Beginning programming with java for dummies, 2nd edition (2005)
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This document provides an overview and table of contents for the book "Beginning Programming with Java For Dummies, 2nd Edition". The book introduces Java programming concepts in an accessible way for beginners. It covers downloading and installing the necessary Java software, writing and running simple Java programs, using variables and data types, control flow statements like if/else, methods, classes and objects. The book is organized into five parts that progressively build Java programming skills from the basics to more advanced topics.
![Chapter 3: Running Programs 43
12. Replace an existing line of code in your new Java program.
Type a line of code in JCreator’s Editor pane. Replace the line
// TODO: Add your code here
with the line
System.out.println(“Chocolate, royalties, sleep”);
Copy the new line of code exactly as you see it in Listing 3-1.
• Spell each word exactly the way I spell it in Listing 3-1.
• Capitalize each word exactly the way I do in Listing 3-1.
• Include all the punctuation symbols — the dots, the quotation
marks, the semicolon, everything.
Listing 3-1: A Program to Display the Things I Like
class MyFirstJavaClass {
/**
* Method main
*
*
* @param args
*
*/
public static void main(String[] args) {
System.out.println(“Chocolate, royalties, sleep”);
}
}
Java is case-sensitive, which means that system.out.printLn isn’t the
same as System.out.println. If you type system.out.printLn, you’re
program won’t work. Be sure to capitalize your code eXactLy as it is in
Listing 3-1.
13. From the menu bar, choose Build➪Compile Project.
If you typed everything correctly, you see the comforting Process com-
pleted message, with no error messages, at the bottom of JCreator’s
work area. The text appears in JCreator’s Build Output pane in the lower
portion of JCreator’s work area. (Refer to Figure 3-4.)
When you choose Build➪Compile Project, JCreator compiles whichever
project is currently active. Only one project at a time is active. So if your
workspace contains several projects, make sure that the project you
want to compile is currently the active project.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-61-320.jpg)
![48 Part II: Writing Your Own Java Programs
Behold! A program!
In Listing 4-1, you get a blast of Java code. Like all novice programmers, you’re
expected to gawk humbly at the code. But don’t be intimidated. When you get
the hang of it, programming is pretty easy. Yes, it’s fun too.
Listing 4-1: A Simple Java Program
/*
* A program to list the good things in life
* Author: Barry Burd, BeginProg2@BurdBrain.com
* February 13, 2005
*/
class ThingsILike {
public static void main(String args[]) {
System.out.println(“Chocolate, royalties, sleep”);
}
}
When I run the program in Listing 4-1, I get the result shown in Figure 4-1:
The computer displays the words Chocolate, royalties, sleep on the
screen. Now I admit that writing and running a Java program is a lot of work
just to get the words Chocolate, royalties, sleep to appear on some-
body’s computer screen, but every endeavor has to start somewhere.
Figure 4-1:
Running the
program in
Listing 4-1.
You can run the code in Listing 4-1 on your computer. Here’s how:
1. Follow the instructions in Chapter 2 for installing the special edition
of JCreator (the edition that you find on this book’s Web site).
2. Next follow the instructions in the first half of Chapter 3.
Those instructions tell you how to run the Chapter03_Example01 project,
which is in the Chapter03 workspace. To run the code in Listing 4-1, select
the Chapter04_Listing01 project in the Chapter04 workspace.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-66-320.jpg)
![Chapter 4: Exploring the Parts of a Program 51
class ThingsILike {
public static void main(String args[]) {
Each Java keyword has a specific meaning — a meaning that remains
unchanged from one program to another. For example, whenever I write a
Java program, the word public always signals a part of the program that’s
accessible to any other piece of code.
The Java programming language is case-sensitive. This means that if you
change a lowercase letter in a word to an uppercase letter, you change the
word’s meaning. Changing case can make the entire word go from being
meaningful to being meaningless. In Listing 4-1, you can’t replace public with
Public. If you do, the whole program stops working.
This chapter has little or no detail about the meanings of the keywords
class, public, static, and void. You can peek ahead at the material in
other chapters, but you can also get along by cheating. When you write a
program, just start with
class SomethingOrOther {
and then paste the text
public static void main(String args[]) {
into your code. In your first few programs, this strategy serves you well.
The Cheat Sheet in the front of this book has a complete list of Java
keywords.
Here’s one thing to remember about keywords: In Java, each keyword has an
official, predetermined meaning. The people at Sun Microsystems, who have
the final say on what constitutes a Java program, have created all of Java’s
keywords. You can’t make up your own meaning for any of the Java keywords.
For example, you can’t use the word public in a calculation:
//This is BAD, BAD CODE:
public = 6;
If you try to use a keyword this way, then the compiler displays an error
message and refuses to translate your source code. It works the same way
in English. Have a baby, and name it “Because.”
“Let’s have a special round of applause for tonight’s master of ceremonies
— Because O. Borel.”
You can do it, but the kid will never lead a normal life.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-69-320.jpg)
![52 Part II: Writing Your Own Java Programs
Identifiers that you or I can define
I like the name Suzanne, but if you don’t like traditional names, then make up
a brand new name. You’re having a new baby. Call her “Deneen” or “Chrisanta.”
Name him “Belton” or “Merk.”
A name is a word that identifies something, so I’ll stop calling these things
names and start calling them identifiers. In computer programming, an identi-
fier is a noun of some kind. An identifier refers to a value, a part of a program,
a certain kind structure, or any number of things.
Listing 4-1 has two identifiers that you or I can define on our own. They’re the
made-up words ThingsILike and args.
class ThingsILike {
public static void main(String args[]) {
Just as the names Suzanne and Chrisanta have no special meaning in English,
so the names ThingsILike and args have no special meaning in Java. In
Listing 4-1, I use ThingsILike for the name of my program, but I could also
have used a name like GooseGrease, Enzyme, or Kalamazoo. I have to put
(String someName[]) in my program, but I could use (String args[]),
(String commandLineArguments[]), or (String cheese[]).
Do as I say, not as I do. Make up sensible, informative names for the things in
your Java programs. Names like GooseGrease are cute, but they don’t help
you keep track of your program-writing strategy.
When I name my Java program, I can use ThinksILike or GooseGrease, but I
can’t use the word public. Words like class, public, static, and void are
keywords in Java.
The args in (String args[]) holds anything extra that you type when you
issue the command to run a Java program. For example, if you get the pro-
gram to run by typing java ThingsILike won too 3, then args stores the
extra values won, too, and 3. As a beginning programmer, you don’t need to
think about this feature of Java. Just paste (String args[]) into each of
your programs.
Identifiers with agreed upon meanings
Many people are named Suzanne, but only one country is named Canada.
That’s because there’s a standard, well-known meaning for the word “Canada.”
It’s the country with a red maple leaf on its flag. If you start your own country,](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-70-320.jpg)
![Chapter 4: Exploring the Parts of a Program 53
you should avoid naming it Canada, because naming it Canada would just
confuse everyone. (I know, a town in Kentucky is named Canada, but that
doesn’t count. Remember, you should ignore exceptions like this.)
Most programming languages have identifiers with agreed upon meanings.
In Java, almost all of these identifiers are defined in the Java API. Listing 4-1
has five such identifiers. They’re the words main, String, System, out, and
println:
public static void main(String args[]) {
System.out.println(“Chocolate, royalties, sleep”);
}
Here’s a quick rundown on the meaning of each of these names (more detailed
descriptions appear throughout this book):
main: The main starting point for execution in every Java program.
String: A bunch of text; a row of characters, one after another.
System: A canned program in the Java API. (This program accesses some
features of your computer that are outside the direct control of the Java
virtual machine.)
out: The place where a text-based program displays its text. (For a pro-
gram running in JCreator, the word out represents the General Output
pane. To read more about text-based programs, check the first several
paragraphs of Chapter 3.)
println: Display text on your computer screen.
Strictly speaking, the meanings of the identifiers in the Java API are not cast
in stone. Although you can make up your own meanings for the words like
System or println, this isn’t a good idea. If you did, you would confuse the
dickens out of other programmers, who are used to the standard API mean-
ings for these familiar identifier names.
Literals
A literal is a chunk of text that looks like whatever value it represents.
In Suzanne’s sentence (refer to Figure 4-2), “eh” is a literal, because “eh”
refers to the word “eh.”
Programming languages have literals too. For example, in Listing 4-1, the stuff
in quotes is a literal:
System.out.println(“Chocolate, royalties, sleep”);](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-71-320.jpg)
![54 Part II: Writing Your Own Java Programs
When you run the ThingsILike program, you see the words Chocolate,
royalties, sleep on the screen. In Listing 4-1, the text “Chocolate,
royalties, sleep” refers to these words, exactly as they appear on the
screen (minus the quotation marks).
Most of the numbers that you use in computer programs are literals. If you
put the statement
mySalary = 1000000.00;
in a computer program, then 1000000.00 is a literal. It stands for the number
1000000.00 (one million).
Punctuation
A typical computer program has lots of punctuation. For example, consider
the program in Listing 4-1:
class ThingsILike {
public static void main(String args[]) {
System.out.println(“Chocolate, royalties, sleep”);
}
}
Each bracket, each brace, each squiggle of any kind plays a role in making the
program meaningful.
In English, you write all the way across one line, and then you wrap your text
to the start of the next line. In programming, you seldom work this way. Instead,
the code’s punctuation guides the indenting of certain lines. The indentation
shows which parts of the program are subordinate to which other parts. It’s
as if, in English, you wrote Suzanne’s sentence like this:
Suzanne says “eh” because
,
as you know
,
she lives in Canada.
The diagrams in Figures 4-3 and 4-4 show you how parts of the ThingsILike
program are contained inside other parts. Notice how a pair of curly braces
acts like a box. To make the program’s structure be visible at a glance, you
indent all the stuff inside of each box.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-72-320.jpg)
![56 Part II: Writing Your Own Java Programs
Comments
A comment is text that’s outside the normal flow. In Figure 4-2, words “A com-
ment:” aren’t part of the Suzanne sentence. Instead, these words are about
the Suzanne sentence.
The same is true of comments in computer programs. The first five lines in
Listing 4-1 form one big comment. The computer doesn’t act on this comment.
There are no instructions for the computer to perform inside this comment.
Instead, the comment tells other programmers something about your code.
Comments are for your own benefit, too. Imagine that you set aside your
code for a while and work on something else. When you return later to work
on the code again, the comments help you remember what you were doing.
The Java programming language has three different kinds of comments:
Traditional comments: The comment in Listing 4-1 is a traditional
comment. The comment begins with /* and ends with */. Everything
between the opening /* and the closing */ is for human eyes only.
Nothing between /* and */ gets translated by the compiler.
The second, third, and fourth lines in Listing 4-1 have extra asterisks.
I call them “extra” because these asterisks aren’t required when you
create a comment. They just make the comment look pretty. I include
them in Listing 4-1 because, for some reason that I don’t entirely under-
stand, most Java programmers add these extra asterisks.
End-of-line comments: Here’s some code with end-of-line comments:
class ThingsILike { //One thing is miss-
ing
public static void main(String args[]) {
System.out.println(“Royalties, sleep”);
//Chocolate
}
}
An end-of-line comment starts with two slashes, and goes to the end of a
line of type.
You may hear programmers talk about “commenting out” certain parts
of their code. When you’re writing a program, and something’s not work-
ing correctly, it often helps to try removing some of the code. If nothing
else, you find out what happens when that suspicious code is removed.
Of course, you may not like what happens when the code is removed, so
you don’t want to delete the code completely. Instead, you turn your
ordinary Java statements into comments. For example, turn System.out.
println(“Sleep”); into /* System.out.println(“Sleep”); */.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-74-320.jpg)
![Chapter 5: Composing a Program 67
Listing 5-1: A Java Program
import java.util.Scanner;
class EchoLine {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
System.out.println(myScanner.nextLine());
}
}
Figure 5-1 shows a run of the EchoLine code (the code in Listing 5-1). The text
in the figure is a mixture of my own typing and the computer’s responses.
Figure 5-1:
What part of
the word
“don’t” do
you not
understand?
To illustrate the situation, I concocted Figure 5-2. In Figure 5-2, I added high-
light to the text that I type. (Everything that I type is highlighted. Everything
the computer displays on its own is in regular, un-highlighted style.)
Figure 5-2:
Whatever
you get
to type is
highlighted.
Here’s what happens when you run the code in Listing 5-1:
1. At first, the computer does nothing. You see a cursor on the left edge of
the General Output pane, as shown in Figure 5-3. The computer is wait-
ing for you to type something.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-85-320.jpg)
![Chapter 5: Composing a Program 73
Also, the table’s story has a surprise ending. To read a single character, you
don’t call nextSomething. Instead, you can call the bizarre findInLine
(“.”).charAt(0) combination of methods. (You’ll have to excuse the folks
who created the Scanner class. They approached the input problem from a
specialized point of view.)
A quick look at the Scanner
In this chapter, I advise you to ignore any mean- readingThingie (or any other name you
ings behind the lines import java.util. want to use) as long as you use the name
Scanner and Scanner myScanner, etc. consistently. So, if you want to be creative,
Just paste these two lines mindlessly in your you can write
code, and then move on. Scanner readingThingie = new
Of course, you may not want to take my advice. Scanner(System.in);
You may not like ignoring things in your code. If
you happen to be such a stubborn person, I have System.out.println(readingThing
ie.nextLine());
a few quick facts for you.
The revised Listing 5-1 (with reading
The word Scanner is defined in the
Thingie instead of myScanner) compiles
Java API.
and runs without a hitch.
A Scanner is something you can use for
The line import java.util.Scanner
getting input.
is an example of an import declaration.
This Scanner class is new in version 5.0 of
An optional import declaration allows you to
the Java API. If you use version Java 1.4.2,
abbreviate names in the rest of your program.
then you don’t have access to the Scanner
You can remove the import declaration from
class. (You get an error when you try to
Listing 5-1. But if you do, you must use
compile Listing 5-1.)
the Scanner class’s fully qualified name
The words System and in are defined in throughout your code. Here’s how:
the Java API.
class EchoLine {
Taken together, the words System.in
stand for the computer keyboard. public static void
main(String args[]) {
In later chapters you see things like new java.util.Scanner
Scanner(new File(“myData.txt”)). myScanner =
In those chapters, I replace System.in with new
the words new File(“myData.txt”) java.util.Scanner(System.in)
because I’m not getting input from the key- ;
board. Instead, I’m getting input from a file
on the computer’s hard drive.
System.out.println(myScanne
The word myScanner doesn’t come from r.nextLine());
the Java API. }
}
The word myScanner is a Barry Burd cre-
ation. Instead of myScanner, you can use](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-91-320.jpg)
![74 Part II: Writing Your Own Java Programs
To see some of the table’s methods in action, check other program listings in
this book. Chapters 6, 7, and 8 have some particularly nice examples.
Type two lines of code, and don’t look back
Buried innocently inside Listing 5-1 are two extra lines of code. These lines
help the computer read input from the keyboard. The two lines are
import java.util.Scanner;
Scanner myScanner = new Scanner(System.in);
Concerning these two lines, I have bad news and good news.
The bad news is, the reasoning behind these lines is difficult to under-
stand. That’s especially true here in Chapter 5, where I introduce Java’s
most fundamental concepts.
The good news is, you don’t have to understand the reasoning behind
these two lines. You can copy and paste these lines into any program
that gets input from the keyboard. You don’t have to change the lines in
any way. These lines work without any modifications in all kinds of Java
programs.
Just be sure to put these lines in the right places:
Make the import java.util.Scanner line be the first line in your
program.
Put the Scanner myScanner = new Scanner(System.in) line inside
your main method immediately after the public static void main
(String args[]) { line.
At some point in the future, you may have to be more careful about the posi-
tioning of these two lines. But for now, the rules I give will serve you well.
Expecting the Unexpected
Not long ago, I met an instructor with an interesting policy. He said, “Sometimes
when I’m lecturing, I compose a program from scratch on the computer. I do
it right in front of my students. If the program compiles and runs correctly on
the first try, I expect the students to give me a big round of applause.”](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-92-320.jpg)
![78 Part II: Writing Your Own Java Programs
A message like the one in Figure 5-8 makes your life much simpler. I don’t have
to explain the message, and you don’t have to puzzle over the message’s mean-
ing. Just take the message ‘;’ expected on its face value. The message says,
“I expect to see a semicolon at this point in your program.” A caret (a ^ thingy)
points to the place in the program where the computer expects to see a semi-
colon. The computer expects a semicolon after the System.out.println
(myScanner.nextLine()) statement and before the close curly brace.
So do what the message tells you to do. Go back to the editor and put a semi-
colon after the System.out.println(myScanner.nextLine()) statement.
That settles it.
Using too much punctuation
In junior high school, my English teacher said I should use a comma when-
ever I would normally pause for a breath. This advice doesn’t work well
during allergy season, when my sentences have more commas in them than
words. Even as a paid author, I have trouble deciding where the commas
should go, so I often add extra commas for good measure. This makes more
work for my editor, Christine, who has a recycle bin full of commas by the
desk in her office.
It’s the same way in a Java program. You can get carried away with punctua-
tion. Consider, for example, the main method header in Listing 5-1. This line
is a dangerous curve for novice programmers.
For information on the terms method header and method body, see Chapter 4.
Normally, you shouldn’t be ending a method header with a semicolon. But
people add semicolons anyway. (Maybe, in some subtle way, a method
header looks like it should end with a semicolon.)
//The following line is incorrect:
public static void main(String args[]); {
If you add this extraneous semicolon to the code in Listing 5-1, you get the
message shown in Figure 5-9.
Figure 5-9:
A not-so-
helpful error
message.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-96-320.jpg)
![82 Part II: Writing Your Own Java Programs
You come to a fork in the road. “The directions said nothing about a fork.
Which way should I go?” You veer right, but a minute later you’re forced onto
a highway. You see a sign that says, “Next Exit 24 Miles.” Now you’re really
lost, and the gas gauge points to “S.” (The “S” stands for “Stranded.”)
So here’s what happened: You made an honest mistake. You shouldn’t have
turned left at the yellow blinking light. That mistake alone wasn’t so terrible.
But that first mistake lead to more confusion, and eventually, your choices
made no sense at all. If you hadn’t turned at the blinking light, you’d never
have encountered that stinking fork in the road. Then, getting on the highway
was sheer catastrophe.
Is there a point to this story? Of course there is. A computer can get itself
into the same sort of mess. The computer notices an error in your program.
Then, metaphorically speaking, the computer takes a fork in the road — a
fork based on the original error — a fork for which none of the alternatives
lead to good results.
Here’s an example. You’re retyping the code in Listing 5-1, and you forget to
type a close parenthesis:
//The following line is incorrect:
public static void main(String args[] {
When you try to compile the code, you get the messages shown in Figure 5-11.
Figure 5-11:
Two error
messages.
The computer reports two errors — one error on line 5, and another error on
line 10. “Let’s see,” you say to yourself. “Line 10 is the last line of the main
method. The message says ‘;’ expected. Why would I want a semicolon at
the end of my main method?”
Well, you better take a step backward. In Figure 5-11, the first error message
says ‘)’ expected. That’s okay. But after that first message, the computer is
really confused. In the second message, the computer suggests that you end
the main method with a semicolon, as if that would fix anything. The computer
is trying to make the best of a bad situation but, at this point, you shouldn’t
believe a word that the computer says.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-100-320.jpg)
![Chapter 5: Composing a Program 85
In fact, that fixes it, because a change from Class to class gets your code
running again.
An interface is like a class. But unlike a class, an interface can’t stand on its
own. For instance, you can’t put your static void main method in a Java
interface. None of the programs in this book use interfaces, so don’t worry
about interfaces until you advance past the beginning programming stage.
Run-time error messages
Up to this point in the chapter, I describe errors that crop up when you com-
pile a program. Another category of errors hides until you run the program.
A case in point is the improper capitalization of the word main.
Assume that, in a moment of wild abandon, you incorrectly spell main with a
capital M:
//The following line is incorrect:
public static void Main(String args[]) {
When you compile the code, everything is hunky-dory. You see a friendly
Process completed message — nothing else.
But then you try to run your program. At this point, the bits hit the fan. The
catastrophe is illustrated in Figure 5-13.
Figure 5-13:
Whadaya
mean
“NoSuch
Method
Error”?
Sure, your program has something named Main, but does it have anything
named main? (Yes, I’ve heard of a famous poet named e. e. cummings, but
who the heck is E. E. Cummings?) The computer doesn’t presume that your
word Main means the same thing as the expected word main. You need to
change Main back to main. Then everything will be okay.
But in the meantime (or in the maintime), how does this improper capitaliza-
tion make it past the compiler? Why don’t you get any error messages when
you compile the program? And if a capital M doesn’t upset the compiler, why
does this capital M mess everything up at run time?
The answer goes back to the different kinds of words in the Java program-
ming language. As it says in Chapter 4, Java has identifiers and keywords.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-103-320.jpg)
![Chapter 5: Composing a Program 87
Spaces and indentation
Java isn’t fussy about the use of spaces and indentation. All you need to do is
keep your program well-organized and readable. Here’s an example:
import java.util.Scanner;
class EchoLine
{
public static void main( String args[] )
{
Scanner myScanner = new Scanner( System.in );
System.out.println
( myScanner.nextLine() );
}
}
How you choose to do things
A program is like a fingerprint. No two programs look very much alike. Say I
discuss a programming problem with a colleague. Then we go our separate
ways and write our own programs to solve the same problem. Sure, we’re
duplicating the effort. But will we create the exact same code? Absolutely
not. Everyone has his or her own style, and everyone’s style is unique.
I asked fellow Java programmer David Herst to write his own EchoLine pro-
gram without showing him my code from Listing 5-1. Here’s what he wrote:
0
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
public class EchoLine {
public static void main(String[] args)
throws IOException {
InputStreamReader isr =
new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
String input = br.readLine();
System.out.println(input);
}
}
Don’t worry about BufferedReader, InputStreamReader, or things like that.
Just notice that, like snowflakes, no two programs are written exactly alike,
even if they accomplish the same task. That’s nice. It means your code, how-
ever different, can be as good as the next person’s. That’s very encouraging.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-105-320.jpg)
![90 Part II: Writing Your Own Java Programs
Go ahead. Click the Order Now! link. Just see what happens. You get an order
form with two items on it. One item is labeled $5.95 (CD-ROM), and the
other item reads $25.00 (shipping and handling). What a rip-off! Thanks
to SnitSoft’s generosity, you can pay $30.95 for ten cents worth of software.
Behind the scenes of the SnitSoft Web page, a computer program does
some scoundrel’s arithmetic. The program looks something like the code
in Listing 6-1.
Listing 6-1: SnitSoft’s Grand Scam
class SnitSoft {
public static void main(String args[]) {
double amount;
amount = 5.95;
amount = amount + 25.00;
System.out.print(“We will bill $”);
System.out.print(amount);
System.out.println(“ to your credit card.”);
}
}
When I run the Listing 6-1 code on my own computer (not the SnitSoft com-
puter), I get the output shown in Figure 6-1.
Figure 6-1:
Running the
code from
Listing 6-1.
Using a variable
The code in Listing 6-1 makes use of a variable named amount. A variable is
a placeholder. You can stick a number like 5.95 into a variable. After you’ve
placed a number in the variable, you can change your mind and put a differ-
ent number, like 30.95, into the variable. (That’s what varies in a variable.)
Of course, when you put a new number in a variable, the old number is no
longer there. If you didn’t save the old number somewhere else, the old
number is gone.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-108-320.jpg)
![Chapter 6: Using the Building Blocks: Variables, Values, and Types 97
I know. I’ll make the code more versatile. I’ll input the amount from the key-
board. Listing 6-2 has the revised code, and Figure 6-7 shows a run of the
new code.
Listing 6-2: Getting a Double Value from the Keyboard
import java.util.Scanner;
class VersatileSnitSoft {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
double amount;
System.out.print(“What’s the price of a CD-ROM? “);
amount = myScanner.nextDouble();
amount = amount + 25.00;
System.out.print(“We will bill $”);
System.out.print(amount);
System.out.println(“ to your credit card.”);
}
}
Figure 6-7:
Getting the
value of
a double
variable.
Though these be methods, yet there is
madness in ’t
Notice the call to the nextDouble method in Listing 6-2. Back in Listing 5-1, I
use nextLine, but here in Listing 6-2, I use nextDouble.
In Java, each type of input requires its own special method. If you’re getting a
line of text, then nextLine works just fine. But if you’re reading stuff from the
keyboard, and you want that stuff to be interpreted as a number, you need a
method like nextDouble.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-115-320.jpg)
![100 Part II: Writing Your Own Java Programs
Moving variables from place to place
It helps to remember the difference between initializations and assignments.
For one thing, you can drag a declaration with its initialization outside of a
method.
//This is okay:
class SnitSoft {
static double amount = 5.95;
public static void main(String args[]) {
amount = amount + 25.00;
System.out.print(“We will bill $”);
System.out.print(amount);
System.out.println(“ to your credit card.”);
}
}
You can’t do the same thing with assignment statements. (See the following
code and Figure 6-8.)
//This does not compile:
class BadSnitSoftCode {
static double amount;
amount = 5.95; //Misplaced statement
public static void main(String args[]) {
amount = amount + 25.00;
System.out.print(“We will bill $”);
System.out.print(amount);
System.out.println(“ to your credit card.”);
}
}
Figure 6-8:
A failed
attempt to
compile
BadSnitSoft
Code.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-118-320.jpg)
![Chapter 6: Using the Building Blocks: Variables, Values, and Types 101
You can’t drag statements outside of methods. (Even though a variable decla-
ration ends with a semicolon, a variable declaration isn’t considered to be a
statement. Go figure!)
The advantage of putting a declaration outside of a method is illustrated in
Chapter 19. While you wait impatiently to reach that chapter, notice how I
added the word static to each declaration that I pulled out of the main
method. I had to do this because the main method’s header has the word
static in it. Not all methods are static. In fact, most methods aren’t static.
But, whenever you pull a declaration out of a static method, you have to
add the word static at the beginning of the declaration. All the mystery sur-
rounding the word static is resolved in Chapter 18.
Combining variable declarations
The code in Listing 6-1 has only one variable (as if variables are in short
supply). You can get the same effect with several variables.
class SnitSoftNew {
public static void main(String args[]) {
double cdPrice;
double shippingAndHandling;
double total;
cdPrice = 5.95;
shippingAndHandling = 25.00;
total = cdPrice + shippingAndHandling;
System.out.print(“We will bill $”);
System.out.print(total);
System.out.println(“ to your credit card.”);
}
}
This new code gives you the same output as the code in Listing 6-1. (Refer to
Figure 6-1.)
The new code has three declarations — one for each of the program’s three
variables. Because all three variables have the same type (the type double), I
can modify the code and declare all three variables in one fell swoop:
double cdPrice, shippingAndHandling, total;](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-119-320.jpg)
![104 Part II: Writing Your Own Java Programs
With 30 gumballs and four kids, there’s no way to divide the gumballs evenly.
Of course, if I get rid of a kid, then I can give ten gumballs to each kid. The
trouble is, gumballs are disposable; kids are not. So my only alternative is to
divvy up what gumballs I can and dispose of the rest. “Okay, think quick,” I
say to myself. “With 30 gumballs and four kids, how many gumballs can I
promise to each kid?”
I waste no time in programming my computer to figure out this problem for
me. When I’m finished, I have the code in Listing 7-1.
Listing 7-1: How to Keep Four Kids from Throwing Tantrums
class KeepingKidsQuiet {
public static void main(String args[]) {
int gumballs;
int kids;
int gumballsPerKid;
gumballs = 30;
kids = 4;
gumballsPerKid = gumballs / kids;
System.out.print(“Each kid gets “);
System.out.print(gumballsPerKid);
System.out.println(“ gumballs.”);
}
}
A run of the KeepingKidsQuiet program is shown in Figure 7-1. If each kid
gets seven gumballs, then the kids can’t complain that I’m playing favorites.
They’ll have to find something else to squabble about.
Figure 7-1:
Fair and
square.
At the core of the gumball problem, I’ve got whole numbers — numbers with
no digits beyond the decimal point. When I divide 30 by 4, I get 71⁄2, but I can’t
take the 1⁄2 seriously. No matter how hard I try, I can’t divide a gumball in half,
at least not without hearing “my half is bigger than his half.” This fact is](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-122-320.jpg)
![Chapter 7: Numbers and Types 105
reflected nicely in Java. In Listing 7-1, all three variables (gumballs, kids, and
gumballsPerKid) are of type int. An int value is a whole number. When you
divide one int value by another (as you do with the slash in Listing 7-1), you
get another int. When you divide 30 by 4, you get 7 — not 71⁄2. You see this in
Figure 7-1. Taken together, the statements
gumballsPerKid = gumballs/kids;
System.out.print(gumballsPerKid);
put the number 7 on the computer screen.
Reading whole numbers from the keyboard
What a life! Yesterday there were four kids in my living room, and I had 30 gum-
balls. Today there are six kids in my house, and I have 80 gumballs. How can I
cope with all this change? I know! I’ll write a program that reads the numbers
of gumballs and kids from the keyboard. The program is in Listing 7-2, and a
run of the program is shown in Figure 7-2.
Listing 7-2: A More Versatile Program for Kids and Gumballs
import java.util.Scanner;
class KeepingMoreKidsQuiet {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int gumballs;
int kids;
int gumballsPerKid;
System.out.print(“How many gumballs? How many kids? “);
gumballs = myScanner.nextInt();
kids = myScanner.nextInt();
gumballsPerKid = gumballs / kids;
System.out.print(“Each kid gets “);
System.out.print(gumballsPerKid);
System.out.println(“ gumballs.”);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-123-320.jpg)
![Chapter 7: Numbers and Types 109
The remainder operator turns out to be fairly useful. After all, a remainder is
the amount you have left over after you divide two numbers. What if you’re
making change for $1.38? After dividing 138 by 25, you have 13 cents left over,
as shown in Figure 7-5.
138 cents
138/25 is 5
Figure 7-5:
Hey, bud!
Got change
for 138 138%25 is 13
sticks?
The code in Listing 7-3 makes use of this remainder idea.
Listing 7-3: Making Change
import java.util.Scanner;
class MakeChange {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int quarters, dimes, nickels, cents;
int whatsLeft, total;
System.out.print(“How many cents do you have? “);
total = myScanner.nextInt();
quarters = total / 25;
whatsLeft = total % 25;
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-127-320.jpg)
![122 Part II: Writing Your Own Java Programs
Too bad I didn’t think of that a few months ago. Anyway, this chapter deals
with letters, truth, falsehood, and other such things.
Characters
In Chapters 6 and 7, you store numbers in all your variables. That’s fine, but
there’s more to life than numbers. For example, I wrote this book with a com-
puter, and this book contains thousands and thousands of non-numeric things
called characters.
The Java type that’s used to store characters is char. Listing 8-1 has a simple
program that uses the char type, and a run of the Listing 8-1 program is shown
in Figure 8-1.
Listing 8-1: Using the char Type
class LowerToUpper {
public static void main(String args[]) {
char smallLetter, bigLetter;
smallLetter = ‘b’;
bigLetter = Character.toUpperCase(smallLetter);
System.out.println(bigLetter);
}
}
Figure 8-1:
Converting
lower- to
uppercase.
In Listing 8-1, the first assignment statement stores the letter b in the
smallLetter variable. In that statement, notice how b is surrounded by
single quote marks. In a Java program, every char literal starts and ends
with a single quote mark.
When you surround a letter with quote marks, you tell the computer that
the letter isn’t a variable name. For example, in Listing 8-1, the incorrect
statement smallLetter = b would tell the computer to look for a variable
named b. Because there’s no variable named b, you’d get a cannot find
symbol message.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-140-320.jpg)
![124 Part II: Writing Your Own Java Programs
I tried it. I cooked up the experiment in Listing 8-2.
Listing 8-2: Investigating the Behavior of toUpperCase
class MyExperiment {
public static void main(String args[]) {
char smallLetter, bigLetter;
smallLetter = ‘R’;
bigLetter = Character.toUpperCase(smallLetter);
System.out.println(bigLetter);
smallLetter = ‘3’;
bigLetter = Character.toUpperCase(smallLetter);
System.out.println(bigLetter);
}
}
In my experiment, I didn’t mix chemicals and blow things up. Here’s what I
did instead:
I assigned ‘R’ to smallLetter.
The toUpperCase method took the uppercase R and gave me back another
uppercase R. (See Figure 8-3.) I got no error message. This told me what
the toUpperCase method does with a letter that’s already uppercase.
The method does nothing.
I assigned ‘3’ to smallLetter.
The toUpperCase method took the digit 3 and gave me back the same
digit 3. (See Figure 8-3.) I got no error message. This told me what the
toUpperCase method does with a character that’s not a letter. It does
nothing, zip, zilch, bupkis.
Figure 8-3:
Running
the code in
Listing 8-2.
I write about this experiment to make an important point. When you don’t
understand something about computer programming, it often helps to write a
test program. Make up an experiment and see how the computer responds.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-142-320.jpg)
![Chapter 8: Numbers? Who Needs Numbers? 127
The code in Listing 8-2 is simple and manageable. In such a small program,
simplicity and manageability don’t matter very much. But in a large program,
it helps to think carefully about the use of each variable.
When not to reuse a variable
The previous section discusses the reuse of variables to make a program
slick and easy to read. This section shows you the flip side. In this section,
the problem at hand forces you to create new variables.
Suppose you’re writing code to reverse the letters in a four-letter word. You
store each letter in its own separate variable. Listing 8-3 shows the code, and
Figure 8-5 shows the code in action.
Listing 8-3: Making a Word Go Backwards
import java.util.Scanner;
class ReverseWord {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
char c1, c2, c3, c4;
c1 = myScanner.findInLine(“.”).charAt(0);
c2 = myScanner.findInLine(“.”).charAt(0);
c3 = myScanner.findInLine(“.”).charAt(0);
c4 = myScanner.findInLine(“.”).charAt(0);
System.out.print(c4);
System.out.print(c3);
System.out.print(c2);
System.out.print(c1);
System.out.println();
}
}
Figure 8-5:
Stop those
pots!](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-145-320.jpg)
![Chapter 8: Numbers? Who Needs Numbers? 131
To represent a lone character in the text of a computer program, you surround
the character with single quote marks. But, when you type a character as part
of a program’s input, you don’t surround the character with quote marks.
Suppose that your program calls nextInt, and then findInLine(“.”).
charAt(0). If you type 80x on the keyboard, you get an error message. (The
message says InputMismatchException. The nextInt method expects you
to type a blank space after each int value.) Now what happens if, instead of
typing 80x, you type 80 x on the keyboard? Then the program gets 80 for the
int value, followed by a blank space for the character value. For the program
to get the x, the program has to call findInLine(“.”).charAt(0) one more
time. It seems wasteful, but it makes sense in the long run.
The boolean Type
I’m in big trouble. I have 140 gumballs, and 15 kids are running around and
screaming in my living room. They’re screaming because each kid wants 10
gumballs, and they’re running because that’s what kids do in a crowded living
room. I need a program that tells me if I can give 10 gumballs to each kid.
I need a variable of type boolean. A boolean variable stores one of two
values — true or false (true, I can give 10 gumballs to each kid; or false, I
can’t give 10 gumballs to each kid). Anyway, the kids are going berserk, so
I’ve written a short program and put it in Listing 8-4. The output of the pro-
gram is shown in Figure 8-9.
Listing 8-4: Using the boolean Type
class CanIKeepKidsQuiet {
public static void main(String args[]) {
int gumballs;
int kids;
int gumballsPerKid;
boolean eachKidGetsTen;
gumballs = 140;
kids = 15;
gumballsPerKid = gumballs / kids;
System.out.print(“True or false? “);
System.out.println(“Each kid gets 10 gumballs.”);
eachKidGetsTen = gumballsPerKid >= 10;
System.out.println(eachKidGetsTen);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-149-320.jpg)
![134 Part II: Writing Your Own Java Programs
You can compare other things (besides numbers and characters) with the ==
and != operators. But when you do, you have to be careful. For more infor-
mation, see Chapter 18.
Comparing numbers
Nothing is more humdrum than comparing numbers. “True or false? Five is
greater than or equal to ten.” False. Five is neither greater than nor equal to
ten. See what I mean? Bo-ring.
Comparing whole numbers is an open-and-shut case. But unfortunately, when
you compare decimal numbers, there’s a wrinkle. Take a program for convert-
ing from Celsius to Fahrenheit. Wait! Don’t take just any such program; take
the program in Listing 8-5.
Listing 8-5: It’s Warm and Cozy in Here
import java.util.Scanner;
class CelsiusToFahrenheit {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
double celsius, fahrenheit;
System.out.print(“Enter the Celsius temperature: “);
celsius = myScanner.nextDouble();
fahrenheit = 9.0 / 5.0 * celsius + 32.0;
System.out.print(“Room temperature? “);
System.out.println(fahrenheit == 69.8);
}
}
If you run the code in Listing 8-5 and input the number 21, the computer finds
the value of 9.0 / 5.0 * 21 + 32.0. Believe it or not, you want to check
the computer’s answer. (Who knows? Maybe the computer gets it wrong!)
You need to do some arithmetic, but please don’t reach for your calculator.
A calculator is just a small computer, and machines of that kind stick up for
one another. To check the computer’s work, you need to do the arithmetic by
hand. What? You say you’re math phobic? Well, don’t worry. I’ve done all the
math in Figure 8-11.
If you do the arithmetic by hand, then the value you get for 9.0 / 5.0 * 21
+ 32.0 is exactly 69.8. So run the code in Listing 8-5, and give celsius the
value 21. You should get true when you display the value of fahrenheit ==
69.8, right?](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-152-320.jpg)
![Chapter 9: Forks in the Road 147
A complete program
Listing 9-2 contains a complete program with a simple if statement. The list-
ing’s code behaves like an electronic oracle. Ask the program a yes or no
question, and the program answers you back. Of course, the answer to
your question is randomly generated. But who cares? It’s fun to ask anyway.
Listing 9-2: I Know Everything
import java.util.Scanner;
import java.util.Random;
class AnswerYesOrNo {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
Random myRandom = new Random();
int randomNumber;
System.out.print(“Type your question, my child: “);
myScanner.nextLine();
randomNumber = myRandom.nextInt(10) + 1;
if (randomNumber > 5) {
System.out.println(“Yes. Isn’t it obvious?”);
} else {
System.out.println(“No, and don’t ask again.”);
}
}
}
Figure 9-4 shows several runs of the program in Listing 9-2. The program’s
action has four parts:
Figure 9-4:
The all-
knowing
Java
program in
action.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-165-320.jpg)
![Chapter 9: Forks in the Road 149
Randomness makes me dizzy
When you call myRandom.nextInt(10) + 1, such thing as a single random number. After
you get a number from 1 to 10. As a test, I wrote all, how random is a number like 9?
a program that calls the myRandom.nextInt
A number is random only when it’s one in a
(10) + 1 twenty times.
very disorderly collection of numbers. More
import java.util.Random; precisely, a number is random if the process
class TwentyNumbers { used to generate the number follows the two
public static void trends listed above. When they’re being
main(String args[]) { careful, scientists avoid the term random
Random myRandom = new
number, and use the term randomly gener-
Random();
ated number instead.
System.out.print(myRandom.n It’s hard to generate numbers randomly.
extInt(10) + 1); Computer programs do the best they can, but
System.out.print(“ “); ultimately, today’s computer programs follow
System.out.print(myRandom.n
a pattern, and that pattern isn’t truly random.
extInt(10) + 1); To generate numbers in a truly random fash-
System.out.print(“ “); ion, you need a big tub of ping-pong balls,
like the kind they use in state lottery draw-
System.out.print(myRandom.n
ings. The problem is, most computers don’t
extInt(10) + 1);
//...And so on. come with big tubs of ping-pong balls among
I ran the program several times, and got the their peripherals. So strictly speaking, the
results shown in the figure below. (Actually, I numbers generated by Java’s Random class
copied the results from JCreator’s General aren’t random. Instead, scientists call these
Output pane to Windows Notepad.) Stare briefly numbers pseudorandom.
at the figure and notice two trends: It surprises us all, but knowing one randomly
There’s no obvious way to predict what generated value is of no help in predicting
number comes next. the next randomly generated value.
No number occurs much more often than For example, if you toss a coin twice, and get
any of the others. heads each time, are you more likely to get
tails on the third flip? No. It’s still fifty-fifty.
If you have three sons, and you’re expect-
ing a fourth child, is the fourth child more
likely to be a girl? No. A child’s gender has
nothing to do with the genders of the older
children. (I’m ignoring any biological
The Java virtual machine jumps through hoops effects, which I know absolutely nothing
to maintain these trends. That’s because crank- about. Wait! I do know some biological
ing out numbers in a random fashion is a very trivia: A newborn child is more likely to be a
tricky business. Here are some interesting facts boy than a girl. For every 21 newborn boys,
about the process: there are only 20 newborn girls. Boys are
weaker, so we die off faster. That’s why
Scientists and non-scientists use the term nature makes more of us at birth.)
random number. But in reality, there’s no](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-167-320.jpg)
![Chapter 9: Forks in the Road 151
Anyway, whether you’re skinning catfish, skinning kitties, or writing computer
programs, the same principle holds true. You always have alternatives. Listing
9-2 shows you one way to write an if statement. The rest of this chapter (and
all of Chapter 10) show you some other ways to create if statements.
. . . Or else what?
You can create an if statement without an else clause. For example, imagine
a Web page on which one in ten randomly chosen visitors receives a special
offer. To keep visitors guessing, I call the Random class’s nextInt method,
and make the offer to anyone whose number is lucky 7.
If myRandom.nextInt(10) + 1 generates the number 7, display a special
offer message.
If myRandom.nextInt(10) + 1 generates any number other than 7, do
nothing. Don’t display a special offer message, and don’t display a dis-
couraging, “Sorry, no offer for you,” message.
The code to implement such a strategy is shown in Listing 9-3. A few runs of the
code are shown in Figure 9-5.
Listing 9-3: Aren’t You Lucky?
import java.util.Random;
class SpecialOffer {
public static void main(String args[]) {
Random myRandom = new Random();
int randomNumber = myRandom.nextInt(10) + 1;
if (randomNumber == 7) {
System.out.println(“An offer just for you!”);
}
System.out.println(randomNumber);
}
}
The if statement in Listing 9-3 has no else clause. This if statement has the
following form:
if (Condition) {
SomeStatements
}
When randomNumber is 7, the computer displays An offer just for you!
When randomNumber isn’t 7, the computer doesn’t display An offer just
for you! The action is illustrated in Figure 9-6.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-169-320.jpg)
![Chapter 9: Forks in the Road 153
Anyway, notice that the value of randomNumber is displayed in every run.
The println for randomNumber isn’t inside the if statement. (This println
comes after the if statement.) So the computer always executes this
println. Whether randomNumber == 7 is true or false, the computer takes
the appropriate if action, and then marches on to execute System.out.
println(randomNumber).
Packing more stuff into an if statement
Here’s an interesting situation: You have two baseball teams — the Hankees
and the Socks. You want to display the teams’ scores on two separate lines,
with the winner’s score coming first. (On the computer screen, the winner’s
score is displayed above the loser’s score. In case of a tie, you display the
two identical scores, one above the other.) Listing 9-4 has the code.
Listing 9-4: May the Best Team Be Displayed First
import java.util.Scanner;
import static java.lang.System.in;
import static java.lang.System.out;
class TwoTeams {
public static void main(String args[]) {
Scanner myScanner = new Scanner(in);
int hankees, socks;
out.print(“Hankees and Socks scores? “);
hankees = myScanner.nextInt();
socks = myScanner.nextInt();
out.println();
if (hankees > socks) {
out.print(“Hankees: “);
out.println(hankees);
out.print(“Socks: “);
out.println(socks);
} else {
out.print(“Socks: “);
out.println(socks);
out.print(“Hankees: “);
out.println(hankees);
}
}
}
Figure 9-7 has a few runs of the code. (To show a few runs in one figure, I
copied the results from JCreator’s General Output pane to Windows Notepad.)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-171-320.jpg)
![160 Part III: Controlling the Flow
Because the kids’ and seniors’ prices are the same, you can combine these
prices into one category. (That’s not always the best programming strategy,
but do it anyway for this example.) To find a particular moviegoer’s ticket
price, you need one or more if statements. There are many ways to struc-
ture the conditions, and I chose one of these ways for the code in Listing 10-1.
Listing 10-1: Are You Paying Too Much?
import java.util.Scanner;
class TicketPrice {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int age;
double price = 0.00;
System.out.print(“How old are you? “);
age = myScanner.nextInt();
if (age >= 12 && age < 65) {
price = 9.25;
}
if (age < 12 || age >= 65) {
price = 5.25;
}
System.out.print(“Please pay $”);
System.out.print(price);
System.out.print(“. “);
System.out.println(“Enjoy the show!”);
}
}
Several runs of the TicketPrice program (Listing 10-1) are shown in Figure
10-2. (For your viewing pleasure, I’ve copied the runs from JCreator’s General
Output pane to Windows Notepad.) When you turn 12, you start paying full
price. You keep paying full price until you become 65. At that point, you pay
the reduced price again.
The first if statement’s condition tests for the regular price group.
Anyone who’s at least 12 years of age and is under 65 belongs in this
group.
The second if statement’s condition tests for the fringe ages. A person
who’s under 12 or is 65 or older belongs in this category.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-178-320.jpg)
![Chapter 10: Which Way Did He Go? 163
Figure 10-4:
The choices
in Listing
10-1.
Listing 10-2: Do You Have a Coupon?
import java.util.Scanner;
class TicketPriceWithDiscount {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int age;
double price = 0.00;
char reply;
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-181-320.jpg)
![Chapter 10: Which Way Did He Go? 165
The second such statement has a hefty condition. The condition tests to
see if the reply variable stores any reasonable value at all. If the reply
isn’t Y, and isn’t y, and isn’t N, and isn’t n, then the computer expresses its
concern by displaying, “Huh?” (As a paying customer, the word “Huh?”
on the automated ticket teller’s screen will certainly get your attention.)
When you create a big multipart condition, you always have several ways to
think about the condition. For example, you can rewrite the last condition in
Listing 10-2 as if (!(reply == ‘Y’ || reply == ‘y’ || reply == ‘N’
|| reply == ‘n’)). “If it’s not the case that the reply is either Y, y, N, or n,
then display ‘Huh?’” So which way of writing the condition is better — the
way I do it in Listing 10-2, or the way I do it in this tip? It depends on your
taste. Whatever makes the logic easiest for you to understand is the best way.
Using boolean variables
No matter how good a program is, you can always make it a little bit better.
Take the code in Listing 10-2. Does the forest of if statements make you ner-
vous? Do you slow to a crawl when you read each condition? Wouldn’t it be
nice if you could glance at a condition and make sense of it very quickly?
To some extent, you can. If you’re willing to create some additional variables,
you can make your code easier to read. Listing 10-3 shows you how.
Listing 10-3: George Boole Would Be Proud
import java.util.Scanner;
class NicePrice {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int age;
double price = 0.00;
char reply;
boolean isKid, isSenior, hasCoupon, hasNoCoupon;
System.out.print(“How old are you? “);
age = myScanner.nextInt();
System.out.print(“Have a coupon? (Y/N) “);
reply = myScanner.findInLine(“.”).charAt(0);
isKid = age < 12;
isSenior = age >= 65;
hasCoupon = reply == ‘Y’ || reply == ‘y’;
hasNoCoupon = reply == ‘N’ || reply == ‘n’;
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-183-320.jpg)
![Chapter 10: Which Way Did He Go? 167
Listing 10-4: No Extra Break for Kids or Seniors
import java.util.Scanner;
class CheckAgeForDiscount {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int age;
double price = 0.00;
char reply;
System.out.print(“How old are you? “);
age = myScanner.nextInt();
System.out.print(“Have a coupon? (Y/N) “);
reply = myScanner.findInLine(“.”).charAt(0);
if (age >= 12 && age < 65) {
price = 9.25;
}
if (age < 12 || age >= 65) {
price = 5.25;
}
if ((reply == ‘Y’ || reply == ‘y’) &&
(age >= 12 && age < 65)) {
price -= 2.00;
}
System.out.print(“Please pay $”);
System.out.print(price);
System.out.print(“. “);
System.out.println(“Enjoy the show!”);
}
}
Figure 10-6:
Running
the code in
Listing 10-4.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-185-320.jpg)
![170 Part III: Controlling the Flow
Building a Nest
The year is 1968, and The Prisoner is on TV. In the last episode, the show’s
hero meets his nemesis “Number One.” At first Number One wears a spooky
happy-face/sad-face mask, and when the mask comes off, there’s a monkey
mask underneath. To find out what’s behind the monkey mask, you have to
watch the series on DVD. But in the meantime, notice the layering; a mask
within a mask. You can do the same kind of thing with if statements. This
section’s example shows you how.
