only in concert, as part of a “big picture” of class relationships. People are often confused by other, non-object-oriented features of Java, like method overloading, which are sometimes presented as object-oriented. Don’t be fooled: If it isn’t late binding, it isn’t polymorphism. Chapter 7: Polymorphism 243
- 244. To use polymorphism, and thus object-oriented techniques, effectively in your programs you must expand your view of programming to include not just members and messages of an individual class, but also the commonality among classes and their relationships with each other. Although this requires significant effort, it’s a worthy struggle, because the results are faster program development, better code organization, extensible programs, and easier code maintenance. Exercises 1. Create an inheritance hierarchy of Rodent: Mouse, Gerbil, Hamster, etc. In the base class, provide methods that are common to all Rodents, and override these in the derived classes to perform different behaviors depending on the specific type of Rodent. Create an array of Rodent, fill it with different specific types of Rodents, and call your base-class methods to see what happens. 2. Change exercise one so that Rodent is an interface. 3. Repair the problem in WindError.java. 4. In GreenhouseControls.java, add Event inner classes that turn fans on and off. 244 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 245. 7 8: Holding your objects It’s a fairly simple program that has only a fixed quantity of objects with known lifetimes. In general your programs will always be creating new objects based on some criteria that will be known only at the time the program is running. In addition, you won’t know until run-time the quantity or even the exact type of the objects you need. To solve the general programming problem, you need to create any number of objects, anytime, anywhere. So you can’t rely on creating a named handle to hold each one of your objects: MyObject myHandle; since you’ll never know how many of these things you’ll actually need. To solve this rather essential problem, Java has several ways to hold objects (or rather, handles to objects). The built-in type is the array, which has been discussed before and will get additional coverage in this chapter. Also, the Java utilities library has some collection classes (also known as container classes, but that term is used by the AWT so “collection” will be used here) which provide more sophisticated ways to hold and even manipulate your objects; this will comprise the remainder of this chapter. Arrays Most of the necessary introduction to arrays is in the last section of Chapter 4, which shows how you define and initialize an array. Holding objects is the focus of this chapter, and an array is just a way to hold objects. But there are a number of other ways to hold objects, so what makes an array special? There are two issues that distinguish arrays from other types of collections: efficiency and type. The array is the most efficient way that Java provides to store and access a sequence of objects (actually, object handles). The array is a simple linear sequence, which makes 245
- 246. element access very fast, but you pay for this speed: when you create an array object, its size is fixed and cannot be changed for the lifetime of that array object. You might suggest creating an array of a particular size and then, if you run out of space, creating a new one and moving all the handles from the old one to the new one. This is the behavior of the Vector class which will be studied later in the chapter. However, because of the overhead of this size flexibility, a Vector is measurably less efficient than an array. The vector class in C++ does know the type of objects it holds, but it has a different drawback when compared with arrays in Java: the C++ vector doesn’t do bounds checking, so you can run past the end (however, it’s possible to ask how big the vector is). In Java, you get bounds checking regardless of whether you’re using an array or a collection – you’ll get a RuntimeException if you exceed the bounds. As you’ll learn in Chapter 9, this type of exception indicates a programmer error and thus you don’t need to check for it in your code. As an aside, the reason the C++ vector doesn’t check bounds with every access is speed – in Java you have the constant performance overhead of bounds checking all the time for both arrays and collections. The other generic collection classes that will be studied in this chapter, Vector, Stack and Hashtable, all deal with objects as if they had no specific type. That is, they treat them as type Object, the root class of all classes in Java. This works fine from one standpoint: you need to build only one collection, and any Java object will go into that collection (except for primitives – these can be placed in collections as constants using the Java primitive wrapper classes, or as changeable values by wrapping in your own class). This is the second place where an array is superior to the generic collections: when you create an array, you create it to hold a specific type. This means you get compile-time type checking to prevent you from putting the wrong type in, or mistaking the type that you’re extracting. Of course, Java will prevent you from sending an inappropriate message to an object one way or another, either at compile-time or at run-time, so it’s not as if it is riskier one way or another, it’s just nicer if the compiler points it out to you, faster at run-time, and there’s less likelihood that the end user will get surprised by an exception. For both of the aforementioned reasons – efficiency and type checking – it’s always worth trying to use an array if you can. However, when you’re trying to solve a more general problem arrays can be too restrictive. After looking at arrays, the rest of this chapter will be devoted to the collection classes provided by Java. Arrays are first-class objects Regardless of what type of array you’re working with, the array identifier is actually a handle to a true object that’s created on the heap. The heap object can be created either implicitly, as part of the array initialization syntax, or explicitly with a new expression. Part of the heap object (in fact, the only field or method you can access) is the read-only length member that tells you how many elements can be stored in that array object. The ‘[]’ syntax is the only other access that you have to the array object. The following example shows the various ways an array can be initialized, and how the array handles can be assigned to different array objects. It also shows that arrays of objects and arrays of primitives are almost identical in their use; the only difference is that arrays of objects hold handles while arrays of primitives hold the primitive values directly (see page 80 if you have trouble executing this program). //: ArraySize.java // Initialization & re-assignment of arrays package c08; class Weeble {} // A small mythical creature 246 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 247. public class ArraySize { public static void main(String args[]) { // Arrays of objects: Weeble a[]; // Null handle Weeble b[] = new Weeble[5]; // Null handles Weeble c[] = new Weeble[4]; for(int i = 0; i < c.length; i++) c[i] = new Weeble(); Weeble d[] = { new Weeble(), new Weeble(), new Weeble() }; // Compile error: variable a not initialized: //!System.out.println("a.length=" + a.length); System.out.println("b.length = " + b.length); // The handles inside the array are // automatically initialized to null: for(int i = 0; i < b.length; i++) System.out.println("b[" + i + "]=" + b[i]); System.out.println("c.length = " + c.length); System.out.println("d.length = " + d.length); a = d; System.out.println("a.length = " + a.length); // Java 1.1 initialization syntax: a = new Weeble[] { new Weeble(), new Weeble() }; System.out.println("a.length = " + a.length); // Arrays of primitives: int e[]; // Null handle int f[] = new int[5]; int g[] = new int[4]; for(int i = 0; i < g.length; i++) g[i] = i*i; int h[] = { 11, 47, 93 }; // Compile error: variable e not initialized: //!System.out.println("e.length=" + e.length); System.out.println("f.length = " + f.length); // The primitives inside the array are // automatically initialized to zero: for(int i = 0; i < f.length; i++) System.out.println("f[" + i + "]=" + f[i]); System.out.println("g.length = " + g.length); System.out.println("h.length = " + h.length); e = h; System.out.println("e.length = " + e.length); // Java 1.1 initialization syntax: e = new int[] { 1, 2 }; System.out.println("e.length = " + e.length); } } ///:~ Here’s the output from the program: b.length = 5 b[0]=null Chapter 8: Holding Your Objects 247
- 248. b[1]=null b[2]=null b[3]=null b[4]=null c.length = 4 d.length = 3 a.length = 3 a.length = 2 f.length = 5 f[0]=0 f[1]=0 f[2]=0 f[3]=0 f[4]=0 g.length = 4 h.length = 3 e.length = 3 e.length = 2 The array a is initially just a null handle, and the compiler prevents you from doing anything with this handle until you’ve properly initialized it. The array b is initialized to point to an array of Weeble handles, but no actual Weeble objects are ever placed in that array. However, you can still ask what the size of the array is, since b is pointing to a legitimate object. This brings up a slight drawback: you can’t find out how many elements are actually in the array, since length tells you only how many elements can be placed in the array; that is, the size of the array object, not the number of elements it actually holds. However, when an array object is created its handles are automatically initialized to null so you can see whether a particular array slot has an object in it by checking to see whether it’s null. Similarly, an array of primitives is automatically initialized to zero for numeric types, null for char and false for boolean. Array c shows the creation of the array object followed by the assignment of Weeble objects to all the slots in the array. Array d shows the “aggregate initialization” syntax that causes the array object to be created (implicitly with new on the heap, just like for Array c) and initialized with Weeble objects, all in one statement. The expression a = d; shows how you can take a handle that’s attached to one array object and assign it to another array object, just as you can do with any other type of object handle. Now both a and d are pointing to the same array object on the heap. Java 1.1 adds a new array initialization syntax, which could be thought of as a “dynamic aggregate initialization.” The Java 1.0 aggregate initialization used by d must be used at the point of d’s definition, but with the new Java 1.1 syntax you can create and initialize an array object anywhere at all. For example, suppose hide( ) is a method that takes an array of Weeble objects. You could call it by saying: hide(d); but in Java 1.1 you can also dynamically create the array you want to pass as the argument: hide(new Weeble[] { new Weeble(), new Weeble() }); This new syntax provides a more convenient way to write code in some situations. 248 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 249. The second part of the above example shows that primitive arrays work just like object arrays except that primitive arrays hold the primitive values directly. Collections of primitives Collection classes can hold only handles to objects. An array, however, can be created to hold primitives directly, as well as handles to objects. It is possible to use the “wrapper” classes such as Integer, Double etc. to place primitive values inside a collection, but as you’ll see later in this chapter in the WordCount.java example, the wrapper classes for primitives are only somewhat useful anyway. Whether you put primitives in arrays or wrap them in a class that’s placed in a collection is a question of efficiency: it’s much more efficient to create and access an array of primitives than a collection of wrapped primitives. Of course, if you’re using a primitive type and you need the flexibility of a collection that automatically expands itself when more space is needed, the array won’t work and you’re forced to use a collection of wrapped primitives. You might think that there should be a specialized type of Vector for each of the primitive data types, but Java doesn’t provide this for you. Some sort of templatizing mechanism might someday provide a better way for Java to handle this problem1 . Returning an array Suppose you’re writing a method and you don’t just want to return one thing, but instead a whole bunch of things. Languages like C and C++ make this difficult because you can’t just return an array, but only a pointer to an array. This introduces problems because it becomes messy to control the lifetime of the array, which easily leads to memory leaks. Java takes a similar approach, but you just “return an array.” Actually, of course, you’re returning a handle to an array but with Java you never worry about responsibility for that array – it will be around as long as you need it, and the garbage collector will clean it up when you’re done. As an example, consider returning an array of String: //: IceCream.java // Returning arrays from methods public class IceCream { static String flav[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; static String[] flavorSet(int n) { // Force it to be positive & within bounds: n = Math.abs(n) % (flav.length + 1); String results[] = new String[n]; int picks[] = new int[n]; for(int i = 0; i < picks.length; i++) picks[i] = -1; 1 This is one of the places where C++ is distinctly superior to Java, since C++ supports parameterized types with the template keyword. Chapter 8: Holding Your Objects 249
- 250. for(int i = 0; i < picks.length; i++) { retry: while(true) { int t = (int)(Math.random() * flav.length); for(int j = 0; j < i; j++) if(picks[j] == t) continue retry; picks[i] = t; results[i] = flav[t]; break; } } return results; } public static void main(String args[]) { for(int i = 0; i < 20; i++) { System.out.println( "flavorSet(" + i + ") = "); String fl[] = flavorSet(flav.length); for(int j = 0; j < fl.length; j++) System.out.println("t" + fl[j]); } } } ///:~ The method flavorSet( ) creates an array of String called results. The size of this array is n, determined by the argument you pass into the method. Then it proceeds to randomly choose flavors from the array flav and place them into results, which it finally returns. Returning an array is just like returning any other object: it’s a handle. It’s not important that the array was created within flavorSet( ), or that the array was created anyplace else, for that matter. The garbage collector takes care of cleaning up the array when you’re done with it, and the array will persist for as long as you need it. As an aside, notice that when flavorSet( ) randomly chooses flavors, it ensures that a random choice hasn’t been picked before. This is performed in a seemingly infinite while loop that keeps making random choices until it finds one that’s not already in the picks array (of course a String comparison could also have been performed to see if the random choice was already in the results array but String comparisons are very inefficient). If it’s successful it adds the entry and breaks out to go find the next one (i gets incremented). But if t is a number that’s already in picks, then a labeled continue is used to jump back two levels which forces a new t to be selected. It’s particularly convincing to watch this happen with a debugger. main( ) prints out 20 full sets of flavors, so you can see that flavorSet( ) chooses the flavors in a random order each time. It’s easiest to see this if you redirect the output into a file. And while you’re looking at the file, remember, you’re not really hungry (You just want the ice cream, you don’t need it). Collections To summarize what we’ve seen so far: your first, most efficient choice to hold a group of objects should be an array, and you’re forced into this choice if you want to hold a group of primitives. In the remainder of the chapter we’ll look at the more general case, when you don’t know at the time you’re writing the program how many objects you’re going to need, or if you need a more sophisticated way to store your objects. Java provides four 250 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 251. types of collection classes to solve this problem: Vector, BitSet, Stack and Hashtable. Although compared to other languages that provide collections this is a fairly meager supply, you can nonetheless solve a surprising number of problems using these tools. Among their other characteristics – Stack, for example, implements a LIFO (last-in, first- out) sequence, and Hashtable is an associative array that lets you associate any object with any other object – the Java collection classes will automatically resize themselves. Thus, you can put in any number of objects and you don’t have to worry about how big to make the collection while you’re writing the program. Disadvantage: unknown type The “disadvantage” to using the Java collections is that you lose type information when you put an object into a collection. This happens because, when the collection was written the programmer of that collection had no idea what specific type you wanted to put in the collection, and making the collection hold only your type would prevent it from being a general-purpose tool. So instead, the collection holds handles to objects of type Object, which is of course every object in Java since it’s the root of all the classes (of course, this doesn’t include primitive types, since they aren’t inherited from anything). This is a great solution, except for a couple of things: 1. Since the type information is thrown away when you put an object handle into a collection, any type of object can be put into your collection, even if you mean it to hold only, say, cats. Someone could just as easily put a dog into the collection. 2. Since the type information is lost, the only thing the collection knows it holds is a handle to an Object. You must perform a cast to the correct type before you use it. On the up side, Java won’t let you misuse the objects that you put into a collection. If you throw a dog into a collection of cats, then go through and try to treat everything in the collection as a cat, you’ll get an exception when you get to the dog. In the same vein, if you try to cast the dog handle that you pull out of the cat collection into a cat, you’ll get an exception at run-time. Here’s an example: //: CatsAndDogs.java // Simple collection example (Vector) import java.util.*; class Cat { private int catNumber; Cat(int i) { catNumber = i; } void print() { System.out.println("Cat #" + catNumber); } } class Dog { private int dogNumber; Dog(int i) { dogNumber = i; } Chapter 8: Holding Your Objects 251
- 252. void print() { System.out.println("Dog #" + dogNumber); } } public class CatsAndDogs { public static void main(String args[]) { Vector cats = new Vector(); for(int i = 0; i < 7; i++) cats.addElement(new Cat(i)); // Not a problem to add a dog to cats: cats.addElement(new Dog(7)); for(int i = 0; i < cats.size(); i++) ((Cat)cats.elementAt(i)).print(); // Dog is detected only at run-time } } ///:~ You can see that using a Vector is straightforward: create one, put objects in using addElement( ) and later get them out with elementAt( ) (notice that Vector has a method size( ) to let you know how many elements have been added so you don’t inadvertently run off the end and cause an exception). The classes Cat and Dog are distinct – they have nothing in common except that they are Objects (if you don’t explicitly say what class you’re inheriting from, you automatically inherit from Object). The Vector class, which comes from java.util, holds Objects, so not only can I put Cat objects into this collection using the Vector method addElement( ), but I can also add Dog objects without complaint at either compile-time or run-time. When I go to fetch out what I think are Cat objects using the Vector method elementAt( ), I get back a handle to an Object that I must cast to a Cat. Then I have to surround the entire expression with parentheses to force the evaluation of the cast before calling the print( ) method for Cat, otherwise I’ll get a syntax error. Then, at run-time, when I try to cast the Dog object to a Cat, I’ll get an exception. This is more than just an annoyance. It’s something that can create some difficult-to-find bugs. If one part (or several parts) of a program inserts objects into a collection, and you discover only in a separate part of the program, through an exception, that a bad object was placed in the collection, then you must find out where the bad insert occurred by code inspection, which is about the worst debugging tool we have. On the upside, it’s very convenient to start with some standardized collection classes for programming, despite the scarcity and awkwardness. Sometimes it works right anyway It turns out that in some cases things seem to work correctly without casting back to your original type. The first case is quite special: the String class has some extra help from the compiler to make it work smoothly. Whenever the compiler expects a String object and it hasn’t got one, it will automatically call the toString( ) method that’s defined in Object and may be overridden by any Java class. This method produces the desired String object, which is then used wherever it was wanted. Thus, all you need to do to make objects of your class magically print out is to override the toString( ) method, as shown in the following example: //: WorksAnyway.java // In special cases, things just seem // to work correctly. 252 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 253. import java.util.*; class Mouse { private int mouseNumber; Mouse(int i) { mouseNumber = i; } // Magic method: public String toString() { return "This is Mouse #" + mouseNumber; } void print(String msg) { if(msg != null) System.out.println(msg); System.out.println( "Mouse number " + mouseNumber); } } class MouseTrap { static void caughtYa(Object m) { Mouse mouse = (Mouse)m; // Cast from Object mouse.print("Caught one!"); } } public class WorksAnyway { public static void main(String args[]) { Vector mice = new Vector(); for(int i = 0; i < 3; i++) mice.addElement(new Mouse(i)); for(int i = 0; i < mice.size(); i++) { // No cast necessary, automatic call // to Object.toString(): System.out.println( "Free mouse: " + mice.elementAt(i)); MouseTrap.caughtYa(mice.elementAt(i)); } } } ///:~ You can see the redefinition of toString( ) in Mouse. In the second for loop in main( ) you find the statement: System.out.println("Free mouse: " + mice.elementAt(i)); After the ‘+’ sign the compiler is expecting to see a String object. elementAt( ) produces an Object, so to get the desired String the compiler implicitly calls toString( ). Unfortunately, you can only this kind of magic work with String; it isn’t available for any other type. A second approach to hiding the cast has been placed inside Mousetrap: the caughtYa( ) method accepts, not a Mouse, but an Object which it then casts to a Mouse. This is quite presumptuous, of course, since by accepting an Object anything could be passed to the method. However, if the cast is incorrect – if you passed the wrong type – you’ll get an exception at run-time. This is not as good as compile-time checking but it’s still robust. Notice that in the use of this method: Chapter 8: Holding Your Objects 253
- 254. MouseTrap.caughtYa(mice.elementAt(i)); no cast is necessary. Making a type-conscious Vector You might not want to give up on this issue just yet. A more ironclad solution is to create a new class using the Vector, such that it will accept only your type and produce only your type: //: GopherVector.java // A type-conscious Vector import java.util.*; class Gopher { private int gopherNumber; Gopher(int i) { gopherNumber = i; } void print(String msg) { if(msg != null) System.out.println(msg); System.out.println( "Gopher number " + gopherNumber); } } class GopherTrap { static void caughtYa(Gopher g) { g.print("Caught one!"); } } class GopherVector { private Vector v = new Vector(); public void addElement(Gopher m) { v.addElement(m); } public Gopher elementAt(int index) { return (Gopher)v.elementAt(index); } public int size() { return v.size(); } public static void main(String args[]) { GopherVector gophers = new GopherVector(); for(int i = 0; i < 3; i++) gophers.addElement(new Gopher(i)); for(int i = 0; i < gophers.size(); i++) GopherTrap.caughtYa(gophers.elementAt(i)); } } ///:~ This is similar to the previous example, except that the new GopherVector class has a private member of type Vector (inheriting from Vector tends to be frustrating, for reasons you’ll see later), and methods just like Vector. However, it doesn’t accept and produce generic Objects, only Gopher objects. Because a GopherVector will accept only a Gopher, if you were to say: 254 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 255. gophers.addElement(new Pigeon()); you would get an error message at compile time. So this approach, while more tedious from a coding standpoint, will tell you immediately if you’re using a type improperly. Notice that no cast is necessary when using elementAt( ) – it’s always a Gopher. Parameterized types This kind of problem isn’t isolated – there are numerous cases where you need to create new types based on other types, and where it is very useful to have specific type information at compile-time. This is the concept of a parameterized type. In C++ this is directly supported by the language in the form of templates. At one point, Java had reserved the keyword generic to someday support parameterized types, but it’s uncertain if this will ever occur. Enumerators (iterators) In any collection class, you must have a way to put things in and a way to get things out. After all, that’s the primary job of a collection – to hold things. In the Vector, addElement( ) is the way you insert objects, and elementAt( ) is one way to get things out. Vector is quite flexible – you can select anything at any time, and select multiple elements at once using different indexes. But if you want to start thinking at a higher level, there’s a drawback: you have to know the exact type of the collection in order to use it. This might not seem bad at first, but what if you start out using a Vector, and later on in your program you decide, for reasons of efficiency, that you want to change to a List (which is part of the Java 1.2 collections library). Or you’d like to write a piece of code that doesn’t know or care what type of collection it’s working with. The concept of an iterator can be used to achieve this next level of abstraction. This is an object whose job is to move through a sequence of objects and select each object in that sequence, without knowing or caring about the underlying structure of that sequence. In addition, an iterator is usually what’s called a “light-weight” object; that is, one that’s cheap to create. For that reason, you’ll often find seemingly strange constraints for iterators; for example, some iterators can move in only one direction. The Java Enumeration2 is an example of an iterator with these kinds of constraints – there’s not much you can do with one except: 1. Ask a collection to hand you an Enumeration using a method called elements( ). This Enumeration will be selecting the first element in the sequence) 2. Get the next object in the sequence with nextElement( ) 3. See if there are any more objects in the sequence with hasMoreElements( ) That’s all. It’s a very simple implementation of an iterator, but still powerful. To see how it works, let’s revisit the CatsAndDogs.java program from earlier in the chapter. In the 2 The term iterator is common in C++ and elsewhere in OOP, so it’s difficult to know why the Java team used a strange name. The collections library in Java 1.2 fixes this as well as many other problems. Chapter 8: Holding Your Objects 255
- 256. original version, the method elementAt( ) was used to select each element, but in the following modified version an enumeration is used: //: CatsAndDogs2.java // Simple collection with Enumeration import java.util.*; class Cat2 { private int catNumber; Cat2(int i) { catNumber = i; } void print() { System.out.println("Cat number " +catNumber); } } class Dog2 { private int dogNumber; Dog2(int i) { dogNumber = i; } void print() { System.out.println("Dog number " +dogNumber); } } public class CatsAndDogs2 { public static void main(String args[]) { Vector cats = new Vector(); for(int i = 0; i < 7; i++) cats.addElement(new Cat2(i)); // Not a problem to add a dog to cats: cats.addElement(new Dog2(7)); Enumeration e = cats.elements(); while(e.hasMoreElements()) ((Cat2)e.nextElement()).print(); // Dog is detected only at run-time } } ///:~ You can see that the only change is in the last few lines. Instead of: for(int i = 0; i < cats.size(); i++) ((Cat)cats.elementAt(i)).print(); an Enumeration is used to step through the sequence: while(e.hasMoreElements()) ((Cat2)e.nextElement()).print(); With the Enumeration, you don’t have to worry about the number of elements in the collection. That’s taken care of for you by hasMoreElements( ) and nextElement( ). As another example, consider the creation of a general-purpose printing method: //: HamsterMaze.java 256 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 257. // Using an Enumeration import java.util.*; class Hamster { private int hamsterNumber; Hamster(int i) { hamsterNumber = i; } public String toString() { return "This is Hamster #" + hamsterNumber; } } class Printer { static void printAll(Enumeration e) { while(e.hasMoreElements()) System.out.println( e.nextElement().toString()); } } public class HamsterMaze { public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 3; i++) v.addElement(new Hamster(i)); Printer.printAll(v.elements()); } } ///:~ Look closely at the printing method: static void printAll(Enumeration e) { while(e.hasMoreElements()) System.out.println( e.nextElement().toString()); } Notice that there’s no information about the type of sequence. All you have is an Enumeration, and that’s all you need to know about the sequence: that you can get the next object, and that you can know when you’re at the end. This idea of taking a collection of objects and passing through it to perform an operation on each one is very powerful and will be seen again and again throughout this book. This particular example is even more generic, since it uses the ubiquitous toString( ) method (ubiquitous only because it’s part of the Object class). An alternative way to call print (although probably slightly less efficient, if you could even notice the difference) is: System.out.println("" + e.nextElement()); which uses the “automatic conversion to String” that’s wired into Java. When the compiler sees a String, followed by a ‘+’, it expects another String to follow and calls toString( ) automatically (in Java 1.1 the first String is unnecessary; any object will be converted to a String). You can also perform a cast, which has the effect of calling toString( ): System.out.println((String)e.nextElement()); Chapter 8: Holding Your Objects 257
- 258. In general, however, you’ll want to do something more than call Object methods, so you’ll run up against the type-casting issue again. That is, you’ll have to assume that you’ve gotten an Enumeration to a sequence of the particular type you’re interested in, and cast the resulting objects to that (getting a run-time exception if you’re wrong). Types of collections The standard Java 1.0 and 1.1 library comes with a bare minimum set of collection classes, but they’re probably enough to get by with for many of your programming projects (as you’ll see at the end of this chapter, Java 1.2 provides a radically-redesigned and filled-out library of collections). Vector The Vector is quite simple to use, as you’ve seen so far. Although most of the time you’ll just use addElement( ) to insert objects, elementAt( ) to get them out one at a time and elements( ) to get an Enumeration to the sequence, there’s also a set of other methods that can be useful. As usual with the Java libraries, we won’t use or talk about them all here, but be sure to look them up in the electronic documentation to get a feel for what they can do. Crashing Java The Java standard collections contain a toString( ) method so they can produce a String representation of themselves, including the objects they hold. Inside of Vector, for example, the toString( ) steps through the elements of the Vector and calls toString( ) for each one. Suppose you’d like to print out the address of your class. It seems to make sense to simply refer to this (in particular, C++ programmers are prone to this approach): //: CrashJava.java // One way to crash Java import java.util.*; public class CrashJava { public String toString() { return "CrashJava address: " + this + "n"; } public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 10; i++) v.addElement(new CrashJava()); System.out.println(v); } } ///:~ It turns out that if you simply create a CrashJava object and print it out, you’ll get an endless sequence of exceptions. However, if you place the CrashJava objects in a Vector and print out that Vector as shown here, it can’t handle it and you don’t even get an exception; Java just crashes (but at least it didn’t bring down my operating system). This was tested with Java 1.1. What’s happening is automatic type conversion for Strings. When you say: "CrashJava address: " + this 258 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 259. The compiler sees a String followed by a ‘+’ and something that’s not a String, so it tries to convert this to a String. It does this conversion by calling toString( ), which produces a recursive call. When this occurs inside a Vector it appears that the stack overflows without the exception-handling mechanism getting a chance to respond. If you really do want to print the address of the object in this case, the solution is to call the Object toString( ) method, which does just that. So instead of saying this, you’d say super.toString( ). BitSet A BitSet is really a Vector of bits, and is used if you want to efficiently store a whole lot of on-off information. It’s efficient only from the standpoint of size; if you’re looking for efficient access it is slightly slower than using an array of some native type. In addition, the minimum size of the BitSet is that of a long: 64 bits. This implies that if you’re storing anything smaller, like 8 bits, a BitSet will be wasteful so you’re better off creating your own class to hold your flags. In a normal Vector, the collection will expand as you add more elements. The BitSet does this as well – sort of. That is, sometimes it works and sometimes it doesn’t, which makes it appear that the Java version 1.0 implementation of BitSet is just badly done (it is fixed in Java 1.1). The following example shows how the BitSet works and demonstrates the version 1.0 bug: //: Bits.java // Demonstration of BitSet import java.util.*; public class Bits { public static void main(String args[]) { Random rand = new Random(); // Take the LSB of nextInt(): byte bt = (byte)rand.nextInt(); BitSet bb = new BitSet(); for(int i = 7; i >=0; i--) if(((1 << i) & bt) != 0) bb.set(i); else bb.clear(i); System.out.println("byte value: " + bt); printBitSet(bb); short st = (short)rand.nextInt(); BitSet bs = new BitSet(); for(int i = 15; i >=0; i--) if(((1 << i) & st) != 0) bs.set(i); else bs.clear(i); System.out.println("short value: " + st); printBitSet(bs); int it = rand.nextInt(); BitSet bi = new BitSet(); for(int i = 31; i >=0; i--) if(((1 << i) & it) != 0) Chapter 8: Holding Your Objects 259
- 260. bi.set(i); else bi.clear(i); System.out.println("int value: " + it); printBitSet(bi); // Test bitsets >= 64 bits: BitSet b127 = new BitSet(); b127.set(127); System.out.println("set bit 127: " + b127); BitSet b255 = new BitSet(65); b255.set(255); System.out.println("set bit 255: " + b255); BitSet b1023 = new BitSet(512); // Without the following, an exception is thrown // in the Java 1.0 implementation of BitSet: // b1023.set(1023); b1023.set(1024); System.out.println("set bit 1023: " + b1023); } static void printBitSet(BitSet b) { System.out.println("bits: " + b); String bbits = new String(); for(int j = 0; j < b.size() ; j++) bbits += (b.get(j) ? "1" : "0"); System.out.println("bit pattern: " + bbits); } } ///:~ The random number generator is used to create a random byte, short and int, and each one is transformed into a corresponding bit pattern in a BitSet. All this works fine because a BitSet is 64 bits, so none of these cause it to increase in size. But in Java 1.0, when the BitSet is greater than 64 bits, some strange behavior occurs. If you set a bit that’s just one greater than the BitSet’s currently-allocated storage, it will expand nicely. But if you try to set bits at higher locations than that without first just touching the boundary, you’ll get an exception, since the BitSet won’t expand properly in Java 1.0. The example shows a BitSet of 512 bits being created. The constructor allocates storage for twice that number of bits. Then if you try to set bit 1024 or greater without first setting bit 1023, you’ll throw an exception in Java 1.0. Fortunately, this is fixed in Java 1.1 but you’ll need to avoid using the BitSet if you must write code for Java 1.0. Stack A Stack is sometimes referred to as a “last-in, first-out” (LIFO) collection. That is, whatever you “push” on the Stack last is the first item you can “pop” out. Like all the other collections in Java, what you push and pop are Objects, so you must cast what you pop. What’s rather odd is that rather than using a Vector as a building block to create a Stack, Stack is inherited from Vector. This means it has all the characteristics and behaviors of a Vector plus some extra Stack behaviors. It’s difficult to know whether the designers explicitly decided that this was an especially useful way to do things, or whether it was just a naïve design. Here’s a simple demonstration of Stack that reads each line from a file and pushes it as a String: 260 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 261. //: Stacks.java // Demonstration of Stack Class import java.util.*; import java.io.*; public class Stacks { static String months[] = { "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; public static void main(String args[]) { Stack stk = new Stack(); for(int i = 0; i < months.length; i++) stk.push(months[i] + "n"); System.out.println("stk = " + stk); // Treating a stack as a Vector: stk.addElement("The last linen"); System.out.println( "element 5 = " + stk.elementAt(5)); System.out.println("popping elements:"); while(!stk.empty()) System.out.print(stk.pop()); } } ///:~ Each line in the months array is inserted into the Stack with push( ), and later fetched from the top of the stack with a pop( ). To make a point, Vector operations are also performed on the Stack object. This is possible because, by virtue of inheritance, a Stack is a Vector. Thus all operations that can be performed on a Vector can also be performed on a Stack, such as elementAt( ). Hashtable A Vector allows you to select from a sequence of objects using a number, so in a sense it associates numbers to objects. But what if you’d like to select from a sequence of objects using some other criterion? A Stack is an example: its selection criterion is “the last thing pushed on the stack.” A very powerful twist on this idea of “selecting from a sequence” is alternately termed a map, a dictionary or an associative array. Conceptually, it seems like a vector, but instead of looking up objects using a number, you look them up using another object! This is very often a key process in a program. The concept shows up in Java as the abstract class Dictionary. The interface for this class is straightforward: size( ) tells you how many elements are within, isEmpty( ) is true if there are no elements, put(Object key, Object value) adds a value (the thing you’ll be wanting) and associates it with a key (the thing you’ll be looking it up with). get(Object key) produces the value given the corresponding key, and remove(Object key) removes the key-value pair from the list. There are enumerations: keys( ) produces an Enumeration of the keys, and elements( ) produces an Enumeration of all the values. That’s all there is to a Dictionary. A Dictionary isn’t terribly difficult to implement. Here’s a simple approach, which uses two Vectors, one for keys and one for values: //: AssocArray.java // Simple version of a Dictionary import java.util.*; Chapter 8: Holding Your Objects 261
- 262. public class AssocArray extends Dictionary { private Vector keys = new Vector(); private Vector values = new Vector(); public int size() { return keys.size(); } public boolean isEmpty() { return keys.isEmpty(); } public Object put(Object key, Object value) { keys.addElement(key); values.addElement(value); return key; } public Object get(Object key) { int index = keys.indexOf(key); // indexOf() Returns -1 if key not found: if(index == -1) return null; return values.elementAt(index); } public Object remove(Object key) { int index = keys.indexOf(key); if(index == -1) return null; keys.removeElementAt(index); Object returnval = values.elementAt(index); values.removeElementAt(index); return returnval; } public Enumeration keys() { return keys.elements(); } public Enumeration elements() { return values.elements(); } // Test it: public static void main(String args[]) { AssocArray aa = new AssocArray(); for(char c = 'a'; c <= 'z'; c++) aa.put(String.valueOf(c), String.valueOf(c) .toUpperCase()); char[] ca = { 'a', 'e', 'i', 'o', 'u' }; for(int i = 0; i < ca.length; i++) System.out.println("Uppercase: " + aa.get(String.valueOf(ca[i]))); } } ///:~ The first thing you see in the definition of AssocArray is that it extends Dictionary. This means that AssocArray is a type of Dictionary, so you can make the same requests of it that you can a Dictionary. If you make your own Dictionary, as is done here, all you have to do is fill in all the methods that are in Dictionary (and you must override all the methods because all of them – with the exception of the constructor – are abstract). The Vectors keys and values are linked by a common index number. That is, if I call put( ) with a key of “roof” and a value of “blue” (assuming I’m associating the various parts of a house with the colors they are to be painted) and there are already 100 elements in the AssocArray, then “roof” will be the 101 element of keys and “blue” will be the 101 262 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 263. element of values. And if you look at get( ), when you pass “roof” in as the key, it produces the index number with keys.indexOf( ), and then uses that index number to produce the value in the associated values vector. The test in main( ) is very simple; it’s just a map of lowercase characters to uppercase characters, which could obviously be done in a number of more efficient ways. But it shows that AssocArray is functional. The standard Java library contains only one embodiment of a Dictionary, called Hashtable3. Java’s Hashtable has the same basic interface as AssocArray (since they both inherit Dictionary), but it differs in one distinct way: efficiency. If you look at what must be done for a get( ), it seems pretty slow to search through a Vector for the key. This is where Hashtable speeds things up: instead of the tedious linear search for the key, it uses a special value called a hash code. The hash code is a way to take some information in the object in question and turn it into a “relatively unique” int for that object. All objects have a hash code, and hashCode( ) is a method in the root class Object. A Hashtable takes the hashCode( ) of the object and uses it to quickly hunt for the key. This results in a dramatic performance improvement. The way that a Hashtable works is beyond the scope of this book4 – all you need to know is that Hashtable is a fast Dictionary, and that a Dictionary is a very useful tool. As an example of the use of a Hashtable, consider a program to check the randomness of Java’s Math.random( ) method. Ideally, it would produce a perfect distribution of random numbers, but to test this we need to generate a bunch of random numbers and count the ones that fall in the various ranges. A Hashtable is perfect for this, since it associates objects with objects (in this case, the values produced by Math.random( ) with the number of times those values appear: //: Statistics.java // Simple demonstration of Hashtable import java.util.*; class Counter { int i = 1; public String toString() { return Integer.toString(i) + "n"; } } class Statistics { public static void main(String args[]) { Hashtable ht = new Hashtable(); for(int i = 0; i < 10000; i++) { // Produce a number between 0 and 20: Integer r = new Integer((int)(Math.random() * 20)); if(ht.containsKey(r)) ((Counter)ht.get(r)).i++; 3 If you’re planning on using RMI (described in Chapter 15) you should be aware that there’s a problem when putting remote objects into a Hashtable. (See Core Java, by Cornell & Horstmann, Prentice-Hall 1997). 4 The best reference I know of is Practical Algorithms for Programmers, by Andrew Binstock and John Rex, Addison-Wesley 1995. Chapter 8: Holding Your Objects 263
- 264. else ht.put(r, new Counter()); } System.out.println(ht); } } ///:~ In main( ), each time a random number is generated it is wrapped inside an Integer object so that handle can be used with the Hashtable (you can’t use a primitive with a collection, only an object handle). The containsKey( ) method checks to see if this key is already in the collection (that is, has the number been found already?). If so, the get( ) methods gets the associated value for the key, which in this case is a Counter object. The value i inside the counter is then incremented to indicate one more of this particular random number has been found. If the key has not been found yet, the method put( ) will place a new key-value pair into the Hashtable. Since Counter automatically initializes its variable i to one when it’s created, it indicates the first occurrence of this particular random number. To display the Hashtable, it is simply printed out. The Hashtable toString( ) method moves through all the key-value pairs and calls the toString( ) for each one. The Integer toString( ) is pre-defined, and you can see the toString( ) for Counter. The output from one run is: {19=526 , 18=533 , 17=460 , 16=513 , 15=521 , 14=495 , 13=512 , 12=483 , 11=488 , 10=487 , 9=514 , 8=523 , 7=497 , 6=487 , 5=480 , 4=489 , 3=509 , 2=503 , 1=475 , 0=505 } You might wonder at the necessity of the class Counter which seems like it doesn’t even have the functionality of the wrapper class Integer. Why not use int or Integer? Well, you can’t use an int because all the collections can hold only Object handles. After seeing collections the wrapper classes might begin to make a little more sense to you, since you can’t put any of the primitive types in collections. However, the only thing you can do with the Java wrappers is to (1) initialize them to a particular value and (2) read that value. That is, there’s no way to change a value once a wrapper object has been created. This makes the Integer wrapper immediately useless to solve our problem, and so we’re forced to create a new class that does satisfy the need. 264 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 265. Creating “key” classes In the previous example, a standard library class (Integer) was used as a key for the Hashtable. It worked fine as a key, because it has all the necessary wiring to make it work correctly as a key. But a common pitfall occurs when using Hashtables when you create your own classes to be used as keys. For example, consider a weather predicting system that matches Groundhog objects to Prediction objects. It seems fairly straightforward: you create the two classes and use Groundhog as the key and Prediction as the value: //: SpringDetector.java // Looks plausible, but doesn't work right. import java.util.*; class Groundhog { int ghNumber; Groundhog(int n) { ghNumber = n; } } class Prediction { boolean shadow = Math.random() > 0.5; public String toString() { if(shadow) return "Six more weeks of Winter!"; else return "Early Spring!"; } } public class SpringDetector { public static void main(String args[]) { Hashtable ht = new Hashtable(); for(int i = 0; i < 10; i++) ht.put(new Groundhog(i), new Prediction()); System.out.println("ht = " + ht + "n"); System.out.println( "Looking up prediction for groundhog #3:"); Groundhog gh = new Groundhog(3); if(ht.containsKey(gh)) System.out.println((Prediction)ht.get(gh)); } } ///:~ Each Groundhog is given an identity number, so you can look up a Prediction in the Hashtable by saying “give me the Prediction associated with Groundhog number 3.” The Prediction class contains a boolean which is initialized using Math.random( ), and a toString( ) that interprets the result for you. In main( ), a Hashtable is filled with Groundhogs and their associated Predictions. The Hashtable is printed so you can see that it has in fact been filled. Then a Groundhog with an identity number of 3 is used to look up the prediction for Groundhog #3. It seems simple enough, but it doesn’t work. The problem is that Groundhog is inherited from the common root class Object (which is what happens if you don’t specify a base class, thus all classes are ultimately inherited from Object). It is Object’s hashCode( ) method that is used to generate the hash code for each object, and by default it just uses the address of its object. Thus the first instance of Groundhog(3) does not produce a hash Chapter 8: Holding Your Objects 265
- 266. code equal to the hash code for the second instance of Groundhog(3) that we tried to use as a lookup. So you might think that all you need to do is write an appropriate override for hashCode( ). But it still won’t work until you’ve done one more thing: overridden the equals( ) that is also part of Object. This method is used by the Hashtable when trying to determine if your key is equal to any of the keys in the table. Again, the default Object.equals( ) simply compares object addresses, so one Groundhog(3) is not equal to another Groundhog(3). Thus, to use your own classes as keys in a Hashtable, you must override both hashCode( ) and equals( ), as shown in the following solution to the above problem: //: SpringDetector2.java // If you create a class that's used as a key in // a Hashtable, you must override hashCode() // and equals(). import java.util.*; class Groundhog2 { int ghNumber; Groundhog2(int n) { ghNumber = n; } public int hashCode() { return ghNumber; } public boolean equals(Object o) { if ((o != null) && (o instanceof Groundhog2)) return ghNumber == ((Groundhog2)o).ghNumber; else return false; } } public class SpringDetector2 { public static void main(String args[]) { Hashtable ht = new Hashtable(); for(int i = 0; i < 10; i++) ht.put(new Groundhog2(i),new Prediction()); System.out.println("ht = " + ht + "n"); System.out.println( "Looking up prediction for groundhog #3:"); Groundhog2 gh = new Groundhog2(3); if(ht.containsKey(gh)) System.out.println((Prediction)ht.get(gh)); } } ///:~ Note that this uses the Prediction class from the previous example, so SpringDetector.java must be compiled first or you’ll get a compile-time error when you try to compile SpringDetector2.java. Groundhog2.hashCode( ) returns the ground hog number as an identifier (in this example, the programmer is responsible for ensuring that no two ground hogs exist with the same ID number). The hashCode( ) is not required to return a unique identifier, but the equals( ) method must be able to strictly determine whether two objects are equivalent. 266 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 267. The equals( ) method does two sanity checks: to see if the object is null, and if not, whether it is an instance of Groundhog2 (using the instanceof keyword, which is fully explained in Chapter 11). It should be a Groundhog2 to even continue executing equals( ). The comparison, as you can see, is based on the actual ghNumbers. This time, when you run the program, you’ll see it produces the correct output. Properties: a type of Hashtable In the very first example in the book, a type of Hashtable was used called Properties. In that example, the lines: Properties p = System.getProperties(); p.list(System.out); called the static method getProperties( ) to get a special Properties object that described the system characteristics. The method list( ) is a method of Properties that sends the contents to any stream output that you choose. In addition there’s a save( ) method to allow you to write your property list to a file in a way that it can be retrieved later with the load( ) method. Although the Properties class is inherited from Hashtable, it also contains a second Hashtable that acts to hold the list of “default” properties. So if a property isn’t found in the primary list, the defaults will be searched. The Properties class is also available for use in your programs. Enumerators revisited We can now demonstrate the true power of the Enumeration: the ability to separate the operation of traversing a sequence from the underlying structure of that sequence. In the following example, the class PrintData uses an Enumeration to move through a sequence and call the toString( ) method for every object. Two different types of collections are created, a Vector and a Hashtable, and they are each filled with, respectively, Mouse and Hamster objects (these classes were defined earlier in the chapter; note you must have compiled HamsterMaze.java and WorksAnyway.java for the following program to compile). Because an Enumeration hides the structure of the underlying collection, PrintData doesn’t know or care what kind of collection the Enumeration comes from: //: Enumerators2.java // Revisiting Enumerations import java.util.*; class PrintData { static void print(Enumeration e) { while(e.hasMoreElements()) System.out.println( e.nextElement().toString()); } } class Enumerators2 { public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 5; i++) v.addElement(new Mouse(i)); Hashtable h = new Hashtable(); for(int i = 0; i < 5; i++) Chapter 8: Holding Your Objects 267
- 268. h.put(new Integer(i), new Hamster(i)); System.out.println("Vector"); PrintData.print(v.elements()); System.out.println("Hashtable"); PrintData.print(h.elements()); } } ///:~ Notice that PrintData.print( ) takes advantage of the fact that the objects in these collections are of class Object so it can call toString( ). It’s more likely that in your problem, you’ll have to make the assumption that your Enumeration is walking through a collection of some specific type. For example, you might assume that everything in the collection is a Shape with a draw( ) method – then you’ll have to downcast from the Object that Enumeration.nextElement() returns to produce a Shape. Sorting One of the things that’s missing in the Java 1.0 and 1.1 libraries is algorithmic operations, even simple sorting. So it makes sense to create a Vector that sorts itself using the classic Quicksort. A problem with writing generic sorting code is that sorting must perform comparisons based on the actual type of the object. Of course, one approach is to write a different sorting method for every different type, but you should be able to recognize that this does not produce code that is easily re-used for new types. A primary goal of programming design is to “separate the things that change from things that stay the same,” and here, the code that stays the same is the general sort algorithm, but the thing that changes from one use to the next is the way objects are compared. So instead of hard-wiring the comparison code into many different sort routines, the technique of the callback will be used. With a callback, the part of the code that varies from case to case is encapsulated inside its own class, and the part of the code that’s always the same will call back to the code that changes. That way you can make different objects to express different ways of comparison and feed them to the same sorting code. The following interface describes how to compare two objects, and thus encapsulates “the things that change” for this particular problem: //: Compare.java // Interface for sorting callback: package c08; interface Compare { boolean lessThan(Object lhs, Object rhs); boolean lessThanOrEqual(Object lhs, Object rhs); } ///:~ For both methods, the lhs represents the “left hand” object and the rhs represents the “right hand” object in the comparison. Now a subclass of Vector can be created that implements the Quicksort using Compare. The algorithm, which is known for its speed, will not be explained here – for details, see Practical Algorithms for Programmers, by Binstock & Rex, Addison-Wesley 1995. //: SortVector.java 268 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 269. // A generic sorting vector package c08; import java.util.*; public class SortVector extends Vector { private Compare compare; // To hold the callback public SortVector(Compare comp) { compare = comp; } public void sort() { quickSort(0, size() - 1); } private void quickSort(int left, int right) { if(right > left) { Object o1 = elementAt(right); int i = left - 1; int j = right; while(true) { while(compare.lessThan( elementAt(++i), o1)) ; while(j > 0) if(compare.lessThanOrEqual( elementAt(--j), o1)) break; // out of while if(i >= j) break; swap(i, j); } swap(i , right); quickSort(left, i-1); quickSort(i+1, right); } } private void swap(int loc1, int loc2) { Object tmp = elementAt(loc1); setElementAt(elementAt(loc2), loc1); setElementAt(tmp, loc2); } } ///:~ You can now see the reason for the term “callback,” since the quickSort( ) method “calls back” to the methods in Compare. You can also see how this technique has produced generic, reusable code. To use the SortVector, you must create a class that implements Compare for the kind of objects that you’re sorting. This is a place where an inner class is not essential, but it can make sense for code organization. Here’s an example for String objects: //: StringSortTest.java // Testing the generic sorting Vector package c08; import java.util.*; public class StringSortTest { static class StringCompare implements Compare { public boolean lessThan(Object l, Object r) { return ((String)l).toLowerCase().compareTo( Chapter 8: Holding Your Objects 269
- 270. ((String)r).toLowerCase()) < 0; } public boolean lessThanOrEqual(Object l, Object r) { return ((String)l).toLowerCase().compareTo( ((String)r).toLowerCase()) <= 0; } } public static void main(String args[]) { SortVector sv = new SortVector(new StringCompare()); sv.addElement("d"); sv.addElement("A"); sv.addElement("C"); sv.addElement("c"); sv.addElement("b"); sv.addElement("B"); sv.addElement("D"); sv.addElement("a"); sv.sort(); Enumeration e = sv.elements(); while(e.hasMoreElements()) System.out.println(e.nextElement()); } } ///:~ The inner class is static because it is invoked inside a static method (main( )). You can see how, once the framework is set up, it’s very easy to reuse a design like this – you simply write the class that encapsulates “the things that change” and hand an object to the SortVector. The comparison forces the strings to lower case, so that the capital A’s end up next to the small a’s, and not in some entirely different place. This example shows, however, a slight deficiency in this approach, since the test code above puts the uppercase and lowercase single letters of the same letter in the order that they appear: A a b B c C d D. This is not usually much of a problem because you’re usually working with longer strings, and in that situation the effect doesn’t show up (the Java 1.2 collections provide sorting functionality that solves this problem). Inheritance (extends) is used here to create a new type of Vector – that is, StrSortVector is a Vector with some added functionality. The use of inheritance here is powerful but it presents problems. It turns out that some methods are final (described in Chapter 7) so you cannot override them. If you want to create a sorted Vector that accepts and produces only String objects you run into a wall, since addElement( ) and elementAt( ) are final, and these are precisely the methods you’d need to override so they accept and produce only String objects. No luck there. On the other hand, consider composition: the placing of an object inside a new class. Rather than rewrite the above code to accomplish this, we can simply use a SortVector inside the new class. In this case, the inner class will be created anonymously: //: StrSortVector.java // Automatically sorted Vector that // accepts and produces only Strings package c08; import java.util.*; 270 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 271. public class StrSortVector { private SortVector v = new SortVector( // Anonymous inner class: new Compare() { public boolean lessThan(Object l, Object r) { return ((String)l).toLowerCase().compareTo( ((String)r).toLowerCase()) < 0; } public boolean lessThanOrEqual(Object l, Object r) { return ((String)l).toLowerCase().compareTo( ((String)r).toLowerCase()) <= 0; } } ); private boolean sorted = false; public void addElement(String s) { v.addElement(s); sorted = false; } public String elementAt(int index) { if(!sorted) { v.sort(); sorted = true; } return (String)v.elementAt(index); } public Enumeration elements() { if(!sorted) { v.sort(); sorted = true; } return v.elements(); } // Test it: public static void main(String args[]) { StrSortVector sv = new StrSortVector(); sv.addElement("d"); sv.addElement("A"); sv.addElement("C"); sv.addElement("c"); sv.addElement("b"); sv.addElement("B"); sv.addElement("D"); sv.addElement("a"); Enumeration e = sv.elements(); while(e.hasMoreElements()) System.out.println(e.nextElement()); } } ///:~ This quickly reuses the code from SortVector to create the desired functionality. However, all the public methods from SortVector and Vector do not appear in StrSortVector – its Chapter 8: Holding Your Objects 271
- 272. methods are only the ones that are explicitly defined. So you can either make a definition for each one, or periodically go back and adjust it when you need new ones until the class design settles down. The advantage to this approach is that it will take only String objects and produce only String objects, and the checking happens at compile time instead of run time. Of course, that’s true only for addElement( ) and elementAt( ); elements( ) still produces an Enumeration which is untyped at compile time. Type checking for the Enumeration and in StrSortVector still happens, of course, it just happens at run-time, by throwing exceptions if you do something wrong. It’s a trade-off: do you find out about something for sure at compile time, or instead probably at run-time? (That is, “probably while you’re testing the code” and “probably not when the program user tries something you didn’t test for”). Given the choices and the hassle, it’s easier to use inheritance and just grit your teeth while casting – again, if parameterized types are ever add to Java they will solve this problem. You can see there’s a flag called sorted in this class. You could sort the vector every time addElement( ) is called, and constantly keep it in a sorted state. But usually people add a lot of elements to a Vector before beginning to read it. So sorting after every addElement( ) would be less efficient than waiting until someone wants to read the vector, and then sorting it, which is what is done here. The technique of delaying a process until it is absolutely necessary is called lazy evaluation. The generic collection library You’ve seen in this chapter that the standard Java library has some fairly useful collections, but far from a complete set. In addition, algorithms like sorting are not supported at all. One of the strengths of C++ is its libraries, in particular the Standard Template Library (STL) which provides a fairly full set of collections as well as many algorithms like sorting and searching that work with those collections. Based on this model, the ObjectSpace company was inspired to create the Generic Collection Library for Java (formerly called the Java Generic Library, but the abbreviation JGL is still used – the old name infringed on Sun’s copyright), which follows the design of the STL as much as possible (given the differences between the two languages) and seems to fulfill many if not all of the needs for a collection library, or as far as one could go in this direction without C++’s template mechanism. The JGL includes linked lists, sets, queues, maps, stacks, sequences, and iterators that are far more functional than Enumeration, along with a full set of algorithms like searching and sorting. ObjectSpace has also made, in some cases, more intelligent design decisions than the Sun library designers. For example, the methods in the JGL collections are not final so it’s easy to inherit and override those methods. The JGL has been included in some vendors’ Java distributions and ObjectSpace has made the JGL freely available for all uses, including commercial use, at http://www.ObjectSpace.com. The online documentation that comes in the JGL package is quite good and should be adequate to get you started. Summary To review the collections provided in the standard Java library: 1. An array associates numerical indices to objects. It holds objects of a known type, so you don’t have to cast the result when you’re looking up an object. It can be multi-dimensional, and it can hold primitives. However, its size cannot be changed once you create it. 272 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 273. 2. A Vector also associates numerical indices to objects – you can think of arrays and Vectors as random-access collections. The Vector automatically resizes itself as you add more elements. But a Vector can hold only Object handles, so it won’t hold primitives and you must always cast the result when you pull an Object handle out of a collection. 3. A Hashtable is a type of Dictionary, which is a way to associate, not numbers, but objects with other objects. A Hashtable also supports random access to objects, in fact, its whole design is focused around rapid access. 4. A Stack is a last-in, first-out (LIFO) queue. If you’re familiar with data structures, you might wonder why there’s not a larger set of collections. From a functionality standpoint, do you really need a larger set of collections? With a Hashtable you can put things in, find them quickly and with an Enumeration, iterate through the sequence and perform an operation on every element in the sequence. That’s a very powerful tool, and maybe it should be enough. But a Hashtable has no concept of order. Vectors and arrays give you a linear order, but it’s expensive to insert an element into the middle of either one. In addition, queues and dequeues and priority queues and trees are about ordering the elements, not just putting them in and later finding them or moving through them linearly. These data structures are also very useful, and that’s why they were included in Standard C++. For this reason, you should consider the collections in the standard Java library only as a starting point, and use the JGL when your needs go beyond that (if you’re using Java 1.2 the new collections are more likely to satisfy all your needs). Exercises 1. Create a new class called Gerbil with an int gerbilNumber that’s initialized in the constructor (similar to the Mouse example in this chapter). Give it a method called hop( ) that prints out which gerbil number this is and that it’s hopping. Create a Vector and add a bunch of Gerbil objects to the Vector. Now use the elementAt( ) method to move through the Vector and call hop( ) for each Gerbil. 2. Modify the exercise one so you use an Enumeration to move through the Vector while calling hop( ). 3. In AssocArray.java, change the example so it uses a Hashtable instead of an AssocArray. 4. Take the Gerbil class in exercise one and put it into a Hashtable instead, associating the name of the Gerbil as a String (the key) for each Gerbil (the value) you put in the table. Get an Enumeration for the keys( ) and use it to move through the Hashtable, looking up the Gerbil for each key and printing out the key and telling the gerbil to hop( ). 5. Change exercise one in Chapter 7 to use a Vector to hold the Rodents and an Enumeration to move through the sequence of Rodents. Remember that a Vector holds only Objects so you’ll have to use a cast (i.e.: RTTI) when accessing individual Rodents. 6. (Intermediate) In Chapter 7, locate the GreenhouseControls.java example, which consists of three files. In Controller.java, the class EventSet is just a collection. Chapter 8: Holding Your Objects 273
- 274. Change the code to use a Stack instead of an EventSet. This will require more than just replacing EventSet with Stack; you’ll also need to use an Enumeration to cycle through the set of events. You’ll probably find it easier if at times you treat the collection as a Stack and at other times as a Vector. 7. (Challenging). Find the source code for Vector in the Java source code library that comes with all Java distributions. Copy this code and make a special version called intVector that holds only ints. Consider what it would take to make a special version of Vector for all the primitive types. Now consider what happens if you want to make a linked list class that works with all the primitive types. If parameterized types are ever implemented in Java, they will provide a way to do this work for you, automatically (as well as many other benefits). 274 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 275. e 9: Error handling with exceptions The basic philosophy of Java is that “badly-formed code will not be run.” And, as with C++, the ideal time to catch the error is at compile time, before you even try to run the program. However, not all errors can be detected at compile time. The rest of the problems must be handled at run-time through some formality that allows the originator of the error to pass appropriate information to a recipient who will know how to properly handle the difficulty. In C and other early languages, there could be several of these formalities, and they were generally established by convention and not part of the programming language. Typically you returned a special value or set a flag, and the recipient was supposed to look at the value or the flag and determine that something was amiss. However, as the years passed it was discovered that programmers who use a library tend to think of themselves as invincible, as in “yes, errors might happen to others but not in my code.” So, not too surprisingly, they wouldn’t check for the error conditions (and sometimes the error conditions were too silly to check for1 ). If you were thorough enough to check for an error every time you called a method, your code could turn into an unreadable nightmare. Because programmers could still coax systems out of these languages they were resistant to admitting the truth: this approach to handling errors was a major limitation to creating large, robust, maintainable programs. 1 The C programmer can look up the return value of printf( ) for an example of this. 275
- 276. The solution is to take the casual nature out of error handling, and to enforce formality. This actually has a long history, since implementations of exception handling go back to operating systems in the 60’s and even to BASIC’s on error goto. But C++ exception handling was based on Ada, and Java’s is based primarily on C++ (although it looks even more like Object Pascal). The word “exception” is meant in the sense of “I take exception to that.” At the point where the problem occurs you might not know what to do with it, but you do know that you can’t just continue merrily on, that you must stop and somebody somewhere must figure out what to do. But you don’t have enough information in the current context to fix the problem. So you hand the problem out to a higher context where someone is qualified to make the proper decision (very much like a chain of command). The other rather significant benefit of exceptions is that they clean up error handling code. Instead of checking for a particular error and dealing with it at multiple places in your program, you no longer need to check at the point of the method call (since the exception will guarantee that someone catches it) and you need to handle the problem in only one place, the so-called exception handler. This saves you code, and it separates the code that describes what you want to do from the code that is executed when things go awry. In general, reading, writing and debugging code becomes much clearer with exceptions than when using the old way. Because exception handling is enforced by the Java compiler, there are only so many examples that can be written in this book without learning about exception handling. This chapter introduces you to the code you need to write to properly handle the exceptions, and the way you can generate your own exceptions if one of your methods gets into trouble. Basic exceptions An exceptional condition is a problem that prevents the continuation of the method or scope that you’re in. It’s important to distinguish an exceptional condition from a normal problem, where you have enough information in the current context to somehow cope with the difficulty. With an exceptional condition, you cannot continue processing because you don’t have the information necessary to deal with the problem in the current context. The only thing you can do is jump out of the current context and relegate that problem to a higher context. This is what happens when you throw an exception. A simple example is a divide. If you’re about to divide by zero, it’s worth checking to make sure you don’t go ahead and perform the divide. But what does it mean that the denominator is zero? Maybe you know, in the context of the problem you’re trying to solve in that particular method, how to deal with a zero denominator. But if it’s an unexpected value, you can’t deal with it and so must throw an exception rather than continuing along that path. When you throw an exception, several things happen. First, the exception object itself is created, in the same way that any Java object is created: on the heap, with new. Then the current path of execution (the one you couldn’t continue, remember) is stopped and the handle for the exception object is ejected from the current context. At this point the exception-handling mechanism takes over and begins to look for an appropriate place to continue executing the program. This appropriate place is the exception handler, whose job is to recover from the problem so the program may either try another tack or simply continue. 276 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 277. As a simple example of throwing an exception, consider an object handle called t. It’s possible you might be passed a handle that hasn’t been initialized, and so you might want to check before trying to call a method using that object handle. You can send information about the error into a larger context by creating an object representing your information and “throwing” it out of your current context. This is called throwing an exception. Here’s what it looks like: if(t == null) throw new NullPointerException(); This throws the exception, which allows you – in the current context – to abdicate responsibility for thinking about the issue further. It’s just magically handled somewhere else. Precisely where will be shown shortly. Exception arguments Like any object in Java, you always create exceptions on the heap using new, and a constructor gets called. There are two constructors in all the standard exceptions; the first is the default constructor, and the second takes a string argument so you can place pertinent information in the exception: if(t == null) throw new NullPointerException("t = null"); This string can later be extracted using various methods, as will be shown later. The keyword throw causes a number of relatively magical things to happen. First it executes the new-expression to create an object that isn’t there under normal program execution, and of course the constructor is called for that object. Then the object is, in effect, “returned” from the method, even though that object type isn’t normally what the method is designed to return. A simplistic way to think about exception handling is as an alternate return mechanism, although you get into trouble if you take the analogy too far. You can also exit from ordinary scopes by throwing an exception. But a value is returned, and the method or scope exits. Any similarity to an ordinary return from a method ends here, because where you return to is someplace completely different than for a normal method call. (You end up in an appropriate exception handler that might be miles away from where the exception was thrown.) In addition, you can throw as many different types of objects as you want. Typically, you’ll throw a different class of exception for each different type of error. The idea is to store the information in the exception object and the type of exception object, so someone in the bigger context can figure out what to do with your exception. Catching an exception If a method throws an exception, it must assume that exception is caught and dealt with. One of the advantages of Java exception handling is that it allows you to concentrate on the problem you’re actually trying to solve in one place, and then deal with the errors from that code in another place. To see how an exception is caught, you must first understand the concept of a guarded region, which is a section of code that may produce exceptions, and which is followed by the code to handle those exceptions. Chapter 9: Error Handling with Exceptions 277
- 278. The try block If you’re inside a method and you throw an exception (or another method you call within this method throws an exception), that method will exit in the process of throwing. If you don’t want a throw to leave a method, you can set up a special block within that method to capture the exception. This is called the try block because you “try” your various method calls there. The try block is an ordinary scope, preceded by the keyword try: try { // code that may generate exceptions } If you were carefully checking for errors in a programming language that didn’t support exception handling, you’d have to surround every method call with setup and error testing code, even if you call the same method several times. With exception handling, you put everything in a try block and capture all the exceptions in one place. This means your code is a lot easier to write and easier to read because the goal of the code is not confused with the error checking. Exception handlers Of course, the thrown exception must end up someplace. This “place” is the exception handler, and there’s one for every exception type you want to catch. Exception handlers immediately follow the try block and are denoted by the keyword catch: try { // code that may generate exceptions } catch(Type1 id1) { // handle exceptions of Type1 } catch(Type2 id2) { // handle exceptions of Type2 } catch(Type3 id3) { // handle exceptions of Type3 } // etc... Each catch clause (exception handler) is like a little method that takes one and only one argument of a particular type. The identifier (id1, id2, and so on) may be used inside the handler, just like a method argument. Sometimes you never use the identifier because the type of the exception gives you enough information to deal with the exception, but the identifier must still be there. The handlers must appear directly after the try block. If an exception is thrown, the exception-handling mechanism goes hunting for the first handler with an argument that matches the type of the exception. Then it enters that catch clause, and the exception is considered handled. (The search for handlers stops once the catch clause is finished.) Only the matching catch clause executes; it’s not like a switch statement where you need a break after each case to prevent the remaining ones from executing. Notice that, within the try block, a number of different method calls might generate the same exception, but you need only one handler. Termination vs. resumption There are two basic models in exception-handling theory. In termination (which is what Java and C++ support) you assume the error is so critical there’s no way to get back to 278 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 279. where the exception occurred. Whoever threw the exception decided there was no way to salvage the situation, and they don’t want to come back. The alternative is called resumption. It means the exception handler is expected to do something to rectify the situation, and then the faulting method is retried, presuming success the second time. If you want resumption, it means you still hope to continue execution after the exception is handled. In this case, your exception is more like a method call – which is how you should set up situations in Java where you want resumption-like behavior (that is, don’t throw an exception; call a method that fixes the problem). Alternatively, place your try block inside a while loop that keeps reentering the try block until the result is satisfactory. Historically, programmers using operating systems that supported resumptive exception handling eventually ended up using termination-like code and skipping resumption. So although resumption sounds attractive at first, it seems it isn’t quite so useful in practice. The dominant reason is probably the coupling that results: your handler must often be aware of where the exception is thrown from and contain non-generic code specific to the throwing location. This makes the code difficult to write and maintain, especially for large systems where the exception can be generated from many points. The exception specification In Java, you’re required to inform the person calling your method of the exceptions that might be thrown out of that method. This is very civilized because it means the caller can know exactly what code to write to catch all potential exceptions. Of course, if source code is available, the client programmer could hunt through and look for throw statements, but very often a library doesn’t come with sources. To prevent this from being a problem, Java provides syntax (and forces you to use that syntax) to allow you to politely tell the client programmer what exceptions this method throws, so the client programmer may handle them. This is the exception specification and it’s part of the method declaration, appearing after the argument list. The exception specification uses an additional keyword, throws, followed by a list of all the potential exception types. So your method definition might look like this: void f() throws tooBig, tooSmall, divZero { //... If you say void f() { // ... it means that no exceptions are thrown from the method (except for the exceptions of type RuntimeException, which can reasonably be thrown anywhere – this is described later). You can’t lie about an exception specification – if your method causes exceptions and doesn’t handle them, the compiler will detect this and tell you that you must either handle the exception or indicate with an exception specification that it may be thrown from your method. By enforcing exception specifications from top to bottom, Java guarantees that exception-correctness can be ensured at compile time2 . There is one place you can lie: you can claim to throw an exception that you don’t. The compiler takes your word for it, and forces the users of your method to treat it as if it really does throw that exception. This has the beneficial effect of being a placeholder for 2 This is a significant improvement over C++ exception handling, which doesn’t catch violations of exception specifications until run time, when it’s not very useful. Chapter 9: Error Handling with Exceptions 279
- 280. that exception, so you can actually start throwing the exception later without requiring changes to existing code. Catching any exception It is possible to create a handler that catches any type of exception. You do this by catching the base-class exception type Exception (there are other types of base exceptions, but Exception is the base that’s pertinent to virtually all programming activities): catch(Exception e) { System.out.println("caught an exception"); } This will catch any exception, so if you use it you’ll want to put it at the end of your list of handlers to avoid pre-empting any exception handlers that follow it. Since the Exception class is the base of all the exception classes that are important to the programmer, you don’t get much specific information about the exception, but you can call the methods that come from its base type Throwable: String getMessage( ) Gets the detail message . String toString( ) Returns a short description of the Throwable, including the detail message if there is one. void printStackTrace( ) void printStackTrace(PrintStream) Prints the Throwable and the Throwable’s call stack trace. The call stack shows the sequence of method calls that brought you to the point where the exception was thrown. The first version prints to standard error, the second prints to a stream of your choice. If you’re working under Windows, you can’t redirect standard error so you might want to use the second version and send the results to System.out; that way the output can be redirected any way you want. In addition, you get some other methods from Throwable’s base type Object (everybody’s base type). The one that might come in handy for exceptions is getClass( ), which returns an object representing the class of this object. You can in turn query this Class object for its name with getName( ) or toString( ). You can also do more sophisticated things with Class objects that aren’t necessary in exception handling. Class objects will be studied later in the book. Here’s an example that shows the use of the Exception methods (see page 80 if you have trouble executing this program): //: ExceptionMethods.java // Demonstrating the Exception Methods package c09; public class ExceptionMethods { public static void main(String args[]) { try { throw new Exception("Here's my Exception"); } catch(Exception e) { System.out.println("Caught Exception"); System.out.println( 280 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 281. "e.getMessage(): " + e.getMessage()); System.out.println( "e.toString(): " + e.toString()); System.out.println("e.printStackTrace():"); e.printStackTrace(); } } } ///:~ The output for this program is: Caught Exception e.getMessage(): Here's my Exception e.toString(): java.lang.Exception: Here's my Exception e.printStackTrace(): java.lang.Exception: Here's my Exception at ExceptionMethods.main You can see that the methods provide successively more information – each is effectively a superset of the previous one. Rethrowing an exception Sometimes you’ll want to rethrow the exception that you just caught, particularly when you use Exception to catch any exception. Since you already have the handle to the current exception, you can simply re-throw that handle: catch(Exception e) { System.out.println("An exception was thrown"); throw e; } Any further catch clauses for the same try block are still ignored – the throw causes the exception to go to the exception handlers in the next-higher context. In addition, everything about the exception object is preserved, so the handler at the higher context that catches the specific exception type can extract all the information from that object. If you just re-throw the current exception, the information that you print about that exception in printStackTrace( ) will pertain to the exception’s origin, not the place where you re-throw it. If you want to install new stack trace information, you can do so by calling fillInStackTrace( ), which returns an exception object that it creates by stuffing the current stack information into the old exception object. Here’s what it looks like: //: Rethrowing.java // Demonstrating fillInStackTrace() public class Rethrowing { public static void f() throws Exception { System.out.println( "originating the exception in f()"); throw new Exception("thrown from f()"); } public static void g() throws Throwable { try { f(); } catch(Exception e) { System.out.println( Chapter 9: Error Handling with Exceptions 281
- 282. "Inside g(), e.printStackTrace()"); e.printStackTrace(); throw e; // 17 // throw e.fillInStackTrace(); // 18 } } public static void main(String args[]) throws Throwable { try { g(); } catch(Exception e) { System.out.println( "Caught in main, e.printStackTrace()"); e.printStackTrace(); } } } ///:~ The important line numbers are marked inside of comments. With line 17 uncommented (as shown), the output is: originating the exception in f() Inside g(), e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:8) at Rethrowing.g(Rethrowing.java:12) at Rethrowing.main(Rethrowing.java:24) Caught in main, e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:8) at Rethrowing.g(Rethrowing.java:12) at Rethrowing.main(Rethrowing.java:24) So the exception stack trace always remembers its true point of origin, no matter how many times it gets rethrown. With line 17 commented and line 18 uncommented, fillInStackTrace( ) is used instead, and the result is: originating the exception in f() Inside g(), e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:8) at Rethrowing.g(Rethrowing.java:12) at Rethrowing.main(Rethrowing.java:24) Caught in main, e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.g(Rethrowing.java:18) at Rethrowing.main(Rethrowing.java:24) Because of fillInStackTrace( ), line 18 becomes the new point of origin of the exception. The class Throwable must appear in the exception specification for g( ) and main( ) because fillInStackTrace( ) produces a handle to a Throwable object. Since Throwable is a base class of Exception, it’s possible to get an object that’s a Throwable but not an Exception, so the handler for Exception in main( ) might miss it. To make sure 282 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 283. everything is in order, the compiler forces an exception specification for Throwable. For example, the exception in the following program is not caught in main( ): //: ThrowOut.java public class ThrowOut { public static void main(String args[]) throws Throwable { try { throw new Throwable(); } catch(Exception e) { System.out.println("Caught in main()"); } } } ///:~ It’s also possible to rethrow a different exception than the one you caught. If you do this, you get a similar effect as when using fillInStackTrace( ): the information about the original site of the exception is lost, and what you’re left with is the information pertaining to the new throw: //: RethrowNew.java // Rethrow a different object than you catch public class RethrowNew { public static void f() throws Exception { System.out.println( "originating the exception in f()"); throw new Exception("thrown from f()"); } public static void main(String args[]) { try { f(); } catch(Exception e) { System.out.println( "Caught in main, e.printStackTrace()"); e.printStackTrace(); throw new NullPointerException("from main"); } } } ///:~ The output is: originating the exception in f() Caught in main, e.printStackTrace() java.lang.Exception: thrown from f() at RethrowNew.f(RethrowNew.java:8) at RethrowNew.main(RethrowNew.java:13) java.lang.NullPointerException: from main at RethrowNew.main(RethrowNew.java:18) The final exception knows only that it came from main( ), and not from f( ). Notice that Throwable isn’t necessary in any of the exception specifications. You never have to worry about cleaning up the previous exception, or any exceptions for that matter: they’re all heap-based objects created with new, so the garbage collector automatically cleans them all up. Chapter 9: Error Handling with Exceptions 283
- 284. Standard java exceptions Java contains a class called Throwable that describes anything that can be thrown as an exception. There are two general types of Throwable objects (“types of” = “inherited from”): Error represents compile-time and system errors that you don’t worry about catching (except in very special cases). Exception is the basic type that can be thrown from any of the standard Java library class methods and from your methods and run-time accidents. The best way to get an overview of the exceptions is to browse on-line Java documentation from http://java.sun.com (of course, it’s easier to download it first). It’s worth doing this once just to get a feel for the various exceptions, but you’ll soon see that there isn’t anything special between one exception and the next except for the name. Also, the number of exceptions in Java keeps expanding; basically it’s pointless to print them in a book. Any new library you get from a third-party vendor will probably have its own exceptions, as well. The important thing to understand is the concept and what you should do with the exceptions. java.lang.Exception This is the basic exception class your program can catch. Other exceptions are derived from this. The basic idea is that the name of the exception represents the problem that occurred and the exception name is intended to be relatively self-explanatory. The exceptions are not all defined in java.lang; some are created to support other libraries like util, net and io, which you can see from their full class names or what they are inherited from; for example, all IO exceptions are inherited from java.io.IOException. The special case of RuntimeException The first example in this chapter was if(t == null) throw new NullPointerException(); It can be a bit horrifying to think that you must check for null on every handle that is passed into a method (since you can’t know if the caller has passed you a valid handle). Fortunately, you don’t – this is part of the standard run-time checking that Java performs for you, and if any call is made to a null handle, Java will automatically throw a NullPointerException. So the above bit of code is always superfluous. There’s a whole group of exception types that are in this category: they’re always thrown automatically by Java and you don’t need to include them in your exception specifications. Conveniently enough, they’re all grouped together by putting them under a single base class called RuntimeException, which is a perfect example of inheritance: it establishes a family of types that have some characteristics and behaviors in common. In addition, you never need to write an exception specification saying that a method may throw a RuntimeException, since that’s just assumed. Because they indicate bugs, you virtually never catch a RuntimeException – it’s dealt with automatically. If you were forced to check for RuntimeExceptions your code could get pretty messy. Even though you don’t typically catch RuntimeExceptions, in your own packages you may choose to throw some of the RuntimeExceptions. What happens when you don’t catch such exceptions? Since the compiler doesn’t enforce exception specifications for these, it’s quite plausible that a RuntimeException could 284 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 285. percolate all the way out to your main( ) method without being caught. To see what happens in this case, try the following example: //: NeverCaught.java // Ignoring RuntimeExceptions public class NeverCaught { static void f() { throw new RuntimeException("From f()"); } static void g() { f(); } public static void main(String args[]) { g(); } } ///:~ You can already see that a RuntimeException (or anything inherited from it) is a special case, since the compiler doesn’t require an exception specification for these types. The output is: java.lang.RuntimeException: From f() at NeverCaught.f(NeverCaught.java:9) at NeverCaught.g(NeverCaught.java:12) at NeverCaught.main(NeverCaught.java:15) So the answer is: if a RuntimeException gets all the way out to main( ) without being caught, printStackTrace( ) is called for that exception as the program exits. Keep in mind that it’s possible to ignore only RuntimeExceptions in your coding, since all other handling is carefully enforced by the compiler. The reasoning is that a RuntimeException represents a programming error: 1. An error you cannot catch (receiving a null handle handed to your method by a client programmer, for example) 2. An error that you, as a programmer, should have checked for in your code (such as ArrayIndexOutOfBoundsException where you should have paid attention to the size of the array). You can see what a tremendous benefit it is to have exceptions in this case, since they assist in the debugging process. It’s interesting to note that you cannot classify Java exception handling as a single- purpose tool. Yes, it is designed to handle those pesky run-time errors that will occur because of forces outside the control of your code, but it’s also essential for certain types of programming bugs that the compiler cannot detect. Creating your own exceptions You’re not stuck using the Java exceptions. This is important because you’ll often need to create your own exceptions to denote a special error that your library is capable of creating, but which was not foreseen when the Java hierarchy was created. Chapter 9: Error Handling with Exceptions 285
- 286. To create your own exception class, you’re forced to inherit from an existing type of exception, preferably one that is very close in meaning to your new exception. Inheriting an exception is quite simple: //: Inheriting.java // Inheriting your own exceptions class MyException extends Exception { public MyException() {} public MyException(String msg) { super(msg); } } public class Inheriting { public static void f() throws MyException { System.out.println( "Throwing MyException from f()"); throw new MyException(); } public static void g() throws MyException { System.out.println( "Throwing MyException from g()"); throw new MyException("Originated in g()"); } public static void main(String args[]) { try { f(); } catch(MyException e) { e.printStackTrace(); } try { g(); } catch(MyException e) { e.printStackTrace(); } } } ///:~ The inheritance occurs in the creation of the new class: class MyException extends Exception { public MyException() {} public MyException(String msg) { super(msg); } } The key phrase here is extends Exception, which says “it’s everything an Exception is, and more.” The added code is very small – the addition of two constructors that define the way MyException is created. Remember that the compiler automatically calls the base- class default constructor if you don't explicitly call a base-class constructor, as in the MyException( ) default constructor. In the second constructor, the base-class constructor with a String argument is invoked by using the super keyword. The output of the program is: 286 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 287. Throwing MyException from f() MyException at Inheriting.f(Inheriting.java:16) at Inheriting.main(Inheriting.java:24) Throwing MyException from g() MyException: Originated in g() at Inheriting.g(Inheriting.java:20) at Inheriting.main(Inheriting.java:29) You can see the absence of the detail message in the MyException thrown from f( ). The process of creating your own exceptions can be taken further: you can add extra constructors and members: //: Inheriting2.java // Inheriting your own exceptions class MyException2 extends Exception { public MyException2() {} public MyException2(String msg) { super(msg); } public MyException2(String msg, int x) { super(msg); i = x; } public int val() { return i; } private int i; } public class Inheriting2 { public static void f() throws MyException2 { System.out.println( "Throwing MyException2 from f()"); throw new MyException2(); } public static void g() throws MyException2 { System.out.println( "Throwing MyException2 from g()"); throw new MyException2("Originated in g()"); } public static void h() throws MyException2 { System.out.println( "Throwing MyException2 from h()"); throw new MyException2("Originated in h()", 47); } public static void main(String args[]) { try { f(); } catch(MyException2 e) { e.printStackTrace(); } try { g(); } catch(MyException2 e) { e.printStackTrace(); } Chapter 9: Error Handling with Exceptions 287
- 288. try { h(); } catch(MyException2 e) { e.printStackTrace(); System.out.println("e.val() = " + e.val()); } } } ///:~ A data member i has been added, along with a method that reads that value, and an additional constructor that sets it. The output is: Throwing MyException2 from f() MyException2 at Inheriting2.f(Inheriting2.java:22) at Inheriting2.main(Inheriting2.java:34) Throwing MyException2 from g() MyException2: Originated in g() at Inheriting2.g(Inheriting2.java:26) at Inheriting2.main(Inheriting2.java:39) Throwing MyException2 from h() MyException2: Originated in h() at Inheriting2.h(Inheriting2.java:30) at Inheriting2.main(Inheriting2.java:44) e.val() = 47 Since an exception is just another kind of object, you can continue this process of embellishing the power of your exception classes. Keep in mind, however, that all this dressing-up might be lost on the client programmers using your packages, since they might simply look for the exception to be thrown and nothing more (that’s the way most of the Java library exceptions are used). If this is the case, it’s possible to create a new exception type with almost no code at all: //: SimpleException.java class SimpleException extends Exception { } ///:~ This relies on the compiler to create of the default constructor (which automatically calls the base-class default constructor). Of course, in this case you don’t get a SimpleException(String) constructor, but in practice that isn’t used very much. Exception restrictions When you override a method, you can throw only the exceptions that have been specified in the base-class version of the method. This is a very useful restriction, since it means that code that works with the base class will automatically work with any object derived from the base class (a fundamental OOP concept, of course), including exceptions. This example demonstrates the kinds of restrictions imposed (at compile time) for exceptions: //: StormyInning.java // Overridden methods may throw only the // exceptions specified in their base-class // versions, or exceptions derived from the // base-class exceptions. 288 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 289. class BaseballException extends Exception {} class Foul extends BaseballException {} class Strike extends BaseballException {} abstract class Inning { Inning() throws BaseballException {} void event () throws BaseballException { // Doesn't actually have to throw anything } abstract void atBat() throws Strike, Foul; void walk() {} // Throws nothing } class StormException extends Exception {} class RainedOut extends StormException {} class PopFoul extends Foul {} interface Storm { void event() throws RainedOut; void rainHard() throws RainedOut; } public class StormyInning extends Inning implements Storm { // OK to add new exceptions for constructors, // but you must deal with the base constructor // exceptions: StormyInning() throws RainedOut, BaseballException {} StormyInning(String s) throws Foul, BaseballException {} // Regular methods must conform to base class: //! void walk() throws PopFoul {} //Compile error // Interface CANNOT add exceptions to existing // methods from the base class: //! public void event() throws RainedOut {} // If the method doesn't already exist in the // base class, the exception is OK: public void rainHard() throws RainedOut {} // You can choose not to throw any exceptions, // even if base version does: public void event() {} // Overridden methods can throw // inherited exceptions: void atBat() throws PopFoul {} public static void main(String args[]) { try { StormyInning si = new StormyInning(); si.atBat(); } catch(PopFoul e) { } catch(RainedOut e) { } catch(BaseballException e) {} // Strike not thrown in derived version. try { // What happens if you upcast? Chapter 9: Error Handling with Exceptions 289
- 290. Inning i = new StormyInning(); i.atBat(); // You must catch the exceptions from the // base-class version of the method: } catch(Strike e) { } catch(Foul e) { } catch(RainedOut e) { } catch(BaseballException e) {} } } ///:~ In Inning, you can see that both the constructor and the event( ) method say they will throw an exception, but they never actually do. This is legal because it allows you to force the user to catch any exceptions that you may add in overridden versions of event( ). The same idea holds for abstract methods, as seen in atBat( ). The interface Storm is interesting because it contains one method (event( ))that is defined in Inning, and one method that isn’t. Both methods throw a new type of exception, RainedOut. When StormyInning extends Inning and implements Storm, you’ll see that the event( ) method in Storm cannot change the exception interface of event( ) in Inning. Again, this makes sense because otherwise you’d never know if you were catching the right thing when working with the base class. Of course, if a method described in an interface is not in the base class, like rainHard( ), then there’s no problem if it throws exceptions. The restriction on exceptions does not apply to constructors. You can see in StormyInning that a constructor can throw anything it wants, regardless of what the base-class constructor throws. However, since a base-class constructor must always be called one way or another (here, the default constructor is called automatically), the derived-class constructor must declare any base-class constructor exceptions in its exception specification. The reason StormyInning.walk( ) will not compile is that it throws an exception, while Inning.walk( ) does not. If this was allowed, then you could write code that called Inning.walk( ) and that didn’t have to handle any exceptions, but then when you substituted an object of a class derived from Inning, exceptions would be thrown so your code would break. By forcing the derived-class methods to conform to the exception specifications of the base-class methods, substitutability of objects is maintained. The overridden event( ) method shows that a derived-class version of a method may choose not to throw any exceptions, even if the base class version does. Again, this is fine since it doesn’t break any code that is written assuming the base-class version throws exceptions. Similar logic applies to atBat( ), which throws PopFoul, an exception that is derived from Foul thrown by the base-class version of atBat( ). This way, if someone writes code that works with Inning and calls atBat( ), they must catch the Foul exception. Since PopFoul is derived from Foul, the exception handler will also catch PopFoul. The last point of interest is in main( ). Here you can see that if you’re dealing with exactly a StormyInning object, the compiler forces you to catch only the exceptions that are specific to that class, but if you upcast to the base type then the compiler (correctly) 290 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 291. forces you to catch the exceptions for the base type. All these constraints produce much more robust exception-handling code3 . It’s useful to realize that, although exception specifications are enforced by the compiler during inheritance, the exception specifications are not part of the type of a method, which is comprised of only the method name and argument types. Therefore, you cannot overload methods based on exception specifications. In addition, because an exception specification exists in a base-class version of a method doesn’t mean it must exist in the derived-class version of the method, and this is quite different from inheriting the methods themselves (that is, a method in the base class must also exist in the derived class). Put another way, the “exception specification interface” for a particular method may narrow during inheritance and overriding, but it may not widen – this is precisely the opposite of the rule for the class interface during inheritance. Performing cleanup with finally There’s often some piece of code that you may want executed whether or not an exception occurs in a try block. This usually pertains to some operation other than memory recovery (since that’s taken care of by the garbage collector). To achieve this effect, you use a finally clause4 at the end of all the exception handlers. The full picture of an exception- handling section is thus: try { // The guarded region: // Dangerous stuff that may throw A, B, or C } catch (A a1) { // Handle A } catch (B b1) { // Handle B } catch (C c1) { // Handle C } finally { // Stuff that happens every time } To demonstrate to yourself that the finally clause always runs, try this program: //: FinallyWorks.java // The finally clause is always executed public class FinallyWorks { static int count = 0; public static void main(String args[]) { while(true) { 3 ANSI/ISO C++ added similar constraints that require derived-method exceptions to be the same as, or derived from, the exceptions thrown by the base-class method. This is one case where C++ is actually able to check exception specifications at compile time. 4 C++ exception handling does not have the finally clause because it relies on destructors to accomplish this sort of cleanup. Chapter 9: Error Handling with Exceptions 291
- 292. try { // post-increment is zero first time: if(count++ == 0) throw new Exception(); System.out.println("No exception"); } catch(Exception e) { System.out.println("Exception thrown"); } finally { System.out.println("in finally clause"); if(count == 2) break; // out of "while" } } } } ///:~ This program also gives a hint for how you can deal with the fact that exceptions in Java (like exceptions in C++) do not allow you to resume back to where the exception was thrown, as discussed earlier. If you place your try block in a loop, you can establish a condition that must be met before continuing the program. You can also add a static counter or some other device to allow the loop to try several different approaches before giving up. This way you can build a greater level of robustness into your programs. The output is: Exception thrown in finally clause No exception in finally clause Whether an exception is thrown or not, the finally clause is always executed. What’s finally for? In a language without garbage collection and without automatic destructor calls,5 finally is important because it allows the programmer to guarantee the release of memory regardless of what happens in the try block. But Java has garbage collection, so releasing memory is virtually never a problem. Also, it has no destructors to call. When do you need to use finally in Java, then? finally is necessary when you need to set something other than memory back to its original state. This is usually something like an open file or network connection, something you’ve drawn on the screen or even a switch in the outside world, as modeled in the following example: //: OnOffSwitch.java // Why use finally? class Switch { boolean state = false; boolean read() { return state; } 5 A destructor is a function that’s always called when an object becomes unused. You always know exactly where and when the destructor gets called. C++ has automatic destructor calls, but Delphi’s Object Pascal versions 1 & 2 do not (which changes the meaning and use of the concept of a destructor for that language). 292 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 293. void on() { state = true; } void off() { state = false; } } public class OnOffSwitch { static Switch sw = new Switch(); public static void main(String args[]) { try { sw.on(); // code that may throw exceptions... sw.off(); } catch(NullPointerException e) { System.out.println("NullPointerException"); sw.off(); } catch(IllegalArgumentException e) { System.out.println("IOException"); sw.off(); } } } ///:~ The goal here is to make sure that the switch is off when main( ) is completed, so sw.off( ) is placed at the end of the try block and the end of each exception handler. But it’s possible that an exception could be thrown that isn’t caught here, and so sw.off( ) would be missed. However, with finally you can place the closure code from a try block in just one place: //: WithFinally.java // Finally Guarantees cleanup class Switch2 { boolean state = false; boolean read() { return state; } void on() { state = true; } void off() { state = false; } } public class WithFinally { static Switch2 sw = new Switch2(); public static void main(String args[]) { try { sw.on(); // code that may throw exceptions... } catch(NullPointerException e) { System.out.println("NullPointerException"); } catch(IllegalArgumentException e) { System.out.println("IOException"); } finally { sw.off(); } } } ///:~ Here the sw.off( ) has been moved to just one place, where it’s guaranteed to run no matter what happens. Chapter 9: Error Handling with Exceptions 293
- 294. Even in cases where the exception is not caught in the current set of catch clauses, finally will be executed before the exception-handling mechanism continues its search for a handler at the next higher level: //: AlwaysFinally.java // Finally is always executed class Ex extends Exception {} public class AlwaysFinally { public static void main(String args[]) { System.out.println( "Entering first try block"); try { System.out.println( "Entering second try block"); try { throw new Ex(); } finally { System.out.println( "finally in 2nd try block"); } } catch(Ex e) { System.out.println( "Caught Ex in first try block"); } finally { System.out.println( "finally in 1st try block"); } } } ///:~ The output for this program shows you what happens: Entering first try block Entering second try block finally in 2nd try block Caught Ex in first try block finally in 1st try block The finally statement will also be executed in situations where break and continue statements are involved. Note that, along with the labeled break and labeled continue, finally eliminates the need for a goto statement in Java. Pitfall: the lost exception In general Java’s exception implementation is quite outstanding, but unfortunately there’s a flaw. Although exceptions are an indication of a crisis in your program and should never be ignored, it’s possible for an exception to simply be lost. This happens with a particular configuration using a finally clause: //: LostMessage.java // How an exception can be lost class VeryImportantException extends Exception { public String toString() { 294 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 295. return "A very important exception!"; } } class HoHumException extends Exception { public String toString() { return "A trivial exception"; } } public class LostMessage { void f() throws VeryImportantException { throw new VeryImportantException(); } void dispose() throws HoHumException { throw new HoHumException(); } public static void main(String args[]) throws Exception { LostMessage lm = new LostMessage(); try { lm.f(); } finally { lm.dispose(); } } } ///:~ The output is: A trivial exception at LostMessage.dispose(LostMessage.java:21) at LostMessage.main(LostMessage.java:29) You can see that there’s no evidence of the VeryImportantException, which is simply replaced by the HoHumException in the finally clause. This is a rather serious pitfall, since it means an exception can be completely lost, and in a far more subtle and difficult- to-detect fashion than the example above. In contrast, C++ treats the situation where a second exception is thrown before the first one is handled as a dire programming error. Perhaps a future version of Java will repair the problem (the above results were produced with Java 1.1). Constructors When writing code with exceptions, it’s particularly important that you always ask: “If an exception occurs, will this be properly cleaned up?” Most of the time you’re fairly safe, but in constructors there’s a problem. The constructor puts the object into a safe starting state, but it may perform some operation – such as opening a file – that doesn’t get cleaned up until the user is finished with the object and calls a special cleanup method. If you throw an exception from inside a constructor, these cleanup behaviors may not occur properly. This means you must be especially diligent while writing your constructor. Since you’ve just learned about finally, you may think that’s the correct solution. But it’s not quite that simple because finally performs the cleanup code every time, even in the situations where we don’t want the cleanup code executed until the cleanup method runs. Thus, if you do perform cleanup in finally, you must set some kind of flag when the Chapter 9: Error Handling with Exceptions 295
- 296. constructor finishes normally and don't do anything in the finally block if the flag is set. Because this isn’t particularly elegant (you are coupling your code from one place to another), it’s best if you try to avoid performing this kind of cleanup in finally unless you are forced to. In the following example, a class called InputFile is created that opens a file and allows you to read it one line (converted into a String) at a time. It uses the classes FileReader and BufferedReader from the Java standard IO library which will be discussed in Chapter 10, but which are simple enough that you probably won’t have any trouble understanding their basic use: //: Cleanup.java // Paying attention to exceptions // in constructors import java.io.*; class InputFile { private BufferedReader in; InputFile(String fname) throws Exception { try { in = new BufferedReader( new FileReader(fname)); // other code that may throw exceptions } catch(FileNotFoundException e) { System.out.println( "Could not open " + fname); // Wasn't open, so don't close it throw e; } catch(Exception e) { // All other exceptions must close it try { in.close(); } catch(IOException e2) { System.out.println( "in.close() unsuccessful"); } throw e; } finally { // Don't close it here!!! } } String getLine() { String s; try { s = in.readLine(); } catch(IOException e) { System.out.println( "readLine() unsuccessful"); s = "failed"; } return s; } void cleanup() { try { in.close(); 296 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 297. } catch(IOException e2) { System.out.println( "in.close() unsuccessful"); } } } public class Cleanup { public static void main(String args[]) { try { InputFile in = new InputFile("Cleanup.java"); String s; int i = 1; while((s = in.getLine()) != null) System.out.println(""+ i++ + ": " + s); in.cleanup(); } catch(Exception e) { System.out.println( "Caught in main, e.printStackTrace()"); e.printStackTrace(); } } } ///:~ This example uses Java 1.1 IO classes. The constructor for InputFile takes a String argument, which is the name of the file you want to open. Inside a try block, it creates a FileReader using the file name. A FileReader isn’t particularly useful until you turn around and use it to create a BufferedReader that you can actually talk to – notice that one of the benefits of InputFile is that it combines these two actions. If the FileReader constructor is unsuccessful it throws a FileNotFoundException, which must be caught separately because that’s the one case where you don’t want to close the file since it wasn’t successfully opened. Any other catch clauses must close the file because it was opened by the time those catch clauses are entered (of course, this is trickier if more than one method can throw a FileNotFoundException. In that case, you may want to break things up into several try blocks). The close( ) method itself throws an exception which is tried and caught even though it’s within the block of another catch clause – it’s just another pair of curly braces to the Java compiler. After performing local operations, the exception is re-thrown, which is appropriate because this constructor failed and you wouldn’t want the calling method to assume that the object had been properly created and was valid. In this example, which doesn’t use the aforementioned flagging technique, the finally clause is definitely not the place to close( ) the file, since that would close it every time the constructor completed. Since we want the file to be open for the useful lifetime of the InputFile object this would not be appropriate. The getLine( ) method returns a String containing the next line in the file. The readLine( ) method that it calls can throw an exception which is caught and dealt with so that getLine( ) doesn’t throw any exceptions. One of the design issues with exceptions is whether to handle an exception completely at this level, to handle it partially and pass the same exception (or a different one) on, or whether to simply pass it on. Passing it on, when appropriate, can certainly simplify coding. The getLine( ) method becomes: String getLine() throws IOException { return in.readLine(); Chapter 9: Error Handling with Exceptions 297
- 298. } But of course, the caller is now responsible for handling any IOException that might arise. The cleanup( ) method must be called by the user when they are finished using the InputFile object, to release the system resources (such as file handles) that are used by the BufferedReader and/or FileReader objects6 . You don’t want to do this until you’re finished with the InputFile object, at the point you’re going to let it go. You might think of putting such functionality into a finalize( ) method, but as mentioned in Chapter 4 you can’t always be sure that finalize( ) will be called (even if you can be sure that it will be called, you don’t know when). This is one of the downsides to Java – all cleanup other than memory cleanup doesn’t happen automatically, so you must inform the client programmer that they are responsible, and possibly guarantee that cleanup occurs using finalize( ). In Cleanup.java an InputFile is created to open the same source file that creates the program, and this file is read in a line at a time, and line numbers are added. All exceptions are caught generically in main( ), although you could choose greater granularity. One of the benefits of this example is to show you why exceptions are introduced at this point in the book. Exceptions are so integral to programming in Java, especially because the compiler enforces them, that you can accomplish only so much without knowing how to work with them. Exception matching When an exception is thrown, the exception-handling system looks through the “nearest” handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs. Matching an exception doesn’t require a perfect match between the exception and its handler. A derived-class object will match a handler for the base class, as shown in this example: //: Human.java // Catching Exception Hierarchies class Annoyance extends Exception {} class Sneeze extends Annoyance {} public class Human { public static void main(String args[]) { try { throw new Sneeze(); } catch(Sneeze s) { System.out.println("Caught Sneeze"); } catch(Annoyance a) { System.out.println("Caught Annoyance"); } } 6 In C++, a destructor would handle this for you. 298 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 299. } ///:~ The Sneeze exception will be caught by the first catch clause that it matches, which is the first one, of course. However, if you remove the first catch clause: try { throw new Sneeze(); } catch(Annoyance a) { System.out.println("Caught Annoyance"); } The remaining catch clause will still work because it’s catching the base class of Sneeze. Put another way, catch(Annoyance e) will catch a Annoyance or any class derived from it. This is very useful, because it means that if you decide to add more exceptions to a method, if they’re all inherited from the same base class then the client programmer’s code will not need changing, assuming they catch the base class, at the very least. If you try to “mask” the derived-class exceptions by putting the base-class catch clause first, like this: try { throw new Sneeze(); } catch(Annoyance a) { System.out.println("Caught Annoyance"); } catch(Sneeze s) { System.out.println("Caught Sneeze"); } The compiler will give you an error message, since it sees that the Sneeze catch-clause can never be reached. Exception guidelines Use exceptions to 1. Fix the problem and call the method (which caused the exception) again. 2. Patch things up and continue without retrying the method. 3. Calculate some alternative result instead of what the method was supposed to produce. 4. Do whatever you can in the current context and rethrow the same exception to a higher context. 5. Do whatever you can in the current context and throw a different exception to a higher context. 6. Terminate the program. 7. Simplify. If your exception scheme makes things more complicated, then it is painful and annoying to use. 8. Make your library and program safer. This is a short-term investment (for debugging) and a long-term investment (for application robustness). Chapter 9: Error Handling with Exceptions 299
- 300. Summary Improved error recovery is one of the most powerful ways you can increase the robustness of your code. Error recovery is a fundamental concern for every program you write, and it’s especially important in Java, where one of the primary goals is to create program components for others to use. To create a robust system, each component must be robust. The goals for exception handling in Java are to simplify the creation of large, reliable programs using less code than currently possible, with more confidence that your application doesn’t have an unhandled error. Exceptions are not terribly difficult to learn, and are one of those features that provide immediate and significant benefits to your project. Fortunately, Java enforces all aspects of exceptions so it’s guaranteed that they will be used consistently, both by library designer and client programmer. Exercises 1. Create a class with a main( ) that throws an object of class Exception inside a try block. Give the constructor for Exception a string argument. Catch the exception inside a catch clause and print out the string argument. Add a finally clause and print a message to prove you were there. 2. Create your own exception class using the extends keyword. Write a constructor for this class that takes a String argument and stores it inside the object with a String handle. Write a method that prints out the stored String. Create a try-catch clause to exercise your new exception. 3. Write a class with a method that throws an exception of the type created in exercise 2. Try compiling it without an exception specification to see what the compiler says. Add the appropriate exception specification. Try out your class and its exception inside a try-catch clause. 300 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 301. v 10: The Java IO system Creating a good input/output (IO) system is one of the more difficult tasks for the language designer. This is evidenced by the number of different approaches taken. The challenge seems to be in covering all eventualities. Not only are there different kinds of IO that you want to communicate with (files, the console, network connections), but you need to talk to them in a wide variety of ways (sequential, random-access, binary, character, by lines, by words, etc.). The Java library designers attacked the problem by creating lots of classes. In fact, there are so many classes for Java’s IO system that it can be intimidating at first. In addition there has been a significant change in the IO library between Java 1.0 and Java 1.1. But instead of simply replacing the old library with a new one, the designers at Sun extended the old library and added the new one alongside it. As a result you may sometimes end up mixing the old and new libraries and creating even more intimidating code. After a while it becomes familiar and possibly even less offensive, and you might gain some insight into library design as you ponder how it could be restructured. This chapter will help you understand the variety of IO classes in the standard Java library and how to use them. The first portion of the chapter will introduce the “old” Java 1.0 IO stream library, since there is a significant amount of existing code that uses that library, and the remainder of the chapter will introduce the new features in the Java 1.1 IO library. Notice that when you compile some of the code in the first part of the chapter with a Java 1.1 compiler you may get a “deprecated feature” warning message at compile time. The code still works; the compiler is just suggesting that you use certain new features that are described in the latter part of the chapter. It is valuable, however, to see 301
- 302. the difference between the old and new way of doing things and that’s why it was left in – to increase your understanding (and to allow you to read code written for Java 1.0). Input and output The Java library classes for IO are divided by input and output, as you can see by looking at the online Java class hierarchy with your Web browser. By inheritance, all classes derived from InputStream have basic methods called read( ) for reading a single byte or array of bytes. Likewise, all classes derived from OutputStream have basic methods called write( ) for writing a single byte or array of bytes. However, you won’t generally use these methods – they exist so more sophisticated classes can use them as they provide a more useful interface. Thus, you’ll rarely create your stream object by using a single class, but instead will layer multiple objects together provide your desired functionality. The fact that you create more than one object to create a single resulting stream is the primary reason that Java’s stream library is confusing. It’s helpful to categorize the classes by their functionality. The library designers started by deciding that all classes that had anything to do with input would be inherited from InputStream and all classes that were associated with output would be inherited from OutputStream. On further inspection you’ll see this is a bit naïve. Types of InputStream This category includes the classes that decide where your input is coming from: 1. An array of bytes 2. A String object 3. A file 4. A “pipe,” which works like a physical pipe: you put things in one end and they come out the other 5. A sequence of other streams, so you can collect them together into a single stream 6. Other sources, such as an Internet connection (this will be discussed in a later chapter). In addition, the FilterInputStream provides a base class for classes that attach (1) attributes or (2) useful interfaces to input streams. This is discussed later. Table 10-1. Types of InputStream Class Function Constructor How to use it Arguments ByteArrayInputStream Allows a buffer The buffer from As a source of data. in memory to be which to extract the Connect it to a used as an bytes. FilterInputStream InputStream. object to provide a useful interface. StringBufferInputStream Converts a A String. The As a source of data. String into an underlying Connect it to a InputStream. implementation FilterInputStream actually uses a object to provide a 302 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 303. Class Function Constructor How to use it Arguments StringBuffer. useful interface. FileInputStream For reading A String representing As a source of data. information the file name, or a Connect it to a from a file. File or FileDescriptor FilterInputStream object. object to provide a useful interface. PipedInputStream Produces the data PipedOutputStream As a source of data that’s being in multithreading. written to the Connect it to a associated FilterInputStream PipedOutputStre object to provide a am. Implements useful interface. the “piping” concept. SequenceInputStream Coverts two or Two InputStream As a source of data. more objects or an Connect it to a InputStream Enumeration for a FilterInputStream objects into a container of object to provide a single InputStream objects. useful interface. InputStream. FilterInputStream Abstract class See Table 10-3. See Table 10-3. providing an interface for useful functionality to the other InputStream classes. See Table 10-3. Types of OutputStream This category includes the classes that decide where your output will go: an array of bytes (no String, however: presumably you can create one yourself using the array of bytes), a file, or a “pipe.” In addition, the FilterOutputStream provides a base class for classes that attach (1) attributes or (2) useful interfaces to output streams. This is discussed later. Table 10-2. Types of OutputStream Class Function Constructor How to use it Arguments ByteArrayOutputStream Creates a buffer in Optional initial To designate the memory. All the size of the buffer. destination of your data you send to the data. Connect it to a stream is placed in FilterOutputStrea this buffer. m object to provide Chapter 10: The Java IO System 303