But first, take a look at Listing 10-4. In that code, the condition age >= 12 &&
age < 65 is tested twice. Both times, the computer sends the numbers 12, 65,
and the age value through its jumble of circuits, and both times, the computer
gets the same answer. This is wasteful, but waste isn’t your only concern.
What if you decide to change the age limit for senior tickets? From now on, no
one under 100 gets a senior discount. You fish through the code and see the
first age >= 12 && age < 65 test. You change 65 to 100, pat yourself on the
back, and go home. The problem is, you’ve changed one of the two age >= 12
&& age < 65 tests, but you haven’t changed the other. Wouldn’t it be better to
keep all the age >= 12 && age < 65 testing in just one place?
Listing 10-5 comes to the rescue. In Listing 10-5, I smoosh all my if statements
together into one big glob. The code is dense, but it gets the job done nicely.
Listing 10-5: Nested if Statements
import java.util.Scanner;
class AnotherAgeCheck {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int age;
double price = 0.00;
char reply;
System.out.print(“How old are you? “);
age = myScanner.nextInt();
System.out.print(“Have a coupon? (Y/N) “);
reply = myScanner.findInLine(“.”).charAt(0);
if (age >= 12 && age < 65) {
price = 9.25;
if (reply == ‘Y’ || reply == ‘y’) {
price -= 2.00;
}
} else {
price = 5.25;
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-188-320.jpg)
![Chapter 10: Which Way Did He Go? 173
Do you give up? The answer is, there’s no right answer. What happens
depends on the way you write the program. Take a look back at Listing 9-4 in
Chapter 9. When the scores are equal, the condition hankees > socks is
false. So the program’s flow of execution drops down to the else clause.
That clause displays the Socks score first and the Hankees score second.
(Refer to Figure 9-7.)
The program doesn’t have to work this way. If I take Listing 9-4 and change
hankees > socks to hankees >= socks then, in case of a tie, the Hankees
score comes first.
Suppose you want a bit more control. When the scores are equal, you want
an It’s a tie message. To do this, think in terms of a three-pronged fork.
You have a prong for a Hankees win, another prong for a Socks win, and a
third prong for a tie. You can write this code in several different ways, but
one way that makes lots of sense is in Listing 10-6. For three runs of the code
in Listing 10-6, see Figure 10-12.
Listing 10-6: In Case of a Tie . . .
import java.util.Scanner;
import static java.lang.System.out;
class WinLoseOrTie {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int hankees, socks;
out.print(“Hankees and Socks scores? “);
hankees = myScanner.nextInt();
socks = myScanner.nextInt();
out.println();
if (hankees > socks) {
out.println(“Hankees win...”);
out.print(“Hankees: “);
out.println(hankees);
out.print(“Socks: “);
out.println(socks);
} else if (socks > hankees) {
out.println(“Socks win...”);
out.print(“Socks: “);
out.println(socks);
out.print(“Hankees: “);
out.println(hankees);
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-191-320.jpg)
![Chapter 10: Which Way Did He Go? 177
Listing 10-8: Proud Winners and Sore Losers
import java.util.Scanner;
import static java.lang.System.out;
class Scoreboard {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int hankees, socks;
WhoWins who;
out.print(“Hankees and Socks scores? “);
hankees = myScanner.nextInt();
socks = myScanner.nextInt();
out.println();
if (hankees > socks) {
who = WhoWins.home;
out.println(“The Hankees win :-)”);
} else if (socks > hankees) {
who = WhoWins.visitor;
out.println(“The Socks win :-(“);
} else {
who = WhoWins.neither;
out.println(“It’s a tie :-|”);
}
out.println();
out.println(“Today’s game is brought to you by”);
out.println(“SnitSoft, the number one software”);
out.println(“vendor in the Hankeeville area.”);
out.println(“SnitSoft is featured proudly in”);
out.println(“Chapter 6. And remember, four out”);
out.println(“of five doctors recommend SnitSoft”);
out.println(“to their patients.”);
out.println();
if (who == WhoWins.home) {
out.println(“We beat ‘em good. Didn’t we?”);
}
if (who == WhoWins.visitor) {
out.println(“The umpire made an unfair call.”);
}
if (who == WhoWins.neither) {
out.println(“The game goes into overtime.”);
}
}
}
Three runs of the program in Listing 10-8 are pictured in Figure 10-13.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-195-320.jpg)
![182 Part III: Controlling the Flow
if (randomNumber == 4)
.
.
.
if (randomNumber < 1 || randomNumber > 10) {
System.out.print(“Sorry, the electronic oracle”);
System.out.println(“ is closed for repairs.”);
}
But that approach seems wasteful. Why not create a statement that checks
the value of randomNumber just once and then takes an action based on the
value that it finds? Fortunately, just such a statement exists: the switch state-
ment. Listing 11-1 has an example of a switch statement.
Listing 11-1: An Answer for Every Occasion
import java.util.Scanner;
import java.util.Random;
import static java.lang.System.out;
class TheOldSwitcheroo {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
Random myRandom = new Random();
int randomNumber;
out.print(“Type your question, my child: “);
myScanner.nextLine();
randomNumber = myRandom.nextInt(10) + 1;
switch (randomNumber) {
case 1:
out.println(“Yes. Isn’t it obvious?”);
break;
case 2:
out.println(“No, and don’t ask again.”);
break;
case 3:
out.print(“Yessir, yessir!”);
out.println(“ Three bags full.”);
break;
case 4:
out.print(“What part of ‘no’”);
out.println(“ don’t you understand?”);
break;](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-200-320.jpg)
![188 Part III: Controlling the Flow
You can’t use a condition as a case value either:
//You can’t do this:
switch (age) {
case age <= 12: //...etc.
To break or not to break
In every Java programmer’s life, a time comes when he or she forgets to use
break statements. At first, the resulting output is confusing, but then the pro-
grammer remembers fall-through. The term fall-through describes what hap-
pens when you end a case without a break statement. What happens is that
execution of the code falls right through to the next case in line. Execution
keeps falling through until you eventually reach a break statement or the end
of the entire switch statement.
If you don’t believe me, just look at Listing 11-2. This listing’s code has a
switch statement gone bad:
Listing 11-2: Please, Gimme a Break!
/*
* This isn’t good code. The programmer forgot some
* of the break statements.
*/
import java.util.Scanner;
import java.util.Random;
import static java.lang.System.out;
class BadBreaks {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
Random myRandom = new Random();
int randomNumber;
out.print(“Type your question, my child: “);
myScanner.nextLine();
randomNumber = myRandom.nextInt(10) + 1;
switch (randomNumber) {
case 1:
out.println(“Yes. Isn’t it obvious?”);
case 2:
out.println(“No, and don’t ask again.”);](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-206-320.jpg)
![190 Part III: Controlling the Flow
Figure 11-3:
Please
make up
your mind.
Using Fall-through to Your Advantage
Often, when you’re using a switch statement, you don’t want fall-through, so
you pepper break statements throughout the switch. But, sometimes, fall-
through is just the thing you need.
Take the number of days in a month. Is there a simple rule for this? Months
containing the letter “r” have 31 days? Months in which “i” comes before “e”
except after “c” have 30 days?
You can fiddle with if conditions all you want. But to handle all the possibili-
ties, I prefer a switch statement. Listing 11-3 demonstrates the idea.
Listing 11-3: Finding the Number of Days in a Month
import java.util.Scanner;
class DaysInEachMonth {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int month, numberOfDays = 0;
boolean isLeapYear;
System.out.print(“Which month? “);
month = myScanner.nextInt();
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
numberOfDays = 31;
break;
case 4:
case 6:](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-208-320.jpg)
![196 Part III: Controlling the Flow
As luck would have it, you can do something that’s even better. You can write
a statement once, and tell the computer to execute the statement many
times. How many times? You can tell the computer to execute a statement as
many times as it needs to be executed.
That’s the big idea. The rest of this chapter has the details.
Repeating Instructions Over and Over
Again (Java while Statements)
Here’s a simple dice game: Keep rolling two dice until you roll 7 or 11. Listing
12-1 has a program that simulates the action in the game, and Figure 12-1
shows two runs of the program.
Listing 12-1: Roll 7 or 11
import java.util.Random;
import static java.lang.System.out;
class SimpleDiceGame {
public static void main(String args[]) {
Random myRandom = new Random();
int die1 = 0, die2 = 0;
while (die1 + die2 != 7 && die1 + die2 != 11) {
die1 = myRandom.nextInt(6) + 1;
die2 = myRandom.nextInt(6) + 1;
out.print(die1);
out.print(“ “);
out.println(die2);
}
out.print(“Rolled “);
out.println(die1 + die2);
}
}
At the core of Listing 12-1 is a thing called a while statement (also known as a
while loop). A while statement has the following form:
while (Condition) {
Statements
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-214-320.jpg)
![206 Part III: Controlling the Flow
Escapism
You can use a neat trick to make a program’s System.out.print(“t”);
output line up correctly. In Figure 12-5, the num- System.out.println(total);
bers 8 8, then 6 14 (and so on) are displayed. I In the first statement, the computer displays
want these numbers to be right under the head- Card, then jumps to the next tab stop on the
ing words Card and Total. Normally, you can screen, and then displays Total. In the next
get perfect vertical columns by pressing the tab three statements, the computer displays a card
key, but a computer program creates the output in number (like the number 6), then jumps to the next
Figure 12-5. How can you get a computer program tab stop (directly under the word Total), and
to press the tab key? then displays a total value (like the number 14).
In Java, there’s a way. You can put t inside The t combination of characters is an example
double quote marks. of an escape sequence. Another of Java’s escape
sequences, the combination n, moves the cursor
System.out.println(“CardtTotal
to a new line. In other words, System.out.
”);
print(“Hellon”) does the same thing as
System.out.print(card); System.out.println(“Hello”).
Listing 12-2: A Simplified Version of the Game Twenty-One
import java.util.Random;
class PlayTwentyOne {
public static void main(String args[]) {
Random myRandom = new Random();
int card, total = 0;
System.out.println(“Card Total”);
while (total < 21) {
card = myRandom.nextInt(10) + 1;
total += card;
System.out.print(card);
System.out.print(“ “);
System.out.println(total);
}
if (total == 21) {
System.out.println(“You win :-)”);
} else {
System.out.println(“You lose :-(“);
}
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-224-320.jpg)
![Chapter 12: Around and Around It Goes 207
If you’ve read this whole section, then you’re probably exhausted. Creating a
loop can be a lot of work. Fortunately, the more you practice, the easier it
becomes.
Thinking About Loops (Priming)
I remember when I was a young boy. We lived on Front Street in Philadelphia,
near where the El train turned onto Kensington Avenue. Come early morning,
I’d have to go outside and get water from the well. I’d pump several times
before any water would come out. Ma and Pa called it “priming the pump.”
These days I don’t prime pumps. I prime while loops. Consider the case of a
busy network administrator. She needs a program that extracts a username
from an e-mail address. For example, the program reads
John@BurdBrain.com
and writes
John
How does the program do it? Like other examples in the chapter, this prob-
lem involves repetition:
Repeatedly do the following:
Read a character.
Write the character.
The program then stops the repetition when it finds the @ sign. I take a stab
at writing this program. My first attempt doesn’t work, but it’s a darn good
start. It’s in Listing 12-3.
Listing 12-3: Trying to Get a Username from an E-mail Address
/*
* This code does NOT work, but I’m not discouraged.
*/
import java.util.Scanner;
class FirstAttempt {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
char symbol = ‘ ‘;
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-225-320.jpg)
![212 Part III: Controlling the Flow
Listing 12-4 (continued)
class SecondAttempt {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
char symbol = ‘ ‘;
while (symbol != ‘@’) {
System.out.print(symbol);
symbol = myScanner.findInLine(“.”).charAt(0);
}
System.out.println();
}
}
A run of the Listing 12-4 code is shown in Figure 12-12. The code is almost
correct, but I still have a slight problem. Notice the blank space before the
user’s input. The program races prematurely into the loop. The first time the
computer executes the statements
System.out.print(symbol);
symbol = myScanner.findInLine(“.”).charAt(0);
the computer displays an unwanted blank space. Then the computer gets the
J in John. In some applications, an extra blank space is no big deal. But in
other applications, extra output can be disastrous.
Figure 12-12:
The
computer
displays an
extra blank
space.
Fixing the problem
Disastrous or not, an unwanted blank space is the symptom of a logical flaw.
The program shouldn’t display results before it has any meaningful results to
display. The solution to this problem is called . . . (drumroll, please) . . . priming
the loop. You pump findInLine(“.”).charAt(0) once to get some values
flowing. Listing 12-5 shows you how to do it.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-230-320.jpg)
![Chapter 12: Around and Around It Goes 213
Listing 12-5: How to Prime a Loop
/*
* This code works correctly!
*/
import java.util.Scanner;
class GetUserName {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
char symbol;
symbol = myScanner.findInLine(“.”).charAt(0);
while (symbol != ‘@’) {
System.out.print(symbol);
symbol = myScanner.findInLine(“.”).charAt(0);
}
System.out.println();
}
}
Listing 12-5 follows the strategy shown in Figure 12-13. First you get a character
(the letter J in John, for example), then you enter the loop. After you’re in the
loop, you test the letter against the @ sign, and print the letter if it’s appropri-
ate to do so. Figure 12-14 shows a beautiful run of the GetUserName program.
Figure 12-13:
The strategy
in Listing
12-5.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-231-320.jpg)
![218 Part III: Controlling the Flow
Listing 13-1: Using the Keyboard and the Screen
import java.util.Scanner;
class ComputeTotal {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
double unitPrice, quantity, total;
unitPrice = myScanner.nextDouble();
quantity = myScanner.nextInt();
total = unitPrice * quantity;
System.out.println(total);
}
}
Figure 13-3:
Read
from the
keyboard;
write to the
screen.
The goal is to write a program like the one in Listing 13-1. But instead of talk-
ing to your keyboard and screen, this new program talks to your hard drive.
The new program reads unit price and quantity from your hard drive, and
writes the total back to your hard drive.
Java’s API has everything you need for interacting with a hard drive. A nice
example is in Listing 13-2.
Listing 13-2: Using Input and Output Files
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class ReadAndWrite {
public static void main(String args[])
throws FileNotFoundException {](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-235-320.jpg)
![Chapter 13: Piles of Files: Dealing with Information Overload 219
Scanner diskScanner =
new Scanner(new File(“rawData.txt”));
PrintStream diskWriter =
new PrintStream(“cookedData.txt”);
double unitPrice, quantity, total;
unitPrice = diskScanner.nextDouble();
quantity = diskScanner.nextInt();
total = unitPrice * quantity;
diskWriter.println(total);
}
}
Creating code that messes
with your hard drive
“I _____ (print your name)___ agree to pay $_____ each month on the ___th
day of the month.”
Fill in the blanks. That’s all you have to do. Reading input from a disk can
work the same way. Just fill in the blanks in Listing 13-3.
Listing 13-3: A Template to Read Data from a Disk File
/*
* Before you can compile this code,
* you must fill in the blanks.
*/
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
class _______________ {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“___________”));
________ = diskScanner.nextInt();
________ = diskScanner.nextDouble();
________ = diskScanner.nextLine();
________ = diskScanner.findInLine(“.”).charAt(0);
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-236-320.jpg)
![220 Part III: Controlling the Flow
Listing 13-3 (continued)
// Etc.
}
}
To use Listing 13-3, make up a name for your class. Insert that name into the
first blank space. Type the name of the input file in the second space (between
the quotation marks). Then, to read a whole number from the input file, call
diskScanner.nextInt. To read a number that has a decimal point, call
diskScanner.nextDouble. You can call any of the Scanner methods in Table
5-1 (the same methods you call when you get keystrokes from the keyboard).
The stuff in Listing 13-3 isn’t a complete program. Instead, it’s a code template —
a half-baked piece of code, with spaces for you to fill in.
With the template in Listing 13-3, you can input data from a disk file. With a sim-
ilar template, you can write output to a file. The template is in Listing 13-4.
Listing 13-4: A Template to Write Data to a Disk File
/*
* Before you can compile this code,
* you must fill in the blanks.
*/
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class _______________ {
public static void main(String args[])
throws FileNotFoundException {
PrintStream diskWriter =
new PrintStream(“___________”);
diskWriter.print(_____);
diskWriter.println(_____);
// Etc.
}
}
To use Listing 13-4, insert the name of your class into the first blank space.
Type the name of the output file in the space between the quotation marks.
Then, to write part of a line to the output file, call diskWriter.print. To
write the remainder of a line to the output file, call diskWriter.println.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-237-320.jpg)
![Chapter 13: Piles of Files: Dealing with Information Overload 225
There’s no quick, sure-fire way to fix this problem. But you should always
check the following things first:
Check again for a file named rawData.txt.
Open My Computer and poke around for a file with that name.
The filenames displayed in My Computer can be misleading. You may
see the name rawData even though the file’s real name is rawData.txt.
To fix this problem once and for all, read the sidebar “Those pesky file-
name extensions” in Chapter 3.
Check the spelling of the file’s name.
Make sure that the name in your program is exactly the same as the
name of the file on your hard drive. Just one misplaced letter can keep
the computer from finding a file.
If you use Unix or Linux, check the capitalization of the file’s name.
In Unix and Linux, the difference between uppercase and lowercase can
baffle the computer.
Check that the file is in the correct directory.
Sure, you have a file named rawData.txt. But, to find the file, don’t
expect the computer to search everywhere on your hard drive.
As a general rule, you should have rawData.txt and the Listing 13-2
code in the same directory on your hard drive. But file locations can be
tricky, especially if you work in an unfamiliar programming environment.
In some situations the general rule may not apply to you.
So here’s a trick you can use: Compile and run this stripped-down ver-
sion of the code in Listing 13-2:
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class JustWrite {
public static void main(String args[])
throws FileNotFoundException {
PrintStream diskWriter =
new PrintStream(“cookedData.txt”);
diskWriter.println(99.75);
}
}
This program has no need for a stinking rawData.txt file. If you run this
code and get no error messages, then search your hard drive for this pro-
gram’s output (the cookedData.txt file). Note the name of the directory
that contains the cookedData.txt file. When you put rawData.txt in
this same directory, then any problem you had running the Listing 13-2
code should go away.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-242-320.jpg)
![230 Part III: Controlling the Flow
Writing, Rewriting, and Re-rewriting
Given my mischievous ways, I tried a little experiment. I asked myself what
would happen if I ran the same file-writing program more than once. So I cre-
ated a tiny program (the program in Listing 13-5) and I ran the program twice.
Then I examined the program’s output file. The output file (shown in Figure
13-8) contains only two letters.
Listing 13-5: A Little Experiment
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class WriteOK {
public static void main(String args[])
throws FileNotFoundException {
PrintStream diskWriter =
new PrintStream(new File(“approval.txt”));
diskWriter.print (‘O’);
diskWriter.println(‘K’);
}
}
Figure 13-8:
Testing the
waters.
Here’s the sequence of events from the start to the end of the experiment:
1. Before I run the code in Listing 13-5, my computer’s hard drive has no
approval.txt file.
That’s okay. Every experiment has to start somewhere.
2. I run the code in Listing 13-5.
The call to new PrintStream in Listing 13-5 creates a file named
approval.txt. Initially, the new approval.txt file contains no charac-
ters. Later in Listing 13-5, calls to print and println put characters in
the file. So after running the code, the approval.txt file contains two
letters — the letters OK.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-247-320.jpg)
![Chapter 14: Creating Loops within Loops 235
So the company’s network administrator wants to distill the content of the
email.txt file. She wants a new file, usernames.txt, that contains user-
names with no host names, as shown in Figure 14-2.
Figure 14-2:
Usernames
extracted
from the list
of e-mail
addresses.
Reworking some existing code
To solve the administrator’s problem, you need to modify the code in Listing
12-5. The new version gets an e-mail address from a disk file and writes a
username to another disk file. The new version is in Listing 14-1.
Listing 14-1: From One File to Another
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class ListOneUsername {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“email.txt”));
PrintStream diskWriter =
new PrintStream(“usernames.txt”);
char symbol;
symbol = diskScanner.findInLine(“.”).charAt(0);
while (symbol != ‘@’) {
diskWriter.print(symbol);
symbol = diskScanner.findInLine(“.”).charAt(0);
}
diskWriter.println();
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-252-320.jpg)
![238 Part III: Controlling the Flow
The first realization of this strategy is in Listing 14-2.
Listing 14-2: The Mechanical Combining of Two Loops
/*
* This code does NOT work (but
* you learn from your mistakes).
*/
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class ListAllUsernames {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“email.txt”));
PrintStream diskWriter =
new PrintStream(“usernames.txt”);
char symbol;
while (diskScanner.hasNext()) {
symbol = diskScanner.findInLine(“.”).charAt(0);
while (symbol != ‘@’) {
diskWriter.print(symbol);
symbol =
diskScanner.findInLine(“.”).charAt(0);
}
diskWriter.println();
}
}
}
When you run the code in Listing 14-2, you get the disappointing response
shown in Figure 14-3.
Figure 14-3:
You goofed.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-255-320.jpg)
![Chapter 14: Creating Loops within Loops 243
Solving the problem
Listing 14-3 has the solution to the problem described with Figures 14-1 and
14-2. The code in this listing is almost identical to the code in Listing 14-2. The
only difference is the added call to nextLine. When the computer reaches an
@ sign, this nextLine call gobbles up the rest of the input line. (In other words,
the nextLine call gobbles up the rest of the e-mail address. The idea works
because each e-mail address is on its own separate line.) After chewing and
swallowing @BurdBrain.com, the computer moves gracefully to the next line
of input.
Listing 14-3: That’s Much Better!
/*
* This code is correct!!
*/
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
class ListAllUsernames {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“email.txt”));
PrintStream diskWriter =
new PrintStream(“usernames.txt”);
char symbol;
while (diskScanner.hasNext()) {
symbol = diskScanner.findInLine(“.”).charAt(0);
while (symbol != ‘@’) {
diskWriter.print(symbol);
symbol =
diskScanner.findInLine(“.”).charAt(0);
}
diskScanner.nextLine();
diskWriter.println();
}
}
}
To run the code in Listing 14-3, you need an email.txt file — a file like the one
shown in Figure 14-1. In the email.txt file, type several e-mail addresses.
Any addresses will do, as long as each address contains an @ sign and each](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-260-320.jpg)
![246 Part III: Controlling the Flow
Mom and Dad thought the whole Fletcher business was silly, but they respected
Aunt Edna and felt that people her age should be humored, not defied. As for
me, I thought I’d explode from the monotony. Each meal lasted forever. Each
mouthful was an ordeal. I can still remember my mantra — the words I’d say
to myself without meaning to do so:
I’ve chewed 0 times so far.
Have I chewed 100 times yet? If not, then
Chew!
Add 1 to the number of times that I’ve chewed.
Go back to “Have I chewed” to find out if I’m done yet.
Repeating Statements a Certain Number
Times (Java for Statements)
Life is filled with examples of counting loops. And computer programming
mirrors life (. . . or is it the other way around?). When you tell a computer
what to do, you’re often telling the computer to print three lines, process ten
accounts, dial a million phone numbers, or whatever. Because counting loops
are so common in programming, the people who create programming lan-
guages have developed statements just for loops of this kind. In Java, the
statement that repeats something a certain number of times is called a for
statement. An example of a for statement is in Listing 15-1.
Listing 15-1: Horace Fletcher’s Revenge
import static java.lang.System.out;
class AuntEdnaSettlesForTen {
public static void main(String args[]) {
for (int count = 0; count < 10; count++) {
out.print(“I’ve chewed “);
out.print(count);
out.println(“ time(s).”);
}
out.println(“10 times! Hooray!”);
out.println(“I can swallow!”);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-263-320.jpg)
![Chapter 15: The Old Runaround 251
To use a variable outside of a for statement, you have to declare that variable
outside the for statement. You can even do this with the for statement’s
counting variable. Listing 15-2 has an example.
Listing 15-2: Using a Variable Declared Outside of a for Loop
import static java.lang.System.out;
class AuntEdnaDoesItAgain {
public static void main(String args[]) {
int count;
for (count = 0; count < 10; count++) {
out.print(“I’ve chewed “);
out.print(count);
out.println(“ time(s).”);
}
out.print(count);
out.println(“ times! Hooray!”);
out.println(“I can swallow!”);
}
}
A run of the code in Listing 15-2 looks exactly like the run for Listing 15-1. The
run is pictured in Figure 15-1. Unlike its predecessor, Listing 15-2 enjoys the
luxury of using the count variable to display the number 10. It can do this
because, in Listing 15-2, the count variable belongs to the entire main method,
and not to the for loop alone.
Notice the words for (count = 0 in Listing 15-2. Because count is declared
above the for statement, you don’t declare count again in the for statement’s
initialization. I tried declaring count twice, as in the following code:
//This does NOT work:
int count;
for (int count = 0; count < 10; count++) {
...etc.
and the compiler told me to clean up my act:
count is already defined in main(java.lang.String[])
for (int count = 0; count < 10; count++) {
^](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-268-320.jpg)
![254 Part III: Controlling the Flow
That does it. That’s all you need. The code to create the display of Figure 15-5
is in Listing 15-3.
Listing 15-3: Hey! Is This a For-by-For?
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import static java.lang.System.out;
class DisplayHotelData {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“hotelData”));
for (int floor = 1; floor <= 9; floor++) {
out.print(“Floor “);
out.print(floor);
out.print(“: “);
for (int roomNum = 1; roomNum <= 20; roomNum++) {
out.print(diskScanner.nextInt());
out.print(‘ ‘);
}
out.println();
}
}
}
The code in Listing 15-3 has the variable floor going from 1 to 9, and has the
variable roomNum going from 1 to 20. Because the roomNum loop is inside the
floor loop, the writing of twenty numbers happens 9 times. That’s good. It’s
exactly what I want.
Repeating Until You Get What You
Need (Java do Statements)
I introduce Java’s while loop in Chapter 12. When you create a while loop,
you write the loop’s condition first. After the condition, you write the code
that gets repeatedly executed.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-270-320.jpg)
![256 Part III: Controlling the Flow
Pause a moment to think about the flow of actions — what should and shouldn’t
happen when the computer executes the loop. A loop of this kind doesn’t need
to check anything before getting the user’s first response. Indeed, before the
user gives the first response, the loop has nothing to check. The loop shouldn’t
start with “as long as the user’s response is invalid, get another response from
the user.” Instead, the loop should just leap ahead, get a response from the
user, and then check the response to see if it made sense. The code to do all
this is in Listing 15-4.
Listing 15-4: Repeat Before You Delete
/*
* DISCLAIMER: Neither the author nor Wiley Publishing, Inc.,
* nor anyone else even remotely connected with the
* creation of this book, assumes any responsibility
* for any damage of any kind due to the use of this code,
* or the use of any work derived from this code, including
* any work created partially or in full by the reader.
*
* Sign here:_______________________________
*/
import java.util.Scanner;
import java.io.IOException;
class IHopeYouKnowWhatYoureDoing {
public static void main(String args[])
throws IOException {
Scanner myScanner = new Scanner(System.in);
char reply;
do {
System.out.print(“Reply with Y or N...”);
System.out.print(“ Delete all .keep files? “);
reply = myScanner.findInLine(“.”).charAt(0);
} while (reply != ‘Y’ && reply != ‘N’);
if (reply == ‘Y’) {
Runtime.getRuntime().exec(“cmd /c del *.keep”);
}
}
}
The code in Listing 15-4 works on all the industrial-strength versions of
Microsoft Windows, including Windows NT, 2000, and XP. To get the same
effect in Windows 95, 98, or Me, you have to change the last line of code as
follows:
Runtime.getRuntime().exec(“start command /c del *.keep”);](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-272-320.jpg)
![Chapter 15: The Old Runaround 257
To work the same magic in Unix or Linux, you can use the following command:
Runtime.getRuntime().exec
(new String[] {“/bin/sh”, “-c”, “rm -f *.keep”});
One way or another, the call to Runtime.getRuntime...yada-yada deletes
all files whose names end with .keep.
In Listing 15-4, the call to Runtime.getRuntime().exec enables the Java
program to execute an operating system command. This Runtime business
can be tricky to use, so don’t fret over the details. Just take my word for it —
the call to Runtime.getRuntime().exec in Listing 15-4 deletes files.
The Runtime.getRuntime().exec method is one of those “you need a
throws clause” methods that I introduce in Chapter 13. Unlike the methods in
Chapter 13, the exec method forces you to throw an IOException. And with
a throws IOException clause comes an import java.io.IOException
declaration.
At this point you may be wondering how I know that the exec method needs
a throws IOException clause. How many other Java API methods require
throws clauses, and how do you find out about all these things? The answer
is, you can find this information in Java’s API documentation. For details, see
the Appendix on this book’s Web site.
Deleting files
A run of the Listing 15-4 program is shown in Figure 15-6. Before deleting a
bunch of files, the program asks the user if it’s okay to do the deletion. If the
user gives one of the two expected answers (Y or N) then the program pro-
ceeds according to the user’s wishes. But if the user enters any other letter
(or any digit, punctuation symbol, or whatever), then the program asks the
user for another response.
Figure 15-6:
No! Don’t
do it!
In Figure 15-6, the user hems and haws for a while, first with the letter U, then
the digit 8, and then with lowercase letters. Finally, the user enters Y, and the
program deletes the .keep files. If you compare the files on your hard drive
(before and after the run of the program), you’ll see that the program trashes
files with names ending in .keep.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-273-320.jpg)
![Chapter 15: The Old Runaround 261
That’s why my writing a Java program about slot machines is such a strange
occurrence. A typical slot machine has three reels, with each reel having
about twenty symbols. But to illustrate this section’s ideas, I don’t need
twenty symbols. Instead I use four symbols — a cherry, a lemon, a kumquat,
and a rutabaga.
Creating an enhanced for loop
When you play my simplified slot machine you can spin any one of over 60
combinations — cherry+cherry+kumquat, rutabaga+rutabaga+rutabaga, or
whatever. This chapter’s goal is to list all possible combinations. But first, I
show you another kind of loop. Listing 15-5 defines an enum type for a slot
machine’s symbols, and Listing 15-6 displays a list of the symbols.
Listing 15-5: Slot Machine Symbols
enum Symbol {cherry, lemon, kumquat, rutabaga}
Listing 15-6: Listing the Symbols
import static java.lang.System.out;
class ListSymbols {
public static void main(String args[]) {
for (Symbol leftReel : Symbol.values()) {
out.println(leftReel);
}
}
}
Listing 15-6 uses Java’s enhanced for loop. The word “enhanced” means
“enhanced compared with the loops in earlier versions of Java.” The enhanced
for loop is new to Java version 5.0. If you run Java version 1.4.2 (or something
like that) then you can’t use an enhanced for loop.
Here’s the format of the enhanced for loop:
for (TypeName variableName : RangeOfValues) {
Statements
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-277-320.jpg)
![Chapter 15: The Old Runaround 263
In general, a someEnumTypeName.values() expression stands for the set
of values that a particular enum type’s variable can have. For example, in
Listings 10-7 and 10-8 you may use the expression WhoWins.values() to
refer to the home, visitor, and neither values.
The difference between a type’s name (like Symbol) and the type’s values
(as in Symbol.values()) is really subtle. Fortunately, you don’t have to
worry about the difference. As a beginning programmer, you can just use
the .values() suffix in an enhanced loop’s RangeOfValues part.
Nesting the enhanced for loops
Listing 15-6 solves a simple problem in a very elegant way. So after reading
about Listing 15-6, you ask about more complicated problems. “Can I list all
possible three-reel combinations of the slot machine’s four symbols?” Yes,
you can. Listing 15-7 shows you how to do it.
Listing 15-7: Listing the Combinations
import static java.lang.System.out;
class ListCombinations {
public static void main(String args[]) {
for (Symbol leftReel : Symbol.values()) {
for (Symbol middleReel : Symbol.values()) {
for (Symbol rightReel : Symbol.values()) {
out.print(leftReel);
out.print(“ “);
out.print(middleReel);
out.print(“ “);
out.println(rightReel);
}
}
}
}
}
When you run the program in Listing 15-7, you get 64 lines of output. Some of
those lines are shown in Figure 15-9.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-279-320.jpg)
![Chapter 16
Using Loops and Arrays
In This Chapter
Using for loops to the max
Storing many values in a single variable
Working with groups of values
T his chapter has seven illustrations. For these illustrations, the people at
Wiley Publishing insist on following numbering: Figure 16-1, Figure 16-2,
Figure 16-3, Figure 16-4, Figure 16-5, Figure 16-6, and Figure 16-7. But I like a
different kind of numbering. I’d like to number the illustrations figure[0],
figure[1], figure[2], figure[3], figure[4], figure[5], and figure[6].
Read on in this chapter and you’ll find out why.
Some for Loops in Action
The Java Motel, with its ten comfortable rooms, sits in a quiet place off the
main highway. Aside from a small, separate office, the motel is just one long
row of ground floor rooms. Each room is easily accessible from the spacious
front parking lot.
Oddly enough, the motel’s rooms are numbered 0 through 9. I could say that
the numbering is a fluke — something to do with the builder’s original design
plan. But the truth is, starting with 0 makes the examples in this chapter easier
to write.
You, as the Java Motel’s manager, store occupancy data in a file on your
computer’s hard drive. The file has one entry for each room in the motel.
For example, in Figure 16-1, Room 0 has one guest, Room 1 has four guests,
Room 2 is empty, and so on.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-287-320.jpg)
![272 Part IV: Using Program Units
Figure 16-1:
Occupancy
data for the
Java Motel.
You want a report showing the number of guests in each room. Because you
know how many rooms you have, this problem begs for a for loop. The code
to solve this problem is in Listing 16-1, and a run of the code is shown in
Figure 16-2.
Listing 16-1: A Program to Generate an Occupancy Report
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import static java.lang.System.out;
class ShowOccupancy {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“occupancy”));
out.println(“RoomtGuests”);
for (int roomNum = 0; roomNum < 10; roomNum++) {
out.print(roomNum);
out.print(“t”);
out.println(diskScanner.nextInt());
}
}
}
Figure 16-2:
Running
the code in
Listing 16-1.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-288-320.jpg)
![274 Part IV: Using Program Units
Deciding on a loop’s limit at runtime
On occasion, you may want a more succinct report than the one in Figure 16-2.
“Don’t give me a long list of rooms,” you say. “Just give me the number of
guests in Room 3.” To get such a report, you need a slightly smarter program.
The program is in Listing 16-2, with runs of the program shown in Figure 16-3.
Listing 16-2: Report on One Room Only, Please
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import static java.lang.System.out;
public class ShowOneRoomOccupancy {
public static void main(String args[])
throws FileNotFoundException {
Scanner myScanner = new Scanner(System.in);
Scanner diskScanner =
new Scanner(new File(“occupancy”));
int whichRoom;
out.print(“Which room? “);
whichRoom = myScanner.nextInt();
for(int roomNum = 0; roomNum < whichRoom; roomNum++){
diskScanner.nextInt();
}
out.print(“Room “);
out.print(whichRoom);
out.print(“ has “);
out.print(diskScanner.nextInt());
out.println(“ guest(s).”);
}
}
If Listing 16-2 has a moral, it’s that the number of for loop iterations can vary
from one run to another. The loop in Listing 16-2 runs on and on as long as the
counting variable roomNum is less than a room number specified by the user.
When the roomNum is the same as the number specified by the user (that is,
when roomNum is the same as whichRoom), the computer jumps out of the
loop. Then the computer grabs one more int value from the occupancy file
and displays that value on the screen.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-290-320.jpg)
![Chapter 16: Using Loops and Arrays 275
Figure 16-3:
A few
one-room
reports.
As you stare at the runs in Figure 16-3, it’s important to remember the unusual
numbering of rooms. Room 3 has two guests because Room 3 is the fourth
room in the occupancy file of Figure 16-1. That’s because the motel’s rooms
are numbered 0 through 9.
Using all kinds of conditions in a for loop
Look at the run in Figure 16-3, and notice the program’s awful behavior when
the user mistakenly asks about a nonexistent room. The motel has no Room
10. If you ask for the number of guests in Room 10, the program tries to read
more numbers than the occupancy file contains. This unfortunate attempt
causes a NoSuchElementException.
Listing 16-3 fixes the end-of-file problem.
Listing 16-3: A More Refined Version of the One-Room Code
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import static java.lang.System.out;
public class BetterShowOneRoom {
public static void main(String args[])
throws FileNotFoundException {
Scanner myScanner = new Scanner(System.in);
Scanner diskScanner =
new Scanner(new File(“occupancy”));
int whichRoom;
(continued)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-291-320.jpg)
![Chapter 16: Using Loops and Arrays 279
What you’re lacking is a uniform way of naming ten variables. That is, it
would be nice to write
/*
* Nice idea, but this is not real Java code:
*/
for (int roomNum = 0; roomNum < 10; roomNum++) {
guestsInroomNum = diskScanner.nextInt(); //Read forwards
}
for (int roomNum = 9; roomNum >= 0; roomNum--) {
if (guestsInroomNum == 0) {
System.out.println(roomNum); //Write backwards
}
}
Well, there is a way to write loops of this kind. All you need is some square
brackets. When you add square brackets to the idea shown in the preceding
code, you get what’s called an array. An array is a row of values, like the row
of rooms in a one-floor motel. To picture the array, just picture the Java Motel:
First, picture the rooms, lined up next to one another.
Next, picture the same rooms with their front walls missing. Inside each
room you can see a certain number of guests.
If you can, forget that the two guests in Room 9 are putting piles of bills
into a big briefcase. Ignore the fact that the guest in Room 5 hasn’t moved
away from the TV set in a day and a half. Instead of all these details, just
see numbers. In each room, see a number representing the count of guests
in that room. (If freeform visualization isn’t your strong point, then take
a look at Figure 16-6.)
In the lingo of Java programming, the entire row of rooms is called an array.
Each room in the array is called a component of the array (also known as an
array element). Each component has two numbers associated with it:
Index: In the case of the Java Motel array, the index is the room number
(a number from 0 to 9).
Value: In the Java Motel array, the value is the number of guests in a
given room (a number stored in a component of the array).
Using an array saves you from having to declare ten separate variables:
guestsIn0, guestsIn1, guestsIn2, and so on. To declare an array with ten
values in it, you can write two fairly short lines of code:
int guestsIn[];
guestsIn = new int[10];](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-295-320.jpg)
![280 Part IV: Using Program Units
A component
whose index is 6,
and whose value is 4
The value 4
1 4 0 2 2 1 4 3 0 2
Figure 16-6:
An abstract 0 1 2 3 4 5 6 7 8 9
snapshot of
rooms in the
Java Motel. The index 6
You can even squish these two lines into one longer line:
int guestsIn[] = new int[10];
In either of these code snippets, notice the use of the number 10. This number
tells the computer to make the guestsIn array have ten components. Each
component of the array has a name of its own. The starting component is
named guestsIn[0], the next is named guestsIn[1], and so on. The last of
the ten components is named guestsIn[9].
In creating an array, you always specify the number of components. The array’s
indices always start with 0 and end with the number that’s one less than the
total number of components. For example, if your array has ten components
(and you declare the array with new int[10]), then the array’s indices go
from 0 to 9.
Storing values in an array
After you’ve created an array, you can put values into the array’s components.
For example, the guests in Room 6 are fed up with all those mint candies that
you put on peoples’ beds. So they check out and Room 6 becomes vacant.
You should put the value 0 into the 6 component. You can do it with this
assignment statement:](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-296-320.jpg)
![Chapter 16: Using Loops and Arrays 281
guestsIn[6] = 0;
On one weekday, business is awful. No one’s staying at the motel. But then
you get a lucky break. A big bus pulls up to the motel. The side of the bus has
a sign that says “Loners’ Convention.” Out of the bus come 25 people, each
walking to the motel’s small office, none paying attention to the others who
were on the bus. Each person wants a private room. Only 10 of them can stay
at the Java Motel, but that’s okay, because you can send the other 15 loners
down the road to the old C-Side Resort and Motor Lodge.
Anyway, to register 10 of the loners into the Java Motel, you put one guest in
each of your 10 rooms. Having created an array, you can take advantage of
the array’s indexing and write a for loop, like this:
for (int roomNum = 0; roomNum < 10; roomNum++) {
guestsIn[roomNum] = 1;
}
This loop takes the place of ten assignment statements, because the com-
puter executes the statement guestsIn[roomNum] = 1 ten times. The first
time around, the value of roomNum is 0, so in effect, the computer executes
guestsIn[0] = 1;
In the next loop iteration, the value of roomNum is 1, so the computer exe-
cutes the equivalent of the following statement:
guestsIn[1] = 1;
During the next iteration, the computer behaves as if it’s executing
guestsIn[2] = 1;
And so on. When roomNum gets to be 9, the computer executes the equivalent
of the following statement:
guestsIn[9] = 1;
Notice how the loop’s counter goes from 0 to 9. Compare this with Figure 16-6,
and remember that the indices of an array go from 0 to one less than the
number of components in the array. Looping with room numbers from 0 to 9
covers all the rooms in the Java Motel.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-297-320.jpg)
![282 Part IV: Using Program Units
When you work with an array, and you step through the array’s components
using a for loop, you normally start the loop’s counter variable at 0. To form
the condition that tests for another iteration, you often write an expression
like roomNum < arraySize, where arraySize is the number of components
in the array.
Creating a report
The code to create the report in Figure 16-5 is shown in Listing 16-4. This new
program uses the idea in the world’s ugliest code (the code from several pages
back with variables guestsIn0, guestsIn1, and so on). But instead of having
ten separate variables, Listing 16-4 uses an array.
Listing 16-4: Traveling through Data Both Forwards and Backwards
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
class VacanciesInReverse {
public static void main(String args[])
throws FileNotFoundException {
Scanner diskScanner =
new Scanner(new File(“occupancy”));
int guestsIn[];
guestsIn = new int[10];
for (int roomNum = 0; roomNum < 10; roomNum++) {
guestsIn[roomNum] = diskScanner.nextInt();
}
for (int roomNum = 9; roomNum >= 0; roomNum--) {
if (guestsIn[roomNum] == 0) {
System.out.print(“Room “);
System.out.print(roomNum);
System.out.println(“ is vacant.”);
}
}
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-298-320.jpg)
![Chapter 16: Using Loops and Arrays 283
Notice the stuff in parentheses in the VacanciesInReverse program’s second
for loop. It’s easy to get these things wrong. You’re aiming for a loop that
checks Room 9, then Room 8, and so on.
if (guestsIn[9] == 0) {
System.out.print(roomNum);
}
if (guestsIn[8] == 0) {
System.out.print(roomNum);
}
if (guestsIn[7] == 0) {
System.out.print(roomNum);
}
...And so on, until you get to...
if (guestsIn[0] == 0) {
System.out.print(roomNum);
}
Some observations about the code:
The loop’s counter must start at 9:
for (int roomNum = 9; roomNum >= 0; roomNum--)
Each time through the loop, the counter goes down by one:
for (int roomNum = 9; roomNum >= 0; roomNum--)
The loop keeps going as long as the counter is greater than or equal to 0:
for (int roomNum = 9; roomNum >= 0; roomNum--)
Think through each of these three items, and you’ll write a perfect for loop.
Working with Arrays
Earlier in this chapter, a busload of loners showed up at your motel. When they
finally left, you were glad to get rid them, even if it meant having all your rooms
empty for a while. But now, another bus pulls into the parking lot. This bus has
a sign that says “Gregarian Club.” Out of the bus come 50 people, each more
gregarious than the next. Now everybody in your parking lot is clamoring to
meet everyone else. While they meet and greet, they’re all frolicking toward
the front desk, singing the club’s theme song. (Oh no! It’s the Gregarian Chant!)](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-299-320.jpg)
![284 Part IV: Using Program Units
The first five Gregarians all want Room 7. It’s a tight squeeze, but you were
never big on fire codes anyway. Next comes a group of three with a yen for
Room 0. (They’re computer programmers, and they think the room number is
cute.) Then there’s a pack of four Gregarians who want Room 3. (The in-room
pool sounds attractive to them.)
Looping in style
Listing 15-7 uses an enhanced for loop to step Enhanced for loops are nice and concise. But
through a bunch of values. In that program, the don’t be too anxious to use enhanced loops with
values belong to an enum type. Well, this chap- arrays. This feature has some nasty limitations.
ter also deals with a bunch of values; namely, For example, my new howMany loop doesn’t
the values in an array. So you’re probably not display room numbers. I avoid room numbers
surprised if I show you an enhanced for loop because the room numbers in my guestsIn
that steps through an array’s values. array are the indices 0 through 9. Unfortunately,
an enhanced loop doesn’t provide easy access
To see such a loop, start with the code in Listing
to an array’s indices.
16-5. The last loop in that program looks some-
thing like this: And here’s another unpleasant surprise. Start
for (int roomNum = 0; roomNum <
with the following loop from Listing 16-4:
10; roomNum++) { for (int roomNum = 0; roomNum <
10; roomNum++) {
out.println(guestsIn[roomNu guestsIn[roomNum] =
m]); diskScanner.nextInt();
} }
To turn this into an enhanced for loop, you Turn this traditional for loop into an enhanced
make up a new variable name. (What about the for loop, and you get the following misleading
name howMany? I like that name.) Whatever code:
name you choose, the new variable ranges over
for (int howMany : guestsIn) {
the values in the guestsIn array. howMany =
for (int howMany : guestsIn) { diskScanner.nextInt();
out.println(howMany); //Don’t do this
} }
This enhanced loop uses the same format as The new enhanced loop doesn’t do what you
the loop in Chapter 15. want it to do. This loop reads values from an
input file and then dumps these values into the
for (TypeName variableName :
RangeOfValues) { garbage can. In the end, the array’s values
Statements remain unchanged.
} It’s sad but true. To make full use of an array, you
In Chapter 15, the RangeOfValues belongs to have to fall back on Java’s plain old for loop.
an enum type. But in this sidebar’s example, the
RangeOfValues belongs to an array.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-300-320.jpg)
![Chapter 16: Using Loops and Arrays 285
With all this traffic, you better switch on your computer. You start a program
that enables you to enter new occupancy data. The program has five parts:
Create an array, and then put 0 in each of the array’s components.
When the Loners’ Club members left, the motel was suddenly empty.
(Heck, even before the Loners’ Club members left, the motel seemed
empty.) To declare an array and fill the array with zeros, you execute
code of the following kind:
int guestsIn[];
guestsIn = new int[10];
for (int roomNum = 0; roomNum < 10; roomNum++) {
guestsIn[roomNum] = 0;
}
Get a room number, and then get the number of guests who will be
staying in that room.
Reading numbers typed by the user is pretty humdrum stuff. Do a little
prompting and a little nextInt calling, and you’re all set:
out.print(“Room number: “);
whichRoom = myScanner.nextInt();
out.print(“How many guests? “);
numGuests = myScanner.nextInt();
Use the room number and the number of guests to change a value in
the array.
Earlier in this chapter, to put one guest in Room 2, you executed
guestsIn[2] = 1;
So now, you have two variables — numGuests and whichRoom. Maybe
numGuests is 5, and whichRoom is 7. To put numGuests in whichRoom
(that is, to put 5 guests in Room 7), you can execute
guestsIn[whichRoom] = numGuests;
That’s the crucial step in the design of your new program.
Ask the user if the program should keep going.
Are there more guests to put in rooms? To find out, execute this code:
out.print(“Do another? “);
} while (myScanner.findInLine(“.”).charAt(0) == ‘Y’);
Display the number of guests in each room.
No problem! You already did this. You can steal the code (almost verba-
tim) from Listing 16-1:](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-301-320.jpg)
![286 Part IV: Using Program Units
out.println(“RoomtGuests”);
for (int roomNum = 0; roomNum < 10; roomNum++) {
out.print(roomNum);
out.print(“t”);
out.println(guestsIn[roomNum]);
}
The only difference between this latest code snippet and the stuff in
Listing 16-1 is that this new code uses the guestsIn array. The first time
through this loop, the code does
out.println(guestsIn[0]);
displaying the number of guests in Room 0. The next time through the
loop, the code does
out.println(guestsIn[1]);
displaying the number of guests in Room 1. The last time through the
loop, the code does
out.println(guestsIn[9]);
That’s perfect.
The complete program (with these five pieces put together) is in Listing 16-5.
A run of the program is shown in Figure 16-7.
Listing 16-5: Storing Occupancy Data in an Array
import java.util.Scanner;
import static java.lang.System.out;
class AddGuests {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
int whichRoom, numGuests;
int guestsIn[];
guestsIn = new int[10];
for (int roomNum = 0; roomNum < 10; roomNum++) {
guestsIn[roomNum] = 0;
}
do {
out.print(“Room number: “);
whichRoom = myScanner.nextInt();
out.print(“How many guests? “);
numGuests = myScanner.nextInt();
guestsIn[whichRoom] = numGuests;](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-302-320.jpg)
![Chapter 16: Using Loops and Arrays 287
out.println();
out.print(“Do another? “);
} while (myScanner.findInLine(“.”).charAt(0) == ‘Y’);
out.println();
out.println(“RoomtGuests”);
for (int roomNum = 0; roomNum < 10; roomNum++) {
out.print(roomNum);
out.print(“t”);
out.println(guestsIn[roomNum]);
}
}
}
Figure 16-7:
Running
the code in
Listing 16-5.
Hey! The program in Listing 16-5 is pretty big! It may be the biggest program
so far in this book. But big doesn’t necessarily mean difficult. If each piece of
the program makes sense, you can create each piece on its own, and then put
all the pieces together. Voilà! The code is manageable.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-303-320.jpg)
![290 Part IV: Using Program Units
Creating a Class
I start with a “traditional” example. The program in Listing 17-1 processes
simple purchase data. Two runs of the program are shown in Figure 17-1.
Listing 17-1: Doing It the Old-Fashioned Way
import java.util.Scanner;
class ProcessData {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
double amount;
boolean taxable;
double total;
System.out.print(“Amount: “);
amount = myScanner.nextDouble();
System.out.print(“Taxable? (true/false) “);
taxable = myScanner.nextBoolean();
if (taxable) {
total = amount * 1.05;
} else {
total = amount;
}
System.out.print(“Total: “);
System.out.println(total);
}
}
If the output in Figure 17-1 looks funny, it’s because I do nothing in the code
to control the number of digits beyond the decimal point. So in the output,
the value $20.00 looks like 20.0. That’s okay. I show you how to fix the prob-
lem in Chapter 18.
Figure 17-1:
Processing
a customer’s
purchase.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-306-320.jpg)
![292 Part IV: Using Program Units
Listing 17-3: Making Use of Your Purchase Class
import java.util.Scanner;
class ProcessPurchase {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
Purchase onePurchase = new Purchase();
System.out.print(“Amount: “);
onePurchase.amount = myScanner.nextDouble();
System.out.print(“Taxable? (true/false) “);
onePurchase.taxable = myScanner.nextBoolean();
if (onePurchase.taxable) {
onePurchase.total = onePurchase.amount * 1.05;
} else {
onePurchase.total = onePurchase.amount;
}
System.out.print(“Total: “);
System.out.println(onePurchase.total);
}
}
The best way to understand the code in Listing 17-3 is to compare it, line by
line, with the code in Listing 17-1. In fact, there’s a mechanical formula for
turning the code in Listing 17-1 into the code in Listing 17-3. Table 17-1
describes the formula.
Table 17-1 Converting Your Code to Use a Class
In Listing 17-1 In Listing 17-3
double amount; Purchase onePurchase = new Purchase();
boolean taxable;
double total;
amount onePurchase.amount
taxable onePurchase.taxable
total onePurchase.total](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-308-320.jpg)
![298 Part IV: Using Program Units
Creating several objects
After you’ve created a Purchase class, you can create as many purchase
objects as you want. For example, in Listing 17-5, I create three purchase
objects.
Listing 17-5: Processing Purchases
import java.util.Scanner;
class ProcessPurchasesss {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
Purchase aPurchase;
for (int count = 0; count < 3; count++) {
aPurchase = new Purchase();
System.out.print(“Amount: “);
aPurchase.amount = myScanner.nextDouble();
System.out.print(“Taxable? (true/false) “);
aPurchase.taxable = myScanner.nextBoolean();
if (aPurchase.taxable) {
aPurchase.total = aPurchase.amount * 1.05;
} else {
aPurchase.total = aPurchase.amount;
}
System.out.print(“Total: “);
System.out.println(aPurchase.total);
System.out.println();
}
}
}
Figure 17-5 has a run of the code in Listing 17-5, and Figure 17-6 illustrates the
concept.
Figure 17-5:
Running the
code in
Listing 17-5.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-314-320.jpg)
![304 Part IV: Using Program Units
In your own code, you can use this String class without ever seeing what’s
inside the String.java file. That’s one of the great things about object-
oriented programming.
A simple example
A String is bunch of characters. It’s like having several char values in a row.
You can declare a variable to be of type String and store several letters in
the variable. Listing 18-1 has a tiny example.
Listing 18-1: I’m Repeating Myself Again (Again)
import java.util.Scanner;
class JazzyEchoLine {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String lineIn;
lineIn = myScanner.nextLine();
System.out.println(lineIn);
}
}
A run of Listing 18-1 is shown in Figure 18-1. This run bears an uncanny resem-
blance to runs of Listing 5-1 from Chapter 5. That’s because Listing 18-1 is a
reprise of the effort in Listing 5-1.
Figure 18-1:
Running
the code in
Listing 18-1.
The new idea in Listing 18-1 is the use of a String. In Listing 5-1, I have no
variable to store the user’s input. But in Listing 18-1, I create the lineIn vari-
able. This variable stores a bunch of letters, like the letters Do as I write,
not as I do.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-320-320.jpg)
![Chapter 18: Using Methods and Variables from a Java Class 305
Putting String variables to good use
The program in Listing 18-1 takes the user’s input and echoes it back on the
screen. This is a wonderful program, but (like many college administrators
that I know) it doesn’t seem to be particularly useful.
So take a look at a more useful application of Java’s String type. A nice one
is in Listing 18-2.
Listing 18-2: Putting a Name in a String Variable
import java.util.Scanner;
import static java.lang.System.out;
class ProcessMoreData {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String fullName;
double amount;
boolean taxable;
double total;
out.print(“Customer’s full name: “);
fullName = myScanner.nextLine();
out.print(“Amount: “);
amount = myScanner.nextDouble();
out.print(“Taxable? (true/false) “);
taxable = myScanner.nextBoolean();
if (taxable) {
total = amount * 1.05;
} else {
total = amount;
}
out.println();
out.print(“The total for “);
out.print(fullName);
out.print(“ is “);
out.print(total);
out.println(“.”);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-321-320.jpg)
![308 Part IV: Using Program Units
Listing 18-3: A Faulty Password Checker
/*
* This code does not work:
*/
import java.util.Scanner;
import static java.lang.System.out;
class TryToCheckPassword {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String password = “swordfish”;
String userInput;
out.print(“What’s the password? “);
userInput = myScanner.next();
if (password == userInput) {
out.println(“You’re okay!”);
} else {
out.println(“You’re a menace.”);
}
}
}
Here are the facts as they appear in this example:
According to the code in Listing 18-3, the value of password is
“swordfish”.
In Figure 18-3, in response to the program’s prompt, the user types
swordfish. So in the code, the value of userInput is “swordfish”.
The if statement checks the condition password == userInput.
Because both variables have the value “swordfish”, the condition
should be true, but. . . .
The condition is not true, because the program’s output is You’re a
menace.
What’s going on here? I try beefing up the code to see if I can find any clues.
An enhanced version of the password-checking program is in Listing 18-4,
with a run of the new version shown in Figure 18-4.
Listing 18-4: An Attempt to Debug the Code in Listing 18-3
import java.util.Scanner;
import static java.lang.System.out;
class DebugCheckPassword {](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-324-320.jpg)
![Chapter 18: Using Methods and Variables from a Java Class 309
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String password = “swordfish”;
String userInput;
out.print(“What’s the password? “);
userInput = myScanner.next();
out.println();
out.print(“You typed “);
out.println(userInput);
out.print(“But the password is “);
out.println(password);
out.println();
if (password == userInput) {
out.println(“You’re okay!”);
} else {
out.println(“You’re a menace.”);
}
}
}
Figure 18-4:
This looks
even worse.
Ouch! I’m stumped this time. The run in Figure 18-4 shows that both the
userInput and password variables have value swordfish. So why doesn’t
the program accept the user’s input?
When you compare two things with a double equal sign, reference types and
primitive types don’t behave the same way. Consider, for example, int versus
String:
You can compare two int values with a double equal sign. When you do,
things work exactly as you would expect. For example, the condition in
the following code is true:
int apples = 7;
int oranges = 7;
if (apples == oranges)
System.out.println(“They’re equal.”);
When you compare two String values with the double equal sign, things
don’t work the way you expect. The computer doesn’t check to see if the
two String values contain the same letters. Instead, the computer checks
some esoteric property of the way variables are stored in memory.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-325-320.jpg)
![310 Part IV: Using Program Units
For your purposes, the term reference type is just a fancy name for a class.
Because String is defined to be a class in the Java API, I call String a refer-
ence type. This terminology highlights the parallel between primitive types
(such as int) and classes (that is, reference types, such as String).
Comparing strings
In the preceding bullets, the difference between int and String is mighty
interesting. But if the double equal sign doesn’t work for String values, how
do you check to see if Joe User enters the correct password? You do it with
the code in Listing 18-5.
Listing 18-5: Calling an Object’s Method
/*
* This program works!
*/
import java.util.Scanner;
import static java.lang.System.out;
class CheckPassword {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String password = “swordfish”;
String userInput;
out.print(“What’s the password? “);
userInput = myScanner.next();
if (password.equals(userInput)) {
out.println(“You’re okay!”);
} else {
out.println(“You’re a menace.”);
}
}
}
A run of the new password-checking code is shown in Figure 18-5 and, let me
tell you, it’s a big relief! The code in Listing 18-5 actually works! When the
user types swordfish, the if statement’s condition is true.
Figure 18-5:
At last,
Joe User
can log in.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-326-320.jpg)
![Chapter 18: Using Methods and Variables from a Java Class 315
Turning strings into numbers
The code in Listing 18-5 introduces a non-static method named equals. To
compare the password string with the userInput string, you preface .equals
with either of the two string objects. In Listing 18-5, I preface .equals with
the password object:
if (password.equals(userInput))
Each string object has an equals method of its own, so I can achieve the
same effect by writing
if (userInput.equals(password))
But Java has another class named Integer, and the whole Integer class has
a static method named parseInt. If someone hands you a string of characters,
and you want to turn that string into an int value, you can call the Integer
class’s parseInt method. Listing 18-6 has a small example.
Listing 18-6: More Chips, Please
import java.util.Scanner;
import static java.lang.System.out;
class AddChips {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
String reply;
int numberOfChips;
out.print(“How many chips do you have?”);
out.print(“ (Type a number,”);
out.print(“ or type ‘Not playing’) “);
reply = myScanner.nextLine();
if (!reply.equals(“Not playing”)) {
numberOfChips = Integer.parseInt(reply);
numberOfChips += 10;
out.print(“You now have “);
out.print(numberOfChips);
out.println(“ chips.”);
}
}
}
Some runs of the code in Listing 18-6 are shown in Figure 18-8. You want to give
each player ten chips. But some party poopers in the room aren’t playing. So
two people, each with no chips, may not get the same treatment. An empty-
handed player gets ten chips, but an empty-handed party pooper gets none.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-331-320.jpg)
![Chapter 18: Using Methods and Variables from a Java Class 317
Figure 18-9:
Do you have
change for
20.33850000
0000003?
The Java API has a class named NumberFormat, and the NumberFormat class
has a static method named getCurrencyInstance. When you call Number
Format.getCurrencyInstance() with nothing inside the parentheses, you
get an object that can mold numbers into U.S. currency amounts. Listing 18-7
has an example.
Listing 18-7: The Right Way to Display a Dollar Amount
import java.text.NumberFormat;
import java.util.Scanner;
class BetterProcessData {
public static void main(String args[]) {
Scanner myScanner = new Scanner(System.in);
double amount;
boolean taxable;
double total;
NumberFormat currency =
NumberFormat.getCurrencyInstance();
String niceTotal;
System.out.print(“Amount: “);
amount = myScanner.nextDouble();
System.out.print(“Taxable? (true/false) “);
taxable = myScanner.nextBoolean();
if (taxable) {
total = amount * 1.05;
} else {
total = amount;
}
niceTotal = currency.format(total);
System.out.print(“Total: “);
System.out.println(niceTotal);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-333-320.jpg)
![Chapter 18: Using Methods and Variables from a Java Class 321
That’s very nice! How about the System.out business that I introduce in
Chapter 3? I can apply my quotation to that too.
The static out variable belongs to the whole System class, not to any
particular instance of the System class. . . . To refer to the static out
variable, you use the System class’s name along with a dot. You write
something like System.out.println().
If you think about what System.out means, this static business makes sense.
After all, the name System.out refers to the place where a Java program
sends text output. (When you use JCreator, the name System.out refers to
JCreator’s General Output pane.) A typical program has only one place to
send its text output. So a Java program has only one out variable. No matter
how many objects you create — three, ten, or none — you have just one out
variable. And when you make something static, you insure that the program
has only one of those things.
Alright, then! The out variable is static.
To abbreviate the name of a static variable (or a static method), you don’t
use an ordinary import declaration. Instead, you use a static import declara-
tion. That’s why, in Chapter 9 and beyond, I use the word static to import
the out variable:
import static java.lang.System.out;
Barry makes good on an age-old promise
In Chapter 6, I pull a variable declaration outside of a main method. I go from
code of the kind in Listing 18-8, to code of the kind that’s in Listing 18-9.
Listing 18-8: Declaring a Variable Inside the main Method
class SnitSoft {
public static void main(String args[]) {
double amount = 5.95;
amount = amount + 25.00;
System.out.println(amount);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-337-320.jpg)
![322 Part IV: Using Program Units
Listing 18-9: Pulling a Variable Outside of the main Method
class SnitSoft {
static double amount = 5.95;
public static void main(String args[]) {
amount = amount + 25.00;
System.out.println(amount);
}
}
In Chapter 6, I promise to explain why Listing 18-9 needs the extra word
static (in static double amount = 5.95). Well, with all the fuss about
static methods in this chapter, I can finally explain everything.
Look back to Figure 18-7. In that figure, you have checks and you have a sort
method. Each individual check has its own number, its own amount, and its
own payee. But the entire Check class has just one sort method.
I don’t know about you, but to sort my cancelled checks, I hang them on my
exotic Yucca Elephantipes tree. I fasten the higher numbered checks to the
upper leaves, and put the lower numbered checks on the lower leaves. When
I find a check whose number comes between two other checks, I select a free
leaf (one that’s between the upper and lower leaves).
A program to mimic my sorting method looks something like this:
class Check {
int number;
double amount;
String payee;
static void sort() {
Yucca tree;
if (myCheck.number > 1700) {
tree.attachHigh(myCheck);
}
// ... etc.
}
}
Because of the word static, the Check class has only one sort method. And
because I declare the tree variable inside the static sort method, this pro-
gram has only one tree variable. (Indeed, I hang all my cancelled checks on
just one Yucca tree.) I can move the tree variable’s declaration outside of
the sort method. But if I do, I may have too many Yucca trees.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-338-320.jpg)
![330 Part IV: Using Program Units
Calling the method
To put the previous section’s ideas into action, you need more code. So the
next listing (see Listing 19-3) creates instances of the Account class.
Listing 19-3: Making Use of the Code in Listing 19-1
import java.util.Random;
class ProcessAccounts {
public static void main(String args[]) {
Random myRandom = new Random();
Account anAccount;
for (int i = 0; i < 3; i++) {
anAccount = new Account();
anAccount.lastName = “” +
(char) (myRandom.nextInt(26) + ‘A’) +
(char) (myRandom.nextInt(26) + ‘a’) +
(char) (myRandom.nextInt(26) + ‘a’);
anAccount.id = myRandom.nextInt(10000);
anAccount.balance = myRandom.nextInt(10000);
anAccount.display();
}
}
}
Here’s a summary of the action in Listing 19-3:
Do the following three times:
Create a new object (an instance of the Account class).
Randomly generate values for the object’s lastName,
id and balance.
Call the object’s display method.
The first of the three display calls prints the first object’s lastName, id,
and balance values. The second display call prints the second object’s
lastName, id, and balance values. And so on.
A run of the code from Listing 19-3 is shown in Figure 19-3.
Figure 19-3:
Running
the code in
Listing 19-3.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-346-320.jpg)
![Chapter 19: Creating New Java Methods 335
void fillWithData() {
Random myRandom = new Random();
lastName = “” +
(char) (myRandom.nextInt(26) + ‘A’) +
(char) (myRandom.nextInt(26) + ‘a’) +
(char) (myRandom.nextInt(26) + ‘a’);
id = myRandom.nextInt(10000);
balance = myRandom.nextInt(10000);
}
void display() {
NumberFormat currency =
NumberFormat.getCurrencyInstance();
out.print(“The account with last name “);
out.print(lastName);
out.print(“ and ID number “);
out.print(id);
out.print(“ has balance “);
out.println(currency.format(balance));
}
}
I wrote some code to use the class in Listing 19-4. This new code is in
Listing 19-5.
Listing 19-5: This Is So Cool!
class ProcessBetterAccounts {
public static void main(String args[]) {
BetterAccount anAccount;
for (int i = 0; i < 3; i++) {
anAccount = new BetterAccount();
anAccount.fillWithData();
anAccount.display();
}
}
}
Listing 19-5 is pretty slick. Because the code in Listing 19-4 is so darn smart,
the new code in Listing 19-5 has very little work to do. This new code just
creates a BetterAccount object, and then calls the methods in Listing 19-4.
When you run all this stuff, you get results like the ones in Figure 19-3.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-351-320.jpg)
![Chapter 19: Creating New Java Methods 337
void display() {
NumberFormat currency =
NumberFormat.getCurrencyInstance();
out.print(“The account with last name “);
out.print(lastName);
out.print(“ and ID number “);
out.print(id);
out.print(“ has balance “);
out.println(currency.format(balance));
}
}
Listing 19-7: Calling the addInterest Method
import java.util.Random;
class ProcessNiceAccounts {
public static void main(String args[]) {
Random myRandom = new Random();
NiceAccount anAccount;
double interestRate;
for (int i = 0; i < 3; i++) {
anAccount = new NiceAccount();
anAccount.lastName = “” +
(char) (myRandom.nextInt(26) + ‘A’) +
(char) (myRandom.nextInt(26) + ‘a’) +
(char) (myRandom.nextInt(26) + ‘a’);
anAccount.id = myRandom.nextInt(10000);
anAccount.balance = myRandom.nextInt(10000);
anAccount.display();
interestRate = myRandom.nextInt(5);
anAccount.addInterest(interestRate);
anAccount.display();
System.out.println();
}
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-353-320.jpg)
![Chapter 19: Creating New Java Methods 343
Listing 19-9: Calling the Method in Listing 19-8
import java.util.Random;
import java.text.NumberFormat;
class ProcessGoodAccounts {
public static void main(String args[]) {
Random myRandom = new Random();
NumberFormat currency =
NumberFormat.getCurrencyInstance();
GoodAccount anAccount;
double interestRate;
double yearlyInterest;
for (int i = 0; i < 3; i++) {
anAccount = new GoodAccount();
anAccount.lastName = “” +
(char) (myRandom.nextInt(26) + ‘A’) +
(char) (myRandom.nextInt(26) + ‘a’) +
(char) (myRandom.nextInt(26) + ‘a’);
anAccount.id = myRandom.nextInt(10000);
anAccount.balance = myRandom.nextInt(10000);
anAccount.display();
interestRate = myRandom.nextInt(5);
yearlyInterest =
anAccount.getInterest(interestRate);
System.out.print(“This year’s interest is “);
System.out.println
(currency.format(yearlyInterest));
System.out.println();
}
}
}
To see a run of code from Listings 19-8 and 19-9, take a look at Figure 19-8.
Figure 19-8:
Running
the code in
Listing 19-9.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-359-320.jpg)
![348 Part IV: Using Program Units
The Java Swing Classes
Java’s Swing classes create graphical objects on a computer screen. The objects
can include buttons, icons, text fields, check boxes, and other good things
that make windows so useful.
The name “Swing” isn’t an acronym. When the people at Sun Microsystems
were first creating the code for these classes, one of the developers named it
“Swing” because swing music was enjoying a nostalgic revival. And yes, in
addition to String and Swing, the standard Java API has a Spring class. But
that’s another story.
Actually, Java’s API has several sets of windowing components. An older set
is called AWT — the Abstract Windowing Toolkit. But to use some of the Swing
classes, you have to call on some of the old AWT classes. Go figure!
Showing an image on the screen
The program in Listing 20-1 displays a window on your computer screen. To
see the window, look at Figure 20-1.
Listing 20-1: Creating a Window with an Image in It
import javax.swing.JFrame;
import javax.swing.ImageIcon;
import javax.swing.JLabel;
import java.awt.Container;
class ShowPicture {
public static void main(String args[]) {
JFrame frame = new JFrame();
ImageIcon icon = new ImageIcon(“j2fd.jpg”);
JLabel label = new JLabel(icon);
Container contentPane = frame.getContentPane();
contentPane.add(label);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
}
The code in Listing 20-1 has very little logic of its own. Instead, this code
pulls together a bunch of classes from the Java API.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-364-320.jpg)
![Chapter 20: Oooey GUI Was a Worm 353
Listing 20-2: Adding Components to a Frame
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.FlowLayout;
class MyLittleGUI {
public static void main(String args[]) {
JFrame frame;
Container contentPane;
JTextField textfield;
JButton button;
FlowLayout layout;
String sorry;
frame = new JFrame();
frame.setTitle(“Interact”);
contentPane = frame.getContentPane();
textfield = new JTextField(“Type your text here.”);
sorry = “This button is temporarily out of order.”;
button = new JButton(sorry);
contentPane.add(textfield);
contentPane.add(button);
layout = new FlowLayout();
contentPane.setLayout(layout);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
}
Figure 20-4:
The frame in
Listing 20-2.](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-369-320.jpg)
![Chapter 20: Oooey GUI Was a Worm 355
Taking Action
The previous section’s code leaves me feeling a little empty. When you click
the button, nothing happens. When you type in the text field, nothing hap-
pens. What a waste!
To make me feel better, I include one more program in this chapter. The pro-
gram (in Listings 20-3 and 20-4) responds to a button click. When you click
the frame’s button, any text in the text field becomes all uppercase. That’s
very nice, but the code is quite complicated. In fact, the code has so many
advanced features that I can’t fully describe them in the space that I’m allot-
ted. So you may have to trust me.
Listing 20-3: Capitalism in Action
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.FlowLayout;
class CapitalizeMe {
public static void main(String args[]) {
JFrame frame;
Container contentPane;
JTextField textfield;
JButton button;
FlowLayout layout;
frame = new JFrame();
frame.setTitle(“Handy Capitalization Service”);
contentPane = frame.getContentPane();
textfield =
new JTextField(“Type your text here.”, 20);
button = new JButton(“Capitalize”);
button.addActionListener
(new MyActionListener(textfield));
contentPane.add(textfield);
contentPane.add(button);
layout = new FlowLayout();
contentPane.setLayout(layout);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
}](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-371-320.jpg)
![Index
% (percent sign) remainder operator,
• Symbols • 108–111
&& (and operator), 158, 161 + (plus sign)
+= (assignment operator), 339 add char value to string, 331
** (asterisk, double) Javadoc comment, 57 addition operator, 108
*/ (close traditional comment), 56 concatenating strings with, 333–334
== (comparing values), 133, 152, 309 ++ (plus sign, double)
?: (conditional operator), 192–194 postincrement operator, 114–117
{} (curly braces) preincrement operator, 111–114
block and, 155 ‘ (quotation mark), 122
class and, 64 “” (quotation marks, double), 53, 307
example using, 54, 55 ; (semicolon)
nested statements and, 171 adding unnecessary, 78
/ (division sign), 108 if statement, 145, 146
. (dot), 312, 314 omitting, 77–78
“” (double backslash inside quotation statement and, 61
marks), 229 [] (square brackets), 279
// (end-of-line comment), 56 t (tab stop), 206
= (equal sign) in assignment statement, 92
> (greater than), 133
>= (greater than or equal to), 132, 133
•A•
< (less than), 133 abbreviating
<= (less than or equal to), 133 class name, 319
- (minus sign), 108 code, 155–156
— (minus sign, double) name of static variable, 320–321
postdecrement operator, 117 abstract method, 80
predecrement operator, 117 Abstract Windowing Toolkit (AWT), 348
* (multiplication sign), 108 Account class
n (move cursor to new line), 206 creating instances of, 330
!= (not equal to), 133 description of, 327
! (not operator), 158 display method, 327, 329
/* (open traditional comment), 56 account value, generating at random,
|| (or operator), 158, 159, 161 326–327
() (parentheses) ActionEvent, 356
conditions and, 168–169 ActionListener code, 356
if statement, 145, 146 actionPerformed method, 356
method and, 328 active project, 36–37
parameters and, 336 AddChips class, 315
) (parenthesis closing) omitting, 82 AddGuests class, 286–287](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-387-320.jpg)
![384 Beginning Programming with Java For Dummies, 2nd Edition
NullPointerException error on-the-fly information, 336
message, 242 OOP (object-oriented programming)
NumberFormat class, 316–318, 369 classes and, 289, 303, 325–326
numbers description of, 63–64
adding, 333 essence of, 312
comparing, 134–135, 137 objects and, 302
pseudorandom, 149 opening
random, generating, 148–149 Javadoc comment (/**), 57
turning into strings, 316–318 traditional comment (/*), 56
turning strings into, 315–316 operators
numbers using decimal assignment, 117–118, 162, 339
comparing, 134–135, 137 comparison, 133–134
definition of, 96 conditional, 192–194
reading from keyboard, 96–97 logical, 158, 166–168
types of, 118–120 math, 108
numbers, whole postincrement, 114–117
comparing, 134 predecrement and postdecrement, 117
overview of, 103–105 preincrement, 111–114
reading from keyboard, 105–106 remainder, 108–111, 112–113
remainder operator and, 112–113 or operator (||), 158, 159, 161
types of, 118–120 out identifier, 53
numeric types, primitive, 120 out variable, 320–321
outer loop, 237
•O• output
blank line, inserting in, 111
object lining up, 206
calling methods of, 310–311 wrapping, 93–94
class compared to, 300–302 output file, viewing, 224
creating multiple, 298–300 overusing punctuation, 78–81
description of, 294–297
instance compared to, 297
methods, using, 307–310, 313
•P•
referring to parts of, 297 pack method, 352
with two methods, 334–335 package
object code, 12–13 java.awt.event, 356
object-oriented programming (OOP) java.lang, 319–320
classes and, 289, 303, 325–326 java.util, 319
description of, 63–64 parameters
essence of, 312 description of, 336
objects and, 302 example of, 338
occupancy report, generating, 272–277 JFrame methods, 351
offline installation, 25 names of, 339
omitting punctuation, 77–78 parentheses [()]
one-statement rule, 155 conditions and, 168–169
online installation, 26 if statement, 145, 146](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-400-320.jpg)
![Index 385
method and, 328 punctuation, 54–55
parameters and, 336 Web sites, 364–365
parenthesis, closing [)], omitting, 82 project
parseInt method, 316 active, 36–37
passing value to method, 336–339 creating in JCreator MyWorkspace, 39–44
percent sign (%) remainder operator, creating with two source files, 179–180
108–111 prompt, 98
Phipps, Simon, blog of, 364 Proud Winners and Sore Losers
Please, Gimme a Break! program, 188–189 program, 177
plus sign (+) pseudorandom number, 149
add char value to string, 331 public keyword, 51
addition operator, 108 Pulling Variable Outside main Method
concatenating strings with, 333–334 program, 322
plus sign double (++) punctuation. See also specific punctuation
postincrement operator, 114–117 finding omitted, 77–78
preincrement operator, 111–114 if statement, 145–146
portability, 18 overview of, 54–55
price variable, 160, 161–162 parenthesis, closing [)], omitting, 82
priming while loop, 207–209, 212–214 for referring to variables and
primitive types methods, 333
non-numeric, 138 using too much, 78–81
numeric, 120 Purchase class, 291, 298, 299–300
object-oriented programming and, 289
reference type compared to, 300, 309
print method, 227–228 •Q•
println identifier, 53 quotation mark (‘), 122
println method, 227–228 quotation marks
PrintStream, 221 double (“”), 53, 307
ProcessAccounts class, 330 double, inside quotation marks
ProcessBetterAccounts class, 335 (“”), 229
ProcessData class, 290
ProcessGoodAccounts class, 343
ProcessMoreData class, 305 •R•
ProcessNiceAccounts class, 337 Random class, 148, 149, 150
ProcessPurchase class, 292 random number, generating, 148–149
ProcessPurchasesss class, 298 random words, generating, 331
programmer, 10–11, 98 reading
programming, goals of, 176 characters, 129–131
programming language data from disk file, 219–220, 227
comments, 56–57 decimal number from keyboard, 96–97
data-gathering features of, 303 String value from keyboard, 306
description of, 11, 49–50 whole number from keyboard, 105–106
identifiers, 52–53 reference type
keywords, 50–51 description of, 310
literals, 53–54 example of, 291
primitive type compared to, 300, 309](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-401-320.jpg)
![386 Beginning Programming with Java For Dummies, 2nd Edition
regular expression, 129 setLayout method, 354
remainder operator (%), 108–111, 112–113 setTitle method, 354
Repeat Before You Delete program, setVisible(true) method, 352
254–257 short value, 120
responding to button click, 355–357 showing image on screen, 348–349
Responding to Button Clicks program, 356 ShowOccupancy class, 272
return type, 341–345 ShowOneRoomOccupancy class, 274
return value, 341–345 ShowPicture class, 348
reusing variable, 125–127 simple name, 319
reversing letters in word, 127–129 Sindell, Kathleen, Managing Your Money
rewriting disk file, 229–231 Online For Dummies, 323
Right Way to Display Dollar Amount SkillMarket Web site, 364
program, 317 skinning cat, origins of phrase, 150
Roll 7 or 11 program, 196 Slashdot Web site, 365
running slot machine program, 261–267
canned program, 33–38 Smaragdis, Mary, blog of, 364
code, 13–17 SnitSoft class, 90, 321–322
code that straddles two files, 293–294 software, open source, Web site for, 365
disk-oriented program, 216–217 sorting, 321–324
own code, 38–44 source code, 12–13
run-time error message, 85–86 source files, creating project with two,
Runtime.getRuntime().exec method, 179–180
256–257 SourceForge Web site, 365
spaces in code, 87
•S• SpecialOffer program, 151–153
square brackets ([]), 279
Scanner class statement. See also specific statements
declarations, 273 assignment, 92–93, 100, 106–107
description of, 73, 369 block as, 155, 200
hasNext method, 237–238, 276 break statement, 184, 188–190
methods, 72, 106 compound, 144, 200
nextBoolean method, 292 description of, 61
Scanner myScanner - new expression and, 115
Scanner(System.in) line, 74 semicolon (;) and, 61
Schwartz, Jonathan, blog of, 364 static import declaration, 155–156, 321
Scoreboard class, 177 static keyword, 51
screen static method, 101, 313–314
sending characters to, 70 static variable, 320–323
showing image on, 348–349 stopping program, 38, 205
semicolon (;) storing data on hard drive
adding unnecessary, 78 keyboard/screen program for, 217–219
if statement, 145, 146 overview of, 215
omitting, 77–78 reading from and writing to file,
statement and, 61 219–220, 222
setDefaultCloseOperation method, 351 rewriting disk file, 229–231
running disk-oriented program, 216–217](https://image.slidesharecdn.com/beginningprogrammingwithjavafordummies2ndedition2005-101221042448-phpapp02/85/Beginning-programming-with-java-for-dummies-2nd-edition-2005-402-320.jpg)
Beginning programming with java for dummies, 2nd edition (2005)
- 3. Beginning Programming with Java ™ FOR DUMmIES ‰ 2ND EDITION by Barry Burd
- 4. Beginning Programming with Java™ For Dummies®, 2nd Edition Published by Wiley Publishing, Inc. 111 River Street Hoboken, NJ 07030-5774 www.wiley.com Copyright © 2005 by Wiley Publishing, Inc., Indianapolis, Indiana Published by Wiley Publishing, Inc., Indianapolis, Indiana Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permis- sion of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8600. Requests to the Publisher for permission should be addressed to the Legal Department, Wiley Publishing, Inc., 10475 Crosspoint Blvd., Indianapolis, IN 46256, (317) 572-3447, fax (317) 572-4355, or online at http://www. wiley.com/go/permissions. Trademarks: Wiley, the Wiley Publishing logo, For Dummies, the Dummies Man logo, A Reference for the Rest of Us!, The Dummies Way, Dummies Daily, The Fun and Easy Way, Dummies.com, and related trade dress are trademarks or registered trademarks of John Wiley & Sons, Inc., and/or its affiliates in the United States and other countries, and may not be used without written permission. Java is a trademark of Sun Microsystems, Inc. All other trademarks are the property of their respective owners. Wiley Publishing, Inc., is not associated with any product or vendor mentioned in this book. LIMIT OF LIABILITY/DISCLAIMER OF WARRANTY: THE PUBLISHER AND THE AUTHOR MAKE NO REP- RESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS WORK AND SPECIFICALLY DISCLAIM ALL WARRANTIES, INCLUDING WITHOUT LIMITATION WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE. NO WARRANTY MAY BE CRE- ATED OR EXTENDED BY SALES OR PROMOTIONAL MATERIALS. THE ADVICE AND STRATEGIES CON- TAINED HEREIN MAY NOT BE SUITABLE FOR EVERY SITUATION. THIS WORK IS SOLD WITH THE UNDERSTANDING THAT THE PUBLISHER IS NOT ENGAGED IN RENDERING LEGAL, ACCOUNTING, OR OTHER PROFESSIONAL SERVICES. IF PROFESSIONAL ASSISTANCE IS REQUIRED, THE SERVICES OF A COMPETENT PROFESSIONAL PERSON SHOULD BE SOUGHT. NEITHER THE PUBLISHER NOR THE AUTHOR SHALL BE LIABLE FOR DAMAGES ARISING HEREFROM. THE FACT THAT AN ORGANIZATION OR WEBSITE IS REFERRED TO IN THIS WORK AS A CITATION AND/OR A POTENTIAL SOURCE OF FUR- THER INFORMATION DOES NOT MEAN THAT THE AUTHOR OR THE PUBLISHER ENDORSES THE INFORMATION THE ORGANIZATION OR WEBSITE MAY PROVIDE OR RECOMMENDATIONS IT MAY MAKE. FURTHER, READERS SHOULD BE AWARE THAT INTERNET WEBSITES LISTED IN THIS WORK MAY HAVE CHANGED OR DISAPPEARED BETWEEN WHEN THIS WORK WAS WRITTEN AND WHEN IT IS READ. For general information on our other products and services, please contact our Customer Care Department within the U.S. at 800-762-2974, outside the U.S. at 317-572-3993, or fax 317-572-4002. For technical support, please visit www.wiley.com/techsupport. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Library of Congress Control Number: 2005923219 ISBN-13: 978-0-7645-8874-7 ISBN-10: 0-7645-8874-5 Manufactured in the United States of America 10 9 8 7 6 5 4 3 2 1 2B/QU/QU/QV/IN
- 5. About the Author Dr. Barry Burd has an M.S. in Computer Science from Rutgers University, and a Ph.D. in Mathematics from the University of Illinois. As a teaching assistant in Champaign-Urbana, Illinois, he was elected five times to the university-wide List of Teachers Ranked as Excellent by their Students. Since 1980, Dr. Burd has been a professor in the Department of Mathematics and Computer Science at Drew University in Madison, New Jersey. When he’s not lecturing at Drew University, Dr. Burd leads training courses for profes- sional programmers in business and industry. He has lectured at conferences in the United States, Europe, Australia, and Asia. He is the author of several articles and books, including Java 2 For Dummies and Eclipse For Dummies, both published by Wiley Publishing, Inc. Dr. Burd lives in Madison, New Jersey, with his wife and two children. For hobbies he enjoys anything that wastes his and everyone else’s time.
- 7. Dedication For Harriet, Sam and Jennie, Sam and Ruth, Abram and Katie, Benjamin and Jennie Author’s Acknowledgments Author’s To-Do List, February 13, 2005: Item: Send chocolate to Christine Berman — the book’s project editor and copy editor. As anyone who reads Chapter 4 learns, chocolate is one of the most precious commodities on earth. So when I give chocolate, I give it thoughtfully and intentionally. The only thing that rivals chocolate’s goodness is the benefit of a good night’s sleep. But with a 19-month-old child in the house, Christine probably isn’t getting enough sleep. Even so, she has the time and patience to work on my manuscript. Yes, Christine deserves special thanks. Item: Have a plaque erected in honor of Steven Hayes, your acquisitions editor at Wiley. While you dragged your heels, Steve kept on insisting that you write this book. (Sure, you wanted a long vacation instead of a big book project, but who cares? He was right; you were wrong.) Item: Send a thank-you note to tech editor Jim Kelly who helped polish your original work and, miraculously, didn’t make a lot of extra work for you. Item: Recommend your agent Laura Lewin to other computer book authors. If it weren’t for Laura, you’d still be roaming the book exhibits and looking needy at the technology conferences. Item: Visit Frank Thornton, Bonnie Averbach, and Herbert Putz at Temple University. Thank them for steering you to a career as a professor. In any other career, you’d have no time left to write. (And by the way, while you’re in Philly, don’t forget to stop for a cheesesteak.) Item: Send e-mail to Gaisi Takeuti at the University of Illinois, and to William Wisdom and Hughes LeBlanc at Temple University. Thank them for teaching you about Symbolic Logic. It’s made your life as a computer scientist and mathematician much richer. Item: Spend more time with your family. (Remind them that you’re the guy who wandered around the house before this book project got started.) Renew your pledge to clean up after yourself. Don’t be so highstrung, and finish each sentence that you start. Remember that you can never fully return the love they’ve given you, but you should always keep trying.
- 8. Publisher’s Acknowledgments We’re proud of this book; please send us your comments through our online registration form located at www.dummies.com/register/. Some of the people who helped bring this book to market include the following: Acquisitions, Editorial, and Composition Services Media Development Project Coordinator: Maridee Ennis Project Editor: Christine Berman Layout and Graphics: Andrea Dahl, Acquisitions Editor: Steve Hayes Joyce Haughey, Lynsey Osborn, Copy Editor: Christine Berman Melanee Prendergast, Heather Ryan Technical Editor: Jim Kelly Proofreaders: Leeann Harney, Jessica Kramer, Carl William Pierce, Dwight Ramsey, Editorial Manager: Carol Sheehan TECHBOOKS Production Services Media Development Manager: Laura VanWinkle Indexer: TECHBOOKS Production Services Media Development Supervisor: Richard Graves Editorial Assistant: Amanda Foxworth Cartoons: Rich Tennant (www.the5thwave.com) Publishing and Editorial for Technology Dummies Richard Swadley, Vice President and Executive Group Publisher Andy Cummings, Vice President and Publisher Mary Bednarek, Executive Acquisitions Director Mary C. Corder, Editorial Director Publishing for Consumer Dummies Diane Graves Steele, Vice President and Publisher Joyce Pepple, Acquisitions Director Composition Services Gerry Fahey, Vice President of Production Services Debbie Stailey, Director of Composition Services
- 9. Contents at a Glance Introduction .................................................................1 Part I: Revving Up........................................................7 Chapter 1: Getting Started.................................................................................................9 Chapter 2: Setting Up Your Computer ...........................................................................23 Chapter 3: Running Programs.........................................................................................33 Part II: Writing Your Own Java Programs .....................45 Chapter 4: Exploring the Parts of a Program ................................................................47 Chapter 5: Composing a Program ..................................................................................65 Chapter 6: Using the Building Blocks: Variables, Values, and Types .........................89 Chapter 7: Numbers and Types ....................................................................................103 Chapter 8: Numbers? Who Needs Numbers?..............................................................121 Part III: Controlling the Flow.....................................139 Chapter 9: Forks in the Road ........................................................................................141 Chapter 10: Which Way Did He Go? .............................................................................157 Chapter 11: How to Flick a Virtual Switch...................................................................181 Chapter 12: Around and Around It Goes .....................................................................195 Chapter 13: Piles of Files: Dealing with Information Overload .................................215 Chapter 14: Creating Loops within Loops...................................................................233 Chapter 15: The Old Runaround...................................................................................245 Part IV: Using Program Units ....................................269 Chapter 16: Using Loops and Arrays ...........................................................................271 Chapter 17: Programming with Objects and Classes.................................................289 Chapter 18: Using Methods and Variables from a Java Class ...................................303 Chapter 19: Creating New Java Methods.....................................................................325 Chapter 20: Oooey GUI Was a Worm ............................................................................347 Part V: The Part of Tens ............................................359 Chapter 21: Ten Sets of Web Links ...............................................................................361 Chapter 22: Ten Useful Classes in the Java API ..........................................................367 Index .......................................................................371
- 11. Table of Contents Introduction..................................................................1 How to Use This Book .....................................................................................1 Conventions Used in This Book .....................................................................2 What You Don’t Have to Read ........................................................................2 Foolish Assumptions .......................................................................................3 How This Book Is Organized...........................................................................4 Part I: Revving Up ...................................................................................4 Part II: Writing Your Own Java Programs ............................................4 Part III: Controlling the Flow .................................................................5 Part IV: Using Program Units ................................................................5 Part V: The Part of Tens.........................................................................5 Icons Used in This Book..................................................................................5 Where to Go from Here....................................................................................6 Part I: Revving Up ........................................................7 Chapter 1: Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 What’s It All About? .........................................................................................9 Telling a computer what to do............................................................10 Pick your poison...................................................................................11 From Your Mind to the Computer’s Processor...........................................12 Translating your code..........................................................................12 Running code ........................................................................................13 Code you can use .................................................................................17 Your Java Programming Toolset ..................................................................19 What’s already on your hard drive?...................................................20 JCreator .................................................................................................21 Chapter 2: Setting Up Your Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 Downloading and Installing the Software You Need ..................................24 Downloading and installing a Java compiler ....................................24 Downloading and installing the Java API documentation...............26 Downloading and installing the JCreator integrated development environment ...........................................28 Running JCreator for the First Time ............................................................29 Is That All There Is to It? ...............................................................................31 Chapter 3: Running Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33 Running a Canned Java Program..................................................................33 Typing and Running Your Own Code...........................................................38
- 12. x Beginning Programming with Java For Dummies, 2nd Edition Part II: Writing Your Own Java Programs ......................45 Chapter 4: Exploring the Parts of a Program . . . . . . . . . . . . . . . . . . . . .47 Checking Out Java Code for the First Time ................................................47 Behold! A program!...............................................................................48 What the program’s lines say .............................................................49 The Elements in a Java Program ..................................................................49 Keywords...............................................................................................50 Identifiers that you or I can define.....................................................52 Identifiers with agreed upon meanings .............................................52 Literals ...................................................................................................53 Punctuation...........................................................................................54 Comments .............................................................................................56 Understanding a Simple Java Program........................................................57 What is a method?................................................................................57 The main method in a program ..........................................................60 How you finally tell the computer to do something ........................61 The Java class .......................................................................................63 Chapter 5: Composing a Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65 A Program to Echo Keyboard Input.............................................................66 Typing and running a program ...........................................................68 How the EchoLine program works.....................................................70 Getting numbers, words, and other things .......................................72 Type two lines of code, and don’t look back ....................................74 Expecting the Unexpected ............................................................................74 Diagnosing a problem ..........................................................................76 What problem? I don’t see a problem................................................86 Chapter 6: Using the Building Blocks: Variables, Values, and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 Using Variables ...............................................................................................89 Using a variable ....................................................................................90 Understanding assignment statements .............................................92 To wrap or not to wrap? ......................................................................93 What Do All Those Zeros and Ones Mean?.................................................94 Types and declarations .......................................................................95 What’s the point?..................................................................................96 Reading Decimal Numbers from the Keyboard..........................................96 Though these be methods, yet there is madness in ’t.....................97 Methods and assignments ..................................................................99 Variations on a Theme...................................................................................99 Moving variables from place to place..............................................100 Combining variable declarations .....................................................101
- 13. Table of Contents xi Chapter 7: Numbers and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103 Using Whole Numbers .................................................................................103 Reading whole numbers from the keyboard...................................105 What you read is what you get .........................................................106 Creating New Values by Applying Operators ...........................................108 Finding a remainder ...........................................................................108 The increment and decrement operators .......................................111 Assignment operators .......................................................................117 Size Matters ..................................................................................................118 Chapter 8: Numbers? Who Needs Numbers? . . . . . . . . . . . . . . . . . . . .121 Characters.....................................................................................................122 I digress . . . .........................................................................................123 One character only, please................................................................125 Variables and recycling......................................................................125 When not to reuse a variable............................................................127 Reading characters ............................................................................129 The boolean Type ........................................................................................131 Expressions and conditions..............................................................132 Comparing numbers; comparing characters..................................133 The Remaining Primitive Types .................................................................138 Part III: Controlling the Flow .....................................139 Chapter 9: Forks in the Road . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141 Making Decisions (Java if Statements)......................................................143 Looking carefully at if statements ....................................................143 A complete program ..........................................................................147 Indenting if statements in your code ...............................................150 Variations on the Theme .............................................................................150 . . . Or else what? ................................................................................151 Packing more stuff into an if statement...........................................153 Some handy import declarations .....................................................155 Chapter 10: Which Way Did He Go? . . . . . . . . . . . . . . . . . . . . . . . . . . . .157 Forming Bigger and Better Conditions......................................................157 Combining conditions: An example .................................................159 When to initialize? ..............................................................................161 More and more conditions ................................................................162 Using boolean variables ....................................................................165 Mixing different logical operators together ....................................166 Using parentheses ..............................................................................168
- 14. xii Beginning Programming with Java For Dummies, 2nd Edition Building a Nest..............................................................................................170 Nested if statements ..........................................................................171 Cascading if statements.....................................................................172 Enumerating the Possibilities.....................................................................175 Creating an enum type.......................................................................176 Using an enum type............................................................................176 Creating a project with two Java source files .................................179 Chapter 11: How to Flick a Virtual Switch . . . . . . . . . . . . . . . . . . . . . .181 Meet the switch Statement .........................................................................181 The cases in a switch statement ......................................................183 The default in a switch statement....................................................184 Picky details about the switch statement .......................................185 To break or not to break....................................................................188 Using Fall-through to Your Advantage.......................................................190 Using a Conditional Operator.....................................................................192 Chapter 12: Around and Around It Goes . . . . . . . . . . . . . . . . . . . . . . . .195 Repeating Instructions Over and Over Again (Java while Statements)...........................................................................196 Following the action in a loop...........................................................197 No early bailout ..................................................................................199 Thinking about Loops (What Statements Go Where)..............................200 Finding some pieces...........................................................................200 Assembling the pieces .......................................................................203 Getting values for variables ..............................................................203 From infinity to affinity ......................................................................205 Thinking About Loops (Priming) ...............................................................207 Working on the problem....................................................................209 Fixing the problem .............................................................................212 Chapter 13: Piles of Files: Dealing with Information Overload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .215 Running a Disk-Oriented Program .............................................................216 A sample program ..............................................................................217 Creating code that messes with your hard drive ...........................219 Running the sample program ...........................................................222 Troubleshooting problems with disk files ......................................224 Writing a Disk-Oriented Program ...............................................................226 Reading from a file..............................................................................227 Writing to a file....................................................................................227 Writing, Rewriting, and Re-rewriting..........................................................230 Chapter 14: Creating Loops within Loops . . . . . . . . . . . . . . . . . . . . . . .233 Paying Your Old Code a Little Visit............................................................234 Reworking some existing code .........................................................235 Running your code .............................................................................236
- 15. Table of Contents xiii Creating Useful Code ...................................................................................236 Checking for the end of a file ............................................................237 How it feels to be a computer...........................................................239 Why the computer accidentally pushes past the end of the file ...................................................................241 Solving the problem ...........................................................................243 Chapter 15: The Old Runaround . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .245 Repeating Statements a Certain Number Times (Java for Statements) ...............................................................................246 The anatomy of a for statement .......................................................248 Initializing a for loop ..........................................................................250 Using Nested for Loops ...............................................................................252 Repeating Until You Get What You Need (Java do Statements).............254 Getting a trustworthy response .......................................................255 Deleting files........................................................................................257 Using Java’s do statement .................................................................258 A closer look at the do statement ....................................................259 Repeating with Predetermined Values (Java’s Enhanced for Statement)............................................................260 Creating an enhanced for loop .........................................................261 Nesting the enhanced for loops .......................................................263 Part IV: Using Program Units.....................................269 Chapter 16: Using Loops and Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . .271 Some for Loops in Action............................................................................271 Deciding on a loop’s limit at runtime...............................................274 Using all kinds of conditions in a for loop.......................................275 Reader, Meet Arrays; Arrays, Meet the Reader ........................................277 Storing values in an array..................................................................280 Creating a report ................................................................................282 Working with Arrays ....................................................................................283 Chapter 17: Programming with Objects and Classes . . . . . . . . . . . . .289 Creating a Class ............................................................................................290 Reference types and Java classes ....................................................291 Using a newly defined class ..............................................................291 Running code that straddles two separate files.............................293 Why bother?........................................................................................294 From Classes Come Objects .......................................................................294 Understanding (or ignoring) the subtleties ....................................297 Making reference to an object’s parts .............................................297 Creating several objects ....................................................................298 Another Way to Think About Classes........................................................300 Classes, objects, and tables ..............................................................301 Some questions and answers ...........................................................302
- 16. xiv Beginning Programming with Java For Dummies, 2nd Edition Chapter 18: Using Methods and Variables from a Java Class . . . . .303 The String Class............................................................................................303 A simple example ...............................................................................304 Putting String variables to good use................................................305 Reading and writing strings ..............................................................306 Using an Object’s Methods .........................................................................307 Comparing strings ..............................................................................310 The truth about classes and methods.............................................311 Calling an object’s methods ..............................................................312 Combining and using data.................................................................313 Static Methods..............................................................................................313 Calling static and non-static methods .............................................314 Turning strings into numbers ...........................................................315 Turning numbers into strings ...........................................................316 How the NumberFormat works ........................................................318 Understanding the Big Picture ...................................................................318 Packages and import declarations...................................................319 Shedding light on the static darkness .............................................320 Barry makes good on an age-old promise.......................................321 Chapter 19: Creating New Java Methods . . . . . . . . . . . . . . . . . . . . . . .325 Defining a Method within a Class...............................................................325 Making a method ................................................................................326 Examining the method’s header.......................................................328 Examining the method’s body ..........................................................328 Calling the method .............................................................................330 The flow of control .............................................................................332 Using punctuation ..............................................................................333 The versatile plus sign.......................................................................333 Let the Objects Do the Work.......................................................................334 Passing Values to Methods .........................................................................336 Handing off a value.............................................................................338 Working with a method header ........................................................340 How the method uses the object’s values.......................................340 Getting a Value from a Method ...................................................................341 An example..........................................................................................342 How return types and return values work ......................................344 Working with the method header (again) .......................................345 Chapter 20: Oooey GUI Was a Worm . . . . . . . . . . . . . . . . . . . . . . . . . . .347 The Java Swing Classes ...............................................................................348 Showing an image on the screen ......................................................348 Just another class...............................................................................350 Keeping the User Busy (Working with Buttons and Text Fields) ...........352 Taking Action ................................................................................................355
- 17. Table of Contents xv Part V: The Part of Tens .............................................359 Chapter 21: Ten Sets of Web Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . .361 The Horse’s Mouth.......................................................................................361 Finding News, Reviews, and Sample Code ................................................362 Improving Your Code with Tutorials .........................................................362 Finding Help on Newsgroups......................................................................362 Reading Documentation with Additional Commentary ..........................363 Checking the FAQs for Useful Info..............................................................363 Opinions and Advocacy ..............................................................................363 Looking for Java Jobs ..................................................................................364 Finding Out More about Other Programming Languages .......................364 Everyone’s Favorite Sites ............................................................................365 Chapter 22: Ten Useful Classes in the Java API . . . . . . . . . . . . . . . . .367 Applet ............................................................................................................367 ArrayList........................................................................................................368 File..................................................................................................................368 Integer............................................................................................................368 Math ...............................................................................................................369 NumberFormat .............................................................................................369 Scanner..........................................................................................................369 String..............................................................................................................369 StringTokenizer ............................................................................................370 System ...........................................................................................................370 Index........................................................................371
- 18. xvi Beginning Programming with Java For Dummies, 2nd Edition
- 19. Introduction W hat’s your story? Are you a working stiff, interested in knowing more about the way your company’s computers work? Are you a student who needs some extra reading in order to survive a beginning computer course? Are you a typical computer user — you’ve done lots of word processing, and you want to do something more interesting with your computer? Are you a job seeker with an interest in entering the fast-paced, glam- orous, high-profile world of computer programming (or at least, the decent-paying world of computer programming)? Well, if you want to write computer programs, this book is for you. This book avoids the snobby “of-course-you-already-know” assumptions, and describes computer programming from scratch. The book uses Java — an exciting, relatively new computer programming lan- guage. But Java’s subtleties and eccentricities aren’t the book’s main focus. Instead, this book emphasizes a process — the process of creating instructions for a computer to follow. Many highfalutin’ books describe the mechanics of this process — the rules, the conventions, and the formalisms. But those other books aren’t written for real people. Those books don’t take you from where you are to where you want to be. In this book, I assume very little about your experience with computers. As you read each section, you get to see inside my head. You see the problems that I face, the things that I think, and the solutions that I find. Some problems are the kind that I remember facing when I was a novice; other problems are the kind that I face as an expert. I help you understand, I help you visualize, and I help you create solutions on your own. I even get to tell a few funny stories. How to Use This Book I wish I could say, “Open to a random page of this book and start writing Java code. Just fill in the blanks and don’t look back.” In a sense, this is true. You can’t break anything by writing Java code, so you’re always free to experiment.