- 304. Class Function Constructor How to use it Arguments a useful interface. FileOutputStream For sending A String To designate the information to a representing the destination of your file. file name, or a File data. Connect it to a or FileDescriptor FilterOutputStrea object. m object to provide a useful interface. PipedOutputStream Any information PipedInputStream To designate the you write to this destination of your automatically ends data for up as input for the multithreading. associated Connect it to a PipedInputStream. FilterOutputStrea Implements the m object to provide “piping” concept. a useful interface. FilterOutputStream Abstract class See Table 10-4. See Table 10-4. providing an interface for useful functionality to the other OutputStream classes. See Table 10-4. Adding attributes & useful interfaces The designers decided that everything should be an InputStream if it had anything to do with input, and an OutputStream if it had anything to do with output. This sounds like it makes sense, as a first cut design, but with deeper thought (and by referring to the C++ iostream library as an example) they would have seen that there were really two separate issues involved: what kind of device you are talking to (disk, console, memory) and the way you want to talk to it (with characters or bytes, random access, formatted, etc.). Then it would have made sense to create two separate class hierarchies, one for each need. The hierarchy dealing with devices wouldn’t have any way for the client programmer to talk to a device until they attached an interface class to the device object. However, the Java IO library doesn’t work this way. The device classes contain rudimentary read/write functionality, thus you can easily be confused into thinking that they should be used on their own in some situations. The interface classes were blindly shoehorned into the InputStream and OutputStream hierarchies, so it’s not very clear they actually have a separate use. This is a good lesson; because of this poor design new Java programmers will always have a struggle to figure out: 1. What classes are supposed to be used in what situations. It’s not clear you’re supposed to mix more than one class of the same type together. 304 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 305. 2. Are they foolish for not understanding why the classes are designed this way? Perhaps there’s something about class design they don’t understand, hidden away in this library. It’s pretty hard to know this as a novice, especially since you’re bound to assume the Java library designers knew what they were doing. The classes that provide the convenient interface to control a particular InputStream or OutputStream are the FilterInputStream and FilterOutputStream – which are in themselves not very intuitive names. They are derived, respectively, from InputStream and OutputStream, and they are themselves abstract classes, in theory to provide a common interface for all the different ways you want to talk to a stream. In fact, FilterInputStream and FilterOutputStream simply mimic their base classes, while the derived classes have wildly different interfaces, another factor to suggest poorly- considered class design (see the summary of this chapter for further analysis). Reading from an InputStream with FilterInputStream On inspection, you’ll discover that FilterInputStream classes fall into the “grab-bag” category. The description in the online documentation states that this base class is “the basis for enhancing input stream functionality.” Translated, this means “we couldn’t figure out where else to put this stuff, but it seemed like it belonged together.” To be somewhat merciful here, this isn’t a bad approach to use as a starting point for library design – as long as, at some point, you perform a design review and discover ideas that really don’t hang together. And it’s important that you realize what went on here, so you don’t mistake this for good library design and emulate it, or feel inadequate because you don’t see the design benefits of this library. The FilterInputStream classes accomplish two significantly different things. DataInputStream allows you to read different types of primitive data as well as String objects. This, along with its companion DataOutputStream, allows you to portably move primitive data from one place to another via a stream. These “places,” are determined by the classes in Table 10-1. If you’re reading data in blocks and parsing it yourself you won’t need DataInputStream, but in most other cases you will want to use it to automatically format the data you read. The remaining classes modify the way an InputStream behaves internally: whether it’s buffered or unbuffered, if it keeps track of the lines it’s reading (allowing you to ask for line numbers, or set the line number), and whether you can push back a single character. The last two classes look an awful lot like support for building a compiler (that is, they were added to support the construction of the Java compiler), so you probably won’t use them in general programming. You’ll probably need to buffer your input almost every time, regardless of the IO device you’re connecting to, so it would have made more sense for the IO library to make a special case for unbuffered input rather than buffered input. Table 10-3. Types of FilterInputStream Class Function Constructor How to use it Arguments DataInputStream Used in concert with InputStream Contains a full interface DataOutputStream, to allow you to read so you can read primitive types. primitives (int, char, Chapter 10: The Java IO System 305
- 306. Class Function Constructor How to use it Arguments long, etc.) from a stream in a portable fashion. BufferedInputStream Use this to prevent a InputStream This doesn’t provide an physical read every , with interface per se, just a time you want more optional requirement that a data. You’re saying buffer size. buffer be used. Attach “use a buffer” an interface object. LineNumberInputStream Keeps track of line InputStream This just adds line numbers in the input numbering, so you’ll stream; you can call probably attach an getLineNumber( ) and interface object. setLineNumber(int). PushbackInputStream Has a one-byte push- InputStream Generally used in the back buffer so you can scanner for a compiler push back the last and probably included character read. because the Java compiler needed it. You probably won’t use this. Writing to an OutputStream with FilterOutputStream The same comments made about FilterInputStream being a poorly-designed “grab bag” apply here as well. The complement to DataInputStream is DataOutputStream, which formats each of the primitive types and String objects onto a stream in such a way that any DataInputStream, on any machine, can read them. All the methods start with “write,” such as writeByte( ), writeFloat( ), etc. If you want to do true formatted output, for example to the console, use a PrintStream. This is the endpoint that allows you to print all the primitive data types and String objects in a viewable format, as opposed to DataOutputStream whose goal is to put them on a stream in a way that DataInputStream can portably reconstruct them. The System.out static object is a PrintStream. The two important methods in PrintStream are print( ) and println( ), which are overloaded to print out all the various types. The difference between print( ) and println( ) is the latter adds a newline when it’s done. BufferedOutputStream is a modifier and tells the stream to use buffering so you don’t get a physical write every time you write to the stream. You’ll probably always want to use this with files, and possibly console IO. Table 10-4. Types of FilterOutputStream Class Function Constructor How to use it Arguments 306 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 307. Class Function Constructor How to use it Arguments DataOutputStream Used in concert with OutputStream Contains full DataInputStream, so interface to allow you can write you to write primitives (int, char, primitive types. long, etc.) to a stream in a portable fashion. PrintStream For producing formatted OutputStream, Should be the output. While with optional “final” wrapping DataOutputStream boolean indicating for your handles the storage of that the buffer is OutputStream data, PrintStream flushed with every object. You’ll handles display. newline. probably use this a lot. BufferedOutputStream Use this to prevent a OutputStream, This doesn’t physical write every with optional provide an time you send a piece of buffer size. interface per se, data. You’re saying “use just a a buffer.” You can call requirement that flush( ) to flush the a buffer is used. buffer. Attach an interface object. Off by itself: RandomAccessFile RandomAccessFile is used for files containing records of known size so you can move from one record to another using seek( ), then read or change the records. The records don’t all have to be the same size, you just have to be able to determine how big they are and where they are placed in the file. At first it’s a little bit hard to believe this: RandomAccessFile is not part of the InputStream or OutputStream hierarchy. It has no association with those hierarchies other than it happens to implement the DataInput and DataOutput interfaces (which are also implemented by DataInputStream and DataOutputStream). It doesn’t even use any of the functionality of the existing InputStream or OutputStream classes – it’s a completely separate class, written from scratch, with all its own (mostly native) methods. The value of access to source code is evident here, since we can discover that the bulk of the stream library was written by Arthur van Hoff and Jonathan Payne (with a little help by James Gosling on the piping classes, which one can infer he wanted for some special purpose). But RandomAccessFile is the sole contribution made by David Brown, which suggests it may have been created in a vacuum and added much later, with no time to properly fit it into the hierarchy. Or perhaps no one could figure out where it belonged. In any event, it stands alone, as a direct descendant of Object. Essentially, a RandomAccessFile works like a DataInputStream pasted together with a DataOutputStream and the methods getFilePointer( ) to find out where you are in the file, seek( ) to move to a new point in the file, and length( ) to determine the maximum Chapter 10: The Java IO System 307
- 308. size of the file. In addition, the constructors require a second argument (identical to fopen( ) in C) indicating whether you are just randomly reading (“r”) or reading and writing (“rw”). There’s no support for write-only files, which could suggest that RandomAccessFile might have worked well if it were inherited from DataInputStream. What’s even more frustrating is that you could easily imagine wanting to seek within other types of streams, such as a ByteArrayInputStream, but the seeking methods are available only in RandomAccessFile, which works for files only. BufferedInputStream does allow you to mark( ) a position (whose value is held in a single internal variable) and reset( ) to that position, but this is very limited and not too useful. The File class The File class has a deceiving name – you might think it refers to an actual file, but it doesn’t. It can represent either the name of a particular file, or the names of a set of files in a directory. If it’s a set of files, you can ask for the set with the list( ) method, and this returns an array of String. It makes sense to return an array rather than one of the flexible collection classes because the number of elements is fixed, and if you want a different directory listing you just create a different File object.. In fact, “FilePath” would have been a better name. This section shows a complete example of the use of this class, including the associated FilenameFilter interface. A directory lister Suppose you’d like to see a directory listing. The File object can be listed in two ways. If you call list( ) with no arguments, you’ll get the full list that the File object contains. However, if you want a restricted list, say for example all the files with an extension of .java, then you use a “directory filter” which is a class that tells how to select the File objects for display. Here’s the code for the example (see page 80 if you have trouble executing this program): //: DirList.java // Displays directory listing package c10; import java.io.*; public class DirList { public static void main(String args[]) { try { File path = new File("."); String[] list; if(args.length == 0) list = path.list(); else list = path.list(new DirFilter(args[0])); for(int i = 0; i < list.length; i++) System.out.println(list[i]); } catch(Exception e) { e.printStackTrace(); } } } 308 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 309. class DirFilter implements FilenameFilter { String afn; DirFilter(String afn) { this.afn = afn; } public boolean accept(File dir, String name) { // Strip path information: String f = new File(name).getName(); return f.indexOf(afn) != -1; } } ///:~ The DirFilter class “implements” the interface FilenameFilter. Interfaces were covered in Chapter 7. It’s useful to see how simple the FilenameFilter interface is: public interface FilenameFilter { boolean accept(File dir, String name); } It says that all that this type of object does is provide a method called accept( ). The whole reason behind the creation of this class is to provide the accept( ) method to the list( ) method, so that list( ) can call back accept( ) to determine which file names should be included in the list. Thus this technique is often referred to as a callback or sometimes a functor (that is, DirFilter is a functor because its job is only to hold a method). Because list( ) takes a FilenameFilter object as its argument, it means you can pass an object of any class that implements FilenameFilter to choose (even at run-time) how the list( ) method will behave. Thus, the purpose of a callback is to provide flexibility in the behavior of code. DirFilter shows that just because an interface contains only a set of methods, you’re not restricted to writing only those methods (you must at least provide definitions for all the methods in an interface, however). In this case, the DirFilter constructor is also created. The accept( ) method must accept a File object representing the directory that a particular file is found in, and a String containing the name of that file. You may choose to use or ignore either of these arguments, but you will probably at least use the file name. Remember that the list( ) method is calling accept( ) for each of the file names in the directory object to see which one should be included – this is indicated by the boolean result returned by accept( ). To make sure that what you’re working with is only the name and contains no path information, all you have to do is take the String object and create a File object out of it, then call getName( ) which strips away all the path information (in a platform- independent way). Then accept( ) uses the String class indexOf( ) method to see if the search string afn appears anywhere in the name of the file. If afn is found within the string, the return value is the starting index of afn, but if it’s not found the return value is -1. Keep in mind that this is a simple string search and does not have regular-expression “wildcard” matching like “fo?.b?r*” which is much more difficult to implement. The list( ) method returns an array. You can query this array for its length and then move through it, selecting the array elements. This ability to easily pass an array in and out of a method is a tremendous improvement over the behavior of C and C++. Anonymous inner classes This example is ideal for rewriting using an anonymous inner class (described in Chapter 7). As a first cut, a method filter( ) is created that returns a handle to a FilenameFilter: //: DirList2.java // Uses Java 1.1 anonymous inner classes import java.io.*; Chapter 10: The Java IO System 309
- 310. public class DirList2 { public static FilenameFilter filter(final String afn) { // Creation of anonymous inner class: return new FilenameFilter() { String fn = afn; public boolean accept(File dir, String n) { // Strip path information: String f = new File(n).getName(); return f.indexOf(fn) != -1; } }; // End of anonymous inner class } public static void main(String args[]) { try { File path = new File("."); String[] list; if(args.length == 0) list = path.list(); else list = path.list(filter(args[0])); for(int i = 0; i < list.length; i++) System.out.println(list[i]); } catch(Exception e) { e.printStackTrace(); } } } ///:~ Notice that the argument to filter( ) must be final. This is required by the anonymous inner class for it to utilize an object from outside its scope. This design is an improvement because the FilenameFilter class is now tightly bound to DirList2. However, you can take this approach one step further and define the anonymous inner class as an argument to list( ), in which case it’s even smaller: //: DirList3.java // Building the anonymous inner class "in-place" import java.io.*; public class DirList3 { public static void main(final String args[]) { try { File path = new File("."); String[] list; if(args.length == 0) list = path.list(); else list = path.list( new FilenameFilter() { public boolean accept(File dir, String n) { String f = new File(n).getName(); return f.indexOf(args[0]) != -1; } }); 310 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 311. for(int i = 0; i < list.length; i++) System.out.println(list[i]); } catch(Exception e) { e.printStackTrace(); } } } ///:~ The argument to main( ) is now final, since the anonymous inner class uses args[0] directly. This shows you how anonymous inner classes allow the creation of quick-and-dirty classes to solve problems. Since everything in Java revolves around classes, this can be a useful coding technique. One benefit is that it keeps the code that solves a particular problem isolated together in one spot. On the other hand, it is not always as easy to read so you must use it judiciously. A sorted directory listing Ah, you say you want the file names sorted? Since there’s no support for sorting in Java 1.0 or Java 1.1 (although sorting is included in Java 1.2) it will have to be added in the program directly, using the SortVector created in Chapter 8: //: SortedDirList.java // Displays sorted directory listing import java.io.*; import c08.*; public class SortedDirList { private File path; private String[] list; public SortedDirList(final String afn) { path = new File("."); if(afn == null) list = path.list(); else list = path.list( new FilenameFilter() { public boolean accept(File dir, String n) { String f = new File(n).getName(); return f.indexOf(afn) != -1; } }); sort(); } void print() { for(int i = 0; i < list.length; i++) System.out.println(list[i]); } private void sort() { StrSortVector sv = new StrSortVector(); for(int i = 0; i < list.length; i++) sv.addElement(list[i]); // The first time an element is pulled from // the StrSortVector the list is sorted: for(int i = 0; i < list.length; i++) list[i] = sv.elementAt(i); Chapter 10: The Java IO System 311
- 312. } // Test it: public static void main(String[] args) { SortedDirList sd; if(args.length == 0) sd = new SortedDirList(null); else sd = new SortedDirList(args[0]); sd.print(); } } ///:~ A few other improvements have been made: instead of creating path and list as local variables to main( ), they are members of the class so their values can be accessible for the lifetime of the object. In fact, main( ) is now just a way to test the class. You can see that the constructor of the class automatically sorts the list once that list has been created. The sort is case-insensitive so you don’t end up with a list of all the words starting with capital letters, followed by the rest of the words starting with all the lowercase letters. However, you’ll notice that within a group of file names that begin with the same letter the capitalized words are listed first, which is still not quite the desired behavior for the sort. This problem will be fixed in Java 1.2. Checking for and creating directories The File class is more than just a representation for an existing directory path, file or group of files. You can also use a File object to create a new directory or an entire directory path if it doesn’t exist. You can also look at the characteristics of files (size, last modification date, read/write), whether a File object represents a file or a directory, and you can delete a file. This program shows the remaining methods available with the File class: //: MakeDirectories.java // Demonstrates the use of the File class to // create directories and manipulate files. import java.io.*; public class MakeDirectories { private final static String usage = "Usage:MakeDirectories path1 ...n" + "Creates each pathn" + "Usage:MakeDirectories -d path1 ...n" + "Deletes each pathn" + "Usage:MakeDirectories -r path1 path2n" + "Renames from path1 to path2n"; private static void usage() { System.err.println(usage); System.exit(1); } private static void fileData(File f) { System.out.println( "Absolute path: " + f.getAbsolutePath() + "n Can read: " + f.canRead() + "n Can write: " + f.canWrite() + "n getName: " + f.getName() + "n getParent: " + f.getParent() + 312 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 313. "n getPath: " + f.getPath() + "n length: " + f.length() + "n lastModified: " + f.lastModified()); if(f.isFile()) System.out.println("it's a file"); else if(f.isDirectory()) System.out.println("it's a directory"); } public static void main(String args[]) { if(args.length < 1) usage(); if(args[0].equals("-r")) { if(args.length != 3) usage(); File old = new File(args[1]), rname = new File(args[2]); old.renameTo(rname); fileData(old); fileData(rname); return; // Exit main } int count = 0; boolean del = false; if(args[0].equals("-d")) { count++; del = true; } for( ; count < args.length; count++) { File f = new File(args[count]); if(f.exists()) { System.out.println(f + " exists"); if(del) { System.out.println("deleting..." + f); f.delete(); } } else { // Doesn't exist if(!del) { f.mkdirs(); System.out.println("created " + f); } } fileData(f); } } } ///:~ In fileData( ) you can see the various file investigation methods put to use to display information about the file or directory path. The first method that’s exercised by main( ) is renameTo( ), which allows you to rename (or move) a file to an entirely new path represented by the argument which is another File object. This also works with directories of any length. If you experiment with the above program, you’ll find you can make a directory path of any complexity because mkdirs( ) will do all the work for you. In Java 1.0, the -d flag reports that the directory is deleted but it’s still there; in Java 1.1 the directory is actually deleted. Chapter 10: The Java IO System 313
- 314. Typical uses of IO streams Although there are lots of IO stream classes in the library that can be combined in many different ways, there are just a few ways that you’ll probably end up using them. However, they require attention to get the right combinations. The following rather long example shows the creation and use of typical IO configurations, so you can use it as a reference when writing your own code. Notice that each configuration begins with a commented number and title that corresponds to the heading for the appropriate explanation that follows in the text. //: IOStreamDemo.java // Typical IO Stream Configurations import java.io.*; import com.eckelobjects.tools.*; public class IOStreamDemo { public static void main(String args[]) { try { // 1. Buffered input file DataInputStream in = new DataInputStream( new BufferedInputStream( new FileInputStream(args[0]))); String s, s2 = new String(); while((s = in.readLine())!= null) s2 += s + "n"; in.close(); // 2. Input from memory StringBufferInputStream in2 = new StringBufferInputStream(s2); int c; while((c = in2.read()) != -1) System.out.print((char)c); // 3. Formatted memory input try { DataInputStream in3 = new DataInputStream( new StringBufferInputStream(s2)); while(true) System.out.print((char)in3.readByte()); } catch(EOFException e) { System.out.println( "End of stream encountered"); } // 4. Line numbering & file output try { LineNumberInputStream li = new LineNumberInputStream( new StringBufferInputStream(s2)); DataInputStream in4 = new DataInputStream(li); 314 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 315. PrintStream out1 = new PrintStream( new BufferedOutputStream( new FileOutputStream("IODemo.out"))); while((s = in4.readLine()) != null ) out1.println( "Line " + li.getLineNumber() + s); out1.close(); // finalize() not reliable! } catch(EOFException e) { System.out.println( "End of stream encountered"); } // 5. Storing & recovering data try { DataOutputStream out2 = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("Data.txt"))); out2.writeBytes( "Here's the value of pi: n"); out2.writeDouble(3.14159); out2.close(); DataInputStream in5 = new DataInputStream( new BufferedInputStream( new FileInputStream("Data.txt"))); System.out.println(in5.readLine()); System.out.println(in5.readDouble()); } catch(EOFException e) { System.out.println( "End of stream encountered"); } // 6. Reading and writing random access files RandomAccessFile rf = new RandomAccessFile("rtest.dat", "rw"); for(int i = 0; i < 10; i++) rf.writeDouble(i*1.414); rf.close(); rf = new RandomAccessFile("rtest.dat", "rw"); rf.seek(5*8); rf.writeDouble(47.0001); rf.close(); rf = new RandomAccessFile("rtest.dat", "r"); for(int i = 0; i < 10; i++) System.out.println( "Value " + i + ": " + rf.readDouble()); rf.close(); // 7. File input shorthand Chapter 10: The Java IO System 315
- 316. InFile in6 = new InFile(args[0]); String s3 = new String(); System.out.println( "First line in file: " + in6.readLine()); in.close(); // 8. Formatted file output shorthand PrintFile out3 = new PrintFile("Data2.txt"); out3.print("Test of PrintFile"); out3.close(); // 9. Data file output shorthand OutFile out4 = new OutFile("Data3.txt"); out4.writeBytes("Test of outDataFilenr"); out4.writeChars("Test of outDataFilenr"); out4.close(); } catch(FileNotFoundException e) { System.out.println( "File Not Found:" + args[0]); } catch(IOException e) { System.out.println("IO Exception"); } } } ///:~ Input streams Of course, one very common thing you’ll want to do is print formatted output to the console, but that’s already been simplified in the package com.eckelobjects.tools created in Chapter 5. Parts 1-4 demonstrate the creation and use of input streams (although part 4 also shows the simple use of an output stream as a testing tool). 1. Buffered input file To open a file for input, you use a FileInputStream with a String or a File object as the file name. For speed, you’ll want that file to be buffered so you give the resulting handle to the constructor for a BufferedInputStream. To read input in a formatted fashion, you give that resulting handle to the constructor for a DataInputStream, which is your final object and the interface you read from. In this example only the readLine( ) method is used, but of course any of the DataInputStream methods are available. When you reach the end of the file, readLine( ) returns null, so that is used to break out of the while loop. The String s2 is used to accumulate the entire contents of the file (including newlines which must be added since readLine( ) strips them off). s2 is then used in the later portions of this program. Finally, close( ) is called to close the file. Technically, close( ) will be called when finalize( ) is run, and this is supposed to happen (whether or not garbage collection occurs) as the program exits. However, Java 1.0 has a rather important bug so this doesn’t happen. In Java 1.1 you must explicitly call System.runFinalizersOnExit(true) to guarantee that finalize( ) will be called for every object in the system. The safest approach is to explicitly call close( ) for files. 316 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 317. 2. Input from memory This piece takes the String s2 that now contains the entire contents of the file and uses it to create a StringBufferInputStream (a String, not a StringBuffer, is required as the constructor argument). Then read( ) is used to read each character one at a time and send it out to the console. Note that read( ) returns the next byte as an int and thus it must be cast to a char to print properly. 3. Formatted memory input The interface for StringBufferInputStream is very limited, so you usually enhance it by wrapping it inside a DataInputStream. However, if you choose to read the characters out a byte at a time using readByte( ), any value is valid so the return value cannot be used to detect the end of input. Instead, you can use the available( ) method to find out how many more characters are available. Here’s an example that shows how to read a file a byte at a time: //: TestEOF.java // Testing for the end of file while reading // a byte at a time. import java.io.*; public class TestEOF { public static void main(String args[]) { try { DataInputStream in = new DataInputStream( new BufferedInputStream( new FileInputStream("TestEof.java"))); while(in.available() != 0) System.out.print((char)in.readByte()); } catch (IOException e) { System.err.println("IOException"); } } } ///:~ Note that available( ) works differently depending on what sort of medium you’re reading from – it’s literally “the number of bytes that can be read without blocking.” With a file this means the whole file, but with a different kind of stream this may not be true, so use it thoughtfully. You could also detect the end of input in cases like these by catching an exception. However, the use of exceptions for control flow is considered a misuse of that feature. 4. Line numbering & file output This example shows the use of the LineNumberInputStream to keep track of the input line numbers. Here, you cannot simply gang all the constructors together, since you have to keep a handle to the LineNumberInputStream (note this is not an inheritance situation, so you cannot simply cast in4 to a LineNumberInputStream). Thus, li holds the handle to the LineNumberInputStream which is then used to create a DataInputStream for easy reading. LineNumberInputStream may be more useful as an example of how to add functionality to a stream by inheriting a new FileInputStream (remember, you have access to the source code for the Java libraries). It’s not particularly hard to keep track of line numbers yourself while reading input lines. Chapter 10: The Java IO System 317
- 318. This example also shows how to write formatted data to a file. First, a FileOutputStream is created to connect to the file. For efficiency, this is made a BufferedOutputStream, which is what you’ll virtually always want to do, but you’re forced to do it explicitly. Then for the formatting it’s turned into a PrintStream. The data file created this way is readable as an ordinary text file. One of the methods that actually indicates when a DataInputStream is exhausted is readLine( ), which returns null when there are no more strings to read. Each line is printed to the file along with its line number, which is acquired through li. You’ll see an explicit close( ) for out1, which would make sense if the program were to turn around and read the same file again. However, this program ends without ever looking at the file IODemo.out. When a program ends, all the finalize( ) methods for all the objects are supposed to be called, and the FileOutputStream finalize( ) flushes the buffers and closes the file, so everything should come to a fine finish. Unfortunately, finalize( ) is not reliable – it does not get called in Java 1.0, and is only guaranteed to be called in Java 1.1 if you call System.runFinalizersOnExit(true). Thus, If you don’t call close( ) for all your output files, you may discover they’re incomplete. Output streams The two primary kinds of output streams are separated by the way they write data: one writes it for human consumption, and the other writes it to be re-acquired by a DataInputStream. The RandomAccessFile stands alone, although its data format is compatible with the DataInputStream and DataOutputStream. 5. Storing & recovering data A PrintStream formats data so it’s readable by a human. To output data so that it can be recovered by another stream, you use a DataOutputStream to write the data, and a DataInputStream to recover the data. Of course these streams could be anything, but here a file is used, buffered for both reading and writing. Note that the character string is written using writeBytes( ) and not writeChars( ). If you use the latter, you’ll be writing the 16-bit Unicode characters. Since there is no complementary “readChars” method in DataInputStream, you’re stuck pulling these characters off one at a time with readChar( ). So for ASCII, it’s easier to write the characters as bytes; then readLine( ) gets a regular ASCII line. The writeDouble( ) stores the double number to the stream and the complementary readDouble( ) recovers it. But for any of the reading methods to work correctly, you must know the exact placement of the data item in the stream, since it would be equally possible to read the stored double as a simple sequence of bytes, or as a char, etc. So you must either have a fixed format for the data in the file or extra information must be stored in the file that you parse to determine where the data is located. 6. Reading and writing random access files As previously noted, the RandomAccessFile is almost totally isolated from the rest of the IO hierarchy, save for the fact that it implements the DataInput and DataOutput interfaces. Thus you cannot combine it with any of the aspects of the InputStream and OutputStream subclasses. Even though it might make sense to treat a ByteArrayInputStream as a random-access element, you can open a file only with a RandomAccessFile. You must assume a RandomAccessFile is properly buffered, since you cannot add that. Again this is an indicator of poor design. 318 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 319. The one option you have is in the second constructor argument: you can open a RandomAccessFile to read (“r”) or read and write (“rw”). Using a RandomAccessFile is like using a combined DataInputStream and DataOutputStream (because it implements the equivalent interfaces). In addition, you can see that seek( ) is used to move about in the file and change one of the values. Shorthand for file manipulation Since there are certain canonical forms that you’ll be using very regularly with files, why do all that typing? This portion shows the creation and use of shorthand versions of typical file reading and writing configurations. These shorthands are placed in the package com.eckelobjects.tools that was begun in Chapter 5 (See page 156). To add each class to the library, you simply place it in the appropriate directory and add the package statement. 7. File input shorthand The creation of an object that reads a file from a buffered DataInputStream can be encapsulated into a class called InFile: //: InFile.java // Shorthand class for opening an input file package com.eckelobjects.tools; import java.io.*; public class InFile extends DataInputStream { public InFile(String filename) throws FileNotFoundException { super( new BufferedInputStream( new FileInputStream(filename))); } public InFile(File file) throws FileNotFoundException { this(file.getPath()); } } ///:~ Both the String versions of the constructor and the File versions are included, to parallel the creation of a FileInputStream. Now you can reduce your chances of repetitive-stress syndrome while creating files, as seen in the example. 8. Formatted file output shorthand The same kind of approach can be taken to create a PrintStream that writes to a buffered file. Here’s the extension to com.eckelobjects.tools: //: PrintFile.java // Shorthand class for opening an output file // for formatted printing. package com.eckelobjects.tools; import java.io.*; public class PrintFile extends PrintStream { public PrintFile(String filename) Chapter 10: The Java IO System 319
- 320. throws IOException { super( new BufferedOutputStream( new FileOutputStream(filename))); } public PrintFile(File file) throws IOException { this(file.getPath()); } } ///:~ Note that it is not possible for a constructor to catch an exception that’s thrown by a base-class constructor. 9. Data file output shorthand Finally, the same kind of shorthand can create a buffered output file for data storage (as opposed to human-readable storage): //: OutFile.java // Shorthand class for opening an output file // for formatted printing. package com.eckelobjects.tools; import java.io.*; public class OutFile extends DataOutputStream { public OutFile(String filename) throws IOException { super( new BufferedOutputStream( new FileOutputStream(filename))); } public OutFile(File file) throws IOException { this(file.getPath()); } } ///:~ It is curious (and unfortunate) that the Java library designers didn’t think to provide these conveniences as part of their standard. Reading from standard input Following the approach pioneered in Unix of “standard input,” “standard output,” and “standard error output,” Java has System.in, System.out and System.err. Throughout the book you’ve seen how to write to standard output, using System.out which is already pre-wrapped as a PrintStream object. System.err is likewise a PrintStream, but System.in is a raw InputStream, with no wrapping. This means that, while you can use System.out and System.err right away, System.in must be wrapped before you can read from it. Typically you’ll want to read input a line at a time using readLine( ), so you’ll want to wrap System.in in a DataInputStream. This is the “old” Java 1.0 way to do line input; a bit later in the chapter you’ll see the new Java 1.1 solution. Here’s an example that simply echoes each line that you type in: //: Echo.java 320 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 321. // How to read from standard input import java.io.*; public class Echo { public static void main(String args[]) { DataInputStream in = new DataInputStream( new BufferedInputStream(System.in)); String s; try { while((s = in.readLine()).length() != 0) System.out.println(s); // An empty line terminates the program } catch(IOException e) { e.printStackTrace(); } } } ///:~ The reason for the try block is that readLine( ) can throw an IOException. Note that System.in should also be buffered, as with most streams It’s a bit inconvenient that you’re forced to wrap System.in in a DataInputStream in each program, but perhaps it was designed this way to allow maximum flexibility. Piped streams The PipedInputStream and PipedOutputStream have been mentioned only briefly in this chapter. This is not to suggest that they aren’t useful, but their value is not apparent until you begin to understand about multithreading, since the piped streams are used to communicate between threads. This is covered along with an example in Chapter 14. StreamTokenizer Although StreamTokenizer is not derived from InputStream or OutputStream, it works only with InputStream objects so it rightfully belongs in the IO portion of the library. The StreamTokenizer class is used to break any InputStream into a sequence of “tokens,” which are bits of text delimited by whatever you choose. For example, your tokens could be words, and then they would be delimited by white space and punctuation. Consider a program to count the occurrence of words in a text file: //: SortedWordCount.java // Counts words in a file, outputs // results in sorted form. import java.io.*; import java.util.*; import c08.*; // Contains StrSortVector class Counter { private int i = 1; int read() { return i; } void increment() { i++; } } Chapter 10: The Java IO System 321
- 322. public class SortedWordCount { private FileInputStream file; private StreamTokenizer st; private Hashtable counts = new Hashtable(); SortedWordCount(String filename) throws FileNotFoundException { try { file = new FileInputStream(filename); st = new StreamTokenizer(file); st.ordinaryChar('.'); st.ordinaryChar('-'); } catch(FileNotFoundException e) { System.out.println( "Could not open " + filename); throw e; } } void cleanup() { try { file.close(); } catch(IOException e) { System.out.println( "st.close() unsuccessful"); } } void countWords() { try { while(st.nextToken() != StreamTokenizer.TT_EOF) { String s; switch(st.ttype) { case StreamTokenizer.TT_EOL: s = new String("EOL"); break; case StreamTokenizer.TT_NUMBER: s = Double.toString(st.nval); break; case StreamTokenizer.TT_WORD: s = st.sval; // Already a String break; default: // single character in ttype s = String.valueOf((char)st.ttype); } if(counts.containsKey(s)) ((Counter)counts.get(s)).increment(); else counts.put(s, new Counter()); } } catch(IOException e) { System.out.println( "nextToken() unsuccessful"); } } Enumeration values() { 322 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 323. return counts.elements(); } Enumeration keys() { return counts.keys(); } Counter getCounter(String s) { return (Counter)counts.get(s); } Enumeration sortedKeys() { Enumeration e = counts.keys(); StrSortVector sv = new StrSortVector(); while(e.hasMoreElements()) sv.addElement((String)e.nextElement()); // This call forces a sort: return sv.elements(); } public static void main(String args[]) { try { SortedWordCount wc = new SortedWordCount(args[0]); wc.countWords(); Enumeration keys = wc.sortedKeys(); while(keys.hasMoreElements()) { String key = (String)keys.nextElement(); System.out.println(key + ": " + wc.getCounter(key).read()); } wc.cleanup(); } catch(Exception e) { e.printStackTrace(); } } } ///:~ It makes sense to present these in a sorted form, but since Java 1.0 and Java 1.1 don’t have any sorting methods, that will have to be mixed in. This is easy enough to do with a StrSortVector (which was created in Chapter 8, and is part of the package created in that chapter – remember that the starting directory for all the subdirectories in this book must be in your class path for the program to compile successfully). To open the file, a FileInputStream is used, and to turn the file into words a StreamTokenizer is created from the FileInputStream. In StreamTokenizer, there is a default list of separators, and you can add more with a set of methods. Here, ordinaryChar( ) is used to say: “this character has no significance that I’m interested in,” so the parser doesn’t include it as part of any of the words it creates. You can find more information in the on-line documentation that comes with Java. In countWords( ), the tokens are pulled one at a time from the stream, and the ttype information is used to determine what to do with each token, since a token can be an end- of-line, a number, a string, or a single character. Once a token is found, the Hashtable counts is queried to see if it already contains the token as a key. If it does, the corresponding Counter object is incremented to indicate that another instance of this word has been found. If not, a new Counter is created – since the Counter constructor initializes its value to one, this also acts to count the word. SortedWordCount is not a type of Hashtable, so it wasn’t inherited. It performs a specific type of functionality, so even though the keys( ) and values( ) methods must be re-exposed, that still doesn’t mean that inheritance should be used since there are a Chapter 10: The Java IO System 323
- 324. number of Hashtable methods that are inappropriate here. In addition, other methods getCounter( ), which get the Counter for a particular String, and sortedKeys( ), which produces an Enumeration, finish the change in the shape of SortedWordCount’s interface. In main( ) you can see the use of a SortedWordCount to open and count the words in a file – it just takes two lines of code. Then an enumeration to a sorted list of keys (words) is extracted, and this is used to pull out each key and associated Count. Note that in this case, cleanup( ) is not necessary, since upon exiting the program the file will be closed. However, you can’t normally assume that the SortedWordCount object is being called from main( ). A second example using StreamTokenizer can be found in Chapter 17. StringTokenizer Although it isn’t part of the IO library, the StringTokenizer has sufficiently similar functionality to StreamTokenizer that it will be described here. The StringTokenizer returns, one at a time, the tokens within a string. These tokens are consecutive characters delimited by tabs, spaces, and newlines. Thus the tokens of the string “Where is my cat?” are “Where”, “is”, “my”, and “cat?” Unlike the StreamTokenizer, however, you cannot tell the StringTokenizer to break up the input in any way that you want. It’s very limited, and you can use only the rules that it has hard- wired into it. Thus, if it’s a very simple tokenization you need, StringTokenizer is fine, but if you need more sophistication you’ll have to use a StreamTokenizer. You ask a StringTokenizer object for the next token in the string using the next( ) method, which either returns the token or an empty string to indicate that no tokens remain. As an example, the following program performs a very limited analysis of a sentence, looking for key phrase sequences to indicate whether happiness or sadness is implied. //: AnalyzeSentence.java // Look for particular sequences // within sentences. import java.util.*; public class AnalyzeSentence { public static void main(String args[]) { analyze("I am happy about this"); analyze("I am not happy about this"); analyze("I am not! I am happy"); analyze("I am sad about this"); analyze("I am not sad about this"); analyze("I am not! I am sad"); analyze("Are you happy about this?"); analyze("Are you sad about this?"); analyze("It's you! I am happy"); analyze("It's you! I am sad"); } static StringTokenizer st; static void analyze(String s) { prt("nnew sentence >> " + s); boolean sad = false; 324 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 325. st = new StringTokenizer(s); while (st.hasMoreTokens()) { String token = next(); // Look until you find one of the // two starting tokens: if(!token.equals("I") && !token.equals("Are")) continue; // Top of while loop if(token.equals("I")) { String tk2 = next(); if(!tk2.equals("am")) // Must be after I break; // Out of while loop else { String tk3 = next(); if(tk3.equals("sad")) { sad = true; break; // Out of while loop } if (tk3.equals("not")) { String tk4 = next(); if(tk4.equals("sad")) break; // Leave sad false if(tk4.equals("happy")) { sad = true; break; } } } } if(token.equals("Are")) { String tk2 = next(); if(!tk2.equals("you")) break; // Must be after Are String tk3 = next(); if(tk3.equals("sad")) sad = true; break; // Out of while loop } } if(sad) prt("Sad detected"); } static String next() { if(st.hasMoreTokens()) { String s = st.nextToken(); prt(s); return s; } else return ""; } static void prt(String s) { System.out.println(s); } } ///:~ Chapter 10: The Java IO System 325
- 326. For each string being analyzed, a while loop is entered and tokens are pulled off the string. Notice the first if statement, which says to continue (go back to the beginning of the loop and start again) if the token is neither an “I” or an “Are.” This means it will get tokens until an “I” or an “Are” is found. You might think to use the == instead of the equals( ) method, but that won’t work correctly, since == compares handle values while equals( ) compares contents. The logic of the rest of the analyze( ) method is that the pattern that’s being searched for is “I am sad” or “I am not happy” or “Are you sad.” Without the break statement, the code for this would be even messier than it is. You should be aware that a typical parser (this is a very primitive example of one) normally has a table of these tokens and a piece of code that moves through the states in the table as new tokens are read. You should think of the StringTokenizer only as shorthand for a very simple and specific kind of StreamTokenizer. However, if you have a String that you want to tokenize and StringTokenizer is too limited, all you have to do is turn it into a stream with StringBufferInputStream and then use that to create a much more powerful StreamTokenizer. Java 1.1 IO streams At this point you may be scratching your head, wondering “is there another design for IO streams that could possibly require more typing? Could someone have come up with an odder design?” Prepare yourself: Java 1.1 makes some significant modifications to the IO stream library. When you see the Reader and Writer classes your first thought (like mine) might be that these were meant to replace the InputStream and OutputStream classes. But that’s not the case. Although some aspects of the original streams library are deprecated (if you use them you will receive a warning from the compiler), the old streams have been left in for backwards compatibility and: 1. New classes have been put into the old hierarchy, so it’s obvious that Sun is not abandoning the old streams. 2. There are times when you’re supposed to use classes in the old hierarchy in combination with classes in the new hierarchy, and to accomplish this there are “bridge” classes: InputStreamReader converts an InputStream to a Reader and OutputStreamWriter converts an OutputStream to a Writer. As a result there are situations where you actually have more layers of wrapping with the new IO stream library than the old. It’s fairly apparent that someone at Sun is quite fond of this design direction so we’ll all have to get used to it. The most important reason for adding the Reader and Writer hierarchies in Java 1.1 is for internationalization. The old IO stream hierarchy supports only 8-bit byte streams, and doesn’t handle the 16-bit Unicode characters very well. Since Unicode is used for internationalization (and Java’s native char is 16-bit Unicode), the Reader and Writer hierarchies were added to support Unicode in all IO operations. In addition, the new libraries are designed for faster operations than the old. As is the practice in this book, I will attempt to provide an overview of the classes but assume that you will use online documentation to determine all the details such as the exhaustive list of methods. 326 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 327. Sources & sinks of data Almost all the Java 1.0 IO stream classes have corresponding Java 1.1 classes to provide native Unicode manipulation. It would be easiest to say: “always use the new classes, never use the old ones” but things are not that simple. Sometimes you are forced into using the Java 1.0 IO stream classes because of the library design; in particular the java.util.zip libraries are new additions to the old stream library and they rely on old stream components. So the most sensible approach to take is to try to use the Reader and Writer classes whenever you can, and you’ll discover the situations when you have to drop back into the old libraries because your code won’t compile. Here is a table that shows the correspondence between the sources and sinks of information (that is, where the data physically comes from or goes to) in the old and new libraries. Sources & Sinks: Corresponding Java 1.1 class Java 1.0 class InputStream Reader converter: InputStreamReader OutputStream Writer converter: OutputStreamWriter FileInputStream FileReader FileOutputStream FileWriter StringBufferInputStream StringReader (no corresponding class) StringWriter ByteArrayInputStream CharArrayReader ByteArrayOutputStream CharArrayWriter PipedInputStream PipedReader PipedOutputStream PipedWriter In general you’ll find that the interfaces in the old library components and the new ones are similar if not identical. Modifying stream behavior In Java 1.0, streams were adapted for particular needs using subclasses of FilterInputStream and FilterOutputStream (in “design patterns” parlance, described later in the book, these would be called adapters, although this particular design shows a poor understanding of that pattern). Java 1.1 IO streams continues the use of this idea, but the model of deriving all the adapters from the same base class is not followed, which can make it a bit confusing if you’re trying to understand it by looking at the class hierarchy. In the following table, the correspondence is a rougher approximation than in the previous table. The difference is because of the class organization: while BufferedOutputStream is a subclass of FilterOutputStream, BufferedWriter is not a subclass of FilterWriter (which, even though it is abstract, has no subclasses and so appears to have been put in Chapter 10: The Java IO System 327
- 328. either as a placeholder or simply so you wouldn’t wonder where it was). However, the interfaces to the classes themselves are quite a close match and it’s apparent that you’re supposed to use the new versions instead of the old whenever possible (that is, except in cases where you’re forced to produce a Stream instead of a Reader or Writer). Filters: Corresponding Java 1.1 class Java 1.0 class FilterInputStream FilterReader FilterOutputStream FilterWriter (abstract class with no subclasses) BufferedInputStream BufferedReader (also has readLine( )) BufferedOutputStream BufferedWriter DataInputStream use DataInputStream (Except when you need to use readLine( ), when you should use a BufferedReader) PrintStream PrintWriter LineNumberInputStream LineNumberReader StreamTokenizer StreamTokenizer (use constructor that takes a Reader instead) PushBackInputStream PushBackReader There’s one direction that’s quite clear: whenever you want to use readLine( ), you shouldn’t do it with a DataInputStream any more (this is met with a deprecation message at compile time), but instead use a BufferedReader. Other than this, DataInputStream is still a “preferred” member of the new Java 1.1 IO library. To make the transition to using a PrintWriter easier, it has constructors that take any OutputStream object. However, PrintWriter itself has no more support for formatting than PrintStream does; the interfaces are virtually the same. Unchanged classes Apparently, the Java library designers felt that they got some of the classes right the first time so there were no changes to these and you can go on using them as they are: Java 1.0 classes w/o corresponding Java 1.1 classes DataOutputStream File RandomAccessFile SequenceInputStream 328 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 329. The DataOutputStream, in particular, is used without change, so for storing and retrieving data in a transportable format you’re forced to stay in the InputStream and OutputStream hierarchies. An example To see the effect of the new classes, let’s look at the appropriate portion of the IOStreamDemo.java example modified to use the Reader and Writer classes: //: NewIODemo.java // Java 1.1 IO typical usage import java.io.*; public class NewIODemo { public static void main(String args[]) { try { // 1. Reading input by lines: BufferedReader in = new BufferedReader( new FileReader(args[0])); String s, s2 = new String(); while((s = in.readLine())!= null) s2 += s + "n"; in.close(); // 1b. Reading standard input: BufferedReader stdin = new BufferedReader( new InputStreamReader(System.in)); System.out.print("Enter a line:"); System.out.println(stdin.readLine()); // 2. Input from memory StringReader in2 = new StringReader(s2); int c; while((c = in2.read()) != -1) System.out.print((char)c); // 3. Formatted memory input try { DataInputStream in3 = new DataInputStream( // Oops: must use deprecated class: new StringBufferInputStream(s2)); while(true) System.out.print((char)in3.readByte()); } catch(EOFException e) { System.out.println("End of stream"); } // 4. Line numbering & file output try { LineNumberReader li = new LineNumberReader( Chapter 10: The Java IO System 329
- 330. new StringReader(s2)); BufferedReader in4 = new BufferedReader(li); PrintWriter out1 = new PrintWriter( new BufferedWriter( new FileWriter("IODemo.out"))); while((s = in4.readLine()) != null ) out1.println( "Line " + li.getLineNumber() + s); out1.close(); } catch(EOFException e) { System.out.println("End of stream"); } // 5. Storing & recovering data try { DataOutputStream out2 = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("Data.txt"))); out2.writeDouble(3.14159); out2.writeBytes("That was pi"); out2.close(); DataInputStream in5 = new DataInputStream( new BufferedInputStream( new FileInputStream("Data.txt"))); BufferedReader in5br = new BufferedReader( new InputStreamReader(in5)); // Must use DataInputStream for data: System.out.println(in5.readDouble()); // Can now use the "proper" readLine(): System.out.println(in5br.readLine()); } catch(EOFException e) { System.out.println("End of stream"); } // 6. Reading and writing random access // files is the same as before. // (not repeated here) } catch(FileNotFoundException e) { System.out.println( "File Not Found:" + args[1]); } catch(IOException e) { System.out.println("IO Exception"); } } } ///:~ In general, you’ll see that the conversion is fairly straightforward and the code looks quite similar. There are some important differences, though. First of all, since random access files have not changed, Section 6 is not repeated. 330 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 331. Section 1 actually shrinks a bit since, if all you’re doing is reading line input you need only to wrap a BufferedReader around a FileReader. Section 1b shows the new way to wrap System.in for reading console input, and this expands because System.in is a DataInputStream and BufferedReader needs a Reader argument, so InputStreamReader is brought in to perform the translation. In section 2 you can see that if you have a String and want to read from it you just use a StringReader instead of a StringBufferInputStream and the rest of the code is identical. Section 3 shows a bug in the design of the new IO stream library. If you have a String and you want to read from it, you’re not supposed to use a StringBufferInputStream any more. When you compile code involving a StringBufferInputStream constructor, you get a deprecation message telling you not to use it. Instead you’re supposed to use a StringReader. However, if you want to do formatted memory input as in Section 3, you’re forced to use a DataInputStream – there is no “DataReader” to replace it – and a DataInputStream constructor requires an InputStream argument. Thus you have no choice but to use the deprecated StringBufferInputStream class. The compiler will give you a deprecation message but there’s nothing you can do about it.1 Section 4 is a reasonably straightforward translation from the old streams to the new, with no surprises. In section 5, you’re forced to use all the old streams classes because DataOutputStream and DataInputStream require them and there are no alternatives. However, you don’t get any deprecation messages at compile time. If a stream is deprecated, typically its constructor produces a deprecation message to prevent you from using the entire class, but in the case of DataInputStream only the readLine( ) method is deprecated since you’re supposed to use a BufferedReader for readLine( ) (but a DataInputStream for all other formatted input). If you compare section 5 with that section in IOStreamDemo.java, you’ll notice that in this version, the data is written before the text. That’s because a bug was introduced in Java 1.1, which is shown in the following code: //: IOBug.java // Java 1.1 (and higher?) IO Bug import java.io.*; public class IOBug { public static void main(String args[]) throws Exception { DataOutputStream out = new DataOutputStream( new BufferedOutputStream( new FileOutputStream("Data.txt"))); out.writeDouble(3.14159); out.writeBytes("That was the value of pin"); out.writeBytes("This is the value of pi/2:n"); out.writeDouble(3.14159/2); out.close(); DataInputStream in = new DataInputStream( new BufferedInputStream( new FileInputStream("Data.txt"))); BufferedReader inbr = new BufferedReader( 1 Perhaps by the time you read this, the bug will be fixed. Chapter 10: The Java IO System 331
- 332. new InputStreamReader(in)); // The doubles written BEFORE the line of text // read back correctly: System.out.println(in.readDouble()); // Read the lines of text: System.out.println(inbr.readLine()); System.out.println(inbr.readLine()); // Trying to read the doubles after the line // produces an end-of-file exception: System.out.println(in.readDouble()); } } ///:~ It appears that anything you write after a call to writeBytes( ) is not recoverable. This is a rather limiting bug, and we can hope that it will be fixed by the time you read this. You should run the above program to test it; if you don’t get an exception and the values print correctly then you’re out of the woods. Redirecting standard IO Java 1.1 has added methods in class System that allow you to redirect the standard input, output and error IO streams using simple static method calls: setIn(InputStream) setOut(PrintStream) setErr(PrintStream) Redirecting output is especially useful if you suddenly start creating a large amount of output on your screen and it’s scrolling past faster than you can read it. Redirecting input is valuable for a command-line program when you want to test a particular user-input sequence again and again. Here’s a simple example that shows the use of these methods: //: Redirecting.java // Demonstrates the use of redirection for // standard IO in Java 1.1 import java.io.*; class Redirecting { public static void main(String args[]) { try { BufferedInputStream in = new BufferedInputStream( new FileInputStream( "Redirecting.java")); // Produces deprecation message: PrintStream out = new PrintStream( new BufferedOutputStream( new FileOutputStream("test.out"))); System.setIn(in); System.setOut(out); System.setErr(out); BufferedReader br = new BufferedReader( new InputStreamReader(System.in)); 332 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 333. String s; while((s = br.readLine()) != null) System.out.println(s); out.close(); // Remember this! } catch(IOException e) { e.printStackTrace(); } } } ///:~ This program simply attaches standard input to a file, and redirects standard output and standard error to another file. This is another example where a deprecation message is inevitable. The message you can get when compiling with the -deprecation flag is: Note: The constructor java.io.PrintStream(java.io.OutputStream) has been deprecated. However, both System.setOut( ) and System.setErr( ) require a PrintStream object as an argument, so you are forced to call the PrintStream constructor. You might wonder, if Java 1.1 deprecates the entire PrintStream class by deprecating the constructor, why would the library designers, at the same time as they added this deprecation, also add new methods to System that required a PrintStream rather than a PrintWriter which is the new and preferred replacement? It’s a mystery. Compression Java 1.1 has also added some classes to support reading and writing streams in a compressed format. These are simply wrapped around existing IO classes to provide compression functionality. One aspect of these new Java 1.1 classes stands out: they are not derived from the new Reader and Writer classes, but instead are part of the InputStream and OutputStream hierarchies. Thus you may be forced to mix the two types of streams (remember that you can use InputStreamReader and OutputStreamWriter to provide easy conversion between one type and another). Java 1.1 Compression Function class CheckedInputStream GetCheckSum( ) produces checksum for any InputStream (not just decompression) CheckedOutputStream GetCheckSum( ) produces checksum for any OutputStream (not just compression) DeflaterOutputStream Base class for compression classes ZipOutputStream A DeflaterOutputStream that compresses data into the Zip file format GZIPOutputStream A DeflaterOutputStream that compresses data into the GZIP file format InflaterInputStream Base class for decompression classes Chapter 10: The Java IO System 333
- 334. Java 1.1 Compression Function class ZipInputStream A DeflaterInputStream that Decompresses data that has been stored in the Zip file format GZIPInputStream A DeflaterInputStream that Decompresses data that has been stored in the GZIP file format Although there are many compression algorithms, these two (Zip and GZIP) are possibly the most commonly used, and as a result there are many tools for reading and writing these formats. Simple compression with GZIP The GZIP interface is very simple and thus is probably more appropriate when you have a single stream of data you want to compress (rather than a collection of dissimilar pieces of data). Here’s an example that compresses a single file: //: GZIPcompress.java // Uses Java 1.1 GZIP compression to compress // a file whose name is passed on the command // line. import java.io.*; import java.util.zip.*; public class GZIPcompress { public static void main(String args[]) { try { BufferedReader in = new BufferedReader( new FileReader(args[0])); BufferedOutputStream out = new BufferedOutputStream( new GZIPOutputStream( new FileOutputStream("test.gz"))); System.out.println("Writing file"); int c; while((c = in.read()) != -1) out.write(c); in.close(); out.close(); System.out.println("Reading file"); BufferedReader in2 = new BufferedReader( new InputStreamReader( new GZIPInputStream( new FileInputStream("test.gz")))); String s; while((s = in2.readLine()) != null) System.out.println(s); } catch(Exception e) { 334 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 335. e.printStackTrace(); } } } ///:~ The use of the compression classes is straightforward – you simply wrap your output stream in a GZIPOutputStream or ZipOutputStream and your input stream in a GZIPInputStream or ZipInputStream. All else is ordinary IO reading and writing. This is, however, a good example of where you’re forced to mix the old IO streams with the new: in uses the Reader classes, whereas GZIPOutputStream’s constructor can accept only an OutputStream object, not a Writer object. Multi-file storage with zip The Java 1.1 library supporting the Zip format is much more extensive. With it you can easily store multiple files, and there’s even a separate class to make the process of reading a Zip file very easy. The library uses the standard Zip format so it works seamlessly with all the tools currently downloadable on the Internet. The following example has the same form as the previous example, but it handles as many command-line arguments as you wish. In addition, it shows the use of the Checksum classes to calculate and verify the checksum for the file. There are two Checksum types: Adler32 (which is faster) and CRC32 (which is slower but slightly more accurate). //: ZipCompress.java // Uses Java 1.1 Zip compression to compress // any number of files whose names are passed // on the command line. import java.io.*; import java.util.*; import java.util.zip.*; public class ZipCompress { public static void main(String args[]) { try { FileOutputStream f = new FileOutputStream("test.zip"); CheckedOutputStream csum = new CheckedOutputStream( f, new Adler32()); ZipOutputStream out = new ZipOutputStream( new BufferedOutputStream(csum)); out.setComment("A test of Java Zipping"); // Can't read the above comment, though for(int i = 0; i < args.length; i++) { System.out.println( "Writing file " + args[i]); BufferedReader in = new BufferedReader( new FileReader(args[i])); out.putNextEntry(new ZipEntry(args[i])); int c; while((c = in.read()) != -1) out.write(c); in.close(); } Chapter 10: The Java IO System 335
- 336. out.close(); // Checksum valid only after the file // has been closed! System.out.println("Checksum: " + csum.getChecksum().getValue()); // Now extract the files: System.out.println("Reading file"); FileInputStream fi = new FileInputStream("test.zip"); CheckedInputStream csumi = new CheckedInputStream( fi, new Adler32()); ZipInputStream in2 = new ZipInputStream( new BufferedInputStream(csumi)); ZipEntry ze; System.out.println("Checksum: " + csumi.getChecksum().getValue()); while((ze = in2.getNextEntry()) != null) { System.out.println("Reading file " + ze); int x; while((x = in2.read()) != -1) System.out.write(x); } in2.close(); // Alternative way to open and read // zip files: ZipFile zf = new ZipFile("test.zip"); Enumeration e = zf.entries(); while(e.hasMoreElements()) { ZipEntry ze2 = (ZipEntry)e.nextElement(); System.out.println("File: " + ze2); // ... and extract the data as before } } catch(Exception e) { e.printStackTrace(); } } } ///:~ For each file to add to the archive, you must call putNextEntry( ) and pass it a ZipEntry object. The ZipEntry object contains an extensive interface that allows you to get and set all the data available on that particular entry in your Zip file: name, compressed and uncompressed sizes, date, CRC checksum, extra field data, comment, compression method, and whether it’s a directory entry. However, even though the Zip format has a way to set a password, this is not supported in Java’s Zip library. In addition, although CheckedInputStream and CheckedOutputStream support both Adler32 and CRC32 checksums, the ZipEntry class supports only an interface for CRC. This is a restriction of the underlying Zip format, but it may limit you from using the faster Adler32. To extract files, ZipInputStream has a getNextEntry( ) method that returns the next ZipEntry if there is one. As a more succinct alternative, you can read the file using a ZipFile object, which has a method entries( ) to return an Enumeration to the ZipEntries. 336 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 337. In order to read the checksum, you must somehow have access to the associated Checksum object. Here, a handle to the CheckedOutputStream and CheckedInputStream objects is retained, but you could also just hold on to a handle to the Checksum object itself. A baffling method in Zip streams is setComment( ). As shown above, you can set a comment when you’re writing a file, but there’s no way to recover the comment in the ZipInputStream. Comments appear to be fully supported on an entry-by-entry basis only via ZipEntry. Of course you are not limited to files when using the GZIP or Zip libraries – you can compress anything, including data to be sent through a network connection. The Java archive (jar) utility The Zip format is also used in the Java 1.1 JAR (Java ARchive) file format, which is a way to collect a group of files into a single compressed file, just like Zip. However, like everything else in Java, JAR files are cross-platform so you don't need to worry about platform issues. You can also include audio and image files as well as class files. JAR files are particularly helpful when dealing with the internet. Before JAR files, your Web browser would have to make repeated requests of a Web server in order to download all the files that make up an applet. In addition, each of these files was uncompressed. By combining all the files for a particular applet into a single JAR file, only one server request is necessary and the transfer is faster because of compression. In addition, each entry in a JAR file may be digitally signed for security. There’s a utility called jar that comes with Sun’s JDK. This automatically compresses the files of your choice. A JAR file consists of a single file containing a collection of zipped files along with a “manifest” that describes them (you can create your own manifest file; otherwise the jar program will do it for you). You can find out more about JAR manifests in the online documentation. You use the jar utility by invoking in on the command line: jar [options] [manifest] destination input-file [input-files] The options are simply a collection of letters (no hyphen or any other indicator is necessary). These are: c Creates a new or empty archive. t Lists the table of contents. x Extracts all files x file Extracts the named file f Says: “I’m going to give you the name of the file.” If you don’t use this, jar assumes that its input will come from standard input, or, if it is creating a file, its output will go to standard output. m Says that the first argument will be the name of the user-created manifest file v Generates verbose output describing what jar is doing Chapter 10: The Java IO System 337
- 338. O Only store the files; doesn’t compress the files (use to create a JAR file that you can put in your classpath) M Don’t automatically create a manifest file If a subdirectory is included in the files to be put into the JAR file, that subdirectory is automatically added, including all its subdirectories, etc. Path information is also preserved. Here are some typical ways to invoke jar: jar cf myJarFile.jar *.class This creates a JAR file called myJarFile.jar which contains all the class files in the current directory, along with an automatically-generated manifest file. jar cmf myManifestFile.mf myJarFile.jar *.class Like the previous example, but also adds a user-created manifest file called myManifestFile.mf. jar tf myJarFile.jar Produces a table of contents of the files in myJarFile.jar. jar tvf myJarFile.jar Adds the “verbose” flag to give more detailed information about the files in myJarFile.jar. jar cvf myApp.jar audio classes image Assuming audio, classes and image are subdirectories, combines all the subdirectories together into the file myApp.jar. The “verbose” flag is also included to give extra feedback while the jar program is working. If you create a JAR file using the O option, that file can be placed in your CLASSPATH: CLASSPATH="lib1.jar;lib2.jar;" Then Java can search lib1.jar and lib2.jar for class files. The jar tool isn’t as useful as a zip utility. For example, you can’t add or update files to an already-created JAR file; you can create JAR files only from scratch. Also, you can’t move files into a JAR file, erasing them as they are moved. However, a JAR file created on one platform will be transparently readable by the jar tool on any other platform (a problem that often plagues zip utilities). As you shall see in Chapter 13, JAR files are also used to package Java Beans. Object serialization Java 1.1 has added a very interesting feature called object serialization that allows you to take any object that implements the Serializable interface and turn it into a sequence of bytes that can later be fully restored into the original object. This is even true across a network, which means that the serialization mechanism automatically compensates for differences in operating systems. That is, you can create an object on a Windows machine, serialize it, and send it across the network to a Unix machine where it will be correctly 338 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 339. reconstructed. You don’t have to worry about the data representations on the different machines, the byte ordering, or any other details. By itself, object serialization is interesting because it allows you to implement lightweight persistence. Remember that persistence means an object’s lifetime is not determined by whether a program is executing or not – the object lives in between invocations of the program. By taking a serializable object and writing it to disk, then restoring that object when the program is re-invoked, you’re able to produce the effect of persistence. The reason it’s called “lightweight” is that you can’t simply define an object using some kind of “persistent” keyword and let the system take care of the details (although this may happen in the future). Object serialization was added to the language to support two major features. Java 1.1’s remote method invocation (RMI) allows objects that live on other machines to behave as if they live on your local machine. When sending messages to remote objects, object serialization is necessary to transport the arguments and return values. RMI is discussed later, in Chapter 15. Object serialization is also necessary for Java Beans, introduced in Java 1.1. When a Bean is used its state information is generally configured at design time. This state information must be stored and later recovered when the program is started; object serialization performs this task. Serializing an object is quite simple, as long as the object implements the Serializable interface (this interface is just a flag, and has no methods). In Java 1.1, many standard library classes have been changed so they’re serializable, including all the wrappers for the primitive types, all the collection classes, and many others. Even Class objects can be serialized (see Chapter 11 for the implications of this). To serialize an object, you create some sort of OutputStream object and then wrap it inside an ObjectOutputStream object. At this point you need only call writeObject( ) and your object is magically serialized and sent to the OutputStream. To reverse the process, you wrap an InputStream inside an ObjectInputStream and call readObject( ). What comes back is, as usual, a handle to an upcast Object, so you must downcast to set things straight. A particularly clever aspect of object serialization is that it not only saves an image of your object but it also follows all the handles contained in your object and saves those objects, and follows all the handles in each of those objects, etc. This is sometimes referred to as the “web of objects” that a single object may be connected to, and it includes arrays of handles to objects as well as member objects. If you had to maintain your own object serialization scheme, maintaining the code to follow all these links would be a bit mind- boggling. However, Java object serialization seems to pull it off flawlessly, no doubt using an optimized algorithm that traverses the web of objects. The following example tests the serialization mechanism by making a “worm” of linked objects, each of which has a link to the next segment in the worm as well as an array of handles to objects of a different class, Data: //: Worm.java // Demonstrates object serialization in Java 1.1 import java.io.*; class Data implements Serializable { private int i; Data(int x) { i = x; } public String toString() { return Integer.toString(i); } Chapter 10: The Java IO System 339
- 340. } public class Worm implements Serializable { // Generate a random int value: private static int r() { return (int)(Math.random() * 10); } private Data[] d = { new Data(r()), new Data(r()), new Data(r()) }; private Worm next; private char c; // Value of i == number of segments Worm(int i, char x) { System.out.println(" Worm constructor: " + i); c = x; if(--i > 0) next = new Worm(i, (char)(x + 1)); } Worm() { System.out.println("Default constructor"); } public String toString() { String s = ":" + c + "("; for(int i = 0; i < d.length; i++) s += d[i].toString(); s += ")"; if(next != null) s += next.toString(); return s; } public static void main(String args[]) { Worm w = new Worm(6, 'a'); System.out.println("w = " + w); try { ObjectOutputStream out = new ObjectOutputStream( new FileOutputStream("worm.out")); out.writeObject("Worm storage"); out.writeObject(w); out.close(); // Also flushes output ObjectInputStream in = new ObjectInputStream( new FileInputStream("worm.out")); String s = (String)in.readObject(); Worm w2 = (Worm)in.readObject(); System.out.println(s + ", w2 = " + w2); } catch(Exception e) { e.printStackTrace(); } try { ByteArrayOutputStream bout = new ByteArrayOutputStream(); ObjectOutputStream out = new ObjectOutputStream(bout); 340 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 341. out.writeObject("Worm storage"); out.writeObject(w); out.flush(); ObjectInputStream in = new ObjectInputStream( new ByteArrayInputStream( bout.toByteArray())); String s = (String)in.readObject(); Worm w3 = (Worm)in.readObject(); System.out.println(s + ", w3 = " + w3); } catch(Exception e) { e.printStackTrace(); } } } ///:~ To make things interesting, the array of Data objects inside Worm are initialized with random numbers (this way you don’t suspect the compiler of keeping some kind of meta- information around). Each Worm segment is labeled with a char that’s automatically generated in the process of recursively generating the linked list of Worms. When you create a Worm, you tell the constructor how long you want it to be. To make the next handle it calls the Worm constructor with a length of one less, etc. The last next handle is left as null, indicating the end of the Worm. The point of all this was to make something reasonably complex that couldn’t easily be serialized. The actual act of serializing, however, is quite simple. Once the ObjectOutputStream is created from some other stream, writeObject( ) serializes the object. Notice the call to writeObject( ) for a String, as well. You can also write all the primitive data types using the same methods as DataOutputStream (they share the same interface). There are two separate try blocks that look very similar. The first writes and reads a file and the second, for variety, writes and reads a ByteArray. You can read and write an object using serialization to any DataInputStream or DataOutputStream including, as you shall see in the networking chapter, a network. The output from one run was: Worm constructor: 6 Worm constructor: 5 Worm constructor: 4 Worm constructor: 3 Worm constructor: 2 Worm constructor: 1 w = :a(262):b(100):c(396):d(480):e(316):f(398) Worm storage, w2 = :a(262):b(100):c(396):d(480):e(316):f(398) Worm storage, w3 = :a(262):b(100):c(396):d(480):e(316):f(398) You can see that the deserialized object really does contain all the links that were in the original object. Notice that no constructor, not even the default constructor, is called in the process of deserializing a Serializable object. The entire object is restored by recovering data from the InputStream. Object serialization is another new Java 1.1 feature that is not part of the new Reader and Writer hierarchies, but instead uses the old InputStream and OutputStream hierarchies. Thus you may encounter situations where you’re forced to mix the two hierarchies. Chapter 10: The Java IO System 341
- 342. Finding the class You may wonder “what’s necessary for an object to be recovered from its serialized state?” For example, suppose you serialize an object and send it as a file or through a network to another machine. Could a program on the other machine reconstruct the object using only the contents of the file? The best way to answer this question is (as usual) by performing an experiment. The following file goes in the subdirectory for this chapter: //: Alien.java // A serializable class import java.io.*; public class Alien implements Serializable { } ///:~ The file that creates and serializes an Alien object goes in the same directory: //: FreezeAlien.java // Create a serialized output file import java.io.*; public class FreezeAlien { public static void main(String args[]) throws Exception { ObjectOutput out = new ObjectOutputStream( new FileOutputStream("file.x")); Alien zorcon = new Alien(); out.writeObject(zorcon); } } ///:~ Rather than catching and handling exceptions, this program takes the quick and dirty approach of just passing the exceptions out of main( ), so they’ll be reported on the command line. Once the program is compiled and run, copy the resulting file.x to a subdirectory called xfiles, where the following code goes: //: ThawAlien.java // Try to recover a serialized file without the // class of object that's stored in that file. package c10.xfiles; import java.io.*; public class ThawAlien { public static void main(String args[]) throws Exception { ObjectInputStream in = new ObjectInputStream( new FileInputStream("file.x")); Object mystery = in.readObject(); System.out.println( mystery.getClass().toString()); } 342 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 343. } ///:~ This program opens the file and reads in the object mystery successfully. However, as soon as you try to find out anything about the object – which requires the Class object for Alien – the Java Virtual Machine (JVM) cannot find Alien.class (unless it happens to be in the Classpath, which it shouldn’t be in this example) and you’ll get a ClassNotFoundException (once again, all evidence of alien life vanishes before proof of its existence can be verified!). This means that if you expect to be able to do much after you’ve recovered an object that has been serialized, you’ll need to make sure that the JVM can find the associated .class file, either in the local class path or somewhere on the Internet. Controlling serialization As you can see, the default serialization mechanism is trivial to use. But what if you have special needs? Perhaps you have special security issues and you don’t want to serialize portions of your object, or perhaps it just doesn’t make sense for one sub-object to be serialized if that part needs to be created anew when the object is recovered. You can control the process of serialization by implementing the Externalizable interface instead of the Serializable interface. The Externalizable interface extends the Serializable interface and adds two methods, writeExternal( ) and readExternal( ) which are automatically called for your object during serialization and deserialization so that you can perform your special operations. The following example shows very simple implementations of the Externalizable interface methods. Notice that Blip1 and Blip2 are nearly identical except for a subtle difference (see if you can discover it by looking at the code): //: Blips.java // Simple use of Externalizable & a pitfall import java.io.*; import java.util.*; class Blip1 implements Externalizable { public Blip1() { System.out.println("Blip1 Constructor"); } public void writeExternal(ObjectOutput out) throws IOException { System.out.println("Blip1.writeExternal"); } public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { System.out.println("Blip1.readExternal"); } } class Blip2 implements Externalizable { Blip2() { System.out.println("Blip2 Constructor"); } public void writeExternal(ObjectOutput out) throws IOException { System.out.println("Blip2.writeExternal"); } Chapter 10: The Java IO System 343
- 344. public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { System.out.println("Blip2.readExternal"); } } public class Blips { public static void main(String args[]) { System.out.println("Constructing objects:"); Blip1 b1 = new Blip1(); Blip2 b2 = new Blip2(); try { ObjectOutputStream o = new ObjectOutputStream( new FileOutputStream("Blips.out")); System.out.println("Saving objects:"); o.writeObject(b1); o.writeObject(b2); o.close(); // Now get them back: ObjectInputStream in = new ObjectInputStream( new FileInputStream("Blips.out")); System.out.println("Recovering b1:"); b1 = (Blip1)in.readObject(); // OOPS! Throws an exception: //! System.out.println("Recovering b2:"); //! b2 = (Blip2)in.readObject(); } catch(Exception e) { e.printStackTrace(); } } } ///:~ The output for this program is: Constructing objects: Blip1 Constructor Blip2 Constructor Saving objects: Blip1.writeExternal Blip2.writeExternal Recovering b1: Blip1 Constructor Blip1.readExternal The reason that the Blip2 object is not recovered is that trying to do so causes an exception. Can you see the difference between Blip1 and Blip2? The constructor for Blip1 is public, while the constructor for Blip2 is not, and that causes the exception upon recovery. Try making Blip2’s constructor public and removing the //! comments to see the correct results. When b1 is recovered, the Blip1 default constructor is called. This is different from recovering a Serializable object, where the object is entirely constructed from its stored bits, with no constructor calls. With an Externalizable object, all the normal default construction behavior occurs (including the initializations at the point of field definition), 344 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 345. and then readExternal( ) is called. You need to be aware of this – in particular the fact that all the default construction always takes place – to produce the correct behavior in your Externalizable objects. Here’s an example that shows what you must do to fully store and retrieve an Externalizable object: //: Blip3.java // Reconstructing an externalizable object import java.io.*; import java.util.*; class Blip3 implements Externalizable { int i; String s; // No initialization public Blip3() { System.out.println("Blip3 Constructor"); // s, i not initialized } public Blip3(String x, int a) { System.out.println("Blip3(String x, int a)"); s = x; i = a; // s & i initialized only in non-default // constructor. } public String toString() { return s + i; } public void writeExternal(ObjectOutput out) throws IOException { System.out.println("Blip3.writeExternal"); // You must do this: out.writeObject(s); out.writeInt(i); } public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { System.out.println("Blip3.readExternal"); // You must do this: s = (String)in.readObject(); i =in.readInt(); } public static void main(String args[]) { System.out.println("Constructing objects:"); Blip3 b3 = new Blip3("A String ", 47); System.out.println(b3.toString()); try { ObjectOutputStream o = new ObjectOutputStream( new FileOutputStream("Blip3.out")); System.out.println("Saving object:"); o.writeObject(b3); o.close(); // Now get it back: ObjectInputStream in = new ObjectInputStream( new FileInputStream("Blip3.out")); System.out.println("Recovering b3:"); b3 = (Blip3)in.readObject(); Chapter 10: The Java IO System 345
- 346. System.out.println(b3.toString()); } catch(Exception e) { e.printStackTrace(); } } } ///:~ The fields s and i are initialized only in the second constructor, but not in the default constructor. This means that if you don’t initialize s and i in readExternal, it will be null (since the storage for the object gets wiped to zero in the very first step of object creation). If you comment out the two lines of code following the phrases “You must do this” and run the program, you’ll see that when the object is recovered, s is null and i is zero. If you are inheriting from an Externalizable object, you’ll typically call the base-class versions of writeExternal( ) and readExternal( ) to provide proper storage and retrieval of the base-class components. Thus, to make things work correctly you must not only write the important data from the object during the writeExternal( ) method (there is no default behavior that writes any of the member objects for an Externalizable object), but you must also recover that data in the readExternal( ) method. This can be a bit confusing at first because the default construction behavior for an Externalizable object can make it seem like some kind of storage and retrieval takes place automatically; it does not. The transient keyword When you’re controlling serialization, there may be a particular subobject that you don’t want Java’s serialization mechanism to automatically save and restore. This is commonly the case if that subobject represents sensitive information that you don’t want to serialize, such as a password. Even if that information is private in the object, once it’s serialized it’s possible for someone to access it by reading a file or intercepting a network transmission. One way to prevent sensitive parts of your object from being serialized is to implement your class as Externalizable, as shown previously. Then, nothing is automatically serialized and you can explicitly serialize only the necessary parts inside writeExternal( ). If you’re working with a Serializable object, however, all serialization happens automatically. To control this, you can turn off serialization on a field-by-field basis using the transient keyword, which says: “don’t bother saving or restoring this – I’ll take care of it.” For example, consider a Login object that keeps information about a particular login session. Suppose that, once you verify the login, you want to store the data but without the password. The easiest way to do this is by implementing Serializable and marking the password field as transient. Here’s what it looks like: //: Logon.java // Demonstrates the "transient" keyword import java.io.*; import java.util.*; class Logon implements Serializable { private Date date = new Date(); private String username; private transient String password; Logon(String name, String pwd) { username = name; 346 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 347. password = pwd; } public String toString() { String pwd = (password == null) ? "(n/a)" : password; return "logon info: n " + "username: " + username + "n date: " + date.toString() + "n password: " + pwd; } public static void main(String args[]) { Logon a = new Logon("Hulk", "myLittlePony"); System.out.println( "logon a = " + a); try { ObjectOutputStream o = new ObjectOutputStream( new FileOutputStream("Logon.out")); o.writeObject(a); o.close(); // Delay: int seconds = 5; long t = System.currentTimeMillis() + seconds * 1000; while(System.currentTimeMillis() < t) ; // Now get them back: ObjectInputStream in = new ObjectInputStream( new FileInputStream("Logon.out")); System.out.println( "Recovering object at " + new Date()); a = (Logon)in.readObject(); System.out.println( "logon a = " + a); } catch(Exception e) { e.printStackTrace(); } } } ///:~ You can see that the date and username fields are ordinary (not transient), and thus are automatically serialized. However, the password is transient, and so is not stored to disk; also the serialization mechanism makes no attempt to recover it. The output is: logon a = logon info: username: Hulk date: Sun Mar 23 18:25:53 PST 1997 password: myLittlePony Recovering object at Sun Mar 23 18:25:59 PST 1997 logon a = logon info: username: Hulk date: Sun Mar 23 18:25:53 PST 1997 password: (n/a) When the object is recovered, the password field is null. Notice that toString( ) must check for a null value of password because if you simply try to assemble a String object using the overloaded ‘+’ operator, and that operator encounters a null handle, you’ll get a NullPointerException (newer versions of Java may contain code to avoid this problem). Chapter 10: The Java IO System 347
- 348. You can also see that the date field is stored to and recovered from disk, and not generated anew. Since Externalizable objects do not store any of their fields by default, the transient keyword is for use only with Serializable objects. An alternative to Externalizable If implementing the Externalizable interface is not to your taste, there’s another approach. You can just implement the Serializable interface and add (and note carefully I say “add” and not “override” or “implement”) methods called writeObject( ) and readObject( ) which will automatically be called when the object is serialized and deserialized, respectively. That is, if you provide these two methods they will be used instead of the default serialization. The methods must have these exact signatures: private void writeObject(ObjectOutputStream stream) throws IOException; private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException From a design standpoint, things get really weird here. First of all, you might think that since these methods are not part of a base class or the Serializable interface, they ought to be defined in their own interface(s). But notice they are defined as private, which means they are to be called only by other members of this class. However, you don't actually call them from other members of this class, but instead the writeObject( ) and readObject( ) methods of the ObjectOutputStream and ObjectInputStream objects call your object’s writeObject( ) and readObject( ) methods (notice my tremendous restraint in not launching into a long diatribe about using the same method names here. In a word: confusing). You may wonder how the ObjectOutputStream and ObjectInputStream objects have access to private methods of your class? We can only assume this is part of the serialization magic. In any event, anything defined in an interface is automatically public so if writeObject( ) and readObject( ) must be private, then they can’t be part of an interface. Since you must follow the signatures exactly, the effect is the same as if you’re implementing an interface. It would appear that when you call ObjectOutputStream.writeObject( ), the Serializable object that you pass it is interrogated (using reflection, no doubt) to see if it implements its own writeObject( ). If so, the normal serialization process is skipped and the writeObject( ) is called. The same sort of situation exists for readObject( ). There’s one other twist. Inside your writeObject( ), you can choose to perform the default writeObject( ) action by calling defaultWriteObject( ). Likewise, inside readObject( ) you can call defaultReadObject( ). Here is a simple example that demonstrates how you can control the storage and retrieval of a Serializable object: //: SerialCtl.java // Controlling serialization by adding your own // writeObject() and readObject() methods. import java.io.*; public class SerialCtl implements Serializable { 348 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 349. String a; transient String b; public SerialCtl(String aa, String bb) { a = "Not Transient: " + aa; b = "Transient: " + bb; } public String toString() { return a + "n" + b; } private void writeObject(ObjectOutputStream stream) throws IOException { stream.defaultWriteObject(); stream.writeObject(b); } private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException { stream.defaultReadObject(); b = (String)stream.readObject(); } public static void main(String args[]) { SerialCtl sc = new SerialCtl("Test1", "Test2"); System.out.println("Before:n" + sc); ByteArrayOutputStream buf = new ByteArrayOutputStream(); try { ObjectOutputStream o = new ObjectOutputStream(buf); o.writeObject(sc); // Now get it back: ObjectInputStream in = new ObjectInputStream( new ByteArrayInputStream( buf.toByteArray())); SerialCtl sc2 = (SerialCtl)in.readObject(); System.out.println("After:n" + sc2); } catch(Exception e) { e.printStackTrace(); } } } ///:~ In this example, one String field is ordinary and the other is transient, to prove that the non-transient field is saved by the defaultWriteObject( ) method and the transient field is saved and restored explicitly. The fields are initialized inside the constructor rather than the point of definition to prove that they are not being initialized by some automatic mechanism during deserialization. If you are going to use the default mechanism to write the non-transient parts of your object, you must call defaultWriteObject( ) as the first operation in writeObject( ), and defaultReadObject( ) as the first operation in readObject( ). These are strange method calls. It would appear, for example, that you are calling defaultWriteObject( ) for an ObjectOutputStream and passing it no arguments, and yet it somehow turns around and knows the handle to your object, and how to write all the non-transient parts. Spooky. Chapter 10: The Java IO System 349
- 350. The storage and retrieval of the transient objects uses more familiar code. And yet, think about what happens here. In main( ), a SerialCtl object is created, and then it’s serialized to an ObjectOutputStream (notice in this case a buffer is used instead of a file – it’s all the same to the ObjectOutputStream). The serialization occurs in the line: o.writeObject(sc); The writeObject( ) method must be examining sc to see if it has its own writeObject( ) method (not by checking the interface – there isn’t one – or the class type, but by actually hunting for the method using reflection). If so, it uses that. A similar approach holds true for readObject( ). Perhaps this was the only practical way they could solve the problem, but it’s certainly strange. Versioning It’s possible you may want to change the version of a serializable class (objects of the original class may be stored in a database, for example). This is supported but you’ll probably do it only in special cases, and it requires an extra depth of understanding that we shall not attempt to achieve here. The JDK1.1 HTML documents downloadable from Sun (which may be part of your Java package’s online documents) cover this topic quite thoroughly. Using persistence It’s quite appealing to utilize serialization technology to store some of the state of your program so that you can later easily restore the program to the current state. But before you can do this, some questions must be answered. What happens if you serialize two objects that both have a handle to a third object? When you restore those two objects from their serialized state, do you get only one of the third object? What if you serialize your two objects to separate files and deserialize them in different parts of your code? Here’s an example that shows the problem: //: MyWorld.java import java.io.*; import java.util.*; class House implements Serializable {} class Animal implements Serializable { String name; House preferredHouse; Animal(String nm, House h) { name = nm; preferredHouse = h; } public String toString() { return name + "[" + super.toString() + "], " + preferredHouse + "n"; } } public class MyWorld { public static void main(String args[]) { House house = new House(); Vector animals = new Vector(); animals.addElement( 350 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 351. new Animal("Bosco the dog", house)); animals.addElement( new Animal("Ralph the hamster", house)); animals.addElement( new Animal("Fronk the cat", house)); System.out.println("animals: " + animals); try { ByteArrayOutputStream buf1 = new ByteArrayOutputStream(); ObjectOutputStream o1 = new ObjectOutputStream(buf1); o1.writeObject(animals); o1.writeObject(animals); // Write a 2nd set // Write to a different stream: ByteArrayOutputStream buf2 = new ByteArrayOutputStream(); ObjectOutputStream o2 = new ObjectOutputStream(buf2); o2.writeObject(animals); // Now get them back: ObjectInputStream in1 = new ObjectInputStream( new ByteArrayInputStream( buf1.toByteArray())); ObjectInputStream in2 = new ObjectInputStream( new ByteArrayInputStream( buf2.toByteArray())); Vector animals1 = (Vector)in1.readObject(); Vector animals2 = (Vector)in1.readObject(); Vector animals3 = (Vector)in2.readObject(); System.out.println("animals1: " + animals1); System.out.println("animals2: " + animals2); System.out.println("animals3: " + animals3); } catch(Exception e) { e.printStackTrace(); } } } ///:~ One thing that’s rather interesting here is that it’s possible to use object serialization to and from a byte array as a way of doing a “deep copy” of any object that’s Serializable (a deep copy means you’re duplicating the entire web of objects, rather than just the basic object and its handles). Copying is covered in depth in Chapter 12. Animal objects contain fields of type House. In main( ), a Vector of these Animals is created and it is serialized, twice to one stream and then again to a separate stream. When these are deserialized and printed, you see the following results (for one run; the objects will be in different memory locations each run): animals: [Bosco the dog[Animal@1cc76c], House@1cc769 , Ralph the hamster[Animal@1cc76d], House@1cc769 , Fronk the cat[Animal@1cc76e], House@1cc769 ] animals1: [Bosco the dog[Animal@1cca0c], House@1cca16 , Ralph the hamster[Animal@1cca17], House@1cca16 Chapter 10: The Java IO System 351
- 352. , Fronk the cat[Animal@1cca1b], House@1cca16 ] animals2: [Bosco the dog[Animal@1cca0c], House@1cca16 , Ralph the hamster[Animal@1cca17], House@1cca16 , Fronk the cat[Animal@1cca1b], House@1cca16 ] animals3: [Bosco the dog[Animal@1cca52], House@1cca5c , Ralph the hamster[Animal@1cca5d], House@1cca5c , Fronk the cat[Animal@1cca61], House@1cca5c ] Of course you expect that the deserialized objects have different addresses than their originals. But notice that in animals1 and animals2 the same addresses appear, including the references to the House object that both share. On the other hand, when animals3 is recovered the system has no way of knowing that the objects in this other stream are aliases of the objects in the first stream, so it makes a completely different web of objects. This means that as long as you’re serializing everything to a single stream, you’ll be able to recover the same web of objects that you wrote, with no accidental duplication of objects. Of course you can change the state of your objects in between the time you write the first of them and the last, but that’s your responsibility – the objects will be written in whatever state they are in (and with whatever connections they have to other objects) at the time you serialize them. The safest thing to do if you want to save the state of a system is to serialize as an “atomic” operation; that is, don’t serialize some things, do some other work, and serialize some more, etc. The simplest way to accomplish this is to put all the objects that comprise the state of your system in a single collection and simply write that collection out in one operation. Then you can restore it with a single method call, as well. The following example is a pretend computer-aided design (CAD) system that demonstrates the approach. In addition, it throws in the issue of static fields – if you look at the documentation you’ll see that Class is Serializable, so it should be easy to store the static fields by simply serializing the Class object. That seems like a sensible approach, anyway. //: CADState.java // Saving and restoring the state of a // pretend CAD system. import java.io.*; import java.util.*; abstract class Shape implements Serializable { public static final int RED = 1, BLUE = 2, GREEN = 3; private int xPos, yPos, dimension; private static Random r = new Random(); private static int counter = 0; abstract public void setColor(int newColor); abstract public int getColor(); public Shape(int xVal, int yVal, int dim) { xPos = xVal; yPos = yVal; dimension = dim; } public String toString() { return getClass().toString() + 352 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 353. " color[" + getColor() + "] xPos[" + xPos + "] yPos[" + yPos + "] dim[" + dimension + "]n"; } public static Shape randomFactory() { int xVal = r.nextInt() % 100; int yVal = r.nextInt() % 100; int dim = r.nextInt() % 100; switch(counter++ % 3) { default: case 0: return new Circle(xVal, yVal, dim); case 1: return new Square(xVal, yVal, dim); case 2: return new Line(xVal, yVal, dim); } } } class Circle extends Shape { private static int color = RED; public Circle(int xVal, int yVal, int dim) { super(xVal, yVal, dim); } public void setColor(int newColor) { color = newColor; } public int getColor() { return color; } } class Square extends Shape { private static int color; public Square(int xVal, int yVal, int dim) { super(xVal, yVal, dim); color = RED; } public void setColor(int newColor) { color = newColor; } public int getColor() { return color; } } class Line extends Shape { private static int color = RED; public static void serializeStaticState(ObjectOutputStream os) throws IOException { os.writeInt(color); } public static void deserializeStaticState(ObjectInputStream os) throws IOException { color = os.readInt(); Chapter 10: The Java IO System 353
- 354. } public Line(int xVal, int yVal, int dim) { super(xVal, yVal, dim); } public void setColor(int newColor) { color = newColor; } public int getColor() { return color; } } public class CADState { public static void main(String args[]) throws Exception { Vector shapeTypes, shapes; if(args.length == 0) { shapeTypes = new Vector(); shapes = new Vector(); // Add handles to the class objects: shapeTypes.addElement(Circle.class); shapeTypes.addElement(Square.class); shapeTypes.addElement(Line.class); // Make some shapes: for(int i = 0; i < 10; i++) shapes.addElement(Shape.randomFactory()); // Set all the static colors to GREEN: for(int i = 0; i < 10; i++) ((Shape)shapes.elementAt(i)) .setColor(Shape.GREEN); // Save the state vector: ObjectOutputStream out = new ObjectOutputStream( new FileOutputStream("CADState.out")); out.writeObject(shapeTypes); Line.serializeStaticState(out); out.writeObject(shapes); } else { // There's a command-line argument ObjectInputStream in = new ObjectInputStream( new FileInputStream(args[0])); // Read in the same order they were written: shapeTypes = (Vector)in.readObject(); Line.deserializeStaticState(in); shapes = (Vector)in.readObject(); } // Display the shapes: System.out.println(shapes); } } ///:~ The Shape class implements Serializable, so anything that is inherited from Shape is automatically Serializable as well. Each Shape contains data, and each derived Shape class contains a static field that determines the color of all of those types of Shapes (placing a static field in the base class would result in only one field, since static fields are not duplicated in derived classes). Methods in the base class can be overridden to set the 354 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 355. color for the various types (static methods are not dynamically bound, so these are normal methods). The randomFactory( ) method creates a different Shape each time you call it, using random values for the Shape data. Circle and Square are straightforward extensions of Shape; the only difference is that Circle initializes color at the point of definition and Square initializes it in the constructor. We’ll leave the discussion of Line for later. In main( ), one Vector is used to hold the Class objects and the other to hold the shapes themselves. If you don’t provide a command-line argument the shapeTypes Vector is created and the Class objects are added, and then the shapes Vector is created and Shape objects are added. Next, all the static color values are set to GREEN, and everything is serialized to the file CADState.out. If you provide a command-line argument (presumably CADState.out), that file is opened and used to restore the state of the program. In both situations, the resulting Vector of Shapes is printed out. The results from one run are: >java CADState [class Circle color[3] xPos[-51] yPos[-99] dim[38] , class Square color[3] xPos[2] yPos[61] dim[-46] , class Line color[3] xPos[51] yPos[73] dim[64] , class Circle color[3] xPos[-70] yPos[1] dim[16] , class Square color[3] xPos[3] yPos[94] dim[-36] , class Line color[3] xPos[-84] yPos[-21] dim[-35] , class Circle color[3] xPos[-75] yPos[-43] dim[22] , class Square color[3] xPos[81] yPos[30] dim[-45] , class Line color[3] xPos[-29] yPos[92] dim[17] , class Circle color[3] xPos[17] yPos[90] dim[-76] ] >java CADState CADState.out [class Circle color[1] xPos[-51] yPos[-99] dim[38] , class Square color[0] xPos[2] yPos[61] dim[-46] , class Line color[3] xPos[51] yPos[73] dim[64] , class Circle color[1] xPos[-70] yPos[1] dim[16] , class Square color[0] xPos[3] yPos[94] dim[-36] , class Line color[3] xPos[-84] yPos[-21] dim[-35] , class Circle color[1] xPos[-75] yPos[-43] dim[22] , class Square color[0] xPos[81] yPos[30] dim[-45] , class Line color[3] xPos[-29] yPos[92] dim[17] , class Circle color[1] xPos[17] yPos[90] dim[-76] ] You can see that the values of xPos, yPos and dim were all stored and recovered successfully, but there’s something wrong with the retrieval of the static information. It’s all ‘3’ going in, but it doesn’t come out that way. Circles have a value of 1 (RED, which is the definition), and Squares have a value of 0 (remember they are initialized in the constructor). It’s as if the statics didn’t get serialized at all! That’s right – even though class Class is Serializable, it doesn’t do what you expect. So if you want to serialize statics, you must do it yourself. This is what the serializeStaticState( ) and deserializeStaticState( ) static methods in Line are for. You can see they are explicitly called as part of the storage and retrieval process (notice that the order of writing to the serialize file and reading back from it must be maintained). Chapter 10: The Java IO System 355
- 356. Another issue you may have to think about is security, since serialization also saves private data. If you have a security issue, those fields should be marked as transient. But then you have to design a secure way to store that information so that when you do a restore you can reset those private variables. Summary The Java IO stream library does seem to satisfy the basic requirements: you can perform reading and writing with the console, a file, with a block of memory, or even across the Internet (as you shall see in Chapter 15). It’s possible (by inheriting from InputStream and OutputStream) to create new types of input and output objects. And you can even add a very simple extensibility to the kinds of objects a stream will accept by redefining the toString( ) method that’s automatically called when you pass an object to a method that’s expecting a String (Java’s limited “automatic type conversion”). The use of layered objects to add responsibilities to individual objects dynamically and transparently is referred to as the decorator pattern (patterns are the subject of Chapter 16). Decorators are often used when subclassing is impractical since subclassing would create a large number of subclasses to support every possible combination needed. Since the Java IO library provides a number of these possibilities, the decorator pattern would seem like a good approach. However, the decorator pattern specifies2 that all objects that wrap around your initial object have the same interface, so the use of the decorators is transparent – you send the same message to an object whether it’s been decorated or not. The Java IO “decorators” change the interfaces, sometimes very significantly. The very reason that the Java IO library is awkward to use is that it has only the appearance of following the decorator pattern, but then immediately breaks that design. The Java 1.1 IO library missed the opportunity to completely change the library design, and instead it has added even more special cases and complexity. There are other questions left unanswered by the documentation and design of the IO stream library. For example, is it simply not possible to fail when you open an output file? Also, some programming systems allow you to specify that you want to open an output file, but only if it doesn’t already exist. In Java it appears you are supposed to use a File object to determine whether a file exists, because if you open it as an FileOutputStream or FileWriter it will always get overwritten. By representing both files and directory paths, the File class also suggests poor design by violating the maxim “don’t try to do too much in a single class.” The IO stream library brings up mixed feelings. It does much of the job and it’s portable. But the design is poor and non-intuitive so there’s extra overhead in learning and teaching it, and it’s incomplete: there’s no support for the kind of output formatting that almost every other language’s IO package supports (and this was not remedied in Java 1.1). The Java 1.1 changes to the IO library haven’t been replacements, but rather additions, and it seems that the library designers couldn’t quite get it straight about which features are deprecated and which are preferred, resulting in annoying deprecation messages that show up the contradictions in the library design. The IO library is usable, but it’s frustrating. At least it might be possible to learn from it, as an example of what not to do in your own class designs. 2 In Design Patterns, Erich Gamma et al., Addison-Wesley 1995. Described later in this book. 356 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 357. Exercises 1. Open a text file so that you can read the file a line at a time. Read each line as a String and place that String object into a Vector. Print out all the lines in the Vector in reverse order. 2. Modify exercise one so that the name of the file you read is provided as a command- line argument. 3. Modify exercise two to also open a text file so you can write text into it. Write the lines in the Vector, along with line numbers, out to the file. 4. Modify exercise two to force all the lines in the Vector to upper case and send the results to System.out. 5. Modify exercise two to take additional arguments of words to find in the file. Print out any lines where the words match. 6. In Blips.java, copy the file and rename it to BlipCheck.java and rename the class Blip2 to BlipCheck (making it public in the process). Remove the //! marks in the file and execute the program including the offending lines. Next, comment out the default constructor for BlipCheck. Run it and explain why it works. 7. In Blip3.java, comment out the two lines after the phrases “You must do this:” and run the program. Explain the result and why it differs from when the 2 lines are in the program. 8. Convert the SortedWordCount.java program to use the Java 1.1 IO Streams. 9. (Intermediate) In Chapter 7, locate the GreenhouseControls.java example, which consists of three files. In GreenhouseControls.java, the Restart( ) inner class has a hard-coded set of events. Change the program so that it reads the events and their relative times from a text file. (Challenging: use a factory method from Chapter 16 to build the events). Chapter 10: The Java IO System 357
- 359. 9 11: Run-time type identification The idea of run-time type identification (RTTI) seems fairly simple at first: it lets you find the exact type of an object when you have a handle to only the base type. However, the need for RTTI uncovers a whole plethora of interesting (and often perplexing) OO design issues, and raises fundamental questions of how you should structure your programs. This chapter looks at the ways that Java allows you to discover information about objects and classes at run-time. This takes two forms: “traditional” RTTI that assumes you have all the types available at compile-time and run-time, and the “reflection” mechanism in Java 1.1 that allows you to discover class information solely at run-time. The “traditional” RTTI will be covered first, followed by a discussion of reflection. The need for RTTI Consider the by-now familiar example of a class hierarchy that uses polymorphism. The generic type is the base class Shape, and the specific derived types are Circle, Square, and Triangle: 359
- 360. Shape draw() Circle Square Triangle This is a typical class-hierarchy diagram, with the base class at the top and the derived classes growing downward. The normal goal in object-oriented programming is for the bulk of your code to manipulate handles to the base type (Shape, in this case) so if you decide to extend the program by adding a new class (Rhomboid, derived from Shape, for example), the bulk of the code is not affected. In this example, the dynamically-bound method in the Shape interface is draw( ), so the intent is for the client programmer to call draw( ) through a generic Shape handle. draw( ) is overridden in all the derived classes, and because it is a dynamically-bound method, the proper behavior will occur even though it is called through a generic Shape handle. That’s polymorphism. Thus, you generally create a specific object (Circle, Square, or Triangle), upcast it to a Shape (forgetting the specific type of the object), and use that anonymous Shape handle in the rest of the program. As a brief review of polymorphism and upcasting, you might code the above example as follows (see page 80 if you have trouble executing this program): //: Shapes.java package c11; import java.util.*; interface Shape { void draw(); } class Circle implements Shape { public void draw() { System.out.println("Circle.draw()"); } } class Square implements Shape { public void draw() { System.out.println("Square.draw()"); } } class Triangle implements Shape { public void draw() { System.out.println("Triangle.draw()"); } 360 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 361. } public class Shapes { public static void main(String args[]) { Vector s = new Vector(); s.addElement(new Circle()); s.addElement(new Square()); s.addElement(new Triangle()); Enumeration e = s.elements(); while(e.hasMoreElements()) ((Shape)e.nextElement()).draw(); } } ///:~ The base class could be coded as an interface, an abstract class or an ordinary class. Since Shape has no concrete members (that is, members with definitions) and it’s not intended that you ever create a plain Shape object, the most appropriate and flexible representation is an interface. It’s also cleaner because you don’t have all those abstract keywords lying about. Each of the derived classes overrides the base-class draw method so it behaves differently. In main( ), specific types of Shape are created and then added to a Vector. This is the point where the upcast occurs because the Vector holds only Objects. Since everything in Java (with the exception of primitives) is an Object, a Vector can also hold Shape objects. But during an upcast to Object, it also loses any specific information, including the fact that the objects are shapes. To the Vector, they are just Objects. At the point you fetch an element out of the Vector with nextElement( ), things get a little busy. Since Vector holds only Objects, nextElement( ) naturally produces an Object handle. But we know it’s actually a Shape handle, and we want to send Shape messages to that object. So a cast to Shape is necessary using the traditional “(Shape)” cast. This is actually the most basic form of RTTI, since in Java all casts are checked at run-time for correctness. That’s exactly what RTTI means: at run-time, the type of an object is identified. In this case, the RTTI cast is only partial: the Object is cast to a Shape, and not all the way to a Circle, Square or Triangle. That’s because the only thing we know at this point is that the Vector is full of Shapes. At compile-time, this is enforced only by your own self- imposed rules, but at run-time the cast ensures it. Now polymorphism takes over and the exact method that’s called for the Shape is determined by whether the handle is for a Circle, Square or Triangle. And this is how it should be, in general: you want the bulk of your code to know as little as possible about specific types of objects, and just deal with the general representation of a family of objects (in this case, Shape). As a result, your code will be easier to write, read and maintain and your designs will be easier to implement, understand and change. So polymorphism is the general goal in object-oriented programming. But what if you have a special programming problem that’s easiest to solve if you know the exact type of a generic handle? For example, suppose you want to allow your users to highlight all the shapes of any particular type by turning them purple. This way, they can find all the triangles on the screen by highlighting them. This is what RTTI accomplishes: you can ask a handle to a Shape exactly what type it’s referring to. Chapter 11: Run-Time Type Identification 361
- 362. The Class object To understand how RTTI works in Java, you must first know how type information is represented at run time. This is accomplished through a special kind of object called the Class object that contains information about the class itself (this is sometimes called a meta-class). In fact, the Class object is used to create all the “regular” objects of your class. There’s a Class object for each class that is part of your program. That is, each time you write a new class, a single Class object is also created (and stored, appropriately enough, in an identically-named .class file). At run time, when you want to make an object of that class, the Java Virtual Machine (JVM) that’s executing your program first checks to see if the Class object for that type is loaded. If not, the JVM loads it by finding the .class file with that name. Thus a Java program isn’t completely loaded before it begins, which is different than many traditional languages. Once the Class object for that type is in memory, it is used to create all objects of that type. If this seems shadowy or you don’t really believe it, here’s a demonstration program to prove it: //: SweetShop.java // Examination of the way the class loader works class Candy { static { System.out.println("Loading Candy"); } } class Gum { static { System.out.println("Loading Gum"); } } class Cookie { static { System.out.println("Loading Cookie"); } } public class SweetShop { public static void main(String args[]) { System.out.println("inside main"); new Candy(); System.out.println("After creating Candy"); try { Class.forName("Gum"); } catch(ClassNotFoundException e) { e.printStackTrace(); } System.out.println( "After Class.forName("Gum")"); new Cookie(); System.out.println("After creating Cookie"); 362 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 363. } } ///:~ Each of the classes Candy, Gum and Cookie has a static clause which is executed as the class is loaded for the first time. This means information will be printed out to tell you when loading occurs for that class. In main( ), the object creations are spread out between print statements to help detect the time of loading. A particularly interesting line is: Class.forName("Gum"); This method is a static member of Class (to which all Class objects belong). A Class object is like any other object and thus you can get and manipulate a handle to it (that’s what the loader does). One of the ways to get a handle to the Class object is forName( ), which takes a String containing the textual name (watch the spelling and capitalization!) of the particular class you want a handle for. It returns a Class handle. The output of this program for one JVM is: inside main Loading Candy After creating Candy Loading Gum After Class.forName("Gum") Loading Cookie After creating Cookie So you can see that each Class object is loaded only as it’s needed, and the static initialization is performed upon class loading. Interestingly enough, a different JVM yielded: Loading Candy Loading Cookie inside main After creating Candy Loading Gum After Class.forName("Gum") After creating Cookie It appears that this JVM anticipated the need for Candy and Cookie by examining the code in main( ), but could not see Gum because it was created by a call to forName( ) and not through a more typical call to new. While this JVM produces the desired effect because it does get the classes loaded before they’re needed, it’s uncertain whether the behavior shown is precisely correct. As you’ll see later in this chapter, the different ways various JVMs load classes and initialize them could cause surprises. Class literals In Java 1.1 you have a second way to produce the handle to the Class object: with the class literal. In the above program this would look like: Gum.class; which is not only simpler, but also safer since it’s checked at compile time. Because it eliminates the method call, it’s also more efficient. Class literals work with regular classes as well as interfaces, arrays and primitive types. In addition, there’s a standard field called TYPE that exists for each of the primitive Chapter 11: Run-Time Type Identification 363
- 364. wrapper classes. The TYPE field produces a handle to the Class object for the associated primitive type, such that: … is equivalent to … boolean.class Boolean.TYPE char.class Character.TYPE byte.class Byte.TYPE short.class Short.TYPE int.class Integer.TYPE long.class Long.TYPE float.class Float.TYPE double.class Double.TYPE void.class Void.TYPE Checking before a cast So far, you’ve seen RTTI forms including: 1. The classic cast, e.g. “(Shape)” which uses RTTI to make sure the cast is correct and throws a ClassCastException if you’ve performed a bad cast. 2. The Class object representing the type of your object. The Class object can be queried for useful runtime information. In C++, the classic cast “(Shape)” does not perform RTTI. It simply tells the compiler to treat the object as the new type. In Java, which does perform the type check, this cast is often called a “type-safe downcast.” The reason for the term “downcast” is the historical arrangement of the class hierarchy diagram. If casting a Circle to a Shape is an upcast, then casting a Shape to a Circle is a downcast. However, you know a Circle is also a Shape, and the compiler freely allows an upcast assignment, but you don’t know that a Shape is necessarily a Circle, so the compiler doesn’t allow you to perform a downcast assignment without using an explicit cast. There’s a third form of RTTI in Java. This is the keyword instanceof which tells you if an object is an instance of a particular type. It returns a boolean so you use it in the form of a question, like this: if(x instanceof Dog) ((Dog)x).bark(); The above if statement checks to see if the object x belongs to the class Dog before casting x to a Dog. It’s important to use instanceof before a downcast when you don’t have other information that tells you the type of the object, otherwise you’ll end up with a ClassCastException. Normally you might be hunting for one type (triangles to turn purple, for example), but the following program shows how to tally all of the objects using instanceof. 364 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 365. //: PetCount.java // Using instanceof package c11.petcount; import java.util.*; class Pet {} class Dog extends Pet {} class Pug extends Dog {} class Cat extends Pet {} class Rodent extends Pet {} class Gerbil extends Rodent {} class Hamster extends Rodent {} class Counter { int i; } public class PetCount { static String[] typenames = { "Pet", "Dog", "Pug", "Cat", "Rodent", "Gerbil", "Hamster", }; public static void main(String args[]) { Vector pets = new Vector(); try { Class[] petTypes = { Class.forName("c11.petcount.Dog"), Class.forName("c11.petcount.Pug"), Class.forName("c11.petcount.Cat"), Class.forName("c11.petcount.Rodent"), Class.forName("c11.petcount.Gerbil"), Class.forName("c11.petcount.Hamster"), }; for(int i = 0; i < 15; i++) pets.addElement( petTypes[ (int)(Math.random()*petTypes.length)] .newInstance()); } catch(InstantiationException e) {} catch(IllegalAccessException e) {} catch(ClassNotFoundException e) {} Hashtable h = new Hashtable(); for(int i = 0; i < typenames.length; i++) h.put(typenames[i], new Counter()); for(int i = 0; i < pets.size(); i++) { Object o = pets.elementAt(i); if(o instanceof Pet) ((Counter)h.get("Pet")).i++; if(o instanceof Dog) ((Counter)h.get("Dog")).i++; if(o instanceof Pug) ((Counter)h.get("Pug")).i++; if(o instanceof Cat) ((Counter)h.get("Cat")).i++; if(o instanceof Rodent) ((Counter)h.get("Rodent")).i++; if(o instanceof Gerbil) ((Counter)h.get("Gerbil")).i++; Chapter 11: Run-Time Type Identification 365
- 366. if(o instanceof Hamster) ((Counter)h.get("Hamster")).i++; } for(int i = 0; i < pets.size(); i++) System.out.println( pets.elementAt(i).getClass().toString()); for(int i = 0; i < typenames.length; i++) System.out.println( typenames[i] + " quantity: " + ((Counter)h.get(typenames[i])).i); } } ///:~ There’s a rather narrow restriction on instanceof in Java 1.0: you can compare it only to a named type, and not to a Class object. In the above example you may feel like it’s tedious to write out all those instanceof expressions, and you’re right. But in Java 1.0 there is no way to cleverly automate it by creating a Vector of Class objects and comparing to those instead. This isn’t as great a restriction as you might think, because you’ll eventually understand that your design is probably flawed if you end up writing a lot of instanceof expressions. Of course this example is contrived – you’d probably put a static data member in each type and increment it in the constructor to keep track of the counts. You would do something like that if you had control of the source code for the class and could change it. Since this is not always the case, RTTI can come in handy. Using class literals It’s interesting to see how the PetCount.java example can be rewritten using Java 1.1 class literals. The result is cleaner in many ways: //: PetCount2.java // Using Java 1.1 class literals package c11.petcount2; import java.util.*; class Pet {} class Dog extends Pet {} class Pug extends Dog {} class Cat extends Pet {} class Rodent extends Pet {} class Gerbil extends Rodent {} class Hamster extends Rodent {} class Counter { int i; } public class PetCount2 { public static void main(String args[]) { Vector pets = new Vector(); Class[] petTypes = { // Class literals work in Java 1.1 only: Pet.class, Dog.class, Pug.class, Cat.class, Rodent.class, Gerbil.class, 366 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 367. Hamster.class, }; try { for(int i = 0; i < 15; i++) { // Offset by one to eliminate Pet.class: int rnd = 1 + (int)( Math.random() * (petTypes.length - 1)); pets.addElement( petTypes[rnd].newInstance()); } } catch(InstantiationException e) {} catch(IllegalAccessException e) {} Hashtable h = new Hashtable(); for(int i = 0; i < petTypes.length; i++) h.put(petTypes[i].toString(), new Counter()); for(int i = 0; i < pets.size(); i++) { Object o = pets.elementAt(i); if(o instanceof Pet) ((Counter)h.get( "class c11.petcount2.Pet")).i++; if(o instanceof Dog) ((Counter)h.get( "class c11.petcount2.Dog")).i++; if(o instanceof Pug) ((Counter)h.get( "class c11.petcount2.Pug")).i++; if(o instanceof Cat) ((Counter)h.get( "class c11.petcount2.Cat")).i++; if(o instanceof Rodent) ((Counter)h.get( "class c11.petcount2.Rodent")).i++; if(o instanceof Gerbil) ((Counter)h.get( "class c11.petcount2.Gerbil")).i++; if(o instanceof Hamster) ((Counter)h.get( "class c11.petcount2.Hamster")).i++; } for(int i = 0; i < pets.size(); i++) System.out.println( pets.elementAt(i).getClass().toString()); Enumeration keys = h.keys(); while(keys.hasMoreElements()) { String nm = (String)keys.nextElement(); Counter cnt = (Counter)h.get(nm); System.out.println( nm.substring(nm.lastIndexOf('.') + 1) + " quantity: " + cnt.i); } } } ///:~ Here, the typenames array has been removed in favor of getting the type name strings from the Class object. Note the extra work for this: the class name is not, for example, Chapter 11: Run-Time Type Identification 367
- 368. Gerbil but instead c11.petcount2.Gerbil since the package name is included. Note also that the system can distinguish between classes and interfaces. You can also see that the creation of petTypes does not need to be surrounded by a try block, since it’s evaluated at compile time and thus won’t throw any exceptions, unlike Class.forName( ). When the Pet objects are dynamically created, you can see that the random number is restricted so it is between 1 and petTypes.length and does not include zero. That’s because zero refers to Pet.class, and presumably a generic Pet object is not interesting. However, since Pet.class is part of petTypes the result is that all the pets get counted. A dynamic instanceof Java 1.1 has added the isInstance method to the class Class. This allows you to dynamically call the instanceof operator which you could do only statically in Java 1.0 (as previously shown). Thus, all those tedious instanceof statements can be removed in the PetCount example: //: PetCount3.java // Using Java 1.1 isInstance() package c11.petcount3; import java.util.*; class Pet {} class Dog extends Pet {} class Pug extends Dog {} class Cat extends Pet {} class Rodent extends Pet {} class Gerbil extends Rodent {} class Hamster extends Rodent {} class Counter { int i; } public class PetCount3 { public static void main(String args[]) { Vector pets = new Vector(); Class[] petTypes = { Pet.class, Dog.class, Pug.class, Cat.class, Rodent.class, Gerbil.class, Hamster.class, }; try { for(int i = 0; i < 15; i++) { // Offset by one to eliminate Pet.class: int rnd = 1 + (int)( Math.random() * (petTypes.length - 1)); pets.addElement( petTypes[rnd].newInstance()); } } catch(InstantiationException e) {} catch(IllegalAccessException e) {} Hashtable h = new Hashtable(); 368 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 369. for(int i = 0; i < petTypes.length; i++) h.put(petTypes[i].toString(), new Counter()); for(int i = 0; i < pets.size(); i++) { Object o = pets.elementAt(i); // Using isInstance to eliminate individual // instanceof expressions: for (int j = 0; j < petTypes.length; ++j) if (petTypes[j].isInstance(o)) { String key = petTypes[j].toString(); ((Counter)h.get(key)).i++; } } for(int i = 0; i < pets.size(); i++) System.out.println( pets.elementAt(i).getClass().toString()); Enumeration keys = h.keys(); while(keys.hasMoreElements()) { String nm = (String)keys.nextElement(); Counter cnt = (Counter)h.get(nm); System.out.println( nm.substring(nm.lastIndexOf('.') + 1) + " quantity: " + cnt.i); } } } ///:~ You can see that the Java 1.1 isInstance( ) method has eliminated the need for the instanceof expressions. In addition, this means that you can add new types of pets by simply changing the petTypes array; the rest of the program does not need modification (as it did when using the instanceof expressions). RTTI syntax Java performs its RTTI using the Class object, even if you’re doing something like a cast. The class Class also has a number of other ways you can use RTTI. First, you must get a handle to the appropriate Class object. One way to do this, as shown in the previous example, is to use a string and the Class.forName( ) method. This is convenient because you don’t need an object of that type in order to get the Class handle. However, if you do already have an object of the type you’re interested in you can fetch the Class handle by calling a method that’s part of the Object root class: getClass( ). This returns the Class handle representing the actual type of the object. Class itself has several interesting and sometimes useful methods, demonstrated in the following example: //: ToyTest.java // Testing class Class interface HasBatteries {} interface Waterproof {} interface ShootsThings {} class Toy { // Comment out the following default // constructor to see // NoSuchMethodError from (*1*) Chapter 11: Run-Time Type Identification 369
- 370. Toy() {} Toy(int i) {} } class FancyToy extends Toy implements HasBatteries, Waterproof, ShootsThings { FancyToy() { super(1); } } public class ToyTest { public static void main(String args[]) { Class c = null; try { c = Class.forName("FancyToy"); } catch(ClassNotFoundException e) {} printInfo(c); Class faces[] = c.getInterfaces(); for(int i = 0; i < faces.length; i++) printInfo(faces[i]); Class cy = c.getSuperclass(); Object o = null; try { // Requires default constructor: o = cy.newInstance(); // (*1*) } catch(InstantiationException e) {} catch(IllegalAccessException e) {} printInfo(o.getClass()); } static void printInfo(Class cc) { System.out.println( "Class name: " + cc.getName() + " is interface? [" + cc.isInterface() + "]"); } } ///:~ You can see that class FancyToy is quite complicated, since it inherits from Toy and implements the interfaces of HasBatteries, Waterproof and ShootsThings. In main( ), a Class handle is created and initialized to the FancyToy Class using forName( ) inside an appropriate try block. The Class.getInterfaces( ) method returns an array of Class objects representing the interfaces that are contained in the Class object of interest. If you have a Class object you can also ask it for its direct base class using getSuperclass( ). This, of course, returns a Class handle which you can further query. This means that, at run time, you can discover an object’s entire class hierarchy. The newInstance( ) method of Class can at first seem like just another way to clone( ) an object. However, you can create a new object with newInstance( ) without an existing object, as seen here, because there is no Toy object, only cy which is a handle to y’s Class object. This is a way to implement a “virtual constructor,” which allows you to say: “I don’t know exactly what type you are, but create yourself properly anyway.” In the above example, cy is just a Class handle with no further type information known at compile time. And when you create a new instance, you get back an Object handle. But that handle 370 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 371. is actually pointing to a Toy object. Of course, before you can send any messages other than those accepted by Object, you have to investigate it a bit and do some casting. In addition, the class that’s being created with newInstance( ) must have a default constructor. There’s no way to use newInstance( ) to create objects that have non-default constructors, so this can be a bit limiting in Java 1. However, the reflection API in Java 1.1 (discussed in the next section) allows you to dynamically use any constructor in a class. The final method in the listing is printInfo( ) which takes a Class handle and gets its name with getName( ) and finds out whether it’s an interface with isInterface( ). The output from this program is: Class name: FancyToy is interface? [false] Class name: HasBatteries is interface? [true] Class name: Waterproof is interface? [true] Class name: ShootsThings is interface? [true] Class name: Toy is interface? [false] Thus, with the Class object you can find out just about everything you want to know about an object. Reflection: run-time class information If you don’t know the precise type of an object, RTTI will tell you that type. However, there’s a limitation: the type must be known at compile time in order for you to be able to detect it using RTTI and do something useful with the information. Put another way, the compiler must know about all the classes you’re working with for RTTI. This doesn’t seem like that much of a limitation at first, but suppose you’re given a handle to an object that’s not in your program space. In fact, the class of the object isn’t even available to your program at compile time. For example, suppose you get a bunch of bytes from a disk file or from a network connection, and you’re told those bytes represent a class. Since the compiler can’t know about the class while it’s compiling the code, how can you possibly use such a class? In a traditional programming environment this seems like a far-fetched scenario. But as we move into a larger programming world there are important cases where this happens. The first is component-based programming where you build projects using Rapid- Application-Development (RAD) in an application builder tool. This is a visual approach to creating a program (which you see on the screen as a form) by moving icons that represent components onto the form. These components are then configured by setting some of their values at program time. This design-time configuration requires that any component be instantiable and that it expose some part of itself and allow its values to be read and set. In addition, components that handle GUI events must expose information about appropriate methods so the RAD environment may assist the programmer in overriding these event- handling methods. Reflection provides the mechanism to detect the available methods and produce the method names. Java 1.1 provides a structure for component-based programming through Java Beans (described in Chapter 13). Another compelling motivation for discovering class information at run-time is to provide the ability to create and execute objects on remote platforms across a network. This is called Remote Method Invocation (RMI) and it allows a Java program (version 1.1 and higher) to have objects distributed across many machines. This distribution may happen for a number of reasons: perhaps you’re doing a computation-intensive task and you want to Chapter 11: Run-Time Type Identification 371
- 372. break it up and put pieces on machines that are idle in order to speed things up. In some situations you may want to place code that handles particular types of tasks (e.g. “Business Rules” in a multi-tier client/server architecture) on a particular machine so that machine becomes a common repository describing those actions and it can be easily changed to affect everyone in the system (This is an interesting development since the machine exists solely to make software changes easy!). Along these lines, distributed computing also supports specialized hardware that may be good at a particular task – matrix inversions, for example – but inappropriate or too expensive for general purpose programming. In Java 1.1, the class Class (described previously in this chapter) is extended to support the concept of reflection, and there’s an additional library java.lang.reflect with classes Field, Method and Constructor (each of which implement the Member interface). Objects of these types are created by the JVM, at run-time, to represent the corresponding member in the unknown class. You may then use the Constructors to create new objects, get( ) and set( ) methods to read and modify the fields associated with Field objects, and the invoke( ) method to call a method associated with a Method object. In addition, you can call the convenience methods getFields( ), getMethods( ), getConstructors( ), etc., to return arrays of the objects representing the fields, methods and constructors (you can find out more by looking up the class Class in your online documentation). Thus, the class information for anonymous objects can be completely determined at run time, and nothing need be known at compile time. It’s important to realize that there’s nothing magic about reflection. When you’re using reflection to interact with an object of an unknown type, the JVM will simply look at the object and see that it belongs to a particular class (just like ordinary RTTI) but then, before it can do anything else, the Class object must be loaded. Thus, the .class file for that particular type must still be available to the JVM, either on the local machine or across the network. So the true difference between RTTI and reflection is that with RTTI, the compiler opens and examines the .class file at compile time – put another way, you can call all the method of an object in the “normal” way. With reflection, the .class file is unavailable at compile time; it is opened and examined only by the run-time environment. A class method extractor You’ll rarely need to use the reflection tools directly – they’re in the language to support the other Java features such as object serialization (described in Chapter 10), Java Beans and RMI (described later in the book). However, there are times when it’s quite useful to be able to dynamically extract information about a class. One extremely useful tool is a class method extractor. As mentioned before, looking at a class definition source code or online documentation shows only the methods that are defined or overridden within that class definition. But there may be dozens more available to you that have come from base classes. To locate these is tedious and time-consuming. Fortunately, reflection provides a way to write a very simple tool that will automatically show you the entire interface. Here’s the way it works: //: ShowMethods.java // Using Java 1.1 reflection to show all the // methods of a class, even if the methods are // defined in the base class. import java.lang.reflect.*; public class ShowMethods { static final String usage = "usage: n" + "ShowMethods qualified.class.namen" + 372 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 373. "To show all methods in class or: n" + "ShowMethods qualified.class.name wordn" + "To search for methods involving 'word'"; public static void main(String args[]) { if(args.length < 1) { System.out.println(usage); System.exit(0); } try { Class c = Class.forName(args[0]); Method[] m = c.getMethods(); Constructor[] ctor = c.getConstructors(); if(args.length == 1) { for (int i = 0; i < m.length; i++) System.out.println(m[i].toString()); for (int i = 0; i < ctor.length; i++) System.out.println(ctor[i].toString()); } else { for (int i = 0; i < m.length; i++) if(m[i].toString() .indexOf(args[1])!= -1) System.out.println(m[i].toString()); for (int i = 0; i < ctor.length; i++) if(ctor[i].toString() .indexOf(args[1])!= -1) System.out.println(ctor[i].toString()); } } catch (ClassNotFoundException e) { System.out.println("No such class: " + e); } } } ///:~ The Class methods getMethods( ) and getConstructors( ) return an array of Method and Constructor, respectively. Each of these classes has further methods to dissect the names, arguments and return values of the methods they represent. But you can also just use toString( ), as is done here, to produce a String with the entire method signature. The rest of the code is just for extracting command-line information, determining if a particular signature matches with your target string (using indexOf( )) and printing the results. This shows reflection in action, since the result produced by Class.forName( ) cannot be known at compile-time, and therefore all the method signature information is being extracted at run-time. If you investigate your online documentation on reflection, you’ll see there is enough support to actually set up and make a method call on an object that’s totally unknown at compile-time. Again, this is something you’ll probably never need to do yourself – the support is there for Java itself and so that a programming environment can manipulate Java Beans – but it’s interesting. An interesting experiment is to run java ShowMethods ShowMethods. This produces a listing that includes a public default constructor, even though you can see from the code that no constructor was defined. The constructor you see is the one that’s automatically synthesized by the compiler. If you then make ShowMethods a non-public class (that is, friendly), the synthesized default constructor no longer shows up in the output. The synthesized default constructor is automatically given the same access as the class itself. Chapter 11: Run-Time Type Identification 373
- 374. The output for ShowMethods is still a little tedious. For example, here’s a portion of the output produced by invoking java ShowMethods java.lang.String: public boolean java.lang.String.startsWith(java.lang.String,int) public boolean java.lang.String.startsWith(java.lang.String) public boolean java.lang.String.endsWith(java.lang.String) It would be even nicer if the qualifiers like java.lang could be stripped off. The StreamTokenizer class introduced in the previous chapter can help solve this problem: //: ShowMethodsClean.java // ShowMethods with the qualifiers stripped // to make the results easier to read import java.lang.reflect.*; import java.io.*; public class ShowMethodsClean { static final String usage = "usage: n" + "ShowMethodsClean qualified.class.namen" + "To show all methods in class or: n" + "ShowMethodsClean qualified.class.name wordn" + "To search for methods involving 'word'"; public static void main(String args[]) { if(args.length < 1) { System.out.println(usage); System.exit(0); } try { Class c = Class.forName(args[0]); Method[] m = c.getMethods(); Constructor[] ctor = c.getConstructors(); // Convert to an array of cleaned Strings: String n[] = new String[m.length + ctor.length]; for(int i = 0; i < m.length; i++) { String s = m[i].toString(); n[i] = StripQualifiers.strip(s); } for(int i = 0; i < ctor.length; i++) { String s = ctor[i].toString(); n[i + m.length] = StripQualifiers.strip(s); } if(args.length == 1) for (int i = 0; i < n.length; i++) System.out.println(n[i]); else for (int i = 0; i < n.length; i++) if(n[i].indexOf(args[1])!= -1) System.out.println(n[i]); } catch (ClassNotFoundException e) { System.out.println("No such class: " + e); 374 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 375. } } } class StripQualifiers { private StreamTokenizer st; public StripQualifiers(String qualified) { st = new StreamTokenizer( new StringReader(qualified)); st.ordinaryChar(' '); // Keep the spaces } public String getNext() { String s = null; try { if(st.nextToken() != StreamTokenizer.TT_EOF) { switch(st.ttype) { case StreamTokenizer.TT_EOL: s = null; break; case StreamTokenizer.TT_NUMBER: s = Double.toString(st.nval); break; case StreamTokenizer.TT_WORD: s = new String(st.sval); break; default: // single character in ttype s = String.valueOf((char)st.ttype); } } } catch(IOException e) { System.out.println(e); } return s; } public static String strip(String qualified) { StripQualifiers sq = new StripQualifiers(qualified); String s = "", si; while((si = sq.getNext()) != null) { int lastDot = si.lastIndexOf('.'); if(lastDot != -1) si = si.substring(lastDot + 1); s += si; } return s; } } ///:~ The class ShowMethodsClean is quite similar to the previous ShowMethods, except that it takes the arrays of Method and Constructor and converts them into a single array of String. Each of these String objects is then passed through StripQualifiers.Strip( ) to remove all the method qualification. As you can see, this uses the StreamTokenizer and String manipulation to do its work. This tool can be a real timesaver while you’re programming, when you can’t remember if a class has a particular method and you don’t want to go walking through the class Chapter 11: Run-Time Type Identification 375
- 376. hierarchy in the online documentation, or if you don’t know whether that class can do anything with (for example) Color objects. Chapter 17 contains a GUI version of this program so you can leave it running while you’re writing code, to allow quick lookups. Summary RTTI allows you to discover type information from an anonymous base-class handle. Thus, it’s ripe for misuse by the novice since it may make sense before polymorphic method calls do. For many people coming from a procedural background, it’s very difficult not to organize their programs into sets of switch statements. They could accomplish this with RTTI and thus lose the very important value of polymorphism in code development and maintenance. The intent of Java is that you use polymorphic method calls throughout your code, and you use RTTI only when you must. However, using polymorphic method calls as they are intended requires that you have control of the base-class definition because at some point in the extension of your program you may discover the base class doesn’t include the method you need. If the base class comes from a library or is otherwise controlled by someone else, a solution to the problem is RTTI: You can inherit a new type and add your extra method. Elsewhere in the code you can detect your particular type and call that special method. This doesn’t destroy the polymorphism and extensibility of the program, because adding a new type will not require you to hunt for switch statements in your program. However, when you add new code in your main body that requires your new feature, you’ll have to use RTTI to detect your particular type. Putting a feature in a base class might mean that, for the benefit of one particular class, all the other classes derived from that base require some meaningless stub of a method. This makes the interface less clear and annoys those who must override abstract methods when they derive from that base class. For example, consider a class hierarchy representing musical instruments. Suppose you wanted to clear the spit valves of all the appropriate instruments in your orchestra. One option is to put a ClearSpitValve( ) method in the base class Instrument, but this is confusing because it implies that Percussion and Electronic instruments also have spit valves. RTTI provides a much more reasonable solution in this case because you can place the method in the specific class (Wind in this case) where it’s appropriate. However, a more appropriate solution is to put a prepareInstrument( ) method in the base class, but you might not see this when you’re first solving the problem and may mistakenly assume that you must use RTTI. Finally, RTTI will sometimes solve efficiency problems. If your code nicely uses polymorphism, but it turns out that one of your objects reacts to this general-purpose code in a horribly inefficient way, you can pick out that type using RTTI and write case- specific code to improve the efficiency. Exercises 1. Write a method that takes an object and recursively prints all the classes in that object’s hierarchy. 2. In ToyTest.java, comment out Toy’s default constructor and explain what happens. 376 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 377. 3. Create a new type of collection that uses a Vector. Capture the type of the first object you put in it, and then allow the user to insert objects of only that type from then on. 4. Write a program to determine whether an array of char is a primitive type or a true object. Chapter 11: Run-Time Type Identification 377
- 379. ^ 12: Passing and returning objects By this time you should be reasonably comfortable with the idea that when you’re “passing” an object, you’re actually passing a handle. In many programming languages, if not all of them, you can use that language’s “regular” way to pass objects around and most of the time everything works fine. But it always seems that there comes a point where you must do something irregular and suddenly things get a bit more complicated (or in the case of C++, quite complicated). Java is no exception here, and it’s important that you understand exactly what’s happening with your object handles as you pass them around and assign to them. This chapter will provide that insight. Another way to pose the question of this chapter, if you’re coming from a programming language so equipped, is “does Java have pointers?” Some have claimed no, pointers are hard and dangerous and therefore bad, and since Java is all goodness and light and will lift your earthly programming burdens it cannot possibly contain such things. However, it’s more accurate to say that Java has pointers; indeed, every object identifier in Java (except for primitives) is one of these pointers, but their use is very restricted and guarded not only by the compiler but by the run-time system. Or to put in another way, Java has pointers, but no pointer arithmetic. These are what I’ve been calling “handles,” and you can think of them as “safety pointers,” not unlike the safety scissors of early elementary school: they aren’t sharp so you cannot hurt yourself without a very great effort, but they can sometimes be slow and tedious. 379
- 380. Passing handles around When you pass a handle into a method, you’re still pointing to the same object. A simple experiment demonstrates this (see page 80 if you have trouble executing this program): //: PassHandles.java // Passing handles around package c12; public class PassHandles { static void f(PassHandles h) { System.out.println("h inside f(): " + h); } public static void main(String args[]) { PassHandles p = new PassHandles(); System.out.println("p inside main(): " + p); f(p); } } ///:~ The method toString( ) is automatically invoked in the print statements, and PassHandles inherits directly from Object with no redefinition of toString( ). Thus, Object’s version of toString( ) is used, which prints out the class of the object followed by the address where that object is located (not the handle, but the actual object storage). The output looks like this: p inside main(): PassHandles@1653748 h inside f(): PassHandles@1653748 You can see that both p and h refer to the same object. This is far more efficient than duplicating a new PassHandles object just so you can send an argument to a method. But it brings up an important issue. Aliasing Aliasing means that more than one handle is tied to the same object, as in the above example. The problem with aliasing occurs when someone writes to that object. If the owners of the other handles aren’t expecting that object to change, they’ll be surprised. This can be demonstrated with a simple example: //: Alias1.java // Aliasing two handles to one object public class Alias1 { int i; Alias1(int ii) { i = ii; } public static void main(String args[]) { Alias1 x = new Alias1(7); Alias1 y = x; // Assign the handle System.out.println("x: " + x.i); System.out.println("y: " + y.i); System.out.println("Incrementing x"); x.i++; System.out.println("x: " + x.i); System.out.println("y: " + y.i); 380 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 381. } } ///:~ In the line: Alias1 y = x; // Assign the handle A new Alias1 handle is created, but instead of being assigned to a fresh object created with new, it’s assigned to an existing handle. This means that the contents of handle x, which is the address of the object x is pointing to, is assigned to y, and thus both x and y are attached to the same object. So when x’s i is incremented in the statement: x.i++; y’s i will be affected as well. This can be seen in the output: x: 7 y: 7 Incrementing x x: 8 y: 8 One very good solution in this case is simply not to do it: don’t consciously alias more than one handle to an object at the same scope. Your code will be much easier to understand and debug. However, when you’re passing a handle in as an argument – which is the way Java is supposed to work – you automatically alias because the local handle that’s created can modify the “outside object” (the object that was created outside the scope of the method). Here’s an example: //: Alias2.java // Method calls implicitly alias their // arguments. public class Alias2 { int i; Alias2(int ii) { i = ii; } static void f(Alias2 handle) { handle.i++; } public static void main(String args[]) { Alias2 x = new Alias2(7); System.out.println("x: " + x.i); System.out.println("Calling f(x)"); f(x); System.out.println("x: " + x.i); } } ///:~ The output is: x: 7 Calling f(x) x: 8 The method is changing its argument, the outside object. When this kind of situation arises, you must decide whether it makes sense, whether the user expects it, and whether it’s going to cause problems. Chapter 12: Passing & Returning Objects 381
- 382. Generally, you call a method in order to produce a return value and/or a change of state in the object that the method is called for (a method is how you “send a message” to that object). It’s much less common to call a method in order to manipulate its arguments; this is referred to as “calling a method for its side effects”. Thus, when you create a method that modifies its arguments the user must be clearly instructed and warned as to the use of that method and its potential surprises. Because of the confusion and pitfalls, it’s much better to avoid changing the argument. If you need to modify an argument during a method call and you don’t intend to modify the outside argument, then you should protect that argument by making a copy inside your method. That’s the subject of much of this chapter. Making local copies To review: all argument passing in Java is performed by passing handles. That is, when you pass “an object,” you’re really passing only a handle to an object outside the method, so if you perform any modifications with that handle, you modify the outside object. In addition: • Aliasing happens automatically during argument passing • There are no local objects, only local handles • Handles have scopes, objects do not • Object lifetime is never an issue in Java • There is no language support (e.g. const) to prevent objects from being modified (to prevent negative effects of aliasing) If you’re only reading information from an object and not modifying it, passing a handle is the most efficient form of argument passing. This is nice: the default way of doing things is also the most efficient. However, sometimes it’s necessary to be able to treat the object as if it were “local” so that changes you make affect only a local copy and do not modify the outside object. Many programming languages support the ability to automatically make a local copy of the outside object, inside the method1 . Java does not, but it allows you to produce this effect. Pass by value This brings up the terminology issue, which always seems good for an argument. The term is “pass by value,” and the meaning really depends on the way you perceive the operation of the program. The general meaning is that you get a local copy of whatever you’re passing, but the real question is “how do you think about what you’re passing?” When it comes to the meaning of “pass by value,” there are two fairly distinct camps: 1. Java passes everything by value. When you’re passing primitives into a method, you get a distinct copy of the primitive. When you’re passing a handle into a method, you get a copy of the handle. Ergo, everything is pass by value. Of 1 In C, which generally handles small bits of data, the default is pass-by-value. C++ had to follow this form, but with objects pass-by-value isn’t usually the most efficient way. In addition, coding classes to support pass-by-value in C++ is a big headache. 382 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 383. course, the assumption is that you’re always thinking (and caring) that handles are being passed, but it seems like the Java design has gone a long way towards allowing you to ignore (most of the time) that you’re working with a handle. That is, it seems to allow you to think of the handle as “the object,” since it implicitly dereferences it whenever you make a method call. 2. Java passes primitives by value (no argument there) but objects are passed by reference. This is the world view that the handle is an alias for the object itself, so you don’t think about passing handles, but instead say “I’m passing the object.” Since you don’t get a local copy of the object when you pass it into a method, objects are clearly not passed by value. There appears to be some support for this view within Sun itself, since one of the “reserved but not implemented” keywords is byvalue (there’s no knowing, however, whether that keyword will ever see the light of day). Having given both camps a good airing and after saying “it depends on how you think of a handle,” I will attempt to sidestep the issue for the rest of the book. In the end, it isn’t that important – what is important is that you understand that passing a handle allows the caller’s object to be changed unexpectedly. Cloning objects The most likely reason for making a local copy of an object is if you’re going to modify that object and you don’t want to modify the caller’s object. If you decide that you want to make a local copy, you simply use the clone( ) method to perform the operation. This is a method that’s defined as protected in the base class Object and which you must override as public in any derived classes that you want to clone. For example, the standard library class Vector overrides clone( ), so we can call clone( ) for Vector: //: Cloning.java // The clone() operation works for only a few // items in the standard Java library. import java.util.*; class Int { private int i; public Int(int ii) { i = ii; } public void increment() { i++; } public String toString() { return Integer.toString(i); } } public class Cloning { public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 10; i++ ) v.addElement(new Int(i)); System.out.println("v: " + v); Vector v2 = (Vector)v.clone(); // Increment all v2's elements: for(Enumeration e = v2.elements(); e.hasMoreElements(); ) ((Int)e.nextElement()).increment(); // See if it changed v's elements: System.out.println("v: " + v); Chapter 12: Passing & Returning Objects 383
- 384. } } ///:~ The clone( ) method produces an Object, which must then be recast to the proper type. This example shows how Vector’s clone( ) method does not automatically try to clone each of the objects that the Vector contains – the old Vector and the cloned Vector are aliased to the same objects. This is often called a shallow copy, since it’s copying only the “surface” portion of an object. The actual object consists of this “surface” plus all the objects that the handles are pointing to, plus all the objects those objects are pointing to, etc. This is often referred to as the “web of objects.” Copying the entire mess is called a deep copy. You can see the effect of the shallow copy in the output, where the actions performed on v2 affect v: v: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] v: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Not trying to clone( ) the objects contained in the Vector is probably a fair assumption because there’s no guarantee that those objects are cloneable. Adding cloneability to a class Even though the clone method is defined in the base-of-all-classes Object, cloning is not automatically available in every class. This would seem to be counterintuitive to the idea that base-class methods are always available in derived classes. Cloning in Java goes against this idea; if you want it to exist for a class, you must specifically add code to make cloning work. Using a trick with protected To prevent default clonability in every class you create, the clone( ) method is protected in the base class Object. Not only does this mean that it’s not available by default to the client programmer who is simply using the class (not subclassing it), but it also means that you cannot call clone( ) via a handle to the base class (although that might seem to be useful in some situations: polymorphically clone a bunch of Objects). It is in effect a way to give you, at compile time, the information that your object is not cloneable – and oddly enough most classes in the standard Java library are not cloneable. Thus, if you say: Integer x = new Integer(1); x = x.clone(); You will get, at compile time, an error message that says clone( ) is not accessible (since Integer doesn’t override it and it defaults to the protected version). If, however, you’re in a class derived from Object (as all classes are) then you have permission to call Object.clone( ) because it’s protected and you’re an inheritor. The base class clone( ) has very useful functionality – it performs the actual bitwise duplication of the derived-class object, thus acting as the common cloning operation. However, you then need to make your clone operation public for it to be accessible. Thus two key issues when you clone are: virtually always call super.clone( ) and make your clone public. You’ll probably want to override clone( ) in any further derived classes, otherwise your (now public) clone( ) will be used, and that may not do the right thing (although, since Object.clone( ) makes a copy of the actual object, it may). Thus the protected trick works only once, the first time you inherit from a class that has no clonability and you want to make a class that’s cloneable. In any classes inherited from your class the clone( ) method is available since it’s not possible in Java to reduce the access of a method during 384 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 385. derivation. That is, once a class is cloneable, everything derived from it is cloneable unless you use provided mechanisms (described later) to “turn off” cloning. Implementing the Cloneable interface There’s one more thing you need to do to complete the clonability of an object: implement the Cloneable interface. This interface is a bit strange because it’s empty! interface Cloneable {} The reason for implementing this empty interface is obviously not because you are going to upcast to Cloneable and call one of its methods. The use of interface here is considered by some to be a “hack” because it’s using a feature for something other than its original intent. Implementing the Cloneable interface acts as a kind of a flag, wired into the type of the class. There are two reasons for the existence of the Cloneable interface. The first is that you may have an upcast handle to a base type and not know whether it’s possible to clone that object. In this case, you can use the instanceof keyword (described in Chapter 11) to find out whether the handle is connected to an object that can be cloned: if(myHandle instanceof Cloneable) // ... The second reason is that mixed into this design for clonability was the thought that maybe you didn’t want all types of objects to be cloneable. So Object.clone( ) verifies that a class implements the Cloneable interface. If not, it throws a CloneNotSupportedException exception. So in general you’re forced to implement Cloneable as part of support for cloning. Successful cloning Once you understand the details of implementing the clone( ) method, you’re able to create classes that can be easily duplicated to provide a local copy: //: LocalCopy.java // Creating local copies with clone() import java.util.*; class MyObject implements Cloneable { int i; MyObject(int ii) { i = ii; } public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) { System.out.println("MyObject can't clone"); } return o; } public String toString() { return Integer.toString(i); } } public class LocalCopy { static MyObject g(MyObject v) { // Passing a handle, modifies outside object: Chapter 12: Passing & Returning Objects 385
- 386. v.i++; return v; } static MyObject f(MyObject v) { v = (MyObject)v.clone(); // Local copy v.i++; return v; } public static void main(String args[]) { MyObject a = new MyObject(11); MyObject b = g(a); // Testing handle equivalence, // not object equivalence: if(a == b) System.out.println("a == b"); else System.out.println("a != b"); System.out.println("a = " + a); System.out.println("b = " + b); MyObject c = new MyObject(47); MyObject d = f(c); if(c == d) System.out.println("c == d"); else System.out.println("c != d"); System.out.println("c = " + c); System.out.println("d = " + d); } } ///:~ First of all, clone( ) must be accessible so you’ll need to make it public. Second, for the initial part of your clone( ) operation you should call the base-class version of clone( ). The clone( ) that’s being called here is the one that’s pre-defined inside Object, and you can call it because it’s protected and thereby accessible in derived classes. Object.clone( ) figures out how big the actual object is, creates enough memory for a new one, and copies all the bits from the old to the new. This is called a bitwise copy, and is typically what you’d expect a clone( ) method to do. But before Object.clone( ) performs its operations, it first checks to see if a class is Cloneable, that is, whether it implements the Cloneable interface. If it doesn’t, Object.clone( ) throws a CloneNotSupportedException to indicate that you can’t clone it. Thus, you’ve got to surround your call to super.clone( ) with a try-catch block, to catch an exception that should never happen (because you’ve implemented the Cloneable interface). In LocalCopy, the two methods g( ) and f( ) demonstrate the difference between the two approaches for argument passing. g( ) shows passing by reference where it modifies the outside object and returns a reference to that same outside object, while f( ) clones the argument, thereby decoupling it and leaving the original object alone. It can then proceed to do whatever it wants, and even to return a handle to this new object without any ill effects to the original. Notice the somewhat curious-looking statement: v = (MyObject)v.clone(); This is where the local copy is created. To keep from being confused by such a statement, you must remember that this rather strange coding idiom is perfectly feasible in Java because everything that has a name is actually a handle. So the handle v is used to clone( ) a copy of what it refers to, and this returns a handle to the base type Object 386 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 387. (because it’s defined that way in Object.clone( )) which must then be cast to the proper type. In main( ), the difference between the effects of the two different argument-passing approaches in the two different methods is tested. The output is: a == b a = 12 b = 12 c != d c = 47 d = 48 It’s important to note that the equivalence tests in Java do not look inside the objects being compared to see if their values are the same. The == and != operators are simply comparing the contents of the handles: if the addresses inside the handles are the same, that means that the handles are pointing to the same object and are therefore “equal.” So what the operators are really testing is whether the handles are aliased to the same object! The effect of Object.clone( ) What actually happens when Object.clone( ) is called that makes it so essential to call super.clone( ) when you override clone( ) in your class? The clone( ) method in the root class is responsible for creating the right amount of storage and making the actual bitwise copy of the bits from the original object into the new object’s storage. That is, it doesn’t just make storage and copy an Object – it actually figures out the size of the precise object that’s being copied, and duplicates that. Since all this is happening from the code in the clone( ) method defined in the root class (that has no idea what’s being inherited from it), you can guess that the process involves RTTI to determine the actual object that’s being cloned. This way, the clone( ) method can create the proper amount of storage and do the correct bitcopy for that type. This means that whatever you do, the first part of the cloning process should normally be a call to super.clone( ). This establishes the groundwork for the cloning operation by making an exact duplicate. At this point you can perform other operations necessary to complete the cloning. To know for sure what those other operations are, you need to understand exactly what Object.clone( ) buys you. In particular, does it automatically clone the destination of all the handles? The following example tests this: //: Snake.java // Tests cloning to see if destination of // handles are also cloned. public class Snake implements Cloneable { private Snake next; private char c; // Value of i == number of segments Snake(int i, char x) { c = x; if(--i > 0) next = new Snake(i, (char)(x + 1)); } void increment() { c++; if(next != null) Chapter 12: Passing & Returning Objects 387
- 388. next.increment(); } public String toString() { String s = ":" + c; if(next != null) s += next.toString(); return s; } public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) {} return o; } public static void main(String args[]) { Snake s = new Snake(5, 'a'); System.out.println("s = " + s); Snake s2 = (Snake)s.clone(); System.out.println("s2 = " + s2); s.increment(); System.out.println( "after s.increment, s2 = " + s2); } } ///:~ A Snake is made up of a bunch of segments, each of type Snake. Thus it’s a singly-linked list. The segments are created recursively, decrementing the first constructor argument for each segment until zero is reached. To give each segment a unique tag, the second argument, a char, is incremented for each recursive constructor call. The increment( ) method recursively increments each tag so you can see the change, and the toString( ) recursively prints each tag. The output is: s = :a:b:c:d:e s2 = :a:b:c:d:e after s.increment, s2 = :a:c:d:e:f This means that only the first segment is duplicated by Object.clone( ), thus it does a shallow copy. If you want the whole snake to be duplicated – a deep copy – you must perform the additional operations inside your overridden clone( ). Therefore, you’ll typically call super.clone( ) in any class derived from a cloneable class, to make sure that all the base-class operations (including Object.clone( )) take place. This is followed by an explicit call to clone( ) for every handle in your object – otherwise those handles will be aliased to those of the original object. It’s very analogous to the way constructors are called – base-class constructor first, then the next-derived constructor and so on to the most-derived constructor. The difference is that clone( ) is not a constructor so there’s nothing to make it happen automatically. You must make sure to do it yourself. Cloning a composed object What other operations should you perform after super.clone( ) is called? This depends on your class, and whether you want to perform a shallow copy or a deep copy. If you want to perform a shallow copy, then the operations performed by Object.clone( ) (bitwise 388 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 389. duplication of the “immediate” object) are adequate. However, if you want to do a deep copy, you must also clone every object handle in your class. In addition, you must be able to assume that the clone( ) method in those objects will in turn perform a deep copy on their handles, and so on. This is quite a commitment. It effectively means that for a deep copy to work you must either control all the code in all the classes, or at least have enough knowledge about all the classes involved in the deep copy to know that they are performing their own deep copy correctly. This example shows what you’ll have to do to accomplish a deep copy when dealing with a composed object: //: DeepCopy.java // Cloning a composed object class DepthReading implements Cloneable { private double depth; public DepthReading(double depth) { this.depth = depth; } public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } return o; } } class TemperatureReading implements Cloneable { private long time; private double temperature; public TemperatureReading(double temperature) { time = System.currentTimeMillis(); this.temperature = temperature; } public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } return o; } } class OceanReading implements Cloneable { private DepthReading depth; private TemperatureReading temperature; public OceanReading(double tdata, double ddata){ temperature = new TemperatureReading(tdata); depth = new DepthReading(ddata); } public Object clone() { OceanReading o = null; Chapter 12: Passing & Returning Objects 389
- 390. try { o = (OceanReading)super.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } // Must clone handles: o.depth = (DepthReading)o.depth.clone(); o.temperature = (TemperatureReading)o.temperature.clone(); return o; // Upcasts back to Object } } public class DeepCopy { public static void main(String args[]) { OceanReading reading = new OceanReading(33.9, 100.5); // Now clone it: OceanReading r = (OceanReading)reading.clone(); } } ///:~ DepthReading and TemperatureReading are quite similar; they both contain only primitives and therefore the clone( ) method can be quite simple: it calls only super.clone( ) and returns the result. Note that the clone( ) code for both classes is identical. OceanReading is composed of DepthReading and TemperatureReading objects and so, to produce a deep copy, its clone( ) must clone the handles inside OceanReading. To accomplish this, the result of super.clone( ) must be cast to an OceanReading object (so you can access the depth and temperature handles). A deep copy with Vector Let’s revisit the Vector example from earlier in this chapter. This time the Int2 class is cloneable so the Vector can be deep copied: //: AddingClone.java // You must go through a few gyrations to // add cloning to your own class. import java.util.*; class Int2 implements Cloneable { private int i; public Int2(int ii) { i = ii; } public void increment() { i++; } public String toString() { return Integer.toString(i); } public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) { System.out.println("Int2 can't clone"); 390 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 391. } return o; } } // Once it's cloneable, inheritance // doesn't remove cloneability: class Int3 extends Int2 { private int j; // Automatically duplicated public Int3(int i) { super(i); } } public class AddingClone { public static void main(String args[]) { Int2 x = new Int2(10); Int2 x2 = (Int2)x.clone(); x2.increment(); System.out.println( "x = " + x + ", x2 = " + x2); // Anything inherited is also cloneable: Int3 x3 = new Int3(7); x3 = (Int3)x3.clone(); Vector v = new Vector(); for(int i = 0; i < 10; i++ ) v.addElement(new Int2(i)); System.out.println("v: " + v); Vector v2 = (Vector)v.clone(); // Now clone each element: for(int i = 0; i < v.size(); i++) v2.setElementAt( ((Int2)v2.elementAt(i)).clone(), i); // Increment all v2's elements: for(Enumeration e = v2.elements(); e.hasMoreElements(); ) ((Int2)e.nextElement()).increment(); // See if it changed v's elements: System.out.println("v: " + v); System.out.println("v2: " + v2); } } ///:~ Int3 is inherited from Int2 and a new primitive member int j is added. You might think you’d need to override clone( ) again to make sure j is copied, but that’s not the case: when Int2’s clone( ) is called as Int3’s clone( ), it calls Object.clone( ) which determines that it’s working with an Int3 and duplicates all the bits in the Int3. Thus, as long as you don’t add handles that need to be cloned, the one call to Object.clone( ) performs all the necessary duplication, regardless of how far down in the hierarchy clone( ) is defined. You can see what’s necessary in order to do a deep copy of a Vector: after the Vector itself is cloned, you have to step through and clone each one of the objects pointed to by the Vector. You’d have to do something similar to this to do a deep copy of a Hashtable. The remainder of the example shows that the cloning did in fact happen by showing that, once an object is cloned, you can change it and the original object is left untouched. Chapter 12: Passing & Returning Objects 391
- 392. Deep copy via serialization When you consider Java 1.1 object serialization (introduced in Chapter 10), you may observe that an object that’s serialized and then deserialized is, in effect, cloned. Indeed, the transient keyword has the same effect for both cloning and serialization (that field is not cloned/serialized). So why not use serialization to perform deep copying? Here’s an example that compares the two approaches by timing them: //: Compete.java import java.io.*; import java.util.*; class Thing1 implements Serializable {} class Thing2 implements Serializable { Thing1 o1 = new Thing1(); } class Thing3 implements Cloneable { public Object clone() { Object o = null; try { o = super.clone(); } catch (CloneNotSupportedException e) { System.out.println("Thing3 can't clone"); } return o; } } class Thing4 implements Cloneable { Thing3 o3 = new Thing3(); public Object clone() { Thing4 o = null; try { o = (Thing4)super.clone(); } catch (CloneNotSupportedException e) { System.out.println("Thing4 can't clone"); } // Clone the field, too: o.o3 = (Thing3)o3.clone(); return o; } } public class Compete implements Serializable { static final int SIZE = 5000; public static void main(String args[]) { Thing2 a[] = new Thing2[SIZE]; for(int i = 0; i < a.length; i++) a[i] = new Thing2(); Thing4 b[] = new Thing4[SIZE]; for(int i = 0; i < b.length; i++) b[i] = new Thing4(); try { 392 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 393. long t1 = System.currentTimeMillis(); ByteArrayOutputStream buf = new ByteArrayOutputStream(); ObjectOutputStream o = new ObjectOutputStream(buf); for(int i = 0; i < a.length; i++) o.writeObject(a[i]); // Now get copies: ObjectInputStream in = new ObjectInputStream( new ByteArrayInputStream( buf.toByteArray())); Thing2 c[] = new Thing2[SIZE]; for(int i = 0; i < c.length; i++) c[i] = (Thing2)in.readObject(); long t2 = System.currentTimeMillis(); System.out.println( "Duplication via serialization: " + (t2 - t1) + " Milliseconds"); // Now try cloning: t1 = System.currentTimeMillis(); Thing4 d[] = new Thing4[SIZE]; for(int i = 0; i < d.length; i++) d[i] = (Thing4)b[i].clone(); t2 = System.currentTimeMillis(); System.out.println( "Duplication via cloning: " + (t2 - t1) + " Milliseconds"); } catch(Exception e) { e.printStackTrace(); } } } ///:~ So that there’s some deep copying going on, Thing2 and Thing4 contain member objects. It’s interesting to note that while Serializable classes are easy to set up, there’s much more work going on to duplicate them. Cloning involves a lot of work to set up the class but the actual duplication of objects is relatively simple. But the results really tell the tale. Here is the output from three different runs: Duplication via serialization: 3400 Milliseconds Duplication via cloning: 110 Milliseconds Duplication via serialization: 3410 Milliseconds Duplication via cloning: 110 Milliseconds Duplication via serialization: 3520 Milliseconds Duplication via cloning: 110 Milliseconds Despite the obviously huge time difference between serialization and cloning, you’ll also notice that the serialization technique seems to vary significantly in its duration, while cloning takes the same amount of time every time. Chapter 12: Passing & Returning Objects 393
- 394. Adding cloneability further down a hierarchy If you create a new class, its base class defaults to Object which defaults to non- clonability (as you’ll see in the next section). As long as you don’t explicitly add clonability, you won’t get it. But you can add it in at any layer, like this: //: HorrorFlick.java // You can insert Cloneability at any // level of inheritance. import java.util.*; class Person {} class Hero extends Person {} class Scientist extends Person implements Cloneable { public Object clone() { try { return super.clone(); } catch (CloneNotSupportedException e) { // this should never happen: // It's Cloneable already! throw new InternalError(); } } } class MadScientist extends Scientist {} public class HorrorFlick { public static void main(String args[]) { Person p = new Person(); Hero h = new Hero(); Scientist s = new Scientist(); MadScientist m = new MadScientist(); // p = (Person)p.clone(); // Compile error // h = (Hero)h.clone(); // Compile error s = (Scientist)s.clone(); m = (MadScientist)m.clone(); } } ///:~ Before clonability was added, the compiler stopped you from trying to clone things. When clonability is added in Scientist, then Scientist and all its descendants are cloneable. Why this strange design? If all this seems to be a strange scheme, that’s because it is. You may wonder why it worked out this way. What is the meaning behind this design? What follows is not a substantiated story – probably because much of the marketing around Java makes it out to be a perfectly-designed language – but it does go a long way towards explaining how things ended up the way they did. Originally, Java was designed as a language to control hardware boxes, and definitely not with the Internet in mind. In a general-purpose language like this, it makes sense that the programmer be able to clone any object. Thus clone( ) was placed in the root class Object, 394 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 395. but it was a public method so you could always clone any object. This seemed to be the most flexible approach, and after all what could it hurt? Well, when Java was seen as the ultimate Internet programming language, things changed. Suddenly, there are security issues, and of course these issues are dealt with using objects, and you don’t necessarily want anyone to be able to clone your security objects. So what you’re seeing is a lot of patches applied on the original simple and straightforward scheme: clone( ) is now protected in Object. You must override it and implement Cloneable and deal with the exceptions. It’s worth noting that you must use the Cloneable interface only if you’re going to call Object’s clone( ), method, since that method checks at run-time to make sure your class implements Cloneable. But for consistency (and since Cloneable is empty anyway) you should implement it. Controlling cloneability You might suggest that, to remove clonability, the clone( ) method simply be made private, but this won’t work since you cannot take a base-class method and make it more private in a derived class. So it’s not that simple. And yet, it’s necessary to be able to control whether an object can be cloned or not. There’s actually a number of attitudes you can take to this in a class that you design: 1. Indifference. You don’t do anything about cloning, which means that your class can’t be cloned but a class that inherits from you can add cloning if it wants. This works only if the default Object.clone( ) will do something reasonable with all the fields in your class. 2. Support clone( ). Follow the standard practice of implementing Cloneable and overriding clone( ). In the overridden clone( ), you call super.clone( ) and catch all exceptions (so your overridden clone( ) doesn’t throw any exceptions). 3. Conditionally support cloning. If your class holds handles to other objects which may or may not be cloneable (an example of this is a collection class) you may try to clone all the objects that you have handles to as part of your cloning, and if they throw exceptions just pass them through. For example, consider a special sort of Vector which tries to clone all the objects it holds. When you write such a Vector, you don't know what sort of objects the client programmer may put into your Vector, so you don't know whether they can be cloned. 4. Don’t implement Cloneable but override clone( ) as protected, producing the correct copying behavior for any fields. This way, anyone inheriting from this class can override clone( ) and call super.clone( ) to produce the correct copying behavior. Note that your implementation can and should invoke super.clone( ) even though that method expects a Cloneable object (it will throw an exception otherwise), because no one will directly invoke it on an object of your type. It will get invoked only through a derived class which, if it is to work successfully, implements Cloneable. 5. Try to prevent cloning by not implementing Cloneable and overriding clone( ) to throw an exception. This is successful only if any class derived from this calls super.clone( ) in it’s redefinition of clone( ). Otherwise a programmer can get around it. Chapter 12: Passing & Returning Objects 395
- 396. 6. Prevent cloning by making your class final. If clone( ) has not been overridden then it can’t be. If it has, then override it again and throw CloneNotSupportedException. Making the class final is the only way to guarantee that cloning is prevented. In addition, when dealing with security objects or other situations where you want to control the number of objects created you should make all constructors private and provide one or more special methods for creating objects. That way, these methods can restrict the number of objects created and the conditions in which they’re created (a particular case of this is the singleton pattern shown in Chapter 16). Here’s an example that shows the various ways cloning can be implemented and then, later in the hierarchy, “turned off:” //: CheckCloneable.java // Checking to see if a handle can be cloned // Can't clone this because it doesn't // override clone(): class Ordinary {} // Overrides clone, but doesn't implement // Cloneable: class WrongClone extends Ordinary { public Object clone() throws CloneNotSupportedException { return super.clone(); // Throws exception } } // Does all the right things for cloning: class IsCloneable extends Ordinary implements Cloneable { public Object clone() throws CloneNotSupportedException { return super.clone(); } } // Turn off cloning by throwing the exception: class NoMore extends IsCloneable { public Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException(); } } class TryMore extends NoMore { public Object clone() throws CloneNotSupportedException { // Calls NoMore.clone(), throws exception: return super.clone(); } } class BackOn extends NoMore { private BackOn duplicate(BackOn b) { 396 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 397. // Somehow make a copy of b // and return that copy. This is a dummy // copy, just to make the point: return new BackOn(); } public Object clone() { // Doesn't call NoMore.clone(): return duplicate(this); } } // Can't inherit from this, so can't override // the clone method like in BackOn: final class ReallyNoMore extends NoMore {} public class CheckCloneable { static Ordinary tryToClone(Ordinary ord) { String id = ord.getClass().getName(); Ordinary x = null; if(ord instanceof Cloneable) { try { System.out.println("Attempting " + id); x = (Ordinary)((IsCloneable)ord).clone(); System.out.println("Cloned " + id); } catch(CloneNotSupportedException e) { System.out.println( "Could not clone " + id); } } return x; } public static void main(String args[]) { // Upcasting: Ordinary ord[] = { new IsCloneable(), new WrongClone(), new NoMore(), new TryMore(), new BackOn(), new ReallyNoMore(), }; Ordinary x = new Ordinary(); // This won't compile, since clone() is // protected in Object: //! x = (Ordinary)x.clone(); // tryToClone() checks first to see if // a class implements Cloneable: for(int i = 0; i < ord.length; i++) tryToClone(ord[i]); } } ///:~ The first class, Ordinary, represents the kinds of classes we’ve been seeing throughout the book: no support for cloning, but as it turns out, no prevention of cloning either. But if you have a handle to an Ordinary object that may have been upcast from a more derived class, you can’t tell if it can be cloned or not. Chapter 12: Passing & Returning Objects 397
- 398. The class WrongClone shows an incorrect way to implement cloning. It does override Object.clone( ) and makes that method public, but it doesn’t implement Cloneable, and so when super.clone( ) is called (which results in a call to Object.clone( )), CloneNotSupportedException is thrown so the cloning doesn’t work. In IsCloneable you can see all the right actions performed for cloning: clone( ) is overridden and Cloneable is implemented. However, this clone( ) method and several others that follow in this example do not catch CloneNotSupportedException but instead pass it through to the caller, who must then put a try-catch block around it. In your own clone( ) methods you will typically catch CloneNotSupportedException inside clone( ) rather than passing it through. As you’ll see, in this example it’s more informative to pass the exceptions through. Class NoMore attempts to “turn off” cloning in the way that the Java designers intended: in the derived class clone( ), you throw CloneNotSupportedException. The clone( ) method in class TryMore properly calls super.clone( ), and this resolves to NoMore.clone( ) which throws an exception and prevents cloning. But what if the programmer doesn’t follow the “proper” path of calling super.clone( ) inside the overridden clone( ) method? In BackOn, you can see how this can happen. This class uses a separate method duplicate( ) to make a copy of the current object, and calls this method inside clone( ) instead of calling super.clone( ). The exception is never thrown and the new class is cloneable. This means that you can’t rely on throwing an exception to prevent making a cloneable class. The only sure-fire solution is shown in ReallyNoMore, which is final and thus cannot be inherited. That means if clone( ) throws an exception in the final class, it cannot be modified with inheritance, and the prevention of cloning is assured (you cannot explicitly call Object.clone( ) from a class that has an arbitrary level of inheritance; you are limited to calling super.clone( ) which has access to only the direct base class). Thus, if you make any objects that involve security issues, you’ll want to make those classes final. The first method you see in class CheckCloneable is tryToClone( ) which takes any Ordinary object and checks to see whether it’s cloneable with instanceof. If so, it casts the object to an IsCloneable, calls clone( ) and casts the result back to Ordinary, catching any exceptions that are thrown. Notice the use of run-time type identification (see Chapter 11) to print out the class name so you can see what’s happening. In main( ), all different types of Ordinary objects are created and upcast to Ordinary in the array definition. The first two lines of code after that create a plain Ordinary object and try to clone it. However, this code will not compile because clone( ) is a protected method in Object. The remainder of the code steps through the array and tries to clone each object, reporting the success or failure of each. The output is: Attempting IsCloneable Cloned IsCloneable Attempting NoMore Could not clone NoMore Attempting TryMore Could not clone TryMore Attempting BackOn Cloned BackOn Attempting ReallyNoMore Could not clone ReallyNoMore So to summarize, if you want a class to be cloneable: 1. Implement the Cloneable interface. 398 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 399. 2. Override clone( ). 3. Call super.clone( ) inside your clone( ). 4. Capture exceptions inside your clone( ). This will produce the most convenient effects. The copy-constructor Cloning may seem to be a complicated process to set up. It may seem like there should be an alternative. One approach that might occur to you (especially if you’re a C++ programmer) is to make a special constructor whose job it is to duplicate an object. In C++, this is called the copy constructor. At first, this seems like the obvious solution. Here’s an example: //: CopyConstructor.java // A constructor for copying an object // of the same type, as an attempt to create // a local copy. class FruitQualities { private int weight; private int color; private int firmness; private int ripeness; private int smell; // etc. FruitQualities() { // Default constructor // do something meaningful... } // Other constructors: // ... // Copy constructor: FruitQualities(FruitQualities f) { weight = f.weight; color = f.color; firmness = f.firmness; ripeness = f.ripeness; smell = f.smell; // etc. } } class Seed { // Members... Seed() { /* Default constructor */ } Seed(Seed s) { /* Copy constructor */ } } class Fruit { private FruitQualities fq; private int seeds; private Seed[] s; Fruit(FruitQualities q, int seedCount) { fq = q; seeds = seedCount; Chapter 12: Passing & Returning Objects 399
- 400. s = new Seed[seeds]; for(int i = 0; i < seeds; i++) s[i] = new Seed(); } // Other constructors: // ... // Copy constructor: Fruit(Fruit f) { fq = new FruitQualities(f.fq); // Call all Seed copy-constructors: for(int i = 0; i < seeds; i++) s[i] = new Seed(f.s[i]); // Other copy-construction activities... } // To allow derived constructors (or other // methods) to put in different qualities: protected void addQualities(FruitQualities q) { fq = q; } protected FruitQualities getQualities() { return fq; } } class Tomato extends Fruit { Tomato() { super(new FruitQualities(), 100); } Tomato(Tomato t) { // Copy-constructor super(t); // Upcast for base copy-constructor // Other copy-construction activities... } } class ZebraQualities extends FruitQualities { private int stripedness; ZebraQualities() { // Default constructor // do something meaningful... } ZebraQualities(ZebraQualities z) { super(z); stripedness = z.stripedness; } } class GreenZebra extends Tomato { GreenZebra() { addQualities(new ZebraQualities()); } GreenZebra(GreenZebra g) { super(g); // Calls Tomato(Tomato) // Restore the right qualities: addQualities(new ZebraQualities()); } void evaluate() { 400 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 401. ZebraQualities zq = (ZebraQualities)getQualities(); // Do something with the qualities // ... } } public class CopyConstructor { public static void ripen(Tomato t) { // Use the "copy constructor": t = new Tomato(t); System.out.println("In ripen, t is a " + t.getClass().getName()); } public static void slice(Fruit f) { f = new Fruit(f); // Hmmm... will this work? System.out.println("In slice, f is a " + f.getClass().getName()); } public static void main(String args[]) { Tomato tomato = new Tomato(); ripen(tomato); // OK slice(tomato); // OOPS! GreenZebra g = new GreenZebra(); ripen(g); // OOPS! slice(g); // OOPS! g.evaluate(); } } ///:~ This seems a bit strange at first. Sure, fruit has qualities, but why not just put data members representing those qualities directly into the Fruit class? There are two potential reasons. The first is that you may want to easily insert or change the qualities. Notice that Fruit has a protected addQualities( ) method to allow derived classes to do this (you might think the logical thing to do is to have a protected constructor in Fruit that takes a FruitQualities argument, but constructors don’t inherit so it wouldn’t be available in second or greater level classes). By making the fruit qualities into a separate class, you have greater flexibility, including the ability to change the qualities midway through the lifetime of a particular Fruit object. The second reason for making FruitQualities a separate object is in case you want to add new qualities or change the behavior, via inheritance and polymorphism. Notice that for GreenZebra (which really is a type of tomato – I’ve grown them and they’re fabulous), the constructor calls addQualities( ) and passes it a ZebraQualities object, which is derived from FruitQualities and so can be attached to the FruitQualities handle in the base class. Of course, when GreenZebra uses the FruitQualities it must downcast it to the right type (as seen in evaluate( )), but it always knows that type is ZebraQualities. You’ll also see that there’s a Seed class, and that Fruit (which by definition carries its own seeds) contains an array of Seeds. Finally, notice that each class has a copy constructor, and that each copy constructor must take care to call the copy constructors for the base class and member objects so as to produce a deep copy. The copy constructor is tested inside the class CopyConstructor. The method ripen( ) takes a Tomato argument and performs copy-construction on it in order to duplicate the object: Chapter 12: Passing & Returning Objects 401
- 402. t = new Tomato(t); while slice( ) takes a more generic Fruit object and also duplicates it: f = new Fruit(f); These are tested with different kinds of Fruit in main( ). Here’s the output: In ripen, t is a Tomato In slice, f is a Fruit In ripen, t is a Tomato In slice, f is a Fruit This is where the problem shows up. After the copy-construction that happens to the Tomato inside slice( ), the result is no longer a Tomato object, but just a Fruit. It has lost all its tomato-ness. Further, when you take a GreenZebra, both ripen( ) and slice( ) turn it into a Tomato and a Fruit, respectively. Thus, unfortunately, the copy constructor scheme is no good to us in Java when attempting to make a local copy of an object. Why does it work in C++ and not Java? The copy constructor is a fundamental part of C++, since it automatically makes a local copy of an object. Yet the above example proves it does not work for Java. Why is this? The issue is that in Java everything that we manipulate is a handle, while in C++ you can have handle-like entities but you can also pass around the objects directly. That’s what the C++ copy constructor is for: when you want to take an actual object and pass it in by value, thus duplicating the object. So it works fine in C++, but you should keep in mind that this scheme fails in Java, so don’t use it. Read-only classes While the local copy produced by clone( ) gives the desired results in the appropriate cases, it is an example of forcing the programmer (the author of the method) to be responsible for preventing the ill effects of aliasing. What if you’re making a library that’s so general-purpose and commonly used that you cannot make the assumption that it will always be cloned in the proper places? Or more likely, what if you want to allow aliasing for efficiency – to prevent the needless duplication of objects – but you don’t want the negative side effects of aliasing? One solution is to create immutable objects. You can define a class such that no methods in the class cause changes to the internal state of the object. In such a class, aliasing has no impact since you can read only the internal state, so if many pieces of code are reading the same object there’s no problem. As a simple example of immutable objects, Java’s standard library contains “wrapper” classes for all the primitive types. You may have already discovered that, if you want to store an int inside a collection like a Vector (which takes only Object handles), you can wrap your int inside the standard library Integer class: //: ImmutableInteger.java // The Integer class cannot be changed import java.util.*; public class ImmutableInteger { public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 10; i++) 402 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 403. v.addElement(new Integer(i)); // But how do you change the int // inside the Integer? } } ///:~ The Integer class (as well as all the primitive “wrapper” classes) implements immutability in a very simple fashion: they have no methods that allow you to change the object. This means that if you do need an object that holds a primitive type that can be modified, you must create it yourself. Fortunately, this is trivial: //: MutableInteger.java // A changeable wrapper class import java.util.*; class IntValue { int n; IntValue(int x) { n = x; } public String toString() { return Integer.toString(n); } } public class MutableInteger { public static void main(String args[]) { Vector v = new Vector(); for(int i = 0; i < 10; i++) v.addElement(new IntValue(i)); System.out.println(v); for(int i = 0; i < v.size(); i++) ((IntValue)v.elementAt(i)).n++; System.out.println(v); } } ///:~ Note that n is friendly to simplify coding. Actually, IntValue can be even simpler if the default initialization to zero is adequate (then you don’t need the constructor) and you don’t care about printing it out (then you don’t need the toString( )): class IntValue { int n; } Fetching the element out and casting it is a bit awkward, but that’s a feature of Vector and not IntValue. Creating read-only classes It’s possible to create your own read-only class. Here’s an example: //: Immutable1.java // Objects that cannot be modified // are immune to aliasing. public class Immutable1 { private int data; public Immutable1(int initVal) { Chapter 12: Passing & Returning Objects 403
- 404. data = initVal; } public int read() { return data; } public boolean nonzero() { return data != 0; } public Immutable1 quadruple() { return new Immutable1(data * 4); } static void f(Immutable1 i1) { Immutable1 quad = i1.quadruple(); System.out.println("i1 = " + i1.read()); System.out.println("quad = " + quad.read()); } public static void main(String args[]) { Immutable1 x = new Immutable1(47); System.out.println("x = " + x.read()); f(x); System.out.println("x = " + x.read()); } } ///:~ All data is private, and you’ll see that none of the public methods modify that data. Indeed, the method that does appear to modify an object is quadruple( ), but this actually creates a new Immutable1 object and leaves the original one untouched. The method f( ) takes an Immutable1 object and performs various operations on it, and the output of main( ) demonstrates that there is no change to x. Thus, x’s object could be aliased many times without harm, because the Immutable1 class is designed to guarantee that objects cannot be changed. The drawback to immutability Creating an immutable class seems at first to provide an elegant solution. However, whenever you do need a modified object of that new type you must suffer the overhead of a new object creation, as well as potentially causing more frequent garbage collections. For some classes this is not a problem, but for others (such as the String class) it is prohibitively expensive. The solution is to create a companion class that can be modified. Then when you’re doing a lot of modifications, you can switch to using the modifiable companion class and then switch back to the immutable class when you’re done. The above example can be modified to show this: //: Immutable2.java // A companion class for making changes // to immutable objects. class Mutable { private int data; public Mutable(int initVal) { data = initVal; } public Mutable add(int x) { data += x; return this; } 404 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 405. public Mutable multiply(int x) { data *= x; return this; } public Immutable2 makeImmutable2() { return new Immutable2(data); } } public class Immutable2 { private int data; public Immutable2(int initVal) { data = initVal; } public int read() { return data; } public boolean nonzero() { return data != 0; } public Immutable2 add(int x) { return new Immutable2(data + x); } public Immutable2 multiply(int x) { return new Immutable2(data * x); } public Mutable makeMutable() { return new Mutable(data); } public static Immutable2 modify1(Immutable2 y){ Immutable2 val = y.add(12); val = val.multiply(3); val = val.add(11); val = val.multiply(2); return val; } // This produces the same result: public static Immutable2 modify2(Immutable2 y){ Mutable m = y.makeMutable(); m.add(12).multiply(3).add(11).multiply(2); return m.makeImmutable2(); } public static void main(String args[]) { Immutable2 i2 = new Immutable2(47); Immutable2 r1 = modify1(i2); Immutable2 r2 = modify2(i2); System.out.println("i2 = " + i2.read()); System.out.println("r1 = " + r1.read()); System.out.println("r2 = " + r2.read()); } } ///:~ Immutable2 contains methods that, as before, preserve the immutability of the objects by producing new objects whenever a modification is desired. These are the add( ) and multiply( ) methods. The companion class is called Mutable, and it also has add( ) and multiply( ) methods, but these modify the Mutable object itself rather than making a new one. In addition, Mutable has a method to use its data to produce an Immutable2 object, and vice versa. The two static methods modify1( ) and modify2( ) show two different approaches to producing the same result. In modify1( ), everything is done within the Immutable2 Chapter 12: Passing & Returning Objects 405
- 406. class and you can see that four new Immutable2 objects are created in the process (and each time val is reassigned, the previous object becomes garbage). In the method modify2( ), you can see that the first action is to take the Immutable2 y and produce a Mutable from it (this is just like calling clone( ) as you saw earlier, but this time a different type of object is created). Then the Mutable object is used to perform a lot of change operations without requiring the creation of many new objects. Finally it’s turned back into an Immutable2. Here, two new objects are created (the Mutable and the result Immutable2) instead of four. This approach makes sense, then, when: 1. You need immutable objects and 2. You often need to make a lot of modifications or 3. It’s very expensive to create new immutable objects Immutable String s Consider the following code: //: Stringer.java public class Stringer { static String upcase(String s) { return s.toUpperCase(); } public static void main(String[] args) { String q = new String("howdy"); System.out.println(q); // howdy String qq = upcase(q); System.out.println(qq); // HOWDY System.out.println(q); // howdy } } ///:~ When q is passed in to upcase( ) it’s actually a copy of the handle to q. The object this handle is connected to stays put in a single physical location. The handles are copied as they are passed around. Looking at the definition for upcase( ), you can see that the handle that’s passed in has the name s, and it exists for only as long as the body of upcase( ) is being executed. When upcase( ) completes, the local handle s vanishes. upcase( ) returns the result which is the original string with all the characters set to uppercase. Of course, it actually returns a handle to the result. But it turns out that the handle that it returns is for a new object, and the original q is left alone. How does this happen? Implicit constants If you say: String s = "asdf"; String x = Stringer.upcase(s); do you really want the upcase( ) method to change the argument? Generally you don’t, because an argument usually looks to the reader of the code as a piece of information 406 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 407. provided to the method, not something to be modified. This is an important guarantee, since it makes code easier to write and understand. In C++, the availability of this guarantee was important enough to put in a special keyword, const, to allow the programmer to ensure that a handle (pointer or reference in C++) could not be used to modify the original object. But then the C++ programmer was required to be very diligent and remember to use const everywhere. It can be confusing and easy to forget. Overloading ‘+’ and the StringBuffer Objects of the String class are designed to be immutable, using the technique shown previously. If you examine the online documentation for the String class (which is summarized a little later in this chapter), you’ll see that every method in the class that appears to modify a String actually creates and returns a brand new String object containing the modification. The original String is left untouched. Thus, there’s no feature in Java like C++’s const to make the compiler support the immutability of your objects. If you want it, you have to wire it in yourself, like String does. Since String objects are immutable, you can alias to a particular String as many times as you want. Because it’s read-only there’s no possibility that one handle will change something that will affect the other handles. So a read-only object solves the aliasing problem nicely. It also seems possible to handle all the cases where you need a modified object by creating a brand new version of the object with the modifications, as String does. However, for some operations this isn’t very efficient. A case in point is the operator ‘+’ which has been overloaded for String objects. Overloading means it has been given an extra meaning when used with a particular class. (The ‘+’ and ‘+=‘ for String are the only operators that are overloaded in Java and Java does not allow the programmer to overload any others2 ). When used with String objects, the ‘+’ allows you to concatenate Strings together: String s = "abc" + foo + "def" + Integer.toString(47); You could imagine how this might work: the String “abc” could have a method append( ) that creates a new String object containing “abc” concatenated with the contents of foo. The new String object would then create another new String that added “def” and so on. This would certainly work, but it requires the creation of a lot of String objects just to put together this new String, and then you have a bunch of the intermediate String objects that need to be garbage-collected. I suspect that the Java designers actually tried this approach first (which is a lesson in software design – you don’t really know anything about a system until you try it out in code and get something working). I also suspect they discovered that it delivered unacceptable performance. The solution is a mutable companion class similar to the one shown previously. For String, this companion class is called StringBuffer, and the compiler automatically creates a StringBuffer to evaluate certain expressions, in particular when the overloaded operators + and += are used with String objects. This example shows what happens: 2 C++ allows the programmer to overload operators at will. Because this can often be a complicated process – see Chapter 10 of my book Thinking in C++ (Prentice-Hall, 1995) – the Java designers deemed it a “bad” feature that shouldn’t be included in Java. It wasn’t so bad that they didn’t end up doing it themselves, and ironically enough, operator overloading would be much easier to use in Java than in C++. Chapter 12: Passing & Returning Objects 407
- 408. //: ImmutableStrings.java // Demonstrating StringBuffer public class ImmutableStrings { public static void main(String args[]) { String foo = "foo"; String s = "abc" + foo + "def" + Integer.toString(47); System.out.println(s); // The "equivalent" using StringBuffer: StringBuffer sb = new StringBuffer("abc"); // Creates String! sb.append(foo); sb.append("def"); // Creates String! sb.append(Integer.toString(47)); System.out.println(sb); } } ///:~ In the creation of String s, the compiler is actually doing the rough equivalent of the subsequent code that uses sb: a StringBuffer is created and append( ) is used to add new characters directly into the StringBuffer object (rather than making new copies each time). While this is more efficient, it’s worth noting that each time you create a quoted character string like “abc” and “def”, the compiler actually turns those into String objects. Thus, there may be more objects created than you expect, despite the efficiency afforded through StringBuffer. The String and StringBuffer classes Here is an overview of the methods available for both String and StringBuffer, so you can get a feel for the way they interact. These tables don’t contain every single method, but rather the ones that are important to this discussion. Methods that are overloaded are summarized in a single row. First, the String class: Method Arguments, Overloading Use Constructor Overloaded: Default, String, Creating String objects. StringBuffer, char arrays, byte arrays. length( ) Number of characters in String. charAt() int Index The char at a location in the String. getChars( ), getBytes( ) The beginning and end from Copy chars or bytes into an which to copy, the array to external array. copy into, an index into the destination array. toCharArray( ) Produces a char[] containing the characters in the String. 408 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 409. Method Arguments, Overloading Use equals( ), A String to compare to. An equality check on the equalsIgnoreCase( ) contents of the two Strings. compareTo( ) A String to compare to. Result is negative, zero or positive depending on the lexicographical ordering of the String and the argument. Uppercase and lowercase are not equal! regionMatches( ) Offset into this String, the Boolean result indicates other String and its offset whether the region matches. and length to compare. Overload adds “ignore case.” startsWith( ) String that it might start Boolean result indicates with. Overload adds offset whether the String starts into argument. with the argument. endsWith( ) String that might be a Boolean result indicates suffix of this String. whether the argument is a suffix. indexOf( ), Overloaded: char, char and Returns -1 if the argument lastIndexOf( ) starting index, String, is not found within this String and starting index String, otherwise returns the index where the argument starts. lastIndexOf( ) searches backward from end. substring( ) Overloaded: Starting index, Returns a new String object starting index and ending containing the specified index. character set. concat( ) The String to concatenate Returns a new String object containing the original String’s characters followed by the characters in the argument. replace( ) The old character to search Returns a new String object for, the new character to with the replacements made. replace it with. Uses the old String if no match is found. toLowerCase( ), Returns a new String object toUpperCase( ) with the case of all letters changed. Uses the old String if no changes need to be made. trim( ) Returns a new String object with the whitespace removed from each end. Uses the old String if no Chapter 12: Passing & Returning Objects 409
- 410. Method Arguments, Overloading Use changes need to be made. valueOf( ) Overloaded: Object, char[], Returns a String containing char[] and offset and count, a character representation boolean, char, int, long, of the argument. float, double. intern( ) Produces one and only one String handle for each unique character sequence You can see that every String method carefully returns a new String object when it’s necessary to change the contents. Also notice that if the contents don’t need changing the method will just return a handle to the original String. This saves storage and overhead. Here’s the StringBuffer class: Method Arguments, overloading Use Constructor Overloaded: default, Create a new StringBuffer length of buffer to create, object. String to create from. toString( ) Creates a String from this StringBuffer. length( ) Number of characters in the StringBuffer. capacity( ) Returns current number of spaces allocated ensureCapacity( ) Integer indicating desired Makes the StringBuffer hold at capacity. least the desired number of spaces. setLength( ) Integer indicating new Truncates or expands the length of character string previous character string. If in buffer. expanding, pads with nulls. charAt( ) Integer indicating the Returns the char at that location location of the desired in the buffer. element. setCharAt( ) Integer indicating the Modifies the value at that location of the desired location. element, and the new char value for the element. getChars( ) The beginning and end Copy chars into an external from which to copy, the array. There’s no getBytes( ) as array to copy into, an in String. index into the destination array. 410 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 411. Method Arguments, overloading Use append( ) Overloaded: Object, The argument is converted to a String, char[], char[] string and appended to the end with offset and length, of the current buffer, increasing boolean, char, int, long, the buffer if necessary. float, double. insert( ) Overloaded, each with a The second argument is first argument of the converted to a string and offset at which to start inserted into the current buffer inserting: Object, String, beginning at the offset. The char[], boolean, char, buffer is increased if necessary. int, long, float, double. reverse( ) The order of the characters in the buffer is reversed. The most commonly-used method is append( ), which is used by the compiler when evaluating String expressions containing the ‘+’ and ‘+=‘ operators. The insert( ) method has a similar form, and both methods perform significant manipulations to the buffer itself rather than creating new objects. String s are special By now you’ve seen that the String class is not just another class in Java – there are a lot of special cases in String, not the least of which is that it’s a built-in class and fundamental to Java. Then there’s the fact that a quoted character string is converted to a String by the compiler, and the special overloaded operators + and +=. In this chapter you’ve seen the remaining special case: the carefully-built immutability using the companion StringBuffer and some extra magic in the compiler. Summary Because everything is a handle in Java, and because every object is created on the heap and garbage collected only when it is no longer used, the flavor of object manipulation changes, especially when passing and returning objects. For example, in C or C++, if you wanted to initialize some piece of storage in a method, you’d probably request that the user pass the address of that piece of storage into the method. Otherwise you’d have to worry about who was responsible for the destruction of that storage. Thus the interface and understanding of such methods is more complicated. But in Java, you never have to worry about responsibility or whether an object will still exist when it is needed, since that is always taken care of for you. This means that your programs can create an object at the point that it is needed, and no sooner, and never worry about the mechanics of passing around responsibility for that object: you simply pass the handle. Sometimes the simplification that this provides is unnoticed, other times it is staggering. The down side to all this underlying magic is twofold: 1. You always take the efficiency hit for the extra memory management (although this can be quite small), and there’s always a slight amount of uncertainty about the time something can take to run (since the garbage collector can be forced into action whenever you get low on memory). For most applications, the benefits outweigh the drawbacks, and particularly time- critical sections can be written using native methods (see Appendix A). Chapter 12: Passing & Returning Objects 411
- 412. 2. Aliasing: sometimes you can accidentally end up with two handles to the same object, which is a problem only if both handles are assumed to point to a distinct object. This is where you need to pay a little closer attention and, when necessary, clone( ) an object to prevent the other handle from being surprised by an unexpected change. Alternatively, you can support aliasing for efficiency by creating immutable objects whose operations may return a new object of the same type or some different type, but never change the original object so that anyone aliased to that object sees no change. Some people say that cloning in Java is a botched design, and to heck with it, so they implement their own version of cloning3 and never call the Object.clone( ) method thus eliminating the need to implement Cloneable and catch the CloneNotSupportedException. This is certainly a reasonable approach and since clone( ) is supported so rarely within the standard Java library itself, apparently a safe one as well. But as long as you don’t call Object.clone( ) you don’t need to implement Cloneable or catch the exception so that would seem acceptable as well. It’s interesting to note that one of the “reserved but not implemented” keywords in Java is byvalue. After seeing the issues of aliasing and cloning, you can imagine that byvalue may someday be used to implement an automatic local copy in Java. This could eliminate the more complex issues of cloning and make coding in these situations simpler and more robust. Exercises 1. Create a class myString containing a String object that you initialize in the constructor using the constructor’s argument. Add a toString( ) method, and a method concatenate( ) that appends a String object to your internal string. Implement clone( ) in myString. Create two static methods that each take a myString x handle as an argument and call x.concatenate(“test”), but in the second method call clone( ) first. Test the two methods and show the different effects. 2. Create a class called Battery containing an int that is a battery number (as a unique identifier). Make it clonable and give it a toString( ) method. Now create a class called Toy that contains an array of Battery and a toString( ) that prints out all the batteries. Write a clone( ) for Toy that automatically clones all its Battery objects. Test this by cloning Toy and printing the result. 3. Change CheckCloneable.java so that all the clone( ) methods catch the CloneNotSupportedException rather than passing it to the caller. 4. Modify Compete.java to add more member objects to classes Thing2 and Thing4 and see if you can determine how the timings vary with complexity; whether it’s a simple linear relationship or if it seems more complicated. 3 Doug Lea, who was helpful in resolving this issue, suggested this to me, saying he just creates a function called duplicate( ) for each class. 412 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 413. 4 13: Creating windows & applets The original design goal of the graphical user interface (GUI) library in Java 1.0 was to allow the programmer to build a GUI that looks good on all platforms. That goal was not achieved; instead the Java 1.0 Abstract Window Toolkit (AWT) produces a GUI that looks equally mediocre on all systems. In addition it’s very restrictive: you can use only four fonts and you cannot access any of the more sophisticated GUI elements that may exist in your operating system (OS) since those elements may not exist in other environments. The Java 1.0 AWT programming model is also awkward and non-object- oriented. Much of this situation has been improved with the new Java 1.1 AWT event model, which takes a much clearer, object-oriented approach, along with the introduction of Java Beans, a component programming model that is particularly oriented towards the easy creation of visual programming environments. Java 1.2 finishes the transformation away from the old Java 1.0 AWT by adding the Java Foundation Classes (JFC), called “Swing.” These are a rich set of easy-to-use, easy-to-understand Java Beans that can be dragged and dropped (as well as hand-programmed) to create a GUI that you can (finally) be satisfied with. The “revision 3” rule of the software industry (it’s not good until rev 3) seems to hold true with programming languages, as well. One of Java’s primary design goals is to create applets, which are little programs that run inside a Web browser. Because they must be safe, applets are limited in what they can accomplish. However, they are a powerful tool in supporting client-side programming, a major issue for the Web. 413
- 414. Programming within an applet is so restrictive it’s often referred to as being “inside the sandbox,” since you always have someone – the Java run-time security system – watching over you. Java 1.1 offers digital signing for applets so you can choose to allow trusted applets to have access to your machine. However, you can also step outside the sandbox and write regular applications, in which case you can access the other features of your OS. We’ve been writing regular applications all along in this book, but they’ve been console applications without any graphical components. The AWT can also be used to build GUI interfaces for regular applications. In this chapter you’ll first learn the use of the original “old” AWT, which is still supported and used by many of the code examples that you will come across. Although it’s a bit painful to learn the old AWT, it’s necessary because you’ll have to read and maintain legacy code that uses the old AWT. Sometimes you’ll even need to write old AWT code to support environments that haven’t upgraded past Java 1.0. In the second part of the chapter you’ll learn about the structure of the “new” AWT in Java 1.1, and see how much better the event model is (if you can, you should use the new Java 1.1 AWT when you’re creating new programs). Finally you’ll learn about the new JFC/Swing components, which can be added to Java 1.1 as a library – this means you can use the library without requiring a full upgrade to Java 1.2. Most of the examples will show the creation of applets, not only because it’s easier but also because that’s where the AWT’s primary usefulness may reside. In addition you’ll see how things are different when you want to create a regular application using the AWT, and how to create programs that are both applets and applications so they can be run either inside a browser or from the command line. Please be aware that this is not a comprehensive glossary of all the methods for the described classes. This chapter will just get you started with the essentials. When you’re looking for more sophistication, make sure you go to your information browser to look for the classes and methods that will solve your problem (if you’re using a development environment your information browser may be built in; if you’re using the Sun JDK then you use your Web browser and start in the java root directory). Appendix E lists other resources for learning library details. Why use the AWT? One of the problems with the “old” AWT that you’ll learn about in this chapter is that it is a poor example of both object-oriented design and GUI development kit design. It throws us back into the dark ages of programming (some suggest that the ‘A’ in AWT stands for “awkward,” “awful,” “abominable,” etc.). You must write lines of code to do everything, including tasks that are accomplished much more easily using resources in other environments. Many of these problems are reduced or eliminated in Java 1.1 because: 1. The new AWT in Java 1.1 is a much better programming model and a significant step towards a better library. Java Beans is the framework for that library. 2. “GUI builders” (visual programming environments) will become de rigeur for all development systems. Java Beans and the new AWT allow the GUI builder to write code for you as you place components onto forms using graphical tools. Other component technologies like ActiveX will be supported in the same fashion. 414 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 415. So why learn to use the old AWT? “Because it’s there.” In this case, “there” has a much more ominous meaning and points to a tenet of object-oriented library design: once you publicize a component in your library, you can never take it out. If you do, you’ll wreck somebody’s existing code. In addition, there are many existing code examples out there that you’ll read as you learn about Java, all using the old AWT. The AWT must reach into the GUI components of the native OS, which means that it performs a task that an applet cannot otherwise accomplish. An untrusted applet cannot make any direct calls into an OS because otherwise it could do bad things to the user’s machine. The only way an untrusted applet can access important functionality like “draw a window on the screen” is through calls in the standard Java library that’s been specially ported and safety-checked for that machine. The original model that Sun created is that this “trusted library” will be provided only by the trusted vendor of the Java system in your Web browser, and that vendor will control what goes into it. But what if you want to extend the system by adding a new component that accesses functionality in the OS? Waiting for Sun to decide that your extension should be incorporated into the standard Java library isn’t going to solve your problem. The new model in Java 1.1 is “trusted code” or “signed code” whereby a special server verifies that a piece of code that you download is in fact “signed” by the stated author using a public-key encryption system. This way, you’ll know for sure where the code comes from, that it’s Bob’s code and not just someone pretending to be Bob. This doesn’t prevent Bob from making mistakes or doing something malicious, but it does prevent Bob from shirking responsibility – anonymity is what makes computer viruses possible. A digitally signed applet – a “trusted applet” – in Java 1.1 can reach into your machine and manipulate it directly, just like any other application you get from a “trusted” vendor and install onto your computer. But the point of all this is that the old AWT is there. There will always be old AWT code floating around, and new Java programmers learning from old books will encounter that code. Also, the old AWT is worth studying as an example of poor library design. The coverage of the old AWT given here will be relatively painless since it won’t go into depth and enumerate every single method and class, but instead give you an overview of the old AWT design. The basic applet Libraries are often grouped according to their functionality. Some libraries, for example, are used as is, off the shelf. The standard Java library String and Vector classes are examples of these. Other libraries are designed specifically as building blocks to build other classes. A certain class of library is the application framework, whose goal is to help you build applications by providing a class or set of classes that produces the basic behavior that you need in every application of a particular type. Then, to customize the behavior to your own needs you inherit from the application class and override the methods of interest. The application framework’s default control mechanism will call your overridden methods at the appropriate time. An application framework is a good example of “separating the things that change from the things that stay the same,” since it attempts to localize all the unique parts of a program in the overridden methods. Applets are built using an application framework. You inherit from class Applet and override the appropriate methods. Most of the time you’ll be concerned with only a few important methods which have to do with how the applet is built and used on a Web page. These methods are: Method Operation Chapter 13: Creating Windows & Applets 415
- 416. Method Operation init( ) Called when the applet is first created, to perform first-time initialization of the applet start( ) Called every time the applet moves into sight on the Web browser, to allow the applet to start up its normal operations (especially those that are shut off by stop( )). Also called after init( ). paint( ) Part of the base class Component (three levels of inheritance up). Called as part of an update( ) to perform special painting on the canvas of an applet. stop( ) Called every time the applet moves out of sight on the Web browser, to allow the applet to shut off expensive operations. Also called right before destroy( ). destroy( ) Called when the applet is being unloaded from the page, to perform final release of resources when the applet is no longer used Consider the paint( ) method. This method is called automatically when the Component (in this case, the applet) decides it needs to update itself – perhaps because it’s being moved back onto the screen or placed on the screen for the first time, or because some other window had been temporarily placed over your Web browser. The applet calls its update( ) method (defined in the base class Component) which goes about restoring everything, and as a part of that restoration calls paint( ). You don’t have to override paint( ) but it turns out to be an easy way to make a very simple applet, so we’ll start out with paint( ). When update( ) calls paint( ) it hands it a handle to a Graphics object that represents the surface on which you can paint. This is important because you’re limited to the surface of that particular component and thus cannot paint outside that area, which is a good thing otherwise you’d be painting outside the lines. In the case of an applet, the surface is the area inside the applet itself. The Graphics object also has a set of operations you can perform on it. These operations revolve around painting on the canvas, so most of them have to do with drawing images, shapes, arcs, etc. (note that you can look all this up in your online Java documentation if you’re curious). There are some methods that allow you to draw characters, however, and the most commonly-used one of these is drawString( ). For this, you must specify the String you want to draw and its starting location on the applet’s drawing surface. This location is given in pixels, so it will look different on different machines, but at least it’s portable. With this information you can create a very simple applet: //: Applet1.java // Very simple applet package c13; import java.awt.*; import java.applet.*; public class Applet1 extends Applet { public void paint(Graphics g) { g.drawString("First applet", 10, 10); 416 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 417. } } ///:~ Notice that applets are not required to have a main( ). That’s all wired in to the application framework; you put any startup code in init( ). To run this program you must place it inside a Web page and view that page inside your Java-enabled Web browser. To place an applet inside a Web page you put a special tag inside the HTML source for that Web page1 , to tell the page how to load and run the applet. This is the applet tag, and it looks like this for Applet1: <applet code=Applet1 width=200 height=200> </applet> The code value gives the name of the .class file where the applet resides. The width and height specify the initial size of the applet (in pixels, as before). There are other items you can place within the applet tag: a place to find other .class files on the Internet (codebase), alignment information (align), a special identifier that makes it possible for applets to communicate with each other (name), and applet parameters to provide information that the applet can retrieve. Parameters are in the form <param name="identifier" value = "information"> and there can be as many as you want. But for simple applets all you need to do is place an applet tag in the above form inside your Web page and that will load and run the applet. Testing applets You can perform a simple test without any network connection by starting up your Web browser and opening the HTML file containing the applet tag (Sun’s JDK also contains a tool called the appletviewer which displays the HTML file and applet without starting up a browser). As the HTML file is loaded, the browser will discover the applet tag and go hunting for the .class file specified by the code value. Of course, it looks at the CLASSPATH to find out where to hunt, and if your .class file isn't in the CLASSPATH then it will give an error message on the status line of the browser to the effect that it couldn't find that .class file. When you want to try this out on your Web site things are a little more complicated. First of all, you must have a Web site, which for most people means a third-party Internet Service Provider (ISP) at a remote location. Then you must have a way to move the HTML files and the .class files from your site to the correct directory (your WWW directory) on the ISP machine; this is typically done with a File Transfer Protocol (FTP) program, of which there are many different types freely available. So it would seem that all you need to do is move the files to the ISP machine with FTP, then connect to the site and HTML file using your browser – if the applet comes up and works, then everything checks out, right? 1 It is assumed that the reader is familiar with the basics of HTML. It’s not too hard to figure out, and there are lots of books and resources. Chapter 13: Creating Windows & Applets 417
- 418. Here's where you can get fooled. If the browser cannot locate the .class file on the server, it will go hunting through the CLASSPATH on your local machine. Thus, the applet may not be loading properly from the server, but to you it looks fine because the browser finds it on your machine. When someone else logs in, however, their browser can't find it. So when you're testing, make sure you erase the relevant .class files on your machine, just to make sure. One of the most insidious places where this happened to me is when I innocently placed an applet inside a package. After uploading the HTML file and applet, it turned out that the server path to the applet was confused because of the package name. However, my browser found it in the local CLASSPATH. Thus, I was the only one that could properly load the applet. It took some time to discover that the package statement was the culprit. In general, you'll want to leave the package statement out of an applet. A more graphical example The above example isn’t too thrilling, so let’s try adding a slightly more interesting graphic component: //: Applet2.java // Easy graphics import java.awt.*; import java.applet.*; public class Applet2 extends Applet { public void paint(Graphics g) { g.drawString("Second applet", 10, 15); g.draw3DRect(0, 0, 100, 20, true); } } ///:~ This puts a box around the string. Of course, all the numbers are hard-coded and are based on pixels, so on some machines the box will fit nicely around the string and on others it will probably be off, because fonts will be different on different machines. There are other interesting things you can find in the documentation for the Graphic class. Any sort of graphics activity is usually entertaining, so further experiments of this sort are left to the reader. Demonstrating the framework methods It’s interesting to see some of the framework methods (this example will look only at init( ), start( ) and stop( ) because paint( ) and destroy( ) are self-evident and not so easily traceable) in action. The following applet keeps track of the number of times these methods are called and displays them using paint( ): //: Applet3.java // Shows init(), start() and stop() activities import java.awt.*; import java.applet.*; public class Applet3 extends Applet { String s; int inits = 0; int starts = 0; int stops = 0; 418 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 419. public void init() { inits++; } public void start() { starts++; } public void stop() { stops++; } public void paint(Graphics g) { s = "inits: " + inits + ", starts: " + starts + ", stops: " + stops; g.drawString(s, 10, 10); } } ///:~ Normally when you override a method you’ll want to look to see whether you need to call the base-class version of that method, in case it does something important. For example, with init( ) you might need to call super.init( ). However, the Applet documentation specifically states that the init( ), start( ) and stop( ) methods in Applet do nothing, so it’s not necessary to call them here. When you experiment with this applet you’ll discover that if you minimize the web browser or cover it up with another window you may not get calls to stop( ) and start( ) (although this behavior seems to vary among implementations; you may wish to contrast the behavior of web browsers with that of applet viewers). The only time the calls will occur is when you move to a different web page and then come back to the one containing the applet. Making a button Making a button is quite simple: you just call the Button constructor with the label you want on the button (you can also use the default constructor if you want a button with no label, but this is not very useful). Normally you’ll want to create a handle for the button so you can refer to it later. The Button is a component, like its own little window that will automatically get repainted as part of an update. This means that you don’t explicitly paint a button or any other kind of control; you simply place them on the form and let them automatically take care of painting themselves. Thus to place a button on a form you override init( ) instead of overriding paint( ): //: Button1.java // Putting buttons on an applet import java.awt.*; import java.applet.*; public class Button1 extends Applet { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public void init() { add(b1); add(b2); } } ///:~ It’s not enough to create the Button (or any other control). You must also call the Applet add( ) method to cause the button to be placed on the applet’s form. This seems a lot simpler than it is, because the call to add( ) actually decides, implicitly, where to place the control on the form. Controlling the layout of a form is examined shortly. Chapter 13: Creating Windows & Applets 419
- 420. Capturing an event You’ll notice that if you compile and run the above applet, nothing happens when you press the buttons. This is where you must step in and write some code to determine what will happen. The basis of event-driven programming, which comprises a lot of what a GUI is about, is tying events to code that responds to those events. After working your way this far through the book and grasping some of the fundamentals of object-oriented programming, you may think that of course there will be some sort of object-oriented approach to handling events. For example, you might have to inherit each button and override some “button pressed” method (this, it turns out, is too tedious and restrictive). You might also think there’s some master “event” class that contains a method for each event you want to respond to. Before objects, the typical approach to handling events was the “giant switch statement.” Each event would have a unique integer value and inside the master event handling method you’d write a switch on that value. The AWT in Java 1.0 doesn’t use any object-oriented approach. Neither does it use a giant switch statement that relies on the assignment of numbers to events. Instead, you must create a cascaded set of if statements. What you’re trying to do with the if statements is detect the object that was the target of the event. That is, if you click on a button, then that particular button is the target. Normally, that’s all you care about – if a button is the target of an event, then it was most certainly a mouse click and you can continue based on that assumption. However, events may contain other information, as well. For example, if you want to find out the pixel location where a mouse click occurred so you can draw a line to that location, the Event object will contain the location. (You should also be aware that Java 1.0 components can be very limited in the kinds of events they generate, while Java 1.1 and Swing/JFC components produce a full set of events). The Java 1.0 AWT method where your cascaded if statement resides is called action( ). Although the whole Java 1.0 Event model has been deprecated in Java 1.1, it is still widely used for simple applets and in systems that do not yet support Java 1.1, so I recommend you become comfortable with it, including the use of the following action() method approach. action( ) has two arguments: the first is of type Event and contains all the information about the event that triggered this call to action( ). For example, it could be a mouse click, a normal keyboard press or release, a special key press or release, the fact that the component got or lost the focus, mouse movements or drags, etc. The second argument is usually the target of the event, which you’ll often ignore. The second argument is also encapsulated in the Event object so it is redundant as an argument. The situations where action( ) gets called are extremely limited: when you place controls on a form, some types of controls (buttons, check boxes, drop-down lists, menus) have a “standard action” that occurs, which causes the call to action( ) with the appropriate Event object. For example, with a button the action( ) method is called when the button is pressed, and at no other time. Normally this is just fine, since that’s what you ordinarily look for with a button. However, it’s possible to deal with many other types of events via the handleEvent( ) method as we shall see later in this chapter. The previous example can be extended to handle button clicks as follows: //: Button2.java // Capturing button presses import java.awt.*; 420 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 421. import java.applet.*; public class Button2 extends Applet { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public void init() { add(b1); add(b2); } public boolean action(Event evt, Object arg) { if(evt.target.equals(b1)) getAppletContext().showStatus("Button 1"); else if(evt.target.equals(b2)) getAppletContext().showStatus("Button 2"); // Let the base class handle it: else return super.action(evt, arg); return true; // We've handled it here } } ///:~ To see what the target is, you ask the Event object what its target member is and then use the equals( ) method to see if it matches the target object handle you’re interested in. When you’ve written handlers for all the objects you’re interested in you must call super.action(evt, arg) in the else statement at the end, as shown above. Remember from Chapter 7 (polymorphism) that your overridden method is called instead of the base class version. However, the base-class version contains code to handle all the cases that you’re not interested in, and it won’t get called unless you call it explicitly. The return value indicates whether you’ve handled it or not, so if you do match an event you should return true, otherwise return whatever the base-class event( ) returns. For this example, the simplest action is just to print what button is pressed. Some systems allow you to pop up a little window with a message in it, but applets discourage this. However, you can put a message at the bottom of the Web browser window on its status line by calling the Applet method getAppletContext( ) to get access to the browser and then showStatus( ) to put a string on the status line. You can print out a complete description of an event the same way, with getAppletContext().showStatus(evt + "" ) (the empty String forces the compiler to convert evt to a String). Both of these reports are really useful only for testing and debugging since the browser itself may overwrite your message. Strange as it may seem, you can also match an event to the text that’s on a button through the second argument in event( ). Using this technique, the above example becomes: //: Button3.java // Matching events on button text import java.awt.*; import java.applet.*; public class Button3 extends Applet { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public void init() { add(b1); Chapter 13: Creating Windows & Applets 421
- 422. add(b2); } public boolean action (Event evt, Object arg) { if(arg.equals("Button 1")) getAppletContext().showStatus("Button 1"); else if(arg.equals("Button 2")) getAppletContext().showStatus("Button 2"); // Let the base class handle it: else return super.action(evt, arg); return true; // We've handled it here } } ///:~ It’s difficult to know exactly what the equals( ) method is doing here, but despite that the biggest problem with this approach is that most new Java programmers who start with this technique spend at least one very frustrating session discovering that they’ve gotten the capitalization or spelling wrong when comparing to the text on a button (I had this experience). Also, if you change the text of the button the code will no longer work (but you won’t get any compile-time or run-time error messages). You should avoid this approach if possible. Text fields A TextField is a one-line area that allows the user to enter and edit text. TextField is inherited from TextComponent which lets you select text, get the selected text as a String, get or set the text, set whether the TextField is editable or not, along with other associated methods that you can find in your online reference. The following example demonstrates some of the functionality of a TextField; you can see that the method names are fairly obvious: //: TextField1.java // Using the text field control import java.awt.*; import java.applet.*; public class TextField1 extends Applet { Button b1 = new Button("Get Text"), b2 = new Button("Set Text"); TextField t = new TextField("Starting text", 30); String s = new String(); public void init() { add(b1); add(b2); add(t); } public boolean action (Event evt, Object arg) { if(evt.target.equals(b1)) { getAppletContext().showStatus(t.getText()); s = t.getSelectedText(); if(s.length() == 0) s = t.getText(); t.setEditable(true); 422 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 423. } else if(evt.target.equals(b2)) { t.setText("Inserted by Button 2: " + s); t.setEditable(false); } // Let the base class handle it: else return super.action(evt, arg); return true; // We've handled it here } } ///:~ There are several ways to construct a TextField; the one shown here provides an initial string and sets the size of the field in characters. Pressing button 1 either gets the text you’ve selected with the mouse or it gets all the text in the field, and places the result in String s. It also allows the field to be edited. Pressing button 2 puts a message and s into the text field and prevents the field from being edited (although you can still select the text). The editability of the text is controlled by passing setEditable( ) a true or false. Text areas A TextArea is like a TextField except that it can have multiple lines and has significantly more functionality. In addition to what you can do with a TextField, you can also append text as well as insert or replace text at a given location. It seems like this functionality could be useful for TextField as well so it’s a little confusing to try to detect how the distinction is made. You might think that if you want TextArea functionality everywhere you can simply use a one-line TextArea in places you would otherwise use a TextField. In Java 1.0, you also got scroll bars with a TextArea even when they weren’t appropriate; that is, you got both vertical and horizontal scroll bars for a one-line TextArea. In Java 1.1 this was remedied with an extra constructor that allows you to select which scroll bars (if any) are present. The following example shows only the Java 1.0 behavior, where the scrollbars are always on. Later in the chapter you’ll see an example that demonstrates Java 1.1 TextAreas. //: TextArea1.java // Using the text area control import java.awt.*; import java.applet.*; public class TextArea1 extends Applet { Button b1 = new Button("Text Area 1"); Button b2 = new Button("Text Area 2"); Button b3 = new Button("Replace Text"); Button b4 = new Button("Insert Text"); TextArea t1 = new TextArea("t1", 1, 30); TextArea t2 = new TextArea("t2", 4, 30); public void init() { add(b1); add(t1); add(b2); add(t2); add(b3); add(b4); Chapter 13: Creating Windows & Applets 423
- 424. } public boolean action (Event evt, Object arg) { if(evt.target.equals(b1)) getAppletContext().showStatus(t1.getText()); else if(evt.target.equals(b2)) { t2.setText("Inserted by Button 2"); t2.appendText(": " + t1.getText()); getAppletContext().showStatus(t2.getText()); } else if(evt.target.equals(b3)) { String s = " Replacement "; t2.replaceText(s, 3, 3 + s.length()); } else if(evt.target.equals(b4)) t2.insertText(" Inserted ", 10); // Let the base class handle it: else return super.action(evt, arg); return true; // We've handled it here } } ///:~ This example also uses a different style: the controls are all created at the point of definition and the init( ) method only adds them to the applet. There are several different TextArea constructors, but the one shown here gives a starting string and the number of rows and columns. The different buttons show appending, replacing and inserting text. Labels A Label does exactly what it sounds like: places a label on the form. This is particularly important for text fields and text areas which don’t have labels of their own, and can also be useful if you simply want to place textual information on a form. You can, as shown in the first example in this chapter, use drawString( ) inside paint( ) to place text in an exact location; when you use a Label it allows you to (approximately) associate the text with some other component via the layout manager (which shall be discussed later in this chapter). With the constructor you can create a blank label, a label with initial text in it (which is what you’ll typically do) and a label with an alignment of CENTER, LEFT or RIGHT (static final ints defined in class Label). You can also change the label and its alignment with setText( ) and setAlignment( ), and if you’ve forgotten what you’ve set these to you can read the values with getText( ) and getAlignment( ). This example shows what you can do with labels: //: Label1.java // Using labels import java.awt.*; import java.applet.*; public class Label1 extends Applet { TextField t1 = new TextField("t1", 10); Label labl1 = new Label("TextField t1"); Label labl2 = new Label(" "); 424 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 425. Label labl3 = new Label(" ", Label.RIGHT); Button b1 = new Button("Test 1"); Button b2 = new Button("Test 2"); public void init() { add(labl1); add(t1); add(b1); add(labl2); add(b2); add(labl3); } public boolean action (Event evt, Object arg) { if(evt.target.equals(b1)) labl2.setText("Text set into Label"); else if(evt.target.equals(b2)) { if(labl3.getText().trim().length() == 0) labl3.setText("labl3"); if(labl3.getAlignment() == Label.LEFT) labl3.setAlignment(Label.CENTER); else if(labl3.getAlignment() == Label.CENTER) labl3.setAlignment(Label.RIGHT); else if(labl3.getAlignment() == Label.RIGHT) labl3.setAlignment(Label.LEFT); } else return super.action(evt, arg); return true; } } ///:~ The first use of the label is the most typical: labeling a TextField or TextArea. In the second part of the example, a bunch of empty spaces are reserved and when you press the “Test 1” button setText( ) is used to insert text into the field. Because a number of blank spaces do not equal the same number of characters (in a proportionally-spaced font) you’ll see that the text gets truncated when inserted into the label. The third part of the example reserves empty space, then the first time you press the “Test 2” button it sees that there are no characters in the label (since trim( ) removes all the blank spaces at each end of a String) and inserts a short label, which is initially left- aligned. The rest of the times you press the button it changes the alignment so you can see the effect. You might think that you could create an empty label and then later put text in it with setText( ). However, you cannot put text into an empty label – presumably because it has zero width – and so creating a label with no text seems to be a useless thing to do. In the above example, the “blank” label is filled with empty spaces so it has enough width to hold text that’s later placed inside. Similarly, setAlignment( ) has no effect on a label that you’d typically create with text in the constructor. The label width is the width of the text, so changing the alignment doesn’t do anything. However, if you start with a long label and then change it to a shorter one, you can see the effect of the alignment. These behaviors occur because of the default layout manager that’s used for applets, which causes things to be squished together to their smallest size. Layout managers will be covered later in this chapter, at which time you’ll see that other layouts don’t have the same effect. Chapter 13: Creating Windows & Applets 425
- 426. Check boxes A check box provides a way to make a single on-off choice; it consists of a tiny box and a label. The box typically holds a little ‘x’ (or some other indication that it is set) or is empty depending on whether that item was selected. You’ll normally create a Checkbox using a constructor that takes the label as an argument. You can get and set the state, and also get and set the label if you want to read or change it after the Checkbox has been created. Note that the capitalization of Checkbox is inconsistent with the other controls, which may catch you by surprise since you might expect it to be “CheckBox.” Whenever a Checkbox is set or cleared an event occurs, which you can capture the same way you do a button. The following example uses a TextArea to enumerate all the check boxes that have been checked: //: CheckBox1.java // Using check boxes import java.awt.*; import java.applet.*; public class CheckBox1 extends Applet { TextArea t = new TextArea(6, 20); Checkbox cb1 = new Checkbox("Check Box 1"); Checkbox cb2 = new Checkbox("Check Box 2"); Checkbox cb3 = new Checkbox("Check Box 3"); public void init() { add(t); add(cb1); add(cb2); add(cb3); } public boolean action (Event evt, Object arg) { if(evt.target.equals(cb1)) trace("1", cb1.getState()); else if(evt.target.equals(cb2)) trace("2", cb2.getState()); else if(evt.target.equals(cb3)) trace("3", cb3.getState()); else return super.action(evt, arg); return true; } void trace(String b, boolean state) { if(state) t.appendText("Check Box " + b + " Setn"); else t.appendText("Check Box " + b + " Clearedn"); } } ///:~ The trace( ) method sends the name of the selected Checkbox and its current state to the TextArea using appendText( ) so you’ll see a cumulative list of which checkboxes were selected and what their state is. 426 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 427. Radio buttons The concept of a radio button in GUI programming comes from pre-electronic car radios with mechanical buttons: when you push one in, any other button that was pressed pops out. Thus it allows you to force a single choice among many. The AWT does not have a separate class to represent the radio button; instead it reuses the Checkbox. However, to put the Checkbox in a radio button group (and to change it’s shape so it’s visually different than an ordinary Checkbox) you must use a special constructor which takes a CheckboxGroup object as an argument (you can also call setCheckboxGroup( ) after the Checkbox has been created). A CheckboxGroup has no constructor argument; its sole reason for existence is to collect together some Checkboxes into a group of radio buttons. One of the Checkbox objects must have its state set to true before you try to display the group of radio buttons, otherwise you’ll get an exception at run time. If you try to set more than one radio button to true then only the final one set will be true. Here’s a simple example of the use of radio buttons. Notice that you capture radio button events like all others: //: RadioButton1.java // Using radio buttons import java.awt.*; import java.applet.*; public class RadioButton1 extends Applet { TextField t = new TextField("Radio button 2", 30); CheckboxGroup g = new CheckboxGroup(); Checkbox cb1 = new Checkbox("one", g, false), cb2 = new Checkbox("two", g, true), cb3 = new Checkbox("three", g, false); public void init() { t.setEditable(false); add(t); add(cb1); add(cb2); add(cb3); } public boolean action (Event evt, Object arg) { if(evt.target.equals(cb1)) t.setText("Radio button 1"); else if(evt.target.equals(cb2)) t.setText("Radio button 2"); else if(evt.target.equals(cb3)) t.setText("Radio button 3"); else return super.action(evt, arg); return true; } } ///:~ To display the state, an edit field is used. This field is set to non-editable because it’s used only to display data, not to collect it. This is shown as an alternative to using a Label. Notice the text in the field is initialized to “Radio button 2” since that’s the initial selected radio button. Chapter 13: Creating Windows & Applets 427
- 428. You can have any number of CheckboxGroups on a form. Drop-down lists Like a group of radio buttons, a drop-down list is a way to force the user to select only one element from a group of possibilities. However, it’s a much more compact way to accomplish this, and it’s easier to change the elements of the list without surprising the user (you can change radio buttons dynamically, but that tends to be visibly jarring). Java’s Choice box is not like the combo box in Windows, which lets you select from a list or type in your own selection. With a Choice box you choose one and only one element from the list. In the following example, the Choice box starts with a certain number of entries, and then new entries are added to the box when a button is pressed. This allows you to see some interesting behaviors in Choice boxes: //: Choice1.java // Using drop-down lists import java.awt.*; import java.applet.*; public class Choice1 extends Applet { String description[] = { "Ebullient", "Obtuse", "Recalcitrant", "Brilliant", "Somnescent", "Timorous", "Florid", "Putrescent" }; TextField t = new TextField(30); Choice c = new Choice(); Button b = new Button("Add items"); int count = 0; public void init() { t.setEditable(false); for(int i = 0; i < 4; i++) c.addItem(description[count++]); add(t); add(c); add(b); } public boolean action (Event evt, Object arg) { if(evt.target.equals(c)) t.setText("index: " + c.getSelectedIndex() + " " + (String)arg); else if(evt.target.equals(b)) { if(count < description.length) c.addItem(description[count++]); } else return super.action(evt, arg); return true; } } ///:~ The TextField displays the “selected index” which is the sequence number of the currently selected element, as well as the String representation of the second argument of action( ), which is in this case the string that was selected. 428 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 429. When you run this applet, pay attention to the way in which the size of the Choice box is determined: in Windows, the size is fixed from the first time you drop down the list. This means that if you drop down the list, then add more elements to the list, the elements will be there but the drop-down list won’t get any longer2 (you can scroll through the elements). However, if you add all the elements before the first time the list is dropped down, then it will be sized correctly. Of course, the user will expect to see the whole list when it’s dropped down, and so this behavior puts some significant limitations on adding elements to Choice boxes. List boxes List boxes are significantly different from Choice boxes, and not just in appearance. While a Choice box drops down when you activate it, a List occupies some fixed number of lines on a screen all the time, and doesn’t change. In addition, a List allows multiple selection: if you click on more than one item the original item stays highlighted and you can select as many as you want. If you want to see the items in a list, you simply call getSelectedItems( ) which produces an array of String of the item that’s been selected. To remove an item from a group you have to click it again. A problem with a List is that the default action is double-clicking, not single clicking. A single click adds or removes elements from the selected group, and a double click calls action( ). One way around this is to re-educate your user, which is the assumption made in the following program: //: List1.java // Using lists with action() import java.awt.*; import java.applet.*; public class List1 extends Applet { String flavors[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; // Show 6 items, allow multiple selection: List lst = new List(6, true); TextArea t = new TextArea(flavors.length, 30); Button b = new Button("test"); int count = 0; public void init() { t.setEditable(false); for(int i = 0; i < 4; i++) lst.addItem(flavors[count++]); add(t); add(lst); add(b); } public boolean action (Event evt, Object arg) { if(evt.target.equals(lst)) { t.setText(""); String[] items = lst.getSelectedItems(); 2 This behavior is apparently a bug and will be fixed in a later version of Java. Chapter 13: Creating Windows & Applets 429