- 20. 2 Beginning Programming with Java For Dummies, 2nd Edition But I have to be honest. If you don’t understand the bigger picture, writing a program is difficult. That’s true with any computer programming language — not just Java. If you’re typing code without knowing what it’s about, and the code doesn’t do exactly what you want it to do, then you’re just plain stuck. So in this book, I divide programming into manageable chunks. Each chunk is (more or less) a chapter. You can jump in anywhere you want — Chapter 5, Chapter 10, or wherever. You can even start by poking around in the middle of a chapter. I’ve tried to make the examples interesting without making one chapter depend on another. When I use an important idea from another chap- ter, I include a note to help you find your way around. In general, my advice is as follows: If you already know something, don’t bother reading about it. If you’re curious, don’t be afraid to skip ahead. You can always sneak a peek at an earlier chapter if you really need to do so. Conventions Used in This Book Almost every technical book starts with a little typeface legend, and Beginning Programming with Java For Dummies, 2nd Edition is no exception. What follows is a brief explanation of the typefaces used in this book: New terms are set in italics. When I want you to type something short or perform a step, I use bold. You’ll also see this computerese font. I use the computerese font for Java code, filenames, Web page addresses (URLs), on-screen messages, and other such things. Also, if something you need to type is really long, it appears in computerese font on its own line (or lines). You need to change certain things when you type them on your own computer keyboard. For example, I may ask you to type class Anyname which means you should type class and then some name that you make up on you own. Words that you need to replace with your own words are set in italicized computerese. What You Don’t Have to Read Pick the first chapter or section that has material you don’t already know and start reading there. Of course, you may hate making decisions as much as I do. If so, here are some guidelines you can follow:
- 21. Introduction 3 If you already know what computer programming is all about, then skip the first half of Chapter 1. Believe me, I won’t mind. If your computer has a Java compiler, and you’re required to use a devel- opment environment other than JCreator, then you can skip Chapter 2. This applies if you plan to use Eclipse, JBuilder, NetBeans, BlueJ, or a number of other development environments. Just make sure that your system uses Java 5.0 or later. This book’s exam- ples don’t work on earlier versions of Java, including versions numbered 1.4.2 and below. So if you’re not sure about your computer’s Java version, or if you have leeway in choosing a development environment, your safest move is to read Chapter 3. And by the way, if Eclipse is your thing, check my Eclipse For Dummies book, published by Wiley. If you’ve already done a little computer programming, be prepared to skim Chapters 6 through 8. Dive fully into Chapter 9, and see if it feels comfortable. (If so, then read on. If not, re-skim Chapters 6, 7, and 8.) If you feel comfortable writing programs in a language other than Java, then this book isn’t for you. Keep this book as a memento, and buy my Java 2 For Dummies book, also published by Wiley Publishing, Inc. If you want to skip the sidebars and the Technical Stuff icons, then please do. In fact, if you want to skip anything at all, feel free. Foolish Assumptions In this book, I make a few assumptions about you, the reader. If one of these assumptions is incorrect, then you’re probably okay. If all these assumptions are incorrect . . . well, buy the book anyway. I assume that you have access to a computer. Here’s good news. You can run the code in this book on almost any computer. The only computers you can’t use to run this code are ancient things that are more than eight years old (give or take a few years). Occasionally, I’m lazy and lapse into Microsoft Windows terminology, but that’s only because so many people run Windows. You can run the latest version of Java on Windows computers, UNIX/Linux computers, and (by some time in 2005) the Macintosh. I assume that you can navigate through your computer’s common menus and dialog boxes. You don’t have to be a Windows, Unix, or Macintosh power user, but you should be able to start a program, find a file, put a file into a certain directory . . . that sort of thing. Most of the time, when you practice the stuff in this book, you’re typing code on your keyboard, not pointing and clicking your mouse.
- 22. 4 Beginning Programming with Java For Dummies, 2nd Edition On those rare occasions when you need to drag and drop, cut and paste, or plug and play, I guide you carefully through the steps. But your computer may be configured in any of several billion ways, and my instructions may not quite fit your special situation. So when you reach one of these platform-specific tasks, try following the steps in this book. If the steps don’t quite fit, send me an e-mail message, or consult a book with instructions tailored to your system. I assume that you can think logically. That’s all there is to computer programming — thinking logically. If you can think logically, you’ve got it made. If you don’t believe that you can think logically, read on. You may be pleasantly surprised. I assume that you know little or nothing about computer programming. This isn’t one of those “all things to all people” books. I don’t please the novice while I tease the expert. I aim this book specifically toward the novice — the person who has never programmed a computer, or has never felt comfortable programming a computer. If you’re one of these people, you’re reading the right book. How This Book Is Organized This book is divided into subsections, which are grouped into sections, which come together to make chapters, which are lumped finally into five parts. (When you write a book, you get to know your book’s structure pretty well. After months of writing, you find yourself dreaming in sections and chapters when you go to bed at night.) The parts of the book are listed here. Part I: Revving Up The chapters in Part I prepare you for the overall programming experience. In these chapters, you find out what programming is all about and get your computer ready for writing and testing programs. Part II: Writing Your Own Java Programs This part covers the basic building blocks — the elements in any Java program, and in any program written using a Java-like language. In this part, you dis- cover how to represent data, and how to get new values from existing values. The program examples are short, but cute.
- 23. Introduction 5 Part III: Controlling the Flow Part III has some of my favorite chapters. In these chapters, you make the computer navigate from one part of your program to another. Think of your program as a big mansion, with the computer moving from room to room. Sometimes the computer chooses between two or more hallways, and some- times the computer revisits rooms. As a programmer, your job is to plan the computer’s rounds through the mansion. It’s great fun. Part IV: Using Program Units Have you ever solved a big problem by breaking it into smaller, more manage- able pieces? That’s exactly what you do in Part IV of this book. You discover the best ways to break programming problems into pieces and to create solu- tions for the newly found pieces. You also find out how to use other people’s solutions. It feels like stealing, but it’s not. This part also contains a chapter about programming with windows, buttons, and other graphical items. If your mouse feels ignored by the examples in this book, read Chapter 20. Part V: The Part of Tens The Part of Tens is a little beginning programmer’s candy store. In the Part of Tens, you can find lists — lists of tips, resources, and all kinds of interesting goodies. I added an Appendix on this book’s web site to help you feel comfortable with Java’s documentation. I can’t write programs without my Java programming documentation. In fact, no Java programmer can write programs without those all-important docs. These docs are in Web page format, so they’re easy to find and easy to navigate. But if you’re not used to all the terminology, the documentation can be overwhelming. Icons Used in This Book If you could watch me write this book, you’d see me sitting at my computer, talking to myself. I say each sentence several times in my head. When I have an extra thought, a side comment, something that doesn’t belong in the regu- lar stream, I twist my head a little bit. That way, whoever’s listening to me (usually nobody) knows that I’m off on a momentary tangent.
- 24. 6 Beginning Programming with Java For Dummies, 2nd Edition Of course, in print, you can’t see me twisting my head. I need some other way of setting a side thought in a corner by itself. I do it with icons. When you see a Tip icon or a Remember icon, you know that I’m taking a quick detour. Here’s a list of icons that I use in this book: A tip is an extra piece of information — something helpful that the other books may forget to tell you. Everyone makes mistakes. Heaven knows that I’ve made a few in my time. Anyway, when I think of a mistake that people are especially prone to make, I write about the mistake in a Warning icon. Sometimes I want to hire a skywriting airplane crew. “Barry,” says the white smoky cloud, “if you want to compare two numbers, use the double equal sign. Please don’t forget to do this.” Because I can’t afford skywriting, I have to settle for something more modest. I create a Remember icon. Occasionally, I run across a technical tidbit. The tidbit may help you understand what the people behind the scenes (the people who developed Java) were thinking. You don’t have to read it, but you may find it useful. You may also find the tidbit helpful if you plan to read other (more geeky) books about Java. This icon calls attention to useful material that you can find online. (You don’t have to wait long to see one of these icons. I use one at the end of this introduction!) Where to Go from Here If you’ve gotten this far, then you’re ready to start reading about computer programming. Think of me (the author) as your guide, your host, your per- sonal assistant. I do everything I can to keep things interesting and, most importantly, help you understand. If you like what you read, send me a note. My e-mail address, which I created just for comments and questions about this book, is BeginProg@BurdBrain. com. And don’t forget — to get the latest information, visit one of this book’s support Web sites. Mine is at www.BurdBrain.com. The Wiley site is at http://www.dummies.com/go/bpjavafd.
- 26. In this part . . . Y ou have to eat before you can cook. You have to wear before you can sew. You have to ride before you can drive. And you have to run computer programs before you can write computer programs. In this part of the book, you run computer programs.
- 27. Chapter 1 Getting Started In This Chapter What computer programming is all about Understanding the software that enables you write programs Revving up to use an integrated development environment C omputer programming? What’s that? Is it technical? Does it hurt? Is it politically correct? Does Bill Gates control it? Why would anyone want to do it? And what about me? Can I learn to do it? What’s It All About? You’ve probably used a computer to do word processing. Type a letter, print it out, and then send the printout to someone you love. If you have easy access to a computer, then you’ve probably surfed the Web. Visit a page, click a link, and see another page. It’s easy, right? Well, it’s easy only because someone told the computer exactly what to do. If you take a computer right from the factory and give no instructions to this computer, the computer can’t do word processing, the computer can’t surf the Web, it can’t do anything. All a computer can do is follow the instructions that people give to it. Now imagine that you’re using Microsoft Word to write the great American novel, and you come to the end of a line. (You’re not at the end of a sentence, just the end of a line.) As you type the next word, the computer’s cursor jumps automatically to the next line of type. What’s going on here?
- 28. 10 Part I: Revving Up Well, someone wrote a computer program — a set of instructions telling the computer what to do. Another name for a program (or part of a program) is code. Listing 1-1 shows you what some of Microsoft Word’s code may look like. Listing 1-1: A Few Lines in a Computer Program if (columnNumber > 60) { wrapToNextLine(); } else { continueSameLine(); } If you translate Listing 1-1 into plain English, you get something like this: If the column number is greater than 60, then go to the next line. Otherwise (if the column number isn’t greater than 60), then stay on the same line. Somebody has to write code of the kind shown in Listing 1-1. This code, along with millions of other lines of code, makes up the program called Microsoft Word. And what about Web surfing? You click a link that’s supposed to take you directly to Yahoo.com. Behind the scenes, someone has written code of the following kind: Go to <a href=http://www.yahoo.com>Yahoo</a>. One way or another, someone has to write a program. That someone is called a programmer. Telling a computer what to do Everything you do with a computer involves gobs and gobs of code. Take a CD-ROM with a computer game on it. It’s really a CD-ROM full of code. At some point, someone had to write the game program: if (person.touches(goldenRing)) { person.getPoints(10); } Without a doubt, the people who write programs have valuable skills. These people have two important qualities:
- 29. Chapter 1: Getting Started 11 They know how to break big problems into smaller step-by-step procedures. They can express these steps in a very precise language. A language for writing steps is called a programming language, and Java is just one of several thousand useful programming languages. The stuff in Listing 1-1 is written in the Java programming language. Pick your poison This book isn’t about the differences among programming languages, but you should see code in some other languages so you understand the bigger picture. For example, there’s another language, Visual Basic, whose code looks a bit different from code written in Java. An excerpt from a Visual Basic program may look like this: If columnNumber > 60 Then Call wrapToNextLine Else Call continueSameLine End If The Visual Basic code looks more like ordinary English than the Java code in Listing 1-1. But, if you think that Visual Basic is like English, then just look at some code written in COBOL: IF COLUMN-NUMBER IS GREATER THAN 60 THEN PERFORM WRAP-TO-NEXT-LINE ELSE PERFORM CONTINUE-SAME-LINE END-IF. At the other end of the spectrum, you find languages like ISETL. Here’s a short ISETL program, along with the program’s output: {x | x in {0..100} | (exists y in {0..10} | y**2=x)}; {81, 64, 100, 16, 25, 36, 49, 4, 9, 0, 1}; Computer languages can be very different from one another but, in some ways, they’re all the same. When you get used to writing IF COLUMN-NUMBER IS GREATER THAN 60, then you can also become comfortable writing if (columnNumber > 60). It’s just a mental substitution of one set of symbols for another.
- 30. 12 Part I: Revving Up From Your Mind to the Computer’s Processor When you create a new computer program, you go through a multistep process. The process involves three important tools: Compiler: A compiler translates your code into computer-friendly (human-unfriendly) instructions. Virtual machine: A virtual machine steps through the computer-friendly instructions. Application programming interface: An application programming inter- face contains useful prewritten code. The next three sections describe each of the three tools. Translating your code You may have heard that computers deal with zeros and ones. That’s cer- tainly true, but what does it mean? Well, for starters, computer circuits don’t deal directly with letters of the alphabet. When you see the word Start on your computer screen, the computer stores the word internally as 01010011 01110100 01100001 01110010 01110100. That feeling you get of seeing a friendly looking five-letter word is your interpretation of the computer screen’s pixels, and nothing more. Computers break everything down into very low-level, unfriendly sequences of zeros and ones, and then put things back together so that humans can deal with the results. So what happens when you write a computer program? Well, the program has to get translated into zeros and ones. The official name for the transla- tion process is compilation. Without compilation, the computer can’t run your program. I compiled the code in Listing 1-1. Then I did some harmless hacking to help me see the resulting zeros and ones. What I saw was the mishmash in Figure 1-1. The compiled mumbo jumbo in Figure 1-1 goes by many different names: Most Java programmers call it bytecode. I often call it a .class file. That’s because, in Java, the bytecode gets stored in files named SomethingOrOther.class. To emphasize the difference, Java programmers call Listing 1-1 the source code, and refer to the zeros and ones in Figure 1-1 as object code.
- 31. Chapter 1: Getting Started 13 Figure 1-1: My computer understands these zeros and ones, but I don’t. To visualize the relationship between source code and object code, see Figure 1-2. You can write source code, and then get the computer to create object code from your source code. To create object code, the computer uses a special software tool called a compiler. Figure 1-2: The computer compiles source code to create object code. Your computer’s hard drive may have a file named javac or javac.exe. This file contains that special software tool — the compiler. (Hey, how about that? The word javac stands for “Java compiler!”) As a Java programmer, you often tell your computer to build some new object code. Your computer fulfills this wish by going behind the scenes and running the instructions in the javac file. Running code Several years ago, I spent a week in Copenhagen. I hung out with a friend who spoke both Danish and English fluently. As we chatted in the public park, I vaguely noticed some kids orbiting around us. I don’t speak a word of Danish, so I assumed that the kids were talking about ordinary kid stuff.
- 32. 14 Part I: Revving Up Then my friend told me that the kids weren’t speaking Danish. “What lan- guage are they speaking?” I asked. “They’re talking gibberish,” she said. “It’s just nonsense syllables. They don’t understand English, so they’re imitating you.” Now to return to present day matters. I look at the stuff in Figure 1-1, and I’m tempted to make fun of the way my computer talks. But then I’d be just like the kids in Copenhagen. What’s meaningless to me can make perfect sense to my computer. When the zeros and ones in Figure 1-1 percolate through my computer’s circuits, the computer “thinks” the thoughts in Figure 1-3. Everyone knows that computers don’t think, but a computer can carry out the instructions depicted in Figure 1-3. With many programming languages (languages like C++ and COBOL, for example), a computer does exactly what I’m describing. A computer gobbles up some object code, and does whatever the object code says to do. Figure 1-3: What the computer gleans from a bytecode file.
- 33. Chapter 1: Getting Started 15 What is bytecode, anyway? Look at Listing 1-1, and at the listing’s translation Don’t bother to absorb the details in my attempt into bytecode in Figure 1-1. You may be tempted at graphical representation in the figure. It’s not to think that a bytecode file is just a cryptogram — worth your time. The thing you should glean from substituting zeros and ones for the letters in my mix of text, boxes, and arrows is that bytecode words like if and else. But it doesn’t work that (the stuff in a .class file) contains a complete way at all. In fact, the most important part of a description of the operations that the computer is bytecode file is the encoding of a program’s logic. to perform. When you write a computer program, your source code describes an overall strategy — The zeros and ones in Figure 1-1 describe the flow a big picture. The compiled bytecode turns the of data from one part of your computer to another. overall strategy into hundreds of tiny, step-by-step I’ve illustrated this flow in the following figure. But details. When the computer “runs your program,” remember, this figure is just an illustration. Your the computer examines this bytecode and carries computer doesn’t look at this particular figure, or out each of the little step-by-step details. at anything like it. Instead, your computer reads a bunch of zeros and ones to decide what to do next.
- 34. 16 Part I: Revving Up That’s how it works in many programming languages, but that’s not how it works in Java. With Java, the computer executes a different set of instruc- tions. The computer executes instructions like the ones in Figure 1-4. Carry out the first instruction in Figure 1-3. Carry out the second instruction in Figure 1-3. Carry out the third instruction in Figure 1-3. Keep going until you encounter an "If." When you encounter an "If," then decide which of the two alternative paths you should follow. Carry out the instructions in the path that you choose. Figure 1-4: How a computer runs a Java program. The instructions in Figure 1-4 tell the computer how to follow other instructions. Instead of starting with Get columnNumber from memory, the computer’s first instruction is, “Do what it says to do in the bytecode file.” (Of course, in the bytecode file, the first instruction happens to be Get columnNumber from memory.) There’s a special piece of software that carries out the instructions in Figure 1-4. That special piece of software is called the Java virtual machine (JVM). The JVM walks your computer through the execution of some bytecode instruc- tions. When you run a Java program, your computer is really running the Java virtual machine. That JVM examines your bytecode, zero by zero, one by one, and carries out the instructions described in the bytecode. Many good metaphors can describe the Java virtual machine. Think of the JVM as a proxy, an errand boy, a go-between. One way or another, you have the situation shown in Figure 1-5. On the (a) side is the story you get with most programming languages — the computer runs some object code. On the (b) side is the story with Java — the computer runs the JVM, and the JVM follows the bytecode’s instructions.
- 35. Chapter 1: Getting Started 17 a bytecode Jav ject code tual Mac ob Vir a hin Jav e Your Your computer computer Figure 1-5: Two ways to run a computer program. (a) (b) Your computer’s hard drive may have a file named java or java.exe. This file contains the instructions illustrated previously in Figure 1-4 — the instruc- tions in the Java virtual machine. As a Java programmer, you often tell your computer to run a Java program. Your computer fulfills this wish by going behind the scenes and running the instructions in the java file. Code you can use During the early 1980s, my cousin-in-law Chris worked for a computer soft- ware firm. The firm wrote code for word processing machines. (At the time, if you wanted to compose documents without a typewriter, you bought a “computer” that did nothing but word processing.) Chris complained about being asked to write the same old code over and over again. “First, I write a search-and-replace program. Then I write a spell checker. Then I write another search-and-replace program. Then, a different kind of spell checker. And then, a better search-and-replace.” How did Chris manage to stay interested in his work? And how did Chris’s employer manage to stay in business? Every few months, Chris had to rein- vent the wheel. Toss out the old search-and-replace program, and write a new program from scratch. That’s inefficient. What’s worse, it’s boring. For years, computer professionals were seeking the Holy Grail — a way to write software so that it’s easy to reuse. Don’t write and rewrite your search-and- replace code. Just break the task into tiny pieces. One piece searches for a single character, another piece looks for blank spaces, a third piece substitutes one letter for another. When you have all the pieces, just assemble these pieces
- 36. 18 Part I: Revving Up to form a search-and-replace program. Later on, when you think of a new fea- ture for your word processing software, you reassemble the pieces in a slightly different way. It’s sensible, it’s cost efficient, and it’s much more fun. The late 1980s saw several advances in software development, and by the early 1990s, many large programming projects were being written from prefab com- ponents. Java came along in 1995, so it was natural for the language’s founders to create a library of reusable code. The library included about 250 programs, including code for dealing with disk files, code for creating windows, and code for passing information over the Internet. Since 1995, this library has grown to include more than 3,000 programs. This library is called the API — the Application Programming Interface. Every Java program, even the simplest one, calls on code in the Java API. This Java API is both useful and formidable. It’s useful because of all the things you can do with the API’s programs. It’s formidable because the API is so extensive. No one memorizes all the features made available by the Java API. Programmers remember the features that they use often, and look up the features that they need in a pinch. They look up these features in an online document called the API Specification (known affectionately to most Java pro- grammers as the API documentation, or the Javadocs). The API documentation describes the thousands of features in the Java API. As a Java programmer, you consult this API documentation on a daily basis. You can bookmark the documentation at the Sun Microsystems Web site and revisit the site whenever you need to look up something. But in the long run (and in the not-so-long run), you can save time by downloading your own copy of the API docs. (For details, see Chapter 2.) Write Once, Run AnywhereTM When Java first hit the tech scene in 1995, the lan- for each operating system. But with the JVM, guage became popular almost immediately. This just one piece of bytecode works on Windows happened in part because of the Java virtual machines, Unix boxes, Macs, or whatever. This machine. The JVM is like a foreign language inter- is called portability, and in the computer pro- preter, turning Java bytecode into whatever native gramming world, portability is a very precious language a particular computer understands. So commodity. Think about all the people using if you hand my Windows computer a Java byte- computers to browse the Internet. These people code file, then the computer’s JVM interprets the don’t all run Microsoft Windows, but each file for the Windows environment. If you hand person’s computer can have its own bytecode the same Java bytecode file to my colleague’s interpreter — its own Java virtual machine. Macintosh, then the Macintosh JVM interprets The marketing folks at Sun Microsystems call it that same bytecode for the Mac environment. the Write Once, Run AnywhereTM model of com- Look again at Figure 1-5. Without a virtual puting. I call it a great way to create software. machine, you need a different kind of object code
- 37. Chapter 1: Getting Started 19 Your Java Programming Toolset To write Java programs, you need the tools described previously in this chapter: You need a Java compiler. (See the section entitled, “Translating your code.”) You need a Java virtual machine. (See the section entitled, “Running code.”) You need the Java API. (See the section entitled, “Code you can use.”) You need the Java API documentation. (Again, see the “Code you can use” section.) You also need some less exotic tools: You need an editor to compose your Java programs. Listing 1-1 contains part of a computer program. When you come right down to it, a computer program is a big bunch of text. So to write a com- puter program, you need an editor — a tool for creating text documents. An editor is a lot like Microsoft Word, or like any other word processing program. The big difference is that an editor adds no formatting to your text — no bold, no italic, no distinctions among fonts. Computer programs have no formatting whatsoever. They have nothing except plain old let- ters, numbers, and other familiar keyboard characters. You need a way to issue commands. You need a way to say things like “compile this program” and “run the Java virtual machine.” Every computer provides ways of issuing commands. (You can double- click icons or type verbose commands in a Run dialog box.) But when you use your computer’s facilities, you jump from one window to another. You open one window to read Java documentation, another window to edit a Java program, and a third window to start up the Java compiler. The process can be very tedious. In the best of all possible worlds, you do all your program editing, documenta- tion reading, and command issuing through one nice interface. This interface is called an integrated development environment (IDE). A typical IDE divides your screen’s work area into several panes — one pane for editing programs, another pane for listing the names of programs, a third pane for issuing commands, and other panes to help you compose and test programs. You can arrange the panes for quick access. Better yet, if you change the information in one pane, the IDE automatically updates the information in all the other panes.
- 38. 20 Part I: Revving Up Some fancy environments give you point-and-click, drag-and-drop, plug-and- play, hop-skip-and-jump access to your Java programs. If you want your program to display a text box, then you click a text box icon and drag it to the workspace on your screen. Figure 1-6 illustrates the use of a drag-and-drop IDE. In Figure 1-6, I create a program that displays two images, two text fields, and two buttons. To help me create the program, I use the Eclipse IDE with the Jigloo graphical plug-in. (For a taste of Eclipse, visit www.eclipse.org. For more info on the neato Jigloo graphical user interface builder, check out www.cloudgarden.com.) Figure 1-6: Using the Eclipse IDE with the Jigloo graphical user inter- face builder. An IDE helps you move seamlessly from one part of the programming endeavor to another. With an IDE, you don’t have to worry about the mechanics of edit- ing, compiling, and running a Java virtual machine. Instead, you can worry about the logic of writing programs. (Wouldn’t you know it? One way or another, you always have something to worry about!) What’s already on your hard drive? You may already have some of the tools you need for creating Java programs. Here are some examples:
- 39. Chapter 1: Getting Started 21 Most versions of Windows come with a Java virtual machine. Look for a file named java.exe in your windowssystem32 directory. Most computers running Mac OS X come with a Java compiler, a Java virtual machine, and a Java API. Some IDEs come with their own Java tools. For example, when you buy the Borland JBuilder IDE you get a compiler, a Java virtual machine, and a copy of the Java API. When you download the free Eclipse IDE you get a Java compiler, but no Java virtual machine and no Java API. You may already have some Java tools, but your tools may be obsolete. This book’s examples use a relatively new version of Java — a version released in September 2004. Even computers and software sold in 2005 may not be up to date with the latest Java features. So if you use the tools that come with your computer, or if you use a commercial product’s software tools, some of this book’s examples may not run. The safest bet is to download tools afresh from the Sun Microsystems Web site. To get detailed instructions on doing the download, see Chapter 2. Many of this book’s examples don’t run on “older” versions of Java, and by “older” I mean versions created before the fall of 2004. If you have trouble running the programs in this book, check to make sure that your version of Java is numbered 5.0, 5.1, or something like that. Older versions (with version numbers like 1.4 or 1.4.2) just don’t cut the muster. JCreator The programs in this book work with any IDE that can run Java 5.0. You can even run the programs without an IDE. But to illustrate the examples in this book, I use JCreator LE (Lite Edition). I chose JCreator LE over other IDEs for several reasons: JCreator LE is free. Among all the Java IDEs, JCreator represents a nice compromise between power and simplicity. Unlike some other Java IDEs, JCreator works with almost any version of Java, from the ancient version 1.0.2 to the new-and-revolutionary ver- sion 5.0. JCreator LE is free. (It’s worth mentioning twice.)
- 40. 22 Part I: Revving Up This book’s Web site has a special edition of JCreator LE — a version that’s customized especially for Beginning Programming with Java For Dummies, 2nd Edition readers! For details on downloading and installing the special edition of JCreator, see Chapter 2. JCreator runs only on Microsoft Windows. If you’re a Unix, Linux, or Macintosh user, please don’t be offended. All the material in this book applies to you, too. You just have to use a different IDE. My personal recommendations include Eclipse and Netbeans. For details, visit this book’s Web site at http://www. dummies.com/go/bpjavafd.
- 41. Chapter 2 Setting Up Your Computer In This Chapter Downloading and installing a Java compiler Downloading and installing an integrated development environment Configuring the integrated development environment Y ou’ve arrived home, fresh from your local computer store. You have a brightly colored box (a box with software in it). You tear open the box, take out a CD-ROM, and put the CD-ROM into your computer. In a few minutes (or maybe a few hours), you’re off and running. The software is installed and you’re typing your heart out. This scenario is typical for software that you buy at your neighborhood computer store. But what about the software you need to begin writing your own computer programs? This book tells you how to write Java programs, but before you can write Java programs, you need several pieces of software. You need a Java compiler and a Java virtual machine (JVM, for short). You also need the Java API documentation and an integrated development environment (IDE). You can get this software in a brightly colored box, but it’s easier (and cheaper) to download the software from the Web. In fact, all the software you need is free. It comes as a few downloads — some from Sun Microsystems, and another from this book’s Web site. Who needs another brightly colored box anyway? This book’s examples work on any system that supports Java 5.0 or later. If your computer already has a Java 5.0 compiler, you can skip the next section’s steps. But if your computer doesn’t have a Java compiler, or if you use an older version of Java (a version numbered 1.3, 1.4.2, or something like that) then many of this book’s examples won’t work with your current software configu- ration. In that case, you must download and install the latest Java compiler. (Even if your computer already has a Java compiler, it never hurts to down- load and install the latest version.) Just follow the steps in the next section.
- 42. 24 Part I: Revving Up Downloading and Installing the Software You Need If you’ve paid for this book, and you already have a working computer, you’ve already spent all the money you need to spend. All the software you need for learning Java is free for the downloading. Downloading and installing a Java compiler When I want the weather to be sunny, I bring an umbrella to work. Bringing an umbrella tells the weather gods to do the opposite of whatever Barry anticipates. The same kind of thing happens with the Java Web site. If I want someone to redesign the Web site, I just write an article describing exactly how to navigate the site. Sometime between the time of my writing and the date of the article’s publication, the people at Sun Microsystems reorganize the entire Web site. It’s as dependable as the tides. Anyway, the Java Web site is in a constant state of flux. That’s why I don’t put detailed instructions for navigating the Java Web site in this book. Instead, I offer some timeless tips. If this section’s “timeless tips” aren’t specific enough for you, visit this book’s Web site at http://www.dummies.com/go/bpjavafd. At the Web site, you can find up-to-date instructions on getting the software you need. What number comes after 1.4.2_06? The numbering of Java’s versions is really con- some people use the term “Java 1.5” when they fusing. First comes Java 1.0, then Java 1.1, then mean “Java 5.0.”) Java 2 Standard Edition 1.2 (J2SE 1.2). Yes, the The formal name for version 5.0 is “Java 2 “Java 2” numbering overlaps partially with the Platform, Standard Edition 5.0.” And to make mat- “1.x” numbering. ters even worse, the people at Sun Microsystems Next come versions 1.3 and 1.4. After version 1.4.1 are thinking about removing the extra “2.” So after comes version 1.4.2 (with intermediate stops at “Java 2, 5.1” you may see plain old “Java, 5.2.” versions like 1.4.1_02). After 1.4.2_06, the next ver- That’s what happens when a company lets mar- sion is version 5.0. (That’s no misprint. Version 5.0 keting people call the shots. comes immediately after the 1.4 versions, although
- 43. Chapter 2: Setting Up Your Computer 25 With all these disclaimers in mind, you can get a Java compiler by following these steps: 1. Visit java.sun.com/j2se. 2. Look for a Download J2SE link (or something like that). The page may have several J2SE version numbers for you to choose from. You may see links to J2SE 1.4.2, J2SE 5.0, and beyond. If you’re not sure which version you want, choosing the highest version number is probably safe, even if that version number is labeled “Beta.” (The Java beta releases are fairly sturdy.) While you wander around, you may notice links labeled J2EE or J2ME. If you know what these are, and you know you need them, then by all means, download these goodies. But if you’re not sure, then bypass both the J2EE and the J2ME. Instead, follow the J2SE (Java 2 Standard Edition) links. The abbreviation J2EE stands for Java 2 Enterprise Edition and J2ME stands for Java 2 Micro Edition. You don’t need the J2EE or the J2ME to run any of the examples in this book. 3. On the J2SE download page, look for an appropriate download link. A download link is “appropriate” as long as the link refers to J2SE (Java 2 Platform, Standard Edition), to JDK (Java Development Kit), and to your particular operating system (such as Windows, Linux, or Solaris). From all possible links, you may have to choose between links labeled for 32- bit systems and links labeled for 64-bit systems. If you don’t know which to choose, and you’re running Windows, then you probably have a 32-bit system. The Sun Microsystems download page offers you a choice between the JDK (Java Development Kit) and the JRE (Java Runtime Environment). The JDK download contains more than the JRE download, and you need more than that feeble JRE download. You need to download the entire JDK. Sun’s regular J2SE page has links for Windows, Linux, and Solaris users. If your favorite operating system isn’t Windows, Linux, or Solaris, don’t despair. You can probably find an appropriate Java compiler by searching on the Web. If you use Macintosh OS X, go straight to developer.apple. com/java. Java 5.0 comes with OS 10.4. Another choice you may have to make is between an offline and online installation: • With the offline installation, you begin by downloading a 50MB setup file. The file takes up space on your hard drive, but if you ever need to install the JDK again, you have the file on your own computer. Until you update your version of the JDK, you don’t need to download the JDK again.
- 44. 26 Part I: Revving Up • With the online installation, you don’t download a big setup file. Instead, you download a teeny little setup file. Then you download (and discard) pieces of the big 50MB file as you need them. Using online installation saves you 50MB of hard drive space. But, if you want to install the same version of the JDK a second time, you have to redo the whole surf/click/download process. Why would anyone want to install the same version of the JDK a second time? Typically, I have two reasons. Either I want to install the software on a second computer, or I mess something up and have to uninstall (and then reinstall) the software. 4. Download whichever file you chose in Step 3. 5. Execute the file that you’ve downloaded. With offline or online installation you download an executable file onto your computer’s hard drive. Execute this file to begin the JDK installation. 6. During the JDK installation, read the dialog boxes and wizards. Watch for the name of the directory in which the JDK is being installed. On my computer, that directory’s name is c:Program FilesJava jdk1.5.0_01, but on your computer, the name may be slightly different. This directory is called your Java home directory. (Depending on whom you ask, this may also be called the JDK home directory.) Write down the directory’s name, because you’ll need that name for stuff that comes later in this chapter. If you don’t catch the Java home directory’s name during the JDK instal- lation, then search your computer’s hard drive for something named jdksomething-or-other. Write down the directory’s name and keep the name in your back pocket. That’s how you put the Java compiler on your computer. But wait! Don’t walk away from your Web browser yet. At the same java.sun.com Web site, you can find the precious Java API documentation. Downloading and installing the Java API documentation I introduced Java’s API documentation in Chapter 1. Without access to the API documentation, you’re a little lost puppy. With access to the documenta- tion, you’re a powerful Java programmer. So follow this section’s steps to get the API documentation (your very own copy on a computer near you).
- 45. Chapter 2: Setting Up Your Computer 27 1. As in the previous section, visit java.sun.com/j2se and look for a Download J2SE link. 2. Find a link to the API documentation for the version of Java that you just downloaded. The way the Sun Microsystems Web site is currently set up, it’s not too hard to find the API documentation. In fact, the download links for the JDK and the Java API documentation are on the same page. This may not be true by the time you read Beginning Programming with Java For Dummies, but it’s certainly true while I’m writing this book. The download page has a big table with the words Download Java 2 on it. Scroll down in the table, and you find a J2SE Documentation heading with an option to download the docs. A language like Java comes with many sets of docs. The documentation that you want is called the “API documentation,” or the “J2SE documen- tation.” If you see links to the “Java Language Specification” or the “Java Virtual Machine Specification,” just ignore these links for now. 3. Download the API documentation. When the download is finished, you have a big ZIP file on your computer’s hard drive. If you use Windows XP or some other ZIP-friendly operating system, you can just double-click the ZIP file’s icon. Your operating system opens the file as if it’s an ordinary directory. If you have Windows 98, Windows 2000, or some other system that doesn’t recognize ZIP files, you need an additional archive handling program. You can find a bunch of these programs by searching on the Web. 4. Extract the API documentation to your Java home directory. The downloaded ZIP file is like a directory on your hard drive. The file contains another directory named docs. Just copy that docs directory (and all of its contents) to your Java home directory. By the time you’re done, you have a Java home directory (with a name like jdk1.5.0_01) and a docs directory immediately inside the Java home directory. (See Figure 2-1.) I give this docs directory a special name. I call it your JavaDoc directory. Figure 2-1: The docs subdirectory of your Java home directory.
- 46. 28 Part I: Revving Up Two bags of goodies Sun’s Web site bundles the basic Java tools in Java virtual machine, and the Application two different ways: Programming Interface. With the JDK, you can create and run your own Java programs. The Java Runtime Environment (JRE): This bundle includes a Java virtual machine and Another name for the JDK is the Java SDK — the the Application Programming Interface. (See Java Software Development Kit. Some people still Chapter 1.) With the JRE, you can run existing use the SDK acronym, even though the folks at Java programs. That’s all. You can’t create Sun Microsystems don’t use it anymore. (Actually, new Java programs, because you don’t have the original name was the JDK. Later Sun changed a Java compiler. it to the SDK. A few years after that, Sun changed back to the name JDK. As an author, this con- The Java Development Kit (JDK): This bundle stant naming and renaming drives me crazy.) includes three tools — a Java compiler, a Downloading and installing the JCreator integrated development environment In the previous sections, you get all the tools your computer needs for pro- cessing Java programs. This section is different. In this section you get the tool that you need for composing and testing your Java programs. You get JCreator — an integrated development environment for Java. JCreator runs only on Microsoft Windows systems. If you use Linux, Unix, Macintosh, or some other non-Windows system, visit this book’s Web site for further instructions. If you have experience installing software, then downloading and installing JCreator is a routine procedure. Here’s what you do: 1. Look for the JCreator download link on this book’s Web site. 2. Click the download link, and save the file to your computer’s hard drive. Like the Java API documentation, the JCreator installation comes to you as a compressed ZIP file. (See Step 3 in the section entitled “Downloading and installing the Java API documentation.”) 3. Unzip the JCreator installation file. You can extract the file’s contents to any directory on your hard drive. (Just make sure you remember the directory’s name.) 4. Open My Computer on your Windows desktop.
- 47. Chapter 2: Setting Up Your Computer 29 5. From My Computer navigate to whatever directory contains extracted contents of JCreator’s installation file. The directory contains a file named Setup.exe (or just plain Setup). 6. Double-click the Setup file’s icon. In response, the computer fires up JCreator’s installation wizard. 7. Follow the instructions in JCreator’s installation wizard. In the end, the installation wizard may offer to launch JCreator for you. (Alternatively, you can scan your Start menu for a new JCreator folder.) One way or another, you start running JCreator. Running JCreator for the First Time The first time you run JCreator, the program asks for some configuration information. Just follow these steps: 1. If you haven’t already done so, launch JCreator. The JCreator Setup Wizard appears on your screen. The wizard’s first page is for File Associations. 2. Accept the File Associations defaults and click Next. The wizard’s next page (the JDK Home Directory page) appears. 3. Look at the text field on the JDK Home Directory page. Make sure that this field displays the name of your Java home directory. (See Figure 2-2.) Figure 2-2: Confirming the location of your Java home directory.
- 48. 30 Part I: Revving Up If the wrong directory name appears in the text field, just click the Browse button and navigate to your computer’s Java home directory. For information on your computer’s Java home directory, see Step 6 of this chapter’s “Downloading and installing a Java compiler” section. 4. When you’re happy with the name in the home directory text field, click Next. The wizard’s last page (the JDK JavaDoc Directory page) appears. 5. Look at the text field on the JDK JavaDoc Directory page. Make sure that this field displays the name of your JavaDoc directory. (See Figure 2-3.) Figure 2-3: Confirming the location of your JavaDoc directory. Normally, your JavaDoc directory’s name is the name of your Java home directory, followed by docs. For information on your computer’s JavaDoc directory, see Step 4 of this chapter’s “Downloading and installing the Java API documentation” section. If the wrong directory name appears in the text field, just click the Browse button and navigate to your computer’s JavaDoc directory. If you do anything wrong in Steps 2 through 5, don’t fret. You can correct your mistake later. See this book’s Web site for details. 6. Click Finish. At this point, the JCreator work area opens. (See Figure 2-4.)
- 49. Chapter 2: Setting Up Your Computer 31 Figure 2-4: JCreator’s work area. In JCreator’s help files, the stuff in Figure 2-4 is called the workspace, not the work area. But elsewhere in these help files, JCreator reuses the word work- space to mean something entirely different. To avoid any confusion, I use two different terms. I use work area for the stuff in Figure 2-4, and I use workspace for that other, entirely different thing. (I explain that entirely different thing in Chapter 3.) Is That All There Is to It? If you’re reading this paragraph, you’ve probably followed some of the instructions in this chapter — instructions for installing a Java compiler, the Java API documentation, and the JCreator IDE on your computer. So the burn- ing question is, have you done the installation correctly? The answer to that question lies in Chapter 3, because in that chapter, you use these tools to run a brand new computer program.
- 50. 32 Part I: Revving Up
- 51. Chapter 3 Running Programs In This Chapter Compiling and running a program Working with a workspace Editing your own Java code I f you’re a programming newbie, for you, running a program probably means clicking a mouse. You want to run Internet Explorer. So you double-click the Internet Explorer icon, or maybe you choose Internet Explorer from the Start menu. That’s all there is to it. When you create your own programs, the situation is a bit different. With a new program, the programmer (or someone from the programmer’s company) creates the icons. Before that, a perfectly good program may not have an icon at all. So what do you do with a brand new Java program? How do you get the program to run? This chapter tells you what you need to know. Running a Canned Java Program The best way to get to know Java is to do Java. When you’re doing Java, you’re writing, testing, and running your own Java programs. This section prepares you by describing how you run and test a program. Instead of writing your own program, you run a program that I’ve already written for you. The program calculates your monthly payments on a home mortgage loan. The mortgage-calculating program doesn’t open its own window. Instead, the program runs in JCreator’s General Output pane. (See Figure 3-1.) A program that operates completely in this General Output pane is called a text-based program.
- 52. 34 Part I: Revving Up Figure 3-1: A run of the text-based mortgage program. If you’re using Linux, Unix, Mac, or some other non-Windows system, the instructions in this section don’t apply to you. Visit this book’s Web site at http://www.dummies.com/go/bpjavafd for an alternative set of instructions. Actually, as you run the mortgage program, you see two things in the General Output pane: Messages and results that the mortgage program sends to you. Messages include things like How much are you borrowing? Results include lines like Your monthly payment is $552.20. Responses that you give to the mortgage program while it runs. If you type 100000.00 in response to the program’s question about how much you’re borrowing, you see that number echoed in the General Output pane. Those pesky filename extensions The filenames displayed in My Computer or in an In Windows 95, 98, or NT: In the Windows Open dialog box can be misleading. You may visit Explorer menu bar, choose View➪Folder the Chapter03_Example01 directory and see Options (or just View➪Options). the name Mortgage. Instead of just Mortgage, In Windows Me or 2000: Choose Start➪ the file’s full name is Mortgage.java. You may Settings➪Control Panel➪Folder Options. even see two Mortgage files. What you don’t see is that one file’s real name is Mortgage.java, In Windows XP with the control panel’s de- and the other file’s real name is Mortgage. fault (category) view: Choose Start➪Control class. Panel➪Performance and Maintenance➪ File Types. The ugly truth is that Windows and its dialog boxes can hide parts of filenames. This awful In Windows XP with the control panel’s feature tends to confuse Java programmers. classic view: Choose Start➪Control Panel➪ So, if you don’t want to be confused, modify the Folder Options. Windows Hide Extensions feature. To do this, In the Folder Options dialog box, click the View you have to open the Folder Options dialog box. tab. Then look for the Hide File Extensions for Here’s how: Known File Types option. Make sure that this check box is not selected.
- 53. Chapter 3: Running Programs 35 Running the mortgage program is easy. Here’s how you do it: 1. Make sure that you’ve followed the instructions in Chapter 2 — instructions for installing the JDK and configuring JCreator. Thank goodness! You don’t have to follow those instructions more than once. 2. Launch JCreator. The big JCreator work area stares at you from your computer screen. (See Figure 3-2.) If this is your first time running JCreator, you don’t see JCreator’s work area. Instead you see the JCreator Setup Wizard. To get past the Setup Wizard, see the instructions in Chapter 2. 3. In JCreator’s menu bar, choose File➪Open Workspace from the main menu. Don’t choose File➪Open. Instead, choose File➪Open Workspace. A familiar-looking Open dialog box appears. This dialog box looks in your MyProjects directory. This MyProjects directory is a subdirectory of the directory in which JCreator is installed. Figure 3-2: An empty work area.
- 54. 36 Part I: Revving Up In this book’s special edition of JCreator the MyProjects directory has subdirectories named Chapter03_Example01, Chapter04_Listing01 and so on. The MyProjects directory also has files with names like Chapter03 and Chapter04. If you set your computer so that it doesn’t hide file extensions, then the names of the files are Chapter03.jcw, Chapter04.jcw, and so on. (See the sidebar entitled “Those pesky file- name extensions.”) 4. Select the file named Chapter03 (or Chapter03.jcw), and then click Open. Clicking Open may coax out a message box asking if you want to “Save the workspace modifications?” If so, click Yes. Clicking Open may coax out another box asking if you want to “. . . close all document Windows?” If so, click Yes. JCreator divides things into workspaces. Each workspace is further subdivided into projects. To organize this book’s examples, I made a workspace for each chapter, and then made a project for each complete Java program. When you open Chapter03.jcw, you get my Chapter03 workspace — a workspace that contains two projects. The project’s names are Chapter03_Example01 and Chapter03_Listing01. That’s why, in JCreator’s File View pane, you see a Chapter03 tree with branches labeled Chapter03_Example01 and Chapter03_Listing01. (See Figure 3-3.) In MyWorkspace.jcw, the extension .jcw stands for “JCreator workspace.” Figure 3-3: Two projects in the File View pane. 5. In the File View’s tree, right-click the Chapter03_Example01 branch. In the resulting context menu, choose Sets as Active Project. (See Figure 3-3.) Choosing Sets as Active Project makes Chapter03_Example01 the active project.