- 430. for(int i = 0; i < items.length; i++) t.appendText(items[i] + "n"); } else if(evt.target.equals(b)) { if(count < flavors.length) lst.addItem(flavors[count++], 0); } else return super.action(evt, arg); return true; } } ///:~ When you press the button it adds items to the top of the list (because of the second argument 0 to addItem( )). Adding elements to a List is more reasonable than the Choice box because users expect to scroll a list box (for one thing, it has a built-in scroll bar) but they don’t expect to have to figure out how to get a drop-down list to scroll, as in the previous example. However, the only way for action( ) to be called is through a double-click. If you need to monitor other activities that the user is doing on your List (in particular, single clicks) you must take an alternative approach. handleEvent( ) So far we’ve been using action( ), but there’s another method that gets first crack at everything: handleEvent( ). Any time an event happens, it happens “over” or “to” a particular object. The handleEvent( ) method for that object is automatically called and an Event object is created and passed to handleEvent( ). The default handleEvent( ) (which is defined in Component, the base class for virtually all the “controls” in the AWT) will call either action( ) as we’ve been using, or other similar methods to indicate mouse activity, keyboard activity or to indicate that the focus has moved. We’ll look at those in a later section in this chapter. What if these other methods – action( ) in particular – don’t satisfy your needs? In the case of List, for example, when you want to catch single mouse clicks but action( ) responds to only double clicks? The solution is to override handleEvent( ) for your applet, which after all is derived from Applet and can therefore override any non-final methods. When you override handleEvent( ) for the applet you’re getting all the applet events before they are routed, so you cannot just assume “it’s to do with my button so I can assume it’s been pressed” since that’s true only for action( ). Inside handleEvent( ) it’s possible that the button has the focus and someone is typing to it. Whether it makes sense or not, those are events that you can detect and act upon in handleEvent( ). To modify the List example so that it will react to single mouse-clicks, the button detection will be left in action( ) but the code to handle the List will be moved into handleEvent( ) as follows: //: List2.java // Using lists with handleEvent() import java.awt.*; import java.applet.*; public class List2 extends Applet { String flavors[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", 430 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 431. "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; // Show 6 items, allow multiple selection: List lst = new List(6, true); TextArea t = new TextArea(flavors.length, 30); Button b = new Button("test"); int count = 0; public void init() { t.setEditable(false); for(int i = 0; i < 4; i++) lst.addItem(flavors[count++]); add(t); add(lst); add(b); } public boolean handleEvent(Event evt) { if(evt.id == Event.LIST_SELECT || evt.id == Event.LIST_DESELECT) { if(evt.target.equals(lst)) { t.setText(""); String[] items = lst.getSelectedItems(); for(int i = 0; i < items.length; i++) t.appendText(items[i] + "n"); } } else return super.handleEvent(evt); return true; } public boolean action(Event evt, Object arg) { if(evt.target.equals(b)) { if(count < flavors.length) lst.addItem(flavors[count++], 0); } else return super.action(evt, arg); return true; } } ///:~ The example is the same as before except for the addition of handleEvent( ). Inside, a check is made to see whether a list selection or deselection has occurred. Now remember handleEvent( ) is being overridden for the applet, so that means this occurrence could be anywhere on the form, so it could be happening to another list. Thus you must also check to see what the target is (although in this case there’s only one list on the applet so we could have made the assumption that all list events must be about that list – this is bad practice since it’s going to be a problem as soon as another list is added). If the list matches the one we’re interested in the same code as before will do the trick. Notice that the form for handleEvent( ) is similar to action( ): if you deal with a particular event you return true, but if you’re not interested in any of the other events via handleEvent( ) you must return super.handleEvent(evt). This is very important because if you don’t, none of the other event-handling code will get called. For example, try commenting out the return super.handleEvent(evt) in the above code. You’ll discover that action( ) never gets called, certainly not what you want. Thus, for both action( ) and handleEvent( ) it’s important to follow the above format and always call the base-class version of the method (fortunately, these kinds of bug-prone details are relegated to Java Chapter 13: Creating Windows & Applets 431
- 432. 1.0. The new design in Java 1.1 that you shall see later in the chapter eliminates these kinds of issues). In Windows, a list box automatically allows multiple selections if you hold down the shift key. This is nice because it allows the user to choose a single or multiple selection rather than fixing it during programming. You might think to be clever and implement this yourself by checking to see if the shift key is held down when a mouse click was made, by testing for evt.shiftDown( ). Alas, the design of the AWT stymies you – you’d have to be able to know which item was clicked on if the shift key wasn’t pressed so you could deselect all the rest and select only that one. However, you cannot figure that out in Java 1.0 (Java 1.1 sends all mouse, keyboard and focus events to a List, so you’ll be able to accomplish this). Controlling layout The way you place components on a form in Java is probably different from any other GUI system you’ve used. First of all, it’s all code; there are no “resources” that control placement of components. Secondly, the way components are placed on a form is controlled by a “layout manager” that decides how the components lie based on the order in which you add( ) them. The size, shape and placement of components will be remarkably different from one layout manager to another. In addition, the layout managers adapt to the dimensions of your applet or application window, so if that window dimension is changed (for example, in the HTML page’s applet specification) the size, shape and placement of the components may change. Both the Applet and Frame classes are derived from Container, whose job it is to contain and display Components (the Container itself is a Component so it can also react to events). In Container there’s a method called setLayout( ) that allows you to choose a different layout manager. In this section we’ll explore the various layout managers by placing buttons into them (since that’s the simplest thing to do). There won’t be any capturing of button events since this is just intended to show how the buttons are laid out. FlowLayout So far, all the applets that have been created seem to have laid out their components using some mysterious internal logic. That’s because the applet uses a default layout scheme: the FlowLayout. This simply “flows” the components onto the form, from left to right until the top space is full, then moves down a row and continues flowing the components on. Here’s an example that explicitly (redundantly) sets the layout manager in an applet to FlowLayout and then places buttons on the form. You’ll notice that with FlowLayout the components take on their “natural” size. A Button, for example, will be the size of its string. //: FlowLayout1.java // Demonstrating the FlowLayout import java.awt.*; import java.applet.*; public class FlowLayout1 extends Applet { public void init() { setLayout(new FlowLayout()); 432 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 433. for(int i = 0; i < 20; i++) add(new Button("Button " + i)); } } ///:~ All components will be compacted to their smallest size in a FlowLayout, so you may get a little bit of surprising behavior. For example, a label will be the size of its string, so if you try right-justifying it you won’t see any result. BorderLayout This layout manager has the concept of four border regions and a center area. When you add something to a panel that’s using a BorderLayout you must use an add( ) method that takes a String object as its first argument, and that string must specify (with proper capitalization) one of: “North” (top), “South” (bottom), “East” (right), “West” (left), or “Center.” If you misspell or mis-capitalize, you won’t get a compile-time error, but the applet simply won’t do what you expect. Fortunately, as you shall see shortly, there’s a much-improved approach in Java 1.1. Here’s a simple example: //: BorderLayout1.java // Demonstrating the BorderLayout import java.awt.*; import java.applet.*; public class BorderLayout1 extends Applet { public void init() { int i = 0; setLayout(new BorderLayout()); add("North", new Button("Button " + i++)); add("South", new Button("Button " + i++)); add("East", new Button("Button " + i++)); add("West", new Button("Button " + i++)); add("Center", new Button("Button " + i++)); } } ///:~ For every placement but "Center," the element that you add is compressed to fit in the smallest amount of space along one dimension while it is stretched to the maximum along the other dimension. "Center," however, spreads out along both dimensions to occupy the middle. The BorderLayout is the default layout manager for applications and dialogs. GridLayout A GridLayout allows you to build a table of components, and as you add them they are placed, left-to-right and top-to-bottom, in the grid. In the constructor you specify the number of rows and columns that you need and these are laid out in equal proportions. //: GridLayout1.java // Demonstrating the FlowLayout import java.awt.*; import java.applet.*; public class GridLayout1 extends Applet { Chapter 13: Creating Windows & Applets 433
- 434. public void init() { setLayout(new GridLayout(7,3)); for(int i = 0; i < 20; i++) add(new Button("Button " + i)); } } ///:~ In this case there are 21 slots but only 20 buttons. The last slot is left empty; no “balancing” goes on with a GridLayout. CardLayout The CardLayout allows you to create the rough equivalent of a “tabbed dialog,” which in more sophisticated environments has actual file-folder tabs running across one edge, and all you have to do is press a tab to bring forward a different dialog. Not so in the AWT: the CardLayout is simply a blank space and you’re responsible for bringing forward new cards (the JFC/Swing library contains tabbed panes which look much better and take care of all the details for you). Combining layouts This example will combine more than one layout type, which seems rather difficult at first since only one layout manager can be operating for an applet or application. This is true, but if you create more Panel objects, each one of those Panels can have its own layout manager and then be integrated into the applet or application as simply another component, using the applet or application’s layout manager. This gives you much greater flexibility as seen in the following example: //: CardLayout1.java // Demonstrating the CardLayout import java.awt.*; import java.applet.Applet; class ButtonPanel extends Panel { ButtonPanel(String id) { setLayout(new BorderLayout()); add("Center", new Button(id)); } } public class CardLayout1 extends Applet { Button first = new Button("First"), second = new Button("Second"), third = new Button("Third"); Panel cards = new Panel(); CardLayout cl = new CardLayout(); public void init() { setLayout(new BorderLayout()); Panel p = new Panel(); p.setLayout(new FlowLayout()); p.add(first); p.add(second); p.add(third); add("North", p); cards.setLayout(cl); 434 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 435. cards.add("First card", new ButtonPanel("The first one")); cards.add("Second card", new ButtonPanel("The second one")); cards.add("Third card", new ButtonPanel("The third one")); add("Center", cards); } public boolean action(Event evt, Object arg) { if (evt.target.equals(first)) { cl.first(cards); } else if (evt.target.equals(second)) { cl.first(cards); cl.next(cards); } else if (evt.target.equals(third)) { cl.last(cards); } else return super.action(evt, arg); return true; } } ///:~ This example begins by creating a new kind of Panel: a ButtonPanel. This contains only a single button, placed at the center of a BorderLayout, which means it will expand to fill the entire panel. The label on the button will let you know which panel you’re on in the CardLayout. In the applet, both the Panel cards where the cards will live and the layout manager cl for the CardLayout must be members of the class because you need to have access to those handles when you want to manipulate the cards. The applet is changed to use a BorderLayout instead of its default FlowLayout, a Panel is created to hold three buttons (using a FlowLayout) and this panel is placed at the “North” end of the applet. The cards panel is added to the “Center” of the applet, thus effectively occupying the rest of the real estate. When you add the ButtonPanels (or whatever other components you want) to the panel of cards, the add( ) method’s first argument is not “North,” “South,” etc. Instead it’s a string that describes the card. Although this string doesn’t show up anywhere on the card itself, you can use it if you want to flip that card using the string. This approach is not used in action( ); instead the first( ), next( ) and last( ) methods are used. Check your documentation for the other approach. In Java, the use of some sort of “tabbed panel” mechanism is quite important because (as you’ll see later) in applet programming the use of pop-up dialogs is heavily discouraged. This means that, for Java 1.0 applets, the CardLayout is the only viable way for the applet to have a number of different forms that “pop up” on command. GridBagLayout Some time ago, it was believed that all the stars, planets, the sun and the moon revolved around the earth. It seemed intuitive, from observation. But then astronomers became more sophisticated and started tracking the motion of individual objects, some of which seemed at times to go backwards in their paths. Since it was known that everything Chapter 13: Creating Windows & Applets 435
- 436. revolved around the earth, those astronomers spent large amounts of their time coming up with equations and theories to explain the motion of the stellar objects. When trying to work with GridBagLayout, you can consider yourself the analog of one of those early astronomers. The basic precept (decreed, interestingly enough, by the designers at “Sun”) is that everything should be done in code. The Copernican revolution (again dripping with irony, the discovery that the planets in the solar system revolve around the sun) is the use of resources to determine the layout and make the programmer’s job easy. Until these are added to Java, you’re stuck (to continue the metaphor) in the Spanish Inquisition of GridBagLayout and GridBagConstraints. My recommendation is to avoid GridBagLayout. Instead, use the other layout managers and especially the technique of combining several panels using different layout managers within a single program – your applets won’t look that different; at least not enough to justify the trouble that GridBagLayout entails. For my part, it’s just too painful to come up with an example for this (and I wouldn’t want to encourage this kind of library design). Instead, I’ll refer you to Core Java by Cornell & Horstmann (2nd ed., Prentice-Hall, 1997) to get started. There’s another light on the horizon: in the JFC/Swing library there is a new layout manager that uses Smalltalk’s popular “Springs and Struts,” and this may very well eliminate any need for GridBagLayout. Alternatives to action() As noted previously, action( ) isn’t the only method that’s automatically called by handleEvent( ) once it sorts everything out for you. There are three other sets of methods that are called, and if you want to capture certain types of events (keyboard, mouse and focus events) all you have to do is override the provided method. These methods are defined in the base class Component, so they’re available in virtually all the controls that you might place on a form. However, you should be very aware that this approach is deprecated in Java 1.1, so although you may see legacy code using this technique you should use the new Java 1.1 approaches (described later in this chapter) instead. Component method When it’s called action (Event evt, Object what) When the “typical” event occurs for this component (for example, when a button is pushed or a drop-down list item is selected) keyDown (Event evt, int key) A key is pressed when this component has the focus. The second argument is the key that was pressed and is redundantly copied from evt.key. keyUp(Event evt, int key) A key is released when this component has the focus. lostFocus(Event evt, Object what) The focus has moved away from the target. Normally, what is redundantly copied from evt.arg. gotFocus(Event evt, Object what) The focus has moved into the target. mouseDown(Event evt, int x, int y) A mouse down has occurred over the component, at the coordinates x, y. 436 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 437. Component method When it’s called mouseUp(Event evt, int x, int y) A mouse up has occurred over the component. mouseMove(Event evt, int x, int y) The mouse has moved while it’s over the component. mouseDrag(Event evt, int x, int y) The mouse is being dragged after a mouseDown occurred over the component. All drag events are reported to the component where the mouseDown occurred until there is a mouseUp. mouseEnter(Event evt, int x, int y) The mouse wasn’t over the component before, but now it is. mouseExit(Event evt, int x, int y) The mouse used to be over the component but now it isn’t. You can see that each method receives an Event object along with some information that you’ll typically need when you’re handling that particular situation – with a mouse event, for example, it’s likely that you’ll want to know the coordinates where the mouse event occurred. It’s interesting to note that when Component’s handleEvent( ) calls any of these methods (the typical case), the extra arguments are always redundant as they are contained within the Event object itself. In fact, if you look at the source code for Component.handleEvent( ) you can see that it explicitly plucks the additional arguments out of the Event object (this might be considered inefficient coding in some languages, but remember that Java’s focus is on safety, not necessarily speed). To prove to yourself that these events are in fact being called and as an interesting experiment it’s worth creating an applet that overrides each of the above methods (except for action( ), which is overridden in many other places in this chapter) and displays data about each of the events as they happen. This example also shows you how to make your own button object, because that’s what is used as the target of all the events of interest. You might first (naturally) assume that to make a new button, you’d inherit from Button. But this doesn’t work. Instead, you inherit from Canvas (a much more generic component) and paint your button on that canvas by overriding the paint( ) method. As you’ll see, it’s really too bad that overriding Button doesn’t work, since there’s a bit of code involved to paint the button (if you don’t believe me, try exchanging Button for Canvas in this example, and remember to call the base-class constructor super(label). You’ll see that the button doesn’t get painted and the events don’t get handled). The myButton class is very specific: it works only with an AutoEvent “parent window” (not a base class, but the window in which this button is created and lives). With this knowledge, myButton can reach into the parent window and manipulate its text fields, which is what’s necessary to be able to write the status information into the fields of the parent. Of course this is a much more limited solution, since myButton can be used only in conjunction with AutoEvent. This kind of code is sometimes called “highly coupled.” However, to make myButton more generic requires a lot more effort which isn’t warranted for this example (and possibly for many of the applets that you will write). Again, keep in mind that the following code uses APIs that are deprecated in Java 1.1. //: AutoEvent.java // Alternatives to action() import java.awt.*; import java.applet.*; import java.util.*; Chapter 13: Creating Windows & Applets 437
- 438. class MyButton extends Canvas { AutoEvent parent; Color color; String label; MyButton(AutoEvent parent, Color color, String label) { this.label = label; this.parent = parent; this.color = color; } public void paint(Graphics g) { g.setColor(color); int rnd = 30; g.fillRoundRect(0, 0, size().width, size().height, rnd, rnd); g.setColor(Color.black); g.drawRoundRect(0, 0, size().width, size().height, rnd, rnd); FontMetrics fm = g.getFontMetrics(); int width = fm.stringWidth(label); int height = fm.getHeight(); int ascent = fm.getAscent(); int leading = fm.getLeading(); int horizMargin = (size().width - width)/2; int verMargin = (size().height - height)/2; g.setColor(Color.white); g.drawString(label, horizMargin, verMargin + ascent + leading); } public boolean keyDown(Event evt, int key) { TextField t = (TextField)parent.h.get("keyDown"); t.setText(evt.toString()); return true; } public boolean keyUp(Event evt, int key) { TextField t = (TextField)parent.h.get("keyUp"); t.setText(evt.toString()); return true; } public boolean lostFocus(Event evt, Object w) { TextField t = (TextField)parent.h.get("lostFocus"); t.setText(evt.toString()); return true; } public boolean gotFocus(Event evt, Object w) { TextField t = (TextField)parent.h.get("gotFocus"); t.setText(evt.toString()); return true; } public boolean mouseDown(Event evt,int x,int y){ 438 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 439. TextField t = (TextField)parent.h.get("mouseDown"); t.setText(evt.toString()); return true; } public boolean mouseDrag(Event evt,int x,int y){ TextField t = (TextField)parent.h.get("mouseDrag"); t.setText(evt.toString()); return true; } public boolean mouseEnter(Event evt,int x,int y){ TextField t = (TextField)parent.h.get("mouseEnter"); t.setText(evt.toString()); return true; } public boolean mouseExit(Event evt,int x,int y){ TextField t = (TextField)parent.h.get("mouseExit"); t.setText(evt.toString()); return true; } public boolean mouseMove(Event evt,int x,int y){ TextField t = (TextField)parent.h.get("mouseMove"); t.setText(evt.toString()); return true; } public boolean mouseUp(Event evt,int x,int y) { TextField t = (TextField)parent.h.get("mouseUp"); t.setText(evt.toString()); return true; } } public class AutoEvent extends Applet { Hashtable h = new Hashtable(); String event[] = { "keyDown", "keyUp", "lostFocus", "gotFocus", "mouseDown", "mouseUp", "mouseMove", "mouseDrag", "mouseEnter", "mouseExit" }; MyButton b1 = new MyButton(this, Color.blue, "test1"), b2 = new MyButton(this, Color.red, "test2"); public void init() { setLayout(new GridLayout(event.length+1,2)); for(int i = 0; i < event.length; i++) { TextField t = new TextField(); t.setEditable(false); add(new Label(event[i], Label.CENTER)); add(t); h.put(event[i], t); Chapter 13: Creating Windows & Applets 439
- 440. } add(b1); add(b2); } } ///:~ You can see the constructor uses the technique of using the same name for the argument as what it’s assigned to, and differentiating between the two using this: this.label = label; The paint( ) method starts out simple: it fills a “round rectangle” with the button’s color, and then draws a black line around it. Notice the use of size( ) to determine the width and height of the component (in pixels, of course). After this, paint( ) seems quite complicated because there’s a lot of calculation going on to figure out how to center the button’s label inside the button using the “font metrics.” You can get a pretty good idea of what’s going on by looking at the method call, and it turns out that this is pretty stock code, so you can just cut and paste it when you want to center a label inside any component. You can’t understand exactly how the keyDown( ), keyUp( ) etc. methods work until you look down at the AutoEvent class. This contains a Hashtable to hold the strings representing the type of event and the TextField where information about that event is held. Of course, these could have been created statically rather than putting them in a Hashtable, but I think you’ll agree that it’s a lot easier to use and change. In particular, if you need to add or remove a new type of event in AutoEvent, you simply add or remove a string in the event[] array – everything else happens automatically. The place where you look up the strings is in the keyDown( ), keyUp( ) etc. methods back in MyButton. Each of these methods uses the parent handle to reach back to the parent window. Since that parent is an AutoEvent it contains the Hashtable h, and the get( ) method, when provided with the appropriate String, will produce a handle to an Object which we happen to know is a TextField – so it is cast to that. Then the Event object is converted to its String representation which is displayed in the TextField. It turns out this example is rather fun to play with since you can really see what’s going on with the events in your program. Applet restrictions For safety’s sake, applets are quite restricted and there are many things you can’t do. You can generally answer the question of what an applet is able to do by looking at what it is supposed to do: extend the functionality of a Web page in a browser. Since, as a net surfer, you never really know if a Web page is from a friendly place or not, you want any code that it runs to be very safe. So the biggest restrictions you’ll notice are probably: 1) An applet can’t touch the local disk. This means writing or reading, since you wouldn’t want an applet to read and transmit important information about you across the Web. Writing is prevented, of course, since that would be an open invitation to a virus. 2) An applet can’t have menus. This is probably less oriented towards safety and more towards reducing confusion. You may have noticed that an applet looks like it blends right in as part of a Web page; you often don’t see the boundaries of the applet. There’s no frame or title bar to hang the menu from, other than the one belonging to the Web browser. Perhaps the design could be changed to allow you to merge your applet menu with the browser menu – that would not only be complicated but would probably get a bit too close to the edge of safety by allowing the applet to affect its environment. 440 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 441. 3) Dialog boxes are “untrusted.” In Java, dialog boxes present a bit of a quandary. First of all, they’re not exactly disallowed in applets but they’re heavily discouraged: if you pop up a dialog box from within an applet you’ll get an “untrusted applet” message attached to that dialog. This is because, in theory, it would be possible to fool the user into thinking that they’re dealing with a regular native application and to get them to type in their credit card number which then goes across the Web (personally, I never type my credit card number into my computer. It does my bookkeeping, and more than that it doesn’t need to know). After seeing the kinds of GUIs that the AWT produces you may have a hard time believing anybody could be fooled that way. But an applet is always attached to a Web page and visible within your Web browser, while a dialog box is detached so in theory it could be possible. As a result it will be rare to see an applet that uses a dialog box. Many applet restrictions may be relaxed for trusted applets (those signed by a trusted source) in newer browsers. There are other issues when thinking about applet development, as well: • Applets take longer to download since you must download the whole thing every time, including a separate server hit for each different class. Your browser may cache the applet, but there are no guarantees. One improvement in Java 1.1 is the JAR (Java ARchive) file that allows packaging all the applet components (including other .class files as well as images and sounds) together into a single compressed file that can be downloaded in a single server transaction. “Digital signing” (the ability to verify the creator of a class) is available for each individual entry in the JAR file. • Because of security issues you must work harder to do certain things such as accessing databases and sending email. In addition, the security restrictions make accessing multiple hosts difficult, since everything has to be routed through the Web server which then becomes a performance bottleneck and a single failure point that can stop the entire process. • An applet within the browser doesn’t have the same kind of control that a native application does. For example, you can’t have a modal dialog box within an applet, since the user can always switch the page. When the user does change from a Web page or even exit the browser, the results can be catastrophic for your applet – there’s no way to save the state so if you’re in the middle of a transaction or other operation the information can be lost. In addition, different browsers do different things to your applet when you leave a Web page so the results are essentially undefined. Applet advantages If you can live within the restrictions, applets have definite advantages, especially when building client/server or other networked applications: • There is no installation issue. An applet has true platform independence (including the ability to easily play audio files, etc.) so you don’t need to make any changes in your code for different platforms nor does anyone have to perform any “tweaking” upon installation. In fact, installation is automatic every time the user loads the Web page along with the applets, so updates happen silently and automatically. In traditional client/server systems, building and installing a new version of the client software is often a nightmare. • Because of the security built into the core Java language and the applet structure, you don’t have to worry about bad code causing damage to Chapter 13: Creating Windows & Applets 441
- 442. someone’s system. This, along with the previous point, makes Java (as well as alternative client-side Web programming tools like JavaScript and VBscript) popular for so-called Intranet client/server applications that live only within the company and don’t move out onto the Internet. • Because applets are automatically integrated with HTML, you have a built-in platform-independent documentation system to support the applet. It’s an interesting twist, since we’re used to having the documentation part of the program rather than vice versa. Windowed applications It’s possible to see that for safety’s sake you can have only very limited behavior within an applet. In a very real sense, the applet is a temporary extension to the Web browser so its functionality must be limited along with its knowledge and control. There are times, however, when you’d like to make a windowed program do something else than sit on a Web page, and perhaps you’d like it to do some of the things a “regular” application can do and yet have the vaunted instant portability provided by Java. In previous chapters in this book we’ve been making command-line applications, but in some operating environments (the Macintosh, for example) there isn’t a command line. So for any number of reasons you’d like to build a windowed, non-applet program using Java. This is certainly a reasonable desire. A Java windowed application can have menus and dialog boxes (impossible or difficult with an applet), and yet if you’re using an older version of Java you sacrifice the native operating environment’s look-and-feel. The JFC/Swing library (as well as Microsoft’s AFC, and other vendor libraries) allows you to make an application that preserves the look and feel of the underlying operating environment. If you want to build windowed applications, it makes sense to do so only if you can use the very latest version of Java and associated tools so you can deliver applications that won’t confound your users. If for some reason you’re forced to use an older version of Java, think very hard before committing to building a significant windowed application. Menus It’s impossible to put a menu directly on an applet (in Java 1.0 and Java 1.1; the Swing library does allow it), so they’re only for regular applications. Go ahead, try it if you don’t believe me and you’re sure that it would make sense to have menus on applets. There’s no setMenuBar( ) method in Applet and that’s the way a menu is attached (you’ll see later that it’s possible to spawn a Frame from within an Applet, and the Frame can contain menus). There are four different types of MenuComponent, all derived from that abstract class: MenuBar (you can have only one MenuBar for a particular Frame), Menu to hold one individual drop-down menu or submenu, MenuItem to represent one single element on a menu, and CheckboxMenuItem which is derived from MenuItem and produces a checkmark to indicate whether that menu item is selected or not. Unlike a system that uses resources, with Java and the AWT you must hand-assemble all the menus in source code. Here are the ice-cream flavors again, used to create menus: //: Menu1.java // Menus work only with Frames. // Shows submenus, checkbox menu items // and swapping menus. 442 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 443. import java.awt.*; public class Menu1 extends Frame { String flavors[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; TextField t = new TextField("No flavor", 30); MenuBar mb1 = new MenuBar(); Menu f = new Menu("File"); Menu m = new Menu("Flavors"); Menu s = new Menu("Safety"); // Alternative approach: CheckboxMenuItem safety[] = { new CheckboxMenuItem("Guard"), new CheckboxMenuItem("Hide") }; MenuItem file[] = { new MenuItem("Open"), new MenuItem("Exit") }; // A second menu bar to swap to: MenuBar mb2 = new MenuBar(); Menu fooBar = new Menu("fooBar"); MenuItem other[] = { new MenuItem("Foo"), new MenuItem("Bar"), new MenuItem("Baz"), }; Button b = new Button("Swap Menus"); public Menu1() { for(int i = 0; i < flavors.length; i++) { m.add(new MenuItem(flavors[i])); // Add separators at intervals: if((i+1) % 3 == 0) m.addSeparator(); } for(int i = 0; i < safety.length; i++) s.add(safety[i]); f.add(s); for(int i = 0; i < file.length; i++) f.add(file[i]); mb1.add(f); mb1.add(m); setMenuBar(mb1); t.setEditable(false); add("Center", t); // Set up the system for swapping menus: add("North", b); for(int i = 0; i < other.length; i++) fooBar.add(other[i]); mb2.add(fooBar); } public boolean handleEvent(Event evt) { if(evt.id == Event.WINDOW_DESTROY) System.exit(0); Chapter 13: Creating Windows & Applets 443
- 444. else return super.handleEvent(evt); return true; } public boolean action(Event evt, Object arg) { if(evt.target.equals(b)) { MenuBar m = getMenuBar(); if(m == mb1) setMenuBar(mb2); else if (m == mb2) setMenuBar(mb1); } else if(evt.target instanceof MenuItem) { if(arg.equals("Open")) { String s = t.getText(); boolean chosen = false; for(int i = 0; i < flavors.length; i++) if(s.equals(flavors[i])) chosen = true; if(!chosen) t.setText("Choose a flavor first!"); else t.setText("Opening "+ s +". Mmm, mm!"); } else if(evt.target.equals(file[1])) System.exit(0); // CheckboxMenuItems cannot use String // matching; you must match the target: else if(evt.target.equals(safety[0])) t.setText("Guard the Ice Cream! " + "Guarding is " + safety[0].getState()); else if(evt.target.equals(safety[1])) t.setText("Hide the Ice Cream! " + "Is it cold? " + safety[1].getState()); else t.setText(arg.toString()); } else return super.action(evt, arg); return true; } public static void main(String args[]) { Menu1 f = new Menu1(); f.resize(300,200); f.show(); } } ///:~ In this program I avoided the typical long lists of add( ) calls for each menu, because that seemed like a lot of unnecessary typing. Instead I placed the menu items into arrays, and then simply stepped through each array calling add( ) in a for loop. This also means that adding or subtracting a menu item is less tedious. As an alternative approach (which I find less desirable since it requires more typing) the CheckboxMenuItems are created in an array of handles called safety; this is true for the arrays file and other as well. This program creates not one but two MenuBars to demonstrate that menu bars can be actively swapped while the program is running. You can see how a MenuBar is made up 444 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 445. of Menus, and each Menu is itself made up of MenuItems, CheckboxMenuItems, or even other Menus (which produce submenus). When a MenuBar is assembled it can be installed into the current program with the setMenuBar( ) method. Note that when the button is pressed, it checks to see which menu is currently installed using getMenuBar( ), then puts the other menu bar in it’s place. The rest of action( ) deals with the various menu items, if it’s not a menu item the string is sent to the TextField. When testing for “Open,” note that spelling and capitalization is critical and it is not checked, so this kind of string comparison is a clear source of programming errors. The checking and un-checking of the menu items is taken care of automatically, but dealing with CheckboxMenuItems can be a bit surprising since for some reason they don’t allow string matching (although string matching isn’t a good approach, this seems inconsistent) so you can match only the target object and not its label). As shown, the getState( ) method can be used to reveal the state. You can also change the state of a CheckboxMenuItem with setState( ). You might think that one menu could reasonably reside on more than one menu bar. This does seem to make sense because all you’re passing to the MenuBar add( ) method is a handle. However, if you try this the behavior will be strange, and not what you expect (it’s difficult to know if this is a bug or if they intended it to work this way). This example also shows what you need to do to create an application instead of an applet (again, because an application can support menus and an applet cannot directly have a menu). Instead of inheriting from Applet, you inherit from Frame. Instead of init( ) to set things up, you make a constructor for your class. Finally, you create a main( ) and in that you build an object of your new type, resize it and then call show( ). It’s different from an applet in only a few small places, but it’s now a standalone windowed application and you’ve got menus. Dialog boxes A dialog box is a window that pops up out of another window. It’s purpose is to deal with some specific issue without cluttering the original window with those details. Dialog boxes are heavily used in windowed programming environments, but as mentioned previously, rarely used in applets. To create a dialog box, you inherit from Dialog, which is just another kind of Window, like a Frame. Unlike a Frame, a Dialog cannot have a menu bar or change the cursor, but other than that they’re quite similar. A dialog has a layout manager (which defaults to BorderLayout) and you override action( ) etc., or handleEvent( ) to deal with events. One significant difference you’ll want to pay attention to in handleEvent( ): when the WINDOW_DESTROY event occurs, you don’t want to shut down the application! Instead, you release the resources used by the dialog’s window by calling dispose( ). In the following example, the dialog box is made up of a grid (using GridLayout) of a special kind of button which is defined here as class ToeButton. This button draws a frame around itself and, depending on its state, a blank, an “x” or an “o” in the middle. It starts out blank, and then depending on whose turn it is, changes to an “x” or an “o.” However, it will also flip back and forth between “x” and “o” when you click on the button (this makes the tic-tac-toe concept only slightly more annoying than it already is). In addition, the dialog box can be set up for any number of rows and columns by changing numbers in the main application window. //: ToeTest.java // Demonstration of dialog boxes Chapter 13: Creating Windows & Applets 445
- 446. // and creating your own components import java.awt.*; class ToeButton extends Canvas { int state = 0; ToeDialog parent; ToeButton(ToeDialog parent) { this.parent = parent; } public void paint(Graphics g) { int x1 = 0; int y1 = 0; int x2 = size().width - 1; int y2 = size().height - 1; g.drawRect(x1, y1, x2, y2); x1 = x2/4; y1 = y2/4; int wide = x2/2; int high = y2/2; if(state == 1) { g.drawLine(x1, y1, x1 + wide, y1 + high); g.drawLine(x1, y1 + high, x1 + wide, y1); } if(state == 2) { g.drawOval(x1, y1, x1 + wide/2, y1 + high/2); } } public boolean mouseDown(Event evt, int x, int y) { if(state == 0) { state = parent.turn; parent.turn = (parent.turn == 1 ? 2 : 1); } else state = (state == 1 ? 2 : 1); repaint(); return true; } } class ToeDialog extends Dialog { // w = number of cells wide // h = number of cells high static final int XX = 1; static final int OO = 2; int turn = XX; // Start with x's turn public ToeDialog(Frame parent, int w, int h) { super(parent, "The game itself", false); setLayout(new GridLayout(w, h)); for(int i = 0; i < w * h; i++) add(new ToeButton(this)); resize(w * 50, h * 50); } public boolean handleEvent(Event evt) { if(evt.id == Event.WINDOW_DESTROY) dispose(); 446 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 447. else return super.handleEvent(evt); return true; } } public class ToeTest extends Frame { TextField rows = new TextField("3"); TextField cols = new TextField("3"); public ToeTest() { setTitle("Toe Test"); Panel p = new Panel(); p.setLayout(new GridLayout(2,2)); p.add(new Label("Rows", Label.CENTER)); p.add(rows); p.add(new Label("Columns", Label.CENTER)); p.add(cols); add("North", p); add("South", new Button("go")); } public boolean handleEvent(Event evt) { if(evt.id == Event.WINDOW_DESTROY) System.exit(0); else return super.handleEvent(evt); return true; } public boolean action(Event evt, Object arg) { if(arg.equals("go")) { Dialog d = new ToeDialog( this, Integer.parseInt(rows.getText()), Integer.parseInt(cols.getText())); d.show(); } else return super.action(evt, arg); return true; } public static void main(String args[]) { Frame f = new ToeTest(); f.resize(200,100); f.show(); } } ///:~ The ToeButton class keeps a handle to its parent, which must be of type ToeDialog. As before, this introduces high coupling because a ToeButton can be used only with a ToeDialog but it solves a number of problems, and in truth it doesn’t seem like such a bad solution because there’s no other kind of dialog that’s keeping track of whose turn it is. Of course, you can take another approach which is to make ToeDialog.turn a static member of ToeButton, which eliminates the coupling, but prevents you from having more than one ToeDialog at a time (more than one that works properly, anyway). The paint( ) method is concerned with the graphics: drawing the square around the button and drawing the “x” or the “o.” This is full of tedious calculations, but it’s straightforward. Chapter 13: Creating Windows & Applets 447
- 448. A mouse click is captured by the overridden mouseDown( ) method, which first checks to see if the button has anything written on it. If not, the parent window is queried to find out whose turn it is and that is used to establish the state of the button. Note that the button then reaches back into the parent and changes the turn. If the button is already displaying an “x” or an “o” then that is flopped. You can see in these calculations the convenient use of the ternary if-else described in Chapter 3. After a button state change, the button is repainted. The constructor for ToeDialog is quite simple: it adds into a GridLayout as many buttons as you request, then resizes it for 50 pixels on a side for each button (if you don’t resize a Window, it won’t show up!). Notice that handleEvent( ) just calls dispose( ) for a WINDOW_DESTROY so the whole application doesn’t go away. ToeTest sets up the whole application by creating the TextFields (for inputting the rows and columns of the button grid) and the “go” button. You’ll see in action( ) that this program uses the less-desirable “string match” technique for detecting the button press (make sure you get spelling and capitalization right!). When the button is pressed, the data in the TextFields must be fetched and, since they are in String form, turned into ints using the static Integer.parseInt( ) method. Once the Dialog is created, the show( ) method must be called to display and activate it. You’ll note that the ToeDialog object is assigned to a Dialog handle d. This is an example of upcasting, although it really doesn’t make much difference here since all that’s happening is the show( ) method is called. However, if you wanted to call some method that existed only in ToeDialog you would want to assign to a ToeDialog handle and not lose the information in an upcast. File dialogs Some operating systems have a number of special built-in dialog boxes to handle the selection of things like fonts, colors, printers and the like. Virtually all graphical operating systems support the opening and saving of files, however, and so Java’s FileDialog encapsulates these for easy use. This, of course, makes no sense at all to use from an applet since an applet can neither read nor write files on the local disk (this will change for trusted applets in newer browsers). The following application exercises the two forms of file dialogs, one for opening and one for saving. Most of the code should by now be familiar, and all the interesting activities happen in action( ) for the two different button clicks: //: FileDialogTest.java // Demonstration of File dialog boxes import java.awt.*; public class FileDialogTest extends Frame { TextField filename = new TextField(); TextField directory = new TextField(); Button open = new Button("Open"); Button save = new Button("Save"); public FileDialogTest() { setTitle("File Dialog Test"); Panel p = new Panel(); p.setLayout(new FlowLayout()); p.add(open); p.add(save); add("South", p); directory.setEditable(false); 448 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 449. filename.setEditable(false); p = new Panel(); p.setLayout(new GridLayout(2,1)); p.add(filename); p.add(directory); add("North", p); } public boolean handleEvent(Event evt) { if(evt.id == Event.WINDOW_DESTROY) System.exit(0); else return super.handleEvent(evt); return true; } public boolean action(Event evt, Object arg) { if(evt.target.equals(open)) { // Two arguments, defaults to open file: FileDialog d = new FileDialog(this, "What file do you want to open?"); d.setFile("*.java"); // Filename filter d.setDirectory("."); // Current directory d.show(); String openFile; if((openFile = d.getFile()) != null) { filename.setText(openFile); directory.setText(d.getDirectory()); } else { filename.setText("You pressed cancel"); directory.setText(""); } } else if(evt.target.equals(save)) { FileDialog d = new FileDialog(this, "What file do you want to save?", FileDialog.SAVE); d.setFile("*.java"); d.setDirectory("."); d.show(); String saveFile; if((saveFile = d.getFile()) != null) { filename.setText(saveFile); directory.setText(d.getDirectory()); } else { filename.setText("You pressed cancel"); directory.setText(""); } } else return super.action(evt, arg); return true; } public static void main(String args[]) { Frame f = new FileDialogTest(); f.resize(250,110); f.show(); } Chapter 13: Creating Windows & Applets 449
- 450. } ///:~ For an “open file” dialog, you use the constructor that takes two arguments; the first is the parent window handle and the second is the title for the title bar of the FileDialog. The method setFile( ) provides an initial file name – presumably the native OS supports wildcards, so in this example all the .java files will initially be displayed. The setDirectory( ) method chooses the directory in which the file selection will begin (generally the OS allows the user to change directories). The show( ) command doesn't return until the dialog is closed. The FileDialog object still exists, so you can read data from it. If you call getFile( ) and it returns null it means the user cancelled out of the dialog. Both the file name and the results of getDirectory( ) are displayed in the TextFields. The button for saving works the same way, except that it uses a different constructor for the FileDialog. This constructor takes three arguments, and the third argument must be either FileDialog.SAVE or FileDialog.OPEN. The new AWT In Java 1.1 a dramatic change has been accomplished in the creation of the new AWT. Most of this change revolves around the new event model used in Java 1.1: as bad and awkward and non-object-oriented as the old event model was, the new event model is possibly the most elegant I have seen, and it’s difficult to understand how such a bad design (the old AWT) and such a good one (the new event model) could come out of the same group. This new way of thinking about events seems to drop so easily into your mind that the issue no longer becomes an impediment; instead it’s a tool that helps you design the system. It’s also essential for Java Beans, described later in the chapter. Instead of the non-object-oriented cascaded if statements in the old AWT, the new approach designates objects as “sources” and “listeners” of events. As you shall see, the use of inner classes is integral to the object-oriented nature of the new event model. In addition, events are now represented in a class hierarchy rather than a single class, and you can create your own event types. You’ll also find, if you’ve programmed with the old AWT, that Java 1.1 has made a number of what may seem like gratuitous name changes, for example setSize( ) replaces resize( ). This will make sense when you learn about Java Beans later in this chapter, because Beans use a particular naming convention so the names had to be modified to make the standard AWT components into Beans. Java 1.1 continues to support the old AWT to ensure backward compatibility with existing programs. Without fully admitting disaster, the online documents for Java 1.1 list all the problems involved with programming the old AWT and describe how those problems are addressed in the new AWT. Clipboard operations are supported in 1.1, although drag-and-drop “will be supported in a future release.” You can access the desktop color scheme so your Java program can fit in with the rest of the desktop. Popup menus are available, and there are some improvements for graphics and images. Mouseless operation is supported. There is a simple API for printing and simplified support for scrolling. 450 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 451. The new event model In the new event model, a component may initiate (“fire”) an event. Each type of event is represented by a distinct class. When an event is fired, it is received by one or more “listeners,” which act on that event. Thus the source of an event and the place where the event is handled may be separate. Each event listener is an object of a class that implements a particular type of listener interface. So as a programmer, all you do is create a listener object and register it with the component that’s firing the event. This registration is performed by calling a addXXXListener( ) method in the event-firing component, where XXX represents the type of event listened for. This means you can easily know what types of events can be handled by noticing the names of the addListener methods, and if you try to listen for the wrong events you’ll find out your mistake at compile time. Java Beans also utilizes the names of the addListener methods to determine what a Bean can do. All of your event logic, then, will go inside a listener class. When you create a listener class, the only restriction is that it must implement the appropriate interface. You can create a global listener class, but this is a situation where inner classes tend to be quite useful, not only because they provide a logical grouping of your listener classes inside the UI or business logic classes they are serving, but because (as you shall see later) the fact that an inner class object keeps a handle to its parent object provides a very nice way to call across class and subsystem boundaries. A simple example will make this clear. Consider the Button2.java example from earlier in this chapter. //: Button2New.java // Capturing button presses import java.awt.*; import java.awt.event.*; // Must add this import java.applet.*; public class Button2New extends Applet { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public void init() { b1.addActionListener(new B1()); b2.addActionListener(new B2()); add(b1); add(b2); } class B1 implements ActionListener { public void actionPerformed(ActionEvent e) { getAppletContext().showStatus("Button 1"); } } class B2 implements ActionListener { public void actionPerformed(ActionEvent e) { getAppletContext().showStatus("Button 2"); } } /* The old way: public boolean action(Event evt, Object arg) { if(evt.target.equals(b1)) getAppletContext().showStatus("Button 1"); Chapter 13: Creating Windows & Applets 451
- 452. else if(evt.target.equals(b2)) getAppletContext().showStatus("Button 2"); // Let the base class handle it: else return super.action(evt, arg); return true; // We've handled it here } */ } ///:~ So you can compare the two approaches, the old code is left in as a comment. In init( ), the only change is the addition of the two lines: b1.addActionListener(new B1()); b2.addActionListener(new B2()); addActionListener( ) tells a button which object to activate when the button is pressed. The classes B1 and B2 are inner classes that implement the interface ActionListener. This interface contains a single method actionPerformed( ) (meaning “this is the action that will be performed when the event is fired”). Notice that actionPerformed( ) does not take a generic event, but rather a specific type of event, ActionEvent, so you don’t need to bother testing and downcasting the argument if you want to extract specific ActionEvent information. One of the nicest things about actionPerformed( ) is how simple it is. It’s just a method that gets called. Compare it to the old action( ) method, where you must figure out what actually happened and act appropriately, and also worry about calling the base class version of action( ) and return a value to indicate whether it’s been handled or not. With the new event model you know that all the event-detection logic is taken care of so you don’t have to figure that out; you just say what happens and you’re done. If you’re not already in love with this approach over the old one, you will be soon. Event and listener types All the AWT components have been changed to include addXXXListener( ) and removeXXXListener( ) methods so that the appropriate types of listeners can be added and removed from each component. You’ll notice that the “XXX” in each case also represents the argument for the method, for example addFooListener(FooListener fl). The following table includes the associated events, listeners, methods and the components that support those particular events by providing the addXXXListener( ) and removeXXXListener( ) methods. Event, listener interface and add- Components supporting this event and remove-methods ActionEvent Button, List, TextField, MenuItem and its ActionListener derivatives including CheckboxMenuItem, Menu addActionListener( ) and PopupMenu removeActionListener( ) AdjustmentEvent Scrollbar AdjustmentListener Anything you create that implements the addAdjustmentListener( ) Adjustable interface removeAdjustmentListener( ) 452 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 453. Event, listener interface and add- Components supporting this event and remove-methods ComponentEvent Component and its derivatives, including ComponentListener Button, Canvas, Checkbox, Choice, Container, addComponentListener( ) Panel, Applet, ScrollPane, Window, Dialog, removeComponentListener( ) FileDialog, Frame, Label, List, Scrollbar, TextArea and TextField ContainerEvent Container and its derivatives, including Panel, ContainerListener Applet, ScrollPane, Window, Dialog, FileDialog addContainerListener( ) and Frame removeContainerListener( ) FocusEvent Component and its derivatives, including FocusListener Button, Canvas, Checkbox, Choice, Container, addFocusListener( ) Panel, Applet, ScrollPane, Window, Dialog, removeFocusListener( ) FileDialog, Frame Label, List, Scrollbar, TextArea and TextField KeyEvent Component and its derivatives, including KeyListener Button, Canvas, Checkbox, Choice, Container, addKeyListener( ) Panel, Applet, ScrollPane, Window, Dialog, removeKeyListener( ) FileDialog, Frame, Label, List, Scrollbar, TextArea and TextField MouseEvent (for both clicks and Component and its derivatives, including motion) Button, Canvas, Checkbox, Choice, Container, MouseListener Panel, Applet, ScrollPane, Window, Dialog, addMouseListener( ) FileDialog, Frame, Label, List, Scrollbar, removeMouseListener( ) TextArea and TextField MouseEvent3 (for both clicks and Component and its derivatives, including motion) Button, Canvas, Checkbox, Choice, Container, MouseMotionListener Panel, Applet, ScrollPane, Window, Dialog, addMouseMotionListener( ) FileDialog, Frame, Label, List, Scrollbar, removeMouseMotionListener( ) TextArea and TextField WindowEvent Window and its derivatives, including Dialog, WindowListener FileDialog and Frame addWindowListener( ) removeWindowListener( ) ItemEvent Checkbox, CheckboxMenuItem, Choice, List and ItemListener anything that implements the ItemSelectable addItemListener( ) interface removeItemListener( ) TextEvent Anything derived from TextComponent, TextListener including TextArea and TextField addTextListener( ) 3 There is no MouseMotionEvent, even though it seems like there ought to be. Clicking and motion is combined into MouseEvent, so this second appearance of MouseEvent in the table is not an error. Chapter 13: Creating Windows & Applets 453
- 454. Event, listener interface and add- Components supporting this event and remove-methods removeTextListener( ) You can see that each type of component supports only certain types of events. It’s helpful to see the events supported by each component, as shown in the following table: Component type Events supported by this component Adjustable AdjustmentEvent Applet ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Button ActionEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Canvas FocusEvent, KeyEvent, MouseEvent, ComponentEvent Checkbox ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent CheckboxMenuItem ActionEvent, ItemEvent Choice ItemEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Component FocusEvent, KeyEvent, MouseEvent, ComponentEvent Container ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Dialog ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent FileDialog ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Frame ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Label FocusEvent, KeyEvent, MouseEvent, ComponentEvent List ActionEvent, FocusEvent, KeyEvent, MouseEvent, ItemEvent, ComponentEvent Menu ActionEvent MenuItem ActionEvent Panel ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent PopupMenu ActionEvent Scrollbar AdjustmentEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent 454 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 455. Component type Events supported by this component ScrollPane ContainerEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent TextArea TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent TextComponent TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent TextField ActionEvent, TextEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Window ContainerEvent, WindowEvent, FocusEvent, KeyEvent, MouseEvent, ComponentEvent Once you know what events a particular component supports, you don’t need to look anything up to react to that event. You simply: 1. Take the name of the event class and remove the word “Event.” Add the word “Listener” to what remains. This is the listener interface you need to implement in your inner class. 2. Implement the above interface, and write out the methods for the events you want to capture. For example, you may be looking for mouse movements, so you write code for the mouseMoved( ) method of the MouseMotionListener interface (you’ll have to implement the other methods, of course, but there’s a shortcut for that which you’ll see soon). 3. Create an object of the listener class in step 2. Register it with your component with the method produced by prepending “add” to your listener name. For example, addMouseMotionListener( ). To finish what you need to know, here are the listener interfaces: Listener interface Methods in interface w/ adapter ActionListener actionPerformed(ActionEvent) AdjustmentListener adjustmentValueChanged(AdjustmentEvent) ComponentListener componentHidden(ComponentEvent) ComponentAdapter componentShown(ComponentEvent) componentMoved(ComponentEvent) componentResized(ComponentEvent) ContainerListener componentAdded(ContainerEvent) ContainerAdapter componentRemoved(ContainerEvent) FocusListener focusGained(FocusEvent) FocusAdapter focusLost(FocusEvent) KeyListener keyPressed(KeyEvent) KeyAdapter keyReleased(KeyEvent) keyTyped(KeyEvent) MouseListener mouseClicked(MouseEvent) Chapter 13: Creating Windows & Applets 455
- 456. Listener interface Methods in interface w/ adapter MouseAdapter mouseEntered(MouseEvent) mouseExited(MouseEvent) mousePressed(MouseEvent) mouseReleased(MouseEvent) MouseMotionListener mouseDragged(MouseEvent) MouseMotionAdapter mouseMoved(MouseEvent) WindowListener windowOpened(WindowEvent) WindowAdapter windowClosing(WindowEvent) windowClosed(WindowEvent) windowActivated(WindowEvent) windowDeactivated(WindowEvent) windowIconified(WindowEvent) windowDeiconified(WindowEvent) ItemListener itemStateChanged(ItemEvent) TextListener textValueChanged(TextEvent) Using listener adapters for simplicity In the above table, you can see that some listener interfaces have only one method. These are trivial to implement, since you’ll implement them only when you want to write that particular method. However, the listener interfaces that have multiple methods could be less pleasant to use. For example, something you must always do when creating an application is provide a WindowListener to the Frame so that when you get the windowClosing( ) event you can call System.exit(0) to exit the application. But since WindowListener is an interface, you must implement all the other methods even if they don’t do anything. This can be annoying. To solve the problem, each of the listener interfaces that have more than one method are provided with adapters, the names of which you can see in the above table. Each adapter provides default methods for each of the interface methods (alas, WindowAdapter does not have a default windowClosing( ) that calls System.exit(0)). Then all you need to do is inherit from the adapter and override only the methods you need to change. For example, the typical WindowListener you’ll use looks like this: class MyWindowListener extends WindowAdapter { public void windowClosing(WindowEvent e) { System.exit(0); } } The whole point of the adapters is to make the creation of listener classes very easy. There is a downside to adapters, however, in the form of a pitfall. Suppose you write a WindowAdapter like the above one: class MyWindowListener extends WindowAdapter { public void WindowClosing(WindowEvent e) { System.exit(0); } 456 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 457. } This doesn’t work, but it will drive you crazy trying to figure out why, since everything will compile and run just fine – except that closing the window won’t exit the program. Can you see the problem? It’s in the name of the method: WindowClosing( ) instead of windowClosing( ). A simple slip in capitalization results in the addition of a completely new method. However, this is not the method that’s called when the window is closing, so you don’t get the desired results. Making windows and applets with the Java 1.1 AWT Often you’ll want to be able to create a class that can be invoked as both a window and an applet. To accomplish this, you simply add a main( ) to your applet that builds an instance of the applet inside a Frame. As a simple example, let’s look at Button2New.java modified to work as both an application and an applet: //: Button2NewB.java // An application and an applet import java.awt.*; import java.awt.event.*; // Must add this import java.applet.*; public class Button2NewB extends Applet { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); TextField t = new TextField(20); public void init() { b1.addActionListener(new B1()); b2.addActionListener(new B2()); add(b1); add(b2); add(t); } class B1 implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText("Button 1"); } } class B2 implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText("Button 2"); } } // To close the application: static class WL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.exit(0); } } // A main() for the application: public static void main(String args[]) { Button2NewB applet = new Button2NewB(); Frame aFrame = new Frame("Button2NewB"); Chapter 13: Creating Windows & Applets 457
- 458. aFrame.addWindowListener(new WL()); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ The inner class WL and the main( ) are the only two elements added to the applet, and the rest of the applet is untouched. In fact, you can generally copy and paste the WL class and main( ) into your own applets with very little modification. The WL class is static so it can be easily created in main( ) (remember that an inner class normally needs an outer class handle when it’s created. Making it static eliminates this need). You can see that in main( ), the applet is explicitly initialized and started since in this case the browser isn’t available to do it for you. Of course, this doesn't provide the full behavior of the browser, which also calls stop( ) and destroy( ), but for most situations it's acceptable. If it’s a problem, you can: 1. Make the handle applet a static member of the class (rather than a local variable of main( )), and then: 2. Call applet.stop( ) and applet.destroy( ) inside WindowAdapter.windowClosing( ) before you call System.exit( ). Notice the last line: aFrame.setVisible(true); This is one of the changes in the Java 1.1 AWT: the show( ) method is deprecated, and setVisible(true) replaces it. These sorts of seemingly capricious changes will make more sense when you learn about Java Beans later in the chapter. This example is also modified to use a TextField rather than printing to the console or to the browser status line. One restriction in making a program that’s both an applet and an application is that you must choose input and output forms that work for both situations. There’s another small new feature of the Java 1.1 AWT shown here. You no longer need to use the error-prone approach of specifying BorderLayout positions using a String. When adding an element to a BorderLayout in Java 1.1, you can say: aFrame.add(applet, BorderLayout.CENTER); You name the location with one of the BorderLayout constants, which can then be checked at compile-time (rather than just quietly doing the wrong thing, as with the old form). This is a definite improvement, and shall be used throughout the rest of the book. Making the window listener an anonymous class Any of the listener classes could be implemented as anonymous classes, but there’s always a chance you may want to use their functionality elsewhere. However, the window listener is used here only to close the application’s window so you can safely make it an anonymous class. Then, in main( ), the line: aFrame.addWindowListener(new WL()); Will become: 458 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 459. aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); This has the advantage that it doesn’t require yet another class name. You’ll have to decide for yourself whether it makes the code easier to understand or more difficult. However, for the remainder of the book an anonymous inner class will usually be used for the window listener. Packaging the applet into a JAR file A very important JAR use is to optimize applet loading. In Java 1.0, people tended to try to cram all their code into a single Applet class so the client would need only a single server hit to download the applet code. Not only did this result in messy, hard to read (and maintain) programs, but the .class file itself was still uncompressed so downloading wasn’t as fast as it could have been. JAR files change all that by compressing all your .class files into a single file which is downloaded by the browser. Now you don’t need to create an ugly design to minimize the number of classes you create, and the user will get a much faster download time. Consider the above example. It looks like Button2NewB is a single class, but in fact it contains three inner classes, so that’s four in all. Once you’ve compiled the program, you package it into a JAR file with the line: jar cf Button2NewB.jar *.class This assumes that the only .class files in the current directory are the ones from Button2NewB.java (otherwise you’ll get extra baggage). Now you can create an HTML page with the new archive tag to indicate the name of the JAR file, like this: <head><title>Button2NewB Example Applet </title></head> <body> <applet code="Button2NewB.class" archive="Button2NewB.jar" width=200 height=150> </applet> </body> Everything else about applet tags in HTML files remains the same. Revisiting the earlier examples So you can see a number of examples using the new event model and so you can study the way a program can be converted from the old to the new event model, the following examples revisit many of the issues demonstrated in the first part of this chapter using the old event model. In addition, each program is now both an applet and an application so you can run it with or without a browser. Text fields This is similar to TextField1.java, but it adds significant extra behavior: Chapter 13: Creating Windows & Applets 459
- 460. //: TextNew.java // Text fields with Java 1.1 events import java.awt.*; import java.awt.event.*; import java.applet.*; public class TextNew extends Applet { Button b1 = new Button("Get Text"), b2 = new Button("Set Text"); TextField t1 = new TextField(30), t2 = new TextField(30), t3 = new TextField(30); String s = new String(); public void init() { b1.addActionListener(new B1()); b2.addActionListener(new B2()); t1.addTextListener(new T1()); t1.addActionListener(new T1A()); t1.addKeyListener(new T1K()); add(b1); add(b2); add(t1); add(t2); add(t3); } class T1 implements TextListener { public void textValueChanged(TextEvent e) { t2.setText(t1.getText()); } } class T1A implements ActionListener { private int count = 0; public void actionPerformed(ActionEvent e) { t3.setText("t1 Action Event " + count++); } } class T1K extends KeyAdapter { public void keyTyped(KeyEvent e) { String ts = t1.getText(); if(e.getKeyChar() == KeyEvent.VK_BACK_SPACE) { // Ensure it's not empty: if( ts.length() > 0) { ts = ts.substring(0, ts.length() - 1); t1.setText(ts); } } else t1.setText( t1.getText() + Character.toUpperCase( e.getKeyChar())); t1.setCaretPosition( 460 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 461. t1.getText().length()); // Stop regular character from appearing: e.consume(); } } class B1 implements ActionListener { public void actionPerformed(ActionEvent e) { s = t1.getSelectedText(); if(s.length() == 0) s = t1.getText(); t1.setEditable(true); } } class B2 implements ActionListener { public void actionPerformed(ActionEvent e) { t1.setText("Inserted by Button 2: " + s); t1.setEditable(false); } } public static void main(String args[]) { TextNew applet = new TextNew(); Frame aFrame = new Frame("TextNew"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ The TextField t3 is included as a place to report when the action listener for the TextField t1 is fired. You’ll see that the action listener for a TextField is fired only when you press the “enter” key. The TextField t1 has several listeners attached to it. The T1 listener copies all text from t1 into t2, and the T1K listener forces all characters to upper case. You’ll notice that the two work together, and if you add the T1K listener after you add the T1 listener, it doesn’t matter: all characters will still be forced to upper case in both text fields. So it would seem that keyboard events are always fired before TextComponent events, and if you want the characters in t2 to retain the original case that was typed in, you’ll have to do some extra work. T1K has some other activities of interest. You must detect a backspace (since you’re controlling everything now) and perform the deletion. The caret must be explicitly set to the end of the field, otherwise it won’t behave as you expect. And finally, to prevent the original character from being handled by the default mechanism, the event must be “consumed” using the consume( ) method that exists for event objects. This tells the system to stop firing the rest of the event handlers for this particular event. This example also quietly demonstrates one of the benefits of the design of inner classes. Notice that in the inner class: Chapter 13: Creating Windows & Applets 461
- 462. class T1 implements TextListener { public void textValueChanged(TextEvent e) { t2.setText(t1.getText()); } } t1 and t2 are not members of T1, and yet they’re accessible without any special qualification. This is because an object of an inner class automatically captures a handle to the outer object that created it, so you can treat members and methods of the enclosing class object as if they’re yours. As you can see, this is quite convenient.4 Text areas The most significant change to text areas in Java 1.1 concerns scroll bars. With the TextArea constructor, you can now control whether a TextArea will have scroll bars: vertical, horizontal, both, or neither. This example modifies the earlier Java 1.0 TextArea1.java to show the Java 1.1 scrollbar constructors: //: TextAreaNew.java // Controlling scrollbars with the TextArea // component in Java 1.1 import java.awt.*; import java.awt.event.*; import java.applet.*; public class TextAreaNew extends Applet { Button b1 = new Button("Text Area 1"); Button b2 = new Button("Text Area 2"); Button b3 = new Button("Replace Text"); Button b4 = new Button("Insert Text"); TextArea t1 = new TextArea("t1", 1, 30); TextArea t2 = new TextArea("t2", 4, 30); TextArea t3 = new TextArea("t3", 1, 30, TextArea.SCROLLBARS_NONE); TextArea t4 = new TextArea("t4", 10, 10, TextArea.SCROLLBARS_VERTICAL_ONLY); TextArea t5 = new TextArea("t5", 4, 30, TextArea.SCROLLBARS_HORIZONTAL_ONLY); TextArea t6 = new TextArea("t6", 10, 10, TextArea.SCROLLBARS_BOTH); public void init() { b1.addActionListener(new B1L()); add(b1); add(t1); b2.addActionListener(new B2L()); add(b2); add(t2); b3.addActionListener(new B3L()); add(b3); b4.addActionListener(new B4L()); add(b4); add(t3); add(t4); add(t5); add(t6); 4 And it solves the problem of “callbacks” without adding any awkward “method pointer” feature to Java. 462 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 463. } class B1L implements ActionListener { public void actionPerformed(ActionEvent e) { t5.append(t1.getText() + "n"); } } class B2L implements ActionListener { public void actionPerformed(ActionEvent e) { t2.setText("Inserted by Button 2"); t2.append(": " + t1.getText()); t5.append(t2.getText() + "n"); } } class B3L implements ActionListener { public void actionPerformed(ActionEvent e) { String s = " Replacement "; t2.replaceRange(s, 3, 3 + s.length()); } } class B4L implements ActionListener { public void actionPerformed(ActionEvent e) { t2.insert(" Inserted ", 10); } } public static void main(String args[]) { TextAreaNew applet = new TextAreaNew(); Frame aFrame = new Frame("TextAreaNew"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,725); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ You’ll notice that you can control the scrollbars only at the time of construction of the TextArea. Also, even if a TextArea doesn’t have a scrollbar, you can move the cursor such that scrolling will be forced (you can see this behavior by playing with the example). Check boxes & radio buttons As noted previously, check boxes and radio buttons are both created with the same class, Checkbox, but radio buttons are Checkboxes placed into a CheckboxGroup. In either case, the interesting event is ItemEvent, for which you create an ItemListener. When dealing with a group of check boxes or radio buttons, you have a choice: either create a new inner class to handle the event for each different Checkbox, or create one inner class that determines which Checkbox was clicked and register a single object of that inner class with each Checkbox object. The following example shows both approaches: //: RadioCheckNew.java Chapter 13: Creating Windows & Applets 463
- 464. // Radio buttons and Check Boxes in Java 1.1 import java.awt.*; import java.awt.event.*; import java.applet.*; public class RadioCheckNew extends Applet { TextField t = new TextField(30); Checkbox cb[] = { new Checkbox("Check Box 1"), new Checkbox("Check Box 2"), new Checkbox("Check Box 3") }; CheckboxGroup g = new CheckboxGroup(); Checkbox cb4 = new Checkbox("four", g, false), cb5 = new Checkbox("five", g, true), cb6 = new Checkbox("six", g, false); public void init() { t.setEditable(false); add(t); ILCheck il = new ILCheck(); for(int i = 0; i < cb.length; i++) { cb[i].addItemListener(il); add(cb[i]); } cb4.addItemListener(new IL4()); cb5.addItemListener(new IL5()); cb6.addItemListener(new IL6()); add(cb4); add(cb5); add(cb6); } // Checking the source: class ILCheck implements ItemListener { public void itemStateChanged(ItemEvent e) { for(int i = 0; i < cb.length; i++) { if(e.getSource().equals(cb[i])) { t.setText("Check box " + (i + 1)); return; } } } } // vs. an individual class for each item: class IL4 implements ItemListener { public void itemStateChanged(ItemEvent e) { t.setText("Radio button four"); } } class IL5 implements ItemListener { public void itemStateChanged(ItemEvent e) { t.setText("Radio button five"); } } class IL6 implements ItemListener { public void itemStateChanged(ItemEvent e) { t.setText("Radio button six"); } 464 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 465. } public static void main(String args[]) { RadioCheckNew applet = new RadioCheckNew(); Frame aFrame = new Frame("RadioCheckNew"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ ILCheck has the advantage that it automatically adapts when you add or subtract Checkboxes. Of course, you can use this with radio buttons as well. It should be used, however, only when your logic is general enough to support this approach, otherwise you’ll end up with a cascaded if statement, a sure sign you should revert to using independent listener classes. Drop-down lists Drop-down lists (Choice) in Java 1.1 also use ItemListeners to notify you when a choice has changed: //: ChoiceNew.java // Drop-down lists with Java 1.1 import java.awt.*; import java.awt.event.*; import java.applet.*; public class ChoiceNew extends Applet { String description[] = { "Ebullient", "Obtuse", "Recalcitrant", "Brilliant", "Somnescent", "Timorous", "Florid", "Putrescent" }; TextField t = new TextField(100); Choice c = new Choice(); Button b = new Button("Add items"); int count = 0; public void init() { t.setEditable(false); for(int i = 0; i < 4; i++) c.addItem(description[count++]); add(t); add(c); add(b); c.addItemListener(new CL()); b.addActionListener(new BL()); } class CL implements ItemListener { public void itemStateChanged(ItemEvent e) { t.setText("index: " + c.getSelectedIndex() + " " + e.toString()); Chapter 13: Creating Windows & Applets 465
- 466. } } class BL implements ActionListener { public void actionPerformed(ActionEvent e) { if(count < description.length) c.addItem(description[count++]); } } public static void main(String args[]) { ChoiceNew applet = new ChoiceNew(); Frame aFrame = new Frame("ChoiceNew"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(750,100); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Nothing else here is particularly new (except that Java 1.1 has significantly fewer bugs in the UI classes). Lists You’ll recall that one of the problems with the Java 1.0 List design is that it took extra work to make it do what you’d expect: react to a single click on one of the list elements. Java 1.1 has solved this problem: //: ListNew.java // Java 1.1 Lists are easier to use import java.awt.*; import java.awt.event.*; import java.applet.*; public class ListNew extends Applet { String flavors[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; // Show 6 items, allow multiple selection: List lst = new List(6, true); TextArea t = new TextArea(flavors.length, 30); Button b = new Button("test"); int count = 0; public void init() { t.setEditable(false); for(int i = 0; i < 4; i++) lst.addItem(flavors[count++]); add(t); add(lst); add(b); 466 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 467. lst.addItemListener(new LL()); b.addActionListener(new BL()); } class LL implements ItemListener { public void itemStateChanged(ItemEvent e) { t.setText(""); String[] items = lst.getSelectedItems(); for(int i = 0; i < items.length; i++) t.append(items[i] + "n"); } } class BL implements ActionListener { public void actionPerformed(ActionEvent e) { if(count < flavors.length) lst.addItem(flavors[count++], 0); } } public static void main(String args[]) { ListNew applet = new ListNew(); Frame aFrame = new Frame("ListNew"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ You can see that no extra logic is required to support a single click on a list item. You just attach a listener like you do everywhere else. Menus The event handling for menus does seem to benefit from the new Java 1.1 event model, but Java’s approach to menus is still very messy and requires a lot of hand-coding. The right medium for a menu seems to be a resource rather than lots of code. Keep in mind that program-building tools will generally handle the creation of menus for you, so that will reduce the pain somewhat (as long as they will also handle the maintenance!). In addition, you’ll find the events for menus are inconsistent and can lead to confusion: MenuItems use ActionListeners, but CheckboxMenuItems use ItemListeners. The Menu objects themselves can also support ActionListeners, but that’s not usually helpful. Generally you’ll just attach listeners to each MenuItem or CheckboxMenuItem, but the following example (revised from the earlier version) also shows ways to combine the capture of multiple menu components into a single listener class. As you’ll see, it’s probably not worth the hassle to do this. //: MenuNew.java // Menus in Java 1.1 import java.awt.*; import java.awt.event.*; Chapter 13: Creating Windows & Applets 467
- 468. public class MenuNew extends Frame { String flavors[] = { "Chocolate", "Strawberry", "Vanilla Fudge Swirl", "Mint Chip", "Mocha Almond Fudge", "Rum Raisin", "Praline Cream", "Mud Pie" }; TextField t = new TextField("No flavor", 30); MenuBar mb1 = new MenuBar(); Menu f = new Menu("File"); Menu m = new Menu("Flavors"); Menu s = new Menu("Safety"); // Alternative approach: CheckboxMenuItem safety[] = { new CheckboxMenuItem("Guard"), new CheckboxMenuItem("Hide") }; MenuItem file[] = { // No menu shortcut: new MenuItem("Open"), // Adding a menu shortcut is very simple: new MenuItem("Exit", new MenuShortcut(KeyEvent.VK_E)) }; // A second menu bar to swap to: MenuBar mb2 = new MenuBar(); Menu fooBar = new Menu("fooBar"); MenuItem other[] = { new MenuItem("Foo"), new MenuItem("Bar"), new MenuItem("Baz"), }; // Initialization code: { ML ml = new ML(); CMI cmi = new CMI(); safety[0].setActionCommand("Guard"); safety[0].addItemListener(cmi); safety[1].setActionCommand("Hide"); safety[1].addItemListener(cmi); file[0].setActionCommand("Open"); file[0].addActionListener(ml); file[1].setActionCommand("Exit"); file[1].addActionListener(ml); other[0].addActionListener(new FooL()); other[1].addActionListener(new BarL()); other[2].addActionListener(new BazL()); } Button b = new Button("Swap Menus"); public MenuNew() { FL fl = new FL(); for(int i = 0; i < flavors.length; i++) { MenuItem mi = new MenuItem(flavors[i]); mi.addActionListener(fl); m.add(mi); // Add separators at intervals: if((i+1) % 3 == 0) 468 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 469. m.addSeparator(); } for(int i = 0; i < safety.length; i++) s.add(safety[i]); f.add(s); for(int i = 0; i < file.length; i++) f.add(file[i]); mb1.add(f); mb1.add(m); setMenuBar(mb1); t.setEditable(false); add(t, BorderLayout.CENTER); // Set up the system for swapping menus: b.addActionListener(new BL()); add(b, BorderLayout.NORTH); for(int i = 0; i < other.length; i++) fooBar.add(other[i]); mb2.add(fooBar); } class BL implements ActionListener { public void actionPerformed(ActionEvent e) { MenuBar m = getMenuBar(); if(m == mb1) setMenuBar(mb2); else if (m == mb2) setMenuBar(mb1); } } class ML implements ActionListener { public void actionPerformed(ActionEvent e) { MenuItem target = (MenuItem)e.getSource(); String actionCommand = target.getActionCommand(); if(actionCommand.equals("Open")) { String s = t.getText(); boolean chosen = false; for(int i = 0; i < flavors.length; i++) if(s.equals(flavors[i])) chosen = true; if(!chosen) t.setText("Choose a flavor first!"); else t.setText("Opening "+ s +". Mmm, mm!"); } else if(actionCommand.equals("Exit")) { dispatchEvent( new WindowEvent(MenuNew.this, WindowEvent.WINDOW_CLOSING)); } } } class FL implements ActionListener { public void actionPerformed(ActionEvent e) { MenuItem target = (MenuItem)e.getSource(); t.setText(target.getLabel()); } } // Alternatively, you can create a different // class for each different MenuItem. Then you // Don't have to figure out which one it is: Chapter 13: Creating Windows & Applets 469
- 470. class FooL implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText("Foo selected"); } } class BarL implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText("Bar selected"); } } class BazL implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText("Baz selected"); } } class CMI implements ItemListener { public void itemStateChanged(ItemEvent e) { CheckboxMenuItem target = (CheckboxMenuItem)e.getSource(); String actionCommand = target.getActionCommand(); if(actionCommand.equals("Guard")) t.setText("Guard the Ice Cream! " + "Guarding is " + target.getState()); else if(actionCommand.equals("Hide")) t.setText("Hide the Ice Cream! " + "Is it cold? " + target.getState()); } } public static void main(String args[]) { MenuNew f = new MenuNew(); f.setSize(300,200); f.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); f.setVisible(true); } } ///:~ This code is very similar to the previous (Java 1.0) version, until you get to the initialization section (marked by the opening brace right after the comment “Initialization code:”). Here you can see the ItemListeners and ActionListeners attached to the various menu components. Java 1.1 supports “menu shortcuts,” so you can select a menu item using the keyboard instead of the mouse. These are quite simple: you just use the overloaded MenuItem constructor that takes as a second argument a MenuShortcut object. The constructor for MenuShortcut takes the key of interest, which magically appears on the menu item when it drops down. The above example adds Control-E to the “Exit” menu item. You can also see the use of setActionCommand( ). This seems a bit strange because in each case the “action command” is exactly the same as the label on the menu component. Why not just use the label, instead of this alternative string? The problem is 470 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 471. internationalization. If you retarget this program to another language, you want to change only the label in the menu, and not go through the code changing all the logic which will no doubt introduce new errors. So to make this easy for code that checks the text string associated with a menu component, the “action command” can be immutable while the menu label can change. All the code works with the “action command,” so it’s unaffected by changes to the menu labels. Notice that in this program, not all the menu components are examined for their action commands, so those that aren’t don’t have their action command set. Much of the constructor is the same as before, with the exception of a couple of calls to add listeners. The bulk of the work happens in the listeners themselves. In BL, the MenuBar swapping happens as in the previous example. In ML, the “figure out who rang” approach is taken by getting the source of the ActionEvent and casting it to a MenuItem, then getting the action command string to pass it through a cascaded if statement. Much of this is the same as before, but notice that if “Exit” is chosen, a new WindowEvent is created, passing in the handle of the enclosing class object (MenuNew.this) and creating a WINDOW_CLOSING event. This is handed to the dispatchEvent( ) method of the enclosing class object, which then ends up calling windowClosing( ) inside WL, just as if the message had been generated the “normal” way. Through this mechanism, you can dispatch any message you want in any circumstances, so it’s quite powerful. The FL listener is simple even though it’s handling all the different flavors in the flavor menu. This approach is useful if you have enough simplicity in your logic, but in general you’ll usually want to take the approach used with FooL, BarL and BazL, where they are each attached to only a single menu component, so no extra detection logic is necessary and you know exactly who called the listener. Even with the profusion of classes generated this way, the code inside tends to be smaller and the process is more foolproof. Dialog boxes This is a direct rewrite of the earlier ToeTest.java. In this version, however, everything is placed inside an inner class. Although this completely eliminates the need to keep track of the object that spawned any class, as was the case in ToeTest.java, it may be taking the concept of inner classes a bit too far. At one point, the inner classes are nested four deep! This is the kind of design where you need to decide whether the benefit of inner classes is worth the increased complexity. In addition, when you create an inner class you’re tying that class to its surrounding class. A standalone class can more easily be reused. //: ToeTestNew.java // Demonstration of dialog boxes // and creating your own components import java.awt.*; import java.awt.event.*; public class ToeTestNew extends Frame { TextField rows = new TextField("3"); TextField cols = new TextField("3"); public ToeTestNew() { setTitle("Toe Test"); Panel p = new Panel(); p.setLayout(new GridLayout(2,2)); p.add(new Label("Rows", Label.CENTER)); p.add(rows); p.add(new Label("Columns", Label.CENTER)); p.add(cols); add(p, BorderLayout.NORTH); Button b = new Button("go"); Chapter 13: Creating Windows & Applets 471
- 472. b.addActionListener(new BL()); add(b, BorderLayout.SOUTH); } static final int XX = 1; static final int OO = 2; class ToeDialog extends Dialog { // w = number of cells wide // h = number of cells high int turn = XX; // Start with x's turn public ToeDialog(int w, int h) { super(ToeTestNew.this, "The game itself", false); setLayout(new GridLayout(w, h)); for(int i = 0; i < w * h; i++) add(new ToeButton()); setSize(w * 50, h * 50); addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { dispose(); } }); } class ToeButton extends Canvas { int state = 0; ToeButton() { addMouseListener(new ML()); } public void paint(Graphics g) { int x1 = 0; int y1 = 0; int x2 = getSize().width - 1; int y2 = getSize().height - 1; g.drawRect(x1, y1, x2, y2); x1 = x2/4; y1 = y2/4; int wide = x2/2; int high = y2/2; if(state == 1) { g.drawLine(x1, y1, x1 + wide, y1 + high); g.drawLine(x1, y1 + high, x1 + wide, y1); } if(state == 2) { g.drawOval(x1, y1, x1 + wide/2, y1 + high/2); } } class ML extends MouseAdapter { public void mousePressed(MouseEvent e) { if(state == 0) { state = turn; turn = (turn == 1 ? 2 : 1); } 472 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 473. else state = (state == 1 ? 2 : 1); repaint(); } } } } class BL implements ActionListener { public void actionPerformed(ActionEvent e) { Dialog d = new ToeDialog( Integer.parseInt(rows.getText()), Integer.parseInt(cols.getText())); d.show(); } } static class WL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.exit(0); } } public static void main(String args[]) { Frame f = new ToeTestNew(); f.setSize(200,100); f.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); f.setVisible(true); } } ///:~ There are some restrictions when using inner classes here. In particular, statics can be at only the outer level of the class, so inner classes cannot have static data or static inner classes. File dialogs Converting from FileDialogTest.java to the new event model is very straightforward: //: FileDialogNew.java // Demonstration of File dialog boxes import java.awt.*; import java.awt.event.*; public class FileDialogNew extends Frame { TextField filename = new TextField(); TextField directory = new TextField(); Button open = new Button("Open"); Button save = new Button("Save"); public FileDialogNew() { setTitle("File Dialog Test"); Panel p = new Panel(); p.setLayout(new FlowLayout()); open.addActionListener(new OpenL()); p.add(open); Chapter 13: Creating Windows & Applets 473
- 474. save.addActionListener(new SaveL()); p.add(save); add(p, BorderLayout.SOUTH); directory.setEditable(false); filename.setEditable(false); p = new Panel(); p.setLayout(new GridLayout(2,1)); p.add(filename); p.add(directory); add(p, BorderLayout.NORTH); } class OpenL implements ActionListener { public void actionPerformed(ActionEvent e) { // Two arguments, defaults to open file: FileDialog d = new FileDialog( FileDialogNew.this, "What file do you want to open?"); d.setFile("*.java"); d.setDirectory("."); // Current directory d.show(); String yourFile = "*.*"; if((yourFile = d.getFile()) != null) { filename.setText(yourFile); directory.setText(d.getDirectory()); } else { filename.setText("You pressed cancel"); directory.setText(""); } } } class SaveL implements ActionListener { public void actionPerformed(ActionEvent e) { FileDialog d = new FileDialog( FileDialogNew.this, "What file do you want to save?", FileDialog.SAVE); d.setFile("*.java"); d.setDirectory("."); d.show(); String saveFile; if((saveFile = d.getFile()) != null) { filename.setText(saveFile); directory.setText(d.getDirectory()); } else { filename.setText("You pressed cancel"); directory.setText(""); } } } public static void main(String args[]) { Frame f = new FileDialogNew(); f.setSize(250,110); f.setVisible(true); f.addWindowListener( new WindowAdapter() { 474 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 475. public void windowClosing(WindowEvent e) { System.exit(0); } }); } } ///:~ It would be nice if all the conversions were this easy, but in general they’re easy enough, and your code benefits from the improved readability. Binding events dynamically One of the benefits of the new AWT event model is flexibility. In the old model you were forced to hard-code the behavior of your program, but with the new model you can add and remove event behavior with single method calls. The following example demonstrates this: //: DynamicEvents.java // The new Java 1.1 event model allows you to // change event behavior dynamically. Also // demonstrates multiple actions for an event. import java.awt.*; import java.awt.event.*; import java.util.*; public class DynamicEvents extends Frame { Vector v = new Vector(); int i = 0; Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public DynamicEvents() { addWindowListener(new BWL()); setLayout(new FlowLayout()); b1.addActionListener(new B()); b1.addActionListener(new B1()); b2.addActionListener(new B()); b2.addActionListener(new B2()); add(b1); add(b2); } class B implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("A button was pressed"); } } class CountListener implements ActionListener { int index; public CountListener(int i) { index = i; } public void actionPerformed(ActionEvent e) { System.out.println( "Counted Listener " + index); } } class B1 implements ActionListener { public void actionPerformed(ActionEvent e) { Chapter 13: Creating Windows & Applets 475
- 476. System.out.println("Button 1 pressed"); ActionListener a = new CountListener(i++); v.addElement(a); b2.addActionListener(a); } } class B2 implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("Button 2 pressed"); int end = v.size() -1; if(end >= 0) { b2.removeActionListener( (ActionListener)v.elementAt(end)); v.removeElementAt(end); } } } class BWL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.out.println("Window Closing"); System.exit(0); } } public static void main(String args[]) { Frame f = new DynamicEvents(); f.setSize(300,200); f.show(); } } ///:~ The new twists in this example are: 1. There is more than one listener attached to each Button. Usually, components handle events as multicast, meaning you can register many listeners for a single event. In the special components where an event is handled as unicast, you’ll get a TooManyListenersException. 2. During the execution of the program, listeners are dynamically added and removed from the Button b2. Adding is accomplished in the way you’ve seen before, but each component also has a removeXXXListener( ) method to remove each type of listener. This kind of flexibility provides much greater power in your programming. You should note that event listeners are not guaranteed to be called in the order they are added (although most implementations do in fact work that way). Separating business logic from ui logic In general you’ll want to design your classes so that each one does “only one thing.” This is particularly important where user-interface code is concerned, since it’s very easy to wrap up “what you’re doing” with “how you’re displaying it.” This kind of coupling prevents code reuse. It’s much more desirable to separate your “business logic” from the GUI. This way, you can not only reuse the business logic more easily, it’s also easier to reuse the GUI. 476 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 477. Another issue is multi-tiered systems, where the “business objects” reside on a completely separate machine. This central location of the business rules allows changes to be instantly effective for all new transactions, and is thus a very compelling way to set up a system. However, these business objects may be used in many different applications and thus should not be tied to any particular mode of display. They should just perform the business operations, nothing more. The following example shows how easy it is to separate the business logic from the GUI code: //: Separation.java // Separating GUI logic and business objects import java.awt.*; import java.awt.event.*; import java.applet.*; class BusinessLogic { private int modifier; BusinessLogic(int mod) { modifier = mod; } public void setModifier(int mod) { modifier = mod; } public int getModifier() { return modifier; } // Some business operations: public int calculation1(int arg) { return arg * modifier; } public int calculation2(int arg) { return arg + modifier; } } public class Separation extends Applet { TextField t = new TextField(20), mod = new TextField(20); BusinessLogic bl = new BusinessLogic(2); Button calc1 = new Button("Calculation 1"), calc2 = new Button("Calculation 2"); public void init() { add(t); calc1.addActionListener(new Calc1L()); calc2.addActionListener(new Calc2L()); add(calc1); add(calc2); mod.addTextListener(new ModL()); add(new Label("Modifier:")); add(mod); } static int getValue(TextField tf) { try { return Integer.parseInt(tf.getText()); } catch(NumberFormatException e) { Chapter 13: Creating Windows & Applets 477
- 478. return 0; } } class Calc1L implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText(Integer.toString( bl.calculation1(getValue(t)))); } } class Calc2L implements ActionListener { public void actionPerformed(ActionEvent e) { t.setText(Integer.toString( bl.calculation2(getValue(t)))); } } class ModL implements TextListener { public void textValueChanged(TextEvent e) { bl.setModifier(getValue(mod)); } } public static void main(String args[]) { Separation applet = new Separation(); Frame aFrame = new Frame("Separation"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(200,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ You can see that BusinessLogic is a very straightforward class that performs its operations without even a hint that it may be used in a GUI environment. It just does its job. Separation keeps track of all the UI details, and it talks only to BusinessLogic through its public interface. All the operations are centered around getting information back and forth through the UI and the BusinessLogic object. So Separation, in turn, just does its job. Since it’s talking only to a BusinessLogic object and doesn’t know much else about it, this could be massaged into talking to other types of objects without much trouble. Thinking in terms of separating UI from business logic also makes life easier when you’re adapting legacy code to work with Java. Recommended coding approaches Inner classes, the new event model, and the fact that the old event model is still supported along with new support for old-style programming has added a new element of confusion. Now there are even more different ways for people to write unpleasant code. Unfortunately, this kind of code is showing up in books and article examples, and even in 478 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 479. documentation and examples distributed from Sun! In this section we’ll look at some misunderstandings about what you should and shouldn’t do with the new AWT, and end by showing that except in very extenuating circumstances you can always use listener classes (written as inner classes) to solve your event-handling needs. Since this is also the simplest and clearest approach, it should be a relief for you to learn this. Before looking at anything else, you should know that although Java 1.1 is backward- compatible with Java 1.0 (that is, you can compile and run 1.0 programs with 1.1), you cannot mix the event models within the same program. That is, you cannot use the old- style action( ) method in the same program where you employ listeners. This can be a problem in a larger program when you're trying to integrate old code with a new program, since you must decide whether to use the old, hard-to-maintain approach with the new program or to update the old code. This shouldn't be too much of a battle since the new approach is so superior to the old. Baseline: the good way to do it To give you something to compare with, here’s an example showing the recommended approach. By now it should be reasonably familiar and comfortable: //: GoodIdea.java // The best way to design classes using the new // Java 1.1 event model: use an inner class for // each different event. This maximizes // flexibility and modularity. import java.awt.*; import java.awt.event.*; import java.util.*; public class GoodIdea extends Frame { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public GoodIdea() { setLayout(new FlowLayout()); addWindowListener(new WL()); b1.addActionListener(new B1()); b2.addActionListener(new B2()); add(b1); add(b2); } public class B1 implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("Button 1 pressed"); } } public class B2 implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("Button 2 pressed"); } } class WL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.out.println("Window Closing"); System.exit(0); } } Chapter 13: Creating Windows & Applets 479
- 480. public static void main(String args[]) { Frame f = new GoodIdea(); f.setSize(300,200); f.setVisible(true); } } ///:~ This is fairly trivial: each button has its own listener which prints something out to the console. But notice there isn’t an if statement in the entire program, or any statement which says “I wonder what caused this event.” Each piece of code is concerned with doing, not type-checking. This is the best way to write your code – not only is it easier to conceptualize, but much easier to read and maintain. Cutting-and-pasting to create new programs is also much easier. Implementing the main class as a listener The first bad idea is a very common and recommended approach. This makes the main class (typically Applet or Frame, but it could be any class) implement the various listeners. Here’s an example: //: BadIdea1.java // Some literature recommends this approach, // but it's missing the point of the new event // model in Java 1.1 import java.awt.*; import java.awt.event.*; import java.util.*; public class BadIdea1 extends Frame implements ActionListener, WindowListener { Button b1 = new Button("Button 1"), b2 = new Button("Button 2"); public BadIdea1() { setLayout(new FlowLayout()); addWindowListener(this); b1.addActionListener(this); b2.addActionListener(this); add(b1); add(b2); } public void actionPerformed(ActionEvent e) { Object source = e.getSource(); if(source == b1) System.out.println("Button 1 pressed"); else if(source == b2) System.out.println("Button 2 pressed"); else System.out.println("Something else"); } public void windowClosing(WindowEvent e) { System.out.println("Window Closing"); System.exit(0); } public void windowClosed(WindowEvent e) {} public void windowDeiconified(WindowEvent e) {} public void windowIconified(WindowEvent e) {} 480 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 481. public void windowActivated(WindowEvent e) {} public void windowDeactivated(WindowEvent e) {} public void windowOpened(WindowEvent e) {} public static void main(String args[]) { Frame f = new BadIdea1(); f.setSize(300,200); f.setVisible(true); } } ///:~ The use of this shows up in the three lines: addWindowListener(this); b1.addActionListener(this); b2.addActionListener(this); Since BadIdea1 implements ActionListener and WindowListener these lines are certainly acceptable, and if you’re still stuck in the mode of trying to make fewer classes to reduce server hits during applet loading, it seems to be a good idea. However: 1. Java 1.1 supports JAR files so all your files can be placed in a single compressed JAR archive which requires only one server hit. You no longer need to reduce class count for Internet efficiency. 2. The above code is much less modular so it’s harder to grab and paste. Notice that you must not only implement the various interfaces for your main class, but in actionPerformed( ) you’ve got to detect which action was performed using a cascaded if statement. Not only is this going backwards, away from the listener model, but you can’t easily reuse the actionPerformed( ) method since it’s very specific to this particular application. Contrast this with GoodIdea.java, where you can just grab one listener class and paste it in anywhere else with minimal fuss. Plus you can register multiple listener classes with a single event, allowing even more modularity in what each listener class does. Mixing the approaches The second bad idea is to mix the two approaches: use inner listener classes, but also implement one or more listener interfaces as part of the main class. This approach has appeared without explanation in books and documentation, and I can assume only that the authors thought they must use the different approaches for different purposes. But you don’t – in your programming you can probably use inner listener classes exclusively. //: BadIdea2.java // An improvement over BadIdea1.java, since it // uses the WindowAdapter as an inner class // instead of implementing all the methods of // WindowListener, but still misses the // valuable modularity of inner classes import java.awt.*; import java.awt.event.*; import java.util.*; public class BadIdea2 extends Frame implements ActionListener { Button b1 = new Button("Button 1"), Chapter 13: Creating Windows & Applets 481
- 482. b2 = new Button("Button 2"); public BadIdea2() { setLayout(new FlowLayout()); addWindowListener(new WL()); b1.addActionListener(this); b2.addActionListener(this); add(b1); add(b2); } public void actionPerformed(ActionEvent e) { Object source = e.getSource(); if(source == b1) System.out.println("Button 1 pressed"); else if(source == b2) System.out.println("Button 2 pressed"); else System.out.println("Something else"); } class WL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.out.println("Window Closing"); System.exit(0); } } public static void main(String args[]) { Frame f = new BadIdea2(); f.setSize(300,200); f.setVisible(true); } } ///:~ Since actionPerformed( ) is still tightly coupled to the main class, it’s hard to reuse that code. It’s also messier and less pleasant to read than the inner class approach. There’s no reason you have to use any of the old thinking for events in Java 1.1 – so why do it? Inheriting a component Another place where you’ll often see variations on the old way of doing things is when creating a new type of component. Here’s a simple example showing that here, too, the new way works: //: GoodTechnique.java // Your first choice when overriding components // should be to install listeners. The code is // much safer, more modular and maintainable. import java.awt.*; import java.awt.event.*; class Display { public static final int EVENT = 0, COMPONENT = 1, MOUSE = 2, MOUSE_MOVE = 3, FOCUS = 4, KEY = 5, ACTION = 6, LAST = 7; 482 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 483. public String evnt[]; Display() { evnt = new String[LAST]; for(int i = 0; i < LAST; i++) evnt[i] = new String(); } public void show(Graphics g) { for(int i = 0; i < LAST; i++) g.drawString(evnt[i], 0, 10 * i + 10); } } class EnabledPanel extends Panel { Color c; int id; Display display = new Display(); public EnabledPanel(int i, Color mc) { id = i; c = mc; setLayout(new BorderLayout()); add(new MyButton(), BorderLayout.SOUTH); addComponentListener(new CL()); addFocusListener(new FL()); addKeyListener(new KL()); addMouseListener(new ML()); addMouseMotionListener(new MML()); } // To eliminate flicker: public void update(Graphics g) { paint(g); } public void paint(Graphics g) { g.setColor(c); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); g.setColor(Color.black); display.show(g); } // Don't need to enable anything for this: public void processEvent(AWTEvent e) { display.evnt[Display.EVENT]= e.toString(); repaint(); super.processEvent(e); } class CL implements ComponentListener { public void componentMoved(ComponentEvent e) { display.evnt[Display.COMPONENT] = "Component moved"; repaint(); } public void componentResized(ComponentEvent e) { display.evnt[Display.COMPONENT] = "Component resized"; repaint(); } Chapter 13: Creating Windows & Applets 483
- 484. public void componentHidden(ComponentEvent e) { display.evnt[Display.COMPONENT] = "Component hidden"; repaint(); } public void componentShown(ComponentEvent e) { display.evnt[Display.COMPONENT] = "Component shown"; repaint(); } } class FL implements FocusListener { public void focusGained(FocusEvent e) { display.evnt[Display.FOCUS] = "FOCUS gained"; repaint(); } public void focusLost(FocusEvent e) { display.evnt[Display.FOCUS] = "FOCUS lost"; repaint(); } } class KL implements KeyListener { public void keyPressed(KeyEvent e) { display.evnt[Display.KEY] = "KEY pressed: "; showCode(e); } public void keyReleased(KeyEvent e) { display.evnt[Display.KEY] = "KEY released: "; showCode(e); } public void keyTyped(KeyEvent e) { display.evnt[Display.KEY] = "KEY typed: "; showCode(e); } void showCode(KeyEvent e) { int code = e.getKeyCode(); display.evnt[Display.KEY] += KeyEvent.getKeyText(code); repaint(); } } class ML implements MouseListener { public void mouseClicked(MouseEvent e) { requestFocus(); // Get FOCUS on click display.evnt[Display.MOUSE] = "MOUSE clicked"; showMouse(e); } public void mousePressed(MouseEvent e) { display.evnt[Display.MOUSE] = 484 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 485. "MOUSE pressed"; showMouse(e); } public void mouseReleased(MouseEvent e) { display.evnt[Display.MOUSE] = "MOUSE released"; showMouse(e); } public void mouseEntered(MouseEvent e) { display.evnt[Display.MOUSE] = "MOUSE entered"; showMouse(e); } public void mouseExited(MouseEvent e) { display.evnt[Display.MOUSE] = "MOUSE exited"; showMouse(e); } void showMouse(MouseEvent e) { display.evnt[Display.MOUSE] += ", x = " + e.getX() + ", y = " + e.getY(); repaint(); } } class MML implements MouseMotionListener { public void mouseDragged(MouseEvent e) { display.evnt[Display.MOUSE_MOVE] = "MOUSE dragged"; showMouse(e); } public void mouseMoved(MouseEvent e) { display.evnt[Display.MOUSE_MOVE] = "MOUSE moved"; showMouse(e); } void showMouse(MouseEvent e) { display.evnt[Display.MOUSE_MOVE] += ", x = " + e.getX() + ", y = " + e.getY(); repaint(); } } } class MyButton extends Button { int clickCounter; String label = ""; public MyButton() { addActionListener(new AL()); } public void paint(Graphics g) { g.setColor(Color.green); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); g.setColor(Color.black); Chapter 13: Creating Windows & Applets 485
- 486. g.drawRect(0, 0, s.width - 1, s.height - 1); drawLabel(g); } private void drawLabel(Graphics g) { FontMetrics fm = g.getFontMetrics(); int width = fm.stringWidth(label); int height = fm.getHeight(); int ascent = fm.getAscent(); int leading = fm.getLeading(); int horizMargin = (getSize().width - width)/2; int verMargin = (getSize().height - height)/2; g.setColor(Color.red); g.drawString(label, horizMargin, verMargin + ascent + leading); } class AL implements ActionListener { public void actionPerformed(ActionEvent e) { clickCounter++; label = "click #" + clickCounter + " " + e.toString(); repaint(); } } } public class GoodTechnique extends Frame { GoodTechnique() { setLayout(new GridLayout(2,2)); add(new EnabledPanel(1, Color.cyan)); add(new EnabledPanel(2, Color.lightGray)); add(new EnabledPanel(3, Color.yellow)); addWindowListener(new WL()); } class WL extends WindowAdapter { public void windowClosing(WindowEvent e) { System.out.println(e); System.out.println("Window Closing"); System.exit(0); } } public static void main(String args[]) { Frame f = new GoodTechnique(); f.setTitle("Good Technique"); f.setSize(700,700); f.setVisible(true); } } ///:~ This example also demonstrates the various events that occur, and displays the information about them. The class Display is a way to centralize that information display. There’s an array of Strings to hold information about each type of event, and the method show( ) takes a handle to whatever Graphics object you have and writes directly on that surface. The scheme is intended to be somewhat reusable. 486 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 487. EnabledPanel represents the new type of component. It’s a colored panel with a button at the bottom, and it captures all the events that happen over it. The MyButton object also captures the events that happen over it. Both components capture all events by using inner listener classes for every single event except where EnabledPanel overrides processEvent( ) in the old style (notice it must also call super.processEvent( )). The only reason for using this method is that it captures every event that happens, so you can view everything that goes on. processEvent( ) does nothing more than show the string representation of each event, otherwise it would have to use a cascade of if statements to figure out what event it was. On the other hand, the inner listener classes already know precisely what event occurred (assuming you register them only to components where you don’t need any control logic, which should be your goal). Thus they don’t have to check anything out; they just do their stuff. Each listener modifies the Display string associated with its particular event and calls repaint( ) so the strings get displayed. You can also see a trick that will usually eliminate flicker: public void update(Graphics g) { paint(g); } You don’t always need to override update( ), but if you write something that flickers, try it. The default version of update clears the background and then calls paint( ) to redraw any graphics. This clearing is usually what causes flicker but is not necessary since paint( ) redraws the entire surface. You can see that there are lots of listeners – however, type checking occurs for the listeners, and you can’t listen for something that the component doesn’t support (unlike BadTechnique.java, which you will see momentarily). Experimenting with this program is quite educational since you learn a lot about the way events occur in Java. For one thing, it shows a flaw in the design of most windowing systems: it’s pretty hard to click and release the mouse without moving it, and the windowing system will often think you’re dragging when you’re actually just trying to click on something. A solution to this is to use mousePressed( ) and mouseReleased( ) instead of mouseClicked( ), and then determine whether to call your own "mouseReallyClicked( )" method based on time and about 4 pixels of mouse hysteresis. Ugly component inheritance The alternative, which you will see put forward in many published works, is to call enableEvents( ) and pass it the masks corresponding to the events you want to handle. This causes those events to be sent to the old-style methods (although they’re new to Java 1.1) with names like processFocusEvent( ). You must also remember to call the base-class version. Here’s what it looks like: //: BadTechnique.java // It's possible to override components this way, // but the listener approach is much better, so // why would you? import java.awt.*; import java.awt.event.*; class Display { public static final int EVENT = 0, COMPONENT = 1, MOUSE = 2, MOUSE_MOVE = 3, FOCUS = 4, KEY = 5, ACTION = 6, Chapter 13: Creating Windows & Applets 487
- 488. LAST = 7; public String evnt[]; Display() { evnt = new String[LAST]; for(int i = 0; i < LAST; i++) evnt[i] = new String(); } public void show(Graphics g) { for(int i = 0; i < LAST; i++) g.drawString(evnt[i], 0, 10 * i + 10); } } class EnabledPanel extends Panel { Color c; int id; Display display = new Display(); public EnabledPanel(int i, Color mc) { id = i; c = mc; setLayout(new BorderLayout()); add(new MyButton(), BorderLayout.SOUTH); // Type checking is lost. You can enable and // process events that the component doesn't // capture: enableEvents( // Panel doesn't handle these: AWTEvent.ACTION_EVENT_MASK | AWTEvent.COMPONENT_EVENT_MASK | AWTEvent.ADJUSTMENT_EVENT_MASK | AWTEvent.ITEM_EVENT_MASK | AWTEvent.TEXT_EVENT_MASK | AWTEvent.WINDOW_EVENT_MASK | // Panel can handle these: AWTEvent.FOCUS_EVENT_MASK | AWTEvent.KEY_EVENT_MASK | AWTEvent.MOUSE_EVENT_MASK | AWTEvent.MOUSE_MOTION_EVENT_MASK | AWTEvent.CONTAINER_EVENT_MASK); // You can enable an event without // overriding its process method. } // To eliminate flicker: public void update(Graphics g) { paint(g); } public void paint(Graphics g) { g.setColor(c); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); g.setColor(Color.black); display.show(g); } public void processEvent(AWTEvent e) { 488 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 489. display.evnt[Display.EVENT]= e.toString(); repaint(); super.processEvent(e); } public void processComponentEvent(ComponentEvent e) { switch(e.getID()) { case ComponentEvent.COMPONENT_MOVED: display.evnt[Display.COMPONENT] = "Component moved"; break; case ComponentEvent.COMPONENT_RESIZED: display.evnt[Display.COMPONENT] = "Component resized"; break; case ComponentEvent.COMPONENT_HIDDEN: display.evnt[Display.COMPONENT] = "Component hidden"; break; case ComponentEvent.COMPONENT_SHOWN: display.evnt[Display.COMPONENT] = "Component shown"; break; default: } repaint(); // Must always remember to call the "super" // version of whatever you override: super.processComponentEvent(e); } public void processFocusEvent(FocusEvent e) { switch(e.getID()) { case FocusEvent.FOCUS_GAINED: display.evnt[Display.FOCUS] = "FOCUS gained"; break; case FocusEvent.FOCUS_LOST: display.evnt[Display.FOCUS] = "FOCUS lost"; break; default: } repaint(); super.processFocusEvent(e); } public void processKeyEvent(KeyEvent e) { switch(e.getID()) { case KeyEvent.KEY_PRESSED: display.evnt[Display.KEY] = "KEY pressed: "; break; case KeyEvent.KEY_RELEASED: display.evnt[Display.KEY] = "KEY released: "; Chapter 13: Creating Windows & Applets 489
- 490. break; case KeyEvent.KEY_TYPED: display.evnt[Display.KEY] = "KEY typed: "; break; default: } int code = e.getKeyCode(); display.evnt[Display.KEY] += KeyEvent.getKeyText(code); repaint(); super.processKeyEvent(e); } public void processMouseEvent(MouseEvent e) { switch(e.getID()) { case MouseEvent.MOUSE_CLICKED: requestFocus(); // Get FOCUS on click display.evnt[Display.MOUSE] = "MOUSE clicked"; break; case MouseEvent.MOUSE_PRESSED: display.evnt[Display.MOUSE] = "MOUSE pressed"; break; case MouseEvent.MOUSE_RELEASED: display.evnt[Display.MOUSE] = "MOUSE released"; break; case MouseEvent.MOUSE_ENTERED: display.evnt[Display.MOUSE] = "MOUSE entered"; break; case MouseEvent.MOUSE_EXITED: display.evnt[Display.MOUSE] = "MOUSE exited"; break; default: } display.evnt[Display.MOUSE] += ", x = " + e.getX() + ", y = " + e.getY(); repaint(); super.processMouseEvent(e); } public void processMouseMotionEvent(MouseEvent e) { switch(e.getID()) { case MouseEvent.MOUSE_DRAGGED: display.evnt[Display.MOUSE_MOVE] = "MOUSE dragged"; break; case MouseEvent.MOUSE_MOVED: display.evnt[Display.MOUSE_MOVE] = "MOUSE moved"; 490 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 491. break; default: } display.evnt[Display.MOUSE_MOVE] += ", x = " + e.getX() + ", y = " + e.getY(); repaint(); super.processMouseMotionEvent(e); } } class MyButton extends Button { int clickCounter; String label = ""; public MyButton() { enableEvents(AWTEvent.ACTION_EVENT_MASK); } public void paint(Graphics g) { g.setColor(Color.green); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); g.setColor(Color.black); g.drawRect(0, 0, s.width - 1, s.height - 1); drawLabel(g); } private void drawLabel(Graphics g) { FontMetrics fm = g.getFontMetrics(); int width = fm.stringWidth(label); int height = fm.getHeight(); int ascent = fm.getAscent(); int leading = fm.getLeading(); int horizMargin = (getSize().width - width)/2; int verMargin = (getSize().height - height)/2; g.setColor(Color.red); g.drawString(label, horizMargin, verMargin + ascent + leading); } public void processActionEvent(ActionEvent e) { clickCounter++; label = "click #" + clickCounter + " " + e.toString(); repaint(); super.processActionEvent(e); } } public class BadTechnique extends Frame { BadTechnique() { setLayout(new GridLayout(2,2)); add(new EnabledPanel(1, Color.cyan)); add(new EnabledPanel(2, Color.lightGray)); add(new EnabledPanel(3, Color.yellow)); // You can also do it for Windows: Chapter 13: Creating Windows & Applets 491
- 492. enableEvents(AWTEvent.WINDOW_EVENT_MASK); } public void processWindowEvent(WindowEvent e) { System.out.println(e); if(e.getID() == WindowEvent.WINDOW_CLOSING) { System.out.println("Window Closing"); System.exit(0); } } public static void main(String args[]) { Frame f = new BadTechnique(); f.setTitle("Bad Technique"); f.setSize(700,700); f.setVisible(true); } } ///:~ Sure, it works. But it’s ugly and hard to write, read, debug, maintain and reuse. So why bother when you can use inner listener classes? New Java 1.1 UI APIs Java 1.1 has also added some important new functionality, including focus traversal, desktop color access, printing “inside the sandbox” and the beginnings of clipboard support. Focus traversal is quite easy, since it’s transparently present in the AWT library components and you don’t have to do anything to make it work. If you make your own components and want them to handle focus traversal, you override isFocusTraversable( ) to return true. If you want to capture the keyboard focus on a mouse click, you catch the mouse down event and call requestFocus( ). Desktop colors The desktop colors provide a way for you to know what the various color choices are on the current user’s desktop. This way, you can use those colors in your program, if you desire. The colors are automatically initialized and placed in static members of class SystemColor, so all you need to do is read the member you’re interested in. The names are intentionally self-explanatory: desktop, activeCaption, activeCaptionText, activeCaptionBorder, inactiveCaption, inactiveCaptionText, inactiveCaptionBorder, window, windowBorder, windowText, menu, menuText, text, textText, textHighlight, textHighlightText, textInactiveText, control, controlText, controlHighlight, controlLtHighlight, controlShadow, controlDkShadow, scrollbar, info (for help), infoText (for help text). Printing Unfortunately, there isn’t much that’s automatic with printing; instead you must go through a number of mechanical, non-OO steps in order to print. Printing a component graphically can be slightly more automatic: by default, the print( ) method calls paint( ) to do its work. There are times when this is satisfactory, but if you want to do anything more specialized you must know that you’re printing so you can in particular find out the page dimensions. 492 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 493. The following example demonstrates the printing of both text and graphics, and the different approaches you can use for printing graphics. In addition, it tests the printing support: //: PrintDemo.java // Printing with Java 1.1 import java.awt.*; import java.awt.event.*; public class PrintDemo extends Frame { Button printText = new Button("Print Text"), printGraphics = new Button("Print Graphics"); TextField ringNum = new TextField(3); Choice faces = new Choice(); Graphics g = null; Plot plot = new Plot3(); // Try different plots Toolkit tk = Toolkit.getDefaultToolkit(); public PrintDemo() { ringNum.setText("3"); ringNum.addTextListener(new RingL()); Panel p = new Panel(); p.setLayout(new FlowLayout()); printText.addActionListener(new TBL()); p.add(printText); p.add(new Label("Font:")); p.add(faces); printGraphics.addActionListener(new GBL()); p.add(printGraphics); p.add(new Label("Rings:")); p.add(ringNum); setLayout(new BorderLayout()); add(p, BorderLayout.NORTH); add(plot, BorderLayout.CENTER); String fontList[] = tk.getFontList(); for(int i = 0; i < fontList.length; i++) faces.add(fontList[i]); faces.select("Serif"); } class PrintData { public PrintJob pj; public int pageWidth, pageHeight; PrintData(String jobName) { pj = getToolkit().getPrintJob( PrintDemo.this, jobName, null); if(pj != null) { pageWidth = pj.getPageDimension().width; pageHeight = pj.getPageDimension().height; g = pj.getGraphics(); } } void end() { pj.end(); } } class ChangeFont { private int stringHeight; ChangeFont(String face, int style, int point) { Chapter 13: Creating Windows & Applets 493
- 494. if(g != null) { g.setFont(new Font(face, style, point)); stringHeight = g.getFontMetrics().getHeight(); } } int stringWidth(String s) { return g.getFontMetrics().stringWidth(s); } int stringHeight() { return stringHeight; } } class TBL implements ActionListener { public void actionPerformed(ActionEvent e) { PrintData pd = new PrintData("Print Text Test"); // Null means print job canceled: if(pd == null) return; String s = "PrintDemo"; ChangeFont cf = new ChangeFont( faces.getSelectedItem(), Font.ITALIC,72); g.drawString(s, (pd.pageWidth - cf.stringWidth(s)) / 2, (pd.pageHeight - cf.stringHeight()) / 3); s = "A smaller point size"; cf = new ChangeFont( faces.getSelectedItem(), Font.BOLD, 48); g.drawString(s, (pd.pageWidth - cf.stringWidth(s)) / 2, (int)((pd.pageHeight - cf.stringHeight())/1.5)); g.dispose(); pd.end(); } } class GBL implements ActionListener { public void actionPerformed(ActionEvent e) { PrintData pd = new PrintData("Print Graphics Test"); if(pd == null) return; plot.print(g); g.dispose(); pd.end(); } } class RingL implements TextListener { public void textValueChanged(TextEvent e) { int i = 1; try { i = Integer.parseInt(ringNum.getText()); } catch(NumberFormatException ex) { i = 1; } plot.rings = i; plot.repaint(); 494 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 495. } } public static void main(String args[]) { Frame pdemo = new PrintDemo(); pdemo.setTitle("Print Demo"); pdemo.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); pdemo.setSize(500, 500); pdemo.setVisible(true); } } class Plot extends Canvas { public int rings = 3; } class Plot1 extends Plot { // Default print() calls paint(): public void paint(Graphics g) { int w = getSize().width; int h = getSize().height; int xc = w / 2; int yc = w / 2; int x = 0, y = 0; for(int i = 0; i < rings; i++) { if(x < xc && y < yc) { g.drawOval(x, y, w, h); x += 10; y += 10; w -= 20; h -= 20; } } } } class Plot2 extends Plot { // To fit the picture to the page, you must // know whether you're printing or painting: public void paint(Graphics g) { int w, h; if(g instanceof PrintGraphics) { PrintJob pj = ((PrintGraphics)g).getPrintJob(); w = pj.getPageDimension().width; h = pj.getPageDimension().height; } else { w = getSize().width; h = getSize().height; } int xc = w / 2; int yc = w / 2; int x = 0, y = 0; Chapter 13: Creating Windows & Applets 495
- 496. for(int i = 0; i < rings; i++) { if(x < xc && y < yc) { g.drawOval(x, y, w, h); x += 10; y += 10; w -= 20; h -= 20; } } } } class Plot3 extends Plot { // Somewhat better. Separate // printing from painting: public void print(Graphics g) { // Assume it's a PrintGraphics object: PrintJob pj = ((PrintGraphics)g).getPrintJob(); int w = pj.getPageDimension().width; int h = pj.getPageDimension().height; doGraphics(g, w, h); } public void paint(Graphics g) { int w = getSize().width; int h = getSize().height; doGraphics(g, w, h); } private void doGraphics( Graphics g, int w, int h) { int xc = w / 2; int yc = w / 2; int x = 0, y = 0; for(int i = 0; i < rings; i++) { if(x < xc && y < yc) { g.drawOval(x, y, w, h); x += 10; y += 10; w -= 20; h -= 20; } } } } ///:~ The program allows you to select fonts from a Choice list (and you’ll see that the number of fonts available in Java 1.1 is still extremely limited, and has nothing to do with any extra fonts you install on your machine). It uses these to print out text in bold and italic and in different sizes. In addition, a new type of component called a Plot is created to demonstrate graphics. A Plot has rings that it will display on the screen and print onto paper, and the three derived classes Plot1, Plot2 and Plot3 perform these tasks in different ways so you can see your alternatives when printing graphics. Also, you can change the number of rings in a plot – this is interesting because it shows the printing fragility in Java 1.1. On my system the printer gave error messages and didn’t print correctly when the ring count got “too high” (whatever that means), but worked fine when the count was “low enough.” You will notice, too, that the page dimensions produced when printing do not seem to correspond to the actual dimensions of the page. This may be fixed in a future release of Java, and you can use this program to test it. 496 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 497. This program encapsulates functionality inside inner classes whenever possible, to facilitate reuse. For example, whenever you want to begin a print job (whether for graphics or text), you must create a PrintJob object, which has its own Graphics object along with the width and height of the page. The creation of a PrintJob and extraction of page dimensions is encapsulated in the PrintData class. Printing text Conceptually, printing text is straightforward: you choose a typeface and size, decide where the string should go on the page, and draw it with Graphics.drawString( ). This means, however, that you must perform all the calculations of exactly where each line will go on the page to make sure it doesn’t run off the end of the page or collide with other lines. If you want to make a word processor, your work is cut out for you. ChangeFont encapsulates a little of the process of changing from one font to another by automatically creating a new Font object with your desired typeface, style (Font.BOLD or Font.ITALIC – there’s no support for underline, strikethrough, etc.) and point size. It also simplifies the calculation of the width and height of a string. When you press the “Print text” button, the TBL listener is activated. You can see that it goes through two iterations of creating a ChangeFont object and calling drawString( ) to print out the string in a calculated position, centered and 1/3 and 2/3 down the page, respectively. Notice whether these calculations actually produce the expected results (they didn’t with the version I used). Printing graphics When you press the “Print graphics” button the GBL listener is activated. The creation of a PrintData object initializes g, and then you simply call print( ) for the component you want to print. To force printing you must call dispose( ) for the Graphics object and end( ) for the PrintData object (which turns around and calls end( ) for the PrintJob). The actual work is going on inside the Plot object. You can see that the base-class Plot is very simple – it extends Canvas and contains an int called rings to indicate how many concentric rings to draw on this particular Canvas. The three derived classes show different approaches to accomplishing the same goal: drawing on both the screen and on the printed page. Plot1 takes the simplest approach to coding: ignore the fact that there are differences in painting and printing, and just override paint( ). The reason this works is that the default print( ) method simply turns around and calls paint( ). However, you’ll notice that the size of the output depends on the size of the on-screen canvas, which makes sense since the width and height are determined by calling Canvas.getSize( ). The other situation where this is acceptable is if your image is always a fixed size. When the size of the drawing surface is important, then you must discover the dimensions. Unfortunately, this turns out to be awkward, as you can see in Plot2. For some possibly very good reason that I don’t know, you cannot simply ask the Graphics object the dimensions of its drawing surface. This would have made the whole process quite elegant. Instead, to see if you’re printing rather than painting, you must detect the PrintGraphics using the RTTI instanceof keyword (described in Chapter 11), then downcast and call the sole PrintGraphics method: getPrintJob( ). Now you have a handle to the PrintJob and you can find out the width and height of the paper. This is a very hacky approach, but perhaps there is some rational reason for it (on the other hand, you’ve seen some of the other library designs by now so you may have the impression that the designers were in fact just hacking around…). You can see that paint( ) in Plot2 goes through both possibilities of printing or painting. But since the print( ) method should be called when printing, why not use that? This Chapter 13: Creating Windows & Applets 497