- 55. Chapter 3: Running Programs 37 In JCreator, only one project at a time can be the active project. To run a particular program, the program’s code has to be in whatever project is currently active. In JCreator’s File View, you can tell which project is active by looking for the project whose name is boldface. (Refer to Figure 3-3.) On some systems, the active project’s name is a hazy, light- gray boldface. If a particular program isn’t in the active project, you can’t run that pro- gram, but you can do some other things with that program. For example, you can see the program in one of JCreator’s panes, make changes to the program, save the program, and so on. For this reason, it’s really easy to get confused and forget which project is active. So always keep the active project in the forefront of your mind. If your code doesn’t do what you think it should do, check to make sure that the project you want to run is the active project. 6. Choose Build➪Compile Project from the main menu. Choosing Compile Project does exactly what it says. It compiles the pro- ject’s code. (To find out what compile means, see Chapter 1.) After some pleasant chirping sounds from your hard drive, JCreator’s Build Output pane displays a Process completed message. (The Build Output pane appears in the lower portion of JCreator’s work area. See Figure 3-4.) Figure 3-4: The compiling process is completed. 7. Choose Build➪Execute Project from the main menu. When you choose Execute Project, the computer runs the project’s code. (In this example, the computer runs a Java program that I wrote.) As part of the run, the message How much are you borrowing? appears in JCreator’s General Output pane. (The General Output pane and the Build Output pane share the lower portion of JCreator’s work area. Refer to Figure 3-1.) 8. Click anywhere inside JCreator’s General Output pane, and then type a number, like 100000.00, and press Enter.
- 56. 38 Part I: Revving Up When you type a number in Step 8, don’t include your country’s currency symbol. (U.S. residents, don’t type a dollar sign.) Things like $100000.00 cause the program to crash. You see a NumberFormatException mes- sage in the General Output pane. After you press Enter, the Java program displays another message (What’s the interest rate?) in JCreator’s General Output pane. 9. In response to the interest rate question, type a number, like 5.25, and press Enter. After you press Enter, the Java program displays another message (How many years . . . ?) in JCreator’s General Output pane. 10. Type a number, like 30, and press Enter. In response to the numbers that you’ve typed, the Java program dis- plays a monthly payment amount. Again, refer to Figure 3-1. Disclaimer: Your local mortgage company charges more than the amount that my Java program calculates. (A lot more.) When you type a number in Step 10, don’t include a decimal point. Things like 30.0 cause the program to crash. You see a NumberFormatException message in the General Output pane. Occasionally you decide in the middle of a program’s run that you’ve made a mistake of some kind. You want to stop the program’s run dead in its tracks. To do this, choose Tools➪Stop Tool from the main menu. If you follow this section’s instructions, and you don’t get the results that I describe, there are three things you can try. I list them in order from best to worst: Check all the steps to make sure you did everything correctly. Send email to me at BeginProg2@BurdBrain.com. If you describe what happened, I can probably figure out what went wrong and tell you how to correct the problem. Panic. Typing and Running Your Own Code The first half of this chapter is about running someone else’s Java code (code that you download from this book’s Web site). But eventually, you’ll write code on your own. This section shows you how to create code with the JCreator development environment.
- 57. Chapter 3: Running Programs 39 Do I see formatting in my Java program? When you use an editor to write a Java program, program file. Instead, the editor displays each you may notice words in various colors. Certain word in a way that makes the Java program words are always blue. Other words are always easy to read. black. You may even see some bold and italic For example, in a Java program, certain words phrases. You may think you see formatting, but (words like class, public, and void) have you don’t. Instead, what you see is called syntax their own special meanings. So JCreator’s editor coloring or syntax highlighting. displays class, public, and void in blue No matter what you call it, the issue is as follows: letters. When I save my Java program file, the computer stores nothing about blue letters in With Microsoft Word, things like bold for- my Java program file. But the editor uses its matting are marked inside a document. discretion to highlight special words with blue When you save MyPersonalDiary.doc, coloring. the instructions to make the words “love” and “hate” bold are recorded inside the Some other editor may display the same words MyPersonalDiary.doc file. in a bold, red font. Another editor (like Windows Notepad) displays all words in plain old black. With a Java program editor, things like bold and coloring aren’t marked inside the Java The version of JCreator that you download from this book’s Web site has a specially customized MyProjects directory. The MyProjects directory contains several readymade workspaces. One of these workspaces (named MyWorkspace) has no projects in it. Here’s how you create a project in MyWorkspace: 1. Launch JCreator. 2. From JCreator’s menu bar, choose File➪Open Workspace. An Open dialog box appears. 3. In the Open dialog box, select MyWorkspace.jcw (or simply MyWorkspace). Then click Open. Clicking Open may coax out a message box asking whether you want to “Save the workspace modifications?” If so, click Yes. Clicking Open may coax out another box asking if you want to “. . . close all document Windows?” If so, click Yes. After clicking Open, you see MyWorkspace in JCreator’s File View pane. The next step is to create a new project within MyWorkspace.
- 58. 40 Part I: Revving Up 4. In the File View pane, right-click MyWorkspace. Then choose Add new Project from the context menu that appears, as shown in Figure 3-5. Figure 3-5: Getting JCreator to add a new project. JCreator’s Project Wizard opens. (See Figure 3-6.) Figure 3-6: The Project Wizard’s Project Template page. 5. On the wizard’s Project Template page, select the Empty Project icon, and then click Next. After clicking Next, you see the wizard’s Project Paths page, as shown in Figure 3-7. 6. In the Name field, type MyFirstProject. You can add blank spaces, making the name My First Project, but I don’t recommend it. In fact, having a blank space in any name (a workspace name, a project name, a filename, or whatever) is generally a bad idea. 7. Make sure that the Add to Current Workspace radio button is selected, and then click Finish. If you click Next instead of Finish, you see some other options that you don’t need right now. So to avoid any confusion, just click Finish.
- 59. Chapter 3: Running Programs 41 Figure 3-7: The Project Wizard’s Project Paths page. Clicking Finish brings you back to JCreator’s work area, with MyFirst Project set in bold. The bold typeface means that MyFirstProject is the active project. The next step is to create a new Java source code file. 8. In the File View pane, right-click MyFirstProject. Then choose Add➪ New Class from the context menu that appears, as shown in Figure 3-8. Figure 3-8: Getting JCreator to add a new class. JCreator’s Class Wizard opens. (See Figure 3-9.) Like every other windowed environment, JCreator provides many ways to accomplish the same task. Instead of right-clicking MyFirstProject and choosing Add➪New Class, you can start at the menu bar and choose File➪New➪Class. But right-clicking a project has a small benefit. If you right-click the name of a project, the newly created class is without a doubt in that project. If you use the menu bar instead, the newly created class goes in whichever project happens to be the active project. So if your workspace contains many projects, you can accidentally put the new class into the wrong project.
- 60. 42 Part I: Revving Up Figure 3-9: The Class Wizard’s Class Settings page. 9. In the Class Wizard’s Name field, type the name of your new class. In this example, use the name MyFirstJavaClass, with no blank spaces between any of the words in the name. (Refer to Figure 3-9.) The name in the Class Wizard must not have blank spaces. And the only allowable punctuation symbol is the underscore character (_). You can name your class MyFirstJavaClass or My_First_Java_Class, but you can’t name it My First Java Class, and you can’t name it JavaClass,MyFirst. 10. Put a checkmark in the Generate Main Method check box. I created most of this book’s examples without putting a checkmark in the Generate Main Method check box. But for this example, just this once, putting a checkmark in the Generate Main Method check box is very helpful. 11. Skip everything in the Class Wizard except the Name field and the Generate Main Method check box. (In other words, click Finish.) Clicking Finish brings you back to JCreator’s work area. Now the Editor pane has a tab named MyFirstJavaClass.java. For your convenience, the MyFirstJavaClass.java tab already has some code in it. (See Figure 3-10.) Figure 3-10: JCreator writes some code in the Editor pane.
- 61. Chapter 3: Running Programs 43 12. Replace an existing line of code in your new Java program. Type a line of code in JCreator’s Editor pane. Replace the line // TODO: Add your code here with the line System.out.println(“Chocolate, royalties, sleep”); Copy the new line of code exactly as you see it in Listing 3-1. • Spell each word exactly the way I spell it in Listing 3-1. • Capitalize each word exactly the way I do in Listing 3-1. • Include all the punctuation symbols — the dots, the quotation marks, the semicolon, everything. Listing 3-1: A Program to Display the Things I Like class MyFirstJavaClass { /** * Method main * * * @param args * */ public static void main(String[] args) { System.out.println(“Chocolate, royalties, sleep”); } } Java is case-sensitive, which means that system.out.printLn isn’t the same as System.out.println. If you type system.out.printLn, you’re program won’t work. Be sure to capitalize your code eXactLy as it is in Listing 3-1. 13. From the menu bar, choose Build➪Compile Project. If you typed everything correctly, you see the comforting Process com- pleted message, with no error messages, at the bottom of JCreator’s work area. The text appears in JCreator’s Build Output pane in the lower portion of JCreator’s work area. (Refer to Figure 3-4.) When you choose Build➪Compile Project, JCreator compiles whichever project is currently active. Only one project at a time is active. So if your workspace contains several projects, make sure that the project you want to compile is currently the active project.
- 62. 44 Part I: Revving Up 14. Check for error messages at the bottom of JCreator’s work area. If, in Step 12, you don’t type the code exactly as it’s shown in Listing 3-1, then in this step you get error messages in JCreator’s Task List pane. (Like so many other things, the Task List pane appears in the lower portion of JCreator’s work area. See Figure 3-11.) Figure 3-11: An error message in the Task List pane. Each error message refers to a specific place in your Java code. To jump the cursor to that place in the Editor pane, double-click the message in the Task List pane. Compare everything you see, character by character, with my code in Listing 3-1. Don’t miss a single detail, including spelling, punctuation, and uppercase versus lowercase. 15. Make any changes or corrections to the code in the Editor pane. Then repeat Steps 13 and 14. When at last you see the Process completed message with no error messages, you’re ready to run the program. 16. From the menu bar choose Build➪Execute Project. That does the trick. Your new Java program runs in JCreator’s General Output pane. If you’re running the code in Listing 3-1, you see the Chocolate, royalties, sleep message in Figure 3-12. It’s like being in heaven! Figure 3-12: Running the program in Listing 3-1.
- 63. Part II Writing Your Own Java Programs
- 64. In this part . . . T his part features some of the world’s simplest pro- grams. And, as simple as they are, these programs illustrate the fundamental ideas behind all computer code. The ideas include things such as variables, values, types, statements, methods, and lots of other important stuff. This part of the book is your springboard, your launch pad, your virtual catapult.
- 65. Chapter 4 Exploring the Parts of a Program In This Chapter Identifying the words in a Java program Using punctuation and indentation Understanding Java statements and methods I work in the science building at a liberal arts college. When I walk past the biology lab, I always say a word of thanks under my breath. I’m thankful for not having to dissect small animals. In my line of work, I dissect computer programs instead. Computer programs smell much better than preserved dead animals. Besides, when I dissect a program, I’m not reminded of my own mortality. In this chapter, I invite you to dissect a program with me. I have a small pro- gram, named ThingsILike. I cut apart the program, and carefully investigate the program’s innards. Get your scalpel ready. Here we go! Checking Out Java Code for the First Time I have a confession to make. The first time I look at somebody else’s computer program, I feel a bit queasy. The realization that I don’t understand some- thing (or many things) in the code makes me nervous. I’ve written hundreds (maybe thousands) of programs, but I still feel insecure when I start reading someone else’s code. The truth is, learning about a computer program is a bootstrapping experience. First I gawk in awe of the program. Then I run the program to see what it does. Then I stare at the program for a while, or read someone’s explanation of the program and its parts. Then I gawk a little more and run the program again. Eventually, I come to terms with the program. Don’t believe the wise guys who say they never go through these steps. Even the experienced programmers approach a new project slowly and carefully.
- 66. 48 Part II: Writing Your Own Java Programs Behold! A program! In Listing 4-1, you get a blast of Java code. Like all novice programmers, you’re expected to gawk humbly at the code. But don’t be intimidated. When you get the hang of it, programming is pretty easy. Yes, it’s fun too. Listing 4-1: A Simple Java Program /* * A program to list the good things in life * Author: Barry Burd, BeginProg2@BurdBrain.com * February 13, 2005 */ class ThingsILike { public static void main(String args[]) { System.out.println(“Chocolate, royalties, sleep”); } } When I run the program in Listing 4-1, I get the result shown in Figure 4-1: The computer displays the words Chocolate, royalties, sleep on the screen. Now I admit that writing and running a Java program is a lot of work just to get the words Chocolate, royalties, sleep to appear on some- body’s computer screen, but every endeavor has to start somewhere. Figure 4-1: Running the program in Listing 4-1. You can run the code in Listing 4-1 on your computer. Here’s how: 1. Follow the instructions in Chapter 2 for installing the special edition of JCreator (the edition that you find on this book’s Web site). 2. Next follow the instructions in the first half of Chapter 3. Those instructions tell you how to run the Chapter03_Example01 project, which is in the Chapter03 workspace. To run the code in Listing 4-1, select the Chapter04_Listing01 project in the Chapter04 workspace.
- 67. Chapter 4: Exploring the Parts of a Program 49 What the program’s lines say If the program in Listing 4-1 ever becomes famous, someone will write a Cliffs Notes book to summarize the program. The book will be really short, because you can summarize the action of Listing 4-1 in just one sentence. Here’s the sentence: Display Chocolate, royalties, sleep on the computer screen. Now compare the sentence above with the bulk in Listing 4-1. Because Listing 4-1 has so many more lines, you may guess that Listing 4-1 has lots of boiler- plate code. Well, your guess is correct. You can’t write a Java program without writing the boilerplate stuff but, fortunately, the boilerplate text doesn’t change much from one Java program to another. Here’s my best effort at summarizing all the Listing 4-1 text in 57 words or less: This program lists the good things in life. Barry Burd wrote this program on February 13, 2005. Barry realizes that you may have questions about this code, so you can reach him at BeginProg2@BurdBrain.com. This code defines a Java class named ThingsILike. Here’s the main starting point for the instructions: Display Chocolate, royalties, sleep on the screen. The rest of this chapter (about 4,500 more words) explains the Listing 4-1 code in more detail. The Elements in a Java Program That both English and Java are called languages is no coincidence. You use a language to express ideas. English expresses ideas to people, and Java expresses ideas to computers. What’s more, both English and Java have things like words, names, and punctuation. In fact, the biggest difference between the two languages is that Java is easier to learn than English. (If English were easy, then computers would understand English. Unfortunately, they can’t.) Take an ordinary English sentence and compare it with the code in Listing 4-1. Here’s the sentence: Suzanne says “eh” because, as you know, she lives in Canada.
- 68. 50 Part II: Writing Your Own Java Programs In your high school grammar class, you worried about verbs, adjectives, and other such things. But in this book, you’ll think in terms of keywords and identifiers, as summarized in Figure 4-2. Suzanne’s sentence has all kinds of things in it. They’re the same kinds of things that you find in a computer program. So here’s the plan: Compare the elements in Figure 4-1 with similar elements in Listing 4-1. You already under- stand English, so you use this understanding to figure out some new things about Java. But first, here’s a friendly reminder: In the next several paragraphs, I draw comparisons between English and Java. As you read these paragraphs, it’s important to keep an open mind. For example, in comparing Java with English, I may write that “names of things aren’t the same as dictionary words.” Sure, you can argue that some dictionaries list proper nouns, and that some people have first names like Hope, Prudence, and Spike, but please don’t. You’ll get more out of the reading if you avoid nitpicking. Okay? Are we still friends? Then read on. Figure 4-2: The things you find in a simple sentence. Keywords A keyword is a dictionary word — a word that’s built right into a language. In Figure 4-2, a word like “says” is a keyword, because “says” plays the same role whenever it’s used in an English sentence. The other keywords in the Suzanne sentence are “because,” “as,” “you,” “know,” “she,” “lives,” and “in.” Computer programs have keywords, too. In fact, the program in Listing 4-1 uses four of Java’s keywords (shown in bold):
- 69. Chapter 4: Exploring the Parts of a Program 51 class ThingsILike { public static void main(String args[]) { Each Java keyword has a specific meaning — a meaning that remains unchanged from one program to another. For example, whenever I write a Java program, the word public always signals a part of the program that’s accessible to any other piece of code. The Java programming language is case-sensitive. This means that if you change a lowercase letter in a word to an uppercase letter, you change the word’s meaning. Changing case can make the entire word go from being meaningful to being meaningless. In Listing 4-1, you can’t replace public with Public. If you do, the whole program stops working. This chapter has little or no detail about the meanings of the keywords class, public, static, and void. You can peek ahead at the material in other chapters, but you can also get along by cheating. When you write a program, just start with class SomethingOrOther { and then paste the text public static void main(String args[]) { into your code. In your first few programs, this strategy serves you well. The Cheat Sheet in the front of this book has a complete list of Java keywords. Here’s one thing to remember about keywords: In Java, each keyword has an official, predetermined meaning. The people at Sun Microsystems, who have the final say on what constitutes a Java program, have created all of Java’s keywords. You can’t make up your own meaning for any of the Java keywords. For example, you can’t use the word public in a calculation: //This is BAD, BAD CODE: public = 6; If you try to use a keyword this way, then the compiler displays an error message and refuses to translate your source code. It works the same way in English. Have a baby, and name it “Because.” “Let’s have a special round of applause for tonight’s master of ceremonies — Because O. Borel.” You can do it, but the kid will never lead a normal life.
- 70. 52 Part II: Writing Your Own Java Programs Identifiers that you or I can define I like the name Suzanne, but if you don’t like traditional names, then make up a brand new name. You’re having a new baby. Call her “Deneen” or “Chrisanta.” Name him “Belton” or “Merk.” A name is a word that identifies something, so I’ll stop calling these things names and start calling them identifiers. In computer programming, an identi- fier is a noun of some kind. An identifier refers to a value, a part of a program, a certain kind structure, or any number of things. Listing 4-1 has two identifiers that you or I can define on our own. They’re the made-up words ThingsILike and args. class ThingsILike { public static void main(String args[]) { Just as the names Suzanne and Chrisanta have no special meaning in English, so the names ThingsILike and args have no special meaning in Java. In Listing 4-1, I use ThingsILike for the name of my program, but I could also have used a name like GooseGrease, Enzyme, or Kalamazoo. I have to put (String someName[]) in my program, but I could use (String args[]), (String commandLineArguments[]), or (String cheese[]). Do as I say, not as I do. Make up sensible, informative names for the things in your Java programs. Names like GooseGrease are cute, but they don’t help you keep track of your program-writing strategy. When I name my Java program, I can use ThinksILike or GooseGrease, but I can’t use the word public. Words like class, public, static, and void are keywords in Java. The args in (String args[]) holds anything extra that you type when you issue the command to run a Java program. For example, if you get the pro- gram to run by typing java ThingsILike won too 3, then args stores the extra values won, too, and 3. As a beginning programmer, you don’t need to think about this feature of Java. Just paste (String args[]) into each of your programs. Identifiers with agreed upon meanings Many people are named Suzanne, but only one country is named Canada. That’s because there’s a standard, well-known meaning for the word “Canada.” It’s the country with a red maple leaf on its flag. If you start your own country,
- 71. Chapter 4: Exploring the Parts of a Program 53 you should avoid naming it Canada, because naming it Canada would just confuse everyone. (I know, a town in Kentucky is named Canada, but that doesn’t count. Remember, you should ignore exceptions like this.) Most programming languages have identifiers with agreed upon meanings. In Java, almost all of these identifiers are defined in the Java API. Listing 4-1 has five such identifiers. They’re the words main, String, System, out, and println: public static void main(String args[]) { System.out.println(“Chocolate, royalties, sleep”); } Here’s a quick rundown on the meaning of each of these names (more detailed descriptions appear throughout this book): main: The main starting point for execution in every Java program. String: A bunch of text; a row of characters, one after another. System: A canned program in the Java API. (This program accesses some features of your computer that are outside the direct control of the Java virtual machine.) out: The place where a text-based program displays its text. (For a pro- gram running in JCreator, the word out represents the General Output pane. To read more about text-based programs, check the first several paragraphs of Chapter 3.) println: Display text on your computer screen. Strictly speaking, the meanings of the identifiers in the Java API are not cast in stone. Although you can make up your own meanings for the words like System or println, this isn’t a good idea. If you did, you would confuse the dickens out of other programmers, who are used to the standard API mean- ings for these familiar identifier names. Literals A literal is a chunk of text that looks like whatever value it represents. In Suzanne’s sentence (refer to Figure 4-2), “eh” is a literal, because “eh” refers to the word “eh.” Programming languages have literals too. For example, in Listing 4-1, the stuff in quotes is a literal: System.out.println(“Chocolate, royalties, sleep”);
- 72. 54 Part II: Writing Your Own Java Programs When you run the ThingsILike program, you see the words Chocolate, royalties, sleep on the screen. In Listing 4-1, the text “Chocolate, royalties, sleep” refers to these words, exactly as they appear on the screen (minus the quotation marks). Most of the numbers that you use in computer programs are literals. If you put the statement mySalary = 1000000.00; in a computer program, then 1000000.00 is a literal. It stands for the number 1000000.00 (one million). Punctuation A typical computer program has lots of punctuation. For example, consider the program in Listing 4-1: class ThingsILike { public static void main(String args[]) { System.out.println(“Chocolate, royalties, sleep”); } } Each bracket, each brace, each squiggle of any kind plays a role in making the program meaningful. In English, you write all the way across one line, and then you wrap your text to the start of the next line. In programming, you seldom work this way. Instead, the code’s punctuation guides the indenting of certain lines. The indentation shows which parts of the program are subordinate to which other parts. It’s as if, in English, you wrote Suzanne’s sentence like this: Suzanne says “eh” because , as you know , she lives in Canada. The diagrams in Figures 4-3 and 4-4 show you how parts of the ThingsILike program are contained inside other parts. Notice how a pair of curly braces acts like a box. To make the program’s structure be visible at a glance, you indent all the stuff inside of each box.
- 73. Chapter 4: Exploring the Parts of a Program 55 Figure 4-3: A pair of curly braces acts like a box. Figure 4-4: The ideas in a computer program are nested inside of one another. I can’t emphasize this point enough. If you don’t indent your code, or if you indent but you don’t do it carefully, then your code still compiles and runs correctly. But this successful run gives you a false sense of confidence. The minute you try to update some poorly indented code, you become hopelessly confused. So take my advice: Keep your code carefully indented at every step in the process. Make your indentation precise, whether you’re scratching out a quick test program, or writing code for a billionaire customer.
- 74. 56 Part II: Writing Your Own Java Programs Comments A comment is text that’s outside the normal flow. In Figure 4-2, words “A com- ment:” aren’t part of the Suzanne sentence. Instead, these words are about the Suzanne sentence. The same is true of comments in computer programs. The first five lines in Listing 4-1 form one big comment. The computer doesn’t act on this comment. There are no instructions for the computer to perform inside this comment. Instead, the comment tells other programmers something about your code. Comments are for your own benefit, too. Imagine that you set aside your code for a while and work on something else. When you return later to work on the code again, the comments help you remember what you were doing. The Java programming language has three different kinds of comments: Traditional comments: The comment in Listing 4-1 is a traditional comment. The comment begins with /* and ends with */. Everything between the opening /* and the closing */ is for human eyes only. Nothing between /* and */ gets translated by the compiler. The second, third, and fourth lines in Listing 4-1 have extra asterisks. I call them “extra” because these asterisks aren’t required when you create a comment. They just make the comment look pretty. I include them in Listing 4-1 because, for some reason that I don’t entirely under- stand, most Java programmers add these extra asterisks. End-of-line comments: Here’s some code with end-of-line comments: class ThingsILike { //One thing is miss- ing public static void main(String args[]) { System.out.println(“Royalties, sleep”); //Chocolate } } An end-of-line comment starts with two slashes, and goes to the end of a line of type. You may hear programmers talk about “commenting out” certain parts of their code. When you’re writing a program, and something’s not work- ing correctly, it often helps to try removing some of the code. If nothing else, you find out what happens when that suspicious code is removed. Of course, you may not like what happens when the code is removed, so you don’t want to delete the code completely. Instead, you turn your ordinary Java statements into comments. For example, turn System.out. println(“Sleep”); into /* System.out.println(“Sleep”); */.
- 75. Chapter 4: Exploring the Parts of a Program 57 This keeps the Java compiler from seeing the code while you try to figure out what’s wrong with your program. Javadoc comments: A special Javadoc comment is any traditional com- ment that begins with an extra asterisk. /** * Print a String and then terminate the line. */ This is a cool Java feature. The software that you can download from java.sun.com includes a little program called javadoc. The javadoc program looks for these special comments in your code. The program uses these comments to create a brand new Web page — a customized documentation page for your code. To find out more about turning Javadoc comments into Web pages, visit this book’s Web site. Understanding a Simple Java Program The following sections present, explain, analyze, dissect, and otherwise demystify the Java program in Listing 4-1. What is a method? You’re working as an auto mechanic in an upscale garage. Your boss, who’s always in a hurry and has a habit of running words together, says, “FixThe Alternator on that junkyOldFord.” Mentally, you run through a list of tasks. “Drive the car into the bay, lift the hood, get a wrench, loosen the alternator belt,” and so on. Three things are going on here: You have a name for the thing you’re supposed to do. The name is FixTheAlternator. In your mind, you have a list of tasks associated with the name FixTheAlternator. The list includes “Drive the car into the bay, lift the hood, get a wrench, loosen the alternator belt,” and so on. You have a grumpy boss who’s telling you to do all this work. Your boss gets you working by saying, “FixTheAlternator.” In other words, your boss gets you working by saying the name of the thing you’re sup- posed to do. In this scenario, using the word method wouldn’t be a big stretch. You have a method for doing something with an alternator. Your boss calls that method into action, and you respond by doing all the things in the list of instructions that you’ve associated with the method.
- 76. 58 Part II: Writing Your Own Java Programs Java methods If you believe all that stuff in the last several paragraphs, then you’re ready to read about Java methods. In Java, a method is a list of things to do. Every method has a name, and you tell the computer to do the things in the list by using the method’s name in your program. I’ve never written a program to get a robot to fix an alternator. But, if I did, the program may include a method named FixTheAlternator. The list of instructions in my FixTheAlternator method would look something like the text in Listing 4-2. Listing 4-2: A Method Declaration void FixTheAlternator() { DriveInto(car, bay); Lift(hood); Get(wrench); Loosen(alternatorBelt); ... } Somewhere else in my Java code (somewhere outside of Listing 4-2), I need an instruction to call my FixTheAlternator method into action. The instruc- tion to call the FixTheAlternator method into action may look like the line in Listing 4-3. Listing 4-3: Calling a Method FixTheAlternator(junkyOldFord); Don’t scrutinize Listings 4-2 and 4-3 too carefully. All the code in Listings 4-2 and 4-3 is fake! I made up this code so that it looks a lot like real Java code, but it’s not real. What’s more important, the code in Listings 4-2 and 4-3 isn’t meant to illustrate all the rules about Java. So if you have a grain of salt handy, take it with Listings 4-2 and 4-3. Almost every computer programming language has something akin to Java’s methods. If you’ve worked with other languages, you may remember things like subprograms, procedures, functions, subroutines, Sub procedures, or PERFORM statements. Whatever you call it in your favorite programming language, a method is a bunch of instructions collected together and given a new name. The declaration, the header, and the call If you have a basic understanding of what a method is and how it works, you can dig a little deeper into some useful terminology:
- 77. Chapter 4: Exploring the Parts of a Program 59 If I’m being lazy, I refer to the code in Listing 4-2 as a method. If I’m not being lazy, I refer to this code as a method declaration. The method declaration in Listing 4-2 has two parts. The first line (the part with the name FixTheAlternator in it, up to but not including the open curly brace) is called a method header. The rest of Listing 4-2 (the part surrounded by curly braces) is a method body. The term method declaration distinguishes the list of instructions in Listing 4-2 from the instruction in Listing 4-3, which is known as a method call. For a handy illustration of all the method terminology, see Figure 4-5. A method’s header and declaration are like an entry in a dictionary. An entry doesn’t really use the word that it defines. Instead, an entry tells you what happens if and when you use the word. chocolate (choc-o-late) n. 1. The most habit-forming substance on earth. 2. Something you pay for with money from royalties. 3. The most impor- tant nutritional element in a person’s diet. FixTheAlternator() Drive the car into the bay, lift the hood, get the wrench, loosen the alternator belt, and then eat some chocolate. Figure 4-5: The terminology describing methods.
- 78. 60 Part II: Writing Your Own Java Programs In contrast, a method call is like the use of a word in a sentence. A method call sets some code in motion. “I want some chocolate, or I’ll throw a fit.” “FixTheAlternator on that junkyOldFord.” A method’s declaration tells the computer what will happen if you call the method into action. A method call (a separate piece of code) tells the com- puter to actually call the method into action. A method’s declaration and the method’s call tend to be in different parts of the Java program. The main method in a program In Listing 4-1, the bulk of the code is the declaration of a method named main. (Just look for the word main in the code’s method header.) For now, don’t worry about the other words in the method header — the words public, static, void, String, and args. I explain these words (on a need-to-know basis) in the next several chapters. Like any Java method, the main method is a recipe: How to make biscuits: Preheat the oven. Roll the dough. Bake the rolled dough. or How to follow the main instructions in the ThingsILike code: Display Chocolate, royalties, sleep on the screen. The word main plays a special role in Java. In particular, you never write code that explicitly calls a main method into action. The word main is the name of the method that is called into action automatically when the pro- gram begins running. When the ThingsILike program runs, the computer automatically finds the program’s main method and executes any instructions inside the method’s body. In the ThingsILike program, the main method’s body has only one instruction. That instruction tells the computer to print Chocolate, royal- ties, sleep on the screen. None of the instructions in a method are executed until the method is called into action. But if you give a method the name main, then that method is called into action automatically.
- 79. Chapter 4: Exploring the Parts of a Program 61 How you finally tell the computer to do something Buried deep in the heart of Listing 4-1 is the single line that actually issues a direct instruction to the computer. The line System.out.println(“Chocolate, royalties, sleep”); tells the computer to display the words Chocolate, royalties, sleep. (If you use JCreator, the computer displays Chocolate, royalties, sleep in the General Output pane.) I can describe this line of code in at least two different ways: It’s a statement: In Java, a direct instruction that tells the computer to do something is called a statement. The statement in Listing 4-1 tells the computer to display some text. The statements in other programs may tell the computer to put 7 in certain memory location, or make a window appear on the screen. The statements in computer programs do all kinds of things. It’s a method call: In the “What is a method?” section, earlier in this chapter, I describe something named a “method call.” The statement FixTheAlternator(junkyOldFord); is an example of a method call, and so is System.out.println(“Chocolate, royalties, sleep”); Java has many different kinds of statements. A method call is just one kind. Ending a statement with a semicolon In Java, each statement ends with a semicolon. The code in Listing 4-1 has only one statement in it, so only one line in Listing 4-1 ends with a semicolon. Take any other line in Listing 4-1, like the method header, for example. The method header (the line with the word main in it) doesn’t directly tell the computer to do anything. Instead, the method header describes some action for future reference. The header announces “Just in case someone ever calls the main method, the next few lines of code tell you what to do in response to that call.” Every complete Java statement ends with a semicolon. A method call is a statement, so it ends with a semicolon, but neither a method header nor a method declaration is a statement.
- 80. 62 Part II: Writing Your Own Java Programs The method named System.out.println The statement in the middle of Listing 4-1 calls a method named System. out.println. This method is defined in the Java API. Whenever you call the System.out.println method, the computer displays text on its screen. Think about names. Believe it or not, I know two people named Pauline Ott. One of them is a nun; the other is physicist. Of course, there are plenty of Paulines in the English-speaking world, just as there are several things named println in the Java API. So to distinguish the physicist Pauline Ott from the film critic Pauline Kael, I write the full name “Pauline Ott.” And, to distinguish the nun from the physicist, I write “Sister Pauline Ott.” In the same way, I write either System.out.println or DriverManager.println. The first (which you use often) writes text on the computer’s screen. The second (which you don’t use at all in this book) writes to a database log file. Just as Pauline and Ott are names in their own right, so System, out, and println are names in the Java API. But to use println, you must write the method’s full name. You never write println alone. It’s always System.out. println or some other combination of API names. The Java programming language is case-sensitive. If you change a lowercase letter to an uppercase letter (or vice versa), you change a word’s meaning. You can’t replace System.out.println with system.out.Println. If you do, your program won’t work. Methods, methods everywhere Two methods play roles in the ThingsILike program. Figure 4-6 illustrates the situation, and the next few bullets give you a guided tour: There’s a declaration for a main method. I wrote the main method myself. This main method is called automatically whenever I start run- ning the ThingsILike program. There’s a call to the System.out.println method. The method call for the System.out.println method is the only statement in the body of the main method. In other words, calling the System.out.println method is the only thing on the main method’s to-do list. The declaration for the System.out.println method is buried inside the official Java API. For a refresher on the Java API, see the Chapter 1. When I say things like “System.out.println is buried inside the API,” I’m not doing justice to the API. True, you can ignore all the nitty-gritty Java code inside the API. All you need to remember is that System.out.println is defined somewhere inside that code. But I’m not being fair when I make the API code sound like something magical. The API is just another bunch of Java code. The statements in the API that tell the computer what it means to carry out a call to System.out.println look a lot like the Java code in Listing 4-1.
- 81. Chapter 4: Exploring the Parts of a Program 63 Figure 4-6: Calling the System.out. println method. The Java class Have you heard the term object-oriented programming (also known as OOP)? OOP is a way of thinking about computer programming problems — a way that’s supported by several different programming languages. OOP started in the 1960s with a language called Simula. It was reinforced in the 1970s with another language named Smalltalk. In the 1980s, OOP took off big time with the language C++. Some people want to change the acronym, and call it COP — class-oriented pro- gramming. That’s because object-oriented programming begins with something called a class. In Java, everything starts with classes, everything is enclosed in classes, and everything is based on classes. You can’t do anything in Java until you’ve created a class of some kind. It’s like being on Jeopardy!, hearing Alex Trebek say, “Let’s go to a commercial,” and then interrupting him by saying, “I’m sorry, Alex. You can’t issue an instruction without putting your instruction inside a class.” It’s important for you to understand what a class really is, so I dare not give a haphazard explanation in this chapter. Instead, I devote much of Chapter 17 to the question, “What is a class?” Anyway, in Java, your main method has to be inside a class. I wrote the code in Listing 4-1, so I got to make up a name for my new class. I chose the name ThingsILike, so the code in Listing 4-1 starts with the words class ThingsILike.
- 82. 64 Part II: Writing Your Own Java Programs Take another look at Listing 4-1, and notice what happens after the line class ThingsILike. The rest of the code is enclosed in curly braces. These braces mark all the stuff inside the class. Without these braces, you’d know where the declaration of the ThingsILike class starts, but you wouldn’t know where the declaration ends. It’s as if the stuff inside the ThingsILike class is in a box. (Refer to Figure 4-3.) To box off a chunk of code, you do two things: You use curly braces: These curly braces tell the compiler where a chunk of code begins and ends. You indent code: Indentation tells your human eye (and the eyes of other programmers) where a chunk of code begins and ends. Don’t forget. You have to do both.
- 83. Chapter 5 Composing a Program In This Chapter Reading input from the keyboard Editing a program Shooting at trouble J ust yesterday, I was chatting with my servant, RoboJeeves. (RoboJeeves is an upscale model in the RJ-3000 line of personal robotic life-forms.) Here’s how the discussion went: Me: RoboJeeves, tell me the velocity of an object after it’s been falling for three seconds in a vacuum. RoboJeeves: All right, I will. “The velocity of an object after it’s been falling for three seconds in a vacuum.” There, I told it to you. Me: RoboJeeves, don’t give me that smart-alecky answer. I want a number. I want the actual velocity. RoboJeeves: Okay! “A number; the actual velocity.” Me: RJ, these cheap jokes are beneath your dignity. Can you or can’t you tell me the answer to my question? RoboJeeves: Yes. Me: “Yes,” what? RoboJeeves: Yes, I either can or can’t tell you the answer to your question. Me: Well, which is it? Can you? RoboJeeves: Yes, I can. Me: Then do it. Tell me the answer. RoboJeeves: The velocity is 153,984,792 miles per hour.
- 84. 66 Part II: Writing Your Own Java Programs Me: (After pausing to think . . .) RJ, I know you never make a mistake, but that number, 153,984,792, is much too high. RoboJeeves: Too high? That’s impossible. Things fall very quickly on the giant planet Mangorrrrkthongo. Now, if you wanted to know about objects falling on Earth, you should have said so in the first place. Sometimes that robot rubs me the wrong way. The truth is, RoboJeeves does whatever I tell him to do — nothing more and nothing less. If I say “Feed the cat,” then RJ says, “Feed it to whom? Which of your guests will be having cat for dinner?” Handy as they are, all computers do the same darn thing. They do exactly what you tell them to do, and that’s sometimes very unfortunate. For exam- ple, in 1962, a Mariner spacecraft to Venus was destroyed just four minutes after its launch. Why? It was destroyed because of a missing keystroke in a FORTRAN program. Around the same time, NASA scientists caught an error that could have trashed the Mercury space flights. (Yup! These were flights with people on board!) The error was a line with a period instead of a comma. (A computer programmer wrote DO 10 I=1.10 instead of DO 10 I=1,10.) With all due respect to my buddy RoboJeeves, he and his computer cousins are all incredibly stupid. Sometimes they look as if they’re second-guessing us humans, but actually they’re just doing what other humans told them to do. They can toss virtual coins and use elaborate schemes to mimic creative behavior, but they never really think on their own. If you say, “Jump,” then they do what they’re programmed to do in response to the letters J-u-m-p. So when you write a computer program, you have to imagine that a genie has granted you three wishes. Don’t ask for eternal love because, if you do, then the genie will give you a slobbering, adoring mate — someone that you don’t like at all. And don’t ask for a million dollars, unless you want the genie to turn you into a bank robber. Everything you write in a computer program has to be very precise. Take a look at an example. . . . A Program to Echo Keyboard Input Listing 5-1 contains a small Java program. The program lets you type one line of characters on the keyboard. As soon as you press Enter, the program dis- plays a second line that copies whatever you typed.
- 85. Chapter 5: Composing a Program 67 Listing 5-1: A Java Program import java.util.Scanner; class EchoLine { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); System.out.println(myScanner.nextLine()); } } Figure 5-1 shows a run of the EchoLine code (the code in Listing 5-1). The text in the figure is a mixture of my own typing and the computer’s responses. Figure 5-1: What part of the word “don’t” do you not understand? To illustrate the situation, I concocted Figure 5-2. In Figure 5-2, I added high- light to the text that I type. (Everything that I type is highlighted. Everything the computer displays on its own is in regular, un-highlighted style.) Figure 5-2: Whatever you get to type is highlighted. Here’s what happens when you run the code in Listing 5-1: 1. At first, the computer does nothing. You see a cursor on the left edge of the General Output pane, as shown in Figure 5-3. The computer is wait- ing for you to type something.
- 86. 68 Part II: Writing Your Own Java Programs Figure 5-3: The computer waits for you to type something. 2. You type one line of text — any text at all. (See Figure 5-4.) Figure 5-4: You type a sentence. 3. You press Enter, and the computer displays another copy of the line that you typed, as shown in Figure 5-5. Figure 5-5: The computer echoes your input. After displaying a copy of your input, the program’s run comes to an end. JCreator adds Process completed to the text in the General Output pane. Typing and running a program This book’s special edition of JCreator has a Chapter05 workspace. Within that workspace you can find a Chapter05_Listing01 project. To test the code in Listing 5-1, you can just open and run the readymade Chapter05_Listing01 project. But instead of running the readymade code, I encourage you to start from scratch — to type Listing 5-1 yourself and then to test your newly-created code. Just follow these steps:
- 87. Chapter 5: Composing a Program 69 1. Launch JCreator. 2. From JCreator’s menu bar, choose File➪Open Workspace. An Open dialog box appears. 3. In the Open dialog box, select MyWorkspace.jcw (or simply MyWorkspace). Then click Open. After clicking Open, you see MyWorkspace in JCreator’s File View pane. The next step is to create a new project within MyWorkspace. 4. In the File View pane, right-click MyWorkspace. Then choose Add new Project from the context menu that appears. JCreator’s Project Wizard opens. 5. In the wizard’s Project Template tab, select the Empty Project icon, and then click Next. After clicking Next, you see the wizard’s Project Paths tab. 6. In the Name field, type MyNewProject. 7. Click Finish. Clicking Finish brings you back to JCreator’s work area, with MyNew Project set in bold. The bold typeface means that MyNewProject is the active project. The next step is to create a new Java source code file. 8. In the File View pane, right-click MyNextProject. Then choose Add➪ New Class from the context menu that appears. JCreator’s Class Wizard opens. 9. In the Class Wizard’s Name field, type the name of your new class. In this example, use the name EchoLine. Spell EchoLine exactly the way I spell it in Listing 5-1, with a capital E, a capital L, and no blank space. In Java, consistent spelling and capitalization are very important. If you’re not consistent within a particular program, then you’ll get error messages when you try to compile the program. 10. Skip everything in the Class Wizard except the Name field. (In other words, click Finish.) Clicking Finish brings you back to JCreator’s work area. Now the Editor pane has a tab named EchoLine.java. 11. Type the program of Listing 5-1 in the Editor pane’s EchoLine.java tab. Copy the code exactly as you see it in Listing 5-1. • Spell each word exactly the way I spell it in Listing 5-1. • Capitalize each word exactly the way I do in Listing 5-1. • Include all the punctuation symbols — the dots, the semicolons, everything.
- 88. 70 Part II: Writing Your Own Java Programs 12. From the menu bar, choose Build➪Compile Project. If you typed everything correctly, you see the comforting Process completed message, with no error messages, in JCreator’s Build Output pane. If you see error messages, then go back to Step 11, and compare every- thing you typed with the stuff in Listing 5-1. Compare every letter, every word, every squiggle, every smudge. 13. Make any changes or corrections to the code in the Editor pane. Then repeat Step 12. When at last you see the Process completed message with no error messages, you’re ready to run the program. 14. From the menu bar choose Build➪Execute Project. Your new Java program runs. A cursor sits on the left edge of JCreator’s General Output pane. (Refer to Figure 5-3.) The computer is waiting for you to type something. 15. Type a line of text, and then press Enter. In response, the computer displays a second copy of your line of text. Then JCreator displays Process completed, and the program’s run comes to an end. (Refer to Figure 5-5.) If this list of steps seems a bit sketchy, you can find much more detail in Chapter 3. (Look first at the section in Chapter 3 about compiling and run- ning a program.) For the most part, the steps here in Chapter 5 are a quick summary of the material in Chapter 3. The big difference is, in Chapter 3, I don’t encourage you to type the program yourself. This section tells you how to type the program with JCreator running on Microsoft Windows. If you don’t use Windows, or if you use Windows but you don’t use JCreator, then visit this book’s Web site at http://www. dummies.com/go/bpjavafd. On that site, I’ve posted some handy tips for creating Java programs in other environments. So what’s the big deal when you type the program yourself? Well, lots of inter- esting things can happen when you apply fingers to keyboard. That’s why the second half of this chapter is devoted to troubleshooting. How the EchoLine program works When you were a tiny newborn, resting comfortably in your mother’s arms, she told you how to send characters to the computer screen: System.out.println(whatever text you want displayed);
- 89. Chapter 5: Composing a Program 71 What she didn’t tell you was how to fetch characters from the computer keyboard. There are lots of ways to do it, but the one I recommend in this chapter is: myScanner.nextLine() Now, here’s the fun part. Calling the nextLine method doesn’t just scoop characters from the keyboard. When the computer runs your program, the computer substitutes whatever you type on the keyboard in place of the text myScanner.nextLine(). To understand this, look at the statement in Listing 5-1: System.out.println(myScanner.nextLine()); When you run the program, the computer sees your call to nextLine and stops dead in its tracks. (Refer to Figure 5-3.) The computer waits for you to type a line of text. So (refer to Figure 5-4) you type the line Hey, there’s an echo in here. The computer substitutes this entire Hey line for the myScanner.nextLine() call in your program. The process is illustrated in Figure 5-6. Figure 5-6: The computer substitutes text in place of the nextLine call.
- 90. 72 Part II: Writing Your Own Java Programs The call to myScanner.nextLine() is nestled inside the System.out. println call. So when all is said and done, the computer behaves as if the statement in Listing 5-1 looks like this: System.out.println(“Hey, there’s an echo in here.”); The computer displays another copy of the text Hey, there’s an echo in here. on the screen. That’s why you see two copies of the Hey line in Figure 5-5. Getting numbers, words, and other things In Listing 5-1, the words myScanner.nextLine() get an entire line of text from the computer keyboard. So if you type Testing 1 2 3 the program in Listing 5-1 echoes back your entire Testing 1 2 3 line of text. Sometimes you don’t want a program to get an entire line of text. Instead, you want the program to get a piece of a line. For example, when you type 1 2 3, you may want the computer to get the number 1. (Maybe the number 1 stands for one customer or something like that.) In such situations, you don’t put myScanner.nextLine() in your program. Instead, you use myScanner. nextInt(). Table 5-1 shows you a few variations on the myScanner.next business. Unfortunately, the table’s entries aren’t very predictable. To read a line of input, you call nextLine. But to read a word of input, you don’t call nextWord. (The Java API has no nextWord method.) Instead, to read a word, you call next. Table 5-1 Some Scanner Methods To Read This . . . . . . Make This Method Call A number with no decimal point in it nextInt() A number with a decimal point in it nextDouble() A word (ending in a blank space, for example) next() A line (or what remains of a line after you’ve nextLine() already read some data from the line) A single character (such as a letter, a digit, findInLine(“.”).charAt(0) or a punctuation character)
- 91. Chapter 5: Composing a Program 73 Also, the table’s story has a surprise ending. To read a single character, you don’t call nextSomething. Instead, you can call the bizarre findInLine (“.”).charAt(0) combination of methods. (You’ll have to excuse the folks who created the Scanner class. They approached the input problem from a specialized point of view.) A quick look at the Scanner In this chapter, I advise you to ignore any mean- readingThingie (or any other name you ings behind the lines import java.util. want to use) as long as you use the name Scanner and Scanner myScanner, etc. consistently. So, if you want to be creative, Just paste these two lines mindlessly in your you can write code, and then move on. Scanner readingThingie = new Of course, you may not want to take my advice. Scanner(System.in); You may not like ignoring things in your code. If you happen to be such a stubborn person, I have System.out.println(readingThing ie.nextLine()); a few quick facts for you. The revised Listing 5-1 (with reading The word Scanner is defined in the Thingie instead of myScanner) compiles Java API. and runs without a hitch. A Scanner is something you can use for The line import java.util.Scanner getting input. is an example of an import declaration. This Scanner class is new in version 5.0 of An optional import declaration allows you to the Java API. If you use version Java 1.4.2, abbreviate names in the rest of your program. then you don’t have access to the Scanner You can remove the import declaration from class. (You get an error when you try to Listing 5-1. But if you do, you must use compile Listing 5-1.) the Scanner class’s fully qualified name The words System and in are defined in throughout your code. Here’s how: the Java API. class EchoLine { Taken together, the words System.in stand for the computer keyboard. public static void main(String args[]) { In later chapters you see things like new java.util.Scanner Scanner(new File(“myData.txt”)). myScanner = In those chapters, I replace System.in with new the words new File(“myData.txt”) java.util.Scanner(System.in) because I’m not getting input from the key- ; board. Instead, I’m getting input from a file on the computer’s hard drive. System.out.println(myScanne The word myScanner doesn’t come from r.nextLine()); the Java API. } } The word myScanner is a Barry Burd cre- ation. Instead of myScanner, you can use
- 92. 74 Part II: Writing Your Own Java Programs To see some of the table’s methods in action, check other program listings in this book. Chapters 6, 7, and 8 have some particularly nice examples. Type two lines of code, and don’t look back Buried innocently inside Listing 5-1 are two extra lines of code. These lines help the computer read input from the keyboard. The two lines are import java.util.Scanner; Scanner myScanner = new Scanner(System.in); Concerning these two lines, I have bad news and good news. The bad news is, the reasoning behind these lines is difficult to under- stand. That’s especially true here in Chapter 5, where I introduce Java’s most fundamental concepts. The good news is, you don’t have to understand the reasoning behind these two lines. You can copy and paste these lines into any program that gets input from the keyboard. You don’t have to change the lines in any way. These lines work without any modifications in all kinds of Java programs. Just be sure to put these lines in the right places: Make the import java.util.Scanner line be the first line in your program. Put the Scanner myScanner = new Scanner(System.in) line inside your main method immediately after the public static void main (String args[]) { line. At some point in the future, you may have to be more careful about the posi- tioning of these two lines. But for now, the rules I give will serve you well. Expecting the Unexpected Not long ago, I met an instructor with an interesting policy. He said, “Sometimes when I’m lecturing, I compose a program from scratch on the computer. I do it right in front of my students. If the program compiles and runs correctly on the first try, I expect the students to give me a big round of applause.”