- 498. approach is used in Plot3, and it eliminates the need to use instanceof since inside print( ) you can just assume that you can cast to a PrintGraphics object. This is a little better. The situation is improved by placing the common drawing code (once the dimensions have been detected) inside a separate method doGraphics( ). Running Frames within applets What if you'd like to print from within an applet? Well, to print anything you must get a PrintJob object through a Toolkit object’s getPrintJob( ) method, which takes only a Frame object, and not an Applet. Thus it would seem that it's possible to print only from within an application, and not an applet. However, it turns out to be possible to create a Frame from within an applet (which is the reverse of what I've been doing for the applet/application examples so far: making an applet and putting inside a Frame). This is a useful technique, since it allows you to use many applications within applets (as long as they don't violate applet security). When the application window comes up within an applet, however, you'll notice that the Web browser sticks a little caveat on it: something to the effect of "Warning: Applet Window." You can see that it's quite straightforward to put a Frame inside an applet. The only thing you must add is code to dispose( ) of the Frame when the user closes it (instead of calling System.exit( )): //: PrintDemoApplet.java // Creating a Frame from within an Applet import java.applet.*; import java.awt.*; import java.awt.event.*; public class PrintDemoApplet extends Applet { public void init() { Button b = new Button("Run PrintDemo"); b.addActionListener(new PDL()); add(b); } class PDL implements ActionListener { public void actionPerformed(ActionEvent e) { final PrintDemo pd = new PrintDemo(); pd.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e){ pd.dispose(); } }); pd.setSize(500, 500); pd.show(); } } } ///:~ There’s some confusion involved with Java 1.1 printing support. Some of the publicity seemed to claim that you’d be able to print from within an applet. However, the Java security system contains a feature that may lock out an applet from initiating its own print job, requiring that the initiation be done via a Web browser or applet viewer. At the time of writing, this seemed to remain an unresolved issue. When I ran this program from within a Web browser, the PrintDemo window came up just fine, but it wouldn't print from the browser. 498 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 499. The clipboard Java 1.1 supports limited operations with the system clipboard. You can copy String objects to the clipboard as text, and you can paste text from the clipboard into String objects. Of course, the clipboard is designed to hold any type of data, but how this data is represented on the clipboard is up to the program doing the cutting and pasting. Although it currently supports only string data, the Java clipboard API provides for extensibility through the concept of a “flavor.” When data comes off the clipboard, it has an associated set of flavors that it can be converted to (for example, a graph might be represented as a string of numbers or as an image) and you can see if that particular clipboard data supports the flavor you’re interested in. The following program is a simple demonstration of cut, copy and paste with String data in a TextArea. One thing you’ll notice is that the keyboard sequences you normally use for cutting, copying and pasting also work. But if you look at any TextField or TextArea in any other program you’ll find they also automatically support the clipboard key sequences. This example simply adds programmatic control of the clipboard, and you could use these techniques if you want to capture clipboard text into some non- TextComponent. //: CutAndPaste.java // Using the clipboard from Java 1.1 import java.awt.*; import java.awt.event.*; import java.awt.datatransfer.*; public class CutAndPaste extends Frame { MenuBar mb = new MenuBar(); Menu edit = new Menu("Edit"); MenuItem cut = new MenuItem("Cut"), copy = new MenuItem("Copy"), paste = new MenuItem("Paste"); TextArea text = new TextArea(20,20); Clipboard clipbd = getToolkit().getSystemClipboard(); public CutAndPaste() { cut.addActionListener(new CutL()); copy.addActionListener(new CopyL()); paste.addActionListener(new PasteL()); edit.add(cut); edit.add(copy); edit.add(paste); mb.add(edit); setMenuBar(mb); add(text, BorderLayout.CENTER); } class CopyL implements ActionListener { public void actionPerformed(ActionEvent e) { String selection = text.getSelectedText(); StringSelection clipString = new StringSelection(selection); clipbd.setContents(clipString, clipString); } } class CutL implements ActionListener { Chapter 13: Creating Windows & Applets 499
- 500. public void actionPerformed(ActionEvent e) { String selection = text.getSelectedText(); StringSelection clipString = new StringSelection(selection); clipbd.setContents(clipString, clipString); text.replaceRange("", text.getSelectionStart(), text.getSelectionEnd()); } } class PasteL implements ActionListener { public void actionPerformed(ActionEvent e) { Transferable clipData = clipbd.getContents(CutAndPaste.this); try { String clipString = (String)clipData. getTransferData( DataFlavor.stringFlavor); text.replaceRange(clipString, text.getSelectionStart(), text.getSelectionEnd()); } catch(Exception ex) { System.out.println("not String flavor"); } } } public static void main(String args[]) { CutAndPaste cp = new CutAndPaste(); cp.setSize(300,200); cp.setVisible(true); cp.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); } } ///:~ The creation and addition of the menu and TextArea should by now seem a pedestrian activity. What’s different is the creation of the Clipboard field clipbd, which is done through the Toolkit. All the action takes place in the listeners. The CopyL and CutL listeners are the same except for the last line of CutL, which erases the line that’s been copied. The special two lines are the creation of a StringSelection object from the String, and the call to setContents( ) with this StringSelection. That’s all there is to putting a String on the clipboard. In PasteL data is pulled off the clipboard using getContents( ). What comes back is a fairly anonymous Transferable object, and you don’t really know what it contains. One way to find out is to call getTransferDataFlavors( ), which returns an array of DataFlavor objects indicating which flavors are supported by this particular object. You can also ask it directly with isDataFlavorSupported( ), passing in the flavor you’re interested in. Here, however, the bold approach is taken: getTransferData( ) is called 500 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 501. assuming that the contents supports the String flavor, and if it doesn’t the problem is sorted out in the exception handler. In the future you can expect more data flavors to be supported. Visual programming & Beans So far in this book you’ve seen how valuable Java is for creating reusable pieces of code. The “most reusable” unit of code has been the class, since it comprises a cohesive unit of characteristics (fields) and behaviors (methods) that can be reused either directly via composition, or through inheritance. Inheritance and polymorphism are essential parts of object-oriented programming, but in the majority of cases when you’re putting together an application, what you really want is components that do exactly what you need. You’d like to drop these parts into your design like the electronic engineer puts together chips on a circuit board (or even, in the case of Java, onto a Web page). It seems, too, that there should be some way to accelerate this “modular assembly” style of programming. “Visual programming” first became successful – very successful – with Microsoft’s Visual Basic (VB), followed by a second-generation design in Borland’s Delphi (the primary inspiration for the Java Beans design). With these programming tools the components are represented visually, which makes sense since they usually display some kind of visual component like a button or a text field. The visual representation, in fact, is often exactly the way the component will look in the running program. So part of the process of visual programming involves dragging a component from a pallet and dropping it onto your form – the application builder tool writes code as you do this, and that code will cause the component to be created in the running program. Simply dropping the component onto a form is normally not enough to complete the program. Usually you’ll need to change the characteristics of a component, such as what color it is or what text is on it or what database it’s connected to, etc. Characteristics that can be modified at design time are referred to as properties. You can manipulate the properties of your component inside the application builder tool, and when you create the program this configuration data is saved so that it can be rejuvenated when the program is started. By now you’re probably used to the idea that an object is more than characteristics; it’s also a set of behaviors. At design-time, the behaviors of a visual component are partially represented by events, meaning “here’s something that can happen to the component.” Normally, you decide what you want to happen when an event occurs by tying code to that event. Here’s the critical part: the application builder tool is able to dynamically interrogate (using reflection) the component to find out what properties and events the component supports. Once it knows what they are, it can display the properties and allow you to change those (saving the state when you build the program), and also display the events. Generally you do something like double clicking on an event and the application builder tool creates a code body and ties it to that particular event. All you have to do at that point is write the code that executes when the event occurs. All this adds up to a lot of work that’s done for you by the application builder tool. As a result you can focus on what the program looks like and what it is supposed to do, and rely on the application builder tool to manage the connection details for you. The reason that visual programming tools have been so successful is that they dramatically speed up Chapter 13: Creating Windows & Applets 501
- 502. the process of building an application – certainly the user interface, but often other portions of the application as well. What is a Bean? After the dust settles, then, a component is really just a block of code, typically embodied in a class. The key issue is the ability for the application builder tool to discover the properties and events for that component. To create a VB component, the programmer had to write a fairly complicated piece of code, following certain conventions to expose the properties and events. Delphi was a second-generation visual programming tool and the language was actively designed around visual programming so it is much easier to create a visual component. However, Java has brought the creation of visual components to its most advanced state with Java Beans, because a Bean is just a class. You don’t have to write any extra code or use special language extensions in order to make something a Bean. The only thing you need to do, in fact, is slightly modify the way you name your methods. It is the method name that tells the application builder tool whether this is a property, an event or just an ordinary method. In the Java documentation, this naming convention is mistakenly termed a “design pattern.” This is unfortunate since design patterns (see Chapter 16) are challenging enough without this sort of confusion. It’s not a design pattern, it’s just a naming convention and it’s fairly simple: 1. For a property named xxx, you typically create two methods: getXxx( ) and setXxx( ). Note that the first letter after get or set is automatically decapitalized to produce the property name. The type produced by the “get” method is the same as the type of the argument to the “set” method. The name of the property and the type for the “get” and “set” are not related. 2. For a boolean property, you can use the “get” and “set” approach above, but you may also use “is” instead of “get.” 3. Ordinary methods of the Bean don’t conform to the above naming convention, but they’re public. 4. For events, you use the “listener” approach. It’s exactly the same as you’ve been seeing: addFooBarListener(FooBarListener) and removeFooBarListener(FooBarListener) to handle a FooBarEvent. Most of the time the built-in events and listeners will satisfy your needs, but you can also create your own events and listener interfaces. Point 1 above answers a question about something you may have noticed in the change from Java 1.0 to Java 1.1: a number of method names have had small, apparently meaningless name changes. Now you can see that most of those changes had to do with adapting to the “get” and “set” naming convention, in order to make that particular component into a Bean. We can use these guidelines to create a very simple Bean: //: Frog.java // A trivial Java Bean package frogbean; import java.awt.*; import java.awt.event.*; class Spots {} 502 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 503. public class Frog { private int jumps; private Color color; private Spots spots; private boolean jmpr; public int getJumps() { return jumps; } public void setJumps(int newJumps) { jumps = newJumps; } public Color getColor() { return color; } public void setColor(Color newColor) { color = newColor; } public Spots getSpots() { return spots; } public void setSpots(Spots newSpots) { spots = newSpots; } public boolean isJumper() { return jmpr; } public void setJumper(boolean j) { jmpr = j; } public void addActionListener( ActionListener l) { //... } public void removeActionListener( ActionListener l) { // ... } public void addKeyListener(KeyListener l) { // ... } public void removeKeyListener(KeyListener l) { // ... } // An "ordinary" public method: public void croak() { System.out.println("Ribbet!"); } } ///:~ First, you can see that it’s just a class. Normally, all your fields will be private, and accessible only through methods. Following the naming convention, the properties are jumps, color, spots and jumper (note the change in case of the first letter in the property name). Although the name of the internal identifier is the same as the name of the property in the first three cases, in jumper you can see that the property name does not force you to use any particular name for internal variables (or indeed, to even have any internal variable for that property). The events this Bean handles are ActionEvent and KeyEvent, based on the naming of the “add” and “remove” methods for the associated listener. Finally you can see the ordinary method croak( ) is still part of the Bean simply because it’s a public method, not because it conforms to any naming scheme. Extracting BeanInfo with the Introspector One of the most critical parts of the Bean scheme occurs when you drag a Bean off a palette and plop it down on a form. The application builder tool must be able to create the Chapter 13: Creating Windows & Applets 503
- 504. Bean (which it can do if there’s a default constructor) and then, without access to the Bean’s source code, extract all the necessary information to create the property sheet and event handlers. Part of the solution is already evident from the end of Chapter 11: Java 1.1 reflection allows all the methods of an anonymous class to be discovered. This is perfect for solving the Bean problem without requiring you to use any extra language keywords like those required in other visual programming languages. In fact, one of the prime reasons reflection was added to Java 1.1 was to support Beans (although reflection also supports object serialization and remote method invocation). So you might expect that the creator of the application builder tool would have to reflect each Bean and hunt through its methods to find the properties and events for that Bean. This is certainly possible, but the Java designers wanted to provide a standard interface for everyone to use, not only to make Beans simpler to use but also to provide a standard gateway to the creation of more complex Beans. This interface is the Introspector class, and the most important method in this class is the static getBeanInfo( ). You pass a Class handle to this method, and it fully interrogates that class and returns a BeanInfo object that you can then dissect to find properties, methods and events. Normally you won’t care about any of this – you’ll probably get most of your Beans off the shelf from vendors and you don’t need to know all the magic that’s going on underneath. You’ll simply drag your Beans onto your form, then configure their properties and write handlers for the events you’re interested in. However, it’s an interesting and educational exercise to use the Introspector to display information about a Bean, so here’s a tool that does it: //: BeanDumper.java // A method to introspect a Bean import java.beans.*; import java.lang.reflect.*; public class BeanDumper { public static void dump(Class bean){ BeanInfo bi = null; try { bi = Introspector.getBeanInfo( bean, java.lang.Object.class); } catch(IntrospectionException ex) { System.out.println("Couldn't introspect " + bean.getName()); System.exit(1); } PropertyDescriptor properties[] = bi.getPropertyDescriptors(); for(int i = 0; i < properties.length; i++) { Class p = properties[i].getPropertyType(); System.out.println( "Property type:n " + p.getName()); System.out.println( "Property name:n " + properties[i].getName()); Method readMethod = properties[i].getReadMethod(); if(readMethod != null) System.out.println( 504 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 505. "Read method:n " + readMethod.toString()); Method writeMethod = properties[i].getWriteMethod(); if(writeMethod != null) System.out.println( "Write method:n " + writeMethod.toString()); System.out.println("===================="); } System.out.println("Public methods:"); MethodDescriptor methods[] = bi.getMethodDescriptors(); for(int i = 0; i < methods.length; i++) System.out.println( methods[i].getMethod().toString()); System.out.println("======================"); System.out.println("Event support:"); EventSetDescriptor events[] = bi.getEventSetDescriptors(); for(int i = 0; i < events.length; i++) { System.out.println("Listener type:n " + events[i].getListenerType().getName()); Method lm[] = events[i].getListenerMethods(); for(int j = 0; j < lm.length; j++) System.out.println( "Listener method:n " + lm[j].getName()); MethodDescriptor lmd[] = events[i].getListenerMethodDescriptors(); for(int j = 0; j < lmd.length; j++) System.out.println( "Method descriptor:n " + lmd[j].getMethod().toString()); Method addListener = events[i].getAddListenerMethod(); System.out.println( "Add Listener Method:n " + addListener.toString()); Method removeListener = events[i].getRemoveListenerMethod(); System.out.println( "Remove Listener Method:n " + removeListener.toString()); System.out.println("===================="); } } // Dump the class of your choice: public static void main(String args[]) { if(args.length < 1) { System.err.println("usage: n" + "BeanDumper fully.qualified.class"); System.exit(0); } Class c = null; Chapter 13: Creating Windows & Applets 505
- 506. try { c = Class.forName(args[0]); } catch(ClassNotFoundException ex) { System.err.println( "Couldn't find " + args[0]); System.exit(0); } dump(c); } } ///:~ BeanDumper.dump( ) is the method that does all the work. First it tries to create a BeanInfo object, and if successful calls the methods of BeanInfo that produce information about properties, methods and events. In Introspector.getBeanInfo( ), you’ll see there is a second argument. This tells the Introspector where to stop in the inheritance hierarchy. Here, it stops before it parses all the methods from Object, since we’re not interested in seeing those. For properties, getPropertyDescriptors( ) returns an array of PropertyDescriptors. For each PropertyDescriptor you can call getPropertyType( ) to find the class of object that is passed in and out via the property methods. Then for each property you can get its pseudonym (extracted from the method names) with getName( ), the method for reading with getReadMethod( ) and the method for writing with getWriteMethod( ). These last two methods return a Method object which can actually be used to invoke the corresponding method on the object (this is part of reflection). For the public methods (including the property methods), getMethodDescriptors( ) returns an array of MethodDescriptors. For each one you can get the associated Method object and print out its name. For the events, getEventSetDescriptors( ) returns an array of (what else?) EventSetDescriptors. Each of these can be queried to find out the class of the listener, the methods of that listener class, and the add- and remove-listener methods. The BeanDumper program prints out all of this information. If you invoke BeanDumper on the Frog class like this: java BeanDumper Frog the output, after removing extra details that are unnecessary here, is: class name: Frog Property type: Color Property name: color Read method: public Color getColor() Write method: public void setColor(Color) ==================== Property type: Spots Property name: spots Read method: public Spots getSpots() 506 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 507. Write method: public void setSpots(Spots) ==================== Property type: boolean Property name: jumper Read method: public boolean isJumper() Write method: public void setJumper(boolean) ==================== Property type: int Property name: jumps Read method: public int getJumps() Write method: public void setJumps(int) ==================== Public methods: public void setJumps(int) public void croak() public void removeActionListener(ActionListener) public void addActionListener(ActionListener) public int getJumps() public void setColor(Color) public void setSpots(Spots) public void setJumper(boolean) public boolean isJumper() public void addKeyListener(KeyListener) public Color getColor() public void removeKeyListener(KeyListener) public Spots getSpots() ====================== Event support: Listener type: KeyListener Listener method: keyTyped Listener method: keyPressed Listener method: keyReleased Method descriptor: public void keyTyped(KeyEvent) Method descriptor: public void keyPressed(KeyEvent) Method descriptor: public void keyReleased(KeyEvent) Add Listener Method: public void addKeyListener(KeyListener) Remove Listener Method: public void removeKeyListener(KeyListener) ==================== Chapter 13: Creating Windows & Applets 507
- 508. Listener type: ActionListener Listener method: actionPerformed Method descriptor: public void actionPerformed(ActionEvent) Add Listener Method: public void addActionListener(ActionListener) Remove Listener Method: public void removeActionListener(ActionListener) ==================== This reveals most of what the Introspector sees as it produces a BeanInfo object from your Bean. You can see that the type of the property and its name are independent. Notice the decapitalization of the property name (the only time this doesn’t occur is when the property name begins with more than one capital letter in a row). And remember that the method names you’re seeing here (such as the read and write methods) are actually produced from a Method object that can be used to invoke the associated method on the object. The public method list includes the methods that are not associated with a property or event, such as croak( ), as well as those that are. These are all the methods that you can call programmatically for a Bean, and the application builder tool can choose to list all these while you’re making method calls, to ease your task. Finally, you can see that the events are fully parsed out into the listener, its methods and the add- and remove-listener methods. Basically, once you have the BeanInfo, you can find out everything of importance for the Bean. You can also call the methods for that Bean, even though you don’t have any other information except the object itself (again, a feature of reflection). A more sophisticated Bean This next example is slightly more sophisticated, albeit frivolous. It’s a canvas that draws a little circle around the mouse whenever the mouse is moved. When you press the mouse, the word “Bang!” appears in the middle of the screen, and an action listener is fired. The properties you can change are the size of the circle as well as the color, size and text of the word that is displayed when you press the mouse. A BangBean also has its own addActionListener( ) and removeActionListener( ) so you can attach your own listener that will be fired when the user clicks on the BangBean. You should be able to recognize the property and event support: //: BangBean.java // A graphical Bean package bangbean; import java.awt.*; import java.awt.event.*; import java.util.*; import java.io.*; public class BangBean extends Canvas implements Serializable { protected int xm, ym; protected int cSize = 20; // Circle size protected String text = "Bang!"; 508 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 509. protected int fontSize = 48; protected Color tColor = Color.red; protected ActionListener actionListener; public BangBean() { addMouseListener(new ML()); addMouseMotionListener(new MM()); } public int getCircleSize() { return cSize; } public void setCircleSize(int newSize) { cSize = newSize; } public String getBangText() { return text; } public void setBangText(String newText) { text = newText; } public int getFontSize() { return fontSize; } public void setFontSize(int newSize) { fontSize = newSize; } public Color getTextColor() { return tColor; } public void setTextColor(Color newColor) { tColor = newColor; } public void paint(Graphics g) { g.setColor(Color.black); g.drawOval(xm - cSize/2, ym - cSize/2, cSize, cSize); } // This is a unicast listener, which is // the simplest form of listener management: public void addActionListener ( ActionListener l) throws TooManyListenersException { if(actionListener != null) throw new TooManyListenersException(); actionListener = l; } public void removeActionListener( ActionListener l) { actionListener = null; } class ML extends MouseAdapter { public void mousePressed(MouseEvent e) { Graphics g = getGraphics(); g.setColor(tColor); g.setFont( new Font( "TimesRoman", Font.BOLD, fontSize)); int width = g.getFontMetrics().stringWidth(text); g.drawString(text, (getSize().width - width) /2, getSize().height/2); g.dispose(); // Call the listener's method: if(actionListener != null) Chapter 13: Creating Windows & Applets 509
- 510. actionListener.actionPerformed( new ActionEvent(BangBean.this, ActionEvent.ACTION_PERFORMED, null)); } } class MM extends MouseMotionAdapter { public void mouseMoved(MouseEvent e) { xm = e.getX(); ym = e.getY(); repaint(); } } public Dimension getPreferredSize() { return new Dimension(200, 200); } // Testing the BangBean: public static void main(String args[]) { BangBean bb = new BangBean(); try { bb.addActionListener(new BBL()); } catch(TooManyListenersException e) {} Frame aFrame = new Frame("BangBean Test"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(bb, BorderLayout.CENTER); aFrame.setSize(300,300); aFrame.setVisible(true); } // During testing, send action information // to the console: static class BBL implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("BangBean action"); } } } ///:~ The first thing you’ll notice is that BangBean implements the Serializable interface. This means that the application builder tool can “pickle” all the information for the BangBean using serialization after the program designer has adjusted the values of the properties. When the Bean is created as part of the running application, these “pickled” properties are restored so you get exactly what you designed. You can see that all the fields are private, which is what you’ll normally do with a Bean – allow access only through methods, usually using the “property” scheme. When you look at the signature for addActionListener( ), you’ll see that it may throw a TooManyListenersException. This indicates that it is unicast, which means it notifies only one listener when the event occurs. Normally, you’ll use multicast events, which means that many listeners may be notified of an event. However, that runs into issues that you won’t be ready for until the next chapter, so it will be revisited there (under the heading “Java Beans revisited”). A unicast event sidesteps the problem. 510 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 511. When you press the mouse, the text is put in the middle of the BangBean, and if the actionListener field is not null, its actionPerformed( ) is called, creating a new ActionEvent object in the process. Whenever the mouse is moved, its new coordinates are captured and the canvas is repainted (erasing, you’ll see, any text that’s on the canvas). The main( ) is added to allow you to test the program from the command line. When a Bean is in a development environment main( ) will not be used, but it’s helpful to have a main( ) in each of your Beans because it provides for rapid testing. main( ) creates a Frame and places a BangBean within it, attaching a simple ActionListener to the BangBean to print to the console whenever an ActionEvent occurs. Normally, of course, the application builder tool would create most of the code that uses the Bean. When you run the BangBean through BeanDumper or put the BangBean inside a Bean- enabled development environment, you’ll notice there are many more properties and actions than are evident from the above code. That’s because BangBean is inherited from Canvas, and Canvas is itself a Bean, so you’re seeing its properties and events as well. Packaging a Bean Before you can bring a Bean into a Bean-enabled visual builder tool, it must be put into the standard Bean container, which is a JAR (Java ARchive) file that includes all the Bean classes as well as a “manifest” file that says “this is a Bean.” A manifest file is simply a text file that follows a particular form. For the BangBean, the manifest file looks like this: Manifest-Version: 1.0 Name: bangbean/BangBean.class Java-Bean: True The first line indicates the version of the manifest scheme, which until further notice from Sun is 1.0. The second line names the BangBean.class file, and the third says “it’s a Bean.” Without the third line, the program builder tool will not recognize the class as a Bean. The only tricky part is that you must make sure you get the proper path in the “Name:” field. If you look back at BangBean.java, you’ll see it’s in package bangbean (and thus in a subdirectory called “bangbean” that’s off of the classpath), and the name in the manifest file must include this package information. In addition, you must place the manifest file in the directory above the root of your package path, which in this case means placing the file in the directory above the “bangbean” subdirectory. Then you must invoke jar from the same directory as the manifest file, as follows: jar cfm BangBean.jar BangBean.mf bangbean This assumes you want the resulting JAR file to be named BangBean.jar and that you’ve put the manifest in a file called BangBean.mf. You may wonder “what about all the other classes that were generated when I compiled BangBean.java?” Well, they all ended up inside the bangbean subdirectory, and you’ll see that the last argument for the above jar command line is the bangbean subdirectory. When you give jar the name of a subdirectory, it packages that entire subdirectory into the jar file (including, in this case, the original BangBean.java source-code file – you may not choose to include the source with your own Beans). In addition, if you turn around and unpack the JAR file you’ve just created, you’ll discover that your manifest file isn’t inside, but that jar has created its own manifest file (based partly on yours) called MANIFEST.MF and placed it inside the subdirectory META-INF (for “meta information”). If you open this Chapter 13: Creating Windows & Applets 511
- 512. manifest file you’ll also note that digital signature information has been added by jar for each file, of the form: Digest-Algorithms: SHA MD5 SHA-Digest: pDpEAG9NaeCx8aFtqPI4udSX/O0= MD5-Digest: O4NcS1hE3Smnzlp2hj6qeg== In general, you don’t need to worry about any of this, and if you make changes you can just modify your original manifest file and re-invoke jar to create a new JAR file for your Bean. You can also add other Beans to the JAR file simply by adding their information to your manifest. One thing to notice is that you’ll probably want to put each Bean in its own subdirectory, since when you create a JAR file you hand the jar utility the name of a subdirectory and it puts everything in that subdirectory into the JAR file. You can see that both Frog and BangBean are in their own subdirectories. Once you have your Bean properly inside a JAR file, you can bring it into a Beans-enabled program-builder environment. The way you do this varies from one tool to the next, but Sun provides a freely-available test bed for Java Beans in their “Beans Development Kit” (BDK) called the “beanbox.” (Download the BDK from www.javasoft.com). To place your Bean in the beanbox, all you need to do is copy the JAR file into the bdk’s “jars” subdirectory before you start up the beanbox. More complex Bean support You can see how remarkably simple it is to make a Bean. But you aren’t limited to what you’ve seen here. The Java Bean design provides a simple point of entry but can also scale to more complex situations. These situations are beyond the scope of this book but they will be briefly introduced here. You can find more details at http://java.sun.com/beans. One place where you can add sophistication is with properties. The above examples have shown only single properties, but it’s also possible to represent multiple properties in an array. This is called an indexed property. You simply provide the appropriate methods (again following a naming convention for the method names) and the Introspector recognizes an indexed property so your application builder tool can respond appropriately. Properties may be bound, which means they will notify other objects via a PropertyChangeEvent. The other objects can then choose to change themselves based on the change to the Bean. Properties may be constrained, which means other objects can veto a change to that property if it is unacceptable. The other objects are notified using a PropertyChangeEvent, and they may throw a ProptertyVetoException to prevent the change from happening and to restore the old values. You can also change the way your Bean is represented at design time: 1. You can provide a custom property sheet for your particular Bean. The ordinary property sheet will be used for all other Beans, but yours is automatically invoked when your Bean is selected. 2. You can create a custom editor for a particular property, so the ordinary property sheet is used but when your special property is being edited, your editor will automatically be invoked. 3. You can provide a custom BeanInfo class for your Bean that produces different information than the default created by the Introspector. 512 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 513. 4. It’s also possible to turn “expert” mode on and off in all FeatureDescriptors to distinguish between basic features and more complicated ones. More to Beans There’s another issue that couldn’t be addressed here. Whenever you create a Bean, you should expect that it will be run in a multithreaded environment. This means that you must understand the issues of threading, which will be introduced in the next chapter. You’ll find a section there called “Java Beans revisited” which will look at the problem and its solution. Summary Of all the libraries in Java, the AWT has seen the most dramatic changes from Java 1.0 to Java 1.2. The Java 1.0 AWT was roundly criticized as being one of the worst designs seen, and while it would allow you to create portable programs, the resulting GUI was “equally mediocre on all platforms.” It was also very limiting, awkward and unpleasant to use compared with native application development tools on a particular platform. When Java 1.1 introduced the new event model and Java Beans, the stage was set – now it was possible to create GUI components that could be easily dragged and dropped inside visual application builder tools. In addition, the design of the event model and Beans clearly shows strong consideration for ease of programming and maintainable code (something that was not evident in the 1.0 AWT). But it wasn’t until the GUI components themselves – the JFC/Swing classes – appeared that the job was finished. With the Swing components, cross-platform GUI programming can be a civilized experience. Actually, the only thing that’s missing is the application builder tool itself, and this is where the real revolution lies. Microsoft’s Visual Basic and Visual C++ require their application builder tools, as does Borland’s Delphi and C++ Builder. If you want the application builder tool to get better, you have to cross your fingers and hope the vendor will give you what you want. But Java is an open environment, and so not only does it allow for competing application builder environments, it encourages them. And for these tools to be taken seriously, they must support Java Beans. This means a leveled playing field: if a better application builder tool comes along, you’re not tied to the one you’ve been using – you can pick up and move to the new one and increase your productivity. This kind of competitive environment for GUI application builder tools has not been seen before, and the resulting competition can generate only positive results for the productivity of the programmer. Exercises 1. Create an applet with a text field and 3 buttons. When you press each button, make some different text appear in the text field. 2. Add a check box to the applet created in exercise one, capture the event and insert different text into the text field. 3. Create an applet and add all the components that cause action( ) to be called, then capture their events and display an appropriate message for each inside a text field. Chapter 13: Creating Windows & Applets 513
- 514. 4. Add to exercise three the components that can be used only with events detected by handleEvent( ). Override handleEvent( ) and display appropriate messages for each inside a text field. 5. Create an applet with a Button and a TextField. Write a handleEvent( ) so that if the button has the focus, characters typed into it will appear in the TextField. 6. Create an application and add to the main frame all the components described in this chapter, including menus and a dialog box. 7. Modify TextNew.java so that the characters in t2 retain the original case that they were typed in, instead of automatically being forced to upper case. 8. Modify CardLayout1.java so that it uses the new Java 1.1 event model. 9. Add Frog.class to the manifest file shown in this chapter, and run jar to create a JAR file containing both Frog and BangBean. Now either download and install the BDK from Sun or use your own Beans-enabled program builder tool and add the JAR file to your environment so you can test the two Beans. 10. Create your own Java Bean called Valve that contains two properties: a Boolean called “on” and an integer called “level.” Create a manifest file, use jar to package your Bean, then load it into the beanbox or into your own Beans-enabled program builder tool so you can test it. 514 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 515. A 14: Multiple threads Objects provide a way to divide a program up into independent sections. Often you also need to turn a program into separate, independently-running subtasks. Each of these independent subtasks is called a thread, and you program as if each thread runs all by itself, and has the CPU all to itself. Some underlying mechanism is actually dividing up the CPU time for you, but in general you don’t have to think about it, which makes programming with multiple threads a much easier task. Some definitions are useful at this point. A process is a self-contained running program with its own address space. A multitasking operating system is capable of running more than one process (program) at a time, while making it look like each one is chugging along by periodically providing CPU cycles to each process. A thread is a single sequential flow of control within a process. A single process can thus have multiple concurrently executing threads. There are many possible uses for multithreading, but in general you’ll have some part of your program tied to a particular event or resource, and you don’t want to hang up the rest of your program because of that. So you create a thread associated with that event or resource, and let it run independently of the main program. A good example is a “quit” button – you don’t want to be forced to poll the quit button in every piece of code you write in your program, and yet you want the quit button to be responsive, as if you were checking it regularly. In fact, one of the most immediately compelling reasons for multithreading is to produce a responsive user interface. 515
- 516. Responsive user interfaces As a starting point, consider a program that performs some CPU-intensive operation and thus ends up ignoring user input and being unresponsive. This one, a combined applet/application, will simply display the result of a running counter: //: Counter1.java // A non-responsive user interface package c14; import java.awt.*; import java.awt.event.*; import java.applet.*; public class Counter1 extends Applet { private int count = 0; private Button onOff = new Button("Toggle"), start = new Button("Start"); private TextField t = new TextField(10); private boolean runFlag = true; public void init() { add(t); start.addActionListener(new StartL()); add(start); onOff.addActionListener(new OnOffL()); add(onOff); } public void go() { while (true) { try { Thread.currentThread().sleep(100); } catch (InterruptedException e){} if(runFlag) t.setText(Integer.toString(count++)); } } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { go(); } } class OnOffL implements ActionListener { public void actionPerformed(ActionEvent e) { runFlag = !runFlag; } } public static void main(String args[]) { Counter1 applet = new Counter1(); Frame aFrame = new Frame("Counter1"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } 516 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 517. }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ At this point the AWT and applet code should be reasonably familiar from Chapter 13. The go( ) method is where the program stays busy: it puts the current value of count into the TextField t, then increments count. Part of the infinite loop inside go( ) is to call sleep( ). sleep( ) must be associated with a Thread object, and it turns out that every application has some thread associated with it (indeed, Java is based on threads and there are always some running along with your application). So regardless of whether you’re explicitly using threads or not you can produce the current thread used by your program with Thread. currentThread() (a static method of the Thread class) and then call sleep( ) for that thread. Note that sleep( ) may throw InterruptedException, although throwing such an exception is considered a hostile way to break from a thread and should be discouraged (once again, exceptions are for exceptional conditions, not normal flow of control). Interrupting a sleeping thread is included to support a future language feature. When the start button is pressed, go( ) is invoked. And upon examining go( ), you may naively think (as I did) that it should allow multithreading because it goes to sleep. That is, while the method is asleep it seems like the CPU could be busy monitoring other button presses. But it turns out the real problem is that go( ) never returns, since it’s in an infinite loop, and this means that actionPerformed( ) never returns. Since you’re stuck inside actionPerformed( ) for the first keypress, the program can’t handle any other events (to get out, you must somehow kill the process; the easiest way to do this is to press Control-C in the console window). The basic problem here is that go( ) needs to continue performing its operations, and at the same time it needs to return so actionPerformed( ) can complete and the user interface can continue responding to the user. But in a conventional method like go( ) it cannot continue and at the same time return control to the rest of the program. This sounds like an impossible thing to accomplish, as if the CPU must be in two places at once, but this is precisely the illusion that threading provides. The thread model (and programming support in Java) is a programming convenience to simplify juggling several operations at the same time within a single program. With threads, the CPU will pop around and give each thread some of its time. Each thread has the consciousness of constantly having the CPU all to itself, but the CPU’s time is actually sliced between all the threads. Threading reduces computing efficiency somewhat, but the net improvement in program design, resource balancing and user convenience is often very valuable. Of course, if you have more than one CPU then the operating system can dedicate each CPU to a set of threads or even a single thread, and then the whole program can run much faster. Multitasking and multithreading tend to be the most reasonable ways to utilize multiprocessor systems. Inheriting from Thread The simplest way to create a thread is to inherit from class Thread, which has all the wiring necessary to create and run threads. The most important method for Thread is Chapter 14: Multiple Threads 517
- 518. run( ), which you must override to make the thread do your bidding. Thus, run( ) is the code that will be executed “simultaneously” with the other threads in a program. The following example creates any number of threads which it keeps track of by assigning each thread a unique number, generated with a static variable. The Thread’s run( ) method is overridden to count down each time it passes through its loop, and to finish when the count is zero (at the point when run( ) returns, the thread is terminated). //: SimpleThread.java // Very simple Threading example public class SimpleThread extends Thread { private int countDown = 5; private int threadNumber; private static int threadCount = 0; public SimpleThread() { threadNumber = ++threadCount; System.out.println("Making " + threadNumber); } public void run() { while(true) { System.out.println("Thread " + threadNumber + "(" + countDown + ")"); if(--countDown == 0) return; } } public static void main(String args[]) { for(int i = 0; i < 5; i++) new SimpleThread().start(); System.out.println("All Threads Started"); } } ///:~ A run( ) method virtually always has some kind of loop that continues until the thread is no longer necessary, so you’ll have to establish the condition on which to break out of this loop (or, in the above case, simply return from run( )). Very often, run( ) is cast in the form of an infinite loop, which means that, barring some external call to stop( ) or destroy( ) for that thread, it will run forever (until the program completes). In main( ) you can see a number of threads being created and run. The special method that comes with the Thread class is start( ), which performs special initialization for the thread and then calls run( ). So the steps are: the constructor is called to build the object, then start( ) configures the thread and calls run( ). If you don’t call start( ) (which you can do in the constructor, if that’s appropriate) the thread will never be started. The output for this program is: Making 1 Making 2 Making 3 Making 4 Making 5 Thread 1(5) Thread 1(4) Thread 1(3) Thread 1(2) Thread 2(5) 518 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 519. Thread 2(4) Thread 2(3) Thread 2(2) Thread 2(1) Thread 1(1) All Threads Started Thread 3(5) Thread 4(5) Thread 4(4) Thread 4(3) Thread 4(2) Thread 4(1) Thread 5(5) Thread 5(4) Thread 5(3) Thread 5(2) Thread 5(1) Thread 3(4) Thread 3(3) Thread 3(2) Thread 3(1) You’ll note that nowhere in this example is sleep( ) called, and yet the output indicates that each thread gets a portion of the CPU’s time in which to execute. This shows that sleep( ), while it relies on the existence of a thread in which to execute, is not involved with either enabling or disabling threading. It’s simply another method. You can also see that the threads are not run in the order that they’re created. In fact, the order in which an existing set of threads is attended to by the CPU is indeterminate, unless you go in and adjust the priorities using Thread’s setPriority( ) method. When main( ) creates the Thread objects it isn’t capturing the handles for any of them. An ordinary object would be fair game for garbage collection, but not a Thread. Each Thread “registers” itself so there is actually a reference to it someplace and the garbage collector can’t clean it up. Threading for a responsive interface Now it’s possible to solve the problem in Counter1.java with a thread. The trick is to place the subtask – that is, the loop that’s inside go( ) – inside the run( ) method of a thread. When the user presses the start button, the thread is started, but then the creation of the thread completes and so even though the thread is running, the main job of the program (watching for and responding to user-interface events) can continue. Here’s the solution: //: Counter2.java // A responsive user interface with threads import java.awt.*; import java.awt.event.*; import java.applet.*; class SeparateSubTask extends Thread { private int count = 0; private Counter2 c2; private boolean runFlag = true; public SeparateSubTask(Counter2 c2) { this.c2 = c2; Chapter 14: Multiple Threads 519
- 520. start(); } public void invertFlag() { runFlag = !runFlag;} public void run() { while (true) { try { sleep(100); } catch (InterruptedException e){} if(runFlag) c2.t.setText(Integer.toString(count++)); } } } public class Counter2 extends Applet { TextField t = new TextField(10); private SeparateSubTask sp = null; private Button onOff = new Button("Toggle"), start = new Button("Start"); public void init() { add(t); start.addActionListener(new StartL()); add(start); onOff.addActionListener(new OnOffL()); add(onOff); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(sp == null) sp = new SeparateSubTask(Counter2.this); } } class OnOffL implements ActionListener { public void actionPerformed(ActionEvent e) { if(sp != null) sp.invertFlag(); } } public static void main(String args[]) { Counter2 applet = new Counter2(); Frame aFrame = new Frame("Counter2"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ 520 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 521. Counter2 is now a very straightforward program, whose job is only to set up and maintain the user interface. But now, when the user presses the start button, a method is not called. Instead a thread of class SeparateSubTask is created (the constructor starts it, in this case), and then the Counter2 event loop continues. Note that the handle to the SeparateSubTask is stored so that when you press the onOff button it can toggle the runFlag inside the SeparateSubTask object, and that thread (when it looks at the flag) can then start and stop itself (this could also have been accomplished by making SeparateSubTask an inner class). The class SeparateSubTask is a simple extension of Thread with a constructor (that stores the Counter2 handle and then runs the thread by calling start( )) and a run( ) that essentially contains the code from inside go( ) in Counter1.java. Because SeparateSubTask knows that it holds a handle to a Counter2, it can reach in and access Counter2’s TextField when it needs to. When you press the onOff button, you’ll see a virtually instant response. Of course, the response isn’t really instant, not like that of a system that’s driven by interrupts. The counter stops only when the thread has the CPU and notices that the flag has changed. Improving the code with an inner class As an aside, look at the coupling that occurs between the SeparateSubTask and Counter2 classes. The SeparateSubTask is intimately tied to Counter2 – it must keep a handle to it’s “parent” Counter2 object so it can call back and manipulate it. And yet the two classes shouldn’t really merged together into a single class (although in the next section you’ll see that Java provides a way to combine them) because they’re doing separate things and are created at different times. They are tightly connected (what I call a “couplet”) and this makes the coding awkward. This is a situation where an inner class can improve the code significantly: //: Counter2i.java // Counter2 using an inner class for the thread import java.awt.*; import java.awt.event.*; import java.applet.*; public class Counter2i extends Applet { private class SeparateSubTask extends Thread { int count = 0; boolean runFlag = true; SeparateSubTask() { start(); } public void run() { while (true) { try { sleep(100); } catch (InterruptedException e){} if(runFlag) t.setText(Integer.toString(count++)); } } } private SeparateSubTask sp = null; private TextField t = new TextField(10); private Button onOff = new Button("Toggle"), start = new Button("Start"); public void init() { add(t); Chapter 14: Multiple Threads 521
- 522. start.addActionListener(new StartL()); add(start); onOff.addActionListener(new OnOffL()); add(onOff); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(sp == null) sp = new SeparateSubTask(); } } class OnOffL implements ActionListener { public void actionPerformed(ActionEvent e) { if(sp != null) sp.runFlag = !sp.runFlag; // invertFlag(); } } public static void main(String args[]) { Counter2i applet = new Counter2i(); Frame aFrame = new Frame("Counter2i"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ This SeparateSubTask name will not collide with the SeparateSubTask in the previous example even though they’re in the same directory, since it’s hidden as an inner class. You can also see that the inner class is private, which means that its fields and methods can be given default access (except for run( ), which must be public since it is public in the base class). The private inner class is not accessible to anyone but Counter2i, and since the two classes are tightly coupled it’s convenient to loosen the access restrictions between them. In SeparateSubTask you can see that the invertFlag( ) method has been removed since Counter2i can now directly access runFlag. Also notice that SeparateSubTask’s constructor has been simplified – now it only starts the thread. The handle to the Counter2i object is still being captured as in the previous version, but instead of doing it by hand and referencing the outer object by hand, the inner class mechanism takes care of it automatically. In run( ), you can see that t is simply accessed, as if it were a field of SeparateSubTask. The t field in the parent class can now be made private since SeparateSubTask can access it without getting any special permission – and it’s always good to make fields “as private as possible” so they cannot be accidentally changed by forces outside your class. Anytime you notice classes that appear to have high coupling with each other, consider the coding and maintenance improvements you might get by using inner classes. 522 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 523. Combining the thread with the main class In the above example you can see that the thread class is separate from the program’s main class. This makes a lot of sense and is relatively easy to understand. There is, however, an alternate form that you will often see used which is not so clear but is usually more concise (which probably accounts for its popularity). This form combines the main program class with the thread class by making the main program class a thread. Since for a GUI program the main program class must be inherited from either Frame or Applet, this means that an interface must be used to paste on the additional functionality. This interface is called Runnable, and it contains the same basic method that Thread does. In fact, Thread also implements Runnable, which specifies only that there be a run( ) method. The use of the combined program/thread is not quite so obvious. When you start the program, you create an object that’s Runnable, but you don’t start the thread. This must be done explicitly. You can see this in the following program, which reproduces the functionality of Counter2: //: Counter3.java // Using the Runnable interface to turn the // main class into a thread. import java.awt.*; import java.awt.event.*; import java.applet.*; public class Counter3 extends Applet implements Runnable { private int count = 0; private boolean runFlag = true; private Thread selfThread = null; private Button onOff = new Button("Toggle"), start = new Button("Start"); private TextField t = new TextField(10); public void init() { add(t); start.addActionListener(new StartL()); add(start); onOff.addActionListener(new OnOffL()); add(onOff); } public void run() { while (true) { try { selfThread.sleep(100); } catch (InterruptedException e){} if(runFlag) t.setText(Integer.toString(count++)); } } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(selfThread == null) { selfThread = new Thread(Counter3.this); selfThread.start(); } Chapter 14: Multiple Threads 523
- 524. } } class OnOffL implements ActionListener { public void actionPerformed(ActionEvent e) { runFlag = !runFlag; } } public static void main(String args[]) { Counter3 applet = new Counter3(); Frame aFrame = new Frame("Counter3"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Now the run( ) is inside the class, but it’s still dormant after init( ) completes. When you press the start button, the thread is created (if it doesn’t already exist) in the somewhat obscure expression: new Thread(Counter3.this); When something has a Runnable interface, it simply means it has a run( ) method, but there’s nothing special about that – it doesn’t produce any innate threading abilities, like those of a class inherited from Thread. So to produce a thread from a Runnable object, you must actually create a thread separately and hand it the Runnable object; there’s a special constructor for this that takes a Runnable as its argument. You can then call start( ) for that thread: selfThread.start(); This performs the usual initialization and then calls run( ). The convenient thing about the Runnable interface is that everything belongs to the same class. If you need to access something, you simply do it without going through a separate object. The penalty for this convenience is strict, though – you can have only a single thread running for that particular object (although you can create more objects of that type, or create other threads in different classes). Note that the Runnable interface is not what imposes this restriction. It’s the combination of Runnable and your main class that does it, since you can have only one object of your main class per application. Making many threads Consider the creation of many different threads. You can’t do this with the previous example, so you’ll have to go back to having separate classes inherited from Thread to encapsulate the run( ). But this is a more general solution and easier to understand, so 524 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 525. while the previous example shows a coding style you’ll often see, I can’t recommend it for most cases because it’s just a little bit more confusing and less flexible. The following example repeats the form of the above examples with counters and toggle buttons. But now all the information for a particular counter, including the button and text field, is inside its own object which is inherited from Thread. All the fields in Ticker are private, which means that the Ticker implementation can be changed at will, including the quantity and type of data components to acquire and display information. When a Ticker object is created, the constructor requires a handle to an AWT Container which Ticker fills with its visual components. This way, if you change the visual components, the code that uses Ticker doesn’t need to be modified. //: Counter4.java // If you separate your thread from the main // class, you can have as many threads as you // want. import java.awt.*; import java.awt.event.*; import java.applet.*; class Ticker extends Thread { private Button b = new Button("Toggle"); private TextField t = new TextField(10); private int count = 0; private boolean runFlag = true; public Ticker(Container c) { b.addActionListener(new ToggleL()); Panel p = new Panel(); p.add(t); p.add(b); c.add(p); } class ToggleL implements ActionListener { public void actionPerformed(ActionEvent e) { runFlag = !runFlag; } } public void run() { while (true) { if(runFlag) t.setText(Integer.toString(count++)); try { sleep(100); } catch (InterruptedException e){} } } } public class Counter4 extends Applet { private Button start = new Button("Start"); private boolean started = false; private Ticker s[]; private boolean isApplet = true; private int size; public void init() { // Get parameter "size" from Web page: if(isApplet) Chapter 14: Multiple Threads 525
- 526. size = Integer.parseInt(getParameter("size")); s = new Ticker[size]; for(int i = 0; i < s.length; i++) s[i] = new Ticker(this); start.addActionListener(new StartL()); add(start); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(!started) { started = true; for(int i = 0; i < s.length; i++) s[i].start(); } } } public static void main(String args[]) { Counter4 applet = new Counter4(); // This isn't an applet, so set the flag and // produce the parameter values from args[]: applet.isApplet = false; applet.size = (args.length == 0 ? 5 : Integer.parseInt(args[0])); Frame aFrame = new Frame("Counter4"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(200, applet.size * 50); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Ticker contains not only its threading equipment but also the way to control and display the thread. You can create as many threads as you want without explicitly creating the windowing components. In Counter4 there’s an array of Ticker objects called s. For maximum flexibility, the size of this array is initialized by reaching out into the Web page itself, using applet parameters. Here’s what the size parameter looks like on the page, embedded inside the applet description: <applet code=Counter4 width=600 height=600> <param name=size value="20"> </applet> The param, name and value are all Web-page keywords. name is what you’ll be referring to in your program, and value can be any string, not just something that resolves to a number. 526 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 527. You’ll notice that the determination of the size of the array s is done inside init( ), and not as part of an inline definition of s. That is, you cannot say as part of the class definition (outside of any methods): int size = Integer.parseInt(getParameter("size")); Ticker s[] = new Ticker[size]; Actually, you can compile this but you’ll get a strange null-pointer exception at run time. It works fine if you move the getParameter( ) initialization inside of init( ). The applet framework performs the necessary startup to grab the parameters before entering init( ). In addition, this code is set up to be either an applet or an application. When it’s an application the size argument is extracted from the command line (or a default value is provided). Once the size of the array is established, new Ticker objects are created; as part of the Ticker constructor the button and text field for each Ticker is added to the applet. Pressing the start button means looping through the whole array of Tickers and calling start( ) for each one. Remember, start( ) performs necessary thread initialization and then calls run( ) for that thread. The ToggleL listener simply inverts the flag in Ticker, and when the associated thread next takes note it can react accordingly. One value of this example is that it allows you to easily create large sets of independent subtasks and to monitor their behavior. In this case, you’ll see that as the number of subtasks gets larger, your machine will probably show more and more divergence in the displayed numbers because of the way that the threads are served. You can also experiment to discover how important the sleep(100) is inside Ticker.run( ). If you remove the sleep( ), things will work fine until you press a toggle button. Then that particular thread has a false runFlag and the run( ) is just tied up in a very tight infinite loop, which appears very difficult to break during multithreading, so the responsiveness and speed of the program really bogs down. Daemon threads A “daemon” thread is one that is supposed to provide a general service in the background as long as the program is running, but is not part of the essence of the program. Thus, when all the non-daemon threads complete then the program is terminated. Conversely, if there are any non-daemon threads still running the program doesn't terminate (there is, for instance, a thread that runs main( )). You can find out if a thread is a daemon by calling isDaemon( ), and you can turn the daemonhood of a thread on and off with setDaemon( ). If a thread is a daemon, then any threads it creates will automatically be daemons. The following example demonstrates daemon threads: //: Daemons.java // Daemonic behavior import java.io.*; class Daemon extends Thread { private static final int SIZE = 10; private Thread t[] = new Thread[SIZE]; public Daemon() { Chapter 14: Multiple Threads 527
- 528. setDaemon(true); start(); } public void run() { for(int i = 0; i < SIZE; i++) t[i] = new DaemonSpawn(i); for(int i = 0; i < SIZE; i++) System.out.println( "t[" + i + "].isDaemon() = " + t[i].isDaemon()); while(true) yield(); } } class DaemonSpawn extends Thread { public DaemonSpawn(int i) { System.out.println( "DaemonSpawn " + i + " started"); start(); } public void run() { while(true) yield(); } } public class Daemons { public static void main(String args[]) { Thread d = new Daemon(); System.out.println( "d.isDaemon() = " + d.isDaemon()); // Allow the daemon threads to finish // their startup processes: BufferedReader stdin = new BufferedReader( new InputStreamReader(System.in)); System.out.println("Waiting for CR"); try { stdin.readLine(); } catch(IOException e) {} } } ///:~ The Daemon thread sets its daemon flag to “true” and then spawns a bunch of other threads to show that they are also daemons. Then it goes into an infinite loop which calls yield( ) to give up control to the other processes. In an earlier version of this program, the infinite loops would increment int counters, but this seemed to bring the whole program to a stop. Using yield( ) makes the program quite peppy. There’s nothing to keep the program from terminating once main( ) finishes its job since there are nothing but daemon threads running. So you can see the results of starting all the daemon threads, System.in is set up to read so the program waits for a carriage return before terminating. Without this you see only some of the results from the creation of the daemon threads (try replacing the readLine( ) code with sleep( ) calls of various lengths to see this behavior). 528 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 529. Sharing limited resources You can think of a single-threaded program as one lonely entity moving around through your problem space and doing one thing at a time. Because there’s only one entity, you never have to think about the problem of two entities trying to use the same resource at the same time, like two people trying to park in the same space, walk through a door at the same time, or even talk at the same time. With multithreading, things aren’t lonely anymore but you now have the possibility of two or more threads trying to use the same limited resource at once. Colliding over a resource must be prevented, or else you’ll have two threads trying to access the same bank account at the same time, or print to the same printer, or adjust the same valve, etc. Improperly accessing resources Consider a variation on the counters that have been used so far in this chapter. In the following example, each thread contains two counters which are incremented and displayed inside run( ). In addition, there’s another thread of class Watcher that is watching the counters to see if they’re always equivalent. This seems like a needless activity, since looking at the code it appears obvious that the counters will always be the same. But that’s where the surprise comes in. Here’s the first version of the program: //: Sharing1.java // Problems with resource sharing while threading import java.awt.*; import java.awt.event.*; import java.applet.*; class TwoCounter extends Thread { private boolean started = false; private TextField t1 = new TextField(5), t2 = new TextField(5); private Label l = new Label("count1 == count2"); private int count1 = 0, count2 = 0; // Add the display components as a panel // to the given container: public TwoCounter(Container c) { Panel p = new Panel(); p.add(t1); p.add(t2); p.add(l); c.add(p); } public void start() { if(!started) { started = true; super.start(); } } public void run() { while (true) { t1.setText(Integer.toString(count1++)); t2.setText(Integer.toString(count2++)); Chapter 14: Multiple Threads 529
- 530. try { sleep(500); } catch (InterruptedException e){} } } public void synchTest() { Sharing1.incrementAccess(); if(count1 != count2) l.setText("Unsynched"); } } class Watcher extends Thread { private Sharing1 p; public Watcher(Sharing1 p) { this.p = p; start(); } public void run() { while(true) { for(int i = 0; i < p.s.length; i++) p.s[i].synchTest(); try { sleep(500); } catch (InterruptedException e){} } } } public class Sharing1 extends Applet { TwoCounter s[]; private static int accessCount = 0; private static TextField aCount = new TextField("0", 10); public static void incrementAccess() { accessCount++; aCount.setText(Integer.toString(accessCount)); } private Button start = new Button("Start"), observer = new Button("Observe"); private boolean isApplet = true; private int numCounters = 0; private int numObservers = 0; public void init() { if(isApplet) { numCounters = Integer.parseInt(getParameter("size")); numObservers = Integer.parseInt( getParameter("observers")); } s = new TwoCounter[numCounters]; for(int i = 0; i < s.length; i++) s[i] = new TwoCounter(this); 530 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 531. Panel p = new Panel(); start.addActionListener(new StartL()); p.add(start); observer.addActionListener(new ObserverL()); p.add(observer); p.add(new Label("Access Count")); p.add(aCount); add(p); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { for(int i = 0; i < s.length; i++) s[i].start(); } } class ObserverL implements ActionListener { public void actionPerformed(ActionEvent e) { for(int i = 0; i < numObservers; i++) new Watcher(Sharing1.this); } } public static void main(String args[]) { Sharing1 applet = new Sharing1(); // This isn't an applet, so set the flag and // produce the parameter values from args[]: applet.isApplet = false; applet.numCounters = (args.length == 0 ? 5 : Integer.parseInt(args[0])); applet.numObservers = (args.length < 2 ? 5 : Integer.parseInt(args[1])); Frame aFrame = new Frame("Sharing1"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e){ System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(350, applet.numCounters *100); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ As before, each counter contains its own display components: two text fields and a label that initially indicates the counts are equivalent. These components are added to the Container in the TwoCounter constructor. Because this thread is started via a button press by the user, it’s possible that start( ) may be called more than once. It’s illegal for Thread.start( ) to be called more than once for a thread (an exception is thrown). You can see the machinery to prevent this in the started flag and the overridden start( ) method. In run( ), count1 and count2 are incremented and displayed in a manner that would seem to keep them exactly identical. Then sleep( ) is called; without this call the program becomes very balky because it becomes hard for the CPU to swap tasks. Chapter 14: Multiple Threads 531
- 532. The synchTest( ) method performs the apparently useless activity of checking to see if count1 is equivalent to count2; if they are not equivalent it sets the label to “Unsynched” to indicate this. But first, it calls a static member of the class Sharing1 that increments and displays an access counter to show how many times this check has successfully occurred (the reason for this will become apparent in future variations of this example). The Watcher class is a thread whose job is to call synchTest( ) for all the TwoCounter objects that are active. It does this by stepping through the array that’s kept in the Sharing1 object. You can think of the Watcher as constantly peeking over the shoulders of the TwoCounter objects. Sharing1 contains an array of TwoCounter objects that it initializes in init( ) and starts as threads when you press the “start” button. Later, when you press the “Observe” button, one or more observers are created and freed upon the unsuspecting TwoCounter threads. Note that to run this as an applet in a browser, your Web page will need to contain the lines: <applet code=Sharing1 width=650 height=500> <param name=size value="20"> <param name=observers value="1"> </applet> You can change the width, height and parameters to suit your experimental tastes. By changing the size and observers you’ll change the behavior of the program. You can also see that this program is set up to run as a stand-alone application, by pulling the arguments from the command line (or providing defaults). Here’s the surprising part. In TwoCounter.run( ), the infinite loop is just passing over and over the adjacent lines: t1.setText(Integer.toString(count1++)); t2.setText(Integer.toString(count2++)); (as well as sleeping, but that’s not important here). When you run the program, however, you’ll discover that count1 and count2 will be observed (by the Watcher) to be unequal at times! This is because of the nature of threads – they can be suspended at any time. So at times, the suspension occurs between the execution of the above two lines, and the Watcher thread happens to come along and perform the comparison at just this moment, thus finding the two counters to be different. This example shows a fundamental problem when using threads. You never know when a thread may be run. Imagine sitting at a table with a fork, about to spear the last piece of food on your plate and as your fork reaches for it, it suddenly vanishes (because your thread was suspended, and another thread came in and stole the food). That’s the problem that you’re dealing with. Sometimes you don’t care if a resource is being accessed at the same time you’re trying to use it (the food is on some other plate). But for multithreading to work, you need some way to prevent two threads from accessing the same resource, at least during critical periods. Preventing this kind of collision is simply a matter of putting a lock on a resource when one thread is using it. The first thread that accesses a resource locks it, and then the other threads cannot access that resource until it is unlocked, at which time another thread locks and uses it, etc. If the front seat of the car is the limited resource, the child who shouts “dibs!” asserts the lock. 532 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 533. How Java shares resources Java has built-in support to prevent collisions over one kind of resource: the memory in an object. Since you typically make the data elements of a class private and access that memory only through methods, you can prevent collisions by making a particular method synchronized. Only one thread at a time may call a synchronized method for a particular object (although that thread may call more than one of the object’s synchronized methods). Here are simple synchronized methods: synchronized void f() { /* ... */ } synchronized void g(){ /* ... */ } Each object contains a single lock (also called a monitor)that’s automatically part of the object (you don’t have to write any special code). When you call any synchronized method, that object is locked and no other synchronized method of that object can be called until the first one finishes and thus releases the lock. In the above example, if f( ) is called for an object, g( ) cannot be called for the same object until f( ) is completed and releases the lock. Thus there’s a single lock that’s shared by all the synchronized methods of a particular object, and this lock prevents common memory from being written by more than one method at a time (i.e. more than one thread at a time). There’s also a single lock per class (as part of the Class object for the class), so that synchronized static methods can lock each other out from static data on a class-wide basis. Note that if you want to guard some other resource from simultaneous access by multiple threads, you can do so by forcing access to that resource through synchronized methods. Synchronizing the counters Armed with this new keyword it appears that the solution is at hand: we’ll simply use the synchronized keyword for the methods in TwoCounter. The following example is the same as before, with the addition of the new keyword: //: Sharing2.java // Using the synchronized keyword to prevent // multiple access to a particular resource. import java.awt.*; import java.awt.event.*; import java.applet.*; class TwoCounter2 extends Thread { private boolean started = false; private TextField t1 = new TextField(5), t2 = new TextField(5); private Label l = new Label("count1 == count2"); private int count1 = 0, count2 = 0; public TwoCounter2(Container c) { Panel p = new Panel(); p.add(t1); p.add(t2); p.add(l); c.add(p); } public void start() { if(!started) { Chapter 14: Multiple Threads 533
- 534. started = true; super.start(); } } public synchronized void run() { while (true) { t1.setText(Integer.toString(count1++)); t2.setText(Integer.toString(count2++)); try { sleep(500); } catch (InterruptedException e){} } } public synchronized void synchTest() { Sharing2.incrementAccess(); if(count1 != count2) l.setText("Unsynched"); } } class Watcher2 extends Thread { private Sharing2 p; public Watcher2(Sharing2 p) { this.p = p; start(); } public void run() { while(true) { for(int i = 0; i < p.s.length; i++) p.s[i].synchTest(); try { sleep(500); } catch (InterruptedException e){} } } } public class Sharing2 extends Applet { TwoCounter2 s[]; private static int accessCount = 0; private static TextField aCount = new TextField("0", 10); public static void incrementAccess() { accessCount++; aCount.setText(Integer.toString(accessCount)); } private Button start = new Button("Start"), observer = new Button("Observe"); private boolean isApplet = true; private int numCounters = 0; private int numObservers = 0; public void init() { if(isApplet) { numCounters = 534 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 535. Integer.parseInt(getParameter("size")); numObservers = Integer.parseInt( getParameter("observers")); } s = new TwoCounter2[numCounters]; for(int i = 0; i < s.length; i++) s[i] = new TwoCounter2(this); Panel p = new Panel(); start.addActionListener(new StartL()); p.add(start); observer.addActionListener(new ObserverL()); p.add(observer); p.add(new Label("Access Count")); p.add(aCount); add(p); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { for(int i = 0; i < s.length; i++) s[i].start(); } } class ObserverL implements ActionListener { public void actionPerformed(ActionEvent e) { for(int i = 0; i < numObservers; i++) new Watcher2(Sharing2.this); } } public static void main(String args[]) { Sharing2 applet = new Sharing2(); // This isn't an applet, so set the flag and // produce the parameter values from args[]: applet.isApplet = false; applet.numCounters = (args.length == 0 ? 5 : Integer.parseInt(args[0])); applet.numObservers = (args.length < 2 ? 5 : Integer.parseInt(args[1])); Frame aFrame = new Frame("Sharing2"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e){ System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(350, applet.numCounters *100); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Chapter 14: Multiple Threads 535
- 536. You’ll notice that both run( ) and synchTest( ) are synchronized. If you synchronize only one of the methods, then the other is free to ignore the object lock and can be called with impunity. This is an important point: every method that accesses a critical shared resource must be synchronized, or it won’t work right. Now a new issue arises. The Watcher2 can never get a peek at what’s going on because the entire run( ) method has been synchronized, and since run( ) is always running for each object the lock is always tied up and synchTest( ) can never be called. You can see this because the accessCount never changes. What we’d like for this example is a way to isolate only part of the code inside run( ). The section of code you want to isolate this way is called a critical section and you use the synchronized keyword in a different way to set up a critical section. Java supports critical sections with the synchronized block; this time synchronized is used to specify the object whose lock is being used to synchronize the enclosed code: synchronized(syncObject) { // This code can be accessed by only // one thread at a time, assuming all // threads respect syncObject's lock } Before the synchronized block can be entered, the lock must be acquired on syncObject. If some other thread already has this lock, then the block cannot be entered until the lock is given up. The Sharing2 example can be modified by removing the synchronized keyword from the entire run( ) method and instead putting a synchronized block around the two critical lines. But what object should be used as the lock? The one that is already respected by synchTest( ), which is the current object (this)! So the modified run( ) looks like this: public void run() { while (true) { synchronized(this) { t1.setText(Integer.toString(count1++)); t2.setText(Integer.toString(count2++)); } try { sleep(500); } catch (InterruptedException e){} } } This is the only change that must be made to Sharing2.java, and you’ll see that while the two counters are never out of synch (according to when the Watcher is allowed to look at them) there is still adequate access provided to the Watcher during the execution of run( ). Of course, all synchronization depends on programmer diligence: every piece of code that may access a shared resource must be wrapped in an appropriate synchronized block. Synchronized efficiency Since having two methods write to the same piece of data never sounds like a particularly good idea, it might seem to make sense for all methods to be automatically synchronized and eliminate the synchronized keyword altogether (of course, the example with a synchronized run( ) shows that this wouldn’t work either). But it turns out that acquiring a lock is not a cheap operation – it multiplies the cost of a method call (that is, entering and exiting from the method, not executing the body of the method) by a 536 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 537. minimum of four times, and may be more depending on your implementation. Thus if you know that a particular method will not cause contention problems it is expedient to leave off the synchronized keyword. Java Beans revisited Now that you understand synchronization you can take another look at Java Beans. Whenever you create a Bean, you must assume that it will run in a multithreaded environment. This means that: 1. Whenever possible, all the public methods of a Bean should be synchronized. Of course this incurs the synchronized runtime overhead. If that’s a problem, methods that will not cause problems in critical sections may be left un- synchronized, but keep in mind this is not always obvious. Methods that qualify tend to be very small (such as getCircleSize( ) in the following example) and/or “atomic,” that is, the method call executes in such a short amount of code that the object cannot be changed during execution. Making such methods un-synchronized may not have a significant effect on the execution speed of your program. You might as well make all public methods of a Bean synchronized and remove the synchronized keyword only when you know for sure that it’s necessary and that it makes a difference. 2. When firing a multicast event to a bunch of listeners interested in that event, you must assume that listeners may be added or removed while moving through the list. The first point is fairly easy to deal with, but the second point requires a little more thought. Consider the BangBean.java example presented in the last chapter. That ducked out of the multithreading question by ignoring the synchronized keyword (which hadn’t been introduced yet) and making the event unicast. Here’s that example modified to work in a multithreaded environment and to use multicasting for events: //: BangBean2.java // You should write your Beans this way so they // can run in a multithreaded environment. import java.awt.*; import java.awt.event.*; import java.util.*; import java.io.*; public class BangBean2 extends Canvas implements Serializable { private int xm, ym; private int cSize = 20; // Circle size private String text = "Bang!"; private int fontSize = 48; private Color tColor = Color.red; private Vector actionListeners = new Vector(); public BangBean2() { addMouseListener(new ML()); addMouseMotionListener(new MM()); } public synchronized int getCircleSize() { return cSize; } public synchronized void setCircleSize(int newSize) { Chapter 14: Multiple Threads 537
- 538. cSize = newSize; } public synchronized String getBangText() { return text; } public synchronized void setBangText(String newText) { text = newText; } public synchronized int getFontSize() { return fontSize; } public synchronized void setFontSize(int newSize) { fontSize = newSize; } public synchronized Color getTextColor() { return tColor; } public synchronized void setTextColor(Color newColor) { tColor = newColor; } public void paint(Graphics g) { g.setColor(Color.black); g.drawOval(xm - cSize/2, ym - cSize/2, cSize, cSize); } // This is a multicast listener, which is // more typically used than the unicast // approach taken in BangBean.java: public synchronized void addActionListener ( ActionListener l) { actionListeners.addElement(l); } public synchronized void removeActionListener( ActionListener l) { actionListeners.removeElement(l); } // Notice this isn't synchronized: public void notifyListeners() { ActionEvent a = new ActionEvent(BangBean2.this, ActionEvent.ACTION_PERFORMED, null); Vector lv = null; // Make a copy of the vector in case someone // adds a listener while we're // calling listeners: synchronized(this) { lv = (Vector)actionListeners.clone(); } // Call all the listener methods: for(int i = 0; i < lv.size(); i++) { ActionListener al = (ActionListener)lv.elementAt(i); 538 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 539. al.actionPerformed(a); } } class ML extends MouseAdapter { public void mousePressed(MouseEvent e) { Graphics g = getGraphics(); g.setColor(tColor); g.setFont( new Font( "TimesRoman", Font.BOLD, fontSize)); int width = g.getFontMetrics().stringWidth(text); g.drawString(text, (getSize().width - width) /2, getSize().height/2); g.dispose(); notifyListeners(); } } class MM extends MouseMotionAdapter { public void mouseMoved(MouseEvent e) { xm = e.getX(); ym = e.getY(); repaint(); } } // Testing the BangBean2: public static void main(String args[]) { BangBean2 bb = new BangBean2(); bb.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e){ System.out.println("ActionEvent" + e); } }); bb.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e){ System.out.println("BangBean2 action"); } }); bb.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e){ System.out.println("More action"); } }); Frame aFrame = new Frame("BangBean2 Test"); aFrame.addWindowListener(new WindowAdapter(){ public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(bb, BorderLayout.CENTER); aFrame.setSize(300,300); aFrame.setVisible(true); } } ///:~ Chapter 14: Multiple Threads 539
- 540. Adding synchronized to the methods is an easy change. However, notice in addActionListener( ) and removeActionListener( ) that the ActionListeners are now added to and removed from a Vector, so you can have as many as you want. You can see that the method notifyListeners( ) is not synchronized. This means that it can be called from more than one thread at a time. It also means that it’s possible for addActionListener( ) or removeActionListener( ) to be called in the middle of a call to notifyListeners( ), which is a problem since it traverses the Vector actionListeners. To alleviate the problem, the Vector is cloned inside a synchronized clause, and the clone is traversed. This way the original Vector can be manipulated without impact on notifyListeners( ). The paint( ) method is also not synchronized. Deciding whether to synchronize overridden methods is not so clear as when you’re just adding your own methods. In this example it turns out that paint( ) seems to work OK whether it’s synchronized or not. But the issues you must consider are: 1. Does the method modify the state of “critical” variables within the object? To discover whether the variables are “critical” you must determine whether they will be read or set by other threads in the program (in this case, the reading or setting is virtually always accomplished via synchronized methods, so you can just examine those). In the case of paint( ), no modification takes place. 2. Does the method depend upon the state of these “critical” variables? If a synchronized method modifies a variable that your method uses, then you may very well want to make your method synchronized as well. Based on this, you may observe that cSize is changed by synchronized methods and therefore paint( ) should be synchronized. Here, however, you can ask “what’s the worst thing that will happen if cSize is changed during a paint( )?” When you see that it’s nothing too bad, and a transient effect at that, it’s best to leave paint( ) un-synchronized to prevent the extra overhead from the synchronized method call. 3. A third clue is to notice whether the base-class version of paint( ) is synchronized, which it isn’t. This isn’t an airtight argument, just a clue. In this case, for example, a field that is changed via synchronized methods (that is cSize) has been mixed into the paint( ) formula and may have changed the situation. Note, however, that synchronized doesn’t inherit – that is, if a method is synchronized in the base class then it is not automatically synchronized in the derived class overridden version. The test code in TestBangBean2 has been modified from that in the previous chapter to demonstrate the multicast ability of BangBean2 by adding extra listeners. Blocking A thread can be in any of four states: 1. New: the thread object has been created but it hasn’t been started yet so it cannot run. 2. Runnable: This means that a thread can be run, when the time-slicing mechanism has CPU cycles available for the thread. Thus the thread may or may not be running, but there’s nothing to prevent it from being run if the scheduler can arrange it: that is, it’s not dead or blocked. 3. Dead: the normal way for a thread to die is by returning from it’s run( ) method. You can also call stop( ), but this throws an exception that’s a 540 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 541. subclass of Error (which means you normally don’t catch it). Remember that throwing an exception should be a special event and not part of normal program execution; thus the use of stop( ) is discouraged (and it’s deprecated in Java 1.2). There’s also a destroy( ) method (which has never been implemented) that you should never call if you can avoid it, since it’s drastic and doesn’t release object locks. 4. Blocked: the thread could be run but there’s something that prevents it. While a thread is in the blocked state the scheduler will simply skip over it and not give it any CPU time. Until a thread re-enters the runnable state it won’t perform any operations. Becoming blocked The blocked state is the most interesting and is worth further examination. A thread can become blocked for five reasons: 1. You’ve put the thread to sleep by calling sleep(milliseconds) in which case it will not be run for the specified time. 2. You’ve suspended the execution of the thread with suspend( ). It will not become runnable again until the thread gets the resume( ) message. 3. You’ve suspended the execution of the thread with wait( ). It will not become runnable again until the thread gets the notify( ) or notifyAll( ) message (yes, this looks just like number 2, but there’s a distinct difference that will be revealed). 4. The thread is waiting for some IO to complete. 5. The thread is trying to call a synchronized method on another object and that object’s lock is not available. You can also call yield( ) (a method of the Thread class) to voluntarily give up the CPU so other threads may run. However, this is no different than if the scheduler decides that your thread has had enough time and jumps to another thread. That is, nothing prevents the scheduler from re-starting your thread. When a thread is blocked, there’s some reason that it cannot continue running. The following example shows all five ways of becoming blocked. It all exists in a single file called Blocking.java but it will be examined here in discrete pieces (you’ll note the “Continued” and “Continuing” tags that allow the tool shown in Chapter 17 to piece everything together). First, the basic framework: //: Blocking.java // Demonstrates the various ways a thread // can be blocked. import java.awt.*; import java.awt.event.*; import java.applet.*; import java.io.*; //////////// The basic framework /////////// class Blockable extends Thread { private Peeker peeker; protected TextField state = new TextField(40); protected int i; public Blockable(Container c) { Chapter 14: Multiple Threads 541
- 542. c.add(state); peeker = new Peeker(this, c); } public synchronized int read() { return i; } protected synchronized void update() { state.setText(getClass().getName() + " state: i = " + i); } public void stopPeeker() { // peeker.stop(); Deprecated in Java 1.2 peeker.terminate(); // The preferred approach } } class Peeker extends Thread { private Blockable b; private int session; private TextField status = new TextField(40); private boolean stop = false; public Peeker(Blockable b, Container c) { c.add(status); this.b = b; start(); } public void terminate() { stop = true; } public void run() { while (!stop) { status.setText(b.getClass().getName() + " Peeker " + (++session) + "; value = " + b.read()); try { sleep(100); } catch (InterruptedException e){} } } } ///:Continued The Blockable class is meant to be a base class for all the classes in this example that demonstrate blocking. A Blockable object contains a TextField called state which is used to display information about the object. The method that displays this information is update( ). You can see it uses getClass( ).getName( ) to produce the name of the class instead of just printing it out; this is because update( ) cannot know the exact name of the class it is called for, since it will be a class derived from Blockable. The indicator of change in Blockable is an int i, which will be incremented by the run( ) method of the derived class. There’s a thread of class Peeker that is started for each Blockable object, and the Peeker’s job is to watch its associated Blockable object to see changes in i by calling read( ) and reporting them in its status TextField. This is important: notice that read( ) and update( ) are both synchronized, which means they require that the object lock be free. Sleeping The first test in this program is with sleep( ) ///:Continuing 542 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 543. ///////////// Blocking via sleep() /////////// class Sleeper1 extends Blockable { public Sleeper1(Container c) { super(c); } public synchronized void run() { while(true) { i++; update(); try { sleep(1000); } catch (InterruptedException e){} } } } class Sleeper2 extends Blockable { public Sleeper2(Container c) { super(c); } public void run() { while(true) { change(); try { sleep(1000); } catch (InterruptedException e){} } } public synchronized void change() { i++; update(); } } ///:Continued In Sleeper1 the entire run( ) method is synchronized. You’ll see that the Peeker associated with this object will run along merrily until you start the thread, and then the Peeker stops cold. This is one form of blocking: since Sleeper1.run( ) is synchronized, and once the thread starts it’s always inside run( ), the method never gives up the object lock and the Peeker is blocked. Sleeper2 provides a solution by making run un-synchronized. Only the change( ) method is synchronized, which means that while run( ) is in sleep( ), the Peeker may access the synchronized method it needs, namely read( ). Here you’ll see that the Peeker continues running when you start the Sleeper2 thread. Suspending and resuming The next part of the example introduces the concept of suspension. The Thread class has a method suspend( ) to temporarily halt the thread and resume( ) that re-starts it at the point it was halted. Presumably, resume( ) is called by some thread outside the suspended one, and in this case there’s a separate class called Resumer which does just that. Each of the classes demonstrating suspend/resume has an associated resumer: ///:Continuing /////////// Blocking via suspend() /////////// class SuspendResume extends Blockable { public SuspendResume(Container c) { super(c); new Resumer(this); } } Chapter 14: Multiple Threads 543
- 544. class SuspendResume1 extends SuspendResume { public SuspendResume1(Container c) { super(c);} public synchronized void run() { while(true) { i++; update(); suspend(); // Deprecated in Java 1.2 } } } class SuspendResume2 extends SuspendResume { public SuspendResume2(Container c) { super(c);} public void run() { while(true) { change(); suspend(); // Deprecated in Java 1.2 } } public synchronized void change() { i++; update(); } } class Resumer extends Thread { private SuspendResume sr; public Resumer(SuspendResume sr) { this.sr = sr; start(); } public void run() { while(true) { try { sleep(1000); } catch (InterruptedException e){} sr.resume(); // Deprecated in Java 1.2 } } } ///:Continued SuspendResume1 also has a synchronized run( ) method. Again, when you start this thread you’ll see that its associated Peeker gets blocked waiting for the lock to become available, which never happens. This is fixed as before in SuspendResume2, which does not synchronize the entire run( ) method, but instead uses a separate synchronized change( ) method. You should be very aware that Java 1.2 deprecates the use of suspend( ) and resume( ), because suspend( ) holds the object’s lock and is thus deadlock-prone. That is, you can easily get a number of locked objects waiting on each other, and this will cause your program to freeze. Although you may see them used in older programs you should not use suspend( ) and resume( ). The proper solution is described later in this chapter. Wait and notify The point with the first two examples is that both sleep( ) and suspend( ) do not release the lock as they are called. You must be aware of this when working with locks. On the 544 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 545. other hand, the method wait( ) does release the lock when it is called, which means that other synchronized methods in the thread object may be called during a wait( ). In the following two classes, you’ll see that the run( ) method is fully synchronized in both cases, however the Peeker still has full access to the synchronized methods during a wait( ). This is because wait( ) releases the lock on the object as it suspends the method it’s called within. You’ll also see that there are two forms of wait( ). The first takes an argument in milliseconds that has the same meaning as in sleep( ): pause for this period of time. The difference is that in wait( ), the object lock is released and you can come out of the wait( ) because of a notify( ) as well as having the clock run out. The second form takes no arguments, and means that the wait( ) will continue until a notify( ) comes along, and will not automatically terminate after a time. One fairly unique aspect of wait( ) and notify( ) is that both methods are part of the base class Object and not part of Thread as are sleep( ), suspend( ) and resume( ). Although this seems a bit strange at first – to have something that’s exclusively for threading as part of the universal base class – it’s essential because they manipulate the lock that’s also part of every object. As a result, you can put a wait( ) inside any synchronized method, regardless of whether there’s any threading going on inside that particular class. In fact, the only place you can call wait( ) is within a synchronized method or block. If you call wait( ) or notify( ) within a method that’s not synchronized the program will compile, but when you run it you’ll get a IllegalMonitorStateException with the somewhat non- intuitive message “current thread not owner.” Note that sleep( ), suspend( ) and resume( ) can all be called within non-synchronized methods since they don’t manipulate the lock. You can call wait( ) or notify( ) for your own lock only. Again, you can compile code that tries to use the wrong lock but it will produce the same IllegalMonitorStateException message as before. You can’t fool with someone else’s lock, but you can ask another object to perform an operation that manipulates its own lock. So one approach is to create a synchronized method that calls notify( ) for its own object. However, in Notifier you’ll see the notify( ) call inside a synchronized block: synchronized(wn2) { wn2.notify(); } where wn2 is the object of type WaitNotify2. This means that this method, which is not part of WaitNotify2, acquires the lock on the wn2 object at which point it’s legal for it to call notify( ) for wn2 and you won’t get the IllegalMonitorStateException. ///:Continuing /////////// Blocking via wait() /////////// class WaitNotify1 extends Blockable { public WaitNotify1(Container c) { super(c); } public synchronized void run() { while(true) { i++; update(); try { wait(1000); } catch (InterruptedException e){} } } } Chapter 14: Multiple Threads 545
- 546. class WaitNotify2 extends Blockable { public WaitNotify2(Container c) { super(c); new Notifier(this); } public synchronized void run() { while(true) { i++; update(); try { wait(); } catch (InterruptedException e){} } } } class Notifier extends Thread { private WaitNotify2 wn2; public Notifier(WaitNotify2 wn2) { this.wn2 = wn2; start(); } public void run() { while(true) { try { sleep(2000); } catch (InterruptedException e){} synchronized(wn2) { wn2.notify(); } } } } ///:Continued wait( ) is typically used when you’ve gotten to the point where you’re waiting for some other condition, under the control of forces outside your thread, to change and you don’t want to just idly wait by inside the thread. So wait( ) allows you to put the thread to sleep while waiting for the world to change, and only when a notify( ) or notifyAll( ) occurs does the thread wake up and check for changes. Thus it provides a way to synchronize between threads. Blocking on IO If a stream is waiting for some IO activity, it will automatically block. In the following portion of the example the two classes work with generic Reader and Writer objects (using the new Java 1.1 Streams), but in the test framework a piped stream will be set up to allow the two threads to safely pass data to each other (which is the purpose of piped streams). The Sender puts data into the Writer and sleeps for a random amount of time. However, Receiver has no sleep( ) or suspend( ) or wait( ). But when it does a read( ) it automatically blocks when there is no more data. ///:Continuing class Sender extends Blockable { // send private Writer out; public Sender(Container c, Writer out) { 546 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 547. super(c); this.out = out; } public void run() { while(true) { for(char c = 'A'; c <= 'z'; c++) { try { i++; out.write(c); state.setText("Sender sent: " + (char)c); sleep((int)(3000 * Math.random())); } catch (InterruptedException e){} catch (IOException e) {} } } } } class Receiver extends Blockable { private Reader in; public Receiver(Container c, Reader in) { super(c); this.in = in; } public void run() { try { while(true) { i++; // Show peeker it's alive // Blocks until characters are there: state.setText("Receiver read: " + (char)in.read()); } } catch(IOException e) { e.printStackTrace();} } } ///:Continued Both classes also put information into their state fields and change i so the Peeker can see that the thread is running. Testing The main applet class is surprisingly simple because most of the work has been put into the Blockable framework. Basically, an array of Blockable objects is created, and since each one is a thread they perform their own activities when you press the “start” button. There’s also a button and actionPerformed( ) clause to stop all the Peeker objects, which provides a demonstration of the alternative to the deprecated (in Java 1.2) stop( ) method of Thread. To set up a connection between the Sender and Receiver objects, a PipedWriter and PipedReader are created. Notice that the PipedReader in must be connected to the PipedWriter out via a constructor argument. After that, anything that’s placed in out can later be extracted from in, as if it passed through a pipe (thus the name). The in and out objects are then passed to the Receiver and Sender constructors, respectively, which treat them as Reader and Writer objects of any type (that is, they are upcast). Chapter 14: Multiple Threads 547
- 548. The array of Blockable handles b[] is not initialized at its point of definition because the piped streams cannot be set up before that definition takes place (the need for the try block prevents this). ///:Continuing /////////// Testing Everything /////////// public class Blocking extends Applet { private Button start = new Button("Start"), stopPeekers = new Button("Stop Peekers"); private boolean started = false; private Blockable b[]; private PipedWriter out; private PipedReader in; public void init() { out = new PipedWriter(); try { in = new PipedReader(out); } catch(IOException e) {} b = new Blockable[] { new Sleeper1(this), new Sleeper2(this), new SuspendResume1(this), new SuspendResume2(this), new WaitNotify1(this), new WaitNotify2(this), new Sender(this, out), new Receiver(this, in) }; start.addActionListener(new StartL()); add(start); stopPeekers.addActionListener( new StopPeekersL()); add(stopPeekers); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(!started) { started = true; for(int i = 0; i < b.length; i++) b[i].start(); } } } class StopPeekersL implements ActionListener { public void actionPerformed(ActionEvent e) { // Demonstration of the preferred // alternative to Thread.stop(): for(int i = 0; i < b.length; i++) b[i].stopPeeker(); } } public static void main(String args[]) { Blocking applet = new Blocking(); Frame aFrame = new Frame("Blocking"); aFrame.addWindowListener( 548 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 549. new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(350,550); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ In init( ), note the loop that moves through the entire array and adds the state and peeker.status text fields to the page. When the Blockable threads are initially created, each one automatically creates and starts its own Peeker. Thus you’ll see the Peekers running before the Blockable threads are started. This is essential, as some of the Peekers will get blocked and stop when the Blockable threads start, and it’s essential to see this to understand that particular aspect of blocking. Deadlock Because threads can become blocked and because objects can have synchronized methods that prevent threads from accessing that object until the synchronization lock is released, it’s possible for one thread to get stuck waiting for another thread, which in turn waits for another thread, etc. until the chain leads back to a thread waiting on the first one. Thus there’s a continuous loop of threads waiting on each other and no one can move. This is called deadlock. The claim is that it doesn’t happen that often, but when it happens to you it’s very frustrating to debug. There is no language support to help prevent deadlock; it’s up to you to avoid it by careful design. These are not very comforting words to the person who’s trying to debug a deadlocking program. The deprecation of stop( ), suspend( ), resume( ) and destroy( ) in Java 1.2 One change that has been made in Java 1.2 to reduce the possibility of deadlock is the deprecation of Thread’s stop( ), suspend( ), resume( ) and destroy( ) methods. The reason the stop( ) method is deprecated is that it is unsafe. It releases all the locks that the thread had acquired, and if the objects are in an inconsistent state (“damaged”) other threads can view and modify them in that state. The resulting problems can be subtle and difficult to detect. Instead of using stop( ), you should follow the example in Blocking.java and use a flag to tell the thread when to terminate itself by exiting its run( ) method. There are times when a thread blocks, such as when it is waiting for input, and it cannot poll a flag as it does in Blocking.java. In these cases, you still shouldn’t use stop( ), but instead you can use the interrupt( ) method in Thread to break out of the blocked code: //: Interrupt.java // The alternative approach to using stop() // when a thread is blocked import java.awt.*; Chapter 14: Multiple Threads 549
- 550. import java.awt.event.*; import java.applet.*; class Blocked extends Thread { public synchronized void run() { try { wait(); // Blocks } catch(InterruptedException e) { System.out.println("InterruptedException"); } System.out.println("Exiting run()"); } } public class Interrupt extends Applet { private Button interrupt = new Button("Interrupt"); private Blocked blocked = new Blocked(); public void init() { add(interrupt); interrupt.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { System.out.println("Button pressed"); if(blocked == null) return; Thread remove = blocked; blocked = null; // to release it remove.interrupt(); } }); blocked.start(); } public static void main(String args[]) { Interrupt applet = new Interrupt(); Frame aFrame = new Frame("Interrupt"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(200,100); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ The wait( ) inside Blocked.run( ) produces the blocked thread. When you press the button, the blocked handle is set to null so the garbage collector will clean it up, and then the object’s interrupt( ) method is called. The first time you press the button you’ll see that the thread quits, but after that there’s no thread to kill so you just see that the button has been pressed. 550 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 551. The suspend( ) and resume( ) methods turn out to be inherently deadlock-prone. When you call suspend( ), the target thread stops but it still holds any locks it has acquired up to that point. This means that no other thread can access the locked resources until the thread is resumed. Any thread that wants to resume the target thread and also tries to use any of the locked resources produces deadlock. You should not use suspend( ) and resume( ), but instead put a flag in your Thread class to indicate whether the thread should be active or suspended. If the flag indicates that the thread is suspended, the thread goes into a wait using wait( ), and when the flag indicates the thread should be resumed the thread is restarted with notify( ). An example can be produced by modifying Counter2.java. Although the effect is similar, you’ll notice that the code organization is quite different – anonymous inner classes are used for all the listeners and the Thread is an inner class, which makes programming slightly more convenient since it eliminates some of the extra bookkeeping necessary in Counter2.java: //: Suspend.java // The alternative approach to using suspend() // and resume(), which have been deprecated // in Java 1.2. import java.awt.*; import java.awt.event.*; import java.applet.*; public class Suspend extends Applet { private TextField t = new TextField(10); private Button suspend = new Button("Suspend"), resume = new Button("Resume"); class Suspendable extends Thread { private int count = 0; private boolean suspended = false; public Suspendable() { start(); } public void fauxSuspend() { suspended = true; } public synchronized void fauxResume() { suspended = false; notify(); } public void run() { while (true) { try { sleep(100); synchronized(this) { while(suspended) wait(); } } catch (InterruptedException e){} t.setText(Integer.toString(count++)); } } } private Suspendable ss = new Suspendable(); public void init() { add(t); suspend.addActionListener( new ActionListener() { Chapter 14: Multiple Threads 551
- 552. public void actionPerformed(ActionEvent e) { ss.fauxSuspend(); } }); add(suspend); resume.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { ss.fauxResume(); } }); add(resume); } public static void main(String args[]) { Suspend applet = new Suspend(); Frame aFrame = new Frame("Suspend"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e){ System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300,100); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ The flag suspended inside Suspendable is used to turn suspension on and off. To suspend, the flag is set to true by calling fauxSuspend( ) and this is detected inside run( ). The wait( ), as described earlier in this chapter, must be synchronized so it has the object lock. In fauxResume( ), the suspended flag is set to false and notify( ) is called – since this wakes up wait( ) inside a synchronized clause the fauxResume( ) method must also be synchronized so that it acquires the lock before calling notify( ) (thus the lock is available for the wait( ) to wake up with). If you follow the style shown in this program you can avoid using wait( ) and notify( ). The destroy( ) method of Thread has never been implemented; it’s like a suspend( ) that cannot resume and thus it has the same deadlock issues as suspend( ). However, this is not a deprecated method and it may be implemented in a future version of Java (after 1.2) for special situations where the risk of a deadlock is acceptable. You may wonder why these methods, now deprecated, were included in Java in the first place. It seems a clear admission of a rather significant mistake to simply remove them outright (and pokes yet another hole in the arguments for Java’s exceptional design and infallibility touted by Sun marketing people). The heartening thing about the change is that it clearly indicates that the technical people and not the marketing people are running the show – they discovered a problem and they are fixing it. I find this much more promising and hopeful than leaving the problem in because fixing it would admit an error. It means that Java will continue to improve, even if it means a little discomfort on the part of Java programmers. I’d rather deal with the discomfort than watch the language stagnate. 552 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 553. Priorities The priority of a thread tells the scheduler how important this thread is. If there are a number of threads blocked and waiting to be run, the scheduler will run the one with the highest priority first. However, this doesn’t mean that threads with lower priority don’t get run (that is, you can’t get deadlocked because of priorities). Lower priority threads just tend to run less often. You can read the priority of a thread with getPriority( ) and change it with setPriority( ). The form of the prior “counter” examples can be used to show the effect of changing the priorities. In this applet you’ll see that the counters slow down as the associated threads have their priorities lowered: //: Counter5.java // Adjusting the priorities of threads import java.awt.*; import java.awt.event.*; import java.applet.*; class Ticker2 extends Thread { private Button b = new Button("Toggle"), incPriority = new Button("up"), decPriority = new Button("down"); private TextField t = new TextField(10), pr = new TextField(3); // Display priority private int count = 0; private boolean runFlag = true; public Ticker2(Container c) { b.addActionListener(new ToggleL()); incPriority.addActionListener(new UpL()); decPriority.addActionListener(new DownL()); Panel p = new Panel(); p.add(t); p.add(pr); p.add(b); p.add(incPriority); p.add(decPriority); c.add(p); } class ToggleL implements ActionListener { public void actionPerformed(ActionEvent e) { runFlag = !runFlag; } } class UpL implements ActionListener { public void actionPerformed(ActionEvent e) { int newPriority = getPriority() + 1; if(newPriority > Thread.MAX_PRIORITY) newPriority = Thread.MAX_PRIORITY; setPriority(newPriority); } } class DownL implements ActionListener { Chapter 14: Multiple Threads 553
- 554. public void actionPerformed(ActionEvent e) { int newPriority = getPriority() - 1; if(newPriority < Thread.MIN_PRIORITY) newPriority = Thread.MIN_PRIORITY; setPriority(newPriority); } } public void run() { while (true) { if(runFlag) { t.setText(Integer.toString(count++)); pr.setText( Integer.toString(getPriority())); } yield(); } } } public class Counter5 extends Applet { private Button start = new Button("Start"), upMax = new Button("Inc Max Priority"), downMax = new Button("Dec Max Priority"); private boolean started = false; private static final int SIZE = 10; private Ticker2 s[] = new Ticker2[SIZE]; private TextField mp = new TextField(3); public void init() { for(int i = 0; i < s.length; i++) s[i] = new Ticker2(this); add(new Label("MAX_PRIORITY = " + Thread.MAX_PRIORITY)); add(new Label("MIN_PRIORITY = " + Thread.MIN_PRIORITY)); add(new Label("Thread Group Max Priority = ")); add(mp); add(start); add(upMax); add(downMax); start.addActionListener(new StartL()); upMax.addActionListener(new UpMaxL()); downMax.addActionListener(new DownMaxL()); showMaxPriority(); // Recursively display parent thread groups: ThreadGroup parent = s[0].getThreadGroup().getParent(); while(parent != null) { add(new Label( "Parent threadgroup max priority = " + parent.getMaxPriority())); parent = parent.getParent(); } } public void showMaxPriority() { mp.setText(Integer.toString( 554 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 555. s[0].getThreadGroup().getMaxPriority())); } class StartL implements ActionListener { public void actionPerformed(ActionEvent e) { if(!started) { started = true; for(int i = 0; i < s.length; i++) s[i].start(); } } } class UpMaxL implements ActionListener { public void actionPerformed(ActionEvent e) { int maxp = s[0].getThreadGroup().getMaxPriority(); if(++maxp > Thread.MAX_PRIORITY) maxp = Thread.MAX_PRIORITY; s[0].getThreadGroup().setMaxPriority(maxp); showMaxPriority(); } } class DownMaxL implements ActionListener { public void actionPerformed(ActionEvent e) { int maxp = s[0].getThreadGroup().getMaxPriority(); if(--maxp < Thread.MIN_PRIORITY) maxp = Thread.MIN_PRIORITY; s[0].getThreadGroup().setMaxPriority(maxp); showMaxPriority(); } } public static void main(String args[]) { Counter5 applet = new Counter5(); Frame aFrame = new Frame("Counter5"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(300, 600); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Ticker2 follows the form established earlier in this chapter, but there’s an extra TextField for displaying the priority of the thread and two more buttons, for incrementing and decrementing the priority. Also notice the use of yield( ), which voluntarily hands control back to the scheduler. Without this the multithreading mechanism still works, but you’ll notice it runs very slowly (try removing the call to yield( )!). You could also call sleep( ), but then the rate of counting would be controlled by the sleep( ) duration instead of the priority. Chapter 14: Multiple Threads 555
- 556. The init( ) in Counter5 creates an array of 10 Ticker2s; their buttons and fields are placed on the form by the Ticker2 constructor. Counter5 adds buttons to start everything up as well as increment and decrement the maximum priority of the threadgroup. In addition, there are labels that display the maximum and minimum priorities possible for a thread, and a TextField to show the thread group’s maximum priority (the next section will fully describe thread groups). Finally, the priorities of the parent thread groups are also displayed as labels. When you press an “up” or “down” button, that Ticker2’s priority is fetched and incremented or decremented, accordingly. When you run this program, you’ll notice several things. First of all, the thread group’s default priority is 5. Even if you decrement the maximum priority below 5 before starting the threads (or before creating the threads, which requires a code change) each thread will have a default priority of 5. The simple test is to take one counter and decrement its priority to one, and observe that it counts much slower. But now try to increment it again. You can get it back up to the thread group’s priority, but no higher. Now decrement the thread group’s priority a couple of times. The thread priorities are unchanged, but if you try to modify them either up or down you’ll see that they’ll automatically pop to the priority of the thread group. Also, new threads will still be given a default priority, even if that’s higher than the group priority (thus the group priority is not a way to prevent new threads from having higher priorities than existing ones). Finally, try to increment the group maximum priority. It can’t be done. You can only reduce thread group maximum priorities, not increase them. Thread groups All threads belong to a thread group. This may be either the default thread group or a group you explicitly specify when you create the thread. At creation, the thread is bound to a group and cannot change to a different group. Each application has at least one thread that belongs to the system thread group. If you create more threads without specifying a group, they will also belong to the system thread group. Thread groups must also belong to other thread groups. The thread group that a new one belongs to must be specified in the constructor. If you create a thread group without specifying a thread group for it to belong to, it will be placed under the system thread group. Thus, all thread groups in your application will ultimately have the system thread group as the parent. The reason for the existence of thread groups is hard to determine from the literature, which tends to be confusing on this subject. It’s often cited as “security reasons.” According to Arnold & Gosling1 “Threads within a thread group can modify the other threads in the group, including any farther down the hierarchy. A thread cannot modify threads outside of its own group or contained groups.” It’s hard to know what “modify” is supposed to mean here. The following example shows a thread in a “leaf” subgroup modifying the priorities of all the threads in its tree of thread groups, as well as calling a method for all the threads in its tree. //: TestAccess.java 1 The Java Programming Language, by Ken Arnold and James Gosling, Addison-Wesley 1996 pp 179. 556 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 557. // How threads can access other threads // in a parent thread group public class TestAccess { public static void main(String args[]) { ThreadGroup x = new ThreadGroup("x"), y = new ThreadGroup(x, "y"), z = new ThreadGroup(y, "z"); Thread one = new TestThread1(x, "one"), two = new TestThread2(z, "two"); } } class TestThread1 extends Thread { private int i; TestThread1(ThreadGroup g, String name) { super(g, name); } void f() { i++; // modify this thread System.out.println(getName() + " f()"); } } class TestThread2 extends TestThread1 { TestThread2(ThreadGroup g, String name) { super(g, name); start(); } public void run() { ThreadGroup g = getThreadGroup().getParent().getParent(); g.list(); Thread gAll[] = new Thread[g.activeCount()]; g.enumerate(gAll); for(int i = 0; i < gAll.length; i++) { gAll[i].setPriority(Thread.MIN_PRIORITY); ((TestThread1)gAll[i]).f(); } g.list(); } } ///:~ In main( ), several ThreadGroups are created, leafing off from each other: x has no argument but its name (a String) and thus it is automatically placed in the “system” thread group, while y is under x and z is under y. Notice that initialization happens in textual order so this code is legal. Two threads are created and placed in different thread groups. TestThread1 doesn’t have a run( ) method, but it does have an f( ) that modifies the thread and prints something so you can see it was indeed called. TestThread2 is a subclass of TestThread1 and it’s run( ) is fairly elaborate: it first gets the thread group of the current thread, and then moves up the heritage tree by two levels using getParent( ) (this is contrived since I purposely place the TestThread2 object two levels down in the hierarchy). At this point, an array of handles to Threads is created, using the method activeCount( ) to ask how many threads Chapter 14: Multiple Threads 557
- 558. are in this thread group and all the child thread groups. The enumerate( ) method places handles to all these threads in the array gAll, and then I simply move through the entire array calling the f( ) method for each thread, as well as modifying the priority. Thus, a thread in a “leaf” thread group modifies threads in parent thread groups. The debugging method list( ) prints all the information about a thread group to standard output, and is very helpful when investigating thread group behavior. Here’s the output of the program: java.lang.ThreadGroup[name=x,maxpri=10] Thread[one,5,x] java.lang.ThreadGroup[name=y,maxpri=10] java.lang.ThreadGroup[name=z,maxpri=10] Thread[two,5,z] one f() two f() java.lang.ThreadGroup[name=x,maxpri=10] Thread[one,1,x] java.lang.ThreadGroup[name=y,maxpri=10] java.lang.ThreadGroup[name=z,maxpri=10] Thread[two,1,z] Not only does list( ) print the class name of ThreadGroup or Thread, but it also prints the thread group name and its maximum priority. For threads, the thread name is printed, followed by the thread priority and the group it belongs to. Note that list( ) indents the threads and thread groups to indicate that they are children of the un-indented thread group. You can see that f( ) is called by the TestThread2 run( ) method, so it’s obvious that all threads in a group are vulnerable. However, you can access only the threads that branch off from your own system thread group tree, and perhaps this is what is meant by “safety.” You cannot access anyone else’s system thread group tree. Controlling thread groups Putting aside the safety issue, one thing thread groups do seem to be useful for is control: you can perform certain operations on an entire thread group with a single command. The following example demonstrates this and the restrictions on priorities within thread groups. The commented numbers in parentheses provide a reference to compare to the output. //: ThreadGroup1.java // How thread groups control priorities // of the threads inside them. public class ThreadGroup1 { public static void main(String[] args) { // Get the system thread & print its Info: ThreadGroup sys = Thread.currentThread().getThreadGroup(); sys.list(); // (1) // Reduce the system thread group priority: sys.setMaxPriority(Thread.MAX_PRIORITY - 1); // Increase the main thread priority: Thread curr = Thread.currentThread(); curr.setPriority(curr.getPriority() + 1); sys.list(); // (2) 558 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 559. // Attempt to set a new group to the max: ThreadGroup g1 = new ThreadGroup("g1"); g1.setMaxPriority(Thread.MAX_PRIORITY); // Attempt to set a new thread to the max: Thread t = new Thread(g1, "A"); t.setPriority(Thread.MAX_PRIORITY); g1.list(); // (3) // Reduce g1's max priority, then attempt // to increase it: g1.setMaxPriority(Thread.MAX_PRIORITY - 2); g1.setMaxPriority(Thread.MAX_PRIORITY); g1.list(); // (4) // Attempt to set a new thread to the max: t = new Thread(g1, "B"); t.setPriority(Thread.MAX_PRIORITY); g1.list(); // (5) // Lower the max priority below the default // thread priority: g1.setMaxPriority(Thread.MIN_PRIORITY + 2); // Look at a new thread's priority before // and after changing it: t = new Thread(g1, "C"); g1.list(); // (6) t.setPriority(t.getPriority() -1); g1.list(); // (7) // Make g2 a child Threadgroup of g1 and // try to increase its priority: ThreadGroup g2 = new ThreadGroup(g1, "g2"); g2.list(); // (8) g2.setMaxPriority(Thread.MAX_PRIORITY); g2.list(); // (9) // Add a bunch of new threads to g2: for (int i = 0; i < 5; i++) new Thread(g2, Integer.toString(i)); // Show information about all threadgroups // and threads: sys.list(); // (10) System.out.println("Starting all threads:"); Thread[] all = new Thread[sys.activeCount()]; sys.enumerate(all); for(int i = 0; i < all.length; i++) if(!all[i].isAlive()) all[i].start(); // Suspends & Stops all threads in // this group and its subgroups: System.out.println("All threads started"); sys.suspend(); // Deprecated in Java 1.2 // Never gets here... System.out.println("All threads suspended"); sys.stop(); // Deprecated in Java 1.2 System.out.println("All threads stopped"); } } ///:~ Chapter 14: Multiple Threads 559
- 560. The output that follows has been edited to allow it to fit on the page (the java.lang. has been removed) and to add numbers to correspond to the commented numbers in the listing above. (1) ThreadGroup[name=system,maxpri=10] Thread[main,5,system] (2) ThreadGroup[name=system,maxpri=9] Thread[main,6,system] (3) ThreadGroup[name=g1,maxpri=9] Thread[A,9,g1] (4) ThreadGroup[name=g1,maxpri=8] Thread[A,9,g1] (5) ThreadGroup[name=g1,maxpri=8] Thread[A,9,g1] Thread[B,8,g1] (6) ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,6,g1] (7) ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,3,g1] (8) ThreadGroup[name=g2,maxpri=3] (9) ThreadGroup[name=g2,maxpri=3] (10)ThreadGroup[name=system,maxpri=9] Thread[main,6,system] ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,3,g1] ThreadGroup[name=g2,maxpri=3] Thread[0,6,g2] Thread[1,6,g2] Thread[2,6,g2] Thread[3,6,g2] Thread[4,6,g2] Starting all threads: All threads started All programs have at least one thread running, and the first action in main( ) is to call the static method of Thread called currentThread( ). From this thread, the thread group is produced, and list( ) is called for the result. The output is: (1) ThreadGroup[name=system,maxpri=10] Thread[main,5,system] You can see that the name of the main thread group is system, and the name of the main thread is main, and it belongs to the system thread group. The second exercise shows that the system group’s maximum priority can be reduced and the main thread can have it’s priority increased: (2) ThreadGroup[name=system,maxpri=9] Thread[main,6,system] 560 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 561. The third exercise creates a new thread group g1, which automatically belongs to the system thread group since it isn’t otherwise specified. A new thread A is placed in g1. After attempting to set this group’s maximum priority to the highest level and A’s priority to the highest level, the result is: (3) ThreadGroup[name=g1,maxpri=9] Thread[A,9,g1] Thus it’s not possible to change the thread group’s maximum priority to be higher than it’s parent thread group. The fourth exercise reduces g1’s maximum priority by two and then tries to increase it up to Thread.MAX_PRIORITY. The result is: (4) ThreadGroup[name=g1,maxpri=8] Thread[A,9,g1] You can see that the increase in maximum priority didn’t work. You can only decrease a thread group’s maximum priority, not increase it. Also, note that thread A’s priority didn’t change, and now it is higher than the thread group’s maximum priority. Changing a thread group’s maximum priority doesn’t affect existing threads. The fifth exercise attempts to set a new thread to maximum priority: (5) ThreadGroup[name=g1,maxpri=8] Thread[A,9,g1] Thread[B,8,g1] The new thread cannot be changed to anything higher than the maximum thread group priority. The default thread priority for this program is 6; that’s the priority a new thread will be created at and where it will stay if you don’t manipulate the priority. Exercise six lowers the maximum thread group priority below the default thread priority to see what happens when you create a new thread under this condition: (6) ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,6,g1] Even though the maximum priority of the thread group is 3, the new thread is still created using the default priority of 6. Thus, maximum thread group priority does not affect default priority (in fact, there appears to be no way to set the default priority for new threads). After changing the priority, attempting to decrement it by one, the result is: (7) ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,3,g1] Only when you attempt to change the priority is the thread group’s maximum priority enforced. A similar experiment is performed in (8) and (9), when a new thread group g2 is created as a child of g1 and it’s maximum priority is changed. You can see that it’s impossible for g2’s maximum to go higher than g1’s: Chapter 14: Multiple Threads 561
- 562. (8) ThreadGroup[name=g2,maxpri=3] (9) ThreadGroup[name=g2,maxpri=3] Also note that g2 is automatically set to the thread group maximum priority of g1 as g2 is created. After all these experiments, the entire system of thread groups and threads is printed out: (10)ThreadGroup[name=system,maxpri=9] Thread[main,6,system] ThreadGroup[name=g1,maxpri=3] Thread[A,9,g1] Thread[B,8,g1] Thread[C,3,g1] ThreadGroup[name=g2,maxpri=3] Thread[0,6,g2] Thread[1,6,g2] Thread[2,6,g2] Thread[3,6,g2] Thread[4,6,g2] So because of the rules of thread groups, a child group must always have a maximum priority that’s less than or equal to its parent’s maximum priority. The last part of this program demonstrates methods for an entire group of threads. First the program moves through the entire tree of threads and starts each one that hasn’t been started. For drama, the system group is then suspended and finally stopped (although it’s interesting to see that suspend( ) and stop( ) work on entire thread groups, you should keep in mind that these methods are deprecated in Java 1.2). But when you suspend the system group you also suspend the main thread and the whole program shuts down, so it never gets to the point where the threads are stopped. Actually, if you do stop the main thread it throws a ThreadDeath exception, so this is not a very typical thing to do. Since ThreadGroup is inherited from Object which contains the wait( ) method, you can also choose to suspend the program for any number of seconds by calling wait(seconds * 1000). This must acquire the lock inside a synchronized block, of course. The ThreadGroup class also has suspend( ) and resume( ) methods so you can stop and start an entire thread group and all its threads and subgroups with a single command. (Again, suspend( ) and resume( ) are deprecated in Java 1.2) Thread groups can seem a bit mysterious at first, but keep in mind you probably won’t be using them directly very often. Runnable revisited Earlier in this chapter, I suggested that you think carefully before making an applet or main Frame as an implementation of Runnable, because then you can make only one of those threads in your program. This limits your flexibility if you decide you want to have more than one thread of that type. Of course, if you must inherit from a class and you want to add threading behavior to the class, Runnable is the correct solution. The final example in this chapter exploits this by making a Runnable Canvas class that paints different colors on itself. This application is set up to take values from the command line to determine how big the grid of colors is and how long to sleep( ) between color changes, and by playing with these values you’ll discover some interesting and possibly inexplicable features of threads: 562 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 563. //: ColorBoxes.java // Using the Runnable interface import java.awt.*; import java.awt.event.*; class CBox extends Canvas implements Runnable { private Thread t; private int pause; private static final Color colors[] = { Color.black, Color.blue, Color.cyan, Color.darkGray, Color.gray, Color.green, Color.lightGray, Color.magenta, Color.orange, Color.pink, Color.red, Color.white, Color.yellow }; private Color cColor = newColor(); private static final Color newColor() { return colors[ (int)(Math.random() * colors.length) ]; } public void paint(Graphics g) { g.setColor(cColor); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); } public CBox(int pause) { this.pause = pause; t = new Thread(this); t.start(); } public void run() { while(true) { cColor = newColor(); repaint(); try { t.sleep(pause); } catch(InterruptedException e) {} } } } public class ColorBoxes extends Frame { public ColorBoxes(int pause, int grid) { setTitle("ColorBoxes"); setLayout(new GridLayout(grid, grid)); for (int i = 0; i < grid * grid; i++) add(new CBox(pause)); addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); } public static void main(String[] args) { Chapter 14: Multiple Threads 563
- 564. int pause = 50; int grid = 8; if(args.length > 0) pause = Integer.parseInt(args[0]); if(args.length > 1) grid = Integer.parseInt(args[1]); Frame f = new ColorBoxes(pause, grid); f.setSize(500, 400); f.setVisible(true); } } ///:~ ColorBoxes is a typical application with a constructor that sets up the GUI. This constructor takes an argument of int grid to set up the GridLayout so it has grid cells in each dimension. Then it adds the appropriate number of CBox objects to fill the grid, passing the pause value to each one. In main( ) you can see how pause and grid have default values that can be changed if you pass in command-line arguments. CBox is where all the work takes place. This is inherited from Canvas and it implements the Runnable interface so each Canvas can also be a Thread. Remember that when you implement Runnable, you don’t make a Thread object, just a class that has a run( ) method. Thus you must explicitly create a Thread object and hand the Runnable object to the constructor, then call start( ) (this happens in the constructor). In CBox this thread is called t. Note the array colors, which is an enumeration of all the colors in class Color. This is used in newColor( ) to produce a randomly-selected color. The current cell color is cColor. paint( ) is quite simple – it just sets the color to cColor and fills the whole canvas with that color. In run( ), you see the infinite loop that sets the cColor to a new random color and then calls repaint( ) to show it. Then the thread goes to sleep( ) for the amount of time specified on the command line. Precisely because this design is flexible and threading is tied to each Canvas element, you can experiment by making as many as you want (in reality, there is a restriction imposed by the number of threads your JVM can comfortably handle). This program also makes an interesting benchmark, since it can show dramatic speed differences between one JVM implementation and another. Too many threads At some point, you’ll find that ColorBoxes bogs down. On my machine, this occurred somewhere after a 10 by 10 grid. Why does this happen? You’re naturally suspicious that the AWT might have something to do with it, so here’s an example that tests the premise by making fewer threads. The code is reorganized so a Vector implements Runnable and that Vector holds a number of color blocks, and randomly chooses ones to update. Then a number of these Vector objects are created, roughly depending on the grid dimension you choose. As a result, you have far fewer threads than color blocks, so if there’s a speedup we’ll know it was because there were too many threads in the previous example: //: ColorBoxes2.java // Balancing thread use import java.awt.*; import java.awt.event.*; 564 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 565. import java.util.*; class CBox2 extends Canvas { private static final Color colors[] = { Color.black, Color.blue, Color.cyan, Color.darkGray, Color.gray, Color.green, Color.lightGray, Color.magenta, Color.orange, Color.pink, Color.red, Color.white, Color.yellow }; private Color cColor = newColor(); private static final Color newColor() { return colors[ (int)(Math.random() * colors.length) ]; } void nextColor() { cColor = newColor(); repaint(); } public void paint(Graphics g) { g.setColor(cColor); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); } } class CBoxVector extends Vector implements Runnable { private Thread t; private int pause; public CBoxVector(int pause) { this.pause = pause; t = new Thread(this); } public void go() { t.start(); } public void run() { while(true) { int i = (int)(Math.random() * size()); ((CBox2)elementAt(i)).nextColor(); try { t.sleep(pause); } catch(InterruptedException e) {} } } } public class ColorBoxes2 extends Frame { private CBoxVector v[]; public ColorBoxes2(int pause, int grid) { setTitle("ColorBoxes2"); setLayout(new GridLayout(grid, grid)); v = new CBoxVector[grid]; for(int i = 0; i < grid; i++) v[i] = new CBoxVector(pause); for (int i = 0; i < grid * grid; i++) { Chapter 14: Multiple Threads 565
- 566. v[i % grid].addElement(new CBox2()); add((CBox2)v[i % grid].lastElement()); } for(int i = 0; i < grid; i++) v[i].go(); addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); } public static void main(String[] args) { // Shorter default pause than ColorBoxes: int pause = 5; int grid = 8; if(args.length > 0) pause = Integer.parseInt(args[0]); if(args.length > 1) grid = Integer.parseInt(args[1]); Frame f = new ColorBoxes2(pause, grid); f.setSize(500, 400); f.setVisible(true); } } ///:~ In ColorBoxes2 an array of CBoxVector is created and initialized to hold grid CBoxVectors, each of which knows how long to sleep. An equal number of Cbox2 objects are then added to each CBoxVector, and each vector is told to go( ), which starts its thread. CBox2 is very similar to CBox: it paints itself with a randomly-chosen color. But that’s all a CBox2 does. All of the threading has been moved into CBoxVector. The CBoxVector could also have inherited Thread and had a member object of type Vector. That design has the advantage that the addElement( ) and elementAt( ) methods could then be given specific argument and return value types rather than just generic Objects (their names could also be changed to something shorter). However, the design used here seemed at first glance to require less code. In addition it automatically retains all the other behaviors of a Vector. With all the casting and parentheses necessary for elementAt( ), this might not be the case as your body of code grows. As before, when you implement Runnable you don’t get all the equipment that comes with Thread, so you actually have to create a new Thread and hand yourself to it’s constructor in order to have something to start( ), as you can see in the CBoxVector constructor and in go( ). The run( ) method simply chooses a random element number within the vector and calls nextColor( ) for that element to cause it to choose a new randomly-selected color. Upon running this program, you see that it does indeed run faster and respond more quickly (for instance, when you interrupt it, it stops more quickly), and it doesn’t seem to bog down as much at higher grid sizes. Thus a new factor is added into the threading equation: you must watch to see that you don’t have “too many threads” (whatever that turns out to mean for your particular program and platform). If so, you must try to use techniques like the one above to “balance” the number of threads in your program. Thus, if you see performance problems in a multithreaded program you now have a number of issues to examine: 566 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 567. 1. Do you have enough calls to sleep( ), yield( ) and/or wait( )? 2. Are calls to sleep( ) long enough? 3. Are you running too many threads? Issues like this are one reason multithreaded programming is often considered an art. Summary It is vital to learn when to use multithreading and when to avoid it. The main reason to use it is to manage a number of tasks whose intermingling will make more efficient use of the computer or be more convenient for the user. The classic example of resource balancing is using the CPU during I/O waits. The classic example of user convenience is monitoring a “stop” button during long downloads. The main drawbacks to multithreading are: 1. Slowdown while waiting for shared resources 2. Additional CPU overhead required to manage threads 3. Unrewarded complexity, like the silly idea of having a separate thread to update each element of an array 4. Pathologies including starving, racing, and deadlock An additional advantage to threads is that they substitute “light” execution context switches (of the order of 100 instructions) for “heavy” process context switches (of the order of 1000s of instructions). Since all threads in a given process share the same memory space, a light context switch changes only program execution and local variables. On the other hand, a process change, the heavy context switch, must exchange the full memory space. Threading is like stepping into an entirely new world and learning a whole new programming language, or at least a new set of language concepts. With the appearance of thread support in most microcomputer operating systems, extensions for threads have also been appearing in programming languages or libraries. In all cases thread programming (1) seems mysterious and requires a shift in the way you think about programming and (2) looks very similar to thread support in other languages, so when you understand threads, you understand a common tongue. And although support for threads can make Java seem like a more complicated language, don’t blame Java. Threads are tricky. One of the biggest difficulties with threads occurs because more than one thread may be sharing a resource, like the memory in an object, and you must make sure that multiple threads don’t try to read & change that resource at the same time. This requires judicious use of the synchronized keyword, which is a very helpful tool but must be understood thoroughly as it can quietly introduce deadlock situations. In addition, there’s a certain art to the application of threads. Java is designed to allow you to create as many objects as you need to solve your problem (at least in theory. Creating millions of objects for an engineering finite-element analysis, for example, may not be practical in Java). However, it seems that there is an upper bound to the number of threads you’ll want to create because at some point a large number of threads seems to become unwieldy. This critical point is not in the many thousands as it might be with objects, but rather in the neighborhood of less than 100. As you may often create only a Chapter 14: Multiple Threads 567
- 568. handful of threads to solve a problem this is typically not much of a limit, yet in a more general design it becomes a constraint. A significant non-intuitive issue in threading is that, because of thread scheduling, you can typically make your applications run faster by inserting calls to sleep( ) inside run( )’s main loop. This definitely makes it feel like an art, in particular when the longer delays seem to speed up performance. Of course, the reason this happens is that shorter delays can cause the end-of-sleep( ) scheduler interrupt to happen before the running thread is ready to go to sleep, thus forcing the scheduler to stop it and later restart it so it can finish what it was doing and then go to sleep. It takes extra thought to realize how messy things can get. One thing you may notice missing in this chapter is an animation example, which is one of the most popular things to do with applets. However, a complete solution (with sound) to this problem comes with the Java JDK (available at java.sun.com) in the demo section. In addition, we can expect better animation support to become part of future versions of Java, while completely different non-Java, non-programming solutions to animation for the Web are appearing that will probably be superior to traditional approaches. For explanations about how Java animation works, see Core Java (by Cornell & Horstmann, Prentice-Hall 1997). Exercises 1. Inherit a class from Thread and override the run( ) method. Inside run( ), print a message, then call sleep( ). Repeat this 3 times, then return from run( ). Put a start-up message in the constructor and override finalize( ) to print a shut-down message. Make a separate thread class that calls System.gc( ) and System.runFinalization( ) inside run( ), printing a message as it does so. Make several thread objects of both types and run them to see what happens. 2. Modify Counter2.java so that the thread is an inner class, and doesn’t need to explicitly store a handle to a Counter2. 3. Modify Sharing2.java to add a synchronized block inside the run( ) method of TwoCounter, instead of synchronizing the entire run( ) method. 4. Create two Thread subclasses, one with a run( ) that starts up, captures the handle of the second Thread object and then calls wait( ). The other class’ run( ) should call notifyAll( ) for the first thread after some number of seconds have passed, so the first thread can print out a message. 5. In Counter5.java inside Ticker2, remove the yield( ) and explain the results. Replace the yield( ) with a sleep( ) and explain the results. 6. In ThreadGroup1.java, replace the call to sys.suspend( ) with a call to wait( ) for the thread group, causing it to wait for 2 seconds. For this to work correctly you’ll need to acquire the lock for sys inside a synchronized block. 7. Change Daemons.java so that main( ) has a sleep( ) instead of a readLine( ). Experiment with different sleep times to see what happens. 568 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 569. 8. (Intermediate) In Chapter 7, locate the GreenhouseControls.java example, which consists of three files. In Event.java, the class Event is based on watching the time. Change Event so it is a Thread, and change the rest of the design so it works with this new Thread-based Event. Chapter 14: Multiple Threads 569
- 571. w 15: Network programming Historically, network programming has been error-prone, difficult, and complex. The programmer had to know many details about the network and sometimes even the hardware. You usually needed to understand the various “layers” of the networking protocol, and there were lots of different functions in each different networking library concerned with connecting, packing and unpacking blocks of information, shipping those blocks back and forth, and handshaking. It was a daunting task. However, the concept of networking is not so very difficult. You want to get some information from that machine over there and move it to this machine here, or vice-versa. It’s quite similar to reading and writing files, except that the file exists on a remote machine and the remote machine can decide exactly what it wants to do about the information you’re requesting or sending. One of Java’s great strengths is painless networking. As much as possible, the underlying details of networking have been abstracted away and taken care of within the JVM and local machine installation of Java. The programming model you use is that of a file; in fact, you actually wrap the network connection (a “socket”) with stream objects, so you end up using exactly the same method calls as you do with all other streams. In addition, Java’s built-in multithreading is exceptionally handy when dealing with another networking issue: handling multiple connections at once. This chapter introduces Java’s networking support using easy-to-understand examples. 571
- 572. Identifying a machine Of course, in order to tell one machine from another and to make sure you are connected with exactly the machine you want, there must be some way of uniquely identifying machines on a network. Early networks were satisfied to provide unique names for machines within the local network. However, Java works within the Internet, which requires a way to uniquely identify a machine from all the others in the world. This is accomplished with the IP (Internet Protocol) address that can exist in two forms: 1. The familiar DNS (Domain Name Service) form. My domain name is eckelobjects.com, so suppose I have a computer called Opus in my domain. Its domain name would be Opus.eckelobjects.com. This is exactly the kind of name that you use when you send email to people, and is often incorporated into a World-Wide-Web address. 2. Alternatively, you can use the “dotted quad” form, which is four numbers separated by dots, such as 123.255.28.120. In both cases, the IP address is represented internally as a 32-bit number1 (so each of the quad numbers cannot exceed 255), and you can get a special Java object to represent this number from either of the above forms by using the static InetAddress.getByName( ) method that’s in java.net. The result is an object of type InetAddress that you can use to build a “socket” as you shall see later. As a simple example of using InetAddress.getByName( ), consider what happens if you have a dial-up Internet Service Provider (ISP). Each time you dial up, you are assigned a temporary IP address. But while you're connected, your IP address has the same validity as any other IP address on the Internet. If someone connects to your machine using your IP address then they can connect to a Web server or FTP server that you have running on your machine. Of course, they need to know your IP address, and since it's assigned each time you dial up, how can you find out what it is? The following program uses InetAddress.getByName( ) to produce your IP address. To use it, you must know the name of your computer. It has been tested only on Windows 95, but there you can go to “Settings,” “Control Panel,” “Network,” and then select the “Identification” tab. “Computer name” is the name to put on the command line. //: WhoAmI.java // Finds out your network address when you're // connected to the internet. package c15; import java.net.*; public class WhoAmI { public static void main(String args[]) throws Exception { if(args.length != 1) { System.err.println( "Usage: WhoAmI MachineName"); System.exit(1); 1 This means a maximum of just over 4 billion numbers, which is rapidly running out. The new standard for IP addresses will use a 128-bit number, which should produce enough unique IP addresses for the foreseeable future. 572 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 573. } InetAddress a = InetAddress.getByName(args[0]); System.out.println(a); } } ///:~ In my case, the machine is called “Colossus” (from the movie of the same name, because I keep putting bigger disks on it). So, once I’ve connected to my ISP I run the program: java WhoAmI Colossus I get back a message like this (of course, the address is different each time): Colossus/199.190.87.75 If I tell my friend this address, he can log onto my personal Web server by going to the URL http://199.190.87.75 (only as long as I continue to stay connected during that session). This can sometimes be a handy way to distribute information to someone else, or to test out a Web site configuration before posting it to a “real” server. Servers and clients The whole point of a network is to allow two machines to connect and talk to each other. Once the two machines have found each other they can have a nice, two-way conversation. But how do they find each other? It’s like getting lost in an amusement park: one machine has to stay in one place and listen while the other machine says “hey, where are you?” The machine that “stays in one place” is called the server, and the one that seeks is called the client. This distinction is important only while the client is trying to connect to the server. Once they’ve connected, it becomes a two-way communication process and it doesn’t matter anymore that one happened to take the role of server and the other happened to take the role of the client. So the job of the server is to listen for a connection, and that’s performed by the special server object that you create. The job of the client is to try to make a connection to a server, and this is performed by the special client object you create. Once the connection is made, you’ll see that at both server and client ends, the connection is just magically turned into an IO stream object, and from then on you can treat the connection as if you were reading from and writing to a file. Thus, after the connection is made you will just use the familiar IO commands from Chapter 10. This is one of the very nice features of Java networking. Testing programs without a network For many reasons, you may not have a client machine, a server machine, and a network available to test your programs. You may be performing exercises in a classroom situation, or you may be writing programs that aren’t yet stable enough to put onto the network. How can you test your code? The creators of the Internet Protocol were aware of this issue, and they created a special address called localhost to be the “local loopback” IP address for testing without a network. The generic way to produce this address in Java is: InetAddress addr = InetAddress.getByName(null); If you hand getByName( ) a null, it defaults to using the localhost. The InetAddress is what you use to refer to the particular machine, and you must produce this before you Chapter 15: Network Programming 573
- 574. can go any further. You can’t manipulate the contents of an InetAddress (but you can print them out, as you’ll see in the next example). The only way you can create an InetAddress is through one of that class’s static member methods getByName( ) (which is what you’ll usually use), getAllByName( ) or getLocalHost( ). You can also produce the local loopback address by handing it the string localhost: InetAddress.getByName("localhost"); Or by using its dotted quad form to name the reserved IP number for the loopback: InetAddress.getByName("127.0.0.1"); All three forms produce the same result. Port: a unique place within the machine An IP address isn’t enough to identify a unique server, since many servers can exist on one machine. Each IP machine also contains ports, and when you’re setting up a client or a server you must choose a port where both client and server agree to connect. As if you’re meeting someone: the IP address is the neighborhood and the port is the bar. The port is not a physical location in a machine, but is instead a software abstraction (mainly for bookkeeping purposes). The client program knows how to connect to the machine via its IP address, but how does it connect to a desired service (potentially one of many on that machine)? That's where the port numbers come in as second level of addressing. The idea is that if you ask for a particular port you’re requesting the service that’s associated with the port number. For example, one service may simply be the time of day. Typically, each service is associated with a unique port number on a given server machine. It's up to the client to know ahead of time which port number the desired service is running on. The system services reserve the use of ports 1 through 1024, so you shouldn’t use those or any other port that you know to be in use. The first choice for examples in this book will be port 8080 (in memory of the venerable old 8-bit Intel 8080 chip in my first computer, a CP/M machine). Sockets The socket is the software abstraction used to represent the “terminals” of a connection between two machines. For a given connection, there’s a socket on each machine, and you can imagine a hypothetical “cable” running between the two machines, with each end of the “cable” plugged into a socket. Of course, the physical hardware and cabling between machines is completely unknown. The whole point of the abstraction is that we don’t have to know more than is necessary. In Java, you create a socket to make the connection to the other machine, then you get an InputStream and OutputStream (or, with the appropriate converters, Reader and Writer) from the socket in order to be able to treat the connection as an IO stream object. There are two stream-based socket classes: a ServerSocket which a server uses to "listen" for incoming connections, and a Socket that a client uses in order to initiate a connection. Once a client makes a socket connection, the ServerSocket returns (via the accept( ) method) a corresponding server side Socket through which direct communications will take place. From then on, you have a true Socket to Socket connection and you treat both ends the same way because they are the same. At this point, you use the methods 574 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 575. getInputStream( ) and getOutputStream( ) to produce the corresponding InputStream and OutputStream objects from each Socket. These must be wrapped inside buffers and formatting classes just like any other stream object described in Chapter 10. The use of the term ServerSocket would seem to be another example of a confusing name scheme in the Java libraries. You might think ServerSocket would be better named “ServerConnector” or something without the word “Socket” in it. You might also think that ServerSocket and Socket should both be inherited from some common base class. Indeed, the two classes do have several methods in common but not enough to give them a common base class. Instead, ServerSocket’s job is to wait until some other machine connects to it, then to return an actual Socket. This is why ServerSocket seems to be a bit misnamed, since its job isn’t really to be a socket but instead to make a Socket object when someone else connects to it. However, the ServerSocket does in fact create a physical “server” or listening socket on the host machine. This socket listens for incoming connections and then returns an “established” socket (with the local and remote endpoints defined) via the accept( ) method. The confusing part is that both these sockets (listening and established) are associated with the same server socket. The listening socket can accept only new connection requests and not data packets. So while ServerSocket doesn't make much sense programmatically, it does “physically.” When you create a ServerSocket, you give it only a port number. You don’t have to give it an IP address because it’s already on the machine it represents. When you create a Socket, however, you must give both the IP address and the port number where you’re trying to connect (on the other hand, the Socket that comes back from ServerSocket.accept( ) already contains all this information). A simple server and client This example makes the simplest use of servers and clients using sockets. All the server does is wait for a connection, then uses the Socket produced by that connection to create an InputStream and OutputStream. After that, everything it reads from the InputStream it echoes to the OutputStream until it receives the line END, at which time it closes the connection. The client makes the connection to the server, then creates an OutputStream. Lines of text are sent through the OutputStream. The client also creates an InputStream to hear what the server is saying (which, in this case, is just the words echoed back). Both the server and client use the same port number and the client uses the local loopback address to connect to the server on the same machine so you don’t have to test it over a network (for some configurations, you may need to be connected to a network for the programs to work, even if you aren’t actually communicating over that network). Here is the server: //: JabberServer.java // Very simple server that just // echoes whatever the client sends. import java.io.*; import java.net.*; public class JabberServer { // Choose a port outside of the range 1-1024: public static final int PORT = 8080; public static void main(String args[]) Chapter 15: Network Programming 575
- 576. throws IOException { ServerSocket s = new ServerSocket(PORT); System.out.println("Started: " + s); try { // Blocks until a connection occurs: Socket socket = s.accept(); try { System.out.println( "Connection accepted: "+ socket); BufferedReader in = new BufferedReader( new InputStreamReader( socket.getInputStream())); // Output is automatically flushed // by PrintWriter: PrintWriter out = new PrintWriter( new BufferedWriter( new OutputStreamWriter( socket.getOutputStream())),true); while (true) { String str = in.readLine(); if (str.equals("END")) break; System.out.println("Echoing: " + str); out.println(str); } // Always close the two sockets... } finally { System.out.println("closing..."); socket.close(); } } finally { s.close(); } } } ///:~ You can see that the ServerSocket just needs a port number, but not an IP address (since it’s running on this machine!). When you call accept( ), the method blocks until some client tries to connect to it. That is, it’s there waiting for a connection but other processes can run (see Chapter 14). When a connection is made, accept( ) returns with a Socket object representing that connection. The responsibility for cleaning up the sockets is carefully crafted here. If the ServerSocket constructor fails, the program just quits (note we must assume that the constructor for ServerSocket doesn’t leave any open network sockets lying around if it fails). For this case, main( ) throws IOException so a try block is not necessary. If the ServerSocket constructor is successful, then all other method calls must be guarded in a try-finally block to ensure that, no matter how the block is left, the ServerSocket is properly closed. The same logic is used for the Socket returned by accept( ). If accept( ) fails, then we must assume the Socket doesn’t exist or hold any resources, so it doesn’t need to be cleaned up. If it’s successful, however, the following statements must be in a try-finally block so if they fail the Socket will still be cleaned up. The reason care is required here is that sockets use important non-memory resources and so you must be diligent in order to clean them up (since there is no destructor in Java to do it for you). 576 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 577. Both the ServerSocket and the Socket produced by accept( ) are printed to System.out. This means their toString( ) methods are automatically called. These produce: ServerSocket[addr=0.0.0.0,PORT=0,localport=8080] Socket[addr=127.0.0.1,PORT=1077,localport=8080] Shortly, you’ll see how these fit together with what the client is doing. The next part of the program looks just like opening files for reading and writing except that the InputStream and OutputStream are created from the Socket object. Both the InputStream and OutputStream objects are converted to Java 1.1 Reader and Writer objects using the “converter” classes InputStreamReader and OutputStreamWriter, respectively. You could also have used the Java 1.0 InputStream and OutputStream classes directly, but with output there’s a distinct advantage to using the Writer approach. This appears with PrintWriter, which has an overloaded constructor that takes a second argument, a boolean flag which indicates whether or not to automatically flush the output at the end of each println( ) (but not print( )) statement. Every time you write to out, its buffer must be flushed so the information actually goes out over the network. Flushing is important for this particular example because the client and server each wait for a line from the other party before proceeding. If flushing doesn’t occur, the information will not be put onto the network until the buffer is full which causes lots of problems in this example. When writing network programs you need to be careful about using automatic flushing. That’s because every time you flush the buffer a packet must be created and sent. In this case, that’s exactly what we want, since if the packet containing the line isn’t sent then the handshaking back and forth between server and client will stop. Put another way, the end of a line is the end of a message. But in many cases messages aren’t delimited by lines so it’s much more efficient not to use auto flushing and to let the built-in buffering decide when to build and send a packet. This way, larger packets can be sent and the process will be faster. Notice that, like virtually all streams you open, these are buffered. There’s an exercise at the end of the chapter to show you what happens if you don’t buffer the streams (things get very slow). The infinite while loop just reads lines from the BufferedReader in and writes information to System.out and to the PrintWriter out. Notice that these could be any streams at all, they just happen to be connected to the network. When the client sends the line consisting of “END” the program breaks out of the loop and closes the Socket. Here’s the client: //: JabberClient.java // Very simple client that just sends // lines to the server and reads lines // that the server sends. import java.net.*; import java.io.*; public class JabberClient { public static void main(String args[]) throws IOException { // Passing null to getByName() produces the // special "Local Loopback" IP address, for // testing on one machine w/o a network: Chapter 15: Network Programming 577
- 578. InetAddress addr = InetAddress.getByName(null); // Alternatively, you can use // the address or name: // InetAddress addr = // InetAddress.getByName("127.0.0.1"); // InetAddress addr = // InetAddress.getByName("localhost"); System.out.println("addr = " + addr); Socket socket = new Socket(addr, JabberServer.PORT); // Guard everything in a try-finally to make // sure that the socket is closed: try { System.out.println("socket = " + socket); BufferedReader in = new BufferedReader( new InputStreamReader( socket.getInputStream())); // Output is automatically flushed // by PrintWriter: PrintWriter out = new PrintWriter( new BufferedWriter( new OutputStreamWriter( socket.getOutputStream())),true); for(int i = 0; i < 10; i ++) { out.println("howdy " + i); String str = in.readLine(); System.out.println(str); } out.println("END"); } finally { System.out.println("closing..."); socket.close(); } } } ///:~ In main( ) you can see all three ways to produce the InetAddress of the local loopback IP address: using null, localhost or the explicit reserved address 127.0.0.1. Of course, if you want to actually connect to a machine across a network you substitute that machine’s IP address. When the InetAddress addr is printed (via the automatic call to its toString( ) method) the result is: localhost/127.0.0.1 By handing getByName( ) a null, it defaulted to finding the localhost, and that produced the special address 127.0.0.1. Note that the Socket called socket is created with both the InetAddress and the port number. To understand what it means when you print out one of these Socket objects, remember that an Internet connection is determined uniquely by these four pieces of data: clientHost, clientPortNumber, serverHost, and serverPortNumber. When the server comes up, it takes up its assigned port (8080) on the localhost (127.0.0.1). When the client comes up, it is allocated the next available port on its machine, 1077 in this case, 578 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 579. which also happens to be on the same machine (127.0.0.1) as the server. Now in order for data to move between the client and server, each side has to know where to send it. Therefore, during the process of connecting to the “known” server, the client sends a “return address” so that the server knows where to send its data. This is what you see in the example output for the server side: Socket[addr=127.0.0.1,port=1077,localport=8080] This means the server just accepted a connection from 127.0.0.1 on port 1077 while listening on its local port (8080). On the client side: Socket[addr=localhost/127.0.0.1,PORT=8080,localport=1077] Which means the client made a connection to 127.0.0.1 on port 8080 using the local port 1077. You’ll notice that every time you start up the client anew, the local port number is incremented. It starts at 1025 (one past the reserved block of ports) and keeps going up until you reboot the machine, at which point it starts back at 1025 again (on UNIX machines, once the upper limit of the socket range is reached the numbers will wrap around to the lowest available number again). Once the Socket object has been created, the process of turning it into a BufferedReader and PrintWriter is the same as in the server (again, in both cases you start with a Socket). Here, the client initiates the conversation by sending the string “howdy” followed by a number. Note that the buffer must again be flushed (which happens automatically via the second argument to the PrintWriter constructor); if you don’t the whole conversation will hang because the initial “howdy” will never get sent (the buffer isn’t full enough to cause the send to happen automatically). Each line that is sent back from the server is written to System.out to verify that everything is working correctly. To terminate the conversation, the agreed-upon “END” is sent. If the client simply hangs up then the server throws an exception. You can see that the same care is taken here to ensure that the network resources represented by the Socket are properly cleaned up, using a try-finally block. Sockets produce a “dedicated” connection which persists until it is explicitly disconnected (the dedicated connection can still be disconnected un-explicitly if one side, or an intermediary link, of the connection crashes). This means the two parties are locked in communication and the connection is constantly open. This seems like a logical approach to networking, but it puts an extra load on the network. Later in the chapter you’ll see a different approach to networking, where the connections are only temporary. Serving multiple clients The JabberServer works, but it can handle only one client at a time. In a typical server, you’ll want to be able to deal with many clients at once. The answer is multithreading, and in languages that don’t directly support multithreading this means all sorts of complications. In Chapter 14 you saw that multithreading in Java is about as simple as possible, considering that multithreading is a rather complex topic. Because threading in Java is reasonably straightforward, making a server that handles multiple clients is relatively easy. The basic scheme is to make a single ServerSocket in the server, and call accept( ) to wait for a new connection. When accept( ) returns, you take the resulting Socket and use it to create a new thread whose job is to serve that particular client, then you call accept( ) again to wait for a new client. Chapter 15: Network Programming 579
- 580. In the following server code, you can see that it looks very similar to the JabberServer.java example except that all the operations to serve a particular client have been moved inside a separate thread class: //: MultiJabberServer.java // A server that uses multithreading to handle // any number of clients. import java.io.*; import java.net.*; class ServeOneJabber extends Thread { private Socket socket; private BufferedReader in; private PrintWriter out; public ServeOneJabber(Socket s) throws IOException { socket = s; in = new BufferedReader( new InputStreamReader( socket.getInputStream())); // Enable auto-flush: out = new PrintWriter( new BufferedWriter( new OutputStreamWriter( socket.getOutputStream())), true); // If any of the above calls throw an // exception, the caller is responsible for // closing the socket. Otherwise the thread // will close it. start(); // Calls run() } public void run() { try { while (true) { String str = in.readLine(); if (str.equals("END")) break; System.out.println("Echoing: " + str); out.println(str); } System.out.println("closing..."); } catch (IOException e) { } finally { try { socket.close(); } catch(IOException e) {} } } } public class MultiJabberServer { static final int PORT = 8080; public static void main(String[] args) throws IOException { ServerSocket s = new ServerSocket(PORT); 580 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 581. System.out.println("Server Started"); try { while(true) { // Blocks until a connection occurs: Socket socket = s.accept(); try { new ServeOneJabber(socket); } catch(IOException e) { // If it fails, close the socket, // otherwise the thread will close it: socket.close(); } } } finally { s.close(); } } } ///:~ The ServeOneJabber thread takes the Socket object that’s produced by accept( ) in main( ) every time a new client makes a connection. Then, as before, it creates a BufferedReader and auto-flushed PrintWriter object using the Socket. Finally, it calls the special Thread method start( ), which performs thread initialization and then calls run( ). This performs the same kind of action as in the previous example: reading something from the socket and then echoing it back until it reads the special “END” signal. The responsibility for cleaning up the socket must again be carefully designed. In this case, the socket is created outside of the ServeOneJabber so the responsibility can be shared. If the ServeOneJabber constructor fails, it will just throw the exception to the caller who will then clean up the thread. But if the constructor succeeds then the ServeOneJabber object takes over responsibility for cleaning up the thread, in its run( ). Notice the simplicity of the MultiJabberServer. As before, a ServerSocket is created and accept( ) is called to allow a new connection. But this time, the return value of accept( ) (a Socket) is passed to the constructor for ServeOneJabber which creates a new thread to handle that connection. When the connection is terminated, the thread simply goes away. If the creation of the ServerSocket fails, the exception is again thrown through main( ). But if it succeeds, the outer try-finally guarantees its cleanup. The inner try-catch is only to guard against the failure of the ServeOneJabber constructor; if the constructor succeeds then the ServeOneJabber thread will close the associated socket. To test that the server really does handle multiple clients, the following program creates many clients (using threads), each of which connects to the same server. Each thread has a limited lifetime, and when it goes away that leaves space for the creation of a new thread. The maximum number of threads allowed is determined by the final int maxthreads. You’ll notice this value is rather critical, since if you make it too high the threads seem to run out of resources and the program mysteriously fails. //: MultiJabberClient.java // Client that tests the MultiJabberServer // by starting up multiple clients. import java.net.*; import java.io.*; class JabberClientThread extends Thread { private Socket socket; private BufferedReader in; Chapter 15: Network Programming 581
- 582. private PrintWriter out; private static int counter = 0; private int id = counter++; private static int threadcount = 0; public static int threadCount() { return threadcount; } public JabberClientThread(InetAddress addr) { System.out.println("Making client " + id); threadcount++; try { socket = new Socket(addr, MultiJabberServer.PORT); } catch(IOException e) { // If the creation of the socket fails, // nothing needs to be cleaned up. } try { in = new BufferedReader( new InputStreamReader( socket.getInputStream())); // Enable auto-flush: out = new PrintWriter( new BufferedWriter( new OutputStreamWriter( socket.getOutputStream())), true); start(); } catch(IOException e) { // The socket should be closed on any // failures other than the socket // constructor: try { socket.close(); } catch(IOException e2) {} } // Otherwise the socket will be closed by // the run() method of the thread. } public void run() { try { for(int i = 0; i < 25; i++) { out.println("Client " + id + ": " + i); String str = in.readLine(); System.out.println(str); } out.println("END"); } catch(IOException e) { } finally { // Always close it: try { socket.close(); } catch(IOException e) {} threadcount--; // Ending this thread 582 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 583. } } } public class MultiJabberClient { static final int MAX_THREADS = 40; public static void main(String args[]) throws IOException, InterruptedException { InetAddress addr = InetAddress.getByName(null); while(true) { if(JabberClientThread.threadCount() < MAX_THREADS) new JabberClientThread(addr); Thread.currentThread().sleep(100); } } } ///:~ The JabberClientThread constructor takes an InetAddress and uses it to open a Socket. You’re probably starting to see the pattern – the Socket is always used to create some kind of Reader and/or Writer (or InputStream and/or OutputStream) object, which is the only way the Socket can be used (you can of course write a class or two to automate this process rather than doing all the typing if it becomes painful). Again, start( ) performs thread initialization and calls run( ). Here, messages are sent to the server and information from the server is echoed to the screen. However, the thread has a limited lifetime and eventually completes. Notice that the socket is cleaned up if the constructor fails after the socket is created but before the constructor completes. Otherwise the responsibility for calling close( ) for the socket is relegated to the run( ) method. The threadcount keeps track of how many JabberClientThread objects currently exist. It is incremented as part of the constructor, and decremented as run( ) exits (which means the thread is terminating). In MultiJabberClient.main( ) you can see that the number of threads is tested, and if there are too many, no more are created. Then the method sleeps. This way, some threads will eventually terminate and more can be created. You can experiment with MAX_THREADS to see where your particular system begins to have trouble with too many connections. Datagrams The examples you’ve seen so far use the Transmission Control Protocol (TCP, also known as stream-based sockets), which is designed for ultimate reliability and guarantees that the data will get there. It allows retransmission of lost data, it provides multiple paths through different routers in case one goes down, and bytes are delivered in the order they are sent. All this control and reliability comes at a cost: TCP has a high overhead. There’s a second protocol, called User Datagram Protocol (UDP), which doesn’t guarantee that the packets will be delivered and doesn’t guarantee that they will arrive in the order they were sent. It’s called an “unreliable protocol” (TCP is a “reliable protocol”), which sounds bad but because it’s much faster it can be very useful. There are some applications, such as an audio signal, where it isn’t so critical if a few packets are dropped here or there but speed is very important. Or consider a time-of-day server, where it really doesn’t matter if one of the messages is lost. Also, some applications may be able to fire off a UDP message to a server and can then assume, if there is no response in a reasonable period of time, that the message was lost. Chapter 15: Network Programming 583
- 584. The support for datagrams in Java has the same feel as its support for TCP sockets, but there are significant differences. With datagrams, you put a DatagramSocket on both the client and server, but there is no analogy to the ServerSocket that waits around for a connection. That’s because there is no “connection,” but instead a datagram just shows up. Another fundamental difference is that with TCP sockets, once you’ve made the connection you don’t need to worry about who’s talking to whom anymore; you just send the data back and forth through conventional streams. However, with datagrams, the datagram packet itself must know where it came from and where it’s supposed to go. That means you’ll have to know these things for each datagram packet that you load up and ship off. A DatagramSocket sends and receives the packets, and the DatagramPacket contains the information itself. When you’re receiving a datagram, you need only provide a buffer where the data will be placed; the information about the Internet address and port number where the information came from will be automatically initialized when the packet arrives through the DatagramSocket. So the constructor for a DatagramPacket to receive datagrams is: DatagramPacket(buf, buf.length) Where buf is an array of byte. Since buf is an array, you might wonder why the constructor couldn’t figure out the length of the array on its own. I wondered this myself, and can guess only that it’s a throwback to C-style programming, where of course arrays can’t tell you how big they are. You can reuse a receiving datagram over and over; you don’t have to make a new one each time. Every time you reuse it, the data in the buffer is overwritten. The maximum size of the buffer is restricted only by the allowable datagram packet size, which limits it to slightly less than 64Kbytes. However, in many applications you’ll want it to be much smaller, certainly when you’re sending data. Your chosen packet size depends on what you need for your particular application. When you send a datagram, the DatagramPacket must contain not only the data, but also the Internet address and port where it will be sent. So the constructor for an outgoing DatagramPacket is: DatagramPacket(buf, length, inetAddress, port) This time, buf (which is a byte array) already contains the data that you want to send out. The length may be the length of buf, but it may also be shorter, indicating that you want to send only that many bytes. The other two arguments are the Internet address where the packet is going and the destination port within that machine2 . You might think that the two constructors create two very different objects: one for receiving datagrams, and one for sending them. Good OO design would suggest that these should be two different classes, rather than one class with different behavior depending on how you construct the object. This is probably true, but fortunately the use of DatagramPackets is simple enough that you’re not tripped up by the problem, as you can see in the following example. This example is similar to the MultiJabberServer and MultiJabberClient example for TCP sockets. Multiple clients will send datagrams to a server, which will echo them back to the same client that sent the message. 2 TCP and UDP ports are considered unique. That is, you can simultaneously run a TCP and UDP server on port 8080 without interference. 584 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 585. To simplify the creation of a DatagramPacket from a String and vice-versa, the example begins with a utility class, Dgram, to do the work for you: //: Dgram.java // A utility class to convert back and forth // Between Strings and DataGramPackets. import java.net.*; public class Dgram { public static DatagramPacket toDatagram( String s, InetAddress destIA, int destPort) { // Deprecated in Java 1.1, but it works: byte buf[] = new byte[s.length() + 1]; s.getBytes(0, s.length(), buf, 0); // The correct Java 1.1 approach, but it's // Broken (it truncates the String): // byte buf[] = s.getBytes(); return new DatagramPacket(buf, buf.length, destIA, destPort); } public static String toString(DatagramPacket p){ // The Java 1.0 approach: // return new String(p.getData(), // 0, 0, p.getLength()); // The Java 1.1 approach: return new String(p.getData(), 0, p.getLength()); } } ///:~ The first method of Dgram takes a String, an InetAddress and a port number and builds a DatagramPacket by copying the contents of the String into a byte buffer and passing the buffer into the DatagramPacket constructor. Note the “+1” in the buffer allocation – this was necessary to prevent truncation. The getBytes( ) method of String is a special operation that copies the chars of a String into a byte buffer. This method is now deprecated; Java 1.1 has a “better” way to do this but it’s commented out here because it truncates the String. So you’ll get a deprecation message when you compile it under Java 1.1, but the behavior will be correct (this bug may very well be fixed by the time you read this). The Dgram.toString( ) method shows both the Java 1.0 approach and the new Java 1.1 approach (which is different because there’s a new kind of String constructor). Here is the server for the datagram demonstration: //: ChatterServer.java // A server that echoes datagrams import java.net.*; import java.io.*; import java.util.*; public class ChatterServer { static final int INPORT = 1711; private byte buf[] = new byte[1000]; private DatagramPacket dp = new DatagramPacket(buf, buf.length); // Can listen & send on the same socket: Chapter 15: Network Programming 585
- 586. private DatagramSocket socket; public ChatterServer() { try { socket = new DatagramSocket(INPORT); System.out.println("Server started"); while(true) { // Block until a datagram appears: socket.receive(dp); String rcvd = Dgram.toString(dp) + ", from address: " + dp.getAddress() + ", port: " + dp.getPort(); System.out.println(rcvd); String echoString = "Echoed: " + rcvd; // Extract the address and port from the // received datagram to find out where to // send it back: DatagramPacket echo = Dgram.toDatagram(echoString, dp.getAddress(), dp.getPort()); socket.send(echo); } } catch(SocketException e) { System.err.println("Can't open socket"); System.exit(1); } catch(IOException e) { System.err.println("Communication error"); e.printStackTrace(); } } public static void main(String[] args) { new ChatterServer(); } } ///:~ The ChatterServer contains a single DatagramSocket for receiving messages, instead of creating one each time you’re ready to receive a new message. The single DatagramSocket can be used over and over. This DatagramSocket has a port number because this is the server and the client must have an exact address where it wants to send the datagram. It is given a port number but not an Internet address because it resides on “this” machine so it knows what its Internet address is (in this case, the default localhost). In the infinite while loop, the socket is told to receive( ), whereupon it blocks until a datagram shows up, and then sticks it into our designated receiver, the DatagramPacket dp. The packet is converted to a String along with information about the Internet address and socket where the packet came from. This information is displayed, and then an extra string is added to indicate that it is being echoed back from the server. Now there’s a bit of a quandary. As you shall see, there are potentially many different Internet addresses and port numbers that the messages may come from – that is, the clients may reside on any machine (in this demonstration they all reside on the localhost, but the port number for each client is different). To send a message back to the client that originated it, you need to know that client’s Internet address and port number. Fortunately, this information is conveniently packaged inside the DatagramPacket that sent the message, so all you have to do is pull it out using getAddress( ) and getPort( ), which are used to build the DatagramPacket echo which is sent back through the same 586 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 587. socket that’s doing the receiving. In addition, when the socket sends the datagram, it automatically adds the Internet address and port information of this machine, so that when the client receives the message, it can use getAddress( ) and getPort( ) to find out where the datagram came from. In fact, the only time that getAddress( ) and getPort( ) don’t tell you where the datagram came from is if you create a datagram to send, and you call getAddress( ) and getPort( ) before you send the datagram (in which case it tells the address and port of this machine, the one the datagram is being sent from). This is an essential part of datagrams: you don’t need to keep track of where a message came from, because it’s always stored inside the datagram. In fact, the most reliable way to program is if you don’t try to keep track, but instead always extract the address and port from the datagram in question (as is done here). To test this server, here’s a program that makes a number of clients, all of which fire datagram packets to the server and wait for the server to echo them back. //: ChatterClient.java // Tests the ChatterServer by starting up multiple // clients, each of which sends datagrams. import java.lang.Thread; import java.net.*; import java.io.*; public class ChatterClient extends Thread { // Can listen & send on the same socket: private DatagramSocket s; private InetAddress hostAddress; private byte buf[] = new byte[1000]; private DatagramPacket dp = new DatagramPacket(buf, buf.length); private int id; public ChatterClient(int identifier) { id = identifier; try { // Auto-assign port number: s = new DatagramSocket(); hostAddress = InetAddress.getByName("localhost"); } catch(UnknownHostException e) { System.err.println("Cannot find host"); System.exit(1); } catch(SocketException e) { System.err.println("Can't open socket"); e.printStackTrace(); System.exit(1); } System.out.println("ChatterClient starting"); } public void run() { try { for(int i = 0; i < 25; i++) { String outMessage = "Client #" + id + ", message #" + i; // Make and send a datagram: s.send(Dgram.toDatagram(outMessage, hostAddress, ChatterServer.INPORT)); Chapter 15: Network Programming 587
- 588. // Block until it echoes back: s.receive(dp); // Print out the echoed contents: String rcvd = "Client #" + id + ", rcvd from " + dp.getAddress() + ", " + dp.getPort() + ": " + Dgram.toString(dp); System.out.println(rcvd); } } catch(IOException e) { e.printStackTrace(); System.exit(1); } } public static void main(String[] args) { for(int i = 0; i < 10; i++) new ChatterClient(i).start(); } } ///:~ ChatterClient is created as a Thread so that multiple clients may be made to bother the server. Here you can see the receiving DatagramPacket looks just like the one used for ChatterServer. In the constructor, the DatagramSocket is created with no arguments since it doesn’t need to advertise itself as being at a particular port number. The Internet address used for this socket will be “this machine” (for the example, localhost) and the port number will be automatically assigned as you will see from the output. This DatagramSocket, like the one for the server, will be used both for sending and receiving. The hostAddress is the Internet address of the host machine you want to talk to. The one place in the program where you must know an exact Internet address and port number is when you make the outgoing DatagramPacket. As is always the case, the host must be at a known address and port number so that clients may originate conversations with the host. Each thread is given a unique identification number (although the port number automatically assigned to the thread would also provide a unique identifier). In run( ), a message String is created that contains the thread’s identification number and the message number this thread is currently sending. This String is used to create a datagram which is sent to the host at its address; the port number is taken directly from a constant in ChatterServer. Once the message is sent, receive( ) blocks until the server replies with an echoing message. All the information that’s shipped around with the message allows you to see that what comes back to this particular thread is derived from the message that originated from this thread. In this example, even though UDP is an “unreliable” protocol, you’ll see that all the datagrams get where they’re supposed to (this will be true for localhost and LAN situations, but you may begin to see some failures for non-local connections). When you run this program, you’ll see that each of the threads finishes, which means that each of the datagram packets sent to the server is actually turned around and echoed to the right recipient, otherwise one or more threads would hang, blocking until their input shows up. You might think that the only correct way to, for example, transfer a file from one machine to another is through TCP sockets, since they’re “reliable.” However, because of the speed of datagrams they can actually be a better solution. You simply break the file up into packets and number each packet. The receiving machine takes the packets and 588 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 589. reassembles them; a “header packet” tells it how many to expect and any other important information. If a packet is lost, the receiving machine sends a datagram back telling the sender to retransmit. A Web application Now let’s consider creating an application to run on the Web, which will show Java in all its glory. Part of this application will be a Java program running on the Web server, and the other part will be an applet that’s downloaded to the browser. The applet collects information from the user and sends it back to the application running on the Web server. The task of the program will be very simple –the applet will ask for the email address of the user, and after verifying that this email address is reasonably legitimate (it doesn’t contain spaces, and it does contain an ‘@’ symbol) the applet will send the email address to the web server. The application running on the server will capture the data and check a data file where all the email addresses are kept. If that address is already in the file, it will send back a message to that effect which is displayed by the applet. If the address isn’t in the file, it is placed in the list and the applet is informed that the address was successfully added. Traditionally, the way to handle such a problem is to create an HTML page with a text field and a “submit” button. The user can type whatever they want into the text field and it will be submitted to the server without question. As it submits the data, the Web page also tells the server what to do with the data by mentioning the Common-Gateway Interface (CGI) program that the server should run after receiving this data. This CGI program is typically written in either Perl or C (and sometimes C++, if the server supports it), and it must handle everything. First it looks at the data and decides whether it’s in the correct format. If not, the CGI program must create an HTML page to describe the problem; this page is handed to the server which sends it back to the user, who must then back up a page and try again. If the data is correct, the CGI program opens the data file and either adds the email address to the file or discovers that the email address is already in the file. In both cases it must format an appropriate HTML page for the server to return to the user. As Java programmers, this seems like an awkward way for us to solve the problem, and naturally we’d like to do the whole thing in Java. First of all, we'll use a Java applet to take care of data validation at the client site, without all that tedious Web traffic and page formatting. Then let’s skip the Perl CGI script in favor of a Java application running on the server. In fact, let’s skip the Web server altogether, and simply make our own network connection from the applet to the Java application on the server! As you’ll see, there are a number of issues that make this a more complicated problem than it seems. First of all, it would be ideal to write the applet using Java 1.1 but that’s hardly practical. At this writing, the number of users running Java 1.1-enabled browsers is very small, and although such browsers are now commonly available you’ll probably need to take into account that a significant number of users will be slow to upgrade. So to be on the safe side, the applet will be programmed using only Java 1.0 code. With this in mind, there will be no JAR files to combine .class files in the applet, so the applet should be designed to create as few .class files as possible to minimize download time. Well, it turns out the Web server (the one available to me when I wrote the example) does have Java on it, but only Java 1.0! So the server application must also be written using Java 1.0. Chapter 15: Network Programming 589
- 590. The server application Now consider the server application, which will be called NameCollector. What happens if more than one user at a time tries to submit their email address? If NameCollector uses TCP/IP sockets, then it must use the multithreading approach shown earlier to handle more than one client at a time. But all these threads will try to write to a single file where all the email addresses will be kept. This would require a locking mechanism to make sure that more than one thread doesn’t access the file at once. A semaphore will do the trick, but perhaps there’s a simpler way. If we use datagrams instead, multithreading is unnecessary. A single datagram socket will listen for incoming datagrams, and when one appears the program will process the message and send the reply as a datagram back to whomever sent the request. If the datagram gets lost, then the user will notice that no reply comes and can then re-submit the request. When the server application receives a datagram and unpacks it, all it must do is extract the email address and check the file to see if that address is there already (and if it isn’t, add it). And now we run into another problem. It turns out that Java 1.0 doesn’t quite have the horsepower to easily manipulate the file containing the email addresses (Java 1.1 does). However, the problem can be solved in C quite readily, and this will provide an excuse to show you the easiest way to connect a non-Java program to a Java program. A Runtime object for a program has a method called exec( ) which will start up a separate program on the machine, and return a Process object. You can get an OutputStream that connects to standard input for this separate program, and an InputStream that connects to standard output. This means all you need to do is write a program using any language that takes its input from standard input and writes the output to standard output. This is a very convenient trick when you run into a problem that can’t be solved easily or quickly enough in Java (or when you have legacy code you don’t want to rewrite). You can also use Java’s native methods (see Appendix A) but those are much more involved. The C program The job of this non-Java application is to manage the list of email addresses. Standard input will accept an email address and the program will look up the name in the list to see if it’s already there. If not, it will add it and report success, but if the name is already there it will report that. Don’t worry if you don’t completely understand what the following code means; it’s just one example of how you can write a program in another language and use it from Java, but the particular programming language doesn’t really matter as long as it can read from standard input and write to standard output. //: Listmgr.c // Used by NameCollector.java to manage // the email list file on the server #include <stdio.h> #include <stdlib.h> #include <string.h> #define BSIZE 250 int alreadyInList(FILE* list, char* name) { char lbuf[BSIZE]; // Go to the beginning of the list: fseek(list, 0, SEEK_SET); // Read each line in the list: while(fgets(lbuf, BSIZE, list)) { // Strip off the newline: char * newline = strchr(lbuf, 'n'); 590 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 591. if(newline != 0) *newline = '0'; if(strcmp(lbuf, name) == 0) return 1; } return 0; } int main() { char buf[BSIZE]; FILE* list = fopen("emlist.txt", "a+t"); if(list == 0) { perror("could not open emlist.txt"); exit(1); } while(1) { gets(buf); /* From stdin */ if(alreadyInList(list, buf)) { printf("Already in list: %s", buf); fflush(stdout); } else { fseek(list, 0, SEEK_END); fprintf(list, "%sn", buf); fflush(list); printf("%s added to list", buf); fflush(stdout); } } } ///:~ This assumes that the C compiler accepts ‘//’ style comments (many do, and you can also compile this program with a C++ compiler). If yours doesn’t, simply delete those comments. The first function in the file checks to see whether the name you hand it as a second argument (a pointer to a char) is in the file. Here, the file is passed as a FILE pointer to an already-opened file (the file is opened inside main( )). The function fseek( ) moves around in the file; here it is used to move to the top of the file. fgets( ) reads a line from the file list into the buffer lbuf, not exceeding the buffer size BSIZE. This is inside a while loop so that each line in the file is read. Next, strchr( ) is used to locate the newline character so that it can be stripped off. Finally, strcmp( ) is used to compare the name you’ve passed into the function to the current line int the file. strcmp( ) returns zero if it finds a match. In this case the function exits and a one is returned to indicate that yes, the name was already in the list (notice the function returns as soon as it discovers the match, so it doesn’t waste time looking at the rest of the list). If you get all the way through the list without a match, the function returns zero. In main( ), the file is opened using fopen( ). The first argument is the file name and the second is the way to open the file; a+ means “append, and open (or create if the file does not exist) for update at the end of the file.” The fopen( ) function returns a FILE pointer which, if it’s zero, means the open was unsuccessful. This is dealt with by printing an error message with perror( ) and terminating the program with exit( ). Assuming the file was opened successfully, the program enters an infinite loop. The function call gets(buf) gets a line from standard input (which will be connected to the Java program, remember) and places it in the buffer buf. This is simply passed to the Chapter 15: Network Programming 591
- 592. alreadyInList( ) function, and if it’s already in the list, printf( ) sends that message to standard output (where the Java program is listening). fflush( ) is a way to flush the output buffer. If the name is not already in the list, fseek( ) is used to move to the end of the list and fprintf( ) “prints” the name to the end of the list. Then printf( ) is used to indicate that the name was added to the list (again flushing standard output) and the infinite loop goes back to waiting for a new name. Remember that you normally cannot compile this program on your computer and just load it onto the Web server machine, since that machine may use a different processor and operating system. For example, my Web server runs on an Intel processor but it uses Linux, so I must download the source code and compile using remote commands (via telnet) with the C compiler that comes with the Linux distribution. The Java program This program will first start the above C program and make the necessary connections to talk to it. Then it will create a datagram socket which will be used to listen for datagram packets from the applet. //: NameCollector.java // Extracts email names from datagrams and stores // them inside a file, using Java 1.02. import java.net.*; import java.io.*; import java.util.*; public class NameCollector { final static int COLLECTOR_PORT = 8080; final static int BUFFER_SIZE = 1000; byte buf[] = new byte[BUFFER_SIZE]; DatagramPacket dp = new DatagramPacket(buf, buf.length); // Can listen & send on the same socket: DatagramSocket socket; Process listmgr; PrintStream nameList; DataInputStream addResult; public NameCollector() { try { listmgr = Runtime.getRuntime().exec("listmgr.exe"); nameList = new PrintStream( new BufferedOutputStream( listmgr.getOutputStream())); addResult = new DataInputStream( new BufferedInputStream( listmgr.getInputStream())); } catch(IOException e) { System.err.println( "Cannot start listmgr.exe"); System.exit(1); } try { socket = 592 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 593. new DatagramSocket(COLLECTOR_PORT); System.out.println( "NameCollector Server started"); while(true) { // Block until a datagram appears: socket.receive(dp); String rcvd = new String(dp.getData(), 0, 0, dp.getLength()); // Send to listmgr.exe standard input: nameList.println(rcvd.trim()); nameList.flush(); byte resultBuf[] = new byte[BUFFER_SIZE]; int byteCount = addResult.read(resultBuf); if(byteCount != -1) { String result = new String(resultBuf, 0).trim(); // Extract the address and port from // the received datagram to find out // where to send the reply: InetAddress senderAddress = dp.getAddress(); int senderPort = dp.getPort(); byte echoBuf[] = new byte[BUFFER_SIZE]; result.getBytes( 0, byteCount, echoBuf, 0); DatagramPacket echo = new DatagramPacket( echoBuf, echoBuf.length, senderAddress, senderPort); socket.send(echo); } else System.out.println( "Unexpected lack of result from " + "listmgr.exe"); } } catch(SocketException e) { System.err.println("Can't open socket"); System.exit(1); } catch(IOException e) { System.err.println("Communication error"); e.printStackTrace(); } } public static void main(String[] args) { new NameCollector(); } } ///:~ The first definitions in NameCollector should look familiar: the port is chosen, a datagram packet is created, and there’s a handle to a DatagramSocket. The next three definitions concern the connection to the C program: a Process object is what comes back when the C program is fired up by the Java program, and that Process object produces the InputStream and OutputStream objects representing, respectively, the standard output Chapter 15: Network Programming 593
- 594. and standard input of the C program. These must of course be “wrapped” as is usual with Java IO, so we end up with a PrintStream and DataInputStream. All the work for this program happens inside the constructor. To start up the C program, the current Runtime object is procured. This is used to call exec( ), which returns the Process object. You can see that there are simple calls to produce the streams from the Process object: getOutputStream( ) and getInputStream( ). From this point on, all you need to consider is sending data to the stream nameList and getting the results from addResult. As before, a DatagramSocket is connected to a port. Inside the infinite while loop, the program calls receive( ), which blocks until a datagram shows up. When the datagram appears, its contents are extracted into the String rcvd. This is trimmed to remove white space at each end, and sent to the C program in the line: nameList.println(rcvd.trim()); This emphasizes how simple it is to interact with a non-Java program (as long as it uses standard input and output). Capturing the result from the C program is slightly more complicated. You must call read( ) and provide a buffer where the results will be placed. The return value for read( ) is the number of bytes that came from the C program, and if this value is -1 it means something is wrong. Otherwise, the resultBuf is turned into a String and the spaces are trimmed off. This string is then placed into a DatagramPacket as before, and shipped back to the same address which sent the request in the first place – notice that the sender’s address is part of the DatagramPacket we received. Remember that although the C program must be compiled on the Web server, the Java program can be compiled anywhere since the resulting byte codes will be the same regardless of the platform on which the program will be running. The NameSender applet As mentioned earlier, the applet must be written with Java 1.0 so that it will run on the largest number of browsers, and this means that it’s best if the number of classes produced is minimized. Thus, instead of using the Dgram class developed earlier, all the datagram manipulations will be placed in line. In addition, the applet needs a thread to listen for the reply from the server, and instead of making this a separate thread it’s integrated into the applet by implementing the Runnable interface. This isn’t as easy to read, but it produces a one-class (and one-server-hit) applet: //: NameSender.java // An applet that sends an email address // as a datagram, using Java 1.02. import java.awt.*; import java.applet.*; import java.net.*; import java.io.*; public class NameSender extends Applet implements Runnable { private Thread pl = null; private Button send = new Button( "Add email address to mailing list"); private TextField t = new TextField( "type your email address here", 40); 594 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 595. private String str = new String(); private Label l = new Label(), l2 = new Label(); private DatagramSocket s; private InetAddress hostAddress; private byte buf[] = new byte[NameCollector.BUFFER_SIZE]; private DatagramPacket dp = new DatagramPacket(buf, buf.length); private int vcount = 0; public void init() { setLayout(new BorderLayout()); Panel p = new Panel(); p.setLayout(new GridLayout(2, 1)); p.add(t); p.add(send); add("North", p); Panel labels = new Panel(); labels.setLayout(new GridLayout(2, 1)); labels.add(l); labels.add(l2); add("Center", labels); try { // Auto-assign port number: s = new DatagramSocket(); hostAddress = InetAddress.getByName( getCodeBase().getHost()); } catch(UnknownHostException e) { l.setText("Cannot find host"); } catch(SocketException e) { l.setText("Can't open socket"); } l.setText("Ready to send your email address"); } public boolean action (Event evt, Object arg) { if(evt.target.equals(send)) { if(pl != null) { // pl.stop(); Deprecated in Java 1.2 Thread remove = pl; pl = null; remove.interrupt(); } l2.setText(""); // Check for errors in email name: str = t.getText().toLowerCase().trim(); if(str.indexOf(' ') != -1) { l.setText("Spaces not allowed in name"); return true; } if(str.indexOf(',') != -1) { l.setText("Commas not allowed in name"); return true; } if(str.indexOf('@') == -1) { l.setText("Name must include '@'"); l2.setText(""); Chapter 15: Network Programming 595
- 596. return true; } if(str.indexOf('@') == 0) { l.setText("Name must preceed '@'"); l2.setText(""); return true; } String end = str.substring(str.indexOf('@')); if(end.indexOf('.') == -1) { l.setText("Portion after '@' must " + "have an extension, such as '.com'"); l2.setText(""); return true; } // Everything's OK, so send the name. Get a // fresh buffer, so it's zeroed. For some // reason you must use a fixed size rather // than calculating the size dynamically: byte sbuf[] = new byte[NameCollector.BUFFER_SIZE]; str.getBytes(0, str.length(), sbuf, 0); DatagramPacket toSend = new DatagramPacket( sbuf, 100, hostAddress, NameCollector.COLLECTOR_PORT); try { s.send(toSend); } catch(Exception e) { l.setText("Couldn't send datagram"); return true; } l.setText("Sent: " + str); send.setLabel("Re-send"); pl = new Thread(this); pl.start(); l2.setText( "Waiting for verification " + ++vcount); } else return super.action(evt, arg); return true; } // The thread portion of the applet watches for // the reply to come back from the server: public void run() { try { s.receive(dp); } catch(Exception e) { l2.setText("Couldn't receive datagram"); return; } l2.setText(new String(dp.getData(), 0, 0, dp.getLength())); } } ///:~ 596 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 597. The UI for the applet is quite simple. There’s a TextField where you type your email address, and a Button to send the email address to the server. Two Labels are used to report status back to the user. By now you can recognize the DatagramSocket, InetAddress, buffer and DatagramPacket as trappings of the network connection. Lastly you can see the run( ) method that implements the thread portion so the applet can listen for the reply sent back by the server. The init( ) method sets up the GUI with the familiar layout tools, then creates the DatagramSocket which will be used both for sending and receiving datagrams. The action( ) method (remember we’re confined to Java 1.0 now, so we can’t use any slick inner listener classes) watches only to see if you press the “send” button. When the button is pressed, the first action is to check the Thread pl to see if it’s null. If it’s not null, there’s a live thread running. The first time the message is sent a thread is started up to watch for the reply. Thus, if a thread is running, it means this is not the first time the user has tried to send the message. The pl handle is set to null and the old listener is interrupted (this is the preferred approach, since stop( ) is deprecated in Java 1.2 as explained in the previous chapter). Regardless of whether this is the first time the button was pressed, the text in l2 is erased. The next group of statements checks the email name for errors. The String.indexOf( ) method is used to search for illegal characters, and if one is found it is reported to the user. Notice that all this happens without any network activity, so it’s fast and it doesn’t bog down the Internet. Once the name is verified, it is packaged into a datagram and sent to the host address and port number in the same way that was described in the earlier datagram example. The first label is changed to show you that the send has occurred, and the button text is changed so that it reads “re-send.” At this point, the thread is started up and the second label informs you that the applet is waiting for a reply from the server. The run( ) method for the thread uses the DatagramSocket that lives in NameSender to receive( ), which blocks until the datagram packet comes from the server. The resulting packet is placed into NameSender’s DatagramPacket dp. The data is retrieved from the packet and placed into the second label in NameSender. At this point, the thread terminates and becomes dead. If the reply doesn’t come back from the server in a reasonable amount of time, the user may become impatient and press the button again, thus terminating the current thread (and, after re-sending the data, starting a new one). Because a thread is used to listen for the reply, the user still has full use of the UI. The Web page Of course, the applet must go inside a Web page. This is the complete Web page; you can see that it’s intended to be used to automatically collect names for my mailing list: <HTML> <HEAD> <META CONTENT="text/html"> <TITLE>Add Yourself to Bruce Eckel's Java Mailing List</TITLE> </HEAD> <BODY LINK="#0000ff" VLINK="#800080" BGCOLOR="#ffffff"> <FONT SIZE=6><P>Add Yourself to Bruce Eckel's Java Mailing List</P></FONT> The applet on this page will automatically add your email address to the mailing list, so you will receive update information about changes to the online version of "Thinking in Java," notification when the book is in print, information about upcoming Java seminars, and notification about the "Hands-on Java Seminar" Multimedia CD. Chapter 15: Network Programming 597
- 598. Type in your email address and press the button to automatically add yourself to this mailing list. <HR> <applet code=NameSender width=400 height=100> </applet> <HR> If after several tries, you do not get verification it means that the Java application on the server is having problems. In this case, you can add yourself to the list by sending email to <A HREF="mailto:Bruce@EckelObjects.com">Bruce@EckelObjects.com</A> </HTML> The applet tag itself is quite trivial, no different from the very first one presented in the book. Problems with this approach This certainly seems like an elegant approach. There’s no CGI programming and so there are no delays while the server starts up a CGI program. The datagram approach seems to produce a nice quick response. In addition, when Java 1.1 is available everywhere, the server portion can be written entirely in Java (although it’s quite interesting to see how easy it is to connect to a non-Java program using standard input and output). There are problems, however. One problem is rather subtle: since the Java application is running constantly on the server and it spends most of its time blocked in the Datagram.receive( ) method, there may be some CPU hogging going on. At least, that’s the way it appeared on the server where I was experimenting. On the other hand, there wasn’t much else happening on that server, and starting the program using “nice” (a Unix program to prevent a process from hogging the CPU) or its equivalent may solve the problem if you have a more heavily-loaded server. In any event, it’s worth keeping your eye on an application like this – a blocked receive( ) may hog the CPU. The second problem is a show-stopper. It concerns firewalls. A firewall is a machine that sits between your network and the Internet. It monitors all traffic coming in from the Internet and going out to the Internet, and makes sure that traffic conforms to what it expects. Firewalls are conservative little beasts. They demand strict conformance to all the rules, and if you’re not conforming they assume you’re doing something sinful, and shut you out (not quite so bad as the Spanish Inquisition, but close). For example, if you are on a network behind a firewall and you start connecting to the Internet using a Web browser, the firewall expects that all your transactions will connect to the server using the accepted http port, which is 80. Now here comes this Java applet NameSender which is trying to send a datagram to port 8080, which is way outside the range of the “protected” ports 0- 1024. The firewall naturally assumes the worst, that someone has a virus, and so it doesn’t allow the transaction to happen. As long as your customers have raw connections to the Internet (for example, using a typical Internet Service Provider) there’s no problem, but you may have some important customers dwelling behind firewalls, and they won’t be able to use your program. This is rather disheartening after learning so much Java, because it would seem that you must give up Java on the server and learn how to write CGI scripts in C or Perl. But as it turns out, despair is not in order. One scenario is part of Sun's grand scheme. If everything goes as planned, Web servers will be equipped with servlet servers. These will take a request from the client (going 598 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 599. through the firewall-accepted port 80) and instead of starting up a CGI program, they will start up a Java program called a servlet. This is a little application that’s designed to run only on the server. A servlet server will automatically start up the servlet to handle the client request, which means you can write all your programs only in Java (further enabling the “100% pure Java initiative”). It is admittedly an appealing idea: once you’re comfortable with Java, you don’t have to switch to a more primitive language to handle requests on the server. Since it’s only for handling requests on the server, the servlet API has no GUI abilities. This fits quite well with NameCollector.java, which doesn’t have a GUI anyway. At this writing, a low-cost servlet server was available from java.sun.com. In addition, Sun is encouraging other Web server manufacturers to add servlet capabilities to their servers. Connecting Java to CGI A more immediate and elegant solution to the problem already exists in the Java networking libraries. A Java program can send a CGI request to a server, just like an HTML page can. As with HTML pages, this request can be either a GET or a POST. In addition, the Java program can intercept the output of the CGI program, so you don’t have to rely on the program to format a new page and force the user to back up from one page to another if something goes wrong. In fact, the appearance of the program can be just the same as the previous version. It also turns out that the code is simpler, and that CGI isn’t very difficult to write after all (an innocent statement that’s true of many things – after you understand them). So in this section you’ll get a crash course in CGI programming. To solve the general problem some CGI tools will be created in C++ which will allow you to easily write a CGI program to solve any problem. The benefit to this approach is portability – the example you are about to see will work on any system that supports CGI, and there’s no problem with firewalls. This example also works out the basics of creating any connection with applets and CGI programs, so you can easily adapt it to your own projects. Encoding data for CGI In this version, the name and the email address will be collected, and stored in the file in the form: First Last <email@domain.com>; This is a convenient form for many mailers. Since two fields are being collected, there are no shortcuts because CGI has a particular format for encoding the data in fields. You can see this for yourself if you make an ordinary HTML page and add the lines: <Form method="GET" ACTION="/cgi-bin/Listmgr2.exe"> <P>Name: <INPUT TYPE = "text" NAME = "name" VALUE = "" size = "40"></p> <P>Email Address: <INPUT TYPE = "text" NAME = "email" VALUE = "" size = "40"></p> <p><input type = "submit" name = "submit" > </p> </Form> This creates two data entry fields called name and email, along with a submit button that collects the data and sends it to a CGI program. Listmgr2.exe is the name of the Chapter 15: Network Programming 599
- 600. executable program that resides in the directory that’s typically called “cgi-bin” on your Web server3 (if the named program is not in the cgi-bin directory, you won’t see any results at all). If you fill out this form and press the “submit” button, you will see in the URL address window of the browser something like: http://www.myhome.com/cgi-bin/Listmgr2.exe? name=First+Last&email=email@domain.com&submit=Submit (Without the line break, of course). Here you see a little bit of the way that data is encoded to send to CGI. For one thing, there are no spaces allowed (since spaces typically separate command-line arguments). Spaces are replaced by ‘+’ signs. In addition, each field contains the field name (which is determined by the HTML page) followed by an ‘=’ and the field data, and terminated by a ‘&’. At this point you may wonder about the ‘+’, ‘=’ and ‘&’. What if those are used in the field, as in “John & Marsha Smith”? This is encoded to: John+%26+Marsha+Smith That is, the special character is turned into a ‘%’ followed by its ASCII value in hex. Fortunately, Java has a tool to perform this encoding for you. It’s a static method of the class URLEncoder called encode( ). You can experiment with this method using the following program: //: EncodeDemo.java // Demonstration of URLEncoder.encode() import java.net.*; public class EncodeDemo { public static void main(String args[]) { String s = ""; for(int i = 0; i < args.length; i++) s += args[i] + " "; s = URLEncoder.encode(s.trim()); System.out.println(s); } } ///:~ This takes the command-line arguments and combines them into a string of words separated by spaces (the final space is removed using String.trim( )). These are then encoded and printed. To invoke a CGI program, then, all the applet needs to do is collect the data from its fields (or wherever it needs to collect the data from), URL-encode each piece of data, and then assemble it into a single string, placing the name of each field followed by an ‘=’, followed by the data, followed by an ‘&’. To form the entire CGI command, this string is placed after the URL of the CGI program and a ‘?’. That’s all it takes to invoke any CGI program, and as you’ll see you can easily do it within an applet. 3 You can test this under Windows32 using the Microsoft Personal Web Server that comes with Microsoft Office 97 and some of their other products. This is a nice way to experiment since you can perform local tests (and it's also very fast). If you're on a different platform or you don't have Office 97 you may be able to find a freeware Web server for testing by searching the Internet. 600 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 601. The applet The applet is actually considerably simpler than NameSender.java, partly because it’s so easy to send a GET request and also because no thread is required to wait for the reply (apparently, the server honors only one CGI request to a particular program at one time. At least, I tried making them collide by running more than one at once at it seemed unfazed). There are now two fields instead of one, but you’ll notice that much of the applet looks familiar, from NameSender.java. //: NameSender2.java // An applet that sends an email address // via a CGI GET, using Java 1.02. import java.awt.*; import java.applet.*; import java.net.*; import java.io.*; public class NameSender2 extends Applet { final String CGIProgram = "Listmgr2.exe"; Button send = new Button( "Add email address to mailing list"); TextField name = new TextField( "type your name here", 40), email = new TextField( "type your email address here", 40); String str = new String(); Label l = new Label(), l2 = new Label(); int vcount = 0; public void init() { setLayout(new BorderLayout()); Panel p = new Panel(); p.setLayout(new GridLayout(3, 1)); p.add(name); p.add(email); p.add(send); add("North", p); Panel labels = new Panel(); labels.setLayout(new GridLayout(2, 1)); labels.add(l); labels.add(l2); add("Center", labels); l.setText("Ready to send email address"); } public boolean action (Event evt, Object arg) { if(evt.target.equals(send)) { l2.setText(""); // Check for errors in data: if(name.getText().trim() .indexOf(' ') == -1) { l.setText( "Please give first and last name"); l2.setText(""); return true; } str = email.getText().trim(); if(str.indexOf(' ') != -1) { Chapter 15: Network Programming 601
- 602. l.setText( "Spaces not allowed in email name"); l2.setText(""); return true; } if(str.indexOf(',') != -1) { l.setText( "Commas not allowed in email name"); return true; } if(str.indexOf('@') == -1) { l.setText("Email name must include '@'"); l2.setText(""); return true; } if(str.indexOf('@') == 0) { l.setText( "Name must preceed '@' in email name"); l2.setText(""); return true; } String end = str.substring(str.indexOf('@')); if(end.indexOf('.') == -1) { l.setText("Portion after '@' must " + "have an extension, such as '.com'"); l2.setText(""); return true; } // Build and encode the email data: String emailData = "name=" + URLEncoder.encode( name.getText().trim()) + "&email=" + URLEncoder.encode( email.getText().trim().toLowerCase()) + "&submit=Submit"; // Send the name using CGI's GET process: try { l.setText("Sending..."); URL u = new URL( getDocumentBase(), "cgi-bin/" + CGIProgram + "?" + emailData); l.setText("Sent: " + email.getText()); send.setLabel("Re-send"); l2.setText( "Waiting for reply " + ++vcount); DataInputStream server = new DataInputStream(u.openStream()); String line; while((line = server.readLine()) != null) l2.setText(line); } catch(MalformedURLException e) { l.setText("Bad URl"); } catch(IOException e) { l.setText("IO Exception"); 602 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 603. } } else return super.action(evt, arg); return true; } } ///:~ The name of the CGI program (which you’ll see later) is Listmgr2.exe. Many Web servers are Unix machines (mine runs Linux) which don’t traditionally use the .exe extension for their executable programs, but you can call the program anything you want under Unix. By using the .exe extension the program can be tested without change under both Unix and Win32. As before, the applet sets up its user interface (with two fields this time, instead of one). The only significant difference occurs inside the action( ) method, which handles the button press. After the name has been checked, you see the lines: String emailData = "name=" + URLEncoder.encode( name.getText().trim()) + "&email=" + URLEncoder.encode( email.getText().trim().toLowerCase()) + "&submit=Submit"; // Send the name using CGI's GET process: try { l.setText("Sending..."); URL u = new URL( getDocumentBase(), "cgi-bin/" + CGIProgram + "?" + emailData); l.setText("Sent: " + email.getText()); send.setLabel("Re-send"); l2.setText( "Waiting for reply " + ++vcount); DataInputStream server = new DataInputStream(u.openStream()); String line; while((line = server.readLine()) != null) l2.setText(line); // ... The name and email data is extracted from their respective text boxes, and the spaces are trimmed off both ends using trim( ). The email name is forced to lower case so all email addresses in the list can be accurately compared (to prevent accidental duplicates based on capitalization). The data from each field is URL-encoded, and then the GET string is assembled in exactly the same way that an HTML page would do it (this way you can use a Java applet in concert with any existing CGI program designed to work with regular HTML GET requests). At this point, some Java magic happens: if you want to connect to any URL, all you need to do is create a URL object and hand the address to the constructor. The constructor makes the connection with the server (and, with Web servers, all the action happens in making the connection, via the string used as the URL). In this case, the URL points to the cgi-bin directory of the current Web site (the base address of the current Web site is produced with getDocumentBase( )). When the web server sees "cgi-bin" in a URL, it expects that to be followed by the name of the program inside the cgi-bin directory that you want it to run. Following the program name is a question mark and the argument Chapter 15: Network Programming 603
- 604. string that the CGI program will look for in the QUERY_STRING environment variable, as you’ll see. Normally, when you make any sort of request, what you get back (what you’re forced to accept in return) is an HTML page. With Java URL objects, however, you can intercept anything that comes back from the CGI program by getting an InputStream from the URL object. This is performed with the URL openStream( ) method, which is in turn wrapped in a DataInputStream. Then you can read lines, and when readLine( ) returns null the CGI program has finished its output. The CGI program you’re about to see returns only one line, a string indicating success or failure (and the details of the failure). This line is captured and placed into the second Label field so the user can see the results. Displaying a Web page from within an applet It’s also possible for the applet to display the result of the CGI program as a Web page, just as if it were running in normal HTML mode. You can do this with the following line: getAppletContext().showDocument(u); Where u is the URL object. Here’s a very simple example which just redirects you to another Web page. The page happens to be the output of a CGI program, but you can as easily go to an ordinary HTML page, so you could build on this applet to produce a password-protected gateway to a particular portion of your Web site: //: ShowHTML.java import java.awt.*; import java.applet.*; import java.net.*; import java.io.*; public class ShowHTML extends Applet { static final String CGIProgram = "MyCGIProgram"; Button send = new Button("Go"); Label l = new Label(); public void init() { add(send); add(l); } public boolean action (Event evt, Object arg) { if(evt.target.equals(send)) { try { // This could be an HTML page instead of // a CGI program. Note that this CGI // program doesn't use arguments, but // you can add them in the usual way. URL u = new URL( getDocumentBase(), "cgi-bin/" + CGIProgram); // Display the output of the URL using // the Web browser, as an ordinary page: getAppletContext().showDocument(u); } catch(Exception e) { l.setText(e.toString()); } } 604 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 605. else return super.action(evt, arg); return true; } } ///:~ The beauty of the URL class is how much it shields you from. You can connect to Web servers without knowing much at all about what’s going on under the covers. The CGI program in C++ At this point you could follow the previous example and write the CGI program for the server using ANSI C. One argument for doing this is that ANSI C can be found virtually everywhere. However, C++ has become quite ubiquitous, especially in the form of the GNU C++ Compiler4 (g++) which can be downloaded free from the Internet for virtually any platform (and often comes pre-installed with operating systems such as Linux). As you will see, this means you can get the benefit of object-oriented programming in a CGI program. To avoid throwing too many new concepts at you all at once, this program will not be a “pure” C++ program; some code will be written in plain C even though C++ alternatives exist. This isn’t a significant issue because the biggest benefit in using C++ for this program is the ability to create classes. Since what we’re concerned with when parsing the CGI information is the field name-value pairs, one class (Pair) will be used to represent a single name-value pair, and a second class (CGI_vector) will automatically parse the CGI string into Pair objects which it will hold (as a vector) so you can fetch each Pair out at your leisure. This program is also interesting because it demonstrates some of the pluses and minuses of C++ in contrast with Java. You’ll see some similarities; for example the class keyword. Access control has identical keywords public and private, but they’re used differently: they control a block instead of a single method or field (that is, if you say private: each following definition is private until you say public:). Also, when you create a class, all the definitions automatically default to private. One of the reasons for using C++ here is the convenience of the C++ Standard Template Library (STL). Among other things, the STL contains a vector class. This is a C++ template, which means it will be configured at compile time so it will hold objects of only a particular type (in this case, Pair objects). Unlike the Java Vector, which will accept anything, the C++ vector template will cause a compile-time error message if you try to put anything but a Pair object into the vector, and when you get something out of the vector it will automatically be a Pair object, without casting. Thus, the checking happens at compile time and produces a more robust program. In addition, the program can run faster since you don’t have to perform run-time casts. The vector also overloads the operator[] so you have a convenient syntax for extracting Pair objects. The vector template will be used in the creation of CGI_vector, which you’ll see is a fairly short definition considering how powerful it is. On the down side, look at the complexity of the definition of Pair in the following code. Pair has more method definitions than you’re used to seeing in Java code, because the C++ programmer must know how to control copying with the copy-constructor and 4 GNU stands for “Gnu’s Not Unix.” The project, created by the Free Software Foundation, was originally intended to replace the Unix operating system with a free version of that OS. Linux appears to have replaced this initiative, but the GNU tools have played an integral part in the development of Linux which comes packaged with many GNU components. Chapter 15: Network Programming 605
- 606. assignment with the overloaded operator=. As described in Chapter 12, occasionally you need to concern yourself with similar things in Java, but in C++ you must be aware of them almost constantly. The project will start with a reusable portion, which consists of Pair and CGI_vector in a C++ header file. Technically, you shouldn’t cram this much into a header file, but for these examples it doesn’t hurt anything and it will also look more Java-like, so it will be easier for you to read: //: CGITools.h // Automatically extracts and decodes data // from CGI GETs and POSTs. Tested with GNU C++ // (available for most server machines). #include <string.h> #include <vector> // STL vector using namespace std; // A class to hold a single name-value pair from // a CGI query. CGI_vector holds Pair objects and // returns them from its operator[]. class Pair { char* nm; char* val; public: Pair() { nm = val = 0; } Pair(char* name, char* value) { // Creates new memory: nm = decodeURLString(name); val = decodeURLString(value); } const char* name() const { return nm; } const char* value() const { return val; } // Test for "emptiness" bool empty() const { return (nm == 0) || (val == 0); } // Automatic type conversion for boolean test: operator bool() const { return (nm != 0) && (val != 0); } // The following constructors & destructor are // necessary for bookkeeping in C++. // Copy-constructor: Pair(const Pair& p) { if(p.nm == 0 || p.val == 0) { nm = val = 0; } else { // Create storage & copy rhs values: nm = new char[strlen(p.nm) + 1]; strcpy(nm, p.nm); val = new char[strlen(p.val) + 1]; strcpy(val, p.val); } } // Assignment operator: Pair& operator=(const Pair& p) { 606 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 607. // Clean up old lvalues: delete nm; delete val; if(p.nm == 0 || p.val == 0) { nm = val = 0; } else { // Create storage & copy rhs values: nm = new char[strlen(p.nm) + 1]; strcpy(nm, p.nm); val = new char[strlen(p.val) + 1]; strcpy(val, p.val); } return *this; } ~Pair() { // Destructor delete nm; // 0 value OK delete val; } // If you use this method outide this class, // you're responsible for calling 'delete' on // the pointer that's returned: static char* decodeURLString(const char* URLstr) { int len = strlen(URLstr); char* result = new char[len + 1]; memset(result, len + 1, 0); for(int i = 0, j = 0; i <= len; i++, j++) { if(URLstr[i] == '+') result[j] = ' '; else if(URLstr[i] == '%') { result[j] = translateHex(URLstr[i + 1]) * 16 + translateHex(URLstr[i + 2]); i += 2; // Move past hex code } else // An ordinary character result[j] = URLstr[i]; } return result; } // Translate a single hex character; used by // decodeURLString(): static char translateHex(char hex) { if(hex >= 'A') return (hex & 0xdf) - 'A' + 10; else return hex - '0'; } }; // Parses any CGI query and turns it // into an STL vector of Pair objects: class CGI_vector : public vector<Pair> { char* qry; const char* start; // Save starting position // Prevent assignment and copy-construction: void operator=(CGI_vector&); Chapter 15: Network Programming 607
- 608. CGI_vector(CGI_vector&); public: // const fields must be initialized in the C++ // "Constructor initializer list": CGI_vector(char* query) : start(new char[strlen(query) + 1]) { qry = (char*)start; // Cast to non-const strcpy(qry, query); Pair p; while((p = nextPair()) != 0) push_back(p); } // Destructor: ~CGI_vector() { delete start; } private: // Produces name-value pairs from the query // string. Returns an empty Pair when there's // no more query string left: Pair nextPair() { char* name = qry; if(name == 0 || *name == '0') return Pair(); // End, return null Pair char* value = strchr(name, '='); if(value == 0) return Pair(); // Error, return null Pair // Null-terminate name, move value to start // of its set of characters: *value = '0'; value++; // Look for end of value, marked by '&': qry = strchr(value, '&'); if(qry == 0) qry = ""; // Last pair found else { *qry = '0'; // Terminate value string qry++; // Move to next pair } return Pair(name, value); } }; ///:~ After the #include statements, you see a line that says using namespace std; Namespaces in C++ solve one of the problems taken care of by the package scheme in Java: hiding library names. The std namespace refers to the Standard C++ library, and vector is in this library so the line is required. The Pair class starts out looking pretty simple: it just holds two (private) character pointers, one for the name and one for the value. The default constructor simply sets these pointers to zero, since in C++ an object’s memory isn’t automatically zeroed. The second constructor calls the method decodeURLString( ) which produces a decoded string in newly-allocated heap memory. This memory must be managed and destroyed by the object, as you shall see in the destructor. The name( ) and value( ) methods produce read-only pointers to the respective fields. The empty( ) method is a way for you to ask the Pair object whether either of its fields are empty; it returns bool which is C++’s 608 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 609. built-in primitive Boolean data type. The operator bool( ) uses a special case of C++ operator overloading, which allows you to control automatic type conversion. If you have a Pair object called p and you use it in an expression where a Boolean result is expected, such as if(p) { //…, then the compiler will recognize that it has a Pair and it needs a Boolean, so it will automatically call operator bool( ) to perform the necessary conversion. The next three methods are part of the bookkeeping that’s necessary when you create a class in C++. In the so-called “canonical form” for a C++ class, you must define the necessary “ordinary” constructors as well as a copy-constructor and the assignment operator, operator= (and the destructor, to clean up the memory). You must define these because they can be quietly called by the compiler when you pass objects in and out of a function (this calls the copy-constructor) or when you assign objects (the assignment operator). Once you’ve gone through the trouble to understand how the copy-constructor and assignment operator work you can write robust classes in C++, but it is a bit of a learning experience5 . The copy-constructor Pair(const Pair&) is automatically called whenever you pass an object into or out of a function by value. That is, you aren’t passing the address of the object, you’re making a copy of the whole object inside the function frame. This isn’t an option in Java since you pass only handles, thus there’s no copy-constructor in Java (if you want to make a local duplicate, you clone( ) the object – see Chapter 12). Likewise, if you assign a handle in Java, it’s simply copied. But assignment in C++ means the entire object is copied. In the copy-constructor, you create new storage and copy the source data, but with the assignment operator you must release the old storage before allocating new storage. What you’re seeing is probably the worst-case complexity scenario for a C++ class, but it’s one of the reasons Java proponents can argue that Java is a lot simpler than C++. In Java you pass handles and there’s a garbage collector, so you don’t have to do this kind of thing. This isn’t quite the whole story. The Pair class is using char* for nm and val, and the worst-case complexity occurs primarily around pointers. If instead you use the more modern Standard C++ string class instead of char*, things get much simpler (however, not all compilers have caught up enough to come with string). Then, the first part of Pair looks like this: class Pair { string nm; string val; public: Pair() { } Pair(char* name, char* value) { // Creates new memory: nm = decodeURLString(name); val = decodeURLString(value); } const char* name() const { return nm.c_str(); } const char* value() const { return val.c_str(); } // Test for "emptiness" bool empty() const { return (nm.length() == 0) || (val.length() == 0); } 5 My book Thinking in C++ (Prentice-Hall, 1995) devotes an entire chapter to this subject. Refer to this if you need further information on the subject. Chapter 15: Network Programming 609
- 610. // Automatic type conversion for boolean test: operator bool() const { return (nm.length() != 0) && (val.length() != 0); } (Also, for this case decodeURLString( ) returns a string instead of a char*). You don’t need to define a copy-constructor, operator= or destructor because the compiler does that for you, and does it correctly. But even if it sometimes works automatically, C++ programmers must still know the details of copy-construction and assignment. The remainder of the Pair class consists of the two methods decodeURLString( ) and a helper method translateHex( ) which is used by decodeURLString( ). After allocating adequate storage (which must be released by the destructor), decodeURLString( ) moves through and replaces each ‘+’ with a space, and each hex code (beginning with a ‘%’) with the appropriate character. CGI_vector parses and holds an entire CGI GET command. It is inherited from the STL vector which is instantiated to hold Pairs. Inheritance in C++ is denoted by using a colon where you’d say extends in Java. In addition, inheritance defaults to private so you’ll almost always need to use the public keyword as was done here. You can also see that CGI_vector has a copy-constructor and an operator=, but they’re both declared as private. This is to prevent the compiler from synthesizing the two functions (which it will do if you don’t declare them yourself), but it also prevents the client programmer from passing a CGI_vector by value or from using assignment. CGI_vector’s job is to take the QUERY_STRING and parse it into name-value pairs, which it will do with the aid of Pair. First it copies the string into locally-allocated memory and keeps track of the starting address with the constant pointer start (this is later used in the destructor to release the memory). Then it uses its method nextPair( ) to parse the string into raw name-value pairs, delimited by ‘=’ and ‘&’ signs. These are handed by nextPair( ) to the Pair constructor so nextPair( ) can return the Pair object which is then added to the vector with push_back( ). When nextPair( ) runs out of QUERY_STRING, it returns zero. Now that the basic tools are defined, they can easily be used in a CGI program, like this: //: Listmgr2.cpp // CGI version of Listmgr.c in C++, which // extracts its input via the GET submission // from the associated applet. Also works as // an ordinary CGI program with HTML forms. #include <stdio.h> #include "CGITools.h" const char* dataFile = "list2.txt"; const char* notify = "Bruce@EckelObjects.com"; #undef DEBUG // Similar code as before, except that it looks // for the email name inside of '<>': int inList(FILE* list, const char* emailName) { const int BSIZE = 255; char lbuf[BSIZE]; char emname[BSIZE]; // Put the email name in '<>' so there's no // possibility of a match within another name: sprintf(emname, "<%s>", emailName); // Go to the beginning of the list: 610 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 611. fseek(list, 0, SEEK_SET); // Read each line in the list: while(fgets(lbuf, BSIZE, list)) { // Strip off the newline: char * newline = strchr(lbuf, 'n'); if(newline != 0) *newline = '0'; if(strstr(lbuf, emname) != 0) return 1; } return 0; } void main() { // You MUST print this out, otherwise the // server will not send the response: printf("Content-type: text/plainnn"); FILE* list = fopen(dataFile, "a+t"); if(list == 0) { printf("error: could not open database. "); printf("Notify %s", notify); return; } // For a CGI "GET," the server puts the data // in the environment variable QUERY_STRING: CGI_vector query(getenv("QUERY_STRING")); #if defined(DEBUG) // Test: dump all names and values for(int i = 0; i < query.size(); i++) { printf("query[%d].name() = [%s], ", i, query[i].name()); printf("query[%d].value() = [%s]n", i, query[i].value()); } #endif(DEBUG) Pair name = query[0]; Pair email = query[1]; if(name.empty() || email.empty()) { printf("error: null name or email"); return; } if(inList(list, email.value())) { printf("Already in list: %s", email.value()); return; } // It's not in the list, add it: fseek(list, 0, SEEK_END); fprintf(list, "%s <%s>;n", name.value(), email.value()); fflush(list); fclose(list); printf("%s <%s> added to listn", name.value(), email.value()); } ///:~ Chapter 15: Network Programming 611
- 612. The alreadyInList( ) function is almost identical to the previous version, except that it assumes all email names are inside ‘<>’. When you use the GET approach (which is normally done in the HTML METHOD tag of the FORM directive, but which is controlled here by the way the data is sent), the Web server grabs everything after the ‘?’ and puts in into the environment variable QUERY_STRING. So to read that information all you have to do is get the value of QUERY_STRING, which you do using the standard C library function getenv( ). In main( ), notice how simple the act of parsing the QUERY_STRING is: you just hand it to the constructor for the CGI_vector object called query and all the work is done for you. From then on you can pull out the names and values from query as if it were an array (this is because the operator[] is overloaded in vector). You can see how this works in the debug code, which is surrounded by the preprocessor directives #if defined(DEBUG) and #endif(DEBUG). Now it’s important to understand something about CGI. A CGI program is handed its input in one of two ways: through QUERY_STRING during a GET (as in this case) or through standard input during a POST. But a CGI program sends its output through standard output, typically using printf( ) in a C program. Where does this output go? Back to the web server, which decides what to do with it. The server makes this decision based on the content-type header, which means that if the content-type header isn’t the first thing it sees, it won’t know what to do with the data. Thus it’s essential that you start the output of all CGI programs with the content-type header. In this case, we want the server to feed all the information directly back to the client program (which is our applet, waiting for its reply). The information should be unchanged, so the content-type is text/plain. Once the server sees this, it will echo all strings right back to the client. Thus each of the strings you see, three for error conditions and one for a successful add, will end up right back at the applet. Adding the email name uses the same code. In the case of the CGI script, however, there isn’t an infinite loop – the program just responds and then terminates. Each time a CGI request comes in, the program is started in response to that request, and then it shuts down. Thus there is no possibility of CPU hogging, and the only performance issue concerns starting the program up and opening the file, which are dwarfed by the overhead of the Web server as it handles the CGI request. One of the advantages of this design is that, now that Pair and CGI_vector are defined, most of the work is done for you so you can easily create your own CGI program simply by modifying main( ). Eventually, Servlet servers will probably be ubiquitous but in the meantime C++ is still pretty handy for creating fast CGI programs. What about POST? Using a GET is fine for many applications. However, GET passes its data to the CGI program through an environment variable, and some Web servers can run out of environment space with long GET strings (you should start worrying at about 200 characters). CGI provides a solution for this: POST. With POST, the data is encoded and concatenated the same way as a GET, but POST uses standard input to pass the encoded query string to the CGI program. All you have to do is determine the length of the query string, and this length is stored in the environment variable CONTENT_LENGTH. Once you know the length, you can allocate storage and read the precise number of bytes from standard input. The Pair and CGI_vector from CGITools.h can be used as is for a CGI program that handles POSTs. The following listing shows how simple it is to write such a CGI program. 612 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 613. In this example, “pure” C++ will be used so the stdio.h library will be dropped in favor of iostreams. With iostreams, two pre-defined objects are available: cin which connects to standard input, and cout which connects to standard output. There are several ways to read from cin and write to cout, but the following program take the common approach of using ‘<<’ to send information to cout, and the use of a member function (in this case, read( )) to read from cin. //: POSTtest.cpp // CGI_vector works as easily with POST as it // does with GET. Written in "pure" C++. #include <iostream.h> #include "CGITools.h" void main() { cout << "Content-type: text/plainn" << endl; // For a CGI "POST," the server puts the length // of the content string in the environment // variable CONTENT_LENGTH: char* clen = getenv("CONTENT_LENGTH"); if(clen == 0) { cout << "Zero CONTENT_LENGTH" << endl; return; } int len = atoi(clen); char* query_str = new char[len + 1]; cin.read(query_str, len); query_str[len] = '0'; CGI_vector query(query_str); // Test: dump all names and values for(int i = 0; i < query.size(); i++) cout << "query[" << i << "].name() = [" << query[i].name() << "], " << "query[" << i << "].value() = [" << query[i].value() << "]" << endl; delete query_str; // Release storage } ///:~ The getenv( ) function returns a pointer to a character string representing the content length. If this pointer is zero, the CONTENT_LENGTH environment variable has not been set, so something is wrong. Otherwise, the character string must be converted to an integer using the ANSI C library function atoi( ). The length is used with new to allocate enough storage to hold the query string (plus its null terminator), and then read( ) is called for cin. The read( ) function takes a pointer to the destination buffer and the number of bytes to read. The query_str is then null-terminated to indicate the end of the character string. At this point the query string is no different from a GET query string, so it is handed to the constructor for CGI_vector. The different fields in the vector are then available just as in the previous example. To test this program, you must compile it in the cgi-bin directory of your host Web server. Then you can perform a simple test by writing an HTML page like this: <HTML> <HEAD> <META CONTENT="text/html"> <TITLE>A test of standard HTML POST</TITLE> Chapter 15: Network Programming 613
- 614. </HEAD> Test, uses standard html POST <Form method="POST" ACTION="/cgi-bin/POSTtest"> <P>Field1: <INPUT TYPE = "text" NAME = "Field1" VALUE = "" size = "40"></p> <P>Field2: <INPUT TYPE = "text" NAME = "Field2" VALUE = "" size = "40"></p> <P>Field3: <INPUT TYPE = "text" NAME = "Field3" VALUE = "" size = "40"></p> <P>Field4: <INPUT TYPE = "text" NAME = "Field4" VALUE = "" size = "40"></p> <P>Field5: <INPUT TYPE = "text" NAME = "Field5" VALUE = "" size = "40"></p> <P>Field6: <INPUT TYPE = "text" NAME = "Field6" VALUE = "" size = "40"></p> <p><input type = "submit" name = "submit" > </p> </Form> </HTML> When you fill this out and submit it, you’ll get back a simple text page containing the parsed results, so you can see that the CGI program works correctly. Of course, it’s a little more interesting to submit the data using an applet. Submitting POST data is a different process, however. After you invoke the CGI program in the usual way, you must make a direct connection to the server so you can feed it the query string. The server then turns around and feeds the query string to the CGI program via standard input. To make a direct connection to the server, you must take the URL you’ve created and call openConnection( ) to produce a URLConnection. Then, because a URLConnection doesn’t normally allow you to send data to it, you must call the magic function setDoOutput(true), along with setDoInput(true) and setAllowUserInteraction(false)6 . Finally, you can call getOutputStream( ) to produce an OutputStream, which you wrap inside a DataOutputStream so you can talk to it conveniently. Here’s an applet that does just that, after collecting data from its various fields: //: POSTtest.java // An applet that sends its data via a CGI POST import java.awt.*; import java.applet.*; import java.net.*; import java.io.*; public class POSTtest extends Applet { final static int SIZE = 10; Button submit = new Button("Submit"); TextField t[] = new TextField[SIZE]; String query = ""; Label l = new Label(); TextArea ta = new TextArea(15, 60); 6 I can’t say I really understand what’s going on here, but I managed to get it working by studying Java Network Programming by Elliotte Rusty Harold (O’Reilly 1997). He alludes to a number of confusing bugs in the Java networking libraries, so this is an area where you can’t just write code and have it working right away. Be warned. 614 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 615. public void init() { Panel p = new Panel(); p.setLayout(new GridLayout(t.length + 2, 2)); for(int i = 0; i < t.length; i++) { p.add(new Label( "Field " + i + " ", Label.RIGHT)); p.add(t[i] = new TextField(30)); } p.add(l); p.add(submit); add("North", p); add("South", ta); } public boolean action (Event evt, Object arg) { if(evt.target.equals(submit)) { query = ""; ta.setText(""); // Encode the query from the field data: for(int i = 0; i < t.length; i++) query += "Field" + i + "=" + URLEncoder.encode( t[i].getText().trim()) + "&"; query += "submit=Submit"; // Send the name using CGI's POST process: try { URL u = new URL( getDocumentBase(), "cgi-bin/POSTtest"); URLConnection urlc = u.openConnection(); urlc.setDoOutput(true); urlc.setDoInput(true); urlc.setAllowUserInteraction(false); DataOutputStream server = new DataOutputStream( urlc.getOutputStream()); // Send the data server.writeBytes(query); server.close(); // Read and display the response. You // cannot use // getAppletContext().showDocument(u); // to display the results as a Web page! DataInputStream in = new DataInputStream( urlc.getInputStream()); String s; while((s = in.readLine()) != null) { ta.appendText(s + "n"); } in.close(); } catch (Exception e) { l.setText(e.toString()); } } else return super.action(evt, arg); Chapter 15: Network Programming 615
- 616. return true; } } ///:~ Once the information is sent to the server, you can call getInputStream( ) and wrap the return value in a DataInputStream so you can read the results. One thing you’ll notice is that the results are displayed as lines of text in a TextArea. Why not simply use getAppletContext().showDocument(u)? Well, this is one of those mysteries. The above code works fine, but if you instead try to use showDocument( ), everything stops working – almost. That is, showDocument( ) does work but what you get back from POSTtest is “Zero CONTENT_LENGTH.” So somehow, showDocument( ) prevents the POST query from being passed on to the CGI program. It’s difficult to know whether this is a bug that will be fixed, or some lack of understanding on my part (the books I looked at were equally abstruse). In any event, if you can stand to limit yourself to looking at the text that comes back from the CGI program, the above applet works fine. Connecting to databases with JDBC It has been estimated that half of all software development involves client/server operations. A great promise of Java has been the ability to build platform-independent client/server database applications. In Java 1.1 this has come to fruition with Java DataBase Connectivity (JDBC). One of the major problems with databases has been the feature wars between the database companies. There is a “standard” database language, Structured Query Language (SQL-92), but usually you must know which database vendor you’re working with despite the standard. JDBC is designed to be platform-independent, so you don’t need to worry about which database you’re using while you’re programming. However, it’s still possible to make vendor-specific calls from JDBC so you aren’t restricted from doing what you must. JDBC, like many of the APIs in Java, is designed for simplicity. The method calls you make correspond to the logical operations you’d think of doing when gathering data from a database: connect to the database, create a statement and execute the query, and look at the result set. To allow this platform independence, JDBC provides a driver manager which dynamically maintains all the driver objects that your database queries will need. Thus, if you have three different kinds of vendor databases to connect to, you’ll need three different driver objects. The driver objects register themselves with the driver manager at the time of loading, and you can force the loading using Class.forName( ). To open a database, you must create a “database URL” that specifies: 1. That you’re using JDBC with “jdbc” 2. The “subprotocol”: the name of the driver or the name of a database connectivity mechanism. Since the design of JDBC was inspired by ODBC, the first subprotocol available is the “jdbc-odbc bridge,” specified by “odbc” 3. The database identifier. This varies with the database driver used, but it generally provides a logical name that is mapped by the database administration software to a physical directory where the database tables are located. This means that for your database identifier to have any meaning, you must register 616 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 617. the name using your database administration software (the process of registration varies from platform to platform). All this information is combined together into one string, the “database URL.” For example, to connect through the ODBC subprotocol to a database identified as “people,” the database URL could be: String dbUrl = "jdbc:odbc:people"; If you’re connecting across a network, the database URL will also contain the information identifying the remote machine. When you’re ready to connect to the database, you call the static method DriverManager.getConnection( ), passing it the database URL, the user name and a password to get into the database. You get back a Connection object which you can then use to query and manipulate the database. The following example opens a database of contact information and looks for a person’s last name as given on the command line. It selects only the names of people that have email addresses, then prints out all the ones that match the given last name: //: Lookup.java // Looks up email addresses in a // local database using JDBC import java.sql.*; public class Lookup { public static void main(String args[]) { String dbUrl = "jdbc:odbc:people"; String user = ""; String password = ""; try { // Load the driver (registers itself) Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver"); Connection c = DriverManager.getConnection( dbUrl, user, password); Statement s = c.createStatement(); // SQL code: ResultSet r = s.executeQuery( "SELECT FIRST, LAST, EMAIL " + "FROM people.csv people " + "WHERE " + "(LAST='" + args[0] + "') " + " AND (EMAIL Is Not Null) " + "ORDER BY FIRST"); while(r.next()) { // Capitalization doesn't matter: System.out.println( r.getString("Last") + ", " + r.getString("fIRST") + ": " + r.getString("EMAIL") ); } s.close(); // Also closes ResultSet } catch(Exception e) { e.printStackTrace(); } Chapter 15: Network Programming 617