- 93. Chapter 5: Composing a Program 75 At first you may think this guy has an enormous ego, but you have to put things in perspective. It’s unusual for a program to compile and run correctly the first time. There’s almost always a typo or another error of some kind. So this section deals with the normal, expected errors that you see when you compile and run a program for the first time. Everyone makes these mistakes, even the most seasoned travelers. The key is keeping a cool head. Here’s my general advice: Don’t expect a program that you type to compile the first time. Be prepared to return to your editor and fix some mistakes. Don’t expect a program that compiles flawlessly to run correctly. Getting a program to compile without errors is the easier of the two tasks. Read what’s in JCreator’s Editor pane, not what you assume is in JCreator’s Editor pane. Don’t assume that you’ve typed words correctly, that you’ve capitalized words correctly, or that you’ve matched curly braces or parentheses correctly. Compare the code you typed with any sample code that you have. Make sure that every detail is in order. Be patient. Every good programming effort takes a long time to get right. If you don’t understand something right away, then be persistent. Stick with it (or put it away for a while and come back to it). There’s nothing you can’t understand if you put in enough time. Don’t become frustrated. Don’t throw your pie crust. Frustration (not lack of knowledge) is your enemy. If you’re frustrated, you can’t accomplish anything. Don’t think you’re the only person who’s slow to understand. I’m slow, and I’m proud of it. (Christine, Chapter 6 will be a week late.) Don’t be timid. If your code isn’t working, and you can’t figure out why it’s not working, then ask someone. Post a message on groups.google.com, or send me an e-mail message. (Send it to BeginProg2@BurdBrain.com.) And don’t be afraid of anyone’s snide or sarcastic answer. (For a list of gestures you can make in response to peoples’ snotty answers, see Appendix Z.)
- 94. 76 Part II: Writing Your Own Java Programs Diagnosing a problem The “Typing and running a program” section, earlier in this chapter, tells you how to run the EchoLine program. If all goes well, your screen ends up looking like the one shown in Figure 5-1. But things don’t always go well. Sometimes your finger slips, inserting a typo into your program. Sometimes you ignore one of the details in Listing 5-1, and you get a nasty error message. Of course, some things in Listing 5-1 are okay to change. Not every word in Listing 5-1 is cast in stone. So here’s a nasty wrinkle — I can’t tell you that you must always retype Listing 5-1 exactly as it appears. Some changes are okay; others are not. Keep reading for some “f’rinstances.” Case sensitivity Java is case-sensitive. Among other things, this means that, in a Java program, the letter P isn’t the same as the letter p. If you send me some fan mail and start with “Dear barry” instead of “Dear Barry,” then I still know what you mean. But Java doesn’t work that way. So change just one character in a Java program, and instead of an uneventful compilation you get a big headache! Change p to P like so: //The following line is incorrect: System.out.Println(myScanner.nextLine()); When you try to compile and run the program, you get the ugliness shown in Figure 5-7. Figure 5-7: The Java compiler understands println, but not Println. When you get messages like the ones in Figure 5-7, your best bet is to stay calm and read the messages carefully. Sometimes, the messages contain useful hints. (Of course sometimes, they don’t.) The messages in Figure 5-7 start with EchoLine.java:8: cannot find symbol. In plain English, this means “There’s something that the Java compiler can’t interpret on line 8 of your EchoLine.java file.”
- 95. Chapter 5: Composing a Program 77 “And what may that something be?” you ask. The answer is also in Figure 5-7. The second line of the message says symbol : method Println, which means, “The Java compiler can’t interpret the word Println.” (The message stops short of saying, “It’s the word Println, you dummy!” In any case, if the computer says you’re one of us Dummies, you should take it as a compliment.) Now, there are plenty of reasons why the compiler may not be able to under- stand a word like Println. But, for a beginning programmer, there are two important things that you should check right away: Have you spelled the word correctly? Did you type prntlin instead of println? Have you capitalized all letters correctly? Did you type Println or PrintLn instead of println? Either of these errors can send the Java compiler into a tailspin. So compare your typing with the approved typing word for word (and letter for letter). When you find a discrepancy, go back to the editor and fix the problem. Then try compiling the program again. When an error message says EchoLine.java:67, you don’t need to count to the program’s 67th line. Just double-click the phrase EchoLine.java:67 in JCreator’s Build Output pane. When you do, JCreator draws a little red arrow next to line 67 (and moves the cursor to line 67) in the Editor pane. Omitting punctuation In English and in, Java using the; proper! punctuation is important) Take, for instance, the semicolons in Listing 5-1. What happens if you forget to type a semicolon? //The following code is incorrect: System.out.println(myScanner.nextLine()) } If you leave off the semicolon, you get the message shown in Figure 5-8. Figure 5-8: A helpful error message.
- 96. 78 Part II: Writing Your Own Java Programs A message like the one in Figure 5-8 makes your life much simpler. I don’t have to explain the message, and you don’t have to puzzle over the message’s mean- ing. Just take the message ‘;’ expected on its face value. The message says, “I expect to see a semicolon at this point in your program.” A caret (a ^ thingy) points to the place in the program where the computer expects to see a semi- colon. The computer expects a semicolon after the System.out.println (myScanner.nextLine()) statement and before the close curly brace. So do what the message tells you to do. Go back to the editor and put a semi- colon after the System.out.println(myScanner.nextLine()) statement. That settles it. Using too much punctuation In junior high school, my English teacher said I should use a comma when- ever I would normally pause for a breath. This advice doesn’t work well during allergy season, when my sentences have more commas in them than words. Even as a paid author, I have trouble deciding where the commas should go, so I often add extra commas for good measure. This makes more work for my editor, Christine, who has a recycle bin full of commas by the desk in her office. It’s the same way in a Java program. You can get carried away with punctua- tion. Consider, for example, the main method header in Listing 5-1. This line is a dangerous curve for novice programmers. For information on the terms method header and method body, see Chapter 4. Normally, you shouldn’t be ending a method header with a semicolon. But people add semicolons anyway. (Maybe, in some subtle way, a method header looks like it should end with a semicolon.) //The following line is incorrect: public static void main(String args[]); { If you add this extraneous semicolon to the code in Listing 5-1, you get the message shown in Figure 5-9. Figure 5-9: A not-so- helpful error message.
- 97. Chapter 5: Composing a Program 79 The error message in Figure 5-9 is a bit misleading. Instead of saying extra semicolon should be removed, the message says missing method body, or declare abstract. What the heck does that mean? Is there life after a failed compilation? In the section entitled “Typing and running a pro- You choose Build➪Compile Project again. gram” you create a file named EchoLine.java. This gives you your favorite cannot find (See Step 10 of the instructions in that section.) symbol error message. The computer When you successfully compile the project, the doesn’t create a new EchoLine.class computer creates another file — a file named file. EchoLine.class. (You don’t see EchoLine. You choose Build➪Execute Project once class in JCreator’s File View pane, but even so, the again. computer creates an EchoLine.class file.) Later, to run the program, the Java virtual machine And what happens when you choose Build➪ follows the instructions in this EchoLine.class Execute Project the second time? If you guessed file. For details, see Chapter 1. that the old EchoLine program runs correctly, then you’re right. That’s what happens when the computer suc- cessfully compiles your program. But what hap- Even though your latest compiler effort was a pens if your program contains errors? Whenever failure, your earlier compilation created a good an attempted compilation isn’t successful, the EchoLine.class file. That EchoLine. computer doesn’t create a .class file. And class file is still on your hard drive. The file without this .class file, an attempted run isn’t wasn’t replaced during the failed compilation. successful either. So when you choose Build➪Execute Project the second time, the computer uses that old But sometimes, the story has a peculiar twist. EchoLine.class file and correctly runs the Imagine this unfortunate sequence of events: code in Listing 5-1. You type Listing 5-1 exactly as it appears in This can be really confusing. When you choose this book. Build➪Execute Project, your gut tells you that On JCreator’s menu bar, you choose Build➪ you’re getting the results of your most recent Compile Project. This creates an Echo compiling attempt. But your gut can be wrong, Line.class file. Very nice! wrong, wrong. In the scenario that I just described, an unsuccessful compilation is fol- On JCreator’s menu bar, you choose Build➪ lowed by what appears to be a successful run. Execute Project. You see the program’s It’s a mess, so you have to keep your wits about output. Wonderful! you. If a compilation fails, then don’t march on You make a harmful change to your Echo and try to run the project. Instead, go back and Line.java file. For instance, you turn the figure out why the compilation failed. p in println into a heinous, unwanted capital P.
- 98. 80 Part II: Writing Your Own Java Programs Well, when the computer tries to compile the bad code (Listing 5-1 with one too many semicolons), it gets confused. I illustrate the confusion in Figure 5-10. Your eye sees an extra semicolon, but the computer’s eye interprets this as a method without a body. So that’s the first part of the error message — the computer says missing method body. Figure 5-10: What’s on this computer’s mind? We all know that a computer is a very patient, very sympathetic machine. That’s why the computer looks at your code, and decides to give you one more chance. The computer remembers that Java has an advanced feature in which you write a method header without writing a method body. When you do this, you get what’s called an abstract method — something that I don’t use at all in this book. Anyway, in Figure 5-10, the computer sees a header with no body. So the computer says to itself, “I know! Maybe the programmer is trying to write an abstract method. The trouble is, an abstract method’s header has to have the word abstract in it. I should remind the programmer about that.” So the computer displays the declare abstract message in Figure 5-9. One way or another, you can’t interpret the message in Figure 5-9 without reading between the lines. So here are some tips to help you decipher murky messages:
- 99. Chapter 5: Composing a Program 81 Avoid the knee-jerk response. Some people see the declare abstract message in Figure 5-9 and wonder how they can declare a method to be abstract. Unfortunately, this isn’t the right approach. If you don’t know what it means to declare abstract then, chances are, you didn’t mean to declare anything to be abstract in the first place. Stare at the bad line of code for a long, long time. If you look carefully at the public static . . . line in Figure 5-9, then eventually you’ll notice that it’s different from the corresponding line in Listing 5-1. The line in Listing 5-1 has no semicolon, but the line in Figure 5-9 has a semicolon. Of course, you won’t always be starting with some prewritten code like the stuff in Listing 5-1. That’s where practice makes perfect. The more code you write, the more sensitive your eyes will become to things like extraneous semicolons and other programming goofs. Often the first message is the best You’re looking for the nearest gas station, so you ask one of the locals. “Go to the first traffic light and make a left,” says the local. You go straight for a few streets, and see a blinking yellow signal. You turn left at the signal, and travel for a mile or so. What? No gas station? Maybe you mistook the blinking signal for a real traffic light. Why can’t the computer fix it? How often do you get to finish someone else’s The answer is simple. The computer isn’t inter- sentence? “Please,” says your supervisor, “go ested in taking any chances. What if you don’t over there and connect the . . .” really want a semicolon before the curly brace on line 9? What if the missing semicolon represents “Wires,” you say. “I’ll connect the wires.” If you a more profound problem? If the computer added know what someone means to say, why wait for the extra semicolon, it could potentially do more them to say it? harm than good. This same question comes up in connection with Returning to you and your supervisor . . . computer error messages. Take a look at the message in Figure 5-8. The computer expects a Boom! A big explosion. “Not the wires, you semicolon before the curly brace on line 9. Well, Dummy. The dots. I wanted you to connect the Mr. Computer, if you know where you want a dots.” semicolon, then just add the semicolon, and be “Sorry,” you say. done with it. Why are you bothering me about it?
- 100. 82 Part II: Writing Your Own Java Programs You come to a fork in the road. “The directions said nothing about a fork. Which way should I go?” You veer right, but a minute later you’re forced onto a highway. You see a sign that says, “Next Exit 24 Miles.” Now you’re really lost, and the gas gauge points to “S.” (The “S” stands for “Stranded.”) So here’s what happened: You made an honest mistake. You shouldn’t have turned left at the yellow blinking light. That mistake alone wasn’t so terrible. But that first mistake lead to more confusion, and eventually, your choices made no sense at all. If you hadn’t turned at the blinking light, you’d never have encountered that stinking fork in the road. Then, getting on the highway was sheer catastrophe. Is there a point to this story? Of course there is. A computer can get itself into the same sort of mess. The computer notices an error in your program. Then, metaphorically speaking, the computer takes a fork in the road — a fork based on the original error — a fork for which none of the alternatives lead to good results. Here’s an example. You’re retyping the code in Listing 5-1, and you forget to type a close parenthesis: //The following line is incorrect: public static void main(String args[] { When you try to compile the code, you get the messages shown in Figure 5-11. Figure 5-11: Two error messages. The computer reports two errors — one error on line 5, and another error on line 10. “Let’s see,” you say to yourself. “Line 10 is the last line of the main method. The message says ‘;’ expected. Why would I want a semicolon at the end of my main method?” Well, you better take a step backward. In Figure 5-11, the first error message says ‘)’ expected. That’s okay. But after that first message, the computer is really confused. In the second message, the computer suggests that you end the main method with a semicolon, as if that would fix anything. The computer is trying to make the best of a bad situation but, at this point, you shouldn’t believe a word that the computer says.
- 101. Chapter 5: Composing a Program 83 One message and none very helpful Look at the following sidebar figure and think expected message, but this time, the message about what happened. The computer got all is all wrong. Sure, you should have typed import upset because it didn’t see one of the words instead of Import. But no, the computer’s that it expected. The computer expected to see suggestion that the word class or interface either class or interface, and it scolded me will fix everything is incorrect. for not using one of these two words. Now unfortunately, in this particular figure, Oddly enough, the computer was going out on you don’t get a message saying cannot find a limb when it suggested the words class and symbol . . . Import. And you certainly interface. The fact is, not all programs begin don’t see a message like ‘import’ expected. with the word class or the word interface. Either of those messages would be more helpful For example, the code in Listing 5-1 begins with than ‘class’ or ‘interface’ expected, the word import. but neither message is destined to appear. That’s just the way the cookie crumbles. Computers Now, what happens if your pinky finger becomes aren’t smart animals, and if someone programs the heavy, and you type Import instead of import? computer to say ‘class’ or ‘interface’ //The following line is incor- expected, then that’s exactly what the com- rect: puter says. Import java.util.Scanner; Some people say that computers make them Then, much to your dismay, you get the message feel stupid. For me, it’s the opposite. A computer shown in the accompanying figure. The figure reminds me how dumb a machine can be, and has yet another ‘class’ or ‘interface’ how smart a person can be. I like that. The moral of this story is simple. The first of the computer’s error messages is often the most reliable. The rest of the messages may be nothing but con- fusing drivel. If you get more than one error message, always look carefully at the first mes- sage in the bunch. That first error message is often the most informative of all the error messages. Occasionally, the first error message isn’t the most informative. Take time reading the messages and look for the most helpful among them.
- 102. 84 Part II: Writing Your Own Java Programs Same kind of error; different kind of message You’ve found an old family recipe for deviled eggs (one of my favorites). You follow every step as carefully as you can, but you leave out the salt because of your grandmother’s high blood pressure. You hand your grandmother an egg (a finished masterpiece). “Not enough pepper,” she says, and she walks away. The next course is beef bourguignon. You take an unsalted slice to dear old Granny. “Not sweet enough,” she groans, and she leaves the room. “But that’s impossible,” you think. “There’s no sugar in beef bourguignon. I left out the salt.” Even so, you go back to the kitchen and prepare mashed potatoes. You use unsalted butter, of course. “She’ll love it this time,” you think. “Sour potatoes! Yuck!” Granny says, as she goes to the sink to spit it all out. Because you have a strong ego, you’re not insulted by your grandmother’s behavior. But you’re somewhat confused. Why is she saying such different things about three unsalted recipes? Maybe there are some subtle differences that you don’t know about. Well, the same kind of thing happens when you’re writing computer programs. You can make the same kind of mistake twice (or at least, make what you think is the same kind of mistake twice), and get different error messages each time. For example, if you read the earlier stuff in this chapter, you may come to believe that every unrecognized word leads to a cannot find symbol mes- sage. Well, I’m sorry. It just doesn’t work that way. Take, for example, the word class in Listing 5-1. Change the lowercase c to an uppercase C: //The following line is incorrect: Class EchoLine { When it sees this line, the compiler doesn’t even bother to tell you that it can’t find a symbol. The compiler just thinks, “Most programs start with the word class, and some start with the word interface. This incorrect pro- gram starts with a word that I don’t understand (because I don’t think about the letters C and c having anything to do with one another). I’ll suggest either class or interface. That should fix it.” So the computer sends you the mes- sage that’s shown in Figure 5-12. Figure 5-12: Spelling “Class” incorrectly (with a capital “C”).
- 103. Chapter 5: Composing a Program 85 In fact, that fixes it, because a change from Class to class gets your code running again. An interface is like a class. But unlike a class, an interface can’t stand on its own. For instance, you can’t put your static void main method in a Java interface. None of the programs in this book use interfaces, so don’t worry about interfaces until you advance past the beginning programming stage. Run-time error messages Up to this point in the chapter, I describe errors that crop up when you com- pile a program. Another category of errors hides until you run the program. A case in point is the improper capitalization of the word main. Assume that, in a moment of wild abandon, you incorrectly spell main with a capital M: //The following line is incorrect: public static void Main(String args[]) { When you compile the code, everything is hunky-dory. You see a friendly Process completed message — nothing else. But then you try to run your program. At this point, the bits hit the fan. The catastrophe is illustrated in Figure 5-13. Figure 5-13: Whadaya mean “NoSuch Method Error”? Sure, your program has something named Main, but does it have anything named main? (Yes, I’ve heard of a famous poet named e. e. cummings, but who the heck is E. E. Cummings?) The computer doesn’t presume that your word Main means the same thing as the expected word main. You need to change Main back to main. Then everything will be okay. But in the meantime (or in the maintime), how does this improper capitaliza- tion make it past the compiler? Why don’t you get any error messages when you compile the program? And if a capital M doesn’t upset the compiler, why does this capital M mess everything up at run time? The answer goes back to the different kinds of words in the Java program- ming language. As it says in Chapter 4, Java has identifiers and keywords.
- 104. 86 Part II: Writing Your Own Java Programs The keywords in Java are cast in stone. If you change class to Class, or change public to Public, then you get something new — something that the computer probably can’t understand. That’s why the compiler chokes on improper keyword capitalizations. It’s the compiler’s job to make sure that all the keywords are used properly. On the other hand, the identifiers can bounce all over the place. Sure, there’s an identifier named main, but you can make up a new identifier named Main. (You shouldn’t do it, though. It’s too confusing to people who know Java’s usual meaning for the word main.) When the compiler sees a mistyped line, like public static void Main, the compiler just assumes that you’re making up a brand new name. So the compiler lets the line pass. You get no complaints from your old friend, the compiler. But then, when you try to run the code, the computer goes ballistic. The Java virtual machine always looks for something spelled main, with a small m. If the JVM doesn’t see anything named main, then the JVM gets upset. “NoSuch Method . . . main,” says the JVM. So now the JVM, and not the compiler, gives you an error message. What problem? I don’t see a problem I end this chapter on an upbeat note by showing you some of the things you can change in Listing 5-1 without rocking the boat. The identifiers that you create If you create an identifier, then that name is up for grabs. For instance, in Listing 5-1, you can change EchoLine to RepeatAfterMe. class RepeatAfterMe { public static void main ... etc. This presents no problem at all, as long as you’re willing to be consistent. Just follow most of the steps in this chapter’s “Typing and running a pro- gram” section. In Step 9, instead of typing EchoLine, type RepeatAfterMe in the Class Wizard’s Name field. In Step 11, when you copy the code from Listing 5-1, don’t type class EchoLine { near the top of the listing. Instead, type the words class RepeatAfterMe {
- 105. Chapter 5: Composing a Program 87 Spaces and indentation Java isn’t fussy about the use of spaces and indentation. All you need to do is keep your program well-organized and readable. Here’s an example: import java.util.Scanner; class EchoLine { public static void main( String args[] ) { Scanner myScanner = new Scanner( System.in ); System.out.println ( myScanner.nextLine() ); } } How you choose to do things A program is like a fingerprint. No two programs look very much alike. Say I discuss a programming problem with a colleague. Then we go our separate ways and write our own programs to solve the same problem. Sure, we’re duplicating the effort. But will we create the exact same code? Absolutely not. Everyone has his or her own style, and everyone’s style is unique. I asked fellow Java programmer David Herst to write his own EchoLine pro- gram without showing him my code from Listing 5-1. Here’s what he wrote: 0 import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.IOException; public class EchoLine { public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); String input = br.readLine(); System.out.println(input); } } Don’t worry about BufferedReader, InputStreamReader, or things like that. Just notice that, like snowflakes, no two programs are written exactly alike, even if they accomplish the same task. That’s nice. It means your code, how- ever different, can be as good as the next person’s. That’s very encouraging.
- 106. 88 Part II: Writing Your Own Java Programs
- 107. Chapter 6 Using the Building Blocks: Variables, Values, and Types In This Chapter Declaring variables Assigning values to variables Working with numbers Using Java types B ack in 1946, John von Neumann wrote a groundbreaking paper about the newly emerging technology of computers and computing. Among other things, he established one fundamental fact: For all their complexity, the main business of computers is to move data from one place to another. Take a number — the balance in a person’s bank account. Move this number from the computer’s memory to the computer’s processing unit. Add a few dollars to the balance, and then move it back to the computer’s memory. The movement of data . . . that’s all there is; there ain’t no more. Good enough! This chapter shows you how to move around your data. Using Variables Here’s an excerpt from a software company’s Web site: SnitSoft recognizes its obligation to the information technology community. For that reason, SnitSoft is making its most popular applications available for a nominal charge. For just $5.95 plus shipping and handling, you receive a CD-ROM containing SnitSoft’s premier products.
- 108. 90 Part II: Writing Your Own Java Programs Go ahead. Click the Order Now! link. Just see what happens. You get an order form with two items on it. One item is labeled $5.95 (CD-ROM), and the other item reads $25.00 (shipping and handling). What a rip-off! Thanks to SnitSoft’s generosity, you can pay $30.95 for ten cents worth of software. Behind the scenes of the SnitSoft Web page, a computer program does some scoundrel’s arithmetic. The program looks something like the code in Listing 6-1. Listing 6-1: SnitSoft’s Grand Scam class SnitSoft { public static void main(String args[]) { double amount; amount = 5.95; amount = amount + 25.00; System.out.print(“We will bill $”); System.out.print(amount); System.out.println(“ to your credit card.”); } } When I run the Listing 6-1 code on my own computer (not the SnitSoft com- puter), I get the output shown in Figure 6-1. Figure 6-1: Running the code from Listing 6-1. Using a variable The code in Listing 6-1 makes use of a variable named amount. A variable is a placeholder. You can stick a number like 5.95 into a variable. After you’ve placed a number in the variable, you can change your mind and put a differ- ent number, like 30.95, into the variable. (That’s what varies in a variable.) Of course, when you put a new number in a variable, the old number is no longer there. If you didn’t save the old number somewhere else, the old number is gone.
- 109. Chapter 6: Using the Building Blocks: Variables, Values, and Types 91 Figure 6-2 gives a before-and-after picture of the code in Listing 6-1. When the computer executes amount = 5.95, the variable amount has the number 5.95 in it. Then, after the amount = amount + 25.00 statement is executed, the variable amount suddenly has 30.95 in it. When you think about a variable, picture a place in the computer’s memory where wires and transistors store 5.95, 30.95, or whatever. In Figure 6-2, imagine that each box is surrounded by millions of other such boxes. Now you need some terminology. (You can follow along in Figure 6-3.) The thing stored in a variable is called a value. A variable’s value can change during the run of a program (when SnitSoft adds the shipping and handling cost, for example). The value stored in a variable isn’t necessarily a number. (You can, for example, create a variable that always stores a letter.) The kind of value stored in a variable is a variable’s type. (You can read more about types in the rest of this chapter and in the next two chapters as well.) There’s a subtle, almost unnoticeable difference between a variable and a variable’s name. Even in formal writing, I often use the word variable when I mean variable name. Strictly speaking, amount is the variable name, and all the memory storage associated with amount (including the value and type of amount) is the variable itself. If you think this distinction between variable and variable name is too subtle for you to worry about, join the club. Figure 6-2: A variable (before and after).
- 110. 92 Part II: Writing Your Own Java Programs Figure 6-3: A variable, its value, and its type. Every variable name is an identifier — a name that you can make up in your own code (for more about this, see Chapter 4). In preparing Listing 6-1, I made up the name amount. Understanding assignment statements The statements with equal signs in Listing 6-1 are called assignment statements. In an assignment statement, you assign a value to something. In many cases, this something is a variable. You should get into the habit of reading assignment statements from right to left. For example, the first assignment statement in Listing 6-1 says, “Assign 5.95 to the amount variable.” The second assignment statement is just a bit more complicated. Reading the second assignment statement from right to left, you get “Add 25.00 to the value that’s already in the amount variable and make that number (30.95) be the new value of the amount variable.” For a graphic, hit-you-over-the-head illustration of this, see Figure 6-4. In an assignment statement, the thing being assigned a value is always on the left side of the equal sign.
- 111. Chapter 6: Using the Building Blocks: Variables, Values, and Types 93 Figure 6-4: Reading an assignment statement from right to left. To wrap or not to wrap? The last three statements in Listing 6-1 use a neat trick. You want the program to display just one line on the screen, but this line contains three different things: The line starts with We will bill $. The line continues with the amount variable’s value. The line ends with to your credit card. These are three separate things, so you put these things in three separate statements. The first two statements are calls to System.out.print. The last statement is a call to System.out.println. Calls to System.out.print display text on part of a line and then leave the cursor at the end of the current line. After executing System.out.print, the cursor is still at the end of the same line, so the next System.out.whatever can continue printing on that same line. With several calls to print capped off by a single call to println, the result is just one nice-looking line of output, as Figure 6-5 illustrates. A call to System.out.print writes some things and leaves the cursor sitting at the end of the line of output. A call to System.out.println writes things and then finishes the job by moving the cursor to the start of a brand new line of output.
- 112. 94 Part II: Writing Your Own Java Programs Figure 6-5: The roles played by System.out. print and System.out. println. What Do All Those Zeros and Ones Mean? Here’s a word: gift The question for discussion is, what does that word mean? Well, it depends on who looks at the word. For example, an English-speaking reader would say that “gift” stands for something one person bestows upon another in a box covered in bright paper and ribbons. Look! I’m giving you a gift! But in German, the word “gift” means “poison.” Let me give you some gift, my dear. And in Swedish, “gift” can mean either “married” or “poison.” As soon as they got gift, she slipped a gift into his drink.
- 113. Chapter 6: Using the Building Blocks: Variables, Values, and Types 95 Then there’s French. In France, there’s a candy bar named “Gift.” He came for the holidays, and all he gave me was a bar of Gift. So what do the letters g-i-f-t really mean? Well, they don’t mean anything until you decide on a way to interpret them. The same is true of the zeros and ones inside a computer’s circuitry. Take, for example, the sequence 01001010. This sequence can stand for the letter J, but it can also stand for the number 74. That same sequence of zeros and ones can stand for 1.0369608636003646×10–43. And when interpreted as screen pixels, the same sequence can represent the dots shown in Figure 6-6. The meaning of 01001010 depends entirely on the way the software interprets this sequence. Figure 6-6: An extreme close-up of eight black- and-white screen pixels. Types and declarations How do you tell the computer what 01001010 stands for? The answer is in the concept called type. The type of a variable describes the kinds of values that the variable is permitted to store. In Listing 6-1, look at the first line in the body of the main method. double amount; This line is called a variable declaration. Putting this line in your program is like saying, “I’m declaring my intention to have a variable named amount in my program.” This line reserves the name amount for your use in the program. In this variable declaration, the word double is a Java keyword. This word double tells the computer what kinds of values you intend to store in amount. In particular, the word double stands for numbers between –1.8×10308 and
- 114. 96 Part II: Writing Your Own Java Programs 1.8×10308. That’s an enormous range of numbers. Without the fancy ×10 nota- tion, the second of these numbers is 1800000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000 0000.0 If the folks at SnitSoft ever charge that much for shipping and handling, they can represent the charge with a variable of type double. What’s the point? More important than the humongous range of the double keyword’s num- bers is the fact that a double value can have digits to the right of the decimal point. After you declare amount to be of type double, you can store all sorts of numbers in amount. You can store 5.95, 0.02398479, or –3.0. In Listing 6-1, if I hadn’t declared amount to be of type double, then I may not have been able to store 5.95. Instead, I would have had to store plain old 5 or dreary old 6, without any digits beyond the decimal point. For more info on numbers without decimal points, see Chapter 7. This paragraph deals with a really picky point, so skip it if you’re not in the mood. People often use the phrase “decimal number” to describe a number with digits to the right of the decimal point. The problem is, the syllable “dec” stands for the number ten, so the word “decimal” implies a base-10 representa- tion. Because computers store base-2 (not base-10) representations, the word “decimal” to describe such a number is a misnomer. But in this book, I just can’t help myself. I’m calling them “decimal numbers” whether the techies like it or not. Reading Decimal Numbers from the Keyboard I don’t believe it! SnitSoft is having a sale! For one week only, you can get the SnitSoft CD-ROM for the low price of just $5.75! Better hurry up and order one. No, wait! Listing 6-1 has the price fixed at $5.95. I have to revise the program.
- 115. Chapter 6: Using the Building Blocks: Variables, Values, and Types 97 I know. I’ll make the code more versatile. I’ll input the amount from the key- board. Listing 6-2 has the revised code, and Figure 6-7 shows a run of the new code. Listing 6-2: Getting a Double Value from the Keyboard import java.util.Scanner; class VersatileSnitSoft { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); double amount; System.out.print(“What’s the price of a CD-ROM? “); amount = myScanner.nextDouble(); amount = amount + 25.00; System.out.print(“We will bill $”); System.out.print(amount); System.out.println(“ to your credit card.”); } } Figure 6-7: Getting the value of a double variable. Though these be methods, yet there is madness in ’t Notice the call to the nextDouble method in Listing 6-2. Back in Listing 5-1, I use nextLine, but here in Listing 6-2, I use nextDouble. In Java, each type of input requires its own special method. If you’re getting a line of text, then nextLine works just fine. But if you’re reading stuff from the keyboard, and you want that stuff to be interpreted as a number, you need a method like nextDouble.
- 116. 98 Part II: Writing Your Own Java Programs To go from Listing 6-1 to Listing 6-2, I added an import declaration and some stuff about new Scanner(System.in). You can find out more about these things by reading the section on input and output in Chapter 5. (You can find out even more about input and output by visiting Chapter 13.) And more exam- ples (more myScanner.nextSomething methods) are in Chapters 7 and 8. Who does what, and how? When you write a program, you’re called a pro- nextLine, nextDouble, or one of the Scanner grammer, but when you run a program, you’re class’s other nextSomething methods. These called a user. So when you test your own code, print and next calls belong in two separate you’re being both the programmer and the user. statements. Java has no commonly used, single statement that does both the prompting and the Suppose that your program contains a my “next-ing.” Scanner.nextSomething() call, like the calls in Listings 5-1 and 6-2. Then your program As the programmer, your job is to combine the gets input from the user. But, when the program prompting and the next-ing. You can combine runs, how does the user know to type some- prompting and next-ing in all kinds of ways. thing on the keyboard? If the user and the pro- Some ways are helpful to the user, and some grammer are the same person, and the program ways aren’t. is fairly simple, then knowing what to type is no If you don’t have a call to print or big deal. For example, when you start running println, then the user sees no prompt. A the code in Listing 5-1, you have this book in blinking cursor sits quietly and waits for the front of you, and the book says “The computer is user to type something. The user has to guess waiting for you to type something . . . You type what kind of input to type. Occasionally one line of text . . .” So you type the text and that’s okay, but usually it isn’t. press Enter. Everything is fine. If you call print or println, but you But very few programs come with their own don’t call a myScanner.nextSomething books. In many instances, when a program starts method, then the computer doesn’t wait for running, the user has to stare at the screen to the user to type anything. The program races figure out what to do next. The code in Listing 6-2 to execute whatever statement comes imme- works in this stare-at-the-screen scenario. In diately after the print or println. Listing 6-2, the first call to print puts an infor- mative message (What’s the price of a If your prompt displays a misleading mes- CD-ROM?) on the user’s screen. A message of sage, then you mislead the user. Java has this kind is called a prompt. no built-in feature that checks the appropri- ateness of a prompt. That’s not surprising. When you start writing programs, you can easily Most computer languages have no prompt- confuse the roles of the prompt and the user’s checking feature. input. So remember, no preordained relationship exists between a prompt and the subsequent So be careful with your prompts and gets. Be nice input. To create a prompt, you call print or to your user. Remember, you were once a humble println. Then, to read the user’s input, you call computer user too.
- 117. Chapter 6: Using the Building Blocks: Variables, Values, and Types 99 Methods and assignments Note how I use myScanner.nextDouble in Listing 6-2. The method myScanner. nextDouble is called as part of assignment statement. If you look in Chapter 5 at the section on how the EchoLine program works, you see that the com- puter can substitute something in place of a method call. The computer does this in Listing 6-2. When you type 5.75 on the keyboard, the computer turns amount = myScanner.nextDouble(); into amount = 5.75; (The computer doesn’t really rewrite the code in Listing 6-2. This amount = 5.75 line just illustrates the effect of the computer’s action.) In the second assignment statement in Listing 6-2, the computer adds 25.00 to the 5.75 that’s stored in amount. Some method calls have this substitution effect, and others (like System. out.println) don’t. To find out more about this, see Chapter 19. Variations on a Theme Look back at Listing 6-1. In that listing, it takes two lines to give the amount variable its first value: double amount; amount = 5.95; You can do the same thing with just one line: double amount=5.95; When you do this, you don’t say that that you’re “assigning” a value to the amount variable. The line double amount=5.95 isn’t called an “assignment statement.” Instead, this line is called a declaration with an initialization. You’re initializing the amount variable. You can do all sorts of things with initializations, even arithmetic. double gasBill = 174.59; double elecBill = 84.21; double H2OBill = 22.88; double total = gasBill + elecBill + H2OBill;
- 118. 100 Part II: Writing Your Own Java Programs Moving variables from place to place It helps to remember the difference between initializations and assignments. For one thing, you can drag a declaration with its initialization outside of a method. //This is okay: class SnitSoft { static double amount = 5.95; public static void main(String args[]) { amount = amount + 25.00; System.out.print(“We will bill $”); System.out.print(amount); System.out.println(“ to your credit card.”); } } You can’t do the same thing with assignment statements. (See the following code and Figure 6-8.) //This does not compile: class BadSnitSoftCode { static double amount; amount = 5.95; //Misplaced statement public static void main(String args[]) { amount = amount + 25.00; System.out.print(“We will bill $”); System.out.print(amount); System.out.println(“ to your credit card.”); } } Figure 6-8: A failed attempt to compile BadSnitSoft Code.
- 119. Chapter 6: Using the Building Blocks: Variables, Values, and Types 101 You can’t drag statements outside of methods. (Even though a variable decla- ration ends with a semicolon, a variable declaration isn’t considered to be a statement. Go figure!) The advantage of putting a declaration outside of a method is illustrated in Chapter 19. While you wait impatiently to reach that chapter, notice how I added the word static to each declaration that I pulled out of the main method. I had to do this because the main method’s header has the word static in it. Not all methods are static. In fact, most methods aren’t static. But, whenever you pull a declaration out of a static method, you have to add the word static at the beginning of the declaration. All the mystery sur- rounding the word static is resolved in Chapter 18. Combining variable declarations The code in Listing 6-1 has only one variable (as if variables are in short supply). You can get the same effect with several variables. class SnitSoftNew { public static void main(String args[]) { double cdPrice; double shippingAndHandling; double total; cdPrice = 5.95; shippingAndHandling = 25.00; total = cdPrice + shippingAndHandling; System.out.print(“We will bill $”); System.out.print(total); System.out.println(“ to your credit card.”); } } This new code gives you the same output as the code in Listing 6-1. (Refer to Figure 6-1.) The new code has three declarations — one for each of the program’s three variables. Because all three variables have the same type (the type double), I can modify the code and declare all three variables in one fell swoop: double cdPrice, shippingAndHandling, total;
- 120. 102 Part II: Writing Your Own Java Programs So which is better, one declaration or three declarations? Neither is better. It’s a matter of personal style. You can even add initializations to a combined declaration. When you do, each initialization applies to only one variable. For example, with the line double cdPrice, shippingAndHandling = 25.00, total; the value of shippingAndHandling becomes 25.00, but the variables cdPrice and total get no particular value.
- 121. Chapter 7 Numbers and Types In This Chapter Processing whole numbers Making new values from old values Understanding Java’s more exotic types N ot so long ago, people thought computers did nothing but big, number- crunching calculations. Computers solved arithmetic problems, and that was the end of the story. In the 1980s, with the widespread use of word-processing programs, the myth of the big metal math brain went by the wayside. But even then, computers made great calculators. After all, computers are very fast and very accurate. Computers never need to count on their fingers. Best of all, computers don’t feel burdened when they do arithmetic. I hate ending a meal in a good restau- rant by worrying about the tax and tip, but computers don’t mind that stuff at all. (Even so, computers seldom go out to eat.) Using Whole Numbers Let me tell you, it’s no fun being an adult. Right now I have four little kids in my living room. They’re all staring at me because I have a bag full of gumballs in my hand. With 30 gumballs in the bag, the kids are all thinking “Who’s the best? Who gets more gumballs than the others? And who’s going to be treated unfairly?” They insist on a complete, official gumball count, with each kid get- ting exactly the same number of tasty little treats. I must be careful. If I’m not, then I’ll never hear the end of it.
- 122. 104 Part II: Writing Your Own Java Programs With 30 gumballs and four kids, there’s no way to divide the gumballs evenly. Of course, if I get rid of a kid, then I can give ten gumballs to each kid. The trouble is, gumballs are disposable; kids are not. So my only alternative is to divvy up what gumballs I can and dispose of the rest. “Okay, think quick,” I say to myself. “With 30 gumballs and four kids, how many gumballs can I promise to each kid?” I waste no time in programming my computer to figure out this problem for me. When I’m finished, I have the code in Listing 7-1. Listing 7-1: How to Keep Four Kids from Throwing Tantrums class KeepingKidsQuiet { public static void main(String args[]) { int gumballs; int kids; int gumballsPerKid; gumballs = 30; kids = 4; gumballsPerKid = gumballs / kids; System.out.print(“Each kid gets “); System.out.print(gumballsPerKid); System.out.println(“ gumballs.”); } } A run of the KeepingKidsQuiet program is shown in Figure 7-1. If each kid gets seven gumballs, then the kids can’t complain that I’m playing favorites. They’ll have to find something else to squabble about. Figure 7-1: Fair and square. At the core of the gumball problem, I’ve got whole numbers — numbers with no digits beyond the decimal point. When I divide 30 by 4, I get 71⁄2, but I can’t take the 1⁄2 seriously. No matter how hard I try, I can’t divide a gumball in half, at least not without hearing “my half is bigger than his half.” This fact is
- 123. Chapter 7: Numbers and Types 105 reflected nicely in Java. In Listing 7-1, all three variables (gumballs, kids, and gumballsPerKid) are of type int. An int value is a whole number. When you divide one int value by another (as you do with the slash in Listing 7-1), you get another int. When you divide 30 by 4, you get 7 — not 71⁄2. You see this in Figure 7-1. Taken together, the statements gumballsPerKid = gumballs/kids; System.out.print(gumballsPerKid); put the number 7 on the computer screen. Reading whole numbers from the keyboard What a life! Yesterday there were four kids in my living room, and I had 30 gum- balls. Today there are six kids in my house, and I have 80 gumballs. How can I cope with all this change? I know! I’ll write a program that reads the numbers of gumballs and kids from the keyboard. The program is in Listing 7-2, and a run of the program is shown in Figure 7-2. Listing 7-2: A More Versatile Program for Kids and Gumballs import java.util.Scanner; class KeepingMoreKidsQuiet { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int gumballs; int kids; int gumballsPerKid; System.out.print(“How many gumballs? How many kids? “); gumballs = myScanner.nextInt(); kids = myScanner.nextInt(); gumballsPerKid = gumballs / kids; System.out.print(“Each kid gets “); System.out.print(gumballsPerKid); System.out.println(“ gumballs.”); } }
- 124. 106 Part II: Writing Your Own Java Programs Figure 7-2: Next thing you know, I’ll have seventy kids and a thousand gumballs. You should notice a couple of things about Listing 7-2. First, you can read an int value with the nextInt method. Second, you can issue successive calls to Scanner methods. In Listing 7-2, I call nextInt twice. All I have to do is separate the numbers I type by blank spaces. In Figure 7-2, I put one blank space between my 80 and my 6, but more blank spaces would work as well. This blank space rule applies to many of the Scanner methods. For example, here’s some code that reads three numeric values: gumballs = myScanner.nextInt(); costOfGumballs = myScanner.nextDouble(); kids = myScanner.nextInt(); Figure 7-3 shows valid input for these three method calls. Figure 7-3: Three numbers for three Scanner method calls. What you read is what you get When you’re writing your own code, you should never take anything for granted. Suppose you accidentally reverse the order of the gumballs and kids assignment statements in Listing 7-2:
- 125. Chapter 7: Numbers and Types 107 //This code is misleading: System.out.print(“How many gumballs? How many kids? “); kids = myScanner.nextInt(); gumballs = myScanner.nextInt(); Then, the line How many gumballs? How many kids? is very misleading. Because the kids assignment statement comes before the gumballs assign- ment statement, the first number you type becomes the value of kids, and the second number you type becomes the value of gumballs. It doesn’t matter that your program displays the message How many gumballs? How many kids?. What matters is the order of the assignment statements in the program. If the kids assignment statement accidentally comes first, you can get a strange answer, like the zero answer in Figure 7-4. That’s how int division works. It just cuts off any remainder. Divide a small number (like 6) by a big number (like 80), and you get 0. Figure 7-4: How to make six kids very unhappy.
- 126. 108 Part II: Writing Your Own Java Programs Creating New Values by Applying Operators What could be more comforting than your old friend, the plus sign? It was the first thing you learned about in elementary school math. Almost everybody knows how to add two and two. In fact, in English usage, adding two and two is a metaphor for something that’s easy to do. Whenever you see a plus sign, one of your brain cells says, “Thank goodness, it could be something much more complicated.” So Java has a plus sign. You can use the plus sign to add two numbers: int apples, oranges, fruit; apples = 5; oranges = 16; fruit = apples + oranges; Of course, the old minus sign is available too: apples = fruit - oranges; Use an asterisk for multiplication, and a forward slash for division: double rate, pay, withholding; int hours; rate = 6.25; hours = 35; pay = rate * hours; withholding = pay / 3.0; When you divide an int value by another int value, you get an int value. The computer doesn’t round. Instead, the computer chops off any remainder. If you put System.out.println(11 / 4) in your program, the computer prints 2, not 2.75. If you need a decimal answer, make either (or both) of the numbers you’re dividing double values. For example, if you put System.out. println(11.0 / 4) in your program, the computer divides a double value, 11.0, by an int value, 4. Because at least one of the two values is double, the computer prints 2.75. Finding a remainder There’s a useful arithmetic operator called the remainder operator. The symbol for the remainder operator is the percent sign (%). When you put System. out.println(11 % 4) in your program, the computer prints 3. It does this because 4 goes into 11 who-cares-how-many times, with a remainder of 3.
- 127. Chapter 7: Numbers and Types 109 The remainder operator turns out to be fairly useful. After all, a remainder is the amount you have left over after you divide two numbers. What if you’re making change for $1.38? After dividing 138 by 25, you have 13 cents left over, as shown in Figure 7-5. 138 cents 138/25 is 5 Figure 7-5: Hey, bud! Got change for 138 138%25 is 13 sticks? The code in Listing 7-3 makes use of this remainder idea. Listing 7-3: Making Change import java.util.Scanner; class MakeChange { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int quarters, dimes, nickels, cents; int whatsLeft, total; System.out.print(“How many cents do you have? “); total = myScanner.nextInt(); quarters = total / 25; whatsLeft = total % 25; (continued)
- 128. 110 Part II: Writing Your Own Java Programs Listing 7-3 (continued) dimes = whatsLeft / 10; whatsLeft = whatsLeft % 10; nickels = whatsLeft / 5; whatsLeft = whatsLeft % 5; cents = whatsLeft; System.out.println(); System.out.println(“From “ + total + “ cents you get”); System.out.println(quarters + “ quarters”); System.out.println(dimes + “ dimes”); System.out.println(nickels + “ nickels”); System.out.println(cents + “ cents”); } } A run of the code in Listing 7-3 is shown in Figure 7-6. You start with a total of 138 cents. The statement quarters = total / 25; divides 138 by 25, giving 5. That means you can make 5 quarters from 138 cents. Next, the statement whatsLeft = total % 25; divides 138 by 25 again, and puts only the remainder, 13, into whatsLeft. Now you’re ready for the next step, which is to take as many dimes as you can out of 13 cents. Figure 7-6: Change for $1.38. You keep going like this until you’ve divided away all the nickels. At that point, the value of whatsLeft is just 3 (meaning 3 cents). When two or more variables have similar types, you can create the variables with combined declarations. For example, Listing 7-3 has two combined declarations — one for the variables quarters, dimes, nickels, and cents (all of type int); another for the variables whatsLeft and total (both of
- 129. Chapter 7: Numbers and Types 111 type int). But to create variables of different types, you need separate decla- rations. For example, to create an int variable named total and a double variable named amount, you need one declaration int total; and another declaration double amount;. Listing 7-3 has a call to System.out.println() with nothing in the paren- theses. When the computer executes this statement, the cursor jumps to a new line on the screen. (I often use this statement to put a blank line in a program’s output.) The increment and decrement operators Java has some neat little operators that make life easier (for the computer’s processor, for your brain, and for your fingers). Altogether there are four such operators — two increment operators and two decrement operators. The increment operators add one, and the decrement operators subtract one. To see how they work, you need some examples. Using preincrement The first example is in Figure 7-7. Figure 7-7: Using pre- increment.
- 130. 112 Part II: Writing Your Own Java Programs A run of the program in Figure 7-7 is shown in Figure 7-8. In this horribly uneventful run, the count of gumballs gets displayed three times. If thine int offends thee, cast it out The run in Figure 7-6 seems artificial. Why would double amount; you start with 138 cents? Why not use the more int total; familiar $1.38? The reason is that the number System.out.print(“How much 1.38 isn’t a whole number, and without whole money do you have? “); numbers, the remainder operator isn’t very amount = myScanner.nextDouble(); useful. For example, the value of 1.38 % 0.25 total = amount * 100; //This is 0.1299999999999999. All those nines are doesn’t quite work. tough to work with. In everyday arithmetic, multiplying by 100 does So if you want to input 1.38, then the program the trick. But computers are fussy. With a com- should take your 1.38 and turn it into 138 cents. puter, you have to be very careful when you mix The question is, how can you get your program int values and double values. (See the first do this? figure in this sidebar.) My first idea is to multiply 1.38 by 100:
- 131. Chapter 7: Numbers and Types 113 To cram a double value into an int variable, you need something called casting. When you cast a value, you essentially say. “I’m aware that I’m trying to squish a double value into an int variable. It’s a tight fit, but I want to do it anyway.” To do casting, you put the name of a type in parentheses, as follows: total = (int) (amount * 100); //This works! This casting notation turns the double value 138.00 into the int value 138, and everybody’s happy. (See the second figure in this sidebar.) Figure 7-8: A run of the preincre- ment code (the code in Figure 7-7). The double plus sign goes under two different names, depending on where you put it. When you put the ++ before a variable, the ++ is called the prein- crement operator. In the word preincrement, the pre stands for before. In this setting, the word before has two different meanings: You’re putting ++ before the variable. The computer adds 1 to the variable’s value before the variable gets used in any other part of the statement. Figure 7-9 has a slow-motion instant replay of the preincrement operator’s action. In Figure 7-9, the computer encounters the System.out.println (++gumballs) statement. First, the computer adds 1 to gumballs (raising the value of gumballs to 29). Then the computer executes System.out. println, using the new value of gumballs (29).