- 618. } } ///:~ You can see the creation of the database URL as previously described. In this example, there is no password protection on the database so the user name and password are empty strings. Once the connection is made with DriverManager.getConnection( ), you can use the resulting Connection object to create a Statement object using the createStatement( ) method. With the resulting Statement, you can call executeQuery( ), passing in a string containing an SQL-92 standard SQL statement (you’ll see shortly how you can generate this statement automatically, so you don’t have to know much about SQL). The executeQuery( ) method returns a ResultSet object, which is quite a bit like an iterator: the next( ) method moves the iterator to the next record in the statement, or returns null if the end of the result set has been reached. You’ll always get a ResultSet object back from executeQuery( ) even if a query results in an empty set (that is, an exception is not thrown). Note that you must call next( ) once before trying to read any record data. If the result set is empty, this first call to next( ) will return false. For each record in the result set, you can select the fields using (among other approaches) the field name as a string. Notice that the capitalization of the field name is ignored – it doesn’t matter with an SQL database. You determine the type you’ll get back by calling getInt( ), getString( ), getFloat( ), etc. At this point, you’ve got your database data in Java native format and can do whatever you want with it using ordinary Java code. Getting the example to work With JDBC, understanding the code is relatively simple. The confusing part is making it work on your particular system. The reason this is confusing is that it requires you to figure out how to get your JDBC driver to load properly, and how to set up a database using your database administration software. Of course, this process may vary radically from machine to machine, but the process I used to make it work under 32-bit Windows may give you clues to help you attack your own situation. Step 1: find the JDBC Driver The above program contains the statement: Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); This implies a directory structure, which is deceiving. With this particular installation of JDK 1.1, there was no file called JdbcOdbcDriver.class, so if you looked at this example and went searching for it you’d be frustrated. Other published examples use a pseudo name, like “myDriver.ClassName,” which is less than helpful. In fact, the load statement above for the jdbc-odbc driver (the only one that actually comes with JDK 1.1) appears in only a few places in the online documentation (in particular, a page labeled “JDBC-ODBC Bridge Driver”). If the above load statement doesn’t work, then the name may have been changed as part of a Java version change so you should hunt through the documentation again. If the load statement is wrong, you’ll get an exception at this point. To test whether your driver load statement is working correctly, comment out the code after the statement and up to the catch clause; if the program throws no exceptions it means the driver is loading properly. 618 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 619. Step 2: configure the database Again, this is specific to 32-bit Windows; you may have to do some research to figure it out for your own platform. First, open the control panel. You may find two icons that say “ODBC.” You must use the one that says “32bit ODBC,” since the other one is for backwards compatibility with 16- bit ODBC software and will produce no results for JDBC. When you open the “32bit ODBC” icon, you’ll see a tabbed dialog with a number of tabs, including “User DSN,” “System DSN,” “File DSN,” etc., where “DSN” means “Data Source Name.” It turns out that for the JDBC-ODBC bridge, the only place where it’s actually important to set up your database is “System DSN,” but you’ll also want to test your configuration and create queries, and for that you’ll also need to set up your database in “File DSN.” This will allow the Microsoft Query tool (that comes with Microsoft Office) to find the database. Note that other query tools are also available from other vendors. The most interesting database is one that you’re already using. Standard ODBC supports a number of different file formats including such venerable workhorses as DBase. However, it also includes the very simple “comma-separated ASCII” format, which virtually every data tool has the ability to write. In my case, I just took my “people” database which I’ve been maintaining for years using various contact-management tools and exported it as a comma-separated ASCII file (these typically have an extension of .csv). In the “File DSN” section I chose “Add,” chose the text driver to handle my comma-separated ASCII file and then un-checked “use current directory” to allow me to specify the directory where I exported the data file. You’ll note when you do this that you don’t actually specify a file, but only a directory. That’s because a database is typically represented as a collection of files under a single directory (although it could be represented in other forms as well). Each file normally contains a single table, and the SQL statements can produce results which are culled from multiple tables in the database (this is called a join). A database which contains only a single table (like this one) is usually called a flat-file database. Most problems that go beyond the simple storage and retrieval of data generally require multiple tables which must be related by joins to produce the desired results, and these are called relational databases. Step 3: test the configuration To test the configuration you’ll need a way to discover whether the database is visible from a program that queries it. Of course, you can simply run the above JDBC program example up to and including the statement: Connection c = DriverManager.getConnection( dbUrl, user, password); If an exception is thrown, your configuration was incorrect. However, it’s useful to get a query-generation tool involved at this point. I used Microsoft Query which came with Microsoft Office, but you may have something else you prefer. The query tool must know where the database is, and Microsoft Query required that I go to the ODBC Administrator’s “File DSN” tab and add a new entry there, again specifying the text driver and the directory where my database lives. You can name the entry anything you want, but it’s helpful to use the same name you used in “System DSN.” Once you’ve done this, you will see that your database is available when you create a new query using your query tool. Chapter 15: Network Programming 619
- 620. Step 4: generate your SQL query The query that I created using Microsoft Query not only showed me that my database was there and in good order, but it also automatically created the SQL code that I needed to insert into my Java program. I wanted a query that would search for records that had the last name that was typed on the command line when starting the Java program. So as a starting point, I searched for a specific last name, ‘Eckel’. I also wanted to display only names that had email addresses associated with them. The steps I took to create this query were: 1. Start a new query and use the Query Wizard. Select the “people” database (this is the equivalent of opening the database connection using the appropriate database URL). 2. Select the “people” table within the database. From within the table, choose the columns FIRST, LAST and EMAIL. 3. Under “Filter Data,” choose LAST and select “equals” with an argument of Eckel. Click the “And” radio button. 4. Choose EMAIL and select “Is not Null.” 5. Under “Sort By,” choose FIRST. The result of this query will show you whether you’re getting what you want. Now you can press the SQL button and without any research on your part, up will pop the correct SQL code, ready for you to cut and paste. For this query, it looked like this: SELECT people.FIRST, people.LAST, people.EMAIL FROM people.csv people WHERE (people.LAST='Eckel') AND (people.EMAIL Is Not Null) ORDER BY people.FIRST With more complicated queries it’s very easy to get things wrong, but with a query tool you can interactively test your queries and automatically generate the correct code. It’s hard to argue the case for doing this by hand. Step 5: modify and paste in your query You’ll notice that the above code looks different than what’s used in the program. That’s because the query tool uses full qualification for all the names, even when there’s only one table involved (when more than one table is involved, the qualification prevents collisions between columns from different tables that have the same names). Since this query involves only one table, you can optionally remove the “people” qualifier from most of the names, like this: SELECT FIRST, LAST, EMAIL FROM people.csv people WHERE (LAST='Eckel') AND (EMAIL Is Not Null) ORDER BY FIRST In addition, you don’t want this program to be hard coded to look for only one name. Instead it should hunt for the name given as the command-line argument. Making these changes and turning the SQL statement into a dynamically-created String produces: "SELECT FIRST, LAST, EMAIL " + 620 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 621. "FROM people.csv people " + "WHERE " + "(LAST='" + args[0] + "') " + " AND (EMAIL Is Not Null) " + "ORDER BY FIRST"); SQL has another way to insert names into a query called stored procedures, which is used for speed. But for much of your database experimentation and for your first cut, building your own query strings in Java is fine. You can see from this example that by using the tools currently available – in particular the query-building tool – database programming with SQL and JDBC can be quite straightforward. A GUI version of the lookup program It’s more useful to leave the lookup program running all the time and simply switch to it and type in a name whenever you want to look someone up. The following program creates the lookup program as an application/applet, and it also adds name completion so the data will show up without forcing you to type the entire last name: //: VLookup.java // GUI version of Lookup.java import java.awt.*; import java.awt.event.*; import java.applet.*; import java.sql.*; public class VLookup extends Applet { String dbUrl = "jdbc:odbc:people"; String user = ""; String password = ""; Statement s; TextField searchFor = new TextField(20); Label completion = new Label(" "); TextArea results = new TextArea(40, 20); public void init() { searchFor.addTextListener(new SearchForL()); Panel p = new Panel(); p.add(new Label("Last name to search for:")); p.add(searchFor); p.add(completion); setLayout(new BorderLayout()); add(p, BorderLayout.NORTH); add(results, BorderLayout.CENTER); try { // Load the driver (registers itself) Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver"); Connection c = DriverManager.getConnection( dbUrl, user, password); s = c.createStatement(); } catch(Exception e) { results.setText(e.getMessage()); } Chapter 15: Network Programming 621
- 622. } class SearchForL implements TextListener { public void textValueChanged(TextEvent te) { ResultSet r; if(searchFor.getText().length() == 0) { completion.setText(""); results.setText(""); return; } try { // Name completion: r = s.executeQuery( "SELECT LAST FROM people.csv people " + "WHERE (LAST Like '" + searchFor.getText() + "%') ORDER BY LAST"); if(r.next()) completion.setText( r.getString("last")); r = s.executeQuery( "SELECT FIRST, LAST, EMAIL " + "FROM people.csv people " + "WHERE (LAST='" + completion.getText() + "') AND (EMAIL Is Not Null) " + "ORDER BY FIRST"); } catch(Exception e) { results.setText( searchFor.getText() + "n"); results.append(e.getMessage()); return; } results.setText(""); try { while(r.next()) { results.append( r.getString("Last") + ", " + r.getString("fIRST") + ": " + r.getString("EMAIL") + "n"); } } catch(Exception e) { results.setText(e.getMessage()); } } } public static void main(String args[]) { VLookup applet = new VLookup(); Frame aFrame = new Frame("Email lookup"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); 622 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 623. aFrame.setSize(500,200); applet.init(); applet.start(); aFrame.setVisible(true); } } ///:~ Much of the database logic is the same, but you can see that a TextListener is added to listen to the TextField, so that whenever you type a new character it first tries to do a name completion by looking up the last name in the database and using the first one that shows up (it places it in the completion Label, and uses that as the lookup text). This way, as soon as you’ve typed enough characters for the program to uniquely find the name you’re looking for, you can stop. Why the JDBC API seems so complex When you browse the online documentation for JDBC it can seem very daunting. In particular, in the DatabaseMetaData interface – which is just huge, contrary to most of the interfaces you see in Java – there are methods like dataDefinitionCausesTransactionCommit( ), getMaxColumnNameLength( ), getMaxStatementLength( ), storesMixedCaseQuotedIdentifiers( ), supportsANSI92IntermediateSQL( ), supportsLimitedOuterJoins( ), and on and on. What’s this all about? As mentioned earlier, databases have seemed from their inception to be in a constant state of turmoil, primarily because the demand for database applications, and thus database tools, is so great. Only recently has there been any convergence on the common language of SQL (and there are plenty of other database languages in common use). But even with an SQL “standard” there are so many variations on that theme that JDBC must provide the large DatabaseMetaData interface so that your code can discover the capabilities of the particular “standard” SQL database that it’s currently connected to. In short, you can write simple, transportable SQL but if you want to optimize speed your coding will multiply tremendously as you investigate the capabilities of a particular vendor’s database. This, of course, is not Java’s fault. The discrepancies between database products are just something that JDBC tries to help compensate for, that’s all. But bear in mind that your life will be easier if you can either write generic queries and not worry too much about performance, or, if you must tune for performance, know the platform you’re writing for so you don’t need to write all that investigation code. There is more JDBC information available in the electronic documents that come as part of the Java 1.1 distribution from Sun. In addition, you can find more in the book JDBC Database Access with Java (Hamilton, Cattel and Fisher, Addison-Wesley 1997). Other JDBC books are appearing regularly. Remote methods Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and that you can send a message to that object and get a result as if the object lived on your local machine. This simplification is exactly what Java 1.1 Remote Method Invocation (RMI) allows you to do. This section walks you through the steps necessary to create your own RMI objects. Chapter 15: Network Programming 623
- 624. Remote interfaces RMI makes heavy use of interfaces. When you want to create a remote object, you mask the underlying implementation by passing around an interface. Thus, when the client gets a handle to a remote object, what they really get is an interface handle, which happens to connect to some local stub code which itself talks across the network. But you don’t think about this, you just send messages via your interface handle. When you create a remote interface, you must follow these guidelines: 1. The remote interface must be public (that is, it cannot have “package access” e.g. “friendly”). Otherwise, a client will get an error when attempting to load a remote object that implements the remote interface. 2. The remote interface must extend the interface java.rmi.Remote. 3. Each method in the remote interface must declare java.rmi.RemoteException in its throws clause, in addition to any application-specific exceptions. 4. A remote object passed as an argument or return value (either directly or embedded within a local object) must be declared as the remote interface, not the implementation class. Here’s a simple remote interface that represents a very accurate time service: //: PerfectTimeI.java // The PerfectTime remote interface package c15.ptime; import java.rmi.*; interface PerfectTimeI extends Remote { long getPerfectTime() throws RemoteException; } ///:~ It looks like any other interface, except that it extends Remote and all its methods throw RemoteException. Remember that an interface and all its methods are automatically public. Implementing the remote interface The server must contain a class that extends UnicastRemoteObject and implements the remote interface. This class may also have additional methods, but only the methods in the remote interface will be available to the client, of course, since the client will get only a handle to the interface, not the actual class that implements it. You must explicitly define the constructor for the remote object, even if you’re only defining a default constructor that just calls the base-class constructor. You must write it out since it must throw RemoteException. Here’s the implementation of the remote interface PerfectTimeI: //: PerfectTime.java // The implementation of the PerfectTime // remote object package c15.ptime; import java.rmi.*; import java.rmi.server.*; 624 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 625. import java.rmi.registry.*; import java.net.*; public class PerfectTime extends UnicastRemoteObject implements PerfectTimeI { // Implementation of the interface: public long getPerfectTime() throws RemoteException { return System.currentTimeMillis(); } // Must implement constructor to throw // RemoteException: public PerfectTime() throws RemoteException { // super(); // Called automatically } // Registration for RMI serving: public static void main(String args[]) { System.setSecurityManager( new RMISecurityManager()); try { PerfectTime pt = new PerfectTime(); Naming.bind( "//colossus:2005/PerfectTime", pt); System.out.println("Ready to do time"); } catch(Exception e) { e.printStackTrace(); } } } ///:~ Here, main( ) handles all the details of setting up the server. When you’re serving RMI objects, at some point in your program you must: 1. Create and install a security manager that supports RMI. The only one available for RMI as part of the Java distribution is RMISecurityManager. 2. Create one or more instances of a remote object. Here, you can see the creation of the PerfectTime object. 3. Register at least one of the remote objects with the RMI remote object registry, for bootstrapping purposes. One remote object may have methods that produce handles to other remote objects. This allows you to set it up so the client must go to the registry only once, to get the first remote object. Setting up the registry Here, you see a call to the static method Naming.bind( ). However, this call requires that the registry be running as a separate process on the computer. The name of the registry server is rmiregistry, and under 32-bit Windows you say: start rmiregistry to start it in the background. On Unix, it is rmiregistry & Like many network programs, the rmiregistry is located at the IP address of whatever machine started it up, but it must also be listening at a port. If you invoke the rmiregistry Chapter 15: Network Programming 625
- 626. as above, with no argument, the registry’s port will default to 1099. If you want it to be at some other port, you add an argument on the command line to specify the port. For this example, the port will be located at 2005, so the rmiregistry should be started like this under 32-bit Windows: start rmiregistry 2005 or for Unix: rmiregistry 2005 & The information about the port must also be given to the bind( ) command, as well as the IP address of the machine where the registry is located. But this brings up what can be a very frustrating problem if you’re expecting to test RMI programs locally the way the network programs have been tested so far in this chapter. In the JDK 1.1.1 release, there are a couple of problems7 : 1. localhost does not work with RMI. Thus, to experiment with RMI on a single machine, you must provide the actual name of the machine. To find out the actual name of your machine under 32-bit Windows, go to the control panel and select “Network.” Select the “Identification” tab, and you’ll see your computer name. In my case, I called my computer “Colossus” (for all the hard disks I’ve had to put on to hold all the different development systems). It appears that capitalization is ignored. 2. RMI will not work unless your computer has an active TCP/IP connection, even if all your components are just talking to each other on the local machine. This means you must connect to your Internet service provider before trying to run the program, or you’ll get some obscure exception messages. Will all this in mind, the bind( ) command becomes: Naming.bind("//colossus:2005/PerfectTime", pt); If you are using the default port 1099, you don’t need to specify a port, so you could say: Naming.bind("//colossus/PerfectTime", pt); In a future release of the JDK (after 1.1) when the localhost bug is fixed, you will be able to perform local testing by leaving off the IP address and using only the identifier: Naming.bind("PerfectTime", pt); The name for the service is arbitrary; it happens to be PerfectTime here, just like the name of the class, but you could call it anything you want. The important thing is that it’s a unique name in the registry that the client knows to look for to procure the remote object. If the name is already in the registry, you’ll get an AlreadyBoundException. To prevent this, you can always use rebind( ) instead of bind( ), since rebind( ) either adds a new entry or replaces the one that’s already there. Even though main( ) exits, your object has been created and registered so it’s kept alive by the registry, waiting for a client to come along and request it. As long as the rmiregistry is running and you don’t call Naming.unbind( ) on your name, the object will be there. For this reason, when you’re developing your code you need to shut down the rmiregistry and restart it when you compile a new version of your remote object. 7 Many brain cells died in agony to discover this information. 626 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 627. You aren’t forced to start up rmiregistry as an external process. If you know that your application is the only one that’s going to use the registry, you can start it up inside your program with the line: LocateRegistry.createRegistry(2005); Where, as before, 2005 is the port number we happen to be using in this example. This is the equivalent of running rmiregistry 2005 from a command line, but can often be more convenient when you’re developing RMI code since it eliminates the extra steps of starting and stopping the registry. Once you’ve executed this code, you can bind( ) using Naming as before. Creating stubs and skeletons If you compile and run PerfectTime.java, it won’t work even if you have the rmiregistry running correctly. That’s because the framework for RMI isn’t all there yet – you must first create the stubs and skeletons that provide the network connection operations and allow you to pretend that the remote object is just another local object on your machine. What’s going on under the covers is complex. Any objects that you pass into or return from a remote object must implement Serializable (if you want to pass remote references instead of the entire objects, the object arguments can implement Remote), so you can imagine that the stubs and skeletons are automatically performing serialization and deserialization as they “marshal” all the arguments across the network and return the result. Fortunately, you don’t have to know about any of this, but you do have to create the stubs and skeletons. This is a simple process: you invoke the rmic tool on your compiled code, and it creates the necessary files. So the only requirement is that another step be added to your compilation process. The rmic tool is particular about packages and classpaths, however. PerfectTime.java is in the package c15.Ptime, and even if you invoke rmic in the same directory where PerfectTime.class is located, rmic won’t find the file since it searches the classpath. So you must specify the location off the class path, like so: rmic c15.PTime.PerfectTime You don’t have to be in the directory containing PerfectTime.class when you execute this command, but the results will be placed in the current directory. When rmic runs successfully, you’ll have two new classes in the directory: PerfectTime_Stub.class PerfectTime_Skel.class corresponding to the stub and skeleton. Now you’re ready to get the server and client to talk to each other. Using the remote object The whole point of RMI is to make the use of remote objects very simple. The only extra thing you must do in your client program is to look up and fetch the remote interface from the server. From then on, it’s just regular Java programming: sending messages to objects. Here’s the program that uses PerfectTime: //: DisplayPerfectTime.java // Uses remote object PerfectTime package c15.ptime; import java.rmi.*; Chapter 15: Network Programming 627
- 628. import java.rmi.registry.*; public class DisplayPerfectTime { public static void main(String args[]) { System.setSecurityManager( new RMISecurityManager()); try { PerfectTimeI t = (PerfectTimeI)Naming.lookup( "//colossus:2005/PerfectTime"); for(int i = 0; i < 10; i++) System.out.println("Perfect time = " + t.getPerfectTime()); } catch(Exception e) { e.printStackTrace(); } } } ///:~ The ID string is the same as the one used to register the object with Naming, and the first part represents the URL and port number. Since you’re using a URL, you can also specify a machine on the Internet. What comes back from Naming.lookup( ) must be cast to the remote interface, not the class itself. If you use the class instead, you’ll get an exception. You can see in the method call t.getPerfectTime( ) that once you have a handle to the remote object, programming with it is indistinguishable from programming with a local object (with one difference: remote methods throw RemoteException). Alternatives to RMI RMI is just one way to create objects that can be distributed across a network. It has the advantage of being a “pure Java” solution, but if you have a lot of code written in some other language, it may not meet your needs. The two most compelling alternatives are Microsoft’s DCOM (which, according to Microsoft’s plan, will eventually be hosted on platforms other than Windows) and CORBA, which is supported in Java 1.1 and was designed from the start to be cross-platform. You can get an in-depth treatment of distributed objects in Java (albeit with a clear bias towards CORBA) in Client/Server Programming with Java and CORBA by Orfali & Harkey (John Wiley & Sons, 1997). Summary There’s actually a lot more to networking than can be covered in this introductory treatment. Java networking also provides fairly extensive support for URLs, including protocol handlers for different types of content that can be discovered at an Internet site. In addition, an up-and-coming technology is the Servlet Server which is an Internet server that uses Java to handle requests instead of the slow and rather awkward CGI (Common Gateway Interface) protocol. This means that to provide services on the server side you’ll be able to write in Java instead of using some other language you may not know as well. 628 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 629. In addition, you’ll get the portability benefits of Java so you won’t have to worry about the particular platform the server is hosted upon. These and other features are fully and carefully described in Java Network Programming by Elliotte Rusty Harold (O’Reilly, 1997). Exercises 1. Compile and run the JabberServer and JabberClient programs in this chapter. Now edit the files to remove all the buffering for the input and output, then compile and run them again to observe the results. 2. Create a server that asks for a password, then opens a file and sends the file over the network connection. Create a client that connects to this server, gives the appropriate password, then captures and saves the file. Test the pair of programs on your machine using the localhost (the local loopback IP address 127.0.0.1 produced by calling InetAddress.getByName(null)). 3. Modify the server in Exercise 2 so that it uses multithreading to handle multiple clients. 4. Modify JabberClient so that output flushing doesn’t occur and observe the effect. 5. Build on ShowHTML.java to produce an applet which is a password-protected gateway to a particular portion of your Web site. 6. (More challenging) Create a client/server pair of programs that uses datagrams to transmit a file from one machine to the other (see the description at the end of the datagram section of this chapter). 7. (More challenging) Take the VLookup.java program and modify it so that when you click on the resulting name it automatically takes that name and copies it to the clipboard (so you can simply paste it in to your email). You’ll have to look back at the IO stream chapter to remember how to use the new Java 1.1 clipboard. Chapter 15: Network Programming 629
- 631. ij 16: Design patterns This chapter introduces the very important and yet non-traditional “patterns” approach to program design. Probably the most important step forward in object-oriented design is the “design patterns” movement, chronicled in Design Patterns, by Gamma, Helm, Johnson & Vlissides (Addison-Wesley 1995)1 . That book shows 23 different solutions to particular classes of problems. In this chapter, the basic concepts of design patterns will be introduced along with several examples. This should whet your appetite to read Design Patterns. The latter part of the chapter contains an example of the design evolution process, starting with an initial solution and moving through the logic and process of evolving the solution to more appropriate designs. The program shown (a trash sorting simulation) has evolved over time, and you can look at that evolution as a prototype for the way your own design can start as an adequate solution to a particular problem and evolve into a flexible approach to a class of problems. The pattern concept Initially, you can think of a pattern as an especially clever and insightful way of solving a particular class of problems. That is, it looks like a lot of people have worked out all the angles of a problem and have come up with the most general, flexible solution for it. The problem may actually be one you have seen and solved before, but your solution probably didn’t have the kind of completeness you’ll see embodied in a pattern. Although they’re called “design patterns,” they really aren’t tied to the realm of design. A pattern actually seems to stand apart from the traditional way of thinking about analysis, 1 But be warned: the examples are in C++. 631
- 632. design and implementation. Instead, a pattern embodies a complete idea within a program, and thus it can sometimes appear at the analysis phase or high-level design phase. This is interesting because a pattern has a direct implementation in code and so you might not expect it to show up before low-level design or implementation (and in fact you may not realize you need a particular pattern until you get to those phases). The basic concept of a pattern can also be seen as the basic concept of program design: adding a layer of abstraction. Whenever you abstract something you’re isolating particular details, and one of the most compelling motivations behind this is to separate things that change from things that stay the same. Another way to put this is that once you find some part of your program that’s likely to change for one reason or another, you’ll want to keep those changes from propagating other changes throughout your code. Not only does this make the code much cheaper to maintain, but it turns out that it is generally simpler to understand (which also results in lowered costs). Often, the most difficult part of developing an elegant and cheap-to-maintain design is in discovering what I call “the vector of change” (here, “vector” refers to the maximum gradient and not a collection class). This means finding the most important thing that changes in your system, or put another way discovering where your greatest cost is. Once you discover the vector of change, you have the focal point around which to structure your design. So the goal of design patterns is to isolate changes in your code. If you look at it this way, you’ve actually been seeing some design patterns already in this book. For example, inheritance can be thought of as a design pattern (albeit one implemented by the compiler). It allows you to express differences in behavior (that’s the thing that changes) in objects that all have the same interface (that’s what stays the same). Composition can also be considered a pattern, since it allows you to change – dynamically or statically – the objects that implement your class, and thus the way that class works. You’ve also already seen another pattern that appears in Design Patterns: the iterator (which Java 1.0 and 1.1 capriciously call the Enumeration; Java 1.2 collections use “iterator”). This hides the particular implementation of the collection as you’re stepping through and selecting the elements one by one. The iterator allows you to write generic code that performs an operation on all the elements in a sequence without regard to the way that sequence is built. Thus your generic code can be used with any collection that can produce an iterator. The singleton Possibly the simplest design pattern is the singleton, which is a way to provide one and only one instance of an object. This is used in the Java libraries, but here’s a more direct example: //: SingletonPattern.java // The Singleton design pattern: you can // never instantiate more than one. package c16; // Since this isn't inherited from a Cloneable // base class and cloneability isn't added, // making it final prevents cloneability from // being added in any derived classes: final class Singleton { private static Singleton s = new Singleton(47); private int i; 632 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 633. private Singleton(int x) { i = x; } public static Singleton getHandle() { return s; } public int getValue() { return i; } public void setValue(int x) { i = x; } } public class SingletonPattern { public static void main(String[] args) { Singleton s = Singleton.getHandle(); System.out.println(s.getValue()); Singleton s2 = Singleton.getHandle(); s2.setValue(9); System.out.println(s.getValue()); try { // Can't do this: compile-time error. // Singleton s3 = (Singleton)s2.clone(); } catch(Exception e) {} } } ///:~ The key to creating a singleton is to prevent the client programmer from having any way to create an object except the ways you provide. Thus, you must make all constructors private, and you must create at least one constructor to prevent the compiler from synthesizing a default constructor for you (which it will create as “friendly”). At this point, you decide how you’re going to create your object. Here, it’s created statically, but you can also wait until the client programmer asks for one and create it on demand. In any case, the object should be stored privately. You provide access through public methods. Here, getHandle( ) produces the handle to the Singleton object. The rest of the interface (getValue( ) and setValue( )) is the normal class interface. Java also allows the creation of objects through cloning. In this example, making the class final prevents cloning. Since Singleton is inherited directly from Object, the clone( ) method remains protected so it cannot be used (doing so produces a compile-time error). However, If you’re inheriting from a class hierarchy that has already overridden clone( ) as public and implemented Cloneable, the way to prevent cloning is to override clone( ) and throw a CloneNotSupportedException as described in Chapter 12. (You could also override clone( ) and simply return this, but that would be deceiving since the client programmer would think they were cloning the object but would instead still be dealing with the original). Note that you aren’t restricted to creating only one object. This is also a technique to create a limited pool of objects. In that situation, however, you may be confronted with the problem of sharing objects in the pool. If this is an issue, you can create a solution involving a check-out and check-in of the shared objects. Classifying patterns In Design Patterns, there are 23 different patterns classified under three purposes (all of which revolve around the particular aspect that can vary). The three purposes are: 1. Creational: how an object can be created. This often involves isolating the details of object creation so your code isn’t dependent on what types of objects there are and thus doesn’t have to be changed when you add a new type of Chapter 16: Design Patterns 633
- 634. object. The aforementioned Singleton is classified as a creational pattern, and later in this chapter you’ll see examples of the Factory Method and Prototype. 2. Structural: designing objects to satisfy particular project constraints. These work with the way objects are connected with other objects to ensure that changes in the system don’t require changes to those connections. 3. Behavioral: objects that handle particular types of actions within a program. These encapsulate processes that you want to perform, like interpreting a language, fulfilling a request, moving through a sequence (as in an iterator) or implementing an algorithm. This chapter contains examples of the Observer and the Visitor patterns. The Design Patterns book has a section on each of their 23 patterns along with an example, typically in C++ but sometimes in Smalltalk (you’ll find this doesn’t matter too much since you can easily translate the concepts from either language into Java). In this book I will not attempt to repeat all the patterns shown in Design Patterns since that book stands on its own and should be studied by the reader separately. Instead, this chapter will give some examples that should provide you with a decent feel for what patterns are about and why they are so important. The observer pattern The observer pattern solves a fairly common problem: what if a group of objects needs to update themselves when some object changes state? This can be seen in the “model-view” aspect of Smalltalk’s MVC (model-view-controller), or the “document-view architecture.” Suppose you have some data (the “document”) and more than one view, say a graph and a textual view. When you change the data, the two views must know to update themselves, and that’s what the observer facilitates. It’s a common enough problem that it’s part of the standard java.util library. There are two types of objects used to implement the observer pattern in Java. The Observable class keeps track of everybody who wants to be informed when a change happens, and whether the “state” has changed or not. When someone says “OK, everybody should check and potentially update themselves,” the Observable class performs this task by calling the notifyObservers( ) method for each one on the list. The notifyObservers( ) method is part of the base class Observable. There are actually two “things that change” in the observer pattern: the quantity of observing objects, and the way an update occurs. That is, the observer pattern allows you to modify both of these without affecting the surrounding code. The following example is similar to the ColorBoxes example from Chapter 14. Boxes are placed in a grid on the screen and each one is initialized to a random color. In addition, each box implements the Observer interface and is registered with an Observable object. When you click on a box, all the other boxes are notified that a change has been made because the Observable object automatically calls each Observer object’s update( ) method. Inside this method, the box checks to see if it’s adjacent to the one that was clicked, and if so it changes its color to match the clicked box. //: BoxObserver.java // Demonstration of Observer pattern using // Java's built-in observer classes. import java.awt.*; import java.awt.event.*; import java.util.*; 634 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 635. // You must inherit a new type of Observable: class BoxObservable extends Observable { public void notifyObservers(Object b) { // Otherwise it won't propagate changes: setChanged(); super.notifyObservers(b); } } public class BoxObserver extends Frame { Observable notifier = new BoxObservable(); public BoxObserver(int grid) { setTitle("Demonstrates Observer pattern"); setLayout(new GridLayout(grid, grid)); for(int x = 0; x < grid; x++) for(int y = 0; y < grid; y++) add(new OCBox(x, y, notifier)); } public static void main(String[] args) { int grid = 8; if(args.length > 0) grid = Integer.parseInt(args[0]); Frame f = new BoxObserver(grid); f.setSize(500, 400); f.setVisible(true); f.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); } } class OCBox extends Canvas implements Observer { Observable notifier; int x, y; // Locations in grid Color cColor = newColor(); static final Color colors[] = { Color.black, Color.blue, Color.cyan, Color.darkGray, Color.gray, Color.green, Color.lightGray, Color.magenta, Color.orange, Color.pink, Color.red, Color.white, Color.yellow }; static final Color newColor() { return colors[ (int)(Math.random() * colors.length) ]; } OCBox(int x, int y, Observable notifier) { this.x = x; this.y = y; notifier.addObserver(this); this.notifier = notifier; Chapter 16: Design Patterns 635
- 636. addMouseListener(new ML()); } public void paint(Graphics g) { g.setColor(cColor); Dimension s = getSize(); g.fillRect(0, 0, s.width, s.height); } class ML extends MouseAdapter { public void mousePressed(MouseEvent e) { notifier.notifyObservers(OCBox.this); } } public void update(Observable o, Object arg) { OCBox clicked = (OCBox)arg; if(nextTo(clicked)) { cColor = clicked.cColor; repaint(); } } private final boolean nextTo(OCBox b) { return Math.abs(x - b.x) <= 1 && Math.abs(y - b.y) <= 1; } } ///:~ When you first look at the online documentation for Observable, it’s a bit confusing because it appears that you can just use an ordinary Observable object to manage the updates. But this doesn’t work (try it – inside BoxObserver, create an Observable object instead of a BoxObservable object and see what happens: nothing). To get an effect, you must inherit from Observable and somewhere in your derived-class code call setChanged( ). This is the method that sets the “changed” flag, which means that when you call notifyObservers( ) all the observers will, in fact, get notified. In the above example setChanged( ) is simply called within notifyObservers( ), but you could use any criterion you want to decide when to call setChanged( ). BoxObserver contains a single Observable object called notifier, and every time an OCBox object is created, it is tied to notifier. In OCBox, whenever you click the mouse the notifyObservers( ) method is called, passing the clicked object in as an argument so that all the boxes receiving the message (in their update( ) method) know who was clicked and can decide whether to change themselves or not. Using a combination of code in notifyObservers( ) and update( ) you can work out some fairly complex schemes. It may appear that the way that observers are notified must be frozen at compile time in the notifyObservers( ) method. However, if you look more closely at the above code you’ll see that the only place in BoxObserver or OCBox that you know you’re working with a BoxObservable is at the point of creation of the Observable object – from then on everything just uses the basic Observable interface. This means that you could inherit other Observable classes and swap them at run-time if you want to change notification behavior. Simulating the trash recycler The nature of this problem is that the trash is thrown unclassified into a single bin, so the specific type information is lost. But later, the specific type information must be recovered to properly sort the trash. In the initial solution, RTTI (described in Chapter 11) is used. 636 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 637. This is not a trivial design because it has an added constraint. That’s what makes it interesting – it’s more like the messy problems you’re likely to encounter in your work. The extra constraint is that the trash arrives at the trash recycling plant all mixed together. The program must model the sorting of that trash. This is where RTTI comes in: you have a bunch of anonymous pieces of trash, and the program figures out exactly what type they are. //: RecycleA.java // Recycling with RTTI package c16.recyclea; import java.util.*; import java.io.*; abstract class Trash { private double weight; Trash(double wt) { weight = wt; } abstract double value(); double weight() { return weight; } // Sums the value of Trash in a bin: static void sumValue(Vector bin) { Enumeration e = bin.elements(); double val = 0.0f; while(e.hasMoreElements()) { // One kind of RTTI: // A dynamically-checked cast Trash t = (Trash)e.nextElement(); // Polymorphism in action: val += t.weight() * t.value(); System.out.println( "weight of " + // Using RTTI to get type // information about the class: t.getClass().getName() + " = " + t.weight()); } System.out.println("Total value = " + val); } } class Aluminum extends Trash { static double val = 1.67f; Aluminum(double wt) { super(wt); } double value() { return val; } static void value(double newval) { val = newval; } } class Paper extends Trash { static double val = 0.10f; Paper(double wt) { super(wt); } double value() { return val; } static void value(double newval) { val = newval; } } Chapter 16: Design Patterns 637
- 638. class Glass extends Trash { static double val = 0.23f; Glass(double wt) { super(wt); } double value() { return val; } static void value(double newval) { val = newval; } } public class RecycleA { public static void main(String args[]) { Vector bin = new Vector(); // Fill up the Trash bin: for(int i = 0; i < 30; i++) switch((int)(Math.random() * 3)) { case 0 : bin.addElement(new Aluminum(Math.random() * 100)); break; case 1 : bin.addElement(new Paper(Math.random() * 100)); break; case 2 : bin.addElement(new Glass(Math.random() * 100)); } Vector glassBin = new Vector(), paperBin = new Vector(), alBin = new Vector(); Enumeration sorter = bin.elements(); // Sort the Trash: while(sorter.hasMoreElements()) { Object t = sorter.nextElement(); // RTTI to show class membership: if(t instanceof Aluminum) alBin.addElement(t); if(t instanceof Paper) paperBin.addElement(t); if(t instanceof Glass) glassBin.addElement(t); } Trash.sumValue(alBin); Trash.sumValue(paperBin); Trash.sumValue(glassBin); Trash.sumValue(bin); } } ///:~ The first thing you’ll notice is the package statement: package c16.recyclea; 638 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 639. What this means is that in the source code listings available for the book, this file will be placed in the subdirectory recyclea which branches off from the subdirectory c16 (for Chapter 16). The unpacking tool in Chapter 17 takes care of placing it in the right subdirectory. The reason for doing this is that this chapter rewrites this particular example a number of times and by putting each version in its own package the class names will not clash. The program uses the classic structure of methods in the base class that are overridden in the derived class. Those overridden methods are called polymorphically from a base-class handle. Several Vector objects are created to hold Trash handles. Of course, Vectors actually hold Objects so they’ll hold anything at all. The reason they hold Trash (or something derived from Trash) is only because you’ve been careful not to put in anything except Trash. If you do put something “wrong” into the Vector, you won’t get any compile-time warnings or errors – you’ll only find out via an exception at run-time. When the Trash handles are added, they lose their specific identities and become simply Object handles (they are upcast). However, because of polymorphism the proper behavior still occurs when the dynamically-bound methods are called through the Enumeration sorter, once the resulting Object has been cast back to Trash. sumValue( ) also uses an Enumeration to perform operations on every object in the Vector. It looks silly to upcast the types of Trash into a collection holding base type handles, and then turn around and downcast. Why not just put the Trash into the appropriate receptacle in the first place? (Indeed, this is the whole enigma of recycling). In this program it would be easy to repair, but sometimes a system’s structure and flexibility can benefit greatly from downcasting. The program satisfies the design requirements: it works. This may be fine as long as it’s a one-shot solution. However, a useful program tends to evolve over time, so you must ask: what if the situation changes? For example, cardboard is now a valuable recyclable commodity, so how will that be integrated into the system (especially if the program is large and complicated). Since the above type-check coding in the switch statement could be scattered throughout the program, you’ll have to go find all that code every time a new type is added, and if you miss one the compiler will not give you any help by pointing out an error. The key to the misuse of RTTI here is that every type is tested. If you’re looking for only a subset of types because that subset needs special treatment, that’s probably fine. But if you’re hunting for every type inside a switch statement, then you’re probably missing an important point, and definitely making your code less maintainable. In the next section we’ll look at how this program evolved over several stages to become much more flexible. This should prove a valuable example in program design. Improving the design The solutions in Design Patterns are organized around the question “what will change as this program evolves?” This is generally the most important question you can ask about any design. If you can build your system around the answer, the results will be two- pronged: not only will your system allow easy (and inexpensive) maintenance, but you may also produce components that are reusable, so that other systems can be built more cheaply. This is the promise of object-oriented programming, but it doesn’t happen automatically – it requires thought and insight on your part. In this section we’ll look at how this process can happen during the refinement of a system. Chapter 16: Design Patterns 639
- 640. The answer to the question “what will change” for the recycling system is a common one: more types will be added to the system. The goal of the design, then, is to make this addition of types as painless as possible. In the recycling program, we’d like to encapsulate all places where specific type information is mentioned, so (if for no other reason) any changes can be localized to those encapsulations. It turns out this process also cleans up the rest of the code considerably. “Make more objects” This brings up a general object-oriented design principle which I first heard spoken by Grady Booch: “if the design is too complicated, make more objects.” This is simultaneously counterintuitive and ludicrously simple, and yet it’s the most useful guideline I’ve found. In general, if you find a place with messy code, consider what sort of class would clean that up. Very often the side effect of cleaning up the code will be a system that has better structure and is more flexible. Consider first the place where Trash objects are created, which is a switch statement inside main( ): for(int i = 0; i < 30; i++) switch((int)(Math.random() * 3)) { case 0 : bin.addElement(new Aluminum(Math.random() * 100)); break; case 1 : bin.addElement(new Paper(Math.random() * 100)); break; case 2 : bin.addElement(new Glass(Math.random() * 100)); } This is definitely messy, and also a place where you must change code whenever a new type is added. If new types are commonly added, a better solution is a single method that takes all the necessary information and produces a handle to an object of the correct type, already upcast to a trash object. In Design Patterns this is broadly referred to as a creational pattern (of which there are several). The specific pattern that will be applied here is a variant of the factory method. Here, the factory method is a static member of Trash, but more commonly it may be a method that is overridden in the derived class. The idea of the factory method is that you pass it the essential information it needs to know to create your object, then stand back and wait for the handle (already upcast to the base type) to pop out as the return value. From then on, you treat the object polymorphically. Thus, you never even need to know the exact type of object that’s created. In fact, the factory method hides it from you to prevent accidental misuse. That is, if you want to use the object without polymorphism, you’ll have to explicitly use RTTI and casting. But there’s a little problem, especially when you use the more complicated approach (not shown here) of making the factory method in the base class and overriding it in the derived classes. What if the information required in the derived class requires more or different arguments? “Creating more objects” solves this problem. To implement the factory method, the Trash class gets a new method called factory. To hide the creational data, there’s a new class called Info that contains all the necessary information for the 640 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 641. factory method to create the appropriate Trash object. Here’s a simple implementation of Info: class Info { int type; // Must change this to add another type: static final int MAX_NUM = 4; double data; Info(int typeNum, double dat) { type = typeNum % MAX_NUM; data = dat; } } An Info object’s only job is to hold information for the factory( ) method. Now if there’s a situation where factory( ) needs more or different information to create a new type of Trash object, the factory( ) interface doesn’t need to be changed. The Info class can be changed by adding new data and new constructors, or in the more typical object-oriented fashion of subclassing. The factory( ) method for this simple example looks like this: static Trash factory(Info i) { switch(i.type) { default: // to quiet the compiler case 0: return new Aluminum(i.data); case 1: return new Paper(i.data); case 2: return new Glass(i.data); case 3: // two lines here: (*1*) return new Cardboard(i.data); } } Here, the determination of the exact type of object is very simple, but you can imagine a more complicated system where factory( ) uses an elaborate algorithm. The point is that it’s now hidden away in one place, and you know to come to this place when you add new types. The creation of new objects is now much simpler in main( ): for(int i = 0; i < 30; i++) bin.addElement( Trash.factory( // (*8*) new Info( (int)(Math.random() * Info.MAX_NUM), Math.random() * 100))); An Info object is created to pass the data into factory( ), which in turn produces some kind of Trash object on the heap and returns the handle that’s added to the Vector bin. Of course, if you change the quantity and type of argument this statement will still need to be modified, but that can be eliminated if the creation of the Info object is itself automated. For example, a Vector of arguments can be passed into the constructor of an Info object (or directly into a factory( ) call, for that matter). This requires that the arguments be parsed and checked at runtime, but it does provide the greatest flexibility. Chapter 16: Design Patterns 641
- 642. You can see from this code what “vector of change” problem the factory is responsible for solving: if you add new types to the system (the change), the only code that must be modified is within the factory itself, so the factory isolates the effect of that change. A pattern for prototyping creation A problem with the above design is that it still requires a central location where all the types of the objects must be known: inside the factory( ) method. If new types are regularly being added to the system, the factory( ) method must be changed for each new type. When you discover something like this, it is useful to try to go one step further and move all the information about the type – including its creation – into the class representing that type. This way, the only thing you need to do to add a new type to the system is to inherit a single class. To move the information about type creation into each specific type of Trash, the “prototype” pattern will be used. The general idea is that you have a master sequence of objects, one of each type you’re interested in making. The objects in this sequence are used only for making new objects, using an operation that’s not unlike the clone( ) scheme built into Java’s root class Object. In this case, the cloning method is called tClone( ). When you’re ready to make a new object, presumably you have some sort of information that establishes the type of object you want to create, then you move through the master sequence comparing your information with whatever appropriate information is in the prototype objects in the master sequence. When you find one that matches your needs, you clone it. In this scheme there is no hard-coded information for creation. Each object knows how to expose appropriate information and how to clone itself. Thus the factory( ) method doesn’t need to be changed when a new type is added to the system. One approach to the problem of prototyping is to add a number of methods to support the creation of new objects. However, in Java 1.1 there’s already support for creating new objects if you have a handle to the Class object. With Java 1.1 reflection (introduced in Chapter 11) you can call a constructor even if you have only a handle to the Class object. This is the perfect solution for the prototyping problem. The list of prototypes will be represented indirectly by a list of handles to all the Class objects you want to create. In addition, if the prototyping fails, the factory( ) method will assume that it’s because a particular Class object wasn’t in the list, and will attempt to load it. By loading the prototypes dynamically like this, the Trash class doesn’t need to know what types it is working with and so doesn’t need any modifications when you add new types. This allows it to be easily reused throughout the rest of the chapter. //: Trash.java // Base class for Trash recycling examples package c16.trash; import java.util.*; import java.lang.reflect.*; public abstract class Trash { private double weight; Trash(double wt) { weight = wt; } Trash() {} public abstract double value(); public double weight() { return weight; } // Sums the value of Trash in a bin: public static void sumValue(Vector bin) { 642 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 643. Enumeration e = bin.elements(); double val = 0.0f; while(e.hasMoreElements()) { // One kind of RTTI: // A dynamically-checked cast Trash t = (Trash)e.nextElement(); val += t.weight() * t.value(); System.out.println( "weight of " + // Using RTTI to get type // information about the class: t.getClass().getName() + " = " + t.weight()); } System.out.println("Total value = " + val); } // Remainder of class provides support for // prototyping: public static class PrototypeNotFoundException extends Exception {} public static class CannotCreateTrashException extends Exception {} private static Vector trashTypes = new Vector(); public static Trash factory(Info info) throws PrototypeNotFoundException, CannotCreateTrashException { for(int i = 0; i < trashTypes.size(); i++) { // Somehow determine the new type // to create, and create one: Class tc = (Class)trashTypes.elementAt(i); if (tc.getName().indexOf(info.id) != -1) { try { // Get the dynamic constructor method // that takes a double argument: Constructor ctor = tc.getConstructor( new Class[] {double.class}); // Call the constructor to create a // new object: return (Trash)ctor.newInstance( new Object[]{new Double(info.data)}); } catch(Exception ex) { ex.printStackTrace(); throw new CannotCreateTrashException(); } } } // Class was not in the list. Try to load it, // but it must be in your class path! try { System.out.println("Loading " + info.id); trashTypes.addElement( Class.forName(info.id)); } catch(Exception e) { Chapter 16: Design Patterns 643
- 644. e.printStackTrace(); throw new PrototypeNotFoundException(); } // Loaded successfully. Recursive call // should work this time: return factory(info); } public static class Info { public String id; public double data; public Info(String name, double data) { id = name; this.data = data; } } } ///:~ The basic Trash class and sumValue( ) are the same as before. The rest of the class supports the prototyping pattern. You can first see two inner classes (which are static, so they are inner classes only for code organization) describing exceptions that can occur. This is followed by a Vector trashTypes which is used to hold the Class handles. In Trash.factory( ), the String inside the Info object id (a different version of the Info class than in the prior discussion) contains the type name of the Trash to be created; this String is compared to the Class names in the list. If there’s a match, then that’s the object to create. Of course, there are many ways to determine what object you want to make. This one is used so that information read in from a file can be turned into objects. Once you’ve discovered which kind of Trash to create, then the reflection methods come into play. The getConstructor( ) method takes an argument that is an array of Class handles. This array represents the arguments, in their proper order, for the constructor that you’re looking for. Here, the array is dynamically created using the Java 1.1 array- creation syntax: new Class[] {double.class} This code assumes that every Trash type has a constructor that takes a double (and note that double.class is distinct from Double.class). It’s also possible, for a more flexible solution, to call getConstructors( ) which returns an array of the possible constructors. What comes back from getConstructor( ) is a handle to a Constructor object (part of java.lang.reflect). You call the constructor dynamically with the method newInstance( ), which takes an array of Object containing the actual arguments. This array is again created using the new Java 1.1 syntax: new Object[]{new Double(info.data)} In this case, however, the double must be placed inside a wrapper class so it can be part of this array of objects. The process of calling newInstance( ) extracts the double, but you can see it is a bit confusing – an argument might be a double or a Double, but when you make the call you must always pass in a Double. Fortunately this issue exists only for the primitive types. Once you understand how to do it, the process of creating a new object given only a Class handle is remarkably simple. Reflection also allows you to call methods in this same dynamic fashion. 644 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 645. Of course, the appropriate Class handle may not be in the trashTypes list. In this case, the return in the inner loop is never executed and you’ll drop out at the end. Here, the program tries to rectify the situation by loading the Class object dynamically and adding it to the trashTypes list. If it still can’t be found something is really wrong, but if the load is successful then the factory method is called recursively to try again. As you’ll see, the beauty of this design is that this code doesn’t need to be changed, regardless of the different situations where it will be used (assuming all Trash subclasses contain a constructor that takes a single double argument). Trash subclasses To fit into the prototyping scheme, the only thing that’s required of each new subclass of Trash is that it contains a constructor that takes a double argument. Java 1.1 reflection handles everything else. Here are the different types of Trash, each in their own file but part of the Trash package (again, to facilitate reuse within the chapter): //: Aluminum.java // The Aluminum class with prototyping package c16.trash; public class Aluminum extends Trash { private static double val = 1.67f; public Aluminum(double wt) { super(wt); } public double value() { return val; } public static void value(double newVal) { val = newVal; } } ///:~ //: Paper.java // The Paper class with prototyping package c16.trash; public class Paper extends Trash { private static double val = 0.10f; public Paper(double wt) { super(wt); } public double value() { return val; } public static void value(double newVal) { val = newVal; } } ///:~ //: Glass.java // The Glass class with prototyping package c16.trash; public class Glass extends Trash { private static double val = 0.23f; public Glass(double wt) { super(wt); } public double value() { return val; } public static void value(double newVal) { val = newVal; } } ///:~ Chapter 16: Design Patterns 645
- 646. And here’s a new type of Trash: //: Cardboard.java // The Cardboard class with prototyping package c16.trash; public class Cardboard extends Trash { private static double val = 0.23f; public Cardboard(double wt) { super(wt); } public double value() { return val; } public static void value(double newVal) { val = newVal; } } ///:~ You can see that, other than the constructor, there’s nothing special about any of these classes. Parsing Trash from an external file The information about Trash objects will be read from an outside file. The file has information about each piece of Trash on a single line in the form Trash:weight, such as: c16.Trash.Glass:54 c16.Trash.Paper:22 c16.Trash.Paper:11 c16.Trash.Glass:17 c16.Trash.Aluminum:89 c16.Trash.Paper:88 c16.Trash.Aluminum:76 c16.Trash.Cardboard:96 c16.Trash.Aluminum:25 c16.Trash.Aluminum:34 c16.Trash.Glass:11 c16.Trash.Glass:68 c16.Trash.Glass:43 c16.Trash.Aluminum:27 c16.Trash.Cardboard:44 c16.Trash.Aluminum:18 c16.Trash.Paper:91 c16.Trash.Glass:63 c16.Trash.Glass:50 c16.Trash.Glass:80 c16.Trash.Aluminum:81 c16.Trash.Cardboard:12 c16.Trash.Glass:12 c16.Trash.Glass:54 c16.Trash.Aluminum:36 c16.Trash.Aluminum:93 c16.Trash.Glass:93 c16.Trash.Paper:80 c16.Trash.Glass:36 c16.Trash.Glass:12 c16.Trash.Glass:60 c16.Trash.Paper:66 c16.Trash.Aluminum:36 c16.Trash.Cardboard:22 646 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 647. Note that the class path must be respected when giving the class names, otherwise the class will not be found. To parse this, the line is read in and the String method indexOf( ) is used to produce the index of the ‘:’. This is first used with the String method substring( ) to extract the name of the Trash type, and next to get the weight which is turned into a double with the static Double.valueOf( ) method. The trim( ) method removes white space at both ends of a string. The Trash parser is placed in a separate file since it will be reused throughout this chapter: //: ParseTrash.java // Open a file and parse its contents into // Trash objects, placing each into a Vector package c16.trash; import java.util.*; import java.io.*; public class ParseTrash { public static void fillBin(String filename, Fillable bin) { try { BufferedReader data = new BufferedReader( new FileReader(filename)); String buf; while((buf = data.readLine())!= null) { String type = buf.substring(0, buf.indexOf(':')).trim(); double weight = Double.valueOf( buf.substring(buf.indexOf(':') + 1) .trim()).doubleValue(); bin.addTrash( Trash.factory( new Trash.Info(type, weight))); } data.close(); } catch(IOException e) { e.printStackTrace(); } catch(Exception e) { e.printStackTrace(); } } // Special case to handle Vector: public static void fillBin(String filename, Vector bin) { fillBin(filename, new FillableVector(bin)); } } ///:~ In RecycleA.java, a Vector was used to hold the Trash objects. However, other types of collections may be used as well. To allow for this, the first version of fillBin( ) takes a handle to a Fillable, which is simply an interface that supports a method called addTrash( ): //: Fillable.java Chapter 16: Design Patterns 647
- 648. // Any object that can be filled with Trash package c16.trash; public interface Fillable { void addTrash(Trash t); } ///:~ Anything that supports this interface can be used with fillBin. Of course, Vector doesn’t implement Fillable, so it won’t work. Since Vector is used in most of the examples, it makes sense to add a second overloaded fillBin( ) method that takes a Vector. To allow the Vector to be used as a Fillable object, it must be adapted. This is easily accomplished using a class: //: FillableVector.java // Adapter that makes a Vector Fillable package c16.trash; import java.util.*; public class FillableVector implements Fillable { private Vector v; public FillableVector(Vector vv) { v = vv; } public void addTrash(Trash t) { v.addElement(t); } } ///:~ You can see that the only job of this class is to connect Fillable’s addTrash( ) method to Vector’s addElement( ). With this class in hand, the overloaded fillBin( ) method can be used with a Vector in ParseTrash.java: public static void fillBin(String filename, Vector bin) { fillBin(filename, new FillableVector(bin)); } This approach works for any collection class that’s used frequently. Alternatively, the collection class can provide its own adapter that implements Fillable (you’ll see this later, in DynaTrash.java). Recycling with prototyping Now you can see the revised version of RecycleA.java using the prototyping technique: //: RecycleAP.java // Recycling with RTTI and Prototypes package c16.recycleap; import c16.trash.*; import java.util.*; public class RecycleAP { public static void main(String args[]) { Vector bin = new Vector(); // Fill up the Trash bin: ParseTrash.fillBin("Trash.dat", bin); Vector glassBin = new Vector(), paperBin = new Vector(), alBin = new Vector(); 648 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 649. Enumeration sorter = bin.elements(); // Sort the Trash: while(sorter.hasMoreElements()) { Object t = sorter.nextElement(); // RTTI to show class membership: if(t instanceof Aluminum) alBin.addElement(t); if(t instanceof Paper) paperBin.addElement(t); if(t instanceof Glass) glassBin.addElement(t); } Trash.sumValue(alBin); Trash.sumValue(paperBin); Trash.sumValue(glassBin); Trash.sumValue(bin); } } ///:~ All the Trash objects, as well as the ParseTrash and support classes, are now part of the package c16.trash so they are simply imported. The process of opening the data file containing Trash descriptions and the parsing of that file has been wrapped in the static method ParseTrash.fillBin( ) so now it’s no longer a part of our design focus. You will see that throughout the rest of the chapter, no matter what new classes are added, ParseTrash.fillBin( ) will continue to work without change, which indicates a good design. In terms of object creation, this design does indeed severely localize the changes you need to make to add a new type to the system. However, there’s a significant problem in the use of RTTI that shows up very clearly here. The program seems to run just fine, and yet it never detects any cardboard, even though there is cardboard in the list! This happens because of the use of RTTI, which looks for only the types that you tell it to look for. The clue that RTTI is being misused is that every type in the system is being tested, rather than just a single type or subset of types. As you will see later, there are ways to use polymorphism instead when you’re testing for every type. But if you use RTTI a lot in this fashion, and you add a new type to your system, you can easily forget to make the necessary changes in your program and produce a difficult-to-find bug. So it’s worth trying to eliminate RTTI in this case, not just for aesthetic reasons – it produces more maintainable code. Abstracting usage With creation out of the way, it’s time to tackle the remainder of the design: where the classes are used. Since it’s the act of sorting into bins that’s particularly ugly and exposed, why not take that process and hide it inside a class? This is the principle of “if you must do something ugly, at least localize the ugliness inside a class.” It looks like this: Chapter 16: Design Patterns 649
- 650. Aluminum Vector Trash Sorter Paper Vector Vector of Trash Bins Glass Vector The TrashSorter object initialization must now be changed whenever a new type of Trash is added to the model. You could imagine that the TrashSorter class might look something like this: class TrashSorter extends Vector { sort(Trash t) { /* ... */ } } That is, TrashSorter is a Vector of handles to Vectors of Trash handles, and with the addElement( ) method you can install another one, like so: TrashSorter ts = new TrashSorter(); ts.addElement(new Vector()); Now, however, the sort( ) method becomes a problem. How does the statically-coded method deal with the fact that a new type has been added? To solve this, the type information must be removed from sort( ) so all it needs to do is call a generic method which takes care of the details of type. This, of course, is another way to describe a dynamically-bound method. So sort( ) will simply move through the sequence and call a dynamically-bound method for each Vector. Since the job of this method is to grab the pieces of trash it is interested in, it’s called grab(Trash). The structure now looks like: Tbins: Aluminum Vector Trash Sorter boolean grab(Trash) Paper Vector Vector of Trash Bins boolean grab(Trash) Glass Vector boolean grab(Trash) TrashSorter needs to call each grab( ) method and get a different result depending on what type of Trash the current Vector is holding. That is, each Vector must be aware of the type it holds. The classic approach to this problem is to create a base “Trash bin” class and inherit a new derived class for each different type you want to hold. If Java had a parameterized type mechanism that would probably be the most straightforward approach. But rather than hand-coding all the classes that such a mechanism should be building for us, further observation can produce a better approach. A basic OOP design principle is “use data members for variation in state, use polymorphism for variation in behavior.” Your first thought may be that the grab( ) 650 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 651. method certainly behaves differently for a Vector that holds Paper than one that holds Glass. But what it does is strictly dependent on the type, and nothing else. This could be interpreted as a different state, and since Java has a class to represent type (Class) this can be used to determine the type of Trash a particular Tbin will hold. The constructor for this Tbin requires that you hand it the Class of your choice. This tells the Vector what type it is supposed to hold. Then the grab( ) method uses Class BinType and RTTI to see if the Trash object you’ve handed it matches the type it’s supposed to grab. Here is the whole program. The commented numbers (e.g. (*1*) ) mark sections that will be described following the code. //: RecycleB.java // Adding more objects to the recycling problem package c16.recycleb; import c16.trash.*; import java.util.*; // A vector that admits only the right type: class Tbin extends Vector { Class binType; Tbin(Class binType) { this.binType = binType; } boolean grab(Trash t) { // Comparing class types: if(t.getClass().equals(binType)) { addElement(t); return true; // Object grabbed } return false; // Object not grabbed } } class TbinList extends Vector { //(*1*) boolean sort(Trash t) { Enumeration e = elements(); while(e.hasMoreElements()) { Tbin bin = (Tbin)e.nextElement(); if(bin.grab(t)) return true; } return false; // bin not found for t } void sortBin(Tbin bin) { // (*2*) Enumeration e = bin.elements(); while(e.hasMoreElements()) if(!sort((Trash)e.nextElement())) System.out.println("Bin not found"); } } public class RecycleB { static Tbin bin = new Tbin(Trash.class); public static void main(String args[]) { // Fill up the Trash bin: ParseTrash.fillBin("Trash.dat", bin); Chapter 16: Design Patterns 651
- 652. TbinList trashBins = new TbinList(); trashBins.addElement( new Tbin(Aluminum.class)); trashBins.addElement( new Tbin(Paper.class)); trashBins.addElement( new Tbin(Glass.class)); // add one line here: (*3*) trashBins.addElement( new Tbin(Cardboard.class)); trashBins.sortBin(bin); // (*4*) Enumeration e = trashBins.elements(); while(e.hasMoreElements()) { Tbin b = (Tbin)e.nextElement(); Trash.sumValue(b); } Trash.sumValue(bin); } } ///:~ 1. TbinList holds a set of Tbin handles, so that sort( ) can iterate through the Tbins when it’s looking for a match for the Trash object you’ve handed it. 2. sortBin( ) allows you to pass an entire Tbin in, and it moves through the Tbin, picks out each piece of Trash and sorts it into the appropriate specific Tbin. Note the genericity of this code: it doesn’t change at all if new types are added. If the bulk of your code doesn’t need changing when a new type is added (or some other change occurs) then you have an easily-extensible system. 3. Now you can see how easy it is to add a new type. Very few lines must be changed to support the addition. If it’s really important, you can squeeze out even more by further manipulating the design. 4. One method call causes the contents of bin to be sorted into the respective specifically-typed bins. Multiple dispatching The above design is certainly satisfactory. Adding new types to the system consists of adding or modifying distinct classes without causing code changes to be propagated throughout the system. In addition, RTTI is not “misused” as it was in RecycleA.java. However, it’s possible to go one step further and take a purist viewpoint about RTTI and say that it should be eliminated altogether from the operation of sorting the trash into bins. To accomplish this, you must first take the perspective that all type-dependent activities – such as detecting the type of a piece of trash and putting it into the appropriate bin – should be controlled through polymorphism and dynamic binding. The previous examples first sorted by type, then acted on sequences of elements that were all of a particular type. But whenever you find yourself picking out particular types, stop 652 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 653. and think. The whole idea of polymorphism (dynamically-bound method calls) is to handle type-specific information for you. So why are you hunting for types? The answer is something you probably don’t think about: Java performs only single dispatching. That is, if you are performing an operation on more than one object whose type is unknown, Java will invoke the dynamic binding mechanism on only one of those types. This doesn’t solve the problem, so you end up detecting some types manually and effectively producing your own dynamic binding behavior. The solution is called multiple dispatching, which means setting up a configuration such that a single method call produces more than one dynamic method call, and thus determines more than one type in the process. To get this effect, then, you need to work with more than one type hierarchy: you’ll need a type hierarchy for each dispatch. The following example works with two hierarchies: the existing Trash family and a hierarchy of the types of trash bins that the trash will be placed into. This second hierarchy isn’t always obvious and in this case it needed to be created in order to produce multiple dispatching (in this case there will be only two dispatches, which is referred to as double dispatching). Implementing the double dispatch Remember that polymorphism can occur only via method calls, so if you want double dispatching to occur there must be two method calls, one to determine the type in each hierarchy. In the Trash hierarchy there will be a new method called addToBin( ), which takes an argument of an array of TypedBin. It uses this array to step through and try to add itself to the appropriate bin, and this is where you’ll see the double dispatch. Trash addToBin(TypedBin[]) Aluminum Paper Glass Cardboard addToBin() addToBin() addToBin() addToBin() The new hierarchy is TypedBin, and it contains its own method called add( ) which is also used polymorphically. But here’s an additional twist: add( ) is overloaded to take arguments of the different types of trash. So an essential part of the double dispatching scheme also involves overloading. Chapter 16: Design Patterns 653
- 654. TypedBin add(Aluminum) add(Paper) add(Glass) add(Cardboard) AluminumBin PaperBin GlassBin CardboardBin add(Aluminum) add(Paper) add(Glass) add(Cardboard) Redesigning the program encounters a dilemma: it’s now necessary that the base class Trash contain a method addToBin( ). One approach to this is to copy all the code and change the base class. Another approach, which is one you can take when you don’t have control of the source code, is to put the addToBin( ) method in an interface, leave Trash alone, and inherit new specific types of Aluminum, Paper, Glass and Cardboard. This is the approach taken here. Most of the classes in this design must be public so they are placed in their own files. Here’s the interface //: TypedBinMember.java // An interface for adding the double dispatching // method to the trash hierarchy without // modifying the original hierarchy. package c16.doubledispatch; interface TypedBinMember { // The new method: boolean addToBin(TypedBin[] tb); } ///:~ In each particular subtype of Aluminum, Paper, Glass and Cardboard, the addToBin( ) method in the interface TypedBinMember is implemented,, but it looks like the code is exactly the same in each case: //: DDAluminum.java // Aluminum for double dispatching package c16.doubledispatch; import c16.trash.*; public class DDAluminum extends Aluminum implements TypedBinMember { public DDAluminum(double wt) { super(wt); } public boolean addToBin(TypedBin[] tb) { for(int i = 0; i < tb.length; i++) if(tb[i].add(this)) return true; return false; } } ///:~ //: DDPaper.java 654 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 655. // Paper for double dispatching package c16.doubledispatch; import c16.trash.*; public class DDPaper extends Paper implements TypedBinMember { public DDPaper(double wt) { super(wt); } public boolean addToBin(TypedBin[] tb) { for(int i = 0; i < tb.length; i++) if(tb[i].add(this)) return true; return false; } } ///:~ //: DDGlass.java // Glass for double dispatching package c16.doubledispatch; import c16.trash.*; public class DDGlass extends Glass implements TypedBinMember { public DDGlass(double wt) { super(wt); } public boolean addToBin(TypedBin[] tb) { for(int i = 0; i < tb.length; i++) if(tb[i].add(this)) return true; return false; } } ///:~ //: DDCardboard.java // Cardboard for double dispatching package c16.doubledispatch; import c16.trash.*; public class DDCardboard extends Cardboard implements TypedBinMember { public DDCardboard(double wt) { super(wt); } public boolean addToBin(TypedBin[] tb) { for(int i = 0; i < tb.length; i++) if(tb[i].add(this)) return true; return false; } } ///:~ The code in each addToBin( ) calls add( ) for each TypedBin object in the array. But notice the argument: this. The type of this is different for each subclass of Trash, so the code is in fact different (although this code will benefit if a parameterized type mechanism is ever added to Java). So this is the first part of the double dispatch, because once you’re inside this method you know you’re Aluminum, or Paper, etc. During the call to add( ), this information is passed via the type of this. The compiler resolves the call to the proper overloaded version of add( ). But since tb[i] produces a handle to the base type TypedBin, this call will end up calling a different method depending on the actual type of TypedBin that’s currently selected. This is the second dispatch. Chapter 16: Design Patterns 655
- 656. Here’s the base class for TypedBin: //: TypedBin.java // Vector that knows how to grab the right type package c16.doubledispatch; import c16.trash.*; import java.util.*; public abstract class TypedBin { Vector v = new Vector(); protected boolean addIt(Trash t) { v.addElement(t); return true; } public Enumeration elements() { return v.elements(); } public boolean add(DDAluminum a) { return false; } public boolean add(DDPaper a) { return false; } public boolean add(DDGlass a) { return false; } public boolean add(DDCardboard a) { return false; } } ///:~ You can see that the overloaded add( ) methods all return false. This means that if the method is not overloaded in a derived class, it will continue to return false and the caller (the method addToBin( ), in this case) will assume that the current Trash object has not successfully been added to a collection, and continue searching for the right collection. In each of the subclasses of TypedBin, only one overloaded method is overridden, according to the type of bin that’s being created. For example, CardboardBin overrides add(DDCardboard). The overridden method adds the trash object to its collection and returns true, while all the rest of the add( ) methods in CardboardBin continue to return false since they haven’t been overridden. You’ll see that this is another case where a parameterized type mechanism in Java would allow automatic generation of code (with C++ templates, you wouldn’t have to explicitly write the subclasses or the addToBin( ) method in Trash). Since for this example the trash types have been customized and placed in a different directory, you’ll need a different trash data file to make it work. Here’s a possible DDTrash.dat: c16.DoubleDispatch.DDGlass:54 c16.DoubleDispatch.DDPaper:22 c16.DoubleDispatch.DDPaper:11 c16.DoubleDispatch.DDGlass:17 c16.DoubleDispatch.DDAluminum:89 c16.DoubleDispatch.DDPaper:88 c16.DoubleDispatch.DDAluminum:76 c16.DoubleDispatch.DDCardboard:96 656 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 657. c16.DoubleDispatch.DDAluminum:25 c16.DoubleDispatch.DDAluminum:34 c16.DoubleDispatch.DDGlass:11 c16.DoubleDispatch.DDGlass:68 c16.DoubleDispatch.DDGlass:43 c16.DoubleDispatch.DDAluminum:27 c16.DoubleDispatch.DDCardboard:44 c16.DoubleDispatch.DDAluminum:18 c16.DoubleDispatch.DDPaper:91 c16.DoubleDispatch.DDGlass:63 c16.DoubleDispatch.DDGlass:50 c16.DoubleDispatch.DDGlass:80 c16.DoubleDispatch.DDAluminum:81 c16.DoubleDispatch.DDCardboard:12 c16.DoubleDispatch.DDGlass:12 c16.DoubleDispatch.DDGlass:54 c16.DoubleDispatch.DDAluminum:36 c16.DoubleDispatch.DDAluminum:93 c16.DoubleDispatch.DDGlass:93 c16.DoubleDispatch.DDPaper:80 c16.DoubleDispatch.DDGlass:36 c16.DoubleDispatch.DDGlass:12 c16.DoubleDispatch.DDGlass:60 c16.DoubleDispatch.DDPaper:66 c16.DoubleDispatch.DDAluminum:36 c16.DoubleDispatch.DDCardboard:22 Here’s the rest of the program: //: DoubleDispatch.java // Using multiple dispatching to handle more // than one unknown type during a method call. package c16.doubledispatch; import c16.trash.*; import java.util.*; class AluminumBin extends TypedBin { public boolean add(DDAluminum a) { return addIt(a); } } class PaperBin extends TypedBin { public boolean add(DDPaper a) { return addIt(a); } } class GlassBin extends TypedBin { public boolean add(DDGlass a) { return addIt(a); } } class CardboardBin extends TypedBin { public boolean add(DDCardboard a) { return addIt(a); Chapter 16: Design Patterns 657
- 658. } } class TrashBinSet { private TypedBin[] binSet = { new AluminumBin(), new PaperBin(), new GlassBin(), new CardboardBin() }; public void sortIntoBins(Vector bin) { Enumeration e = bin.elements(); while(e.hasMoreElements()) { TypedBinMember t = (TypedBinMember)e.nextElement(); if(!t.addToBin(binSet)) System.err.println("Couldn't add " + t); } } public TypedBin[] binSet() { return binSet; } } public class DoubleDispatch { public static void main(String args[]) { Vector bin = new Vector(); TrashBinSet bins = new TrashBinSet(); // ParseTrash still works, without changes: ParseTrash.fillBin("DDTrash.dat", bin); // Sort from the master bin into the // individually-typed bins: bins.sortIntoBins(bin); TypedBin[] tb = bins.binSet(); // Perform sumValue for each bin... for(int i = 0; i < tb.length; i++) Trash.sumValue(tb[i].v); // ... and for the master bin Trash.sumValue(bin); } } ///:~ TrashBinSet encapsulates all the different types of TypedBins, along with the sortIntoBins( ) method which is where all the double dispatching takes place. You can see that once the structure is set up, sorting into the various TypedBins is remarkably easy. In addition, the efficiency of two dynamic method calls is probably better than any other way you could sort. Note the ease of use of this system inside main( ), as well as the complete independence of any specific type information within main( ). All other methods that talk only to the Trash base-class interface will be equally invulnerable to changes in Trash types. The code changes necessary to add a new type are relatively isolated: you inherit the new type of Trash with its addToBin( ) method, then you inherit a new TypedBin (this is really just a copy and simple edit) and finally you add a new type in the aggregate initialization for TrashBinSet. 658 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 659. The “visitor” pattern Now consider applying a design pattern with an entirely different goal to the trash- sorting problem. For this pattern, we are no longer concerned with optimizing the addition of new types of Trash to the system. In fact, this pattern makes adding a new type of Trash more complicated. The assumption is actually that you have a primary class hierarchy that is fixed; perhaps it is from another vendor and you cannot make changes to that hierarchy. However, you’d like to add new polymorphic methods to that hierarchy, which means that normally you’d have to add something to the base class interface. So the dilemma is that you need to add methods to the base class, but you can’t touch the base class. How do you get around this? The design pattern that solves this kind of problem is called a “visitor” (the last one in the Design Patterns book) and it builds on the double dispatching scheme shown in the last section. The visitor pattern allows you to extend the interface of the primary type by creating a separate class hierarchy of type Visitor to virtualize the operations performed upon the primary type. The objects of the primary type simply “accept” the visitor, then call the visitor’s dynamically-bound method. It looks like this: Trash Visitor accept(Visitor) Visit(Aluminum) Visit(Paper) Visit(Glass) Paper Glass WeightVisitor etc. Aluminum accept(Visitor v) { PriceVisitor v.visit(this); } visit(Aluminum) { // Perform Aluminum- // specific work } visit(Paper) { // Perform Paper- // specific work } // etc. Now, if v is a Visitable handle to an Aluminum object, the code: PriceVisitor pv = new PriceVisitor(); v.accept(pv); causes two polymorphic method calls: the first to select Aluminum’s version of accept( ), and the second within accept( ) when the specific version of visit( ) is called dynamically using the base-class Visitor handle v. Chapter 16: Design Patterns 659
- 660. This configuration means that new functionality can be added to the system in the form of new subclasses of Visitor. The Trash hierarchy doesn’t need to be touched. This is the prime benefit of the visitor pattern: you can add new polymorphic functionality to a class hierarchy without touching that hierarchy (once the accept( ) methods have been installed). Notice the benefit is helpful here but not exactly what we started out to accomplish, so at first blush you might decide this isn’t the desired solution. But look at one thing that’s been accomplished. The visitor solution avoids sorting from the master Trash sequence into individual typed sequences. Thus, you can leave everything in the single master sequence and simply pass through that sequence with the appropriate visitor to accomplish the goal. Although this behavior seems to be a side effect of visitor it does give us what we want (avoiding RTTI). The double dispatching in the visitor pattern takes care of determining both the type of Trash and the type of Visitor. In the following example, there are two implementations of Visitor, PriceVisitor to determine and sum the price and WeightVisitor to keep track of the weights. You can see all of this implemented in the new, improved version of the recycling program. As with DoubleDispatch.java, the Trash class is left alone and a new interface is created to add the accept( ) method: //: Visitable.java // An interface to add visitor functionality to // the Trash hierarchy without modifying the // base class. package c16.trashvisitor; import c16.trash.*; interface Visitable { // The new method: void accept(Visitor v); } ///:~ The subtypes of Aluminum, Paper, Glass and Cardboard just implement the accept( ) method: //: VAluminum.java // Aluminum for the visitor pattern package c16.trashvisitor; import c16.trash.*; public class VAluminum extends Aluminum implements Visitable { public VAluminum(double wt) { super(wt); } public void accept(Visitor v) { v.visit(this); } } ///:~ //: VPaper.java // Paper for the visitor pattern package c16.trashvisitor; import c16.trash.*; public class VPaper extends Paper implements Visitable { 660 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 661. public VPaper(double wt) { super(wt); } public void accept(Visitor v) { v.visit(this); } } ///:~ //: VGlass.java // Glass for the visitor pattern package c16.trashvisitor; import c16.trash.*; public class VGlass extends Glass implements Visitable { public VGlass(double wt) { super(wt); } public void accept(Visitor v) { v.visit(this); } } ///:~ //: VCardboard.java // Cardboard for the visitor pattern package c16.trashvisitor; import c16.trash.*; public class VCardboard extends Cardboard implements Visitable { public VCardboard(double wt) { super(wt); } public void accept(Visitor v) { v.visit(this); } } ///:~ Since there’s nothing concrete in the Visitor base class, it can be created as an interface: //: Visitor.java // The base interface for visitors package c16.trashvisitor; import c16.trash.*; interface Visitor { void visit(VAluminum a); void visit(VPaper p); void visit(VGlass g); void visit(VCardboard c); } ///:~ Once again custom Trash types have been created in a different subdirectory. The new Trash data file is called VTrash.dat and it looks like this: c16.TrashVisitor.VGlass:54 c16.TrashVisitor.VPaper:22 c16.TrashVisitor.VPaper:11 c16.TrashVisitor.VGlass:17 c16.TrashVisitor.VAluminum:89 c16.TrashVisitor.VPaper:88 c16.TrashVisitor.VAluminum:76 c16.TrashVisitor.VCardboard:96 c16.TrashVisitor.VAluminum:25 Chapter 16: Design Patterns 661
- 662. c16.TrashVisitor.VAluminum:34 c16.TrashVisitor.VGlass:11 c16.TrashVisitor.VGlass:68 c16.TrashVisitor.VGlass:43 c16.TrashVisitor.VAluminum:27 c16.TrashVisitor.VCardboard:44 c16.TrashVisitor.VAluminum:18 c16.TrashVisitor.VPaper:91 c16.TrashVisitor.VGlass:63 c16.TrashVisitor.VGlass:50 c16.TrashVisitor.VGlass:80 c16.TrashVisitor.VAluminum:81 c16.TrashVisitor.VCardboard:12 c16.TrashVisitor.VGlass:12 c16.TrashVisitor.VGlass:54 c16.TrashVisitor.VAluminum:36 c16.TrashVisitor.VAluminum:93 c16.TrashVisitor.VGlass:93 c16.TrashVisitor.VPaper:80 c16.TrashVisitor.VGlass:36 c16.TrashVisitor.VGlass:12 c16.TrashVisitor.VGlass:60 c16.TrashVisitor.VPaper:66 c16.TrashVisitor.VAluminum:36 c16.TrashVisitor.VCardboard:22 The rest of the program creates specific Visitor types and sends them through a single list of Trash objects: //: TrashVisitor.java // The "visitor" pattern package c16.trashvisitor; import c16.trash.*; import java.util.*; // Specific group of algorithms packaged // in each implementation of Visitor: class PriceVisitor implements Visitor { private double alSum; // Aluminum private double pSum; // Paper private double gSum; // Glass private double cSum; // Cardboard public void visit(VAluminum al) { double v = al.weight() * al.value(); System.out.println( "value of Aluminum= " + v); alSum += v; } public void visit(VPaper p) { double v = p.weight() * p.value(); System.out.println( "value of Paper= " + v); pSum += v; } public void visit(VGlass g) { double v = g.weight() * g.value(); 662 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 663. System.out.println( "value of Glass= " + v); gSum += v; } public void visit(VCardboard c) { double v = c.weight() * c.value(); System.out.println( "value of Cardboard = " + v); cSum += v; } void total() { System.out.println( "Total Aluminum: $" + alSum + "n" + "Total Paper: $" + pSum + "n" + "Total Glass: $" + gSum + "n" + "Total Cardboard: $" + cSum); } } class WeightVisitor implements Visitor { private double alSum; // Aluminum private double pSum; // Paper private double gSum; // Glass private double cSum; // Cardboard public void visit(VAluminum al) { alSum += al.weight(); System.out.println("weight of Aluminum = " + al.weight()); } public void visit(VPaper p) { pSum += p.weight(); System.out.println("weight of Paper = " + p.weight()); } public void visit(VGlass g) { gSum += g.weight(); System.out.println("weight of Glass = " + g.weight()); } public void visit(VCardboard c) { cSum += c.weight(); System.out.println("weight of Cardboard = " + c.weight()); } void total() { System.out.println("Total weight Aluminum:" + alSum); System.out.println("Total weight Paper:" + pSum); System.out.println("Total weight Glass:" + gSum); System.out.println("Total weight Cardboard:" + cSum); } } Chapter 16: Design Patterns 663
- 664. public class TrashVisitor { public static void main(String args[]) { Vector bin = new Vector(); // ParseTrash still works, without changes: ParseTrash.fillBin("VTrash.dat", bin); // You could even iterate through // a list of visitors! PriceVisitor pv = new PriceVisitor(); WeightVisitor wv = new WeightVisitor(); Enumeration it = bin.elements(); while(it.hasMoreElements()) { Visitable v = (Visitable)it.nextElement(); v.accept(pv); v.accept(wv); } pv.total(); wv.total(); } } ///:~ Notice that the shape of main( ) has changed again. Now there’s only a single Trash bin. The two Visitor objects are accepted into every element in the sequence, and they perform their operations. The visitors keep their own internal data to tally the total weights and prices. Finally, there’s no run-time type identification other than the inevitable cast to Trash when pulling things out of the sequence. This, too, could be eliminated with the implementation of parameterized types in Java. One way you can distinguish this solution from the double dispatching solution described previously is to note that in the double dispatching solution only one of the overloaded methods add( ) was overridden when each subclass was created, while here each one of the overloaded visit( ) methods is overridden in every subclass of Visitor. More coupling? There’s a lot more code here, and there’s definite coupling between the Trash hierarchy and the Visitor hierarchy. However there’s also very high cohesion within the respective sets of classes: they each do only one thing (Trash describes Trash, while Visitor describes actions performed on Trash), which is an indicator of a good design. Of course, in this case it works well only if you’re adding new Visitors, but it gets in the way when you add new types of Trash. Low coupling between classes and high cohesion within a class is definitely an important design goal. Applied mindlessly, though, it can prevent you from achieving a more elegant design. In particular it seems that some classes inevitably have a certain intimacy with each other. These often occur in pairs that could perhaps be called couplets, for example collections and iterators (Enumerations). The above Trash-Visitor pair appears to be another such couplet. RTTI considered harmful? Various designs in this chapter attempt to remove RTTI, and this could give you the impression that it’s “considered harmful” (the condemnation used for poor, ill-fated goto which was thus never put into Java). This isn’t true; the misuse of RTTI is the problem. The reason these designs removed RTTI is because the misapplication of that feature 664 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 665. prevented extensibility, and the stated goal was to be able to add a new type to the system with as little impact on surrounding code as possible. Since RTTI is often misused to look for every single type in your system, it causes code to be non-extensible: when you add a new type, you have to go hunting for all the code where RTTI is used, and if you miss any you won’t get help from the compiler. However, using RTTI doesn’t automatically create non-extensible code. Let’s revisit the trash recycler once more. This time, a new tool will be introduced: a Hashtable that holds Vectors. The keys for this Hashtable, which I shall call a TypeMap, are the types of trash in the associated Vector. The beauty of this design (suggested by Larry O’Brien) is that the TypeMap dynamically adds a new pair whenever it encounters a new type, so whenever you add a new type to the system (even if you add the new type at run-time) it adapts. The example will again build on the structure of the Trash types in package c16.Trash (and the Trash.dat file used there can be utilized here without change): //: DynaTrash.java // Using a Hashtable of Vectors and RTTI // to automatically sort trash into // vectors. This solution, despite the // use of RTTI, is extensible. package c16.dynatrash; import c16.trash.*; import java.util.*; // Generic TypeMap works in any situation: class TypeMap { private Hashtable t = new Hashtable(); public void add(Object o) { Class type = o.getClass(); if(t.containsKey(type)) ((Vector)t.get(type)).addElement(o); else { Vector v = new Vector(); v.addElement(o); t.put(type,v); } } public Vector get(Class type) { return (Vector)t.get(type); } public Enumeration keys() { return t.keys(); } // Returns handle to adapter class to allow // callbacks from ParseTrash.fillBin(): public Fillable filler() { // Anonymous inner class: return new Fillable() { public void addTrash(Trash t) { add(t); } }; } } public class DynaTrash { public static void main(String args[]) { TypeMap bin = new TypeMap(); ParseTrash.fillBin("Trash.dat",bin.filler()); Enumeration keys = bin.keys(); Chapter 16: Design Patterns 665
- 666. while(keys.hasMoreElements()) Trash.sumValue( bin.get((Class)keys.nextElement())); } } ///:~ Although powerful, the definition for TypeMap is very simple. It contains a Hashtable, and the add( ) method does most of the work. When you add( ) a new object, the handle for the Class object for that type is extracted. This is used as a key to determine whether a Vector holding objects of that type is already present in the Hashtable. If so, that Vector is extracted and the object is added to the Vector. If not, the Class object and a new Vector are added as a key-value pair. You can get an Enumeration of all the Class objects with keys( ), and use each Class object to fetch the corresponding Vector with get( ). And that’s all there is to it. The filler( ) method is interesting because it takes advantage of the design of ParseTrash.fillBin( ), which doesn’t just try to fill a Vector but instead anything that implements the Fillable interface with its addTrash( ) method. All filler( ) needs to do is return a handle to an interface that implements Fillable, and then this handle can be used as an argument to fillBin( ) like this: ParseTrash.fillBin("Trash.dat", bin.filler()); To produce this handle, an anonymous inner class (described in Chapter 7) is used. You never need a named class to implement Fillable, you just need a handle to an object of that class, thus this is an appropriate use of anonymous inner classes. An interesting thing about this design is that even though it wasn’t created to handle the sorting, fillBin( ) is actually performing a sort every time it inserts a Trash object into bin. Much of class DynaTrash should be familiar from the previous examples. This time, instead of putting the new Trash objects into a bin of type Vector, the bin is of type TypeMap, and so when the trash is thrown into bin it’s immediately sorted by TypeMap’s internal sorting mechanism. Stepping through the TypeMap and operating on each individual Vector becomes a very simple matter: Enumeration keys = bin.keys(); while(keys.hasMoreElements()) Trash.sumValue( bin.get((Class)keys.nextElement())); As you can see, adding a new type to the system won’t affect this code, or the code in TypeMap, at all. This is certainly the smallest solution to the problem, and arguably the most elegant as well. It does rely heavily on RTTI, but notice that each key-value pair in the Hashtable is looking for only one type. In addition, there’s no way you can “forget” to add the proper code to this system when you add a new type, since there isn’t any code you need to add. Summary Coming up with a design such as TrashVisitor.java that contains a larger amount of code than the earlier designs can seem at first to be counterproductive. It pays to notice what you’re trying to accomplish with different designs. Design patterns in general strive to separate the things that change from the things that stay the same. The “things that change” 666 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 667. can refer to many different kinds of changes. Perhaps the change occurs because the program is placed in a new environment, or because something in the current environment changes (this could be: “the user wants to add a new shape to the diagram on the screen”). Or, as in this case, the change could be the evolution of the code body itself. While previous versions of the trash-sorting example emphasized the addition of new types of Trash to the system, TrashVisitor.java allows you to easily add new functionality without disturbing the Trash hierarchy. There’s more code in TrashVisitor.java, but adding new functionality to Visitor is very cheap. If this is something that happens a lot, then it’s worth the extra effort and code to make it happen more easily. The discovery of the vector of change is no trivial matter; it’s not something that an analyst can usually detect before the program sees its initial design. The necessary information will probably not appear until later phases in the project: sometimes only at the design or implementation phases do you discover a deeper or more subtle need in your system. In the case of adding new types (which was the focus of most of the “recycle” examples) you might realize that you need a particular inheritance hierarchy only when you are in the maintenance phase and you begin extending the system! One of the most important things you will learn by studying design patterns seems to be an about-face from what has been promoted so far in this book. That is: “OOP is all about polymorphism.” This statement can produce the “two-year-old with a hammer” syndrome (everything looks like a nail). That is, it’s hard enough to “get” polymorphism, and once you do you try to cast all your designs into that one particular mold. What design patterns say is that OOP isn’t just about polymorphism. It’s about “separating the things that change from the things that stay the same.” Polymorphism is an especially important way to do this, and it turns out to be very helpful if the programming language directly supports polymorphism (so you don’t have to wire it in yourself, which would tend to make it prohibitively expensive). But design patterns in general show other ways to accomplish the basic goal, and once your eyes have been opened to this you will begin to search for more creative designs. Since the Design Patterns book came out and made such an impact, people have been searching for other patterns. You can expect to see more of these appear as time goes on. Here are some sites recommended by Jim Coplien (http://www.bell-labs.com/~cope), who is one of the main proponents of the patterns movement: http://st-www.cs.uiuc.edu/users/patterns http://c2.com/cgi/wiki http://c2.com/ppr http://www.bell-labs.com/people/cope/Patterns/Process/index.html http://www.bell-labs.com/cgi-user/OrgPatterns/OrgPatterns http://st-www.cs.uiuc.edu/cgi-bin/wikic/wikic http://www.cs.wustl.edu/~schmidt/patterns.html http://www.espinc.com/patterns/overview.html Exercises 1. Using SingletonPattern.java as a starting point, create a class that manages a fixed number of its own objects. 2. Add a class Plastic to TrashVisitor.java. 3. Add a class Plastic to DynaTrash.java. Chapter 16: Design Patterns 667
- 669. Q 17: Projects This chapter includes a set of projects that build on the material presented in this book, or otherwise didn’t fit in earlier chapters. Most of these projects are significantly more complex than the examples in the rest of the book, and they often demonstrate new techniques and uses of class libraries. Text processing If you come from a C or C++ background, you may be skeptical at first of Java’s power when it comes to handling text. Indeed, one drawback is that execution speed is slower and that may hinder some of your efforts. However, the tools (in particular the String class) are quite powerful as the examples in this section show. As you’ll see, these examples were created to solve problems that arose in the creation of this book. However, they are not restricted to that and the solutions they offer can easily be adapted to other situations. In addition they show the power of Java in an area that has not previously been emphasized. Extracting code listings You’ve no doubt noticed that each complete code listing (not code fragment) in this book begins and ends with special comment tag marks ‘//:’ and ‘///:~’. This meta-information is included so that the code can be automatically extracted from the book into compilable source-code files. In my previous book, I had a system that allowed me to automatically incorporate tested code files into the book. In this book, however, I discovered that it was often easier to paste the code into the book once it was initially tested and (since you never get it right the first time) perform edits to the code within the book. But how to extract it and test the code? This program is the answer, and it may come in handy when you set 669
- 670. out to solve a text processing problem. In addition, it demonstrates many of the String class features. I first save the entire book in ASCII text format, in a separate file. The CodePackager program has two modes (which you can see described in usageString): if you use the -p flag, it expects to see an input file containing the ASCII text from the book. It will go through this file and use the comment tag marks to extract the code, and it uses the file name on the first line to determine the name of the file. In addition, it looks for the package statement in case it needs to put the file in a special directory (via the class path). But that’s not all. It also watches for the change in chapters by keeping track of the package names. Since all packages for each chapter begin with c02, c03, c04, etc. to indicate the chapter in which they belong (except for those beginning with com, which are ignored for the purpose of keeping track of chapters), as long as the first listing in each chapter contains a package statement containing the chapter number, the CodePackager program can keep track of when the chapter changed, and put all the subsequent files in the new chapter subdirectory. As each file is extracted, it is placed in a SourceCodeFile object which is then placed in a collection (this process will be more thoroughly described later). These SourceCodeFile objects could simply be stored in files, but that brings us to the second use for this project. If you invoke CodePackager without the -p flag it expects a “packed” file as input, which it will then extract into separate files. So the -p flag means that the extracted files will be “packed” into this single file. Why bother with the packed file? Because different computer platforms have different ways of storing text information in files. A big issue is the end-of-line character or characters, but other issues may also exist. However, Java has a special type of IO stream – the DataOutputStream – which promises that, regardless of what machine the data is coming from, the storage of that data will be in a form that can be correctly retrieved by any other machine by using a DataInputStream. That is, Java itself handles all the platform-specific details, which is a large part of the promise of Java. So the -p flag stores everything into a single file in a universal format. You download this file and the Java program from the Web, and when you run CodePackager on this file without the -p flag the files will all be extracted to appropriate places on your system (you can specify an alternate subdirectory; otherwise the subdirectories will just be created off the current directory). Some extra work ensures that no system-specific formats remain by using File objects everywhere a path or a file is described. In addition, there’s a sanity check: an empty file is placed in each subdirectory and the name of that file indicates how many files you should find in that subdirectory. Here is the code, which will be described in detail at the end of the listing: //: CodePackager.java // "Packs" and "unpacks" the code in "Thinking // in Java" for cross-platform distribution. /* Commented so CodePackager sees it and starts a new chapter directory, but so you don't have to worry about the directory where this program lives: package c17; */ import java.util.*; import java.io.*; class Pr { static void error(String e) { 670 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 671. System.err.println("ERROR: " + e); System.exit(1); } } class IO { static BufferedReader disOpen(File f) { BufferedReader in = null; try { in = new BufferedReader( new FileReader(f)); } catch(IOException e) { Pr.error("could not open " + f); } return in; } static BufferedReader disOpen(String fname) { return disOpen(new File(fname)); } static DataOutputStream dosOpen(File f) { DataOutputStream in = null; try { in = new DataOutputStream( new BufferedOutputStream( new FileOutputStream(f))); } catch(IOException e) { Pr.error("could not open " + f); } return in; } static DataOutputStream dosOpen(String fname) { return dosOpen(new File(fname)); } static PrintWriter psOpen(File f) { PrintWriter in = null; try { in = new PrintWriter( new BufferedWriter( new FileWriter(f))); } catch(IOException e) { Pr.error("could not open " + f); } return in; } static PrintWriter psOpen(String fname) { return psOpen(new File(fname)); } static void close(Writer os) { try { os.close(); } catch(IOException e) { Pr.error("closing " + os); } } static void close(DataOutputStream os) { try { Chapter 17: Projects 671
- 672. os.close(); } catch(IOException e) { Pr.error("closing " + os); } } static void close(Reader os) { try { os.close(); } catch(IOException e) { Pr.error("closing " + os); } } } class SourceCodeFile { public static final String startMarker = "//:", // Start of source file endMarker = "} ///:~", // End of source endMarker2 = "}; ///:~", // C++ file end beginContinue = "} ///:Continued", endContinue = "///:Continuing", packMarker = "###", // Packed file header tag eol = // Line separator on current system System.getProperty("line.separator"), filesep = // System's file path separator System.getProperty("file.separator"); public static String copyright = ""; static { try { BufferedReader cr = new BufferedReader( new FileReader("Copyright.txt")); String crin; while((crin = cr.readLine()) != null) copyright += crin + "n"; cr.close(); } catch(Exception e) { copyright = ""; } } private String filename, dirname, contents = new String(); private static String chapter = "c02"; // The file name separator from the old system: public static String oldsep; public String toString() { return dirname + filesep + filename; } // Constructor for parsing from document file: public SourceCodeFile(String firstLine, BufferedReader in) { dirname = chapter; // Skip past marker: filename = firstLine.substring( startMarker.length()).trim(); 672 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 673. // Find space that terminates file name: if(filename.indexOf(' ') != -1) filename = filename.substring( 0, filename.indexOf(' ')); System.out.println("found: " + filename); contents = firstLine + eol; if(copyright.length() != 0) contents += copyright + eol; String s; boolean foundEndMarker = false; try { while((s = in.readLine()) != null) { if(s.startsWith(startMarker)) Pr.error("No end of file marker for " + filename); // For this program, no spaces before // the "package" keyword are allowed // in the input source code: else if(s.startsWith("package")) { // Extract package name: String pdir = s.substring( s.indexOf(' ')).trim(); pdir = pdir.substring( 0, pdir.indexOf(';')).trim(); // Capture the chapter from the package // ignoring the 'com' subdirectories: if(!pdir.startsWith("com")) { int firstDot = pdir.indexOf('.'); if(firstDot != -1) chapter = pdir.substring(0,firstDot); else chapter = pdir; } // Convert package name to path name: pdir = pdir.replace( '.', filesep.charAt(0)); System.out.println("package " + pdir); dirname = pdir; } contents += s + eol; // Move past continuations: if(s.startsWith(beginContinue)) while((s = in.readLine()) != null) if(s.startsWith(endContinue)) { contents += s + eol; break; } // Watch for end of code listing: if(s.startsWith(endMarker) || s.startsWith(endMarker2)) { foundEndMarker = true; break; } } if(!foundEndMarker) Chapter 17: Projects 673
- 674. Pr.error( "End marker not found before EOF"); System.out.println("Chapter: " + chapter); } catch(IOException e) { Pr.error("Error reading line"); } } // For recovering from a packed file: public SourceCodeFile(BufferedReader pFile) { try { String s = pFile.readLine(); if(s == null) return; if(!s.startsWith(packMarker)) Pr.error("Can't find " + packMarker + " in " + s); s = s.substring( packMarker.length()).trim(); dirname = s.substring(0, s.indexOf('#')); filename = s.substring(s.indexOf('#') + 1); dirname = dirname.replace( oldsep.charAt(0), filesep.charAt(0)); filename = filename.replace( oldsep.charAt(0), filesep.charAt(0)); System.out.println("listing: " + dirname + filesep + filename); while((s = pFile.readLine()) != null) { // Watch for end of code listing: if(s.startsWith(endMarker) || s.startsWith(endMarker2)) { contents += s; break; } contents += s + eol; } } catch(IOException e) { System.err.println("Error reading line"); } } public boolean hasFile() { return filename != null; } public String directory() { return dirname; } public String filename() { return filename; } public String contents() { return contents; } // To write to a packed file: public void writePacked(DataOutputStream out) { try { out.writeBytes( packMarker + dirname + "#" + filename + eol); out.writeBytes(contents); } catch(IOException e) { Pr.error("writing " + dirname + filesep + filename); } 674 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 675. } // To generate the actual file: public void writeFile(String rootpath) { File path = new File(rootpath, dirname); path.mkdirs(); PrintWriter p = IO.psOpen(new File(path, filename)); p.print(contents); IO.close(p); } } class DirMap { private Hashtable t = new Hashtable(); private String rootpath; DirMap() { rootpath = System.getProperty("user.dir"); } DirMap(String alternateDir) { rootpath = alternateDir; } public void add(SourceCodeFile f){ String path = f.directory(); if(!t.containsKey(path)) t.put(path, new Vector()); ((Vector)t.get(path)).addElement(f); } public void writePackedFile(String fname) { DataOutputStream packed = IO.dosOpen(fname); try { packed.writeBytes("###Old Separator:" + SourceCodeFile.filesep + "###n"); } catch(IOException e) { Pr.error("Writing separator to " + fname); } Enumeration e = t.keys(); while(e.hasMoreElements()) { String dir = (String)e.nextElement(); System.out.println( "Writing directory " + dir); Vector v = (Vector)t.get(dir); for(int i = 0; i < v.size(); i++) { SourceCodeFile f = (SourceCodeFile)v.elementAt(i); f.writePacked(packed); } } IO.close(packed); } // Write all the files in their directories: public void write() { Enumeration e = t.keys(); while(e.hasMoreElements()) { String dir = (String)e.nextElement(); Vector v = (Vector)t.get(dir); for(int i = 0; i < v.size(); i++) { Chapter 17: Projects 675
- 676. SourceCodeFile f = (SourceCodeFile)v.elementAt(i); f.writeFile(rootpath); } // Add file indicating file quantity // written to this directory as a check: IO.close(IO.dosOpen( new File(new File(rootpath, dir), Integer.toString(v.size())+".files"))); } } } public class CodePackager { private static final String usageString = "usage: java CodePackager packedFileName" + "nExtracts source code files from packed n" + "version of Tjava.doc sources into " + "directories off current directoryn" + "java CodePackager packedFileName newDirn" + "Extracts into directories off newDirn" + "java CodePackager -p source.txt packedFile" + "nCreates packed version of source files" + "nfrom text version of Tjava.doc"; private static void usage() { System.err.println(usageString); System.exit(1); } public static void main(String args[]) { if(args.length == 0) usage(); if(args[0].equals("-p")) { if(args.length != 3) usage(); createPackedFile(args); } else { if(args.length > 2) usage(); extractPackedFile(args); } } private static String currentLine; private static BufferedReader in; private static DirMap dm; private static void createPackedFile(String args[]) { dm = new DirMap(); in = IO.disOpen(args[1]); try { while((currentLine = in.readLine()) != null) { if(currentLine.startsWith( SourceCodeFile.startMarker)) { dm.add(new SourceCodeFile( currentLine, in)); 676 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 677. } else if(currentLine.startsWith( SourceCodeFile.endMarker)) Pr.error("file has no start marker"); // Else ignore the input line } } catch(IOException e) { Pr.error("Error reading " + args[1]); } IO.close(in); dm.writePackedFile(args[2]); } private static void extractPackedFile(String args[]) { if(args.length == 2) // Alternate directory dm = new DirMap(args[1]); else // Current directory dm = new DirMap(); in = IO.disOpen(args[0]); String s = null; try { s = in.readLine(); } catch(IOException e) { Pr.error("Cannot read from " + in); } // Capture the separator used in the system // that packed the file: if(s.indexOf("###Old Separator:") != -1 ) { String oldsep = s.substring( "###Old Separator:".length()); oldsep = oldsep.substring( 0, oldsep. indexOf('#')); SourceCodeFile.oldsep = oldsep; } SourceCodeFile sf = new SourceCodeFile(in); while(sf.hasFile()) { dm.add(sf); sf = new SourceCodeFile(in); } dm.write(); } } ///:~ You’ll first notice the package statement which is commented out. Since this is the first program in the chapter, the package statement is necessary to tell CodePackager that the chapter has changed, but actually putting it in a package would be a problem. When you create a package, you tie the resulting program to a particular directory structure, which is fine for most of the examples in this book. Here, however, the CodePackager program must be compiled and run from an arbitrary directory, so the package statement is commented out. It will still look like an ordinary package statement to CodePackager, though, since the program isn’t sophisticated enough to detect multi-line comments (it has no need for such sophistication, a fact which comes in handy here). The first two classes are support/utility classes designed to make the rest of the program more consistent to write and more easily readable. The first, Pr, is very similar to the ANSI C library perror, since it prints an error message (but also exits the program). The second class encapsulates the creation of files, a process that was shown in Chapter 10 as Chapter 17: Projects 677
- 678. one that rapidly becomes verbose and annoying. In Chapter 10, the proposed solution created new classes, but here static method calls are used. Within those methods the appropriate exceptions are caught and dealt with. These methods make the rest of the code much cleaner to read. The first class that helps solve the actual problem is SourceCodeFile, which represents all the information (including the contents, file name and directory) for one source code file in the book. It also contains a set of String constants representing the markers that start and end a file, a marker used inside the packed file, the current system’s end-of-line separator and file path separator (notice the use of System.getProperty( ) to get the local version), and a copyright notice, which is extracted from the following file Copyright.txt. ////////////////////////////////////////////////// // Copyright (c) Bruce Eckel, 1997 // Source code file from the book "Thinking in Java" // All rights reserved EXCEPT as allowed by the // following statements: You may freely use this file // for your own work (personal or commercial), // including modifications and distribution in // executable form only. Permission is granted to use // this file in classroom situations, including its // use in presentation materials, as long as the book // "Thinking in Java" is cited as the source. // Except in classroom situations, you may not copy // and distribute this code; instead, the sole // distribution point is http://www.EckelObjects.com // (and official mirror sites) where it is // freely available. You may not remove this // copyright and notice. You may not distribute // modified versions of the source code in this // package. You may not use this file in printed // media without the express permission of the // author. Bruce Eckel makes no representation about // the suitability of this software for any purpose. // It is provided "as is" without express or implied // warranty of any kind, including any implied // warranty of merchantability, fitness for a // particular purpose or non-infringement. The entire // risk as to the quality and performance of the // software is with you. Bruce Eckel and the // publisher shall not be liable for any damages // suffered by you or any third party as a result of // using or distributing software. In no event will // Bruce Eckel or the publisher be liable for any // lost revenue, profit or data, or for direct, // indirect, special, consequential, incidental or // punitive damages, however caused and regardless of // the theory of liability, arising out of the use of // or inability to use software, even if Bruce Eckel // and the publisher have been advised of the // possibility of such damages. Should the software // prove defective, you assume the cost of all // necessary servicing, repair, or correction. If you // think you've found an error, please email all // modified files with clearly commented changes to: // Bruce@EckelObjects.com. (please use the same 678 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 679. // address for non-code errors found in the book). ////////////////////////////////////////////////// When extracting files from a packed file, the file separator of the system that packed the file is also noted, so it can be replaced with the correct one for the local system. The subdirectory name for the current chapter is kept in the field chapter, which is initialized to c02 (you’ll notice that the listing in Chapter 2 does not contain a package statement). The only time the chapter field changes is when a package statement is discovered in the current file. Building a packed file The first constructor is used to extract a file from the ASCII text version of this book. The calling code (which appears further down in the listing) reads each line in until it finds one that matches the beginning of a listing. At that point, it creates a new SourceCodeFile object, passing it the first line (which has already been read by the calling code) and the BufferedReader object from which to extract the rest of the source code listing. At this point, you begin to see heavy use of the String methods. To extract the file name, the overloaded version of substring( ) is called that takes the starting offset and just goes to the end of the String. This starting index is produced by finding the length( ) of the startMarker. The trim( ) method removes white space from both ends of the String. The first line may also have words after the name of the file; these are detected using the indexOf( ) method which returns -1 if it cannot find the character you’re looking for and the value where the first instance of that character is found if it does. Note there is also an overloaded version of indexOf( ) that takes a String instead of a character. Once the file name is parsed and stored, the first line is placed into the contents String (which is used to hold the entire text of the source code listing). At this point the rest of the lines are read and concatenated into the contents String. It’s not quite that simple, since certain situations require special handling. One case is error checking: if you run into a startMarker, it means that no end marker was placed at the end of the listing that’s currently being collected. This is an error condition which aborts the program. The second special case is the package keyword. Although Java is a free-form language, this program requires that the package keyword be at the beginning of the line. When the package keyword is discovered, the package name is extracted by looking for the space at the beginning and the semicolon at the end (note this could also have been performed in a single operation by using the overloaded substring( ) that takes both the starting and ending indexes). Then the dots in the package name are replaced by the file separator, although an assumption is made here that the file separator is only one character long. This is probably true on all systems, but it’s a place to look if there are problems. The default behavior is to concatenate each line to contents, along with the end-of-line string, until the endMarker is discovered which indicates that the constructor should terminate. If the end of the file is encountered before the endMarker is seen, that’s an error. Extracting from a packed file The second constructor is used to recover the source code files from a packed file. Here, the calling method doesn’t have to worry about skipping over the intermediate text. The file just contains all the source-code files, placed end-to-end. Thus, all you need to hand to this constructor is the BufferedReader where the information is coming from, and the constructor takes it from there. There is some meta-information, however, at the beginning of each listing, and this is denoted by the packMarker. If the packMarker isn’t there, it means the caller is mistakenly trying to use this constructor where it isn’t appropriate. Chapter 17: Projects 679
- 680. Once the packMarker is found, it is stripped off and the directory name (terminated by a ‘#’) and the file name (which goes to the end of the line) are extracted. In both cases, the old separator character is replaced by the one that is current to this machine, using the String replace( ) method. The old separator is placed at the beginning of the packed file, and you’ll see how that is extracted later in the listing. The rest of the constructor is quite simple. It reads and concatenates each line to the contents until the endMarker is found. Accessing and writing the listings The next set of methods are simple accessors: directory( ), filename( ) (notice the method can have the same spelling and capitalization as the field) and contents( ), and hasFile( ) to indicate whether this object contains a file or not (the need for this will be seen later). The last three methods are concerned with writing this code listing to a file, either a packed file via writePacked( ) or an actual Java source file via writeFile( ). All that writePacked( ) needs is the DataOutputStream which was opened elsewhere and represents the file that’s being written. It puts the header information on the first line and then calls writeBytes( ) to write contents in a “universal” format. When writing the actual Java source file, the file itself must be created. This is done using IO.psOpen( ), handing the method a File object that contains not just the file name but also the path. But the question is now: does this path exist? The user has the option of placing all the source code directories in a completely different subdirectory, which may not exist. So before each file is written, the File.mkdirs( ) method is called with the path that you want to write the file into. This will make the entire path, all at once. Containing the entire collection of listings It is very convenient to organize the listings as subdirectories while the whole collection is being built in memory. One reason is another sanity check: as each subdirectory of listings is created, an additional file is added whose name contains the number of files in that directory. The DirMap class produces this effect and demonstrates the concept of a “multimap.” This is implemented with a Hashtable whose keys are the subdirectories being created and whose values are Vector objects containing the SourceCodeFile objects in that particular directory. Thus, instead of just mapping a key to a single value, the “multimap” maps a key to a set of values via the associated Vector. Although this sounds complex it’s actually remarkably straightforward to implement. You’ll see that most of the size of the DirMap class has to do with writing to files, and not to the “multimap” implementation. There are two ways you can make a DirMap: the default constructor assumes you want to make the directories off of the current one, and the second constructor lets you specify an alternate starting directory path. The add( ) method is where quite a bit of dense action occurs. First, the directory( ) is extracted from the SourceCodeFile you want to add, and then the Hashtable is examined to see if it contains that key already. If not, a new Vector is added to the Hashtable and associated with that key. At this point, the Vector is there, one way or another, and it is extracted so the SourceCodeFile can be added. Because Vectors can be easily combined with Hashtables like this, the power of both is amplified. Writing a packed file involves opening the file to write (as a DataOutputStream so the data is universally recoverable) and writing the header information about the old separator on the first line. Next, an Enumeration of the Hashtable keys is produced and stepped 680 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 681. through to select each directory, and fetch the Vector associated with that directory so each SourceCodeFile in that Vector can be written to the packed file. Writing the Java source files to their directories in write( ) is almost identical to writePackedFile( ) since both methods simply call the appropriate method in SourceCodeFile. Here, however, the root path is passed into SourceCodeFile.writeFile( ), and when all the files have been written the additional file with the name containing the number of files is also written. The main program The previously-described classes are used within CodePackager. First you see the usage string which is printed out whenever the end user invokes the program incorrectly, along with the usage( ) method to call it and exit the program. All main( ) does is determine whether you want to create a packed file or extract from one, then make sure the arguments are correct and call the appropriate method. When a packed file is created, it’s assumed to be made in the current directory so the DirMap is created using the default constructor. After the file is opened each line is read and examined for particular conditions: 1. If the line starts with the starting marker for a source code listing, a new SourceCodeFile object is created. The constructor reads the rest of the source listing in. The handle that results is directly added to the DirMap. 2. If the line starts with the end marker for a source code listing, something has gone wrong, since end markers should be found only by the SourceCodeFile constructor. When extracting a packed file, the extraction may be to the current directory or to an alternate directory, so the DirMap object is created accordingly. The file is opened and the first line is read to extract the old file path separator information. Then the input is used to create the first SourceCodeFile object, which is added to the DirMap. New SourceCodeFile objects are created and added as long as they contain a file (the last one created will simply return when it runs out of input and then hasFile( ) will return false). Checking capitalization style Although the previous example may come in handy as a guide for some project of your own that involves text processing, this project is directly useful because it performs a style check to make sure that your capitalization conforms to the de-facto Java style. It opens each .java file in the current directory and extracts all the class names and identifiers, and then tells you if any of them don’t meet the Java style. To make this program operate correctly, you must first build a class name repository to hold all the class names in the standard Java library. You do this by moving into all the source code subdirectories for the standard Java library and running ClassScanner in each subdirectory, providing as arguments the name of the repository file (using the same path and name each time) and the -a flag to indicate that the class names should be added to the repository. To use the program to check your code, you simply run it and hand it the path and name of the repository to use. It will check all the classes and identifiers in the current directory and tell you which ones don’t follow the typical Java capitalization style. You should be aware that the program isn’t perfect; there a few times when it will point out what it thinks is a problem but on looking at the code you’ll see that nothing needs to Chapter 17: Projects 681