- 132. 114 Part II: Writing Your Own Java Programs Figure 7-9: The prein- crement operator in action. With System.out.println(++gumballs), the computer adds 1 to gum- balls before printing the new value of gumballs on the screen. Using postincrement An alternative to preincrement is postincrement. With postincrement, the post stands for after. The word after has two different meanings: You put ++ after the variable. The computer adds 1 to the variable’s value after the variable gets used in any other part of the statement. Figure 7-10 has a close-up view of the postincrement operator’s action. In Fig- ure 7-10, the computer encounters the System.out.println(gumballs++) statement. First, the computer executes System.out.println, using the old value of gumballs (28). Then the computer adds 1 to gumballs (raising the value of gumballs to 29). Figure 7-10: The postin- crement operator in action.
- 133. Chapter 7: Numbers and Types 115 Look at the bold line of code in Figure 7-11. The computer prints the old value of gumballs (28) on the screen. Only after printing this old value does the computer add 1 to gumballs (raising the gumballs value from 28 to 29). Statements and expressions Any part of a computer program that has a value expressions are described earlier in this sidebar.) is called an expression. If you write Which category does gumballs++ belong to? gumballs = 30; The surprising answer is both. The Java code gumballs++ is both a statement and an then 30 is an expression (an expression whose expression. value is the quantity 30). If you write Suppose that, before executing the code amount = 5.95 + 25.00; System.out.println(gumballs++), the then 5.95 + 25.00 is an expression (because value of gumballs is 28: 5.95 + 25.00 has the value 30.95). If you write As a statement, gumballs++ tells the com- gumballsPerKid = gumballs / puter to add 1 to gumballs. kids; As an expression, the value of gumballs++ then gumballs / kids is an expression. (The is 28, not 29. value of the expression gumballs / kids So even though gumballs gets 1 added to it, the depends on whatever values the variables code System.out.println(gumballs++) gumballs and kids have when the statement really means System.out.println(28). with the expression in it is executed.) (See the figure in this sidebar.) This brings us to the subject of the pre- and Now, almost everything you just read about postincrement and decrement operators. There gumballs++ is true about ++gumballs. The are two ways to think about these operators: the only difference is, as an expression, ++gum- way everyone understands it, and the right way. balls behaves in a more intuitive way. Suppose The way I explain it in most of this section (in that, before executing the code System.out. terms of time, with before and after) is the way println(++gumballs), the value of gum- everyone understands the concept. Unfortunately, balls is 28: the way everyone understands the concept isn’t really the right way. When you see ++ or --, As a statement, ++gumballs tells the com- you can think in terms of time sequence. But puter to add 1 to gumballs. occasionally some programmer uses ++ or -- As an expression, the value of ++gumballs in a convoluted way, and the notions of before is 29. and after break down. So if you’re ever in a tight spot, you should think about these operators in So with System.out.println(++gum- terms of statements and expressions. balls), the variable gumballs gets 1 added to it, and the code System.out.println First, remember that a statement tells the com- (++gumballs) really means System.out. puter to do something, and an expression has a println(29). value. (Statements are described in Chapter 4, and
- 134. 116 Part II: Writing Your Own Java Programs Figure 7-11: Using postin- crement. With System.out.println(gumballs++), the computer adds 1 to gumballs after printing the old value that gumballs already had. A run of the code in Figure 7-11 is shown in Figure 7-12. Compare Figure 7-12 with the run in Figure 7-8. With preincrement in Figure 7-8, the second number that gets displayed is 29. With postincrement in Figure 7-12, the second number that gets displayed is 28. In Figure 7-12, the number 29 doesn’t show up on the screen until the end of the run, when the computer executes one last System.out.println (gumballs). Figure 7-12: A run of the postincre- ment code (the code in Figure 7-11).
- 135. Chapter 7: Numbers and Types 117 Are you trying to decide between using preincrement or postincrement? Ponder no longer. Most programmers use postincrement. In a typical Java program, you often see things like gumballs++. You seldom see things like ++gumballs. In addition to preincrement and postincrement, Java has two operators that use --. These operators are called predecrement and postdecrement: With predecrement (--gumballs), the computer subtracts 1 from the variable’s value before the variable gets used in the rest of the statement. With postdecrement (gumballs--), the computer subtracts 1 from the variable’s value after the variable gets used in the rest of the statement. Assignment operators If you read the previous section — the section about operators that add 1 — you may be wondering if you can manipulate these operators to add 2, or add 5, or add 1000000. Can you write gumballs++++, and still call yourself a Java programmer? Well, you can’t. If you try it, then the compiler will give you an error message: unexpected type required: variable found : value gumballs++++; ^ So how can you add values other than 1? As luck would have it, Java has plenty of assignment operators you can use. With an assignment operator, you can add, subtract, multiply, or divide by anything you want. You can do other cool operations too. For example, you can add 1 to the kids variable by writing kids += 1; Is this better than kids++ or kids = kids + 1? No, it’s not better. It’s just an alternative. But you can add 5 to the kids variable by writing kids += 5; You can’t easily add 5 with pre- or postincrement. And what if the kids get stuck in an evil scientist’s cloning machine? The statement kids *= 2; multiplies the number of kids by 2.
- 136. 118 Part II: Writing Your Own Java Programs With the assignment operators, you can add, subtract, multiply, or divide a variable by any number. The number doesn’t have to be a literal. You can use a number-valued expression on the right side of the equal sign: double amount = 5.95; double shippingAndHandling = 25.00, discount = 0.15; amount += shippingAndHandling; amount -= discount * 2; The code above adds 25.00 (shippingAndHandling) to the value of amount. Then, the code subtracts 0.30 (discount * 2) from the value of amount. How generous! Size Matters Here are today’s new vocabulary words: foregift (fore-gift) n. A premium that a lessee pays to the lessor upon the taking of a lease. hereinbefore (here-in-be-fore) adv. In a previous part of this document. Now imagine yourself scanning some compressed text. In this text, all blanks have been removed to conserve storage space. You come upon the following sequence of letters: hereinbeforegiftedit The question is, what do these letters mean? If you knew each word’s length, you could answer the question. here in be foregift edit hereinbefore gifted it herein before gift Ed it A computer faces the same kind of problem. When a computer stores several numbers in memory or on a disk, the computer doesn’t put blank spaces between the numbers. So imagine that a small chunk of the computer’s memory looks like the stuff in Figure 7-13. (The computer works exclusively with zeros and ones, but Figure 7-13 uses ordinary digits. With ordinary digits, it’s easier to see what’s going on.)
- 137. Chapter 7: Numbers and Types 119 Figure 7-13: Storing the digits 4221. What number or numbers are stored in Figure 7-13? Is it two numbers, 42 and 21? Or is it one number, 4,221? And what about storing four numbers, 4, 2, 2, and 1? It all depends on the amount of space each number consumes. Imagine a variable that stores the number of paydays in a month. This number never gets bigger than 31. You can represent this small number with just eight zeros and ones. But what about a variable that counts stars in the universe? That number could easily be more than a trillion, and to represent one trillion accurately, you need 64 zeros and ones. At this point, Java comes to the rescue. Java has four types of whole numbers. Just as in Listing 7-1, I declare int gumballsPerKid; I can also declare byte paydaysInAMonth; short sickDaysDuringYourEmployment; long numberOfStars; Each of these types (byte, short, int, and long) has its own range of possi- ble values. (See Table 7-1.) Java has two types of decimal numbers (numbers with digits to the right of the decimal point). Just as in Listing 6-1, I declare double amount; I can also declare float monthlySalary; Given the choice between double and float, I always choose double. A vari- able of type double has a greater possible range of values and much greater accuracy. (See Table 7-1.)
- 138. 120 Part II: Writing Your Own Java Programs Table 7-1 Java’s Primitive Numeric Types Type Name Range of Values Whole Number Types Byte –128 to 127 Short –32768 to 32767 Int –2147483648 to 2147483647 Long –9223372036854775808 to 9223372036854775807 Decimal Number Types Float –3.4×1038 to 3.4×1038 Double –1.8×10308 to 1.8×10308 Table 7-1 lists six of Java’s primitive types (also known as simple types). Java has only eight primitive types, so only two of Java’s primitive types are miss- ing from Table 7-1. Chapter 8 describes the two remaining primitive types. Chapter 17 introduces types that aren’t primitive. As a beginning programmer, you don’t have to choose among the types in Table 7-1. Just use int for whole numbers and double for decimal numbers. If, in your travels, you see something like short or float in someone else’s program, just remember the following: The types byte, short, int, and long represent whole numbers. The types float and double represent decimal numbers. Most of the time, that’s all you need to know.
- 139. Chapter 8 Numbers? Who Needs Numbers? In This Chapter Working with characters Dealing with “true” or “false” values Rounding out your knowledge of Java’s primitive types I don’t particularly like fax machines. They’re so inefficient. Send a short fax and what do you have? You have two slices of tree — one at the sending end, and another at the receiving end. You also have millions of dots — dots that scan tiny little lines across the printed page. The dots distinguish patches of light from patches of darkness. What a waste! Compare a fax with an e-mail message. Using e-mail, I can send a 25-word con- test entry with just 2500 zeros and ones, and I don’t waste any paper. Best of all, an e-mail message doesn’t describe light dots and dark dots. An e-mail message contains codes for each of the letters — a short sequence of zeros and ones for the letter A, a different sequence of zeros and ones for the letter B, and so on. What could be simpler? Now imagine sending a one-word fax. The word is “true,” which is understood to mean, “true, I accept your offer to write Beginning Programming with Java For Dummies, 2nd Edition.” A fax with this message sends a picture of the four letters t-r-u-e, with fuzzy lines where dirt gets on the paper and little white dots where the cartridge runs short on toner. But really, what’s the essence of the “true” message? There are just two possi- bilities, aren’t there? The message could be “true” or “false,” and to represent those possibilities, I need very little fanfare. How about 0 for “false” and 1 for “true?” They ask, “Do you accept our offer to write Beginning Programming with Java For Dummies, 2nd Edition?” “1,” I reply.
- 140. 122 Part II: Writing Your Own Java Programs Too bad I didn’t think of that a few months ago. Anyway, this chapter deals with letters, truth, falsehood, and other such things. Characters In Chapters 6 and 7, you store numbers in all your variables. That’s fine, but there’s more to life than numbers. For example, I wrote this book with a com- puter, and this book contains thousands and thousands of non-numeric things called characters. The Java type that’s used to store characters is char. Listing 8-1 has a simple program that uses the char type, and a run of the Listing 8-1 program is shown in Figure 8-1. Listing 8-1: Using the char Type class LowerToUpper { public static void main(String args[]) { char smallLetter, bigLetter; smallLetter = ‘b’; bigLetter = Character.toUpperCase(smallLetter); System.out.println(bigLetter); } } Figure 8-1: Converting lower- to uppercase. In Listing 8-1, the first assignment statement stores the letter b in the smallLetter variable. In that statement, notice how b is surrounded by single quote marks. In a Java program, every char literal starts and ends with a single quote mark. When you surround a letter with quote marks, you tell the computer that the letter isn’t a variable name. For example, in Listing 8-1, the incorrect statement smallLetter = b would tell the computer to look for a variable named b. Because there’s no variable named b, you’d get a cannot find symbol message.
- 141. Chapter 8: Numbers? Who Needs Numbers? 123 In the second assignment statement of Listing 8-1, the program calls an API method whose name is Character.toUpperCase. The method Character. toUpperCase does what its name suggests — the method produces the uppercase equivalent of a lowercase letter. In Listing 8-1, this uppercase equivalent (the letter B) is assigned to the variable bigLetter, and the B that’s in bigLetter is printed on the screen, as illustrated in Figure 8-2. smallLetter = 'b'; b bigLetter = Character.toUpperCase(smallLetter); B B Figure 8-2: The action in Listing 8-1. System.out.printIn(bigLetter); When the computer displays a char value on the screen, the computer does not surround the character with single quote marks. I digress . . . A while ago, I wondered what would happen if I called the Character.to UpperCase method and fed the method a character that isn’t lowercase to begin with. I yanked out the Java API documentation, but I found no useful information. The documentation said that toUpperCase “converts the char- acter argument to uppercase using case mapping information from the UnicodeData file.” Thanks, but that’s not useful to me. Silly as it seems, I asked myself what I’d do if I were the toUpperCase method. What would I say if someone handed me a capital R and told me to capitalize that letter? I’d say, “Take back your stinking capital R.” In the lingo of comput- ing, I’d send that person an error message. So I wondered if I’d get an error message when I applied Character.toUpperCase to the letter R.
- 142. 124 Part II: Writing Your Own Java Programs I tried it. I cooked up the experiment in Listing 8-2. Listing 8-2: Investigating the Behavior of toUpperCase class MyExperiment { public static void main(String args[]) { char smallLetter, bigLetter; smallLetter = ‘R’; bigLetter = Character.toUpperCase(smallLetter); System.out.println(bigLetter); smallLetter = ‘3’; bigLetter = Character.toUpperCase(smallLetter); System.out.println(bigLetter); } } In my experiment, I didn’t mix chemicals and blow things up. Here’s what I did instead: I assigned ‘R’ to smallLetter. The toUpperCase method took the uppercase R and gave me back another uppercase R. (See Figure 8-3.) I got no error message. This told me what the toUpperCase method does with a letter that’s already uppercase. The method does nothing. I assigned ‘3’ to smallLetter. The toUpperCase method took the digit 3 and gave me back the same digit 3. (See Figure 8-3.) I got no error message. This told me what the toUpperCase method does with a character that’s not a letter. It does nothing, zip, zilch, bupkis. Figure 8-3: Running the code in Listing 8-2. I write about this experiment to make an important point. When you don’t understand something about computer programming, it often helps to write a test program. Make up an experiment and see how the computer responds.
- 143. Chapter 8: Numbers? Who Needs Numbers? 125 I guessed that handing a capital R to the toUpperCase method would give me an error message, but I was wrong. See? The answers to questions aren’t handed down from heaven. The people who created the Java API made decisions. They made some obvious choices, and but they also made some unexpected choices. No one knows everything about Java’s features, so don’t expect to cram all the answers into your head. The Java documentation is great, but for every question that the documenta- tion answers, it ignores three other questions. So be bold. Don’t be afraid to tinker. Write lots of short, experimental programs. You can’t break the com- puter, so play tough with it. Your inquisitive spirit will always pay off. Reading and understanding Java’s API documentation is an art, not a science. For advice on making the most of these docs, take a look at the Appendix on this book’s web site. One character only, please A char variable stores only one character. So if you’re tempted to write the following statements char smallLetters; smallLetters = ‘barry’; //Don’t do this please resist the temptation. You can’t store more than one letter at a time in a char variable, and you can’t put more than one letter between a pair of single quotes. If you’re trying to store words or sentences (not just single letters), then you need to use something called a String. For a look at Java’s String type, see Chapter 18. Variables and recycling In Listing 8-2, I use smallLetter twice and I use bigLetter twice. That’s why they call these things variables. First the value of smallLetter is R. Later, I vary the value of smallLetter so that the value of smallLetter becomes 3. When I assign a new value to smallLetter, the old value of smallLetter gets obliterated. For example, in Figure 8-4, the second smallLetter assign- ment puts 3 into smallLetter. When the computer executes this second assignment statement, the old value R is gone.
- 144. 126 Part II: Writing Your Own Java Programs Figure 8-4: Varying the value of small Letter. Is that okay? Can you afford to forget the value that smallLetter once had? Yes, in Listing 8-2, it’s okay. After you’ve assigned a value to bigLetter with the statement bigLetter = Character.toUpperCase(smallLetter); you can forget all about the existing smallLetter value. You don’t need to do this: // This code is cumbersome. // The extra variables are unnecessary. char smallLetter1, bigLetter1; char smallLetter2, bigLetter2; smallLetter1 = ‘R’; bigLetter1 = Character.toUpperCase(smallLetter1); System.out.println(bigLetter1); smallLetter2 = ‘3’; bigLetter2 = Character.toUpperCase(smallLetter2); System.out.println(bigLetter2); You don’t need to store the old and new values in separate variables. Instead, you can reuse the variables smallLetter and bigLetter as in Listing 8-2. This reuse of variables doesn’t save you from a lot of extra typing. It doesn’t save much memory space either. But reusing variables keeps the program uncluttered. When you look at Listing 8-2, you can see at a glance that the code has two parts, and you see that both parts do roughly the same thing.
- 145. Chapter 8: Numbers? Who Needs Numbers? 127 The code in Listing 8-2 is simple and manageable. In such a small program, simplicity and manageability don’t matter very much. But in a large program, it helps to think carefully about the use of each variable. When not to reuse a variable The previous section discusses the reuse of variables to make a program slick and easy to read. This section shows you the flip side. In this section, the problem at hand forces you to create new variables. Suppose you’re writing code to reverse the letters in a four-letter word. You store each letter in its own separate variable. Listing 8-3 shows the code, and Figure 8-5 shows the code in action. Listing 8-3: Making a Word Go Backwards import java.util.Scanner; class ReverseWord { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); char c1, c2, c3, c4; c1 = myScanner.findInLine(“.”).charAt(0); c2 = myScanner.findInLine(“.”).charAt(0); c3 = myScanner.findInLine(“.”).charAt(0); c4 = myScanner.findInLine(“.”).charAt(0); System.out.print(c4); System.out.print(c3); System.out.print(c2); System.out.print(c1); System.out.println(); } } Figure 8-5: Stop those pots!
- 146. 128 Part II: Writing Your Own Java Programs The trick in Listing 8-3 is as follows: Assign values to variables c1, c2, c3, and c4 in that order. Display these variables’ values on the screen in reverse order: c4, c3, c2, and then c1, as illustrated in Figure 8-6. Keyboard input: p o t s c1 c2 c3 c4 The computer's memory: p o t s Figure 8-6: Using four variables. Screen output: s t o p If you don’t use four separate variables, then you don’t get the result that you want. For example, imagine that you store characters in only one variable. You run the program and type the word pots. When it’s time to display the word in reverse, the computer remembers the final s in the word pots. But the computer doesn’t remember the p, the o, or the t, as shown in Figure 8-7. Keyboard input: p o t s c1 Figure 8-7: The computer's memory: pots Getting things wrong because you used only one variable. Screen output: s s s s
- 147. Chapter 8: Numbers? Who Needs Numbers? 129 I wish I could give you twelve simple rules to help you decide when and when not to reuse variables. The problem is, I can’t. It all depends on what you’re trying to accomplish. So how do you figure out on your own when and when not to reuse variables? Like the guy says to the fellow who asks how to get to Carnegie Hall, “Practice, practice, practice.” Reading characters The people who created Java’s Scanner class didn’t create a next method for reading a single character. So to input a single character, I paste two Java API methods together. I use the findInLine and charAt methods. What’s behind all this findInLine(“.”). charAt(0) nonsense? Without wallowing in too much detail, here’s how not necessarily a dot.”) So findInLine(“.”) the findInLine(“.”).charAt(0) tech- tells the computer to find the next character of nique works: any kind that the user types on the keyboard. When you’re trying to input a single character, Java’s findInLine method looks for things in a findInLine(“.”) is mighty useful. line of input. The things the method finds depend on the stuff you put in parentheses. For example, But wait! To grab a single character from the key- a call to findInLine(“ddd”) looks board, I call findInLine(“.”).charAt(0). for a group consisting of three digits. With the What’s the role of charAt(0) in reading a single following line of code character? Unfortunately, any findInLine System.out.println(myScanner.fi call behaves as if it’s finding a bunch of charac- ndInLine(“ddd”)); ters, not just a single character. Even when you call findInLine(“.”), and the computer I can type fetches just one letter from the keyboard, the Testing 123 Testing Testing Java program treats that letter as one of possi- bly many input characters. and the computer responds by displaying The call to charAt(0) takes care of the multi- 123 character problem. This charAt(0) call tells In the call findInLine(“ddd”), Java to pick the initial character from any of the each d stands for a single digit. This d characters that findInLine fetches. business is one of many abbreviations in spe- Yes, it’s complicated. And yes, I don’t like having cial code called regular expressions. to explain it. But no, you don’t have to under- Now here’s something strange. In the world of stand any of the details in this sidebar. Just read regular expressions, a dot stands for any charac- the details if you want to read them, and skip the ter at all. (That is, a dot stands for “any character, details if you don’t care.
- 148. 130 Part II: Writing Your Own Java Programs Table 5-1 in Chapter 5 introduces this findInLine(“.”).charAt(0) tech- nique for reading a single input character, and Listing 8-3 uses the technique to read one character at a time. (In fact, Listing 8-3 uses the technique four times to read four individual characters.) Notice the format for the input in Figure 8-5. To enter the characters in the word pots, I type four letters, one after another, with no blank spaces between the letters and no quote marks. The findInLine(“.”).charAt(0) technique works that way, but don’t blame me or my technique. Other developers’ char- acter reading methods work the same way. No matter whose methods you use, reading a character differs from reading a number. Here’s how: With methods like nextDouble and nextInt, you type blank spaces between numbers. If I type 80 6, then two calls to nextInt read the number 80, followed by the number 6. If I type 806, then a single call to nextInt reads the number 806 (eight hundred six), as illustrated in Figure 8-8. With findInLine(“.”).charAt(0), you don’t type blank spaces between characters. If I type po, then two successive calls to findInLine(“.”).charAt(0) read the letter p, followed by the letter o. If I type p o, then two calls to findInLine(“.”).charAt(0) read the letter p, followed by a blank space character. (Yes, the blank space is a character!) Again, see Figure 8-8. Figure 8-8: Reading numbers and characters.
- 149. Chapter 8: Numbers? Who Needs Numbers? 131 To represent a lone character in the text of a computer program, you surround the character with single quote marks. But, when you type a character as part of a program’s input, you don’t surround the character with quote marks. Suppose that your program calls nextInt, and then findInLine(“.”). charAt(0). If you type 80x on the keyboard, you get an error message. (The message says InputMismatchException. The nextInt method expects you to type a blank space after each int value.) Now what happens if, instead of typing 80x, you type 80 x on the keyboard? Then the program gets 80 for the int value, followed by a blank space for the character value. For the program to get the x, the program has to call findInLine(“.”).charAt(0) one more time. It seems wasteful, but it makes sense in the long run. The boolean Type I’m in big trouble. I have 140 gumballs, and 15 kids are running around and screaming in my living room. They’re screaming because each kid wants 10 gumballs, and they’re running because that’s what kids do in a crowded living room. I need a program that tells me if I can give 10 gumballs to each kid. I need a variable of type boolean. A boolean variable stores one of two values — true or false (true, I can give 10 gumballs to each kid; or false, I can’t give 10 gumballs to each kid). Anyway, the kids are going berserk, so I’ve written a short program and put it in Listing 8-4. The output of the pro- gram is shown in Figure 8-9. Listing 8-4: Using the boolean Type class CanIKeepKidsQuiet { public static void main(String args[]) { int gumballs; int kids; int gumballsPerKid; boolean eachKidGetsTen; gumballs = 140; kids = 15; gumballsPerKid = gumballs / kids; System.out.print(“True or false? “); System.out.println(“Each kid gets 10 gumballs.”); eachKidGetsTen = gumballsPerKid >= 10; System.out.println(eachKidGetsTen); } }
- 150. 132 Part II: Writing Your Own Java Programs Figure 8-9: Oh, no! In Listing 8-4, the variable eachKidGetsTen is of type boolean. So the value stored in the eachKidGetsTen variable can be either true or false. (I can’t store a number or a character in the eachKidGetsTen variable.) To find a value for the variable eachKidGetsTen, the program checks to see if gumballsPerKid is greater than or equal to ten. (The symbols >= stand for “greater than or equal to.” What a pity! There’s no _ key on the standard com- puter keyboard.) Because gumballsPerKid is only nine, gumballsPerKid _ 10 is false. So eachKidGetsTen becomes false. Yikes! The kids will tear the house apart! (Before they do, take a look at Figure 8-10.) Figure 8-10: Assigning a value to the eachKid GetsTen variable. Expressions and conditions In Listing 8-4, the code gumballsPerKid >= 10 is an expression. The expres- sion’s value depends on the value stored in the variable gumballsPerKid. On a bad day, the value of gumballsPerKid >= 10 is false. So the variable eachKidGetsTen is assigned the value false. An expression like gumballsPerKid >= 10, whose value is either true or false, is sometimes called a condition.
- 151. Chapter 8: Numbers? Who Needs Numbers? 133 Values like true and false may look as if they contain characters, but they really don’t. Internally, the Java Virtual Machine doesn’t store boolean values with the letters t-r-u-e or f-a-l-s-e. Instead, the JVM stores codes, like 0 for false and 1 for true. When the computer displays a boolean value (as in System.out.println(eachKidGetsTen)), the Java virtual machine converts a code like 0 into the five-letter word false. Comparing numbers; comparing characters In Listing 8-4, I compare a variable’s value with the number 10. I use the >= operator in the expression gumballsPerKid >= 10 Of course, the greater-than-or-equal comparison gets you only so far. Table 8-1 shows you the operators you can use to compare things with one another. Table 8-1 Comparison Operators Operator Meaning Example Symbol == is equal to yourGuess == winningNumber != is not equal to 5 != numberOfCows < is less than strikes < 3 > is greater than numberOfBoxtops > 1000 <= is less than or equal to numberOfCows + numberOfBulls <= 5 >= is greater than or equal to gumballsPerKid >= 10 With the operators in Table 8-1, you can compare both numbers and characters. Notice the double equal sign in the first row of Table 8-1. Don’t try to use a single equal sign to compare two values. The expression yourGuess = winningNumber (with a single equal sign) doesn’t compare yourGuess with winningNumber. Instead yourGuess = winningNumber changes the value of yourGuess. (It assigns the value of winningNumber to the variable yourGuess.)
- 152. 134 Part II: Writing Your Own Java Programs You can compare other things (besides numbers and characters) with the == and != operators. But when you do, you have to be careful. For more infor- mation, see Chapter 18. Comparing numbers Nothing is more humdrum than comparing numbers. “True or false? Five is greater than or equal to ten.” False. Five is neither greater than nor equal to ten. See what I mean? Bo-ring. Comparing whole numbers is an open-and-shut case. But unfortunately, when you compare decimal numbers, there’s a wrinkle. Take a program for convert- ing from Celsius to Fahrenheit. Wait! Don’t take just any such program; take the program in Listing 8-5. Listing 8-5: It’s Warm and Cozy in Here import java.util.Scanner; class CelsiusToFahrenheit { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); double celsius, fahrenheit; System.out.print(“Enter the Celsius temperature: “); celsius = myScanner.nextDouble(); fahrenheit = 9.0 / 5.0 * celsius + 32.0; System.out.print(“Room temperature? “); System.out.println(fahrenheit == 69.8); } } If you run the code in Listing 8-5 and input the number 21, the computer finds the value of 9.0 / 5.0 * 21 + 32.0. Believe it or not, you want to check the computer’s answer. (Who knows? Maybe the computer gets it wrong!) You need to do some arithmetic, but please don’t reach for your calculator. A calculator is just a small computer, and machines of that kind stick up for one another. To check the computer’s work, you need to do the arithmetic by hand. What? You say you’re math phobic? Well, don’t worry. I’ve done all the math in Figure 8-11. If you do the arithmetic by hand, then the value you get for 9.0 / 5.0 * 21 + 32.0 is exactly 69.8. So run the code in Listing 8-5, and give celsius the value 21. You should get true when you display the value of fahrenheit == 69.8, right?
- 153. Chapter 8: Numbers? Who Needs Numbers? 135 Figure 8-11: The Fahrenheit temperature is exactly 69.8. Well, no. Take a look at the run in Figure 8-12. When the computer evaluates fahrenheit == 69.8, the value turns out to be false, not true. What’s going on here? Figure 8-12: A run of the code in Listing 8-5. A little detective work can go a long way. So review the facts: Fact: The value of fahrenheit should be exactly 69.8. Fact: If fahrenheit is 69.8, then fahrenheit == 69.8 is true. Fact: In Figure 8-12, the computer displays the word false. So the expression fahrenheit == 69.8 isn’t true. How do you reconcile these facts? There can be little doubt that fahrenheit == 69.8 is false, so what does that say about the value of fahrenheit? Nowhere in Listing 8-5 is the value of fahrenheit displayed. Could that be the problem? At this point, I use a popular programmer’s trick. I add statements to display the value of fahrenheit. fahrenheit = 9.0 / 5.0 * celsius + 32.0; System.out.print(“fahrenheit: “); //Added System.out.println(fahrenheit); //Added
- 154. 136 Part II: Writing Your Own Java Programs Automated debugging If your program isn’t working correctly, you can try of clicking your mouse, you debug a program by something called a debugger. A debugger auto- typing commands. matically adds invisible print and println Of course, typing commands can be tedious. So calls to your suspicious code. In fact, debuggers some integrated development environments have all kinds of features to help you diagnose enhance jdb with additional graphical tools. For problems. For example, a debugger can pause a example, the Pro version of JCreator provides run of your program and accept special com- menus for issuing jdb commands. (See the figure.) mands to display variables’ values. With some debuggers, you can pause a run and change a In this book, I don’t promote the use of an auto- variable’s value (just to see if things go better mated debugger. But for any large programming when you do). project, automated debugging is an essential tool. So if you plan to write bigger and better The debugger jdb comes with the free JDK programs, please give jdb a try. To get started, download from Sun Microsystems. The only visit www.jcreator.com and download problem is, jdb is entirely text-based. When you JCreator Pro. use jdb, you leave your mouse behind. Instead
- 155. Chapter 8: Numbers? Who Needs Numbers? 137 A run of the enhanced code is shown in Figure 8-13. As you can see, the com- puter misses its mark. Instead of the expected value 69.8, the computer’s value for 9.0 / 5.0 * 21 + 32.0 is 69.80000000000001. That’s just the way the cookie crumbles. The computer does all its arithmetic with zeros and ones, so the computer’s arithmetic doesn’t look like the base-10 arithmetic in Figure 8-11. The computer’s answer isn’t wrong. The answer is just slightly inaccurate. Figure 8-13: The Fahrenheit variable’s full value. So be careful when you compare two numbers for equality (with ==) or for inequality (with !=). When you compare two double values, the values are almost never dead-on equal to one another. If your program isn’t doing what you think it should do, then check your sus- picions about the values of variables. Add print and println statements to your code. Comparing characters The comparison operators in Table 8-1 work overtime for characters. Roughly speaking, the operator < means “comes earlier in the alphabet.” But you have to be careful of the following: Because B comes alphabetically before H, the condition ‘B’ < ‘H’ is true. That’s not surprising. Because b comes alphabetically before h, the condition ‘b’ < ‘h’ is true. That’s no surprise either. Every uppercase letter comes before any of the lowercase letters, so the condition ‘b’ < ‘H’ is false. Now that’s a surprise. (See Figure 8-14.) In practice, you seldom have reason to compare one letter with another. But in Chapter 18, you can read about Java’s String type. With the String type, you can compare words, names, and other good stuff. At that point, you have to think carefully about alphabetical ordering, and the ideas in Figure 8-14 come in handy.
- 156. 138 Part II: Writing Your Own Java Programs Figure 8-14: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz The ordering lesser greater of the letters. Under the hood, the letters A through Z are stored with numeric codes 65 through 90. The letters a through z are stored with codes 97 through 122. That’s why each uppercase letter is “less than” any of the lowercase letters. The Remaining Primitive Types In Chapter 7, I tell you that Java has eight primitive types, but Table 7-1 lists only six out of eight types. Table 8-2 describes the remaining two types — the types char and boolean. Table 8-2 isn’t too exciting, but I can’t just leave you with the incomplete story in Table 7-1. Table 8-2 Java’s Primitive Non-numeric Types Type Name Range of Values Character Type char Thousands of characters, glyphs, and symbols Logical Type boolean Only true or false If you dissect parts of the Java virtual machine, you find that Java considers char to be a numeric type. That’s because Java represents characters with something called Unicode — an international standard for representing alpha- bets of the world’s many languages. For example the Unicode representation of an uppercase letter C is 67. The representation of a Hebrew letter aleph is 1488. And (to take a more obscure example) the representation for the voiced retroflex approximant in phonetics is 635. But don’t worry about all this. The only reason I’m writing about the char type’s being numeric is to save face among my techie friends.
- 157. Part III Controlling the Flow
- 158. In this part . . . A computer program is like a role-playing video game. It’s not the kind of game that involves shooting, punching, or racing. It’s a game that involves strategies. Find the golden ring to open the secret passageway. Save the princess by reciting the magic words. It’s that sort of thing. So in this part of the book, you create passageways. As your program weaves its way from one virtual room to another, the computer gets closer and closer to the solution of an important problem. Hey, admit it. This sounds like fun!
- 159. Chapter 9 Forks in the Road In This Chapter Writing statements that choose between alternatives Putting statements inside one another Writing several kinds of decision making statements H ere’s an excerpt from Beginning Programming with Java For Dummies, 2nd Edition, Chapter 2: If your computer already has a Java 5.0 compiler, you can skip the next section’s steps.* The excerpt illustrates two important points: First, you may not have to follow some of the steps in Chapter 2. Second, your choice of action can depend on something being true or false. If it’s true that your computer already has a Java 5.0 compiler, skip certain steps in Chapter 2. So picture yourself walking along a quiet country road. You’re enjoying a pleasant summer day. It’s not too hot, and a gentle breeze from the north makes you feel fresh and alert. You’re holding a copy of this book, opened to the start of Chapter 2. You read the paragraph about having a Java 5.0 com- piler, and then you look up. You see a fork in the road. You see two signs — one pointing to the right; the other pointing to the left. One sign reads, “Have a Java 5.0 compiler? True.” The other sign reads, “Have a Java 5.0 compiler? False.” You evaluate the compiler situation and march on, veering right or left depending on your soft- ware situation. A diagram of this story is shown in Figure 9-1. * This excerpt is reprinted with permission from Wiley Publishing, Inc. If you can’t find a copy of Beginning Programming with Java For Dummies, 2nd Edition in your local bookstore, visit www. wiley.com.
- 160. 142 Part III: Controlling the Flow Figure 9-1: Which way to go? Life is filled with forks in the road. Take an ordinary set of directions for heat- ing up a frozen snack: Microwave cooking directions: Place on microwave safe plate. Microwave on high for 2 minutes. Turn product. Microwave on high for 2 more minutes. Conventional oven directions: Preheat oven to 350 degrees. Place product on baking sheet. Bake for 25 minutes. Again, you choose between alternatives. If you use a microwave oven, do this. Otherwise, do that. In fact, it’s hard to imagine useful instructions that don’t involve choices. If you’re a homeowner with two dependents earning more than $30,000 per year, check here. If you don’t remember how to use curly braces in Java pro- grams, see Chapter 4. Did the user correctly type his or her password? If yes,
- 161. Chapter 9: Forks in the Road 143 then let the user log in; if no, then kick the bum out. If you think the market will go up, then buy stocks; otherwise, buy bonds. And if you buy stocks, which should you buy? And when should you sell? This chapter deals with decision-making, which plays a fundamental role in the creation of instructions. With the material in this chapter, you expand your programming power by leaps and bounds. Making Decisions (Java if Statements) When you work with computer programs, you make one decision after another. Almost every programming language has a way of branching in one of two directions. In Java (and in many other languages) the branching fea- ture is called an if statement. Check out Listing 9-1 to see an if statement. Listing 9-1: An if Statement if (randomNumber > 5) { System.out.println(“Yes. Isn’t it obvious?”); } else { System.out.println(“No, and don’t ask again.”); } The if statement in Listing 9-1 represents a branch, a decision, two alternative courses of action. In plain English, this statement has the following meaning: If the randomNumber variable’s value is greater than 5, display “Yes. Isn’t it obvious?” on the screen. Otherwise, display “No, and don’t ask again.” on the screen. Pictorially, you get the fork shown in Figure 9-2. Looking carefully at if statements An if statement can take the following form: if (Condition) { SomeStatements } else { OtherStatements }
- 162. 144 Part III: Controlling the Flow randomNumber > 5 e tru fa lse Figure 9-2: A random number decides Display Display your fate. "Yes. Isn't it obvious?" "No, and don't ask again." To get a real-life if statement, substitute meaningful text for the three place- holders Condition, SomeStatements, and OtherStatements. Here’s how I make the substitutions in Listing 9-1: I substitute randomNumber > 5 for Condition. I substitute System.out.println(“Yes. Isn’t it obvious?”); for SomeStatements. I substitute System.out.println(“No, and don’t ask again.”); for OtherStatements. The substitutions are illustrated in Figure 9-3. Sometimes I need alternate names for parts of an if statement. I call them the if clause and the else clause. if (Condition) { if clause } else { else clause } An if statement is an example of a compound statement — a statement that includes other statements within it. The if statement in Listing 9-1 includes two println calls and these calls to println are statements.
- 163. Chapter 9: Forks in the Road 145 Figure 9-3: An if statement and its format. Notice how I use parentheses and semicolons in the if statement of Listing 9-1. In particular, notice the following: The condition must be in parentheses. Statements inside the if clause end with semicolons. So do statements inside the else clause. There’s no semicolon immediately after the condition. There’s no semicolon immediately after the word else. As a beginning programmer, you may think these rules are arbitrary. But they’re not. These rules belong to a very carefully crafted grammar. They’re like the grammar rules for English sentences, but they’re even more logical! (Sorry, Christine.) Table 9-1 shows you the kinds of things that can go wrong when you break the if statement’s punctuation rules. The table’s last two items are the most notorious. In these two situations, the compiler doesn’t catch the error. This lulls you into a false sense of security. The trouble is, when you run the pro- gram, the code’s behavior isn’t what you expect it to be.
- 164. 146 Part III: Controlling the Flow Table 9-1 Common if Statement Error Messages Error Example Message or result Missing parentheses if randomNumber ‘(‘ expected surrounding the > 5 { condition Missing semicolon System.out. ‘;’ expected after a statement println() that’s inside the if clause or the else clause Semicolon immediately if (randomNumber ‘else’ without after the condition > 5); ‘if’ Semicolon immediately } else; The program compiles after the word else without errors, but the state- ment after the word else is always executed, whether the condition is true or false. Missing curly braces if (randomNumber The program sometimes > 5) compiles without errors, but the program’s run may not do what you expect it to do. (So the bottom line is, don’t omit the curly braces.) System.out. println(“Yes”); else System.out.println (“No”); As you compose your code, it helps to think of an if statement as one indi- visible unit. Instead of typing the whole first line (condition and all), try typing the if statement’s skeletal outline. if () { //To do: Fill in the condition. //To do: Fill in SomeStatements. } else { //To do: Fill in OtherStatements. } With the entire outline in place, you can start working on the items on your to-do list. When you apply this kind of thinking to a compound statement, it’s harder to make a mistake.
- 165. Chapter 9: Forks in the Road 147 A complete program Listing 9-2 contains a complete program with a simple if statement. The list- ing’s code behaves like an electronic oracle. Ask the program a yes or no question, and the program answers you back. Of course, the answer to your question is randomly generated. But who cares? It’s fun to ask anyway. Listing 9-2: I Know Everything import java.util.Scanner; import java.util.Random; class AnswerYesOrNo { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); Random myRandom = new Random(); int randomNumber; System.out.print(“Type your question, my child: “); myScanner.nextLine(); randomNumber = myRandom.nextInt(10) + 1; if (randomNumber > 5) { System.out.println(“Yes. Isn’t it obvious?”); } else { System.out.println(“No, and don’t ask again.”); } } } Figure 9-4 shows several runs of the program in Listing 9-2. The program’s action has four parts: Figure 9-4: The all- knowing Java program in action.
- 166. 148 Part III: Controlling the Flow 1. Prompt the user. Call System.out.print, telling the user to type a question. 2. Get the user’s question from the keyboard. In Figure 9-4, I run the AnswerYesOrNo program four times, and I type a different question each time. Meanwhile, back in Listing 9-2, the statement myScanner.nextLine(); swallows up my question, and does absolutely nothing with it. This is an anomaly, but you’re smart so you can handle it. Normally, when a program gets input from the keyboard, the program does something with the input. For instance, the program can assign the input to a variable: amount = myScanner.nextDouble(); Alternatively, the program can display the input on the screen: System.out.println(myScanner.nextLine()); But the code in Listing 9-2 is different. When this AnswerYesOrNo program runs, the user has to type something. (The call to getLine waits for the user to type some stuff, and then press Enter.) But the AnswerYesOrNo program has no need to store the input for further analysis. (The computer does what I do when my wife asks me if I plan to clean up after myself. I ignore the question and make up an arbitrary answer.) So the program doesn’t do anything with the user’s input. The call to myScanner.nextLine just sits there in a statement of its own, doing nothing, behaving like a big black hole. It’s unusual for a program to do this, but an electronic oracle is an unusual thing. It calls for some slightly unusual code. 3. Get a random number — any int value from 1 to 10. Okay, wise guys. You’ve just trashed the user’s input. How will you answer yes or no to the user’s question? No problem! None at all! You’ll display an answer randomly. The user won’t know the difference. (Hah, hah!) You can do this as long as you can generate random numbers. The numbers from 1 to 10 will do just fine. In Listing 9-2, the stuff about Random and myRandom looks very much like the familiar Scanner code. From a beginning programmer’s point of view, Random and Scanner work almost the same way. Of course, there’s an important difference. A call to the Random class’s nextInt(10) method doesn’t fetch anything from the keyboard. Instead, this nextInt(10) method gets a number out of the blue. The name Random is defined in the Java API. The call to myRandom. nextInt(10) in Listing 9-2 gets a number from 0 to 9. Then my code adds 1 (making a number from 1 to 10) and assigns that number to the
- 167. Chapter 9: Forks in the Road 149 Randomness makes me dizzy When you call myRandom.nextInt(10) + 1, such thing as a single random number. After you get a number from 1 to 10. As a test, I wrote all, how random is a number like 9? a program that calls the myRandom.nextInt A number is random only when it’s one in a (10) + 1 twenty times. very disorderly collection of numbers. More import java.util.Random; precisely, a number is random if the process class TwentyNumbers { used to generate the number follows the two public static void trends listed above. When they’re being main(String args[]) { careful, scientists avoid the term random Random myRandom = new number, and use the term randomly gener- Random(); ated number instead. System.out.print(myRandom.n It’s hard to generate numbers randomly. extInt(10) + 1); Computer programs do the best they can, but System.out.print(“ “); ultimately, today’s computer programs follow System.out.print(myRandom.n a pattern, and that pattern isn’t truly random. extInt(10) + 1); To generate numbers in a truly random fash- System.out.print(“ “); ion, you need a big tub of ping-pong balls, like the kind they use in state lottery draw- System.out.print(myRandom.n ings. The problem is, most computers don’t extInt(10) + 1); //...And so on. come with big tubs of ping-pong balls among I ran the program several times, and got the their peripherals. So strictly speaking, the results shown in the figure below. (Actually, I numbers generated by Java’s Random class copied the results from JCreator’s General aren’t random. Instead, scientists call these Output pane to Windows Notepad.) Stare briefly numbers pseudorandom. at the figure and notice two trends: It surprises us all, but knowing one randomly There’s no obvious way to predict what generated value is of no help in predicting number comes next. the next randomly generated value. No number occurs much more often than For example, if you toss a coin twice, and get any of the others. heads each time, are you more likely to get tails on the third flip? No. It’s still fifty-fifty. If you have three sons, and you’re expect- ing a fourth child, is the fourth child more likely to be a girl? No. A child’s gender has nothing to do with the genders of the older children. (I’m ignoring any biological The Java virtual machine jumps through hoops effects, which I know absolutely nothing to maintain these trends. That’s because crank- about. Wait! I do know some biological ing out numbers in a random fashion is a very trivia: A newborn child is more likely to be a tricky business. Here are some interesting facts boy than a girl. For every 21 newborn boys, about the process: there are only 20 newborn girls. Boys are weaker, so we die off faster. That’s why Scientists and non-scientists use the term nature makes more of us at birth.) random number. But in reality, there’s no
- 168. 150 Part III: Controlling the Flow variable randomNumber. When that’s done, you’re ready to answer the user’s question. In Java’s API, the word Random is the name of a Java class, and nextInt is the name of a Java method. For more information on the relationship between classes and methods, see Chapters 17, 18, and 19. 4. Answer yes or no. Calling myRandom.nextInt(10) is like spinning a wheel on a TV game show. The wheel has slots numbered 1 to 10. The if statement in Listing 9-2 turns your number into a yes or no alternative. If you roll a number that’s greater than 5, the program answers yes. Otherwise (if you roll a number that’s less than or equal to 5), the program answers no. You can trust me on this one. I’ve made lots of important decisions based on my AnswerYesOrNo program. Indenting if statements in your code Notice how, in Listing 9-2, the println calls inside the if statement are indented. Strictly speaking, you don’t have to indent the statements that are inside an if statement. For all the compiler cares, you can write your whole program on a single line or place all your statements in an artful, misshapen zigzag. The problem is, if you don’t indent your statements in some logical fashion, then neither you nor anyone else can make sense of your code. In Listing 9-2, the indenting of the println calls helps your eye (and brain) see quickly that these statements are subordinate to the overall if/else flow. In a small program, unindented or poorly indented code is barely tolerable. But in a complicated program, indentation that doesn’t follow a neat, logical pattern is a big, ugly nightmare. Always indent your code to make the program’s flow apparent at a glance. Variations on the Theme I don’t like to skin cats. But I’ve heard that, if I ever need to skin one, I have a choice of several techniques. I’ll keep that in mind the next time my cat Muon mistakes the carpet for a litter box.* *Rick Ross, who read about skinning cats in one of my other books, sent me this information via e-mail: “. . . on page 10 you refer to ‘skinning the cat’ and go on to discuss litter boxes and whatnot. Please note that the phrase ‘more than one way to skin a cat’ refers to the difficulty in removing the inedible skin from catfish, and that there is more than one way to do the same. These range from nailing the critter’s tail to a board and taking a pair of pliers to peel it down, to letting the furry kind of cat have the darn thing and just not worrying about it. I grew up on The River (the big one running north/south down the US that begins with ‘M’ and has so many repeated letters), so it’s integral to our experience there. A common misconception (if inhumane and revolting). Just thought you’d want to know.”
- 169. Chapter 9: Forks in the Road 151 Anyway, whether you’re skinning catfish, skinning kitties, or writing computer programs, the same principle holds true. You always have alternatives. Listing 9-2 shows you one way to write an if statement. The rest of this chapter (and all of Chapter 10) show you some other ways to create if statements. . . . Or else what? You can create an if statement without an else clause. For example, imagine a Web page on which one in ten randomly chosen visitors receives a special offer. To keep visitors guessing, I call the Random class’s nextInt method, and make the offer to anyone whose number is lucky 7. If myRandom.nextInt(10) + 1 generates the number 7, display a special offer message. If myRandom.nextInt(10) + 1 generates any number other than 7, do nothing. Don’t display a special offer message, and don’t display a dis- couraging, “Sorry, no offer for you,” message. The code to implement such a strategy is shown in Listing 9-3. A few runs of the code are shown in Figure 9-5. Listing 9-3: Aren’t You Lucky? import java.util.Random; class SpecialOffer { public static void main(String args[]) { Random myRandom = new Random(); int randomNumber = myRandom.nextInt(10) + 1; if (randomNumber == 7) { System.out.println(“An offer just for you!”); } System.out.println(randomNumber); } } The if statement in Listing 9-3 has no else clause. This if statement has the following form: if (Condition) { SomeStatements } When randomNumber is 7, the computer displays An offer just for you! When randomNumber isn’t 7, the computer doesn’t display An offer just for you! The action is illustrated in Figure 9-6.
- 170. 152 Part III: Controlling the Flow Figure 9-5: Three runs of the code in Listing 9-3. Always (I mean always) use a double equal sign when you compare two num- bers or characters in an if statement’s condition. Never (that’s never, ever, ever) use a single equal sign to compare two values. A single equal sign does assignment, not comparison. In Listing 9-3, I took the liberty of adding an extra println. This println (at the end of the main method) displays the random number generated by my call to nextInt. On a Web page with special offers, you probably wouldn’t see the randomly generated number, but I can’t test my SpecialOffer code without knowing what numbers the code generates. randomNumber ==7 e tru false Display "An offer just for you!" Figure 9-6: If you have nothing good to say, then don’t say anything. Display randomNumber
- 171. Chapter 9: Forks in the Road 153 Anyway, notice that the value of randomNumber is displayed in every run. The println for randomNumber isn’t inside the if statement. (This println comes after the if statement.) So the computer always executes this println. Whether randomNumber == 7 is true or false, the computer takes the appropriate if action, and then marches on to execute System.out. println(randomNumber). Packing more stuff into an if statement Here’s an interesting situation: You have two baseball teams — the Hankees and the Socks. You want to display the teams’ scores on two separate lines, with the winner’s score coming first. (On the computer screen, the winner’s score is displayed above the loser’s score. In case of a tie, you display the two identical scores, one above the other.) Listing 9-4 has the code. Listing 9-4: May the Best Team Be Displayed First import java.util.Scanner; import static java.lang.System.in; import static java.lang.System.out; class TwoTeams { public static void main(String args[]) { Scanner myScanner = new Scanner(in); int hankees, socks; out.print(“Hankees and Socks scores? “); hankees = myScanner.nextInt(); socks = myScanner.nextInt(); out.println(); if (hankees > socks) { out.print(“Hankees: “); out.println(hankees); out.print(“Socks: “); out.println(socks); } else { out.print(“Socks: “); out.println(socks); out.print(“Hankees: “); out.println(hankees); } } } Figure 9-7 has a few runs of the code. (To show a few runs in one figure, I copied the results from JCreator’s General Output pane to Windows Notepad.)