- 682. be changed. This is a little annoying, but it’s still much easier than trying to find all these cases by staring at your code. Here’s the listing, which will be explained immediately following the code: //: ClassScanner.java // Scans all files in directory for classes // and identifiers, to check capitalization. // Assumes properly compiling code listings. // Doesn't do everything right, but is a very // useful aid. import java.io.*; import java.util.*; class MultiStringMap extends Hashtable { public void add(String key, String value) { if(!containsKey(key)) put(key, new Vector()); ((Vector)get(key)).addElement(value); } public Vector getVector(String key) { if(!containsKey(key)) { System.err.println( "ERROR: cannot find key: " + key); System.exit(1); } return (Vector)get(key); } public void printValues(PrintStream p) { Enumeration k = keys(); while(k.hasMoreElements()) { String oneKey = (String)k.nextElement(); Vector val = getVector(oneKey); for(int i = 0; i < val.size(); i++) p.println((String)val.elementAt(i)); } } } public class ClassScanner { private File path; private String[] fileList; private Properties classes = new Properties(); private MultiStringMap classMap = new MultiStringMap(), identMap = new MultiStringMap(); private StreamTokenizer in; public ClassScanner() { path = new File("."); fileList = path.list(new JavaFilter()); for(int i = 0; i < fileList.length; i++) { System.out.println(fileList[i]); scanListing(fileList[i]); } } void scanListing(String fname) { 682 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 683. try { in = new StreamTokenizer( new BufferedReader( new FileReader(fname))); // Doesn't seem to work: // in.slashStarComments(true); // in.slashSlashComments(true); in.ordinaryChar('/'); in.ordinaryChar('.'); in.wordChars('_', '_'); in.eolIsSignificant(true); while(in.nextToken() != StreamTokenizer.TT_EOF) { if(in.ttype == '/') eatComments(); else if(in.ttype == StreamTokenizer.TT_WORD) { if(in.sval.equals("class") || in.sval.equals("interface")) { // Get class name: while(in.nextToken() != StreamTokenizer.TT_EOF && in.ttype != StreamTokenizer.TT_WORD) ; classes.put(in.sval, in.sval); classMap.add(fname, in.sval); } if(in.sval.equals("import") || in.sval.equals("package")) discardLine(); else // It's an identifier or keyword identMap.add(fname, in.sval); } } } catch(IOException e) { e.printStackTrace(); } } void discardLine() { try { while(in.nextToken() != StreamTokenizer.TT_EOF && in.ttype != StreamTokenizer.TT_EOL) ; // Throw away tokens to end of line } catch(IOException e) { e.printStackTrace(); } } // StreamTokenizer's comment removal seemed // to be broken. This extracts them: void eatComments() { try { if(in.nextToken() != StreamTokenizer.TT_EOF) { Chapter 17: Projects 683
- 684. if(in.ttype == '/') discardLine(); else if(in.ttype != '*') in.pushBack(); else while(true) { if(in.nextToken() == StreamTokenizer.TT_EOF) break; if(in.ttype == '*') if(in.nextToken() != StreamTokenizer.TT_EOF && in.ttype == '/') break; } } } catch(IOException e) { e.printStackTrace(); } } public String[] classNames() { String[] result = new String[classes.size()]; Enumeration e = classes.keys(); int i = 0; while(e.hasMoreElements()) result[i++] = (String)e.nextElement(); return result; } public void checkClassNames() { Enumeration files = classMap.keys(); while(files.hasMoreElements()) { String file = (String)files.nextElement(); Vector cls = classMap.getVector(file); for(int i = 0; i < cls.size(); i++) { String className = (String)cls.elementAt(i); if(Character.isLowerCase( className.charAt(0))) System.out.println( "class capitalization error, file: " + file + ", class: " + className); } } } public void checkIdentNames() { Enumeration files = identMap.keys(); Vector reportSet = new Vector(); while(files.hasMoreElements()) { String file = (String)files.nextElement(); Vector ids = identMap.getVector(file); for(int i = 0; i < ids.size(); i++) { String id = (String)ids.elementAt(i); if(!classes.contains(id)) { 684 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 685. // Ignore identifiers of length 3 or // longer that are all uppercase // (probably static final values): if(id.length() >= 3 && id.equals( id.toUpperCase())) continue; // Check to see if first char is upper: if(Character.isUpperCase(id.charAt(0))){ if(reportSet.indexOf(file + id) == -1){ // Not reported yet reportSet.addElement(file + id); System.out.println( "Ident capitalization error in:" + file + ", ident: " + id); } } } } } } static final String usage = "Usage: n" + "ClassScanner classnames -an" + "tAdds all the class names in this n" + "tdirectory to the repository file n" + "tcalled 'classnames'n" + "ClassScanner classnamesn" + "tChecks all the java files in this n" + "tdirectory for capitalization errors, n" + "tusing the repository file 'classnames'"; private static void usage() { System.err.println(usage); System.exit(1); } public static void main(String[] args) { if(args.length < 1 || args.length > 2) usage(); ClassScanner c = new ClassScanner(); File old = new File(args[0]); if(old.exists()) { try { // Try to open an existing // properties file: InputStream oldlist = new BufferedInputStream( new FileInputStream(old)); c.classes.load(oldlist); oldlist.close(); } catch(IOException e) { System.err.println("Could not open " + old + " for reading"); System.exit(1); } } if(args.length == 1) { Chapter 17: Projects 685
- 686. c.checkClassNames(); c.checkIdentNames(); } // Write the class names to a repository: if(args.length == 2) { if(!args[1].equals("-a")) usage(); try { BufferedOutputStream out = new BufferedOutputStream( new FileOutputStream(args[0])); c.classes.save(out, "Classes found by ClassScanner.java"); out.close(); } catch(IOException e) { System.err.println( "Could not write " + args[0]); System.exit(1); } } } } class JavaFilter implements FilenameFilter { public boolean accept(File dir, String name) { // Strip path information: String f = new File(name).getName(); return f.trim().endsWith(".java"); } } ///:~ The class MultiStringMap is a tool that allows you to map a group of strings onto each key entry. As in the previous example, it uses a Hashtable (this time with inheritance) with the key as the single string that’s mapped onto the Vector value. The add( ) method simply checks to see if there’s a key already in the Hashtable, and if not it puts one there. The getVector( ) method produces a Vector for a particular key, and printValues( ), which is primarily useful for debugging, prints out all the values, Vector by Vector. To keep life simple, the class names from the standard Java libraries are all put into a Properties object (from the standard Java library). Remember that a Properties object is a Hashtable that holds only String objects for both the key and value entries. However, it can be saved to disk and restored from disk in one method call, so it’s ideal for the repository of names. Actually, we need only a list of names, and a Hashtable cannot accept null for either its key or value entry. So the same object will be used for both the key and the value. For the classes and identifiers that are discovered for the files in a particular directory, two MultiStringMaps are used: classMap and identMap. In addition, when the program starts up it loads the standard class name repository into the Properties object called classes, and when a new class name is found in the local directory that is also added to classes as well as to classMap. This way, classMap can be used to step through all the classes in the local directory, and classes can be used to see if the current token is a class name (which indicates a definition of an object or method is beginning, so grab the next tokens – until a semicolon – and put them into identMap). 686 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 687. The default constructor for ClassScanner creates a list of file names (using the JavaFilter implementation of FilenameFilter as described in Chapter 10). Then it calls scanListing( ) for each file name. Inside scanListing( ) the source code file is opened and turned into a StreamTokenizer. In the documentation, passing true to slashStarComments( ) and slashSlashComments( ) is supposed to strip those comments out but this seems to be a bit flawed (it doesn’t quite work in Java 1.0), so those lines are commented out and instead, as you shall see, the comments are extracted by another method. To do this, the ‘/’ must be captured as an ordinary character rather than letting the StreamTokenizer absorb it as part of a comment, and the ordinaryChar( ) method tells the StreamTokenizer to do this. This is also true for dots (‘.’), since we want to have the method calls pulled apart into individual identifiers. However, the underscore, which is ordinarily treated by StreamTokenizer as an individual character, should be left as part of identifiers since it appears in such static final values as TT_EOF etc., used in this very program. The wordChars( ) method takes a range of characters you want to add to those that are left inside a token that is being parsed as a word. Finally, when parsing for one-line comments or discarding a line we need to know when an end-of-line occurs, so by calling eolIsSignificant(true) the eol will show up rather than being absorbed by the StreamTokenizer. The rest of scanListing( ) reads and reacts to tokens until the end of the file, signified when nextToken( ) returns the final static value StreamTokenizer.TT_EOF. If the token is a ‘/’ this is potentially a comment, and eatComments( ) is called to deal with it. The only other situation we’re interested in here is if it’s a word, of which there are some special cases. If the word is class or interface then the next token represents a class or interface name, and this is put into classes and classMap. If the word is import or package, then we don’t want the rest of the line. Anything else must be an identifier (which we’re interested in) or a keyword (which we’re not, but they’re all lowercase anyway so it won’t spoil things just to put those in). These are added to identMap. The discardLine( ) method is a simple tool which just looks for the end of a line. Notice that any time you get a new token, you must check for the end of the file. The eatComments( ) method is called whenever a forward slash is encountered in the main parsing loop. However, that doesn’t necessarily mean a comment has been found, so the next token must be extracted to see if it’s another forward slash (in which case the line is discarded) or an asterisk. But if it’s neither of those, it means the token you’ve just pulled out is needed back in the main parsing loop! Fortunately, the pushBack( ) method allows you to “push back” the current token onto the input stream so that when the main parsing loop calls nextToken( ) it will get the one you just pushed back. For convenience, the classNames( ) method produces an array of all the names in the classes collection. This method is not used in the program but is helpful for debugging. The next two methods are where the actual checking takes place. In checkClassNames( ), the class names are extracted from the classMap (which, remember, contains only the names in this directory, organized by file name so the file name can be printed along with the errant class name). This is accomplished by pulling each associated Vector and stepping through that, looking to see if the first character is lower case. If so, the appropriate error message is printed. In checkIdentNames( ), a similar approach is taken: each identifier name is extracted from identMap. If the name is not in the classes list, it is assumed to be an identifier or keyword. A special case is checked: if the identifier length is 3 or more and all the characters are uppercase, this identifier is ignored because it’s probably a static final Chapter 17: Projects 687
- 688. value like TT_EOF. Of course, this is not a perfect algorithm but it assumes you’ll eventually notice any all-uppercase identifiers that are out of place. Rather than reporting every identifier that starts with an uppercase character, this method keeps track of which ones have already been reported in a Vector called reportSet( ). This treats the Vector as a “set” which tells you whether an item is already in the set. The item is produced by concatenating the file name and identifier. If the element isn’t in the set, it is added and then the report is made. The rest of the listing is comprised of main( ), which busies itself by handling the command line arguments and figuring out whether you’re building a repository of class names from the standard Java library or checking the validity of code you’ve written. In both cases it makes a ClassScanner object. Whether you’re building a repository or using one, you must try to open the existing repository. By making a File object and testing for existence, you can decide whether to open the file and load( ) the Properties list classes inside ClassScanner (the classes from the repository add to rather than overwrite the classes found by the ClassScanner constructor). If you provide only one command-line argument it means you just want to perform a check of the class names and identifier names, but if you provide two arguments (the second being “-a”) you’re building a class name repository. In this case an output file is opened and the method Properties.save( ) is used to write the list to a file along with a string which provides header file information. A method lookup tool Chapter 11 introduced the Java 1.1 concept of reflection and used that feature to look up methods for a particular class – either the entire list of methods, or a subset that matches a keyword you provide. The magic of this is that it automatically shows you all the methods for a class without forcing you to walk up the inheritance hierarchy examining the base classes at each level. Thus it provides a valuable timesaving tool for programming: since most Java methods are verbose and descriptive, you can look for the methods that contain a word and when you find what you think you’re looking for, check the online documentation. However, you hadn’t seen the AWT by Chapter 11 so that tool was developed as a command-line application. Here is the more useful GUI version, which dynamically updates the output as you type and also allows you to cut and paste from the output: //: DisplayMethods.java // Display the methods of any class inside // a window. Dynamically narrows your search. import java.awt.*; import java.awt.event.*; import java.applet.*; import java.lang.reflect.*; import java.io.*; public class DisplayMethods extends Applet { Class cl; Method m[]; Constructor ctor[]; String n[] = new String[0]; TextField name = new TextField(40), 688 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 689. searchFor = new TextField(30); Checkbox strip = new Checkbox("Strip Qualifiers"); TextArea results = new TextArea(40, 65); public void init() { strip.setState(true); name.addTextListener(new NameL()); searchFor.addTextListener(new SearchForL()); strip.addItemListener(new StripL()); Panel top = new Panel(), lower = new Panel(), p = new Panel(); top.add(new Label("Qualified class name:")); top.add(name); lower.add( new Label("String to search for:")); lower.add(searchFor); lower.add(strip); p.setLayout(new BorderLayout()); p.add(top, BorderLayout.NORTH); p.add(lower, BorderLayout.SOUTH); setLayout(new BorderLayout()); add(p, BorderLayout.NORTH); add(results, BorderLayout.CENTER); } class NameL implements TextListener { public void textValueChanged(TextEvent e) { String nm = name.getText().trim(); if(nm.length() == 0) { results.setText("No match"); n = new String[0]; return; } try { cl = Class.forName(nm); } catch (ClassNotFoundException ex) { results.setText("No match"); return; } m = cl.getMethods(); ctor = cl.getConstructors(); // Convert to an array of Strings: n = new String[m.length + ctor.length]; for(int i = 0; i < m.length; i++) n[i] = m[i].toString(); for(int i = 0; i < ctor.length; i++) n[i + m.length] = ctor[i].toString(); reDisplay(); } } void reDisplay() { // Create the result set: String rs[] = new String[n.length]; String find = searchFor.getText(); int j = 0; Chapter 17: Projects 689
- 690. // Select from the list if find exists: for (int i = 0; i < n.length; i++) { if(find == null) rs[j++] = n[i]; else if(n[i].indexOf(find) != -1) rs[j++] = n[i]; } results.setText(""); if(strip.getState() == true) for (int i = 0; i < j; i++) results.append( StripQualifiers.strip(rs[i]) + "n"); else // Leave qualifiers on for (int i = 0; i < j; i++) results.append(rs[i] + "n"); } class StripL implements ItemListener { public void itemStateChanged(ItemEvent e) { reDisplay(); } } class SearchForL implements TextListener { public void textValueChanged(TextEvent e) { reDisplay(); } } public static void main(String args[]) { DisplayMethods applet = new DisplayMethods(); Frame aFrame = new Frame("Display Methods"); aFrame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(applet, BorderLayout.CENTER); aFrame.setSize(500,750); applet.init(); applet.start(); aFrame.setVisible(true); } } class StripQualifiers { private StreamTokenizer st; public StripQualifiers(String qualified) { st = new StreamTokenizer( new StringReader(qualified)); st.ordinaryChar(' '); } public String getNext() { String s = null; try { if(st.nextToken() != StreamTokenizer.TT_EOF) { 690 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 691. switch(st.ttype) { case StreamTokenizer.TT_EOL: s = null; break; case StreamTokenizer.TT_NUMBER: s = Double.toString(st.nval); break; case StreamTokenizer.TT_WORD: s = new String(st.sval); break; default: // single character in ttype s = String.valueOf((char)st.ttype); } } } catch(IOException e) { System.out.println(e); } return s; } public static String strip(String qualified) { StripQualifiers sq = new StripQualifiers(qualified); String s = "", si; while((si = sq.getNext()) != null) { int lastDot = si.lastIndexOf('.'); if(lastDot != -1) si = si.substring(lastDot + 1); s += si; } return s; } } ///:~ Some things you’ve seen before. As with many of the GUI programs in this book, this is created as both an application and an applet. Also, the StripQualifiers class is exactly the same as it was in Chapter 11. The GUI contains a TextField name for you to type the fully-qualified class name you want to look up, and another one searchFor where you can enter the optional text to search for within the list of methods. The Checkbox allows you to say whether you want to use the fully-qualified names in the output or if you want the qualification stripped off. Finally, the results are displayed in a TextArea. You’ll notice there are no buttons or other components for you to indicate that you want the search to start. That’s because both of the TextFields as well as the Checkbox are monitored by their listener objects. Whenever you make a change, the list is immediately updated. If you change the text within the name field, the new text is captured in class NameL. If the text isn’t empty, it is used inside Class.forName( ) to try to look up the class. As you’re typing, of course, the name will be incomplete and Class.forName( ) will fail, which means it throws an exception. This is trapped and the TextArea is set to “No match”. But as soon as you type in a correct name (capitalization counts), Class.forName( ) is successful and getMethods( ) and getConstructors( ) will return arrays of Method and Constructor objects, respectively. Each of the objects in these arrays is turned into a String with toString( ) (this produces the complete method or constructor signature) and both lists are combined into a single array of String called n. The array n is a member of class DisplayMethods and is used for updating the display whenever reDisplay( ) is called. Chapter 17: Projects 691
- 692. If you change the Checkbox or searchFor components, their listeners simply call reDisplay( ). Inside reDisplay( ), a temporary array of String is created called rs (for “result set”). The result set is either copied directly from n if there is no find word, or conditionally copied from the Strings in n that contain the find word. Finally, the strip Checkbox is interrogated to see if the user wants the names to be stripped (the default is “yes”). If so, StripQualifiers.strip( ) does the job, if not, the list is simply displayed. In init( ), you may think that there’s a lot of busy work involved in setting up the layout. In fact, it is possible to lay out the components with less work, but the advantage of using BorderLayouts this way is that it allows the user to resize the window and make – in particular – the TextArea larger, which means you can resize to allow you to see longer names without scrolling. You may find that you’ll keep this tool running all the time when you’re programming, since it provides one of the best “first lines of attack” when you’re trying to figure out what method to call. Complexity theory This program was modified from code originally created by Larry O’Brien, and is based on the “Boids” program created by Craig Reynolds in 1986 to demonstrate an aspect of complexity theory called “emergence.” The goal here is to produce a reasonably lifelike reproduction of flocking or herding behavior in animals by establishing a small set of very simple rules for each animal. Each animal can look at the entire scene and all the other animals in the scene, but it reacts only to a set of nearby “flockmates.” The animal moves according to three simple steering behaviors: 1. Separation: avoid crowding local flockmates. 2. Alignment: follow the average heading of local flockmates. 3. Cohesion: move toward the center of the group of local flockmates. More elaborate models can include obstacles and the ability for the animals to predict collisions and avoid them, so the animals can flow around fixed objects in the environment. In addition, the animals may also be given a goal, which can cause the herd to follow a desired path. For simplicity, obstacle avoidance and goal-seeking is not included in the model presented here. Emergence means that, despite the limited nature of computers and the simplicity of the steering rules, the result seems realistic. That is, remarkably lifelike behavior “emerges” from this simple model. The program is presented as a combined application/applet: //: FieldOBeasts.java // Demonstration of complexity theory; simulates // herding behavior in animals. Adapted from // a program by Larry O'Brien lobrien@msn.com import java.awt.*; import java.awt.event.*; import java.applet.*; import java.util.*; 692 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 693. class Beast { int x, y, // Screen position currentSpeed; // Pixels per second float currentDirection; // Radians Color color; // Fill color FieldOBeasts field; // Where the Beast roams static final int GSIZE = 10; // Graphic size public Beast(FieldOBeasts f, int x, int y, float cD, int cS, Color c) { field = f; this.x = x; this.y = y; currentDirection = cD; currentSpeed = cS; color = c; } public void step() { // You move based on those within your sight: Vector seen = field.beastListInSector(this); // If you're not out in front if(seen.size() > 0) { // Gather data on those you see int totalSpeed = 0; float totalBearing = 0.0f; float distanceToNearest = 100000.0f; Beast nearestBeast = (Beast)seen.elementAt(0); Enumeration e = seen.elements(); while(e.hasMoreElements()) { Beast aBeast = (Beast) e.nextElement(); totalSpeed += aBeast.currentSpeed; float bearing = aBeast.bearingFromPointAlongAxis( x, y, currentDirection); totalBearing += bearing; float distanceToBeast = aBeast.distanceFromPoint(x, y); if(distanceToBeast < distanceToNearest) { nearestBeast = aBeast; distanceToNearest = distanceToBeast; } } // Rule 1: Match average speed of those // in the list: currentSpeed = totalSpeed / seen.size(); // Rule 2: Move towards the perceived // center of gravity of the herd: currentDirection = totalBearing / seen.size(); // Rule 3: Maintain a minimum distance // from those around you: if(distanceToNearest <= field.minimumDistance){ currentDirection = Chapter 17: Projects 693
- 694. nearestBeast.currentDirection; currentSpeed = nearestBeast.currentSpeed; if(currentSpeed > field.maxSpeed) { currentSpeed = field.maxSpeed; } } } else { // You are in front, so slow down currentSpeed = (int)(currentSpeed * field.decayRate); } // Make the beast move: x += (int)(Math.cos(currentDirection) * currentSpeed); y += (int)(Math.sin(currentDirection) * currentSpeed); x %= field.xExtent; y %= field.yExtent; if(x < 0) x += field.xExtent; if(y < 0) y += field.yExtent; } public float bearingFromPointAlongAxis ( int originX, int originY, float axis) { // Returns bearing angle of the current Beast // in the world coordiante system try { double bearingInRadians = Math.atan( (this.y - originY) / (this.x - originX)); // Inverse tan has two solutions, so you // have to correct for other quarters: if(x < originX) { if(y < originY) { bearingInRadians += - (float)Math.PI; } else { bearingInRadians = (float)Math.PI - bearingInRadians; } } // Just subtract the axis (in radians): return (float) (axis - bearingInRadians); } catch(ArithmeticException aE) { // Divide by 0 error possible on this if(x > originX) { return 0; } else return (float) Math.PI; } } public float distanceFromPoint(int x1, int y1){ 694 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 695. return (float) Math.sqrt( Math.pow(x1 - x, 2) + Math.pow(y1 - y, 2)); } public Point position() { return new Point(x, y); } // Beasts know how to draw themselves: public void draw(Graphics g) { g.setColor(color); int directionInDegrees = (int)( (currentDirection * 360) / (2 * Math.PI)); int startAngle = directionInDegrees - FieldOBeasts.halfFieldOfView; int endAngle = 90; g.fillArc(x, y, GSIZE, GSIZE, startAngle, endAngle); } } public class FieldOBeasts extends Applet implements Runnable { private Vector beasts; static float fieldOfView = (float) (Math.PI / 4), // In radians // Deceleration % per second: decayRate = 1.0f, minimumDistance = 10f; // In pixels static int halfFieldOfView = (int)( (fieldOfView * 360) / (2 * Math.PI)), xExtent = 0, yExtent = 0, numBeasts = 50, maxSpeed = 20; // Pixels/second boolean uniqueColors = true; Thread thisThread; int delay = 25; public void init() { if (xExtent == 0 && yExtent == 0) { xExtent = Integer.parseInt( getParameter("xExtent")); yExtent = Integer.parseInt( getParameter("yExtent")); } beasts = makeBeastVector(numBeasts, uniqueColors); // Now start the beasts a-rovin': thisThread = new Thread(this); thisThread.start(); } public void run() { while(true) { for(int i = 0; i < beasts.size(); i++){ Beast b = (Beast) beasts.elementAt(i); Chapter 17: Projects 695
- 696. b.step(); } try { thisThread.sleep(delay); } catch(InterruptedException ex){} repaint(); // Otherwise it won't update } } Vector makeBeastVector( int quantity, boolean uniqueColors) { Vector newBeasts = new Vector(); Random generator = new Random(); // Used only if uniqueColors is on: double cubeRootOfBeastNumber = Math.pow((double)numBeasts, 1.0 / 3.0); float colorCubeStepSize = (float) (1.0 / cubeRootOfBeastNumber); float r = 0.0f; float g = 0.0f; float b = 0.0f; for(int i = 0; i < quantity; i++) { int x = (int) (generator.nextFloat() * xExtent); if(x > xExtent - Beast.GSIZE) x -= Beast.GSIZE; int y = (int) (generator.nextFloat() * yExtent); if(y > yExtent - Beast.GSIZE) y -= Beast.GSIZE; float direction = (float)( generator.nextFloat() * 2 * Math.PI); int speed = (int)( generator.nextFloat() * (float)maxSpeed); if(uniqueColors) { r += colorCubeStepSize; if(r > 1.0) { r -= 1.0f; g += colorCubeStepSize; if( g > 1.0) { g -= 1.0f; b += colorCubeStepSize; if(b > 1.0) b -= 1.0f; } } } newBeasts.addElement( new Beast(this, x, y, direction, speed, new Color(r,g,b))); } return newBeasts; } public Vector beastListInSector(Beast viewer) { Vector output = new Vector(); Enumeration e = beasts.elements(); 696 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 697. Beast aBeast = (Beast)beasts.elementAt(0); int counter = 0; while(e.hasMoreElements()) { aBeast = (Beast) e.nextElement(); if(aBeast != viewer) { Point p = aBeast.position(); Point v = viewer.position(); float bearing = aBeast.bearingFromPointAlongAxis( v.x, v.y, viewer.currentDirection); if(Math.abs(bearing) < fieldOfView / 2) output.addElement(aBeast); } } return output; } public void paint(Graphics g) { Enumeration e = beasts.elements(); while(e.hasMoreElements()) { ((Beast)e.nextElement()).draw(g); } } public static void main(String args[]) { FieldOBeasts field = new FieldOBeasts(); field.xExtent = 640; field.yExtent = 480; Frame frame = new Frame("Field 'O Beasts"); // Optionally use a command-line argument // for the sleep time: if(args.length >= 1) field.delay = Integer.parseInt(args[0]); frame.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); frame.add(field, BorderLayout.CENTER); frame.setSize(640,480); field.init(); field.start(); frame.setVisible(true); } } ///:~ Although this isn’t a perfect reproduction of the behavior in Craig Reynold’s “Boids” example, it exhibits its own fascinating characteristics, which you can modify by adjusting the numbers. You can find out more about the modeling of flocking behavior and see a spectacular 3-D version of Boids at Craig Reynold’s page http://www.hmt.com/cwr/boids.html. To run this program as an applet, put the following applet tag in an HTML file: <applet code=FieldOBeasts width=640 height=480> Chapter 17: Projects 697
- 698. <param name="xExtent" value = "640"> <param name="yExtent" value = "480"> </applet> Summary This chapter shows some of the more sophisticated things you can do with Java. It also makes the point that while Java must certainly have its limits, those limits are primarily relegated to performance (when the text-processing programs were written, for example, C++ versions were much faster – this may be partly due to an inefficient implementation of the IO library, which should change in time). The limits of Java do not seem to be in the area of expressiveness. Not only does it seem possible to express just about everything you can imagine, Java seems oriented towards making that expression easy to write and read, so you don’t run into the wall of complexity that often occurs with languages that are more trivial to use than Java (at least they seem that way, at first). And with Java 1.2’s JFC/Swing library, even the expressiveness and ease of use of the AWT is improving dramatically. Exercises 1. (Challenging) Re-write FieldOBeasts so that its state can be persistent. Implement buttons to allow you to save and recall different state files and continue running them where they left off. Use CADState.java from chapter 10 as an example of how to do this. 2. (Term project) Taking FieldOBeasts.java as a starting point, build an automobile traffic simulation system. 3. (Term project) Using ClassScanner.java as a starting point, build a tool that points out methods and fields that are defined but never used. 4. (Term project) Using JDBC, build a contact management program using a flat-file database containing names, addresses, phone numbers, email addresses, etc. You should be able to easily add new names to the database. When typing in the name to be looked up, use automatic name completion as shown in VLookup.java in Chapter 15. 698 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 699. ) A: Using non-Java code This appendix contains pointers to other resources for connecting Java to non-Java code. In Chapter 15, the section titled “a Web application” contains an example of connecting to non-Java code using standard input and output. Native methods For information about programming native methods in Java 1.1, see the electronic version of The Java Tutorial by Campione and Walrath at http://java.sun.com. CORBA CORBA is designed to support computing with distributed objects, but to accomplish this it also allows you to program in any language that has CORBA support. Thus, you can write code in one language and connect it to Java via CORBA. Full details can be found in Client/Server Programming with Java and CORBA by Orfali & Harkey (John Wiley & Sons, 1997). The official source for technical references is the Object Management Group (OMG) Web site: http://www.omg.org. 699
- 701. # $ B: Comparing C++ and Java As a C++ programmer you already have the basic idea of object- oriented programming, and the syntax of Java no doubt looks very familiar to you. This makes sense since Java was derived from C++. However, there are a surprising number of differences between C++ and Java. These differences are intended to be significant improvements, and if you understand the differences you’ll see why Java is such a beneficial programming language. This appendix takes you through the important features that make Java distinct from C++. 1. The biggest potential stumbling block is speed: interpreted Java runs something like 20 times slower than C. Nothing prevents the Java language from being compiled and there are just-in-time compilers appearing at this writing which offer significant speed-ups. It is not inconceivable that full native compilers will appear for the more popular platforms, but without those there are classes of problems that will be insoluble with Java because of the speed issue. 2. Java has both kinds of comments like C++ does. 3. Everything must be in a class. There are no global functions or global data. If you want the equivalent of globals, make static methods and static data within a class. There are no structs or enumerations or unions. Only classes. 4. All method definitions are defined in the body of the class. Thus, in C++ it would look like all the functions are inlined, but they’re not (inlines are noted later). 701
- 702. 5. Class definitions are roughly the same form in Java as in C++, but there’s no closing semicolon. There are no class declarations of the form class foo; only class definitions. class aType { void aMethod( ) { /* method body */ } } 6. There’s no scope resolution operator :: in Java. Java uses the dot for everything, but can get away with it since you can define elements only within a class. Even the method definitions must always occur within a class so there is no need for scope resolution there, either. One place where you’ll notice the difference is in the calling of static methods: you say ClassName.methodName( );. In addition, package names are established using the dot, and to perform the equivalent of a C++ #include you use the import keyword. For example: import java.awt.*;. 7. Java, like C++, has primitive types for efficient access. In Java, these are boolean, char, byte , short, int, long, float, and double. All the primitive types have specified sizes that are machine-independent for portability (this must have some impact on performance, varying with the machine). Type- checking and type requirements are much tighter in Java. For example: A. Conditional expressions can be only boolean, not integral B. The result of an expression like X + Y must be used; you can’t just say “X + Y” for the side effect. 8. The char type uses the international 16-bit Unicode character set, so it can automatically represent most national characters. 9. Static quoted strings are automatically converted into String objects. There is no independent static character array string as there is in C/C++. 10. Java adds the triple right shift >>> to act as a “logical” right shift by inserting zeroes at the top end; the >> inserts the sign bit as it shifts (an “arithmetic” shift). 11. Arrays are quite different in Java. There’s a read-only length member that tells you how big the array is, and run-time checking throws an exception if you go out of bounds. All arrays are created on the heap, and you can assign one array to another (the array handle is simply copied). The array identifier itself is a first-class object, with all the methods commonly available to all other objects. 12. All objects of non-primitive types can be created only via new. There’s no equivalent to creating non-primitive objects “on the stack” as in C++. All primitive types can be created only on the stack, without new. There are wrapper classes for all primitive classes so you can create equivalent heap- based objects with new. (Arrays of primitives are a special case: they can be allocated via aggregate initialization as in C++, or by using new). 13. No forward declarations are necessary in Java. If you want to use a class or a method before it is defined in, you simply use it – the compiler ensures that the appropriate definition exists. Thus you don’t have any of the forward referencing issues that you do in C++. 14. Java has no preprocessor. If you want to use classes in another library, you say import and the name of the library. There are no preprocessor-like macros. 702 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 703. 15. Java uses packages in place of namespaces. The name issue is taken care of by (1) putting everything in a class and (2) a facility called “packages” that performs the equivalent namespace breakup for class names. Packages also collect library components under a single library name. You simply import a package and the compiler takes care of the rest. 16. Object handles defined as class members are automatically initialized to null. Initialization of primitive class data members is guaranteed in Java; if you don’t explicitly initialize them they get a default value (a zero or equivalent). You can initialize them directly when you define them in the class, or you can do it in the constructor. The syntax makes more sense than C++, and is consistent for static and non-static members alike. You don’t need to externally define storage for static members like you do in C++. 17. There are no Java pointers in the sense of C and C++. When you create an object with new, you get back a reference (which I’ve been calling a handle in this book). For example: String s = new String(“howdy”); However, unlike C++ references that must be initialized when created and cannot be rebound to a different location, Java references don’t have to be bound at the point of creation, and they can be rebound at will, which eliminates part of the need for pointers. The other reason for pointers is to select any place in memory (which makes them unsafe, which is why Java doesn’t support them). Pointers are often seen as an efficient way to move through an array of primitive variables; Java arrays allow you to do that in a safer fashion. The final solution for pointer problems is native methods (discussed in Appendix A). Passing pointers to methods isn’t a problem since there are no global functions, only classes, and you can pass references to objects. The Java language promoters initially said “no pointers!” but when many programmers questioned “how can you work without pointers?” they began saying “restricted pointers.” You can make up your mind whether it’s “really” a pointer or not. In any event, there’s no pointer arithmetic. 18. Java has constructors, similar to constructors in C++. You get a default constructor if you don’t define one, and if you define a non-default constructor, there’s no automatic default constructor defined for you, just like C++. There are no copy-constructors, since all arguments are passed by reference. 19. There are no destructors in Java. There is no “scope” of a variable per se, to indicate when the object’s lifetime is ended – the lifetime of an object is determined instead by the garbage collector. There is a finalize( ) method that’s a member of each class, like a destructor, but finalize( ) is called by the garbage collector and is supposed to be responsible only for releasing resources. If you need something done at a specific point, you must create a special method and call it, not rely upon finalize( ). Put another way, all objects in C++ will be (or rather, should be) destroyed, but not all objects in Java are garbage collected. Because Java doesn’t support destructors, you must be careful to create a cleanup method if necessary, and to explicitly call all the cleanup methods for the base class and member objects in your class. 20. Java has method overloading that works virtually identically to C++ function overloading. 21. Java does not support default arguments. Appendix B: Comparing C++ and Java 703
- 704. 22. There’s no goto in Java. The one unconditional jump mechanism is the break label or continue label, which is used to jump out of the middle of multiply- nested loops. 23. Java uses a singly-rooted hierarchy, so all objects are ultimately inherited from the root class Object. In C++ you can start a new inheritance tree anywhere, so you end up with a forest of trees. In Java you get a single ultimate hierarchy. This can seem restrictive, but it gives a great deal of power since you know that every object is guaranteed to have at least the Object interface. C++ appears to be the only OO language that does not impose a singly-rooted hierarchy. 24. Java has no templates or other implementation of parameterized types. There is a set of collections: Vector, Stack and Hashtable that hold Object references, and through which you can satisfy your collection needs, but these collections are not designed for efficiency like the C++ Standard Template Library (STL). For a more complete set of collections, there’s a freely-available library called the Generic Collection Library for Java (www.ObjectSpace.com) which shows signs of eventually being incorporated into the standard Java language. 25. Garbage collection means memory leaks are much harder to cause in Java, but not impossible. However, many memory leaks and resouce leaks may be tracked to a badly written finalize( ) or not releasing a resource at the end of the block where it is allocated (a place where a destructor would certainly come in handy). The garbage collector is a huge improvement over C++, and makes a lot of programming problems simply vanish. It may make Java unsuitable for solving a small subset of problems that cannot tolerate a garbage collector, but the advantage of a garbage collector seems to greatly outweigh this potential drawback. 26. Java has built in multithreading support. There’s a Thread class that you inherit to create a new thread (you override the run( ) method). Mutual exclusion occurs at the level of objects using the synchronized keyword as a type qualifier for methods. Only one thread may use a synchronized method of a particular object at any one time. Put another way, when a synchronized method is entered, it first “locks” the object against any other synchronized method using that object, and “unlocks” the object only upon exiting the method. There are no explicit locks; they happen automatically. You’re still responsible for implementing more sophisticated synchronization between threads by creating your own “monitor” class. Recursive synchronized methods work correctly. Time slicing is not guaranteed between equal priority threads. 27. Instead of controlling blocks of declarations like C++ does, access specifiers (public, private and protected) are placed on each definition for each member of a class. Without an explicit access specifier, the element defaults to “friendly,” which means it is accessible to other elements in the same package (equivalent to them all being friends) but inaccessible outside the package. The class, and each method within the class, has an access specifier to determine whether it’s visible outside the file. Sometimes the private keyword is used less in Java because “friendly” access is often more useful than excluding access from other classes in the same package (however, with multithreading the proper use of private is essential). The Java protected keyword means “accessible to inheritors and to others in this package.” There is no equivalent to the C++ protected keyword which means “accessible to inheritors only” (private protected used to do this, but it was removed). 704 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 705. 28.Nested classes. In C++, nesting a class is an aid to name hiding and code organization (but C++ namespaces eliminate the need for name hiding). Java packaging provides the equivalence of namespaces, so that isn't an issue. Java 1.1 has inner classes which look just like nested classes. However, an object of an inner class secretly keeps a handle to the object of the outer class that was involved in the creation of the inner-class object. This means that the inner- class object may access members of the outer-class object without qualification, as if those members belonged directly to the inner-class object. This provides a much more elegant solution to the problem of callbacks, solved with pointers to members in C++. 29.Because of inner classes described in the previous point, there are no pointers to members in Java. 30. No inline methods. The Java compiler may decide on its own to inline a method, but you don’t have much control over this. You may suggest inlining in Java by using the final keyword for a method. However, inline functions are only suggestions to the C++ compiler, as well. 31.Inheritance in Java has the same effect as in C++, but the syntax is different. Java uses the extends keyword to indicate inheritance from a base class, and the super keyword to specify methods to be called in the base class that have the same name as the method you’re in (however, the super keyword in Java allows you to access methods only in the parent class, one level up in the hierarchy. Base-class scoping in C++ allows you to access methods that are deeper in the hierarchy). The base-class constructor is also called using the super keyword. As mentioned before, all classes are ultimately, automatically inherited from Object. There’s no explicit constructor initializer list like in C++ but the compiler forces you to perform all base-class initialization at the beginning of the constructor body and it won’t let you perform these later in the body. Member initialization is guaranteed through a combination of automatic initialization and exceptions for uninitialized object handles. public class Foo extends Bar { public Foo(String msg) { super(msg); // Calls base constructor } public baz(int i) { // Override super.baz(i); // Calls base method } } 32. Inheritance in Java doesn’t change the protection level of the members in the base class. You cannot specify public, private or protected inheritance in Java as you can in C++. Also, overridden methods in a derived class cannot reduce the access of the method in the base class. For example, if a method is public in the base class and you override it, your overridden method must also be public (the compiler ensures this). 33. Java provides the interface keyword which creates the equivalent of an abstract base class filled with abstract methods and with no data members. This makes a clear distinction between something designed to be just an interface versus an extension of existing functionality using the extends keyword. It’s worth noting that the abstract keyword produces a similar effect, in that you can’t create an object of that class. However, an abstract class may contain abstract methods but it can also contain implementations, so it is restricted to single inheritance. Together with interfaces, this scheme prevents the need for some mechanism like virtual base classes in C++. Appendix B: Comparing C++ and Java 705
- 706. To create a version of the interface that can be instantiated, you use the implements keyword, whose syntax looks like inheritance: public interface Face { public void smile(); } public class Baz extends Bar implements Face { public void smile( ) { System.out.println("a warm smile"); } } 34. There’s no virtual keyword in Java because all non-static methods always use dynamic binding. In Java, the programmer doesn’t have to decide whether or not to use dynamic binding. The reason virtual exists in C++ is so you can leave it off for a slight increase in efficiency when you’re tuning for performance (or, put another way, “if you don’t use it you don’t pay for it”), but this often results in confusion and unpleasant surprises. The final keyword provides some latitude for efficiency tuning – it tells the compiler that this method may not be overridden, and thus that it can be statically bound (and made inline, thus using the equivalent of a C++ non-virtual call). These optimizations are up to the compiler. 35. Java doesn’t provide multiple inheritance (MI), at least not in the same sense C++ does. Like protected, MI seems like a good idea but you know you need it only when you are face to face with the right design problem. Since Java uses a singly-rooted hierarchy, you’ll probably run into fewer situations where MI is necessary. The aforementioned interface keyword takes care of combining multiple interfaces. 36. Run-time type identification functionality is quite similar to C++. To get information about handle X you can say, for example: X.getClass().getName(); To perform a type-safe downcast you say: derived d = (derived)base; just like an old-style C cast. The compiler automatically invokes the dynamic casting mechanism without requiring extra syntax. Although this doesn’t have the benefit of easy location of casts as in C++ “new casts,” Java checks usage and throws exceptions so it doesn’t allow bad casts like C++ does. 37. Exception handling in Java is different because there are no destructors. A finally clause can be added to force execution of statements that perform necessary cleanup. All exceptions in Java are inherited from the base class Throwable, so you’re guaranteed a common interface. public void f(Obj b) throws IOException { myresource mr = b.createResource(); try { mr.UseResource(); } catch (MyException e) { // handle my exception } catch (Throwable e) { // handle all other exceptions } finally { mr.dispose(); // special cleanup } } 706 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 707. 38. Exception specifications in Java are vastly superior to those in C++. Instead of the C++ approach of calling a function at run-time when the wrong exception is thrown, Java exception specifications are checked and enforced at compile- time. In addition, overridden methods must conform to the exception specification of the base-class version of that method: they can throw the specified exceptions, or exceptions derived from those. This provides much more robust exception-handling code. 39. There is method overloading, but no operator overloading in Java. The String class does use the + and += operators to concatenate strings and String expressions use automatic type conversion, but that’s a special built-in case. 40.The const issues in C++ are avoided in Java by convention. You pass only handles to objects, and local copies are never made for you automatically. If you want the equivalent of C++’s pass-by-value, you call clone( ) to produce a local copy of the argument (although the clone( ) mechanism is somewhat poorly designed). There’s no copy-constructor that’s automatically called. To create a compile-time constant value, you say: static final int SIZE = 255; static final int BSIZE = 8 * SIZE; 41. Because of security issues, programming an “application” is quite different from programming an “applet.” A significant issue is that an applet won’t let you write to disk, because that would allow a program downloaded from an unknown machine to trash your disk. This changes somewhat with Java 1.1 digital signing, which allows you to unequivocally know everyone that wrote all the programs that have special access to your system (one of which may have trashed your disk; you still have to figure out which one and what to do about it…). 42. Since Java can be too restrictive in some cases, you may be prevented from doing important tasks like directly accessing hardware. Java solves this with native methods that allow you to call a function written in another language (currently only C/C++ are supported). Thus you can always solve a platform- specific problem (in a relatively non-portable fashion, but then that code is isolated). Applets cannot call native methods, only applications. 43. Java has built-in support for comment documentation, so the source code file can also contain its own documentation, which is stripped out and reformatted into HTML using a separate program. This is a boon for documentation maintenance and use. 44. Java contains standard libraries for solving specific tasks. C++ relies on non- standard third-party libraries. These tasks include (or will soon include): – Networking – Database Connection (via JDBC) – Multithreading – Distributed Objects (via RMI and CORBA) – Compression – Commerce The availability and standard nature of these libraries allow for more rapid application development. 45. Java 1.1 includes the Java Beans standard, which is a way to create components that can be used in visual programming environments. This promotes visual components that can be used under all vendor’s development environments. Since you aren’t tied to a particular vendor’s design for visual components, this should result in greater selection and availability of Appendix B: Comparing C++ and Java 707
- 708. components. In addition, the design for Java Beans is simpler for programmers to understand; vendor-specific component frameworks tend to involve a steeper learning curve. 46. If the access to a Java handle fails, an exception is thrown. This test doesn’t have to occur right before the use of a handle; the Java specification just says that the exception must somehow be thrown. Many C++ runtime systems can also throw exceptions for bad pointers. 47. Generally, Java is more robust, via: – Object handles initialized to null (a keyword) – Handles are always checked and exceptions are thrown for failures – All array accesses are checked for bounds violations – Automatic garbage collection prevents memory leaks – Clean, relatively fool-proof exception handling – Simple language support for multi-threading – Bytecode verification of network applets 708 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 709. K C: Java programming guidelines This appendix contains suggestions to help guide you while performing low-level program design, and also while writing code. 1. Capitalize the first letter of class names. The first letter of fields, methods and objects (handles) should be lowercase. All identifiers should run their words together, and capitalize the first letter of all intermediate words. For example ThisIsAClassName thisIsAMethodOrFieldName Capitalize all the letters of static final primitive identifiers that have constant initializers in their definitions. This indicates they are compile-time constants. Packages are a special case: they are all lowercase letters, even for intermediate words. The domain extension (com, org, net, edu, etc.) should also be lowercase (this was a change between Java 1.1 and Java 1.2). 2. For each class you create, consider including a main( ) that contains code to test that class. You don’t need to remove the test code to use the class in a project, and if you make any changes you can easily re-run the tests. 3. Methods should be kept to brief, functional units that describe and implement a discrete part of a class interface. Ideally, methods should be concise; if they are long you may want to search for a way to break them up into several shorter methods; this will also foster reuse within your class. (Sometimes methods must be large, but they should still do just one thing). 4. Try to keep classes small and focused. Clues to suggest redesign of a class are: A) A complicated switch statement: consider using polymorphism 709
- 710. B) A large number of methods that cover broadly different types of operations: consider using several classes C) A large number of member variables that concern broadly different characteristics: consider using several classes 5. Keep things as “private as possible.” Once you publicize an aspect of your library (a method, a class, a field), you can never take it out. If you do, you’ll wreck somebody’s existing code, forcing them to rewrite and redesign. If you publicize only what you must, you can change everything else with impunity, and since designs tend to evolve this is an important freedom. 6. Watch out for “giant object syndrome.” This is often an affliction of procedural programmers who are new to OOP, and who end up writing a procedural program and sticking it inside one or two giant objects. With the exception of application frameworks, objects represent concepts in your application, not the application itself. 7. If you must do something ugly, at least localize the ugliness inside a class. 8. Anytime you notice classes that appear to have high coupling with each other, consider the coding and maintenance improvements you might get by using inner classes (see “Improving the code with an inner class” on page 521). 9. Use comments liberally, and use the javadoc comment-documentation syntax to produce your program documentation. 10. Constructors & exceptions. You’ll generally want to re-throw any exceptions that you catch in a constructor if it causes failure of the creation of that object, so the caller doesn’t blindly continue, thinking that the object was created correctly. 11. If your class requires any cleanup when the client programmer is finished with the object, place the cleanup code in a single, well defined method with a name like cleanup( ) that clearly suggests its purpose. In addition, place a boolean flag in the class to indicate whether the object has been cleaned up or not. In the finalize( ) method for the class, check to make sure the object has been cleaned up, and throw a class derived from RuntimeException if it hasn’t, to indicate a programming error. Before relying on such a scheme, ensure that finalize( ) works on your system (you may need to call System.runFinalizersOnExit(true) to ensure this behavior. 12. If an object must be cleaned up (other than garbage collection) within a particular scope, use the following approach: initialize the object and, if successful, immediately enter a try block with a finally clause that performs the cleanup. 13. When overriding finalize( ) during inheritance, remember to call super.finalize( ) (this is not necessary if Object is your immediate superclass). You should call super.finalize( ) as the last act of your overridden finalize( ) rather than the first, to ensure that base-class components are still valid if you need them. 14. When you are creating a fixed-size collection of objects, transfer them to an array (especially if you’re returning this collection from a method). This way you get the benefit of the array’s compile-time type checking and the recipient of the array may not have to cast the objects in the array to use them. 710 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 711. 15. Choose interfaces over abstract classes. If you know something is going to be a base class, your first choice should be to make it an interface, and only if you’re forced to have method definitions or member variables should you change to an abstract class. 16. Inside constructors, do only what is necessary to set the object into the proper state. Actively avoid calling other methods (except for final methods) since those methods may be overridden to produce unexpected results during construction (see Chapter 7 for details). 17. Objects should not simply hold some data; they should also have well-defined behaviors. 18. Choose composition first when creating new classes from existing classes. Only if inheritance is required by your design should it be used. If you use inheritance where composition will work, your designs will become needlessly complicated. 19. Use inheritance and method overriding to express differences in behavior, and fields to express variations in state. An extreme example of what not to do is inheriting different classes to represent colors instead of using a “color” field. 20. To avoid a highly frustrating experience, make sure there’s only one class of each name anywhere in your classpath. Otherwise the compiler can find the identically-named other class first, and report error messages that make no sense. If you suspect you are having a classpath problem, try looking for .class files with the same names at each of the starting points in your classpath. 21. When using the event “adapters” in the new Java 1.1 AWT, there’s a particularly easy pitfall you can encounter. If you override one of the adapter methods and you don’t quite get the spelling right, you’ll end up adding a new method rather than overriding an existing method. However, this is perfectly legal and so you won’t get any error message from the compiler or run-time system – your code simply won’t work correctly. 22. Use design patterns to eliminate “naked functionality.” That is, if only one object of your class should be created, don’t bolt ahead to the application and write a comment “make only one of these.” Wrap it in a singleton. If you have a lot of messy code in your main program that creates your objects, look for a creational pattern like a factory method where you can encapsulate that creation. Eliminating “naked functionality” will not only make your code much easier to understand and maintain, it will make it more bulletproof against the well-intentioned maintainers that come after you. 23. Watch out for “analysis paralysis.” Remember that you must usually move forward in a project before you know everything, and that often the best and fastest way to learn about some of your unknown factors is to go to the next step rather than trying to figure it out in your head. 24. Remember that code is read much more than it is written. Clean designs make for easy-to-understand programs, but comments, detailed explanations and examples are invaluable. They will help both you and everyone who comes after you. If nothing else, the frustration of trying to ferret out useful information from the on-line Java documentation should convince you. 25. Elegance always pays off. In the short term it may seem like it takes much longer to come up with a truly graceful solution to a problem, but when it works the first time and easily adapts to new situations instead of requiring Appendix C: Java Programming Guidelines 711
- 712. hours, days or months of struggle you’ll see the rewards (even if no one can measure them). And there’s nothing that matches the feeling when you know you’ve got an amazing design. Resist the urge to hurry, it will only slow you down. 712 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 713. J D: A bit about garbage collection It’s hard to believe that Java could possibly be as fast or faster than C++. This assertion has yet to be proven to my satisfaction. However, I’ve begun to see that many of my doubts about speed come from early implementations that were not particularly efficient so there was no model at which to point and explain how Java can be fast. Part of the way I’ve thought about speed has come from being cloistered with the C++ model. C++ is very focused on everything happening statically, at compile time, so that the run-time image of the program is very small and fast. C++ is also based very heavily on the C model, primarily for backwards compatibility, but sometimes simply because it worked a particular way in C so it was the easiest approach in C++. One of the most important cases is the way memory is managed in C and C++, and this has to do with one of my more fundamental assertions about why Java must be slow: in Java, all objects must be created on the heap. In C++, creating objects on the stack is very fast because when you enter a particular scope the stack pointer is moved down once to allocate storage for all the stack-based objects created in that scope, and when you leave the scope (after all the local destructors have been called) the stack pointer is moved up once. However, creating heap objects in C++ is typically much slower because it’s based on the C concept of a heap as a big pool of memory which (and this is essential) must be recycled. When you call delete in C++ the released memory leaves a hole in the heap and so when you call new, the storage allocation mechanism must go seeking to try to fit the storage for your object into any existing holes in the heap, or else you’ll rapidly run out of heap storage. Searching for 713
- 714. available pieces of memory is the reason that allocating heap storage has such a performance impact in C++, so it’s far faster to create stack-based objects. Again, because so much of C++ is based on doing everything at compile-time this makes sense. But in Java there are certain places where things happen more dynamically and it changes the model. When it comes to creating objects, it turns out that the garbage collector can have a significant impact on the speed of object creation. This sounds a bit odd at first – that storage release affects storage allocation – but it’s the way some JVMs work and it means that allocating storage for heap objects in Java can be nearly as fast as creating storage on the stack in C++. You can think of the C++ heap (and a slow implementation of a Java heap) as a yard where each object stakes out its own piece of turf. This real estate can become available sometime later and must be reused. In some JVMs, the Java heap is quite different; it’s more like a conveyor belt which moves forward every time you allocate a new object. This means that object storage allocation is remarkably rapid. The “heap pointer” is simply moved forward into virgin territory, so it’s effectively the same as C++’s stack allocation (of course, there’s a little extra overhead for bookkeeping but it’s nothing like searching for storage). Now you might observe that the heap isn’t in fact a conveyor belt and if you treat it that way you’ll eventually start paging memory a lot (which is a big performance hit) and later run out. But the trick is that the garbage collector steps in and while it collects the garbage, it compacts all the objects in the heap so that you’ve effectively moved the “heap pointer” closer to the beginning of the conveyor belt and further away from a page fault. So the garbage collector rearranges things and makes it possible for the high-speed, infinite-free-heap model to be used while allocating storage. To understand how this works, you need to get a little better idea of the way the different garbage collector (GC) schemes work. A very simple but very slow GC technique is reference counting. This means that each object contains a reference counter, and every time a handle is attached to an object the reference count is increased. Every time a handle goes out of scope or is set to null, the reference count is decreased. Thus, managing reference counts is a small but constant overhead that happens throughout the lifetime of your program. The garbage collector moves through the entire list of objects and when it finds one with a reference count of zero it releases that storage. The one complication is that if objects circularly refer to each other they can have non-zero reference counts while still being garbage. Locating such self-referential groups requires significant extra work on the part of the garbage collector. Reference counting is commonly used to explain one kind of garbage collection but it doesn’t seem to be used in any JVM implementations. In faster schemes, garbage collection is not based on reference counting. Instead, it is based on the idea that any non-dead object must ultimately be traceable back to a handle that lives either on the stack or in static storage. The chain may go through several layers of objects. Thus, if you start in the stack and the static storage area and walk through all the handles you’ll find all the live objects. For each handle that you find you must trace into the object that it points to, and then follow all the handles in that object, tracing into the objects they point to, etc., until you’ve moved through the entire Web that originated with the handle on the stack or in static storage. Each object that you move through must still be alive. Note that there is no problem with detached self-referential groups – these are simply not found, and are therefore automatically garbage. In the approach described here, the JVM uses an adaptive garbage-collection scheme, and what it does with the live objects that it locates depends on the variant currently being used. One of these variants is stop-and-copy. This means that, for reasons that will become apparent, the program is first stopped (this is not a background collection scheme). Then, each live object that is found is copied from one heap to another, leaving behind all the 714 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 715. garbage. In addition, as the objects are copied into the new heap they are packed end-to- end, thus compacting the new heap (and allowing new storage to simply be reeled off the end as previously described). Of course, when an object is moved from one place to another, the handles to that object must be changed. The handle that comes from tracing to the object from the heap or the static storage area can be changed right away, but there may be other handles pointing to this object that will be encountered later during the “walk,” and these are fixed up as they are found (you could imagine a hash table mapping old addresses to new ones). There are two issues that make copy collectors inefficient. The first is the idea that you have two heaps and you slosh all the memory back and forth between these two separate heaps, meaning you must maintain twice as much memory as you actually need. Some JVMs deal with this by allocating the heap in chunks as needed and simply copying from one chunk to another. The second issue is the copying itself. Once your program becomes stable it may be generating little or no garbage. Despite that, a copy collector will still copy all the memory from one place to another, which is wasteful. To prevent this, some JVMs detect that no new garbage is being generated and switch to a different scheme (this is the “adaptive” part). This other scheme is called mark and sweep, and it’s what Sun’s JVM uses all the time. For general use mark and sweep is fairly slow, but when you know you’re generating little or no garbage it’s very fast. Mark and sweep follows the same logic of starting from the stack and static storage and tracing through all the handles to find live objects. However, each time it finds a live object that object is marked by setting a flag in it, but the object isn’t collected yet. Only when the marking process is finished does the sweep occur. During the sweep, the dead objects are released. However, no copying happens so if the collector chooses to compact a fragmented heap it does so by shuffling objects around. The “stop-and-copy” refers to the idea that this type of garbage collection is not done in the background; instead, the program is stopped while the GC occurs. In the Sun literature you’ll find many references to garbage collection as a low-priority background process, but it turns out that this was a theoretical experiment that didn’t work out. In practice the Sun garbage collector is run when memory gets low. In addition, mark-and-sweep requires that the program be stopped. As previously mentioned, in the JVM described here memory is allocated in big blocks. If you allocate a large object, it gets its own block. Strict stop-and-copy requires copying every live object from the source heap to a new heap before you could free the old one, which translates to lots of memory. With blocks the GC can typically use dead blocks to copy objects to as it collects. Each block has a generation count to keep track of whether it’s alive or not. In the normal case, only the blocks created since the last GC are compacted, all other blocks just get their generation count bumped if they have been referenced from somewhere. This handles the normal case of lots of short-lived temporary objects. Periodically, a full sweep is made – large objects are still not copied (just get their generation count bumped) and blocks containing small objects are copied and compacted. The JVM monitors the efficiency of GC and if it becomes a waste of time because all objects are long-lived then it switches to mark-and-sweep. Similarly, the JVM keeps track of how successful mark-and-sweep is and if the heap starts to become fragmented it switches back to stop-and-copy. This is where the “adaptive” part comes in, so you end up with a mouthful: “adaptive generational stop-and-copy mark-and-sweep.” There are a number of additional speedups possible in a JVM. An especially important one involves the operation of the loader and Just-In-Time (JIT) compiler. When a class must be loaded (typically, the first time you want to create an object of that class), the .class file is located and the byte codes for that class are brought into memory. At this point one Appendix D: A Bit about Garbage Collection 715
- 716. approach is to simply JIT all the code, but this has two drawbacks: it takes a little more time which, compounded throughout the life of the program, can add up, and it increases the size of the executable (byte codes are significantly more compact than expanded JIT code) and thus may cause paging which definitely slows down a program. An alternative approach is lazy evaluation which means that the code is not JIT compiled until necessary. Thus, code that never gets executed may never get JIT compiled. Because JVMs are external to browsers, you might expect that you could benefit from the speedups of some JVMs while using any browser. Unfortunately, JVMs don’t currently interoperate with different browsers. Thus, to get the benefits of a particular JVM you must either use the browser with that JVM built in or run standalone Java applications. 716 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 717. E E: Recommended reading Java in a Nutshell: A Desktop Quick Reference, 2nd Edition, by David Flanagan, O'Reilly & Assoc. 1997. A compact summary of the on-line documentation of Java 1.1. Personally, I prefer to browse the docs online, especially since they change so often. However, many folks still like printed documentation and this fits the bill; it also provides more discussion than the online docs. The Java Class Libraries: An Annotated Reference, by Patrick Chan and Rosanna Lee, Addison Wesley 1997. What the online reference should have been: enough description to make it usable. One of the technical reviewers for Thinking in Java said: “If I had only one Java book, this would be it (well, in addition to yours, of course).” I’m not as thrilled with it as he is: it’s big, it’s expensive, and the quality of the examples doesn’t satisfy me. But it’s a place to look when you’re stuck and seems to have more depth (and sheer size) than Java in a Nutshell. Java Network Programming, by Elliote Rusty Harold, O’Reilly 1997. I didn’t begin to understand Java networking until I found this book. I also find his Web site, Café au Lait, to be a stimulating, opinionated and up-to-date perspective on Java developments, unencumbered by allegiances to any vendors. His almost daily updating keeps up with fast-changing news about Java. See http://sunsite.unc.edu/javafaq/. Core Java, 3nd Edition, by Cornell & Horstmann, Prentice-Hall 1997. A good place to go for questions you can’t find the answers to in Thinking in Java. Note: the Java 1.1 revision is Core Java 1.1 Volume 1 – Fundamentals & Core Java 1.1 Volume 2 – Advanced Features. JDBC Database Access with Java, by Hamilton, Cattell & Fisher (Addison-Wesley, 1997). If you know nothing about SQL and databases, this is a nice, gentle introduction. It also contains some of the details as well as an “annotated reference” to the API (again, what the 717
- 718. online reference should have been). The drawback, as with all books in The Java Series (“The ONLY Books Authorized by JavaSoft”) is that it’s been whitewashed so that it says only wonderful things about Java – you won’t find out about any dark corners in this series. Client/Server Programming with Java and CORBA by Orfali & Harkey (John Wiley & Sons, 1997). Compares the different approaches to distributed computing, with a definite leaning to the “CORBA is the best way to do things” camp. Design Patterns, by Gamma, Helm, Johnson & Vlissides (Addison-Wesley 1995). The seminal book that started the patterns movement in programming. Practical Algorithms for Programmers, by Binstock & Rex (Addison-Wesley 1995). The algorithms are in C, so they're fairly easy to translate into Java. Each algorithm is thoroughly explained. 718 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 719. Index 719
- 720. H Index Please note that some names will be duplicated in capitalized form. Following Java style, the capitalized names refer to Java classes, while lowercase names refer to a general concept. -, 84 Abstract Window Toolkit (AWT), 413 !=, 85; operator, 387 abstraction, 30; in program design, 632 &, 88 accept( ), 574 &&, 86 access: class, 164; control, 33, 151, 166; inner &=, 89 classes & access rights, 221; package access and [ ]: indexing operator [ ], 142 friendly, 159; specifiers, 33, 151, 159; within a ^, 89 directory, via the unnamed package, 160 ^=, 89 action( ), 420, 430, 436, 597; cannot combine with |, 88 listeners, 479 ||, 86 ActionEvent, 452, 454, 471, 511 |=, 89 ActionListener, 452 ‘+’: operator + for String, 407 actionPerformed( ), 455 +, 84 adapters: listener adapters, 456 <, 85 add( ), 432 <<, 89 addActionListener( ), 452, 508, 540 <<=, 90 addAdjustmentListener( ), 452 <=, 85 addComponentListener( ), 452 ==, 85; operator, 387; vs. equals( ), 326 addContainerListener( ), 452 >, 85 addElement( ), Vector, 252, 255 >=, 85 addFocusListener( ), 452 >>, 89 addItemListener( ), 452 >>=, 90 addition, 82 abstract: class, 203; inheriting from an abstract addKeyListener( ), 452 class, 204; vs. interface, 210 addListener, 451 abstract keyword, 204 addMouseListener( ), 452 720 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 721. addMouseMotionListener( ), 452 AWT: Abstract Window Toolkit, 413; action( ), 420, addTextListener( ), 452 430, 436, 597; action( ), cannot combine with addWindowListener( ), 452 listeners, 479; ActionEvent, 452, 454, 471, 511; AdjustmentEvent, 452, 454 ActionListener, 452; actionPerformed( ), 455; AdjustmentListener, 452 add( ), 432; addActionListener( ), 452, 508; adjustmentValueChanged( ), 455 addAdjustmentListener( ), 452; addComponentListener( ), 452; Adler32, 335 addContainerListener( ), 452; addFocusListener( ), aliasing, 81; and String, 407; during a method call, 452; addItemListener( ), 452; addKeyListener( ), 380 452; addListener, 451; addMouseListener( ), 452; align, 417 addMouseMotionListener( ), 452; AlreadyBoundException, 626 addTextListener( ), 452; addWindowListener( ), analysis: & design, object-oriented, 52; requirements 452; AdjustmentEvent, 452, 454; analysis, 53 AdjustmentListener, 452; AND: bitwise, 94; logical (&&), 86 adjustmentValueChanged( ), 455; appendText( ), anonymous inner class, 216, 309, 458, 551, 666; 426; Applet, 432; applet advantages for and constructors, 219 client/server systems, 441; applet restrictions, appendText( ), 426 440; archive tag, for HTML and JAR files, 459; applet, 415, 589; advantages for client/server BorderLayout, 433, 458; Button, 419, 432; systems, 441; align, 417; and packages, 418; Button, creating your own, 437; Canvas, 437; archive tag, for HTML and JAR files, 459; CardLayout, 434; Checkbox, 426, 427; classpath, 418; codebase, 417; combined applets Checkbox, Java 1.1, 463; CheckboxGroup, 427; and applications, 457; creating a Frame from CheckboxMenuItem, 442; CheckboxMenuItem, within an applet, 498; destroy( ), 416; displaying Java 1.1, 467; Choice, 428; Choice, Java 1.1, a Web page from within an applet, 604; init( ), 465; combining layouts, 434; Component, 416, 416, 418; name, 417; packaging applets in a JAR 430, 432, 436; ComponentAdapter, 455; file to optimize loading, 459; paint( ), 416; componentAdded( ), 455; ComponentEvent, 452, parameter, 417; placing inside a Web page, 417; 454; componentHidden( ), 455; restrictions, 440; start( ), 416, 418; stop( ), 416, ComponentListener, 452; componentMoved( ), 418; submitting a POST with an applet, 614; 455; componentRemoved( ), 455; tag, for HTML page, 417; update( ), 416 componentResized( ), 455; componentShown( ), Applet, 432; combined with application, 527; 455; consume( ), 461; Container, 432; initialization parameters, 527 ContainerAdapter, 455; ContainerEvent, 452, appletviewer, 417 454; ContainerListener, 452; controlling layout, application: application builder, 501; application 432; DataFlavor, 500; desktop colors, 492; framework, 227; combined applets and Dialog, 445; dialog box, 445; Dialog, Java 1.1, applications, 457; combined with Applet, 527; 471; dispatchEvent( ), 471; dispatching messages, running an application from within an applet, 471; dispose( ), 445, 497, 498; drawString( ), 498; standalone windowed application, 445; 416; drop-down list, 428; East, 433; eliminating windowed applications, 442 flicker, 487; enableEvents( ), 487; end( ), 497; application framework, 415 event listener, 451; Event object, 420, 430; event archive tag, for HTML and JAR files, 459 target, 420; event-driven programming, 420; argument: constructor, 120; final, 187, 310; FileDialog, 448; FileDialog, Java 1.1, 473; flavor, passing a handle into a method, 380; variable clipboard, 499; FlowLayout, 432; Focus argument lists (unknown quantity and type of traversal, 492; FocusAdapter, 455; FocusEvent, arguments), 145 452, 454; focusGained( ), 455; FocusListener, 452; focusLost( ), 455; Font, 497; Frame, 432, array, 245, 272; associative array, 261; bounds 442; getAlignment( ), 424; getAppletContext( ), checking, 143; dynamic creation, 644; first-class 421; getContents( ), 500; getDirectory( ), 450; objects, 246; initialization, 142; length, 246; getFile( ), 450; getPrintJob( ), 497, 498; multidimensional, 146; of objects, 246; of getSelectedItems( ), 429; getState( ), 445; primitives, 246; returning an array, 249 getText( ), 424; getTransferData( ), 500; assigning objects, 80 getTransferDataFlavors( ), 500; gotFocus( ), 436; assignment, 80 graphics, 447; Graphics, 416, 497; associative array, 251, 261 GridBagLayout, 436; GridLayout, 433; auto-decrement operator, 84 handleEvent( ), 420, 430, 436; inner classes, 451; auto-increment operator, 84 isDataFlavorSupported( ), 500; automatic compilation, 156 isFocusTraversable( ), 492; ItemEvent, 452, 454, automatic type conversion, 170 463; ItemListener, 452, 463; itemStateChanged( ), available( ), 317 455; JAR, packaging applets to optimize loading, Index 721
- 722. 459; KeyAdapter, 455; keyDown( ), 436; 452, 454, 471; windowIconified( ), 455; KeyEvent, 452, 454; KeyListener, 452; WindowListener, 452; windowOpened( ), 455 keyPressed( ), 455; keyReleased( ), 455; base 16, 95 keyTyped( ), 455; keyUp( ), 436; Label, 424; base 8, 95 layout manager, 425; List, 429; list boxes, 429; base class, 162, 172, 196; abstract base class, 203; List, Java 1.1, 466; listener adapters, 456; base-class interface, 199; constructor, 234; lostFocus( ), 436; match an event to the text on constructors and exceptions, 176; initialization, a button, 421; Menu, 442; menu shortcuts, 470; 174 Menu, Java 1.1, 467; MenuBar, 442, 471; Basic: Microsoft Visual Basic, 501 MenuComponent, 442; MenuItem, 442, 471; beanbox Bean testing tool, 512 MenuItem, Java 1.1, 467; menus, 442; BeanInfo: custom BeanInfo, 512 MouseAdapter, 455; mouseClicked( ), 455; Beans: and Borland’s Delphi, 501; and Microsoft’s mouseDown( ), 436, 448; mouseDrag( ), 437; Visual Basic, 501; and multithreading, 537; mouseDragged( ), 455; mouseEnter( ), 437; application builder, 501; beanbox Bean testing mouseEntered( ), 455; MouseEvent, 452, 454; tool, 512; bound properties, 512; component, mouseExit( ), 437; mouseExited( ), 455; 502; constrained properties, 512; custom MouseListener, 452; MouseMotionAdapter, 456; BeanInfo, 512; custom property editor, 512; MouseMotionListener, 452; mouseMove( ), 437; custom property sheet, 512; events, 501; mouseMoved( ), 455; mousePressed( ), 455; EventSetDescriptors, 506; FeatureDescriptor, 513; mouseReleased( ), 455; mouseUp( ), 437; getBeanInfo( ), 504; getEventSetDescriptors( ), multicast, 510; multicast events, 476; multiple 506; getMethodDescriptors( ), 506; getName( ), selection in a List, 432; new event model, 450; 506; getPropertyDescriptors( ), 506; North, 433; paint( ), 437, 447, 492, 497; Panel, getPropertyType( ), 506; getReadMethod( ), 506; 434; print( ), 492, 497; PrintGraphics, 497; getWriteMethod( ), 506; indexed property, 512; printing, 498; printing text, 497; PrintJob, 497; Introspector, 504; JAR files for packaging, 511; processEvent( ), 487; proportionally-spaced font, manifest file, 511; Method, 506; 425; radio button, 427; removeActionListener( ), MethodDescriptors, 506; naming convention, 452, 508; removeAdjustmentListener( ), 452; 502; properties, 501; PropertyChangeEvent, 512; removeComponentListener( ), 452; PropertyDescriptors, 506; removeContainerListener( ), 452; ProptertyVetoException, 512; reflection, 501, removeFocusListener( ), 452; 504; Serializable, 510; visual programming, 501 removeItemListener( ), 452; removeKeyListener( ), behavioral design patterns, 634 452; removeMouseListener( ), 452; Bill Joy, 85 removeMouseMotionListener( ), 452; removeTextListener( ), 452; binary: numbers, 95; operators, 89 removeWindowListener( ), 452; repaint( ), 487; bind( ), 625 requestFocus( ), 492; setActionCommand( ), 470; binding: dynamic binding, 196; dynamic, late or setAlignment( ), 424; setCheckboxGroup( ), 427; run-time binding, 193; late binding, 196; method setContents( ), 500; setDirectory( ), 450; call binding, 196; run-time binding, 196 setEditable( ), 423; setFile( ), 450; setLayout( ), BitSet, 251, 259 432; setMenuBar( ), 442; setText( ), 424; show( ), bitwise: AND, 94; AND operator (&), 88; 450; showStatus( ), 421; South, 433; standalone EXCLUSIVE OR XOR (^), 89; NOT ~, 89; windowed application, 445; StringSelection, 500; operators, 88; OR, 94; OR operator (|), 88 super.action( ), 421; super.handleEvent( ), 431; bitwise copy, 386 system clipboard, 499; target of an event, 421; blank final, in Java 1.1, 187 TextArea, 423, 499; TextArea, Java 1.1, 462; blocking: and available( ), 317; and threads, 540; TextComponent, 422; TextComponent, Java 1.1, on IO, 546 461; TextEvent, 452, 454; TextField, 422; Booch, Grady, 640 TextField, Java 1.1, 461; TextListener, 452; book: errors, reporting, 26; updates of the book, textValueChanged( ), 455; Toolkit, 498; 25 TooManyListenersException, 476, 510; Boolean, 104; algebra, 88; and casting, 94; vs. C Transferable, 500; unicast, 510; unicast events, and C++, 87 476; update( ), 487; Web browser window status BorderLayout, 433, 458 line, 421; West, 433; WINDOW_CLOSING, 471; Borland, 513; Delphi, 501 WINDOW_DESTROY, 445; windowActivated( ), bound properties, 512 455; WindowAdapter, 456; windowClosed( ), bounds checking, array, 143 455; windowClosing( ), 455; windowDeactivated( ), 455; windowDeiconified( ), break keyword, 108 455; windowed applications, 442; WindowEvent, browser: class browser, 164 722 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 723. BufferedInputStream, 306, 316 access rights, 221; inner classes and overriding, BufferedOutputStream, 307, 318 225; inner classes and super, 224; inner classes BufferedReader, 296, 328, 679 and the AWT, 451; inner classes and upcasting, BufferedWriter, 328 214; inner classes in methods & scopes, 216; inner business objects/logic, 477 classes, identifiers and .class files, 226; intializing button, 419, 432; creating your own, 437; radio the derived class, 174; loading, 191; member button, 427 initialization, 171; order of initialization, 137; poorly-considered class design, 305; private inner ByteArrayInputStream, 302 classes, 229; public class, and compilation units, ByteArrayOutputStream, 303 152; read-only classes, 402; referring to the outer C++, 29, 85, 713; CGI programming in C++, class object in an inner class, 223; static inner 605; comparing C++ and Java, 701; copy classes, 222; style of creating classes, 163; constructor, 399; copy-constructor, 609; GNU subobject, 174 C++ Compiler, 605; in contrast with Java, 605; Class: Class object, 352, 362, 533; forName( ), 363, operator overloading, 609; Standard Template 691; getClass( ), 280; getConstructors( ), 373; Library (STL), 272; Standard Template Library getInterfaces( ), 370; getMethods( ), 373; (STL)., 605; template, 656; templates, 255; getName( ), 371; getSuperclass( ), 370; isInstance, vector class, vs. array and Java Vector, 246 368; isInterface( ), 371; newInstance( ), 370; callback, 268, 309 printInfo( ), 371; reflection, 644; RTTI using the Canvas, 437, 564 Class object, 369 capitalization: Java capitalization style source-code ClassCastException, 243, 364 checking tool, 681; of package names, 69 classpath, 80, 154, 418; and rmic, 627; pitfall, 158 CardLayout, 434 cleanup: and garbage collector, 177; performing, case statement, 114 132; with finally, 292 cast, 126, 361; and collections, 251; and primitive client programmer, 33, 151 types, 105; from float or double to integral, client, network, 573 truncation, 115; operators, 94 clipboard: system clipboard, 499 catch: catching an exception, 277; catching any clone( ), 384, 642; and composition, 388; and exception, 280; keyword, 278 inheritance, 394; Object.clone( ), 387; CGI: Common-Gateway Interface, 589; connecting removing/turning off clonability, 395; Java to CGI, 599; crash course in CGI super.clone( ), 387, 398; supporting cloning in programming, 599; GET, 599, 612; HTML derived classes, 395 content-type, 612; intercepting results, 604; Cloneable interface, 385 invoking a CGI program from Java, 600; POST, CloneNotSupportedException, 386 599, 612; programming in C++, 605 close( ), 316 change: vector of change, 229, 632, 642 code: coding standards, 24, 709; extracting code CharArrayReader, 327 listings, 669; organization, 159; re-use, 169 CharArrayWriter, 327 codebase, 417 Checkbox, 426, 427; Java 1.1, 463 coding: improving code with inner classes, 521 CheckboxGroup, 427 collection: class, 245, 251; Hashtable, 680; of CheckboxMenuItem, 442; Java 1.1, 467 primitives, 249 CheckedInputStream, 333 collision: name, 156 CheckedOutputStream, 333 colors: desktop colors, 492 Checksum, 335 comma operator, 93, 108 Choice, 428; Java 1.1, 465 common interface, 203 class, 163; abstract class, 203; access, 164; common pitfalls when using operators, 93 anonymous inner class, 216, 309, 458, 551, 666; Common-Gateway Interface (CGI), 589 anonymous inner class and constructors, 219; base class, 162, 172, 196; browser, 164; class compilation unit, 152 hierarchies and exception handling, 298; class compile-time constant, 185 literal, 363, 366; derived class, 196; final classes, compiling, automatic compilation, 156 188; inheritance diagrams, 184; inheriting from complexity theory, Boids example, 692 an abstract class, 204; inheriting from inner Component, 416, 430, 432, 436 classes, 224; initialization & class loading, 190; component, and Java Beans, 502 initialization of data members, 135; initializing ComponentAdapter, 455 members at point of definition, 136; initializing componentAdded( ), 455 the base class, 174; inner class, 213, 461, 521, ComponentEvent, 452, 454 644; inner class nesting within any arbitrary componentHidden( ), 455 scope, 217; inner classes, 479; inner classes & ComponentListener, 452 Index 723
- 724. componentMoved( ), 455 CRC32, 335 componentRemoved( ), 455 createStatement( ), 618 componentResized( ), 455 creational design patterns, 633, 640 componentShown( ), 455 critical section, and synchronized block, 536 composition, 169, 270; and cloning, 388; and currentThread(), 517 design, 239; and design patterns, 632; and daemon threads, 527 dynamic behavior change, 240; choosing data: final, 185; primitive data types and use with composition vs. inheritance, 181; combining operators, 97; static initialization, 138 composition & inheritance, 176; vs. inheritance, database: flat-file database, 619; Java DataBase 184, 323 Connectivity (JDBC), 616; relational database, compression: Java 1.1 compression library, 333 619; URL, 616 conditional operator, 92 DatabaseMetaData, 623 conference, Software Development Conference, 18 DataFlavor, 500 console input, 331 Datagram, 583, 590, 594; receive( ), 598; User const, in C++, 407 Datagram Protocol (UDP), 583 constant: compile-time constant, 185; folding, 185; DatagramPacket, 584, 586, 597 groups of constant values, 212; implicit DatagramSocket, 584, 597 constants, and String, 406 DataInput, 307 constrained properties, 512 DataInputStream, 305, 316, 317, 318, 328, 670 constructor, 119; and anonymous inner classes, DataOutput, 307 216; and exception handling, 295; and DataOutputStream, 307, 318, 328, 670, 680 exceptions, 290; and finally, 295; and DCOM, 628 overloading, 121; and polymorphism, 232; dead, Thread, 540 arguments, 120; base-class constructor, 234; deadlock: multithreading, 549 base-class constructors and exceptions, 176; deadlock multithreading, 544 behavior of polymorphic methods inside decoupling through polymorphism, 193 constructors, 237; C++ copy constructor, 399; calling base-class constructors with arguments, decrement operator, 84 175; calling from other constructors, 129; deep copy, 384, 388; and Vector, 390; using default, 127; default constructor synthesized by serialization to perform deep copying, 392 the compiler, 633; default constructors, 123; default constructor, 123, 127; synthesized by the initialization during inheritance and composition, compiler, 633; synthesizing a default constructor, 176; name, 120; no-arg constructors, 123; order 175 of constructor calls with inheritance, 233; private default keyword, in a switch statement, 114 constructor, 633; return value, 121; static defaultReadObject( ), 349 construction clause, 140 defaultWriteObject( ), 349 Constructor: for reflection, 372, 644 DeflaterOutputStream, 333 consulting & mentoring provided by Bruce Eckel, Delphi, from Borland, 501 25 derived: derived class, 196; derived class, consume( ), 461 initializing, 174 Container, 432 design, 29, 241; abstraction in program design, container class, 245 632; adding more methods to a design, 167; ContainerAdapter, 455 analysis & design, object-oriented, 52; and ContainerEvent, 452, 454 composition, 239; and inheritance, 239; and ContainerListener, 452 mistakes, 167; library design, 151; of object content-type, HTML, 612 hierarchies, 192; poorly-considered class design, 305 continue keyword, 108 design patterns, 56, 166, 631; adapter, 327; control framework, and inner classes, 227 behavioral, 634; creational, 633, 640; factory controlling access, 33, 166 method, 640; observer, 634; prototype, 642, conversion: automatic, 170; narrowing conversion, 648; singleton, 166; structural, 634; vector of 94, 126; widening conversion, 94 change, 632, 642; visitor, 659 copy: deep copy, 384; shallow copy, 384 desktop colors, 492 copy-constructor, in C++, 609 destroy( ), 416, 552; and deadlocks, 549 CORBA, 628 destructor, 131, 132, 292; Java doesn’t have one, couplet, 521 177 couplets, 664 development, incremental, 182 coupling, 279 diagram, class inheritance diagrams, 184 724 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 725. Dialog, 445; Java 1.1, 471 catching an exception, 277; catching any dialog box, 445 exception, 280; changing the point of origin of Dictionary, 261 the exception, 282; class hierarchies, 298; directory: and packages, 158; creating directories constructors, 295; creating your own, 285; and paths, 312; lister, 308 design issues, 297; Error class, 284; Exception dispatchEvent( ), 471 class, 284; exception handler, 278; exception dispatching: double dispatching, 653, 660; multiple handling, 275; exception matching, 298; dispatching, 653 FileNotFoundException, 297; fillInStackTrace( ), 281; finally, 291; guarded region, 277; handler, dispatching messages, AWT, 471 276; handling, 177; losing an exception, pitfall, dispose( ), 445, 497, 498 294; NullPointerException, 284; division, 82 printStackTrace( ), 281; restrictions, 288; re- Domain Name Service (DNS), 572 throwing an exception, 281; RuntimeException, dotted quad, 572 284; specification, 279; termination vs. double dispatching, 653, 660 resumption, 278; Throwable, 280; throwing an double, literal value marker (D), 95 exception, 276, 277; try, 292; try block, 278; Double.valueOf( ), 647 typical uses of exceptions, 299 do-while, 107 exceptional condition, 276 downcast, 184, 242, 364; type-safe downcast in exec( ), 590, 594 run-time type identification, 364 executeQuery( ), 618 drawString( ), 416 Exponential notation, 95 DriverManager, getConnection( ), 617 extends, 162, 174, 241; and interface, 211; drop-down list, 428 keyword, 172 dynamic: array creation, 644; behavior change with extensible, 665; program, 199 composition, 240; binding, 193, 196 extension: pure inheritance vs. extension, 240 early binding, 196 Externalizable, 343; alternative approach to using, East, 433 348 efficiency: and arrays, 245; and final, 189; and extracting code listings, 669 threads, 517; when using the synchronized factory method, 640 keyword, 536 false, 86 elegance, in programming, 55 FeatureDescriptor, 513 elementAt( ), Vector, 252, 255 Field, for reflection, 372 else keyword, 105 fields, initializing fields in interfaces, 212 emergent behavior, 692 file: characteristics of files, 312; data file output enableEvents( ), 487 shorthand, 320; formatted file output encapsulation, 163 shorthand, 319; input shorthand, 319; JAR file, encode( ), URLEncoder, 600 153 end( ), 497 File, 303, 328, 680, 688; class, 308; File.list( ), 308; enum, groups of constant values in C & C++, 212 mkdirs( ), 680 Enumeration, 255, 267, 272, 639; File Transfer Protocol (FTP), 417 hasMoreElements( ), 256; nextElement( ), 256 FileDescriptor, 303 equals( ), 85, 421; overriding for Hashtable, 266; FileDialog, 448; Java 1.1, 473 vs. ==, 326 FileInputStream, 303, 316 equivalence: ==, 85; object equivalence, 85 FilenameFilter, 308, 687 error: handling with exceptions, 275; recovery, 300; FileNotFoundException, 297 reporting errors in book, 26 FileOutputStream, 304, 318 Euler's constant, 95 FileReader, 296, 327 event: Event object, 430; Event object, AWT, 420; FileWriter, 327 event-driven programming, 420; event-driven fillInStackTrace( ), 281 system, 227; Java Beans, 501; listener, 451; FilterInputStream, 303 match an event to the text on a button, 421; FilterOutputStream, 304 model, new in Java 1.1, 450; multicast, 476; FilterReader, 328 multicast event and Java Beans, 537; target, 420; FilterWriter, 328 unicast, 476 final, 206; and efficiency, 189; and static, 185; event listener: order of execution, 476 argument, 187, 310; blank finals in Java 1.1, EventSetDescriptors, 506 187; classes, 188; data, 185; keyword, 185; exception: and base-class constructors, 176; and method, 197; methods, 188; static primitives, constructors, 290; and inheritance, 288, 298; 186; with object handles, 185 Index 725
- 726. finalize( ), 131, 179, 298, 318; and inheritance, getFile( ), 450 234; and super, 236; calling directly, 132; order getFloat( ), 618 of finalization of objects, 237; getInputStream( ), 575, 616 runFinalizersOnExit( ), 134 getInt( ), 618 finally, 177, 179; and constructors, 295; keyword, getInterfaces( ), 370 291; pitfall, 294 getMethodDescriptors( ), 506 finding .class files during loading, 154 getMethods( ), 373 firewall, 598 getName( ), 371, 506 flat-file database, 619 getOutputStream( ), 575, 614 flavor, clipboard, 499 getPort( ), 586 flicker, eliminating, 487 getPrintJob( ), 497, 498 float, literal value marker(F), 95 getPriority( ), 553 floating point: true and false, 87 getProperties( ), 267 flocking, simulating behavior, 692 getPropertyDescriptors( ), 506 FlowLayout, 432 getPropertyType( ), 506 Focus traversal, 492 getReadMethod( ), 506 FocusAdapter, 455 getSelectedItems( ), 429 FocusEvent, 452, 454 getState( ), 445 focusGained( ), 455 getString( ), 618 FocusListener, 452 getSuperclass( ), 370 focusLost( ), 455 getText( ), 424 folding, constant, 185 getTransferData( ), 500 Font, 497 getTransferDataFlavors( ), 500 font, proportionally-spaced, 425 getWriteMethod( ), 506 for keyword, 107 GNU C++ Compiler, 605 forName( ), 363, 691 gotFocus( ), 436 FORTRAN, 95 goto: lack of goto in Java, 110 forward referencing, 137 graphical user interface (GUI), 227, 413 Frame, 432, 442; creating a Frame from within an graphics, 447; Graphics object, 416, 497 applet, 498 greater than (>), 85 framework: application framework, 415; control greater than or equal to (>=), 85 framework and inner classes, 227 GridBagLayout, 436 friendly, 151, 214; and interface, 206 GridLayout, 433, 564 FTP: File Transfer Protocol (FTP), 417 guarded region, in exception handling, 277 functor, 309 GUI: builders, 414; graphical user interface, 227, garbage collection, 130, 132, 234; and cleanup, 413 177; forcing finalization, 180; how it works, guidelines, coding standards, 709 713; order of object reclamation, 179 GZIPInputStream, 334 garbage collector: setting handles to null to allow GZIPOutputStream, 333 cleanup, 229 handle: assigning objects by copying handles, 80; generic, 255 equivalence vs object equivalence, 85; final, 185; Generic Collection Library for Java (JGL), 272 finding exact type of a base handle, 361; handle GET, 599; CGI, 612; request, 601 equivalence vs. object equivalence, 387; vs. get( ), Hashtable, 264 pointer in Java, 379 getAddress( ), 586 handleEvent( ), 420, 430, 436 getAlignment( ), 424 handler, exception, 278 getAppletContext( ), 421 has-a relationship, composition, 182 getBeanInfo( ), 504 hashCode( ), 263; overriding for Hashtable, 266 getClass( ), 280, 369 Hashtable, 246, 251, 261, 267, 273, 323, 440, getConnection( ): DriverManager, 617 680; combined with Vector, 680; used with getConstructor( ), reflection, 644 Vector, 686 getConstructors( ), 373; reflection, 644 hasMoreElements( ), Enumeration, 256 getContents( ), 500 herding, simulating behavior, 692 getDirectory( ), 450 Hexadecimal, 95 getDocumentBase( ), 603 hiding, implementation, 163 getEventSetDescriptors( ), 506 726 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 727. HTML, 589, 599; name, 526; param, 526; value, instance: instance initialization in Java 1.1, 220; 526 non-static instance initialization, 141 if-else statement, 92, 105 instanceof: dynamic instanceof, 368; keyword, 364 IllegalMonitorStateException, 545 Integer: parseInt( ), 448 immutable objects, 402 interface: and implementation, separation, 163; and implementation: and interface, 181, 206; and inheritance, 211; base-class interface, 199; interface, separation, 33, 163; hiding, 163, 214 Cloneable interface used as a flag, 385; common implements keyword, 206 interface, 203; graphical user interface (GUI), 227, import keyword, 152 413; initializing fields in interfaces, 212; keyword, increment operator, 84 206; Runnable, 524; separation of interface and incremental development, 56, 182 implementation, 33; upcasting to an interface, 209; vs. abstract, 210; vs. implemenation, 181 indexed property, 512 internationalization, in IO library, 326 indexing operator [ ], 142 Internet: Internet Protocol, 572; Internet Service indexOf( ), 647, 679; String, 309, 373, 597 Provider (ISP), 417 InetAddress, 597 interrupt( ), 549 InflaterInputStream, 333 InterruptedException, 517 inheritance, 162, 169, 172, 193; and cloning, 394; Intranet, 442 and design patterns, 632; and final, 189; and finalize( ), 234; and synchronized, 540; choosing Introspector, 504 composition vs. inheritance, 181; class inheritance IO: and threads, blocking, 541; available( ), 317; diagrams, 184; combining composition & blocking on IO, 546; blocking, and available( ), inheritance, 176; designing with inheritance, 239; 317; BufferedInputStream, 306, 316; extending interfaces with inheritance, 211; from BufferedOutputStream, 307, 318; BufferedReader, an abstract class, 204; from inner classes, 224; 296, 328, 679; BufferedWriter, 328; inheritance and method overloading vs. ByteArrayInputStream, 302; overriding, 180; initialization with inheritance, ByteArrayOutputStream, 303; characteristics of 190; multiple inheritance in C++ and Java, 209; files, 312; CharArrayReader, 327; pure inheritance vs. extension, 240; specialization, CharArrayWriter, 327; CheckedInputStream, 333; 182; vs composition, 323; vs. composition, 184 CheckedOutputStream, 333; close( ), 316; init( ), 416, 418 connecting to a non-Java program, 590; console input, 331; controlling the process of initialization: and class loading, 190; array serialization, 343; data file output shorthand, initialization, 142; base class, 174; class member, 320; DataInput, 307; DataInputStream, 305, 171; constructor initialization during inheritance 316, 317, 318, 328, 670; DataOutput, 307; and composition, 176; initializing class members DataOutputStream, 307, 318, 328, 670, 680; at point of definition, 136; initializing with the DeflaterOutputStream, 333; deprecated, 301; constructor, 119; instance initialization, 141; directory lister, 308; directory, creating directories instance initialization in Java 1.1, 220; member and paths, 312; exec( ), 590; Externalizable, 343; initializers, 234; non-static instance initialization, File, 303, 328, 680, 688; File class, 308; file 141; of class data members, 135; of method input shorthand, 319; File.list( ), 308; variables, 135; order of initialization, 137, 238; FileDescriptor, 303; FileInputStream, 303, 316; static, 191; with inheritance, 190 FilenameFilter, 308, 687; FileOutputStream, 304, inline method calls, 188 318; FileReader, 296, 327; FileWriter, 327; inner class, 213, 461, 479, 521, 644; access rights, FilterInputStream, 303; FilterOutputStream, 304; 221; and super, 224; and overriding, 225; and FilterReader, 328; FilterWriter, 328; finalize( ), control frameworks, 227; and the AWT, 451; and 318; formatted file output shorthand, 319; from upcasting, 214; anonymous, 666; anonymous standard input, 320; GZIPInputStream, 334; inner class, 309, 551; anonymous inner class and GZIPOutputStream, 333; InflaterInputStream, constructors, 219; hidden reference to the object 333; input, 302; InputStream, 302, 577, 590; of the enclosing class, 222; identifiers and .class InputStreamReader, 326, 327, 577; files, 226; in methods & scopes, 216; inheriting internationalization, 326; Java 1.1 compression from inner classes, 224; nesting within any library, 333; library, 301; lightweight persistence, arbitrary scope, 217; private, 522; private inner 339; LineNumberInputStream, 306, 317; classes, 229; referring to the outer class object, LineNumberReader, 328; mark( ), 308; mkdirs( ), 223; static, 222; static inner classes, 222 313, 680; nextToken( ), 687; input: console input, 331; file input shorthand, 319 ObjectOutputStream, 339; output, 302; InputStream, 302, 577, 590 OutputStream, 302, 303, 577, 590; InputStreamReader, 326, 327, 577 OutputStreamWriter, 326, 327, 577; pipe, 302; piped stream, 546; PipedInputStream, 303, 321; Index 727
- 728. PipedOutputStream, 303, 304, 321; PipedReader, 318, 320, 323, 326, 327, 328, 331, 332, 333, 327; PipedWriter, 327; PrintStream, 307, 318; 335, 338, 339, 341, 356, 357, 359, 363, 366, PrintWriter, 328, 577; Process, 590; pushBack( ), 368, 369, 371, 372, 392, 413, 414, 420, 423, 687; PushbackInputStream, 306; 432, 433, 436, 437, 441, 458, 462, 465, 466, PushBackReader, 328; RandomAccessFile, 307, 467, 470, 479, 481, 482, 487, 492, 496, 498, 318, 328; read( ), 302; readChar( ), 318; 499, 502, 504, 513, 546, 577, 585, 589, 590, readDouble( ), 318; Reader, 326, 327, 577; 598, 616, 623, 628, 644, 645, 699, 705, 707, readExternal( ), 343; readLine( ), 297, 316, 318, 709, 711, 717; AWT, 414; IO streams, 326; JAR 321, 328; readObject( ), 339; redirecting standard utility, 337; new array initialization syntax, 248; IO, 332; renameTo( ), 313; reset( ), 308; seek( ), new AWT, 450; reflection, 371, 642, 688; String 307, 319; SequenceInputStream, 303, 328; behavior, 257 Serializable, 343; setErr(PrintStream), 332; Java 1.2, 25, 69, 154, 255, 258, 270, 311, 312, setIn(InputStream), 332; setOut(PrintStream), 413, 414, 513, 541, 544, 547, 549, 562, 597, 332; StreamTokenizer, 321, 328, 374, 687; 632, 698, 709 StringBuffer, 302; StringBufferInputStream, 302, Java Beans: see Beans, 501 317; StringReader, 327; StringWriter, 327; Java Foundation Classes (JFC/Swing), 413 System.err, 320; System.in, 320, 331; Java Generic Library (former name of the JGL), 272 System.out, 320; text file manipulation, 669; Java operators, 79 transient, 346; typical IO configurations, 314; Java Virtual Machine, 362 Unicode, 326; write( ), 302; writeBytes( ), 318; JDBC: createStatement( ), 618; database URL, 616; writeChars( ), 318; writeDouble( ), 318; DatabaseMetaData, 623; writeExternal( ), 343; writeObject( ), 339; Writer, DriverManager.getConnection( ), 617; 326, 327, 577; ZipEntry, 336; ZipInputStream, executeQuery( ), 618; flat-file database, 619; 334; ZipOutputStream, 333 getFloat( ), 618; getInt( ), 618; getString( ), 618; IP (Internet Protocol), 572 Java DataBase Connectivity, 616; join, 619; is-a, 241; relationship, inheritance, 182; relational database, 619; ResultSet, 618; SQL relationship, inheritance & upcasting, 183 stored procedures, 621; Statement, 618; isDaemon( ), 527 Structured Query Language (SQL), 616 isDataFlavorSupported( ), 500 JFC: Java Foundation Classes (JFC/Swing), 413 isFocusTraversable( ), 492 JGL: Generic Collection Library for Java, 272 isInstance, 368 JIT: Just-In Time compilers, 57 isInterface( ), 371 join, 619 is-like-a, 241 JVM (Java Virtual Machine), 362 ISP (Internet Service Provider), 417 KeyAdapter, 455 ItemEvent, 452, 454, 463 keyDown( ), 436 ItemListener, 452, 463 KeyEvent, 452, 454 itemStateChanged( ), 455 KeyListener, 452 iteration, during software development, 55 keyPressed( ), 455 iterator, 255, 632 keyReleased( ), 455 JAR, 511; archive tag, for HTML and JAR files, keyTyped( ), 455 459; file, 153, 441; files, 481; Java 1.1 JAR keyUp( ), 436 utility, 337; packaging applets to optimize label, 110 loading, 459 Label, 424 Java: and classpath, 80; and pointers, 379; and set- labeled break, 110 top boxes, 88; C++ in contrast with Java, 605; labeled continue, 110 capitalization style source-code checking tool, late binding, 193, 196 681; comparing C++ and Java, 701; connecting to a non-Java program, 590; crashing Java, 258; layout: combining layouts, 434; controlling public Java seminars, 18; speed, 713; versions, layout, 432; layout manager, 425, 432 25 lazy evaluation, 272 Java 1.0, 25, 69, 89, 134, 145, 179, 248, 258, left-shift operator (<<), 89 260, 268, 301, 302, 313, 316, 318, 320, 323, length( ), 679 327, 328, 366, 368, 413, 414, 420, 423, 432, length, for arrays, 246 435, 462, 466, 470, 479, 502, 513, 577, 585, less than (<), 85 589, 594, 597, 632, 687 less than or equal to (<=), 85 Java 1.1, 21, 25, 45, 49, 50, 61, 63, 69, 76, 89, library: design, 151; use, 152 134, 141, 145, 153, 180, 187, 213, 220, 227, lightweight persistence, 339 258, 259, 260, 295, 297, 301, 311, 313, 316, LineNumberInputStream, 306, 317 728 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 729. LineNumberReader, 328 mouseDrag( ), 437 list: boxes, 429; drop-down list, 428 mouseDragged( ), 455 List, 429; Java 1.1, 466 mouseEnter( ), 437 listener adapters, 456 mouseEntered( ), 455 listener classes, 479 MouseEvent, 452, 454 literal: class literal, 363, 366; double, 95; float, 95; mouseExit( ), 437 long, 95; values, 94 mouseExited( ), 455 loading: .class files, 154; initialization & class MouseListener, 452 loading, 190; loding a class, 191 MouseMotionAdapter, 456 local loopback IP address, 573 MouseMotionListener, 452 localhost, 573; and RMI, 626 mouseMove( ), 437 lock, for multithreading, 533 mouseMoved( ), 455 logarithms: natural logarithms, 95 mousePressed( ), 455 logical: AND, 94; operator and short-circuiting, 87; mouseReleased( ), 455 operators, 86; OR, 94 mouseUp( ), 437 long, literal value marker (L), 95 multicast, 510; event, and Java Beans, 537; lostFocus( ), 436 multicast events, 476 lvalue, 80 multidimensional arrays, 146 main( ), 173 multiple dispatching, 653 manifest file, for JAR files, 337, 511 multiple inheritance, in C++ and Java, 209 map, 261 multiple selection, AWT List, 432 mark( ), 308 multiplication, 82 Math.random( ), 263; values produced by, 116 multitasking, 515 mathematical operators, 82 multithreading, 515, 579; and Java Beans, 537; member initializers, 234 blocking, 540; deciding what methods to Menu, 442; Java 1.1, 467 synchronize, 540; drawbacks, 567; Runnable, menu shortcuts, 470 562; stop( ), 597; when to use it, 567 MenuBar, 442, 471 multi-tiered systems, 477 MenuComponent, 442 name, 417; clash, 152; collisions, 156; creating MenuItem, 442, 471; Java 1.1, 467 unique package names, 154; spaces, 152 meta-class, 362 name, HTML keyword, 526 method: adding more methods to a design, 167; Naming: bind( ), 625; rebind( ), 626; unbind( ), 626 aliasing during a method call, 380; aliasing narrowing conversion, 94, 126 during method calls, 82; behavior of native code for connecting to a non-Java program, polymorphic methods inside constructors, 237; 590 distinguishing overloaded methods, 123; final, natural logarithms, 95 197; final methods, 188; initialization of method network programming, 571; accept( ), 574; applet, variables, 135; inline method calls, 188; inner 589; CGI GET, 612; CGI POST, 612; CGI classes in methods & scopes, 216; lookup tool, programming in C++, 605; client, 573; 688; method call binding, 196; overloading, 121; Common-Gateway Interface (CGI), 589; passing a handle into a method, 380; connecting Java to CGI, 599; crash course in CGI polymorphic method call, 193; polymorphic programming, 599; datagram, 590, 594; method calls, 639; protected methods, 182; Datagram.receive( ), 598; DatagramPacket, 584, recursive, 259; recursive method calls, 645; 586, 597; datagrams, 583; DatagramSocket, returning an array, 249; static, 130; synchronized 584, 597; dedicated connection, 579; displaying a method and blocking, 541 Web page from within an applet, 604; DNS Method, 506; for reflection, 372 (Domain Name Service), 572; dotted quad, 572; MethodDescriptors, 506 firewall, 598; GET request, 601; getAddress( ), methodology, software development, 52 586; getDocumentBase( ), 603; Microsoft, 513; Visual Basic, 501 getInputStream( ), 575, 616; mistakes, and design, 167 getOutputStream( ), 575, 614; getPort( ), 586; mkdirs( ), 313, 680 HTML, 589, 599; HTML content-type, in CGI modulus, 82 programming, 612; identifying machines, 572; InetAddress, 597; intercepting results from a CGI monitor, for multithreading, 533 program, 604; Internet Protocol (IP), 572; MouseAdapter, 455 invoking a CGI program from Java, 600; Java mouseClicked( ), 455 DataBase Connectivity (JDBC), 616; local mouseDown( ), 436, 448 loopback IP address, 573; localhost, 573; Index 729
- 730. multithreading, 579; openConnection( ), 614; openConnection( ), 614 port, 574; POST using an applet, 614; openStream( ), URL, 604 QUERY_STRING, 612; receive( ), 586, 597; operator, 79; + and += overloading for String, reliable protocol, 583; server, 573; ServerSocket, 173; +, for String, 407; == and !=, 387; binary, 574; serving multiple clients, 579; Servlet Server, 89; bitwise, 88; casting, 94; comma, 93; comma 628; servlet servers, 598; operator, 108; common pitfalls, 93; indexing setAllowUserInteraction(false), 614; operator [ ], 142; logical, 86; logical operators setDoInput(true), 614; setDoOutput(true), 614; and short-circuiting, 87; ones-complement, 89; showDocument( ), 604, 616; Socket, 574, 578; operator overloading for String, 407; stream-based sockets, 583; testing programs overloading, 93; precedence, 80; precedence without a network, 573; Transmission Control mnemonic, 96; relational, 85; shift, 89; ternary, Protocol (TCP), 583; unreliable protocol, 583; 92; unary, 84, 89 URL, 603, 605; URL openStream( ), 604; operator overloading: in C++, 609 URLConnection, 614; URLEncoder.encode( ), 600; OR, 94; (||), 86 User Datagram Protocol (UDP), 583; Web order: of constructor calls with inheritance, 233; of application, 589 finalization of objects, 237; of initialization, 137, new operator, 130; and primitives, array, 144 190, 238 newInstance( ): reflection, 370 organization, code, 159 newInstance( ), reflection, 644 output: data file output shorthand, 320; formatted nextElement( ), Enumeration, 256 file output shorthand, 319 nextToken( ), 687 OutputStream, 302, 303, 577, 590 no-arg: constructors, 123 OutputStreamWriter, 326, 327, 577 non-Java: connecting to a non-Java program, 590 overflow: and primitive types, 104; stack overflow, North, 433 259 not equivalent (!=), 85 overloading: and constructors, 121; distinguishing notify( ), 541 overloaded methods, 123; lack of name hiding notifyAll( ), 541 during inheritance, 180; method overloading, notifyListeners( ), 540 121; on return values, 127; operator + and += notifyObservers( ), 634, 636 overloading for String, 173; operator null, 248 overloading, 93; operator overloading for String, NullPointerException, 284 407; overloading vs. overriding, 180; vs. numbers, binary, 95 overriding, 202 object, 30; aliasing, 81; arrays are first-class objects, overriding: and inner classes, 225; overloading vs. 246; assigning objects by copying handles, 80; overriding, 180; vs. overloading, 202 assignment, 80; business object/logic, 477; Class package, 152, 638; access, and friendly, 159; and object, 352, 362, 533; creation, 120; equals( ) applets, 418; and directory structure, 158; method, 85; equivalence, 85; equivalence vs creating unique package names, 154; names, handle equivalence, 85; final, 185; handle capitalization, 69; naming convention, 154; equivalence vs. object equivalence, 387; statement and book chapter subdirectories, 80; immutable objects, 402; lock, for unnamed package, 160; visibility, friendly, 214 multithreading, 533; object-oriented paint( ), 416, 437, 447, 492, 497 programming, 360; order of finalization of Panel, 434 objects, 237; process of creation, 140; param, HTML keyword, 526 serialization, 338; web of objects, 339, 384 parameter, applet, 417 Object, 246, 263, 639; clone( ), 384, 387; parameterized type, 255, 272, 655 getClass( ), 369 parseInt( ), 448 object-oriented: analysis & design, 52 pass: pass by value, 382; passing a handle into a object-oriented programming, 29 method, 380 ObjectOutputStream, 339 patterns, design patterns, 166, 631 ObjectSpace, 272 performance, 713; and final, 189 Observable, 634 persistence, 350; lightweight persistence, 339 Observer, 634 pipe, 302 observer design pattern, 634 piped stream, 546 Octal, 95 PipedInputStream, 303, 321 ODBC, 616 PipedOutputStream, 303, 304, 321 ones complement operator, 89 PipedReader, 327 OOP, 163; protocol, 206 PipedWriter, 327 730 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 731. planning, software development, 53 PushbackInputStream, 306 platform-independent file manipulation, 670 PushBackReader, 328 pointers, and Java, 379 put( ), Hashtable, 264 polymorphism, 193, 243, 360, 376, 639, 650, 667; QUERY_STRING, 612 and constructors, 232; behavior of polymorphic Quicksort, 268 methods inside constructors, 237 RAD (Rapid Application Development), 371 port, 574 radio button, 427 portability in C, C++ and Java, 96 random number generator, values produced by, POST, 599; CGI, 612; using an applet, 614 116 precedence: operator precedence mnemonic, 96 random( ), 263 primitive: , dealing with the immutability of RandomAccessFile, 307, 318, 328 primitive wrapper classes, 402; collections of read( ), 302, 594 primitives, 249; comparison, 85; data types, and readChar( ), 318 use with operators, 97; final, 185; final static readDouble( ), 318 primitives, 186; initialization of class data Reader, 326, 327, 546, 577 members, 135; wrappers, 264 readExternal( ), 343 print( ), 492, 497 reading from standard input, 320 PrintGraphics, 497 readLine( ), 297, 316, 318, 321, 328 printInfo( ), 371 readObject( ), 339; with Serializable, 348 printing, 498; text, 497 rebind( ), 626 PrintJob, 497 receive( ), 586, 597; Datagram, 598 println( ), 257 recursive, 259; method calls, 645 printStackTrace( ), 280, 281 redirecting standard IO, 332 PrintStream, 307, 318 referencing, forward referencing, 137 PrintWriter, 328, 577 reflection, 371, 504, 644, 688; and Beans, 501; priority: default priority for a Thread group, 556; difference between RTTI and reflection, 372; Java thread, 553 1.1 reflection, 642 private, 33, 151, 159, 161, 182, 533; and the final registry: remote object registry, 625 specifier, 188; constructor, 633; inner class, 522; relational: database, 619; operators, 85 inner classes, 229 reliable protocol, 583 problem space, 30, 183 Remote Method Invocation (RMI), 623 Process, 590 RemoteException, 628 process, and threading, 515 removeActionListener( ), 452, 508, 540 processEvent( ), 487 removeAdjustmentListener( ), 452 programmer, client, 33, 151 removeComponentListener( ), 452 programming: coding standards, 709; event-driven removeContainerListener( ), 452 programming, 420; object-oriented, 29, 360 removeFocusListener( ), 452 promotion: of primitive types, 104; type removeItemListener( ), 452 promotion, 96 removeKeyListener( ), 452 Properties, 267, 686 removeMouseListener( ), 452 property, 501; bound properties, 512; constrained removeMouseMotionListener( ), 452 properties, 512; custom property editor, 512; custom property sheet, 512; indexed property, removeTextListener( ), 452 512 removeWindowListener( ), 452 PropertyChangeEvent, 512 renameTo( ), 313 PropertyDescriptors, 506 repaint( ), 487 proportionally-spaced font, 425 replace( ), 680 ProptertyVetoException, 512 reporting errors in book, 26 protected, 151, 159, 162, 182; and friendly, 182; requestFocus( ), 492 use in clone( ), 384 requirements analysis, 53 protocol, 206; unreliable protocol, 583 reset( ), 308 prototype, 642; design pattern, 648 resources, 414 public, 33, 151, 159; and interface, 206; class, and ResultSet, 618 compilation units, 152 resume( ), 541, 543; and deadlocks, 549; pure inheritance, vs. extension, 240 deprecation in Java 1.2, 551 pure substitution, 241 resumption, termination vs. resumption, exception pushBack( ), 687 handling, 278 re-throwing an exception, 281 Index 731
- 732. return: constructor return value, 121; overloading Servlet Server, 628 on return value, 127; returning an array, 249 servlet servers, 598 reuse, 192; code reuse, 169; reusable code, 501 setActionCommand( ), 470 right-shift operator (>>), 89 setAlignment( ), 424 RMI: AlreadyBoundException, 626; bind( ), 625; setAllowUserInteraction(false), 614 CORBA, 628; localhost, 626; rebind( ), 626; setChanged( ), 636 Remote, 624; remote interface, 624; Remote setCheckboxGroup( ), 427 Method Invocation, 623; remote object registry, setContents( ), 500 625; RemoteException, 624, 628; rmic, 627; rmic setDaemon( ), 527 and classpath, 627; rmiregistry, 625; setDirectory( ), 450 RMISecurityManager, 625; Serializable setDoInput(true), 614 arguments, 627; skeleton, 627; stub, 627; setDoOutput(true), 614 TCP/IP, 626; unbind( ), 626; setEditable( ), 423 UnicastRemoteObject, 624 setErr(PrintStream), 332 rmic, 627 setFile( ), 450 rmiregistry, 625 setIn(InputStream), 332 RMISecurityManager, 625 setLayout( ), 432 RTTI: and cloning, 387; cast, 361; Class object, 362; ClassCastException, 364; Constructor, 372; setMenuBar( ), 442 difference between RTTI and reflection, 372; setOut(PrintStream), 332 downcast, 364; eliminating from your design, setPriority( ), 553 652; Field, 372; instanceof keyword, 364; setText( ), 424 isInstance, 368; meta-class, 362; Method, 372; shallow copy, 384, 388 misuse of RTTI, 639, 649, 664; reflection, 371; shape: example, 197; example, and run-time type run-time type identification (RTTI), 242; type-safe identification, 359 downcast, 364; using the Class object, 369 shift operators, 89 runFinalizersOnExit( ), 236, 316 shortcut, menu shortcuts, 470 Runnable, 562, 594; interface, 524; Thread, 540 show( ), 450 running programs and the classpath, 80 showDocument( ), 604, 616 Runtime, 590 showStatus( ), 421 run-time binding, 196 side effect, 79, 85, 127, 382 run-time binding, polymorphism, 193 signed two’s complement, 92 run-time type identification: (RTTI), 242; misuse, Simula-67, 163 376; shape example, 359; when to use it, 376 simulation, 692 RuntimeException, 246, 284 singleton, 632; design pattern, 166 rvalue, 80 size( ), Vector, 252 safety, and applet restrictions, 440 sizeof( ): lack of in Java, 96 scheduling, software development, 54 skeleton, RMI, 627 scope: inner class nesting within any arbitrary sleep( ), 517, 531, 541, 542 scope, 217; inner classes in methods & scopes, Smalltalk, 29, 31, 130 216 Socket, 574, 578 section, critical section and synchronized block, 536 sockets, stream-based, 583 seek( ), 307, 319 Software Development Conference, 18 selection, multiple selection in an AWT List, 432 software development, process, 52 seminars: public Java seminars, 18; training, sorting, 268 provided by Bruce Eckel, 25 source code copyright notice, 23 separation of interface and implementation, 33, 163 South, 433 SequenceInputStream, 303, 328 specialization, 182 Serializable, 338, 343, 346, 354, 510; readObject( ), specification: system specification, 53 348; writeObject( ), 348 specification, exception, 279 serialization: and object storage, 350; and transient, specifier: access specifiers, 33, 151, 159 346; controlling the process of serialization, 343; speed, and Java, 713 defaultReadObject( ), 349; defaultWriteObject( ), SQL: stored procedures, 621; Structured Query 349; RMI arguments, 627; to perform deep Language, 616 copying, 392; Versioning, 350 stack: and object creation, 713; overflow, 259 server, 573 Stack, 246, 251, 260, 273 ServerSocket, 574 standard input: Reading from standard input, 320 732 Thinking in Java Bruce Eckel - www.eckelobjects.com
- 733. Standard Template Library (STL) for C++, 272 system specification, 53 standards: coding standards, 709 System.err, 320 standards for coding, 24 System.in, 320, 331 start( ), 416, 418 System.out, 320 Statement, 618 System.out.println( ), 257 static, 206; and final, 185; and inner classes, 222; tag, applet, in HTML, 417 block, 140; clause, 363; construction clause, target, of an event, 421 140; data initialization, 138; final static TCP, Transmission Control Protocol, 583 primitives, 186; initialization, 191; inner classes, TCP/IP, and RMI, 626 222; keyword, 130; method, 130; synchronized template: C++ Standard Template Library (STL), static, 533 605; in C++, 255, 656 status line, Web browser, 421 termination vs. resumption, exception handling, STL: C++ Standard Template Library, 605; Standard 278 Template Library for C++, 272 ternary operator, 92 stop( ), 416, 418, 597; and deadlocks, 549; testing: unit testing, 173 deprecation in Java 1.2, 549 text: file manipulation, 669; processing, 669 stored procedures in SQL, 621 TextArea, 423, 499; Java 1.1, 462 stream-based sockets, 583 TextComponent, 422; Java 1.1, 461 StreamTokenizer, 321, 328, 374, 687 TextEvent, 452, 454 String, 669; automatic type conversion, 253; class TextField, 422; Java 1.1, 461 methods, 406; concatenation with operator +, TextListener, 452 93; immutability, 406; indexOf( ), 309, 373, textValueChanged( ), 455 597, 647, 679; length( ), 679; methods, 408; this keyword, 128 operator +, 253; Operator +, 93; operator + Thread, 515, 517; and Java Beans, 537; and and += overloading, 173; replace( ), 680; Runnable, 562; balancing threads for substring( ), 679; substring( ), 647; toString( ), performance, 566; blocked, 540; combined with 170, 252; trim( ), 603, 679; trim( ), 647 main class, 523; daemon threads, 527; dead, 540; StringBuffer, 302, 317; methods, 410 deadlock, 549; deciding what methods to StringBufferInputStream, 302, 317 synchronize, 540; destroy( ), 552; destroy( ), and StringReader, 327 deadlocks, 549; drawbacks, 567; getPriority( ), StringSelection, 500 553; interrupt( ), 549; IO and threads, blocking, StringTokenizer, 324 541; isDaemon( ), 527; new Thread, 540; StringWriter, 327 notify( ), 541; notifyAll( ), 541; order of Stroustrup, Bjarne (C++ inventor), 24 execution of threads, 519; priority, 553; properly structural design patterns, 634 suspending & resuming, 551; resume( ), 541, stub, RMI, 627 543; resume( ) , deprecation in Java 1.2, 551; style of creating classes, 163 resume( ), and deadlocks, 549; run( ), 518; subobject, 174, 181 Runnable, 540; Runnable interface, 524; substring( ), 647, 679 setDaemon( ), 527; setPriority( ), 553; sharing subtraction, 82 limited resources, 529; sleep( ), 531, 541, 542; super, 175; and finalize( ), 236; and inner classes, start( ), 518; states, 540; stop( ) , deprecation in 224 Java 1.2, 549; stop( ), and deadlocks, 549; stopping, 549; suspend( ), 541, 543; suspend( ) , super keyword, 174 deprecation in Java 1.2, 551; suspend( ), and super.action( ), 421 deadlocks, 549; synchronized method and super.clone( ), 384, 387, 398 blocking, 541; thread group, 556; thread group, super.handleEvent( ), 431 default priority, 556; threads and efficiency, 517; superclass, 174 wait( ), 541, 545; when they can be suspended, suspend( ), 541, 543; and deadlocks, 549; 532; when to use threads, 567; yield( ), 541 deprecation in Java 1.2, 551 throw keyword, 277 Swing, Java Foundation Classes (JFC), 413 Throwable, 282; base class for Exception, 280 switch keyword, 113 throwing an exception, 277 synchronized, 533; and inheritance, 540; and wait( ) token, 321 & notify( ), 545; deciding what methods to Toolkit, 498 synchronize, 540; efficiency, 536; method, and TooManyListenersException, 476, 510 blocking, 541; static, 533; synchronized block, toString( ), 170, 252, 257, 267 536 training seminars provided by Bruce Eckel, 25 system clipboard, 499 Transferable, 500 Index 733
- 734. transient, 346, 392; for both cloning and versioning, serialization, 350 serialization, 392 versions of Java, 25 translation unit, 152 visibility, package visibility, (friendly), 214 Transmission Control Protocol (TCP), 583 visitor pattern, 659 trim( ), 425, 603, 647, 679 visual: programming, 501; programming true, 86 environments, 414 try, 179, 292; try block in exceptions, 278 Visual Basic, Microsoft, 501 two’s complement, signed, 92 wait( ), 541, 545 type: finding exact type of a base handle, 361; Web: application, 589; displaying a Web page from parameterized type, 255, 655; primitive data within an applet, 604; placing an applet inside a types and use with operators, 97; type checking Web page, 417; safety, and applet restrictions, and arrays, 245; type safety in Java, 93; type- 440 safe downcast in run-time type identification, 364 Web browser: status line, 421 TYPE field, for primitive class literals, 363 web of objects, 339, 384 type-check coding, 639 West, 433 type-conscious Vector, 254 while, 106 UDP, User Datagram Protocol, 583 widening conversion, 94 UML, Unified Modeling Language, 54 window: Abstract Window Toolkit (see AWT), 413; unary: minus (-), 84; operator, 89; operators, 84; standalone windowed application, 445; Web plus (+), 84 browser status line, 421 unbind( ), 626 WINDOW_CLOSING, 471 unicast, 510; unicast events, 476 WINDOW_DESTROY, 445 UnicastRemoteObject, 624 windowActivated( ), 455 Unicode, 326 WindowAdapter, 456 unit testing, 173 windowClosed( ), 455 unnamed package, 160 windowClosing( ), 455 upcasting, 183, 193, 360, 639; and interface, 209; windowDeactivated( ), 455 inner classes and upcasting, 214 windowDeiconified( ), 455 update( ), 416, 487 windowed applications, 442 updates of the book, 25 WindowEvent, 452, 454, 471 URL, 603, 605; openStream( ), 604 windowIconified( ), 455 URLConnection, 614 WindowListener, 452 URLEncoder: encode( ), 600 windowOpened( ), 455 use case, 53 wrapper, dealing with the immutability of primitive User Datagram Protocol (UDP), 583 wrapper classes, 402 user interface: and threads, for responsiveness, 519; write( ), 302 responsive, with threading, 516 writeBytes( ), 318 value: preventing change at run-time, 185 writeChars( ), 318 value, HTML keyword, 526 writeDouble( ), 318 variable: defining a variable, 108; initialization of writeExternal( ), 343 method variables, 135; variable argument lists writeObject( ), 339; with Serializable, 348 (unknown quantity and type of arguments), 145 Writer, 326, 327, 546, 577 Vector, 246, 251, 255, 258, 260, 267, 273, 639, XOR, 89 680; addElement( ), 252, 255; and deep copying, yield( ), 541 390; combined with Hashtable, 680; ZipEntry, 336 elementAt( ), 252, 255; size( ), 252; type- ZipInputStream, 334 conscious Vector, 254; used with Hashtable, 686 ZipOutputStream, 333 vector of change, 55, 229, 632, 642, 667 734 Thinking in Java Bruce Eckel - www.eckelobjects.com