- 172. 154 Part III: Controlling the Flow Figure 9-7: See? The code in Listing 9-4 really works! With curly braces, a bunch of print and println calls are tucked away safely inside the if clause. Another group of print and println calls are squished inside the else clause. This creates the forking situation shown in Figure 9-8. hankees > socks e tru fa lse Display hankees Display socks Display socks Display hankees Figure 9-8: Cheer for your favorite team.
- 173. Chapter 9: Forks in the Road 155 Statements and blocks An elegant way to think about if statements is many different kinds of statements). So in Listing to realize that you can put only one statement 9-4, the block inside each clause of an if statement. { if (Condition) System.out.print(“Hankees: aStatement “); else System.out.println(hankees); anotherStatement System.out.print(“Socks: “); On first reading of this one-statement rule, System.out.println(socks); you’re probably thinking that there’s a misprint. } After all, in Listing 9-4, each clause (the if clause and the else clause) seems to contain is a single statement. It’s a statement that has four statements, not just one. within it, four smaller statements. So this big block, this single statement, serves as the one But technically, the if clause in Listing 9-4 has and only statement inside the if clause in only one statement, and the else clause in Listing 9-4. Listing 9-4 has only one statement. The trick is, when you surround a bunch of statements with That’s how the one-statement rule works. In an curly braces, you get what’s called a block, and if statement, when you want the computer to a block behaves, in all respects, like a single execute several statements, you combine those statement. In fact, the official Java documenta- statements into one big statement. To do this, tion lists a block as a kind of statement (one of you make a block using curly braces. Some handy import declarations When I wrote this section’s example, I was tired of writing the word System. After all, Listing 9-4 has ten System.out.print lines. By this point in the book, shouldn’t my computer remember what out.print means? Of course, computers don’t work that way. If you want a computer to “know” what out.print means, you have to code that knowledge somewhere inside the Java compiler. Fortunately for me, the ability to abbreviate things like System.out.print is part of Java 5.0. (This relatively new version of Java wasn’t available until September 2004. An older Java compiler just chokes on the code in Listing 9-4.) This ability to abbreviate things is called static import. It’s illustrated in the second and third lines of Listing 9-4. Whenever I start a program with the line import static java.lang.System.out;
- 174. 156 Part III: Controlling the Flow I can replace System.out with plain out in the remainder of the program. The same holds true of System.in. With an import declaration near the top of Listing 9-4, I can replace new Scanner(System.in) with the simpler new Scanner(in). You may be wondering what all the fuss is about. If I can abbreviate java. util.Scanner by writing Scanner, what’s so special about abbreviating System.out? And why do I have to write out.print? Can I trim System. out.print down to the single word print? Look again at the first few lines of Listing 9-4. When do I need the word static? And what’s the difference between java.util and java.lang? I’m sorry. My response to these questions won’t thrill you. The fact is, I can’t explain away any of these issues until Chapter 18. Before I can explain static import declarations, I need to introduce some ideas. I need to describe classes, packages, and static members. So until you reach Chapter 18, please bear with me. Just paste three import declarations to the top of your Java programs, and trust that everything will work well. You can abbreviate System.out with the single word out. And you can abbreviate System.in with the single word in. Just be sure to copy the import declarations exactly as you see them in Listing 9-4. With any deviation from the lines in Listing 9-4, you may get a compiler error.
- 175. Chapter 10 Which Way Did He Go? In This Chapter Untangling complicated conditions Writing cool conditional code Intertwining your if statements I t’s tax time again. At the moment, I’m working on Form 12432-89B. Here’s what it says: If you’re married with fewer than three children, and your income is higher than the EIQ (Estimated Income Quota), or if you’re single and living in a non-residential area (as defined by Section 10, Part iii of the Uniform Zoning Act), and you’re either self-employed as an LLC (Limited Liability Company) or you qualify for veterans benefits, then skip Steps 3 and 4 or 4, 5, and 6, depending on your answers to Questions 2a and 3d. No wonder I have no time to write! I’m too busy interpreting these tax forms. Anyway, this chapter deals with the potential complexity of if statements. This chapter has nothing as complex as Form 12432-89B, but if you ever encounter something that complicated, you’ll be ready for it. Forming Bigger and Better Conditions In Listing 9-2, the code chooses a course of action based on one call to the Random class’s nextInt method. That’s fine for the electronic oracle program described in Chapter 9, but what if you’re rolling a pair of dice? In Backgammon and other dice games, rolling 3 and 5 isn’t the same as rolling 4 and 4, even though the total for both rolls is 8. The next move varies, depending on whether or not you roll doubles. To get the computer to roll two dice, you exe- cute myRandom.nextInt(6) + 1 two times. Then you combine the two rolls into a larger, more complicated if statement.
- 176. 158 Part III: Controlling the Flow So to simulate a Backgammon game (and many other, more practical situations) you need to combine conditions. If die1 + die2 equals 8 and die1 equals die2, ... You need things like and and or — things that can wire conditions together. Java has operators to represent these concepts, which are described in Table 10-1 and illustrated in Figure 10-1. Table 10-1 Logical Operators Operator Meaning Example Illustration Symbol && and 4 < age && age < 8 Figure 10-1(a) || or age < 4 || 8 < age Figure 10-1(b) ! not !eachKidGetsTen Figure 10-1(c) Combined conditions, like the ones in Table 10-1, can be mighty confusing. That’s why I tread carefully when I use such things. Here’s a short explana- tion of each example in the table: 4 < age && age < 8 The value of the age variable is greater than 4 and is less than 8. The numbers 5, 6, 7, 8, 9 . . . are all greater than 4. But among these numbers, only 5, 6, and 7 are less than 8. So only the numbers 5, 6, and 7 satisfy this combined condition. Figure 10-1: When you satisfy a condition, you’re happy.
- 177. Chapter 10: Which Way Did He Go? 159 age < 4 || 8 < age The value of the age variable is less than 4 or is greater than 8. To create the or condition, you use two pipe symbols. On many U.S. English key- boards, you can find the pipe symbol immediately above the Enter key (the same key as the backslash, but shifted). In this combined condition, the value of the age variable is either less than 4 or is greater than 8. So for example, if a number is less than 4, then the number satisfies the condition. Numbers like 1, 2, and 3 are all less than 4, so these numbers satisfy the combined condition. Also, if a number is greater than 8, then the number satisfies the com- bined condition. Numbers like 9, 10, and 11 are all greater than 8, so these numbers satisfy the condition. !eachKidGetsTen If I weren’t experienced with computer programming languages, I’d be confused by the exclamation point. I’d think that !eachKidGetsTen means, “Yes, each kid does get ten.” But that’s not what this expression means. This expression says, “The variable eachKidGetsTen does not have the value true.” In Java and other programming languages, an exclamation point stands for negative, for no way, for not. Listing 8-4 has a boolean variable named eachKidGetsTen. A boolean variable’s value is either true or false. Because ! means not, the expressions eachKidGetsTen and !eachKidGetsTen have opposite values. So when eachKidGetsTen is true, !eachKidGetsTen is false (and vice versa). Java’s || operator is inclusive. This means that you get true whenever the thing on the left side is true, the thing on the right side is true, or both things are true. For example, the condition 2 < 10 || 20 < 30 is true. In Java, you can’t combine comparisons the way you do in ordinary English. In English, you may say, “We’ll have between three and ten people at the dinner table.” But in Java, you get an error message if you write 3 <= people <= 10. To do this comparison, you need to something like 3 <= people && people <= 10. Combining conditions: An example Here’s a handy example of the use of logical operators. A movie theater posts its prices for admission. Regular price: $9.25 Kids under 12: $5.25 Seniors (65 and older): $5.25
- 178. 160 Part III: Controlling the Flow Because the kids’ and seniors’ prices are the same, you can combine these prices into one category. (That’s not always the best programming strategy, but do it anyway for this example.) To find a particular moviegoer’s ticket price, you need one or more if statements. There are many ways to struc- ture the conditions, and I chose one of these ways for the code in Listing 10-1. Listing 10-1: Are You Paying Too Much? import java.util.Scanner; class TicketPrice { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int age; double price = 0.00; System.out.print(“How old are you? “); age = myScanner.nextInt(); if (age >= 12 && age < 65) { price = 9.25; } if (age < 12 || age >= 65) { price = 5.25; } System.out.print(“Please pay $”); System.out.print(price); System.out.print(“. “); System.out.println(“Enjoy the show!”); } } Several runs of the TicketPrice program (Listing 10-1) are shown in Figure 10-2. (For your viewing pleasure, I’ve copied the runs from JCreator’s General Output pane to Windows Notepad.) When you turn 12, you start paying full price. You keep paying full price until you become 65. At that point, you pay the reduced price again. The first if statement’s condition tests for the regular price group. Anyone who’s at least 12 years of age and is under 65 belongs in this group. The second if statement’s condition tests for the fringe ages. A person who’s under 12 or is 65 or older belongs in this category.
- 179. Chapter 10: Which Way Did He Go? 161 Figure 10-2: Admission prices for Beginning Programm- ing with Java For Dummies: The Movie. The pivotal part of Listing 10-1 is the lump of if statements in the middle, which are illustrated in Figure 10-3. Figure 10-3: The meanings of the conditions in Listing 10-1. When you form the opposite of an existing condition, you can often follow the pattern in Listing 10-1. The opposite of >= is <. The opposite of < is >=. The opposite of && is ||. If you change the dollar amounts in Listing 10-1, you can get into trouble. For example, with the statement price = 5.00, the program displays Please pay $5.0. Enjoy the show! This happens because Java doesn’t store the two zeros to the right of the decimal point (and Java doesn’t know or care that 5.00 is a dollar amount). To fix this kind of thing, see the discussion of NumberFormat.getCurrencyInstance in Chapter 18. When to initialize? Take a look at Listing 10-1, and notice the price variable’s initialization. double price = 0.00;
- 180. 162 Part III: Controlling the Flow This line declares the price variable and sets the variable’s starting value to 0.00. When I omit this initialization, I get an error message: variable price might not have been initialized System.out.print(price); ^ What’s the deal here? I don’t initialize the age variable, but the compiler doesn’t complain about that. Why is the compiler fussing over the price variable? The answer is in the placement of the code’s assignment statements. Consider the following two facts: The statement that assigns a value to age (age = myScanner. nextInt()) is not inside an if statement. That assignment statement always gets executed and (as long as nothing extraordinary happens) the variable age is sure to be assigned a value. Both statements that assign a value to price (price = 9.25 and price = 5.25) are inside if statements. If you look at Figure 10-3, you see that every age group is covered. No one shows up at the ticket counter with an age that forces both if con- ditions to be false. So whenever you run the TicketPrice program, either the first or the second price assignment is executed. The problem is that the compiler isn’t smart enough to check all this. The compiler just sees the structure in Figure 10-4 and becomes scared that the computer won’t take either of the true detours. If (for some unforeseen reason) both of the if statements’ conditions are false, then the variable price doesn’t get assigned a value. So with- out an initialization, price has no value. (More precisely, price has no value that’s intentionally given to it in the code.) Eventually, the computer reaches the System.out.print(price) state- ment. It can’t display price unless price has a meaningful value. So at that point, the compiler throws up its virtual hands in disgust. More and more conditions Last night I had a delicious meal at the neighborhood burger joint. As part of a promotion, I got a discount coupon along with the meal. The coupon is good for $2.00 off the price of a ticket at the local movie theater. To make use of the coupon in the TicketPrice program, I have to tweak the code in Listing 10-1. The revised code is in Listing 10-2. In Figure 10-5, I take that new code around the block a few times.
- 181. Chapter 10: Which Way Did He Go? 163 Figure 10-4: The choices in Listing 10-1. Listing 10-2: Do You Have a Coupon? import java.util.Scanner; class TicketPriceWithDiscount { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int age; double price = 0.00; char reply; (continued)
- 182. 164 Part III: Controlling the Flow Listing 10-2 (continued) System.out.print(“How old are you? “); age = myScanner.nextInt(); System.out.print(“Have a coupon? (Y/N) “); reply = myScanner.findInLine(“.”).charAt(0); if (age >= 12 && age < 65) { price = 9.25; } if (age < 12 || age >= 65) { price = 5.25; } if (reply == ‘Y’ || reply == ‘y’) { price -= 2.00; } if (reply != ‘Y’ && reply != ‘y’ && reply!=’N’ && reply!=’n’) { System.out.println(“Huh?”); } System.out.print(“Please pay $”); System.out.print(price); System.out.print(“. “); System.out.println(“Enjoy the show!”); } } Figure 10-5: Running the code in Listing 10-2. Listing 10-2 has two if statements whose conditions involve characters: In the first such statement, the computer checks to see if the reply vari- able stores the letter Y or the letter y. If either is the case, then it sub- tracts 2.00 from the price. (For information on operators like -=, see Chapter 7.)
- 183. Chapter 10: Which Way Did He Go? 165 The second such statement has a hefty condition. The condition tests to see if the reply variable stores any reasonable value at all. If the reply isn’t Y, and isn’t y, and isn’t N, and isn’t n, then the computer expresses its concern by displaying, “Huh?” (As a paying customer, the word “Huh?” on the automated ticket teller’s screen will certainly get your attention.) When you create a big multipart condition, you always have several ways to think about the condition. For example, you can rewrite the last condition in Listing 10-2 as if (!(reply == ‘Y’ || reply == ‘y’ || reply == ‘N’ || reply == ‘n’)). “If it’s not the case that the reply is either Y, y, N, or n, then display ‘Huh?’” So which way of writing the condition is better — the way I do it in Listing 10-2, or the way I do it in this tip? It depends on your taste. Whatever makes the logic easiest for you to understand is the best way. Using boolean variables No matter how good a program is, you can always make it a little bit better. Take the code in Listing 10-2. Does the forest of if statements make you ner- vous? Do you slow to a crawl when you read each condition? Wouldn’t it be nice if you could glance at a condition and make sense of it very quickly? To some extent, you can. If you’re willing to create some additional variables, you can make your code easier to read. Listing 10-3 shows you how. Listing 10-3: George Boole Would Be Proud import java.util.Scanner; class NicePrice { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int age; double price = 0.00; char reply; boolean isKid, isSenior, hasCoupon, hasNoCoupon; System.out.print(“How old are you? “); age = myScanner.nextInt(); System.out.print(“Have a coupon? (Y/N) “); reply = myScanner.findInLine(“.”).charAt(0); isKid = age < 12; isSenior = age >= 65; hasCoupon = reply == ‘Y’ || reply == ‘y’; hasNoCoupon = reply == ‘N’ || reply == ‘n’; (continued)
- 184. 166 Part III: Controlling the Flow Listing 10-3 (continued) if (!isKid && !isSenior) { price = 9.25; } if (isKid || isSenior) { price = 5.25; } if (hasCoupon) { price -= 2.00; } if (!hasCoupon && !hasNoCoupon) { System.out.println(“Huh?”); } System.out.print(“Please pay $”); System.out.print(price); System.out.print(“. “); System.out.println(“Enjoy the show!”); } } Runs of the Listing 10-3 code look like the stuff in Figure 10-5. The only differ- ence between Listings 10-2 and 10-3 is the use of boolean variables. In Listing 10-3, you get past all the less than signs and double equal signs before the start of any if statements. By the time you encounter the two if statements, the conditions can use simple words — words like isKid, isSenior, and hasCoupon. With all these boolean variables, expressing each if statement’s condition is a snap. You can read more about boolean variables in Chapter 8. Adding a boolean variable can make your code more manageable. But some programming languages don’t have boolean variables, so many program- mers prefer to create if conditions on the fly. That’s why I mix the two tech- niques (conditions with and without boolean variables) in this book. Mixing different logical operators together If you read about Listing 10-2, you know that my local movie theater offers discount coupons. The trouble is, I can’t use a coupon along with any other dis- count. I tried to convince the ticket taker that I’m under 12 years of age, but he didn’t buy it. When that didn’t work, I tried combining the coupon with the senior citizen discount. That didn’t work either. The theater must use some software that checks for people like me. It looks something like the code in Listing 10-4. To watch the code run, take a look at Figure 10-6.
- 185. Chapter 10: Which Way Did He Go? 167 Listing 10-4: No Extra Break for Kids or Seniors import java.util.Scanner; class CheckAgeForDiscount { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int age; double price = 0.00; char reply; System.out.print(“How old are you? “); age = myScanner.nextInt(); System.out.print(“Have a coupon? (Y/N) “); reply = myScanner.findInLine(“.”).charAt(0); if (age >= 12 && age < 65) { price = 9.25; } if (age < 12 || age >= 65) { price = 5.25; } if ((reply == ‘Y’ || reply == ‘y’) && (age >= 12 && age < 65)) { price -= 2.00; } System.out.print(“Please pay $”); System.out.print(price); System.out.print(“. “); System.out.println(“Enjoy the show!”); } } Figure 10-6: Running the code in Listing 10-4.
- 186. 168 Part III: Controlling the Flow Listing 10-4 is a lot like its predecessors, Listings 10-1 and 10-2. The big differ- ence is the bolded if statement. This if statement tests two things, and each thing has two parts of its own: 1. Does the customer have a coupon? That is, did the customer reply with either Y or with y? 2. Is the customer in the regular age group? That is, is the customer at least 12 years old and younger than 65? In Listing 10-4, I join items 1 and 2 using the && operator. I do this because both items (item 1 and item 2) must be true in order for the customer to qualify for the $2.00 discount, as illustrated in Figure 10-7. Using parentheses Listing 10-4 demonstrates something important about conditions. Sometimes, you need parentheses to make a condition work correctly. Take, for example, the following incorrect if statement: //This code is incorrect: if (reply == ‘Y’ || reply == ‘y’ && age >= 12 && age < 65) { price -= 2.00; } Figure 10-7: Both the reply and the age criteria must be true.
- 187. Chapter 10: Which Way Did He Go? 169 Compare this code with the correct code in Listing 10-4. This incorrect code has no parentheses to group reply == ‘Y’ with reply == ‘y’, or to group age >= 12 with age < 65. The result is the bizarre pair of runs in Figure 10-8. Figure 10-8: A capital offense. In Figure 10-8, notice how the y and Y inputs yield different ticket prices, even though the age is 85 in both runs. This happens because, without parenthe- ses, any && operator gets evaluated before any || operator. (That’s the rule in the Java programming language — evaluate && before ||.) When reply is Y, the condition in the bad if statement takes the following form: reply == ‘Y’ || some-other-stuff-that-doesn’t-matter Whenever reply == ‘Y’ is true, the whole condition is automatically true, as illustrated in Figure 10-9. Figure 10-9: “True or false” makes “true.”
- 188. 170 Part III: Controlling the Flow Building a Nest The year is 1968, and The Prisoner is on TV. In the last episode, the show’s hero meets his nemesis “Number One.” At first Number One wears a spooky happy-face/sad-face mask, and when the mask comes off, there’s a monkey mask underneath. To find out what’s behind the monkey mask, you have to watch the series on DVD. But in the meantime, notice the layering; a mask within a mask. You can do the same kind of thing with if statements. This section’s example shows you how. But first, take a look at Listing 10-4. In that code, the condition age >= 12 && age < 65 is tested twice. Both times, the computer sends the numbers 12, 65, and the age value through its jumble of circuits, and both times, the computer gets the same answer. This is wasteful, but waste isn’t your only concern. What if you decide to change the age limit for senior tickets? From now on, no one under 100 gets a senior discount. You fish through the code and see the first age >= 12 && age < 65 test. You change 65 to 100, pat yourself on the back, and go home. The problem is, you’ve changed one of the two age >= 12 && age < 65 tests, but you haven’t changed the other. Wouldn’t it be better to keep all the age >= 12 && age < 65 testing in just one place? Listing 10-5 comes to the rescue. In Listing 10-5, I smoosh all my if statements together into one big glob. The code is dense, but it gets the job done nicely. Listing 10-5: Nested if Statements import java.util.Scanner; class AnotherAgeCheck { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int age; double price = 0.00; char reply; System.out.print(“How old are you? “); age = myScanner.nextInt(); System.out.print(“Have a coupon? (Y/N) “); reply = myScanner.findInLine(“.”).charAt(0); if (age >= 12 && age < 65) { price = 9.25; if (reply == ‘Y’ || reply == ‘y’) { price -= 2.00; } } else { price = 5.25; }
- 189. Chapter 10: Which Way Did He Go? 171 System.out.print(“Please pay $”); System.out.print(price); System.out.print(“. “); System.out.println(“Enjoy the show!”); } } Nested if statements A run of the code in Listing 10-5 looks identical to a run for Listing 10-4. You can see several runs in Figure 10-6. The main idea in Listing 10-5 is to put an if statement inside another if statement. After all, Chapter 9 says that an if statement can take the following form: if (Condition) { SomeStatements } else { OtherStatements } Who says SomeStatements can’t contain an if statement? For that matter, OtherStatements can also contain an if statement. And, yes, you can create an if statement within an if statement within an if statement. There’s no predefined limit on the number of if statements that you can have. if (age >= 12 && age < 65) { price = 9.25; if (reply == ‘Y’ || reply == ‘y’) { if (isSpecialFeature) { price -= 1.00; } else { price -= 2.00; } } } else { price = 5.25; } When you put one if statement inside another, you create nested if state- ments. Nested statements aren’t difficult to write, as long as you take things slowly, and keep a clear picture of the code’s flow in your mind. If it helps, draw yourself a diagram like the one shown in Figure 10-10. When you nest statements, you must be compulsive about the use of indenta- tion and braces (see Figure 10-11). When code has misleading indentation, no one (not even the programmer who wrote the code) can figure out how the code works. A nested statement with sloppy indentation is a programmer’s nightmare.
- 190. 172 Part III: Controlling the Flow Figure 10-10: The flow in Listing 10-5. Figure 10-11: Be careful about adding the proper indentation and braces. Cascading if statements Here’s a riddle: You have two baseball teams — the Hankees and the Socks. You want to display the teams’ scores on two separate lines, with the winner’s score coming first. (On the computer screen, the winner’s score is displayed above the loser’s score.) What happens when the scores are tied?
- 191. Chapter 10: Which Way Did He Go? 173 Do you give up? The answer is, there’s no right answer. What happens depends on the way you write the program. Take a look back at Listing 9-4 in Chapter 9. When the scores are equal, the condition hankees > socks is false. So the program’s flow of execution drops down to the else clause. That clause displays the Socks score first and the Hankees score second. (Refer to Figure 9-7.) The program doesn’t have to work this way. If I take Listing 9-4 and change hankees > socks to hankees >= socks then, in case of a tie, the Hankees score comes first. Suppose you want a bit more control. When the scores are equal, you want an It’s a tie message. To do this, think in terms of a three-pronged fork. You have a prong for a Hankees win, another prong for a Socks win, and a third prong for a tie. You can write this code in several different ways, but one way that makes lots of sense is in Listing 10-6. For three runs of the code in Listing 10-6, see Figure 10-12. Listing 10-6: In Case of a Tie . . . import java.util.Scanner; import static java.lang.System.out; class WinLoseOrTie { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int hankees, socks; out.print(“Hankees and Socks scores? “); hankees = myScanner.nextInt(); socks = myScanner.nextInt(); out.println(); if (hankees > socks) { out.println(“Hankees win...”); out.print(“Hankees: “); out.println(hankees); out.print(“Socks: “); out.println(socks); } else if (socks > hankees) { out.println(“Socks win...”); out.print(“Socks: “); out.println(socks); out.print(“Hankees: “); out.println(hankees); (continued)
- 192. 174 Part III: Controlling the Flow Listing 10-6 (continued) } else { out.println(“It’s a tie...”); out.print(“Hankees: “); out.println(hankees); out.print(“Socks: “); out.println(socks); } } } Figure 10-12: Go, team, go! Listing 10-6 illustrates a way of thinking about a problem. You have one ques- tion with more than two answers. (In this section’s baseball problem, the question is “Who wins?” and the answers are “Hankees,” “Socks,” or “Neither.”) The problem begs for an if statement, but an if statement has only two branches — the true branch and the false branch. So you combine alterna- tives to form cascading if statements. In Listing 10-6, the format for the cascading if statements is if (Condition1) { SomeStatements } else if (Condition2) { OtherStatements } else { EvenMoreStatements } In general, you can use else if as many times as you want:
- 193. Chapter 10: Which Way Did He Go? 175 if (hankeesWin) { out.println(“Hankees win...”); out.print(“Hankees: “); out.println(hankees); out.print(“Socks: “); out.println(socks); } else if (socksWin) { out.println(“Socks win...”); out.print(“Socks: “); out.println(socks); out.print(“Hankees: “); out.println(hankees); } else if (isATie) { out.println(“It’s a tie...”); out.print(“Hankees: “); out.println(hankees); out.print(“Socks: “); out.println(socks); } else if (gameCancelled) { out.println(“Sorry, sports fans.”); } else { out.println(“The game isn’t over yet.”); } Nothing is special about cascading if statements. This isn’t a new program- ming language feature. Cascading if statements take advantage of a loophole in Java — a loophole about omitting curly braces in certain circumstances. Other than that, cascading if statements just gives you a new way to think about decisions within your code. Note: Listing 10-6 uses a static import declaration to avoid needless repeti- tion of the words System.out. To read a little bit about the static import declaration (along with an apology for my not explaining this concept more thoroughly), see Chapter 9. Then to get the real story on static import decla- rations, see Chapter 18. Enumerating the Possibilities Chapter 8 describes Java’s boolean type — the type with only two values (true and false). The boolean type is very handy, but sometimes you need more values. After all, a traffic light’s values can be green, yellow, or red. A playing card’s suit can be spade, club, heart, or diamond. And a weekday can be Monday, Tuesday, Wednesday, Thursday, or Friday. Life is filled with small sets of possibilities, and Java has a feature that can reflect these possibilities. The feature is called an enum type. It’s new in Java version 5.0.
- 194. 176 Part III: Controlling the Flow Creating an enum type The story in Listing 10-6 has three possible endings — the Hankees win, the Socks win, or the game is tied. You can represent the possibilities with a one- line Java program. The program is in Listing 10-7. Listing 10-7: Three Possibilities enum WhoWins {home, visitor, neither} This week’s game is played at Hankeeville’s SnitSoft Stadium, so the value home represents a win for the Hankees, and the value visitor represents a win for the Socks. I wonder what thoughts you have on your first encounter with Listing 10-7. “What good is a one-line program?” you ask. “Three values? Who cares?” you say. “Does this code do anything useful?” you think. “Does the code do any- thing at all?” One of the goals in computer programming is for each program’s structure to mirror whatever problem the program solves. When a program reminds you of its underlying problem, the program is easy to understand, and inexpen- sive to maintain. For instance, a program to tabulate customer accounts should use names like customer and account. And a program that deals with three possible outcomes (home wins, visitor wins, and tie) should have a variable with three possible values. So in Listing 10-7, I create a type to store three values. The WhoWins type defined in Listing 10-7 is called an enum type. Think of the new WhoWins type as a boolean on steroids. Instead of two values (true and false) the WhoWins type has three values (home, visitor, and neither). You can create a variable of type WhoWins WhoWins who; and then assign a value to the new variable. who = WhoWins.home; The fact that you define WhoWins in a file all its own may be a bit unsettling, so I deal with that issue in the this chapter’s final section. Using an enum type Listing 10-8 shows you how to use the brand new WhoWins type.
- 195. Chapter 10: Which Way Did He Go? 177 Listing 10-8: Proud Winners and Sore Losers import java.util.Scanner; import static java.lang.System.out; class Scoreboard { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int hankees, socks; WhoWins who; out.print(“Hankees and Socks scores? “); hankees = myScanner.nextInt(); socks = myScanner.nextInt(); out.println(); if (hankees > socks) { who = WhoWins.home; out.println(“The Hankees win :-)”); } else if (socks > hankees) { who = WhoWins.visitor; out.println(“The Socks win :-(“); } else { who = WhoWins.neither; out.println(“It’s a tie :-|”); } out.println(); out.println(“Today’s game is brought to you by”); out.println(“SnitSoft, the number one software”); out.println(“vendor in the Hankeeville area.”); out.println(“SnitSoft is featured proudly in”); out.println(“Chapter 6. And remember, four out”); out.println(“of five doctors recommend SnitSoft”); out.println(“to their patients.”); out.println(); if (who == WhoWins.home) { out.println(“We beat ‘em good. Didn’t we?”); } if (who == WhoWins.visitor) { out.println(“The umpire made an unfair call.”); } if (who == WhoWins.neither) { out.println(“The game goes into overtime.”); } } } Three runs of the program in Listing 10-8 are pictured in Figure 10-13.
- 196. 178 Part III: Controlling the Flow Figure 10-13: Joy in Hankeeville? Here’s what happens in Listing 10-8: I create a variable to store values of type WhoWins. Just as the line double amount; declares amount to store double values (values like 5.95 and 30.95), the line WhoWins who; declares who to store WhoWins values (values like home, visitor, and neither). I assign a value to the who variable. I execute one of the who = WhoWins.something; assignment statements. The statement that I execute depends on the outcome of the if statement’s hankees > socks comparison.
- 197. Chapter 10: Which Way Did He Go? 179 Notice how I refer to each of the WhoWins values in Listing 10-8. I write WhoWins.home, WhoWins.visitor, or WhoWins.neither. If I forget the WhoWins prefix and type who = home; //This assignment doesn’t work! then the compiler gives me a cannot find symbol error message. That’s just the way enum types work. I compare the variable’s value with each of the WhoWins values. In one if statement, I check the who == WhoWins.home condition. In the remaining two if statements I check for the other WhoWins values. Near the end of Listing 10-8, I could have done without enum values. I could have tested things like hankees > socks a second time. if (hankees > socks) { out.println(“The Hankees win :-)”); } // And later in the program... if (hankees > socks) { out.println(“We beat ‘em good. Didn’t we?”); } But that tactic would be clumsy. In a more complicated program, I may end up checking hankees > socks a dozen times. It would be like asking the same question over and over again. Instead of repeatedly checking the hankees > socks condition, I store the game’s outcome as an enum value. Then I check the enum value as many times as I want. That’s a very tidy way to solve the repeated checking problem. Creating a project with two Java source files You can’t run Listing 10-8 without Listing 10-7. And Listing 10-7 does nothing on its own. To do anything useful, you have to put these two files in the same JCreator project. Here’s how: 1. Create a new JCreator project. For details, see Chapter 3. 2. Right-click the new project’s name in the File View tree. Then, in the resulting context menu, select Add➪New Class. JCreator’s Class Wizard appears.
- 198. 180 Part III: Controlling the Flow 3. In the wizard’s Name field, type the name of your enum type. To create the code in Listing 10-7, type the name WhoWins. 4. Click Finish. The wizard disappears to reveal JCreator’s work area. The editor pane has a new WhoWins.java tab. Delete any code that you see in the WhoWins.java pane, and replace that code with the line in Listing 10-7. (See Figure 10-14.) Figure 10-14: Editing the WhoWins. java file. 5. Follow the usual steps to add a new Scoreboard class. In Scoreboard.java file’s editor pane, type the code in Listing 10-8. 6. Choose Build➪Compile Project. 7. Choose Build➪Execute Project. Voila! The code runs as it does in Figure 10-13.
- 199. Chapter 11 How to Flick a Virtual Switch In This Chapter Dealing with many alternatives Jumping out from the middle of a statement Handling alternative assignments I magine playing Let’s Make a Deal with ten different doors. “Choose door number 1, door number 2, door number 3, door number 4. . . . Wait! Let’s break for a commercial. When we come back, I’ll say the names of the other six doors.” Meet the switch Statement The code back in Listing 9-2 in Chapter 9 simulates a simple electronic oracle. Ask the program a question, and the program randomly generates a yes or no answer. But, as toys go, the code in Listing 9-2 isn’t much fun. The code has only two possible answers. There’s no variety. Even the earliest talking dolls could say about ten different sentences. Suppose that you want to enhance the code of Listing 9-2. The call to myRandom.nextInt(10) + 1 generates numbers from 1 to 10. So maybe you can display a different sentence for each of the ten numbers. A big pile of if statements should do the trick: if (randomNumber == 1) { System.out.println(“Yes. Isn’t it obvious?”); } if (randomNumber == 2) { System.out.println(“No, and don’t ask again.”); } if (randomNumber == 3) { System.out.print(“Yessir, yessir!”); System.out.println(“ Three bags full.”); }
- 200. 182 Part III: Controlling the Flow if (randomNumber == 4) . . . if (randomNumber < 1 || randomNumber > 10) { System.out.print(“Sorry, the electronic oracle”); System.out.println(“ is closed for repairs.”); } But that approach seems wasteful. Why not create a statement that checks the value of randomNumber just once and then takes an action based on the value that it finds? Fortunately, just such a statement exists: the switch state- ment. Listing 11-1 has an example of a switch statement. Listing 11-1: An Answer for Every Occasion import java.util.Scanner; import java.util.Random; import static java.lang.System.out; class TheOldSwitcheroo { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); Random myRandom = new Random(); int randomNumber; out.print(“Type your question, my child: “); myScanner.nextLine(); randomNumber = myRandom.nextInt(10) + 1; switch (randomNumber) { case 1: out.println(“Yes. Isn’t it obvious?”); break; case 2: out.println(“No, and don’t ask again.”); break; case 3: out.print(“Yessir, yessir!”); out.println(“ Three bags full.”); break; case 4: out.print(“What part of ‘no’”); out.println(“ don’t you understand?”); break;
- 201. Chapter 11: How to Flick a Virtual Switch 183 case 5: out.println(“No chance, Lance.”); break; case 6: out.println(“Sure, whatever.”); break; case 7: out.print(“Yes, but only if”); out.println(“ you’re nice to me.”); break; case 8: out.println(“Yes (as if I care).”); break; case 9: out.print(“No, not until”); out.println(“ Cromwell seizes Dover.”); break; case 10: out.print(“No, not until”); out.println(“ Nell squeezes Rover.”); break; default: out.print(“You think you have”); out.print(“ problems?”); out.print(“ My random number”); out.println(“ generator is broken!”); break; } out.println(“Goodbye”); } } The cases in a switch statement Figure 11-1 shows three runs of the program in Listing 11-1. Here’s what hap- pens during one of these runs: The user types a heavy question, and the variable randomNumber gets a value. In the second run of Figure 11-1, this value is 2. Execution of the code in Listing 11-1 reaches the top of the switch state- ment, so the computer starts checking this statement’s case clauses. The value 2 doesn’t match the topmost case clause (the case 1 clause), so the computer moves on to the next case clause.
- 202. 184 Part III: Controlling the Flow The value in the next case clause (the number 2) matches the value of the randomNumber variable, so the computer executes the statements in this case 2 clause. These two statements are out.println(“No, and don’t ask again.”); break; The first of the two statements displays No, and don’t ask again on the screen. The second statement is called a break statement. (What a surprise!) When the computer encounters a break statement, the com- puter jumps out of whatever switch statement it’s in. So in Listing 11-1, the computer skips right past case 3, case 4, and so on. The computer jumps to the statement just after the end of the switch statement. The computer displays Goodbye, because that’s what the statement after the switch statement tells the computer to do. Figure 11-1: Running the code of Listing 11-1. The overall idea behind the program in Listing 11-1 is illustrated in Figure 11-2. The default in a switch statement What if something goes terribly wrong during a run of the Listing 11-1 pro- gram? Suppose the expression myRandom.nextInt(10) + 1 generates a number that’s not in the 1 to 10 range. Then the computer responds by drop- ping past all the case clauses. Instead of landing on a case clause, the com- puter jumps to the default clause. In the default clause, the computer displays You think you have problems?..., and then breaks out of the switch statement. After the computer is out of the switch statement, the computer displays Goodbye. You don’t really need to put a break at the very end of a switch statement. In Listing 11-1, the last break (the break that’s part of the default clause) is just for the sake of overall tidiness.
- 203. Chapter 11: How to Flick a Virtual Switch 185 1 2 3 4 5 6 7 8 9 10 other Yes. Isn't it obvious? No, and don't ask again. Yessir, yessir! Three bags full. What part of 'no' don't you under... No chance, Lance. Sure, whatever. Yes, but only if you're nice to me. Yes (as if I care). No, not until Cromwell seizes D... No, not until Nell squeezes Rover. You think you have problems? ... Figure 11-2: A fork with eleven prongs. Goodbye Picky details about the switch statement A switch statement can take the following form: switch (Expression) { case FirstValue: Statements case SecondValue: MoreStatements // ... more cases... default: EvenMoreStatements } Here are some tidbits about switch statements: The Expression doesn’t have to have an int value. It can be char, byte, short, or int. For example, the following code works nicely:
- 204. 186 Part III: Controlling the Flow char letterGrade; letterGrade = myScanner.findInLine(“.”).charAt(0); switch (letterGrade) { case ‘A’: System.out.println(“Excellent”); break; case ‘B’: System.out.println(“Good”); break; case ‘C’: System.out.println(“Average”); break; } The Expression doesn’t have to be a single variable. It can be any expression of type char, byte, short, or int. For example, you can sim- ulate the rolling of two dice with the following code: int die1, die2; die1 = myRandom.nextInt(6) + 1; die2 = myRandom.nextInt(6) + 1; switch (die1 + die2) { //...etc. The cases in a switch statement don’t have to be in order. Here’s some acceptable code: switch (randomNumber) { case 2: System.out.println(“No, and don’t ask again.”); break; case 1: System.out.println(“Yes. Isn’t it obvious?”); break; case 3: System.out.print(“Yessir, yessir!”); System.out.println(“ Three bags full.”); break; //...etc. This mixing of cases may slow you down when you’re trying to read a program, but it’s legal nonetheless. You don’t need a case for each expected value of the Expression. You can leave some expected values to the default. Here’s an example:
- 205. Chapter 11: How to Flick a Virtual Switch 187 switch (randomNumber) { case 1: System.out.println(“Yes. Isn’t it obvious?”); break; case 5: System.out.println(“No chance, Lance.”); break; case 7: System.out.print(“Yes, but only if”); System.out.println(“ you’re nice to me.”); break; case 10: System.out.print(“No, not until”); System.out.println(“ Nell squeezes Rover.”); break; default: System.out.print(“Sorry,”); System.out.println(“ I just can’t decide.”); break; } The default clause is optional. switch (randomNumber) { case 1: System.out.println(“Yes. Isn’t it obvious?”); break; case 2: System.out.println(“No, and don’t ask again.”); break; case 3: System.out.print(“I’m too tired.”); System.out.println(“ Go ask somebody else.”); } System.out.println(“Goodbye”); If you have no default clause, and a value that’s not covered by any of the cases comes up, then the switch statement does nothing. For exam- ple, if randomNumber is 4, then the code shown above displays Goodbye, and nothing else. In some ways, if statements are more versatile than switch statements. For example, you can’t use a condition in a switch statement’s Expression: //You can’t do this: switch (age >= 12 && age < 65)
- 206. 188 Part III: Controlling the Flow You can’t use a condition as a case value either: //You can’t do this: switch (age) { case age <= 12: //...etc. To break or not to break In every Java programmer’s life, a time comes when he or she forgets to use break statements. At first, the resulting output is confusing, but then the pro- grammer remembers fall-through. The term fall-through describes what hap- pens when you end a case without a break statement. What happens is that execution of the code falls right through to the next case in line. Execution keeps falling through until you eventually reach a break statement or the end of the entire switch statement. If you don’t believe me, just look at Listing 11-2. This listing’s code has a switch statement gone bad: Listing 11-2: Please, Gimme a Break! /* * This isn’t good code. The programmer forgot some * of the break statements. */ import java.util.Scanner; import java.util.Random; import static java.lang.System.out; class BadBreaks { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); Random myRandom = new Random(); int randomNumber; out.print(“Type your question, my child: “); myScanner.nextLine(); randomNumber = myRandom.nextInt(10) + 1; switch (randomNumber) { case 1: out.println(“Yes. Isn’t it obvious?”); case 2: out.println(“No, and don’t ask again.”);
- 207. Chapter 11: How to Flick a Virtual Switch 189 case 3: out.print(“Yessir, yessir!”); out.println(“ Three bags full.”); case 4: out.print(“What part of ‘no’”); out.println(“ don’t you understand?”); break; case 5: out.println(“No chance, Lance.”); case 6: out.println(“Sure, whatever.”); case 7: out.print(“Yes, but only if”); out.println(“ you’re nice to me.”); case 8: out.println(“Yes (as if I care).”); case 9: out.print(“No, not until”); out.println(“ Cromwell seizes Dover.”); case 10: out.print(“No, not until”); out.println(“ Nell squeezes Rover.”); default: out.print(“You think you have”); out.print(“ problems?”); out.print(“ My random number”); out.println(“ generator is broken!”); } out.println(“Goodbye”); } } I’ve put two runs of this code in Figure 11-3. In the first run, the randomNumber is 7. The program executes cases 7 through 10, and the default. In the second run, the randomNumber is 3. The program executes cases 3 and 4. Then, because case 4 has a break statement, the program jumps out of the switch and displays Goodbye. The switch statement in Listing 11-2 is missing some break statements. Even without these break statements, the code compiles with no errors. But when you run the code in Listing 11-2, you don’t get the results that you want.
- 208. 190 Part III: Controlling the Flow Figure 11-3: Please make up your mind. Using Fall-through to Your Advantage Often, when you’re using a switch statement, you don’t want fall-through, so you pepper break statements throughout the switch. But, sometimes, fall- through is just the thing you need. Take the number of days in a month. Is there a simple rule for this? Months containing the letter “r” have 31 days? Months in which “i” comes before “e” except after “c” have 30 days? You can fiddle with if conditions all you want. But to handle all the possibili- ties, I prefer a switch statement. Listing 11-3 demonstrates the idea. Listing 11-3: Finding the Number of Days in a Month import java.util.Scanner; class DaysInEachMonth { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int month, numberOfDays = 0; boolean isLeapYear; System.out.print(“Which month? “); month = myScanner.nextInt(); switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12: numberOfDays = 31; break; case 4: case 6:
- 209. Chapter 11: How to Flick a Virtual Switch 191 case 9: case 11: numberOfDays = 30; break; case 2: System.out.print(“Leap year (true/false)? “); isLeapYear = myScanner.nextBoolean(); if (isLeapYear) { numberOfDays = 29; } else { numberOfDays = 28; } } System.out.print(numberOfDays); System.out.println(“ days”); } } Figure 11-4 shows several runs of the program in Listing 11-3. For month number 6, the computer jumps to case 6. There are no statements inside the case 6 clause, so that part of the program’s run is pretty boring. Figure 11-4: How many days until the next big deadline? But with no break in the case 6 clause, the computer marches right along to case 9. Once again, the computer finds no statements and no break, so the computer ventures to the next case, which is case 11. At that point, the com- puter hits pay dirt. The computer assigns 30 to numberOfDays, and breaks out of the entire switch statement. (See Figure 11-5.) February is the best month of all. For one thing, the February case in Listing 11-3 contains a call to the Scanner class’s nextBoolean method. The method expects me to type either true or false. The code uses whatever word I type to assign a value to a boolean variable. (In Listing 11-3, I assign true or false to the isLeapYear variable.) February also contains its own if statement. In Chapter 10, I nest if state- ments within other if statements. But in February, I nest an if statement within a switch statement. That’s cool.
- 210. 192 Part III: Controlling the Flow Figure 11-5: Follow the bouncing ball. Using a Conditional Operator Java has a neat feature that I can’t resist writing about. Using this feature, you can think about alternatives in a very natural way. And what do I mean by “a natural way?” If I think out loud as I imitate the if statement near the end of Listing 11-3, I come up with this: //The thinking in Listing 11-3: What should I do next? If this is a leap year, I’ll make the numberOfDays be 29; Otherwise, I’ll make the numberOfDays be 28. I’m wandering into an if statement without a clue about what I’m doing next. That seems silly. It’s February, and everybody knows what you do in February. You ask how many days the month has. In my opinion, the code in Listing 11-3 doesn’t reflect the most natural way to think about February. So here’s a more natural way:
- 211. Chapter 11: How to Flick a Virtual Switch 193 //A more natural way to think about the problem: The value of numberOfDays is... Wait! Is this a leap year? If yes, 29 If no, 28 In this second, more natural way of thinking, I know from the start that I’m picking a number of days. So by the time I reach a fork in the road (Is this a leap year?), the only remaining task is to choose between 29 and 28. I can make the choice with finesse: case 2: System.out.print(“Leap year (true/false)? “); isLeapYear = myScanner.nextBoolean(); numberOfDays = isLeapYear ? 29 : 28; The ? : combination is called a conditional operator. In Figure 11-6, I show you how my natural thinking about February can morph into the conditional operator’s format. Figure 11-6: From your mind to the computer’s code. Taken as a whole, isLeapYear ? 29 : 28 is an expression with a value. And what value does this expression have? Well, the value of isLeapYear ? 29 : 28 is either 29 or 28. It depends on whether isLeapYear is or isn’t true. That’s how the conditional operator works: If the stuff before the question mark is true, then the whole expression’s value is whatever comes between the question mark and the colon. If the stuff before the question mark is false, then the whole expression’s value is whatever comes after the colon. Figure 11-7 gives you a goofy way to visualize these ideas.
- 212. 194 Part III: Controlling the Flow If it's true, my value is the stuff Is this stuff before between the If it's false, my the question mark question mark value is the true or false? and the colon. stuff after the colon. 29 Figure 11-7: Have you ever seen an expression It's true! talking to itself? So the conditional operator’s overall effect is as if the computer is executing numberOfDays = 29; or numberOfDays = 28; One way or another, numberOfDays gets a value, and the code solves the problem with style.
- 213. Chapter 12 Around and Around It Goes In This Chapter Creating program loops Formulating solutions to problems with loops Diagnosing loop problems C hapter 8 has code to reverse the letters in a four-letter word that the user enters. In case you jumped over Chapter 8 or you just don’t want to flip back, here’s a quick recap of the code: c1 = myScanner.findInLine(“.”).charAt(0); c2 = myScanner.findInLine(“.”).charAt(0); c3 = myScanner.findInLine(“.”).charAt(0); c4 = myScanner.findInLine(“.”).charAt(0); System.out.print(c4); System.out.print(c3); System.out.print(c2); System.out.print(c1); The code is just dandy for words with exactly four letters, but how do you reverse a five-letter word? As the code stands, you have to add two new statements: c1 = myScanner.findInLine(“.”).charAt(0); c2 = myScanner.findInLine(“.”).charAt(0); c3 = myScanner.findInLine(“.”).charAt(0); c4 = myScanner.findInLine(“.”).charAt(0); c5 = myScanner.findInLine(“.”).charAt(0); System.out.print(c5); System.out.print(c4); System.out.print(c3); System.out.print(c2); System.out.print(c1); What a drag! You add statements to a program whenever the size of a word changes! You remove statements when the input shrinks! That can’t be the best way to solve the problem. Maybe you can command a computer to add statements automatically. (But then again, maybe you can’t.)
- 214. 196 Part III: Controlling the Flow As luck would have it, you can do something that’s even better. You can write a statement once, and tell the computer to execute the statement many times. How many times? You can tell the computer to execute a statement as many times as it needs to be executed. That’s the big idea. The rest of this chapter has the details. Repeating Instructions Over and Over Again (Java while Statements) Here’s a simple dice game: Keep rolling two dice until you roll 7 or 11. Listing 12-1 has a program that simulates the action in the game, and Figure 12-1 shows two runs of the program. Listing 12-1: Roll 7 or 11 import java.util.Random; import static java.lang.System.out; class SimpleDiceGame { public static void main(String args[]) { Random myRandom = new Random(); int die1 = 0, die2 = 0; while (die1 + die2 != 7 && die1 + die2 != 11) { die1 = myRandom.nextInt(6) + 1; die2 = myRandom.nextInt(6) + 1; out.print(die1); out.print(“ “); out.println(die2); } out.print(“Rolled “); out.println(die1 + die2); } } At the core of Listing 12-1 is a thing called a while statement (also known as a while loop). A while statement has the following form: while (Condition) { Statements }
- 215. Chapter 12: Around and Around It Goes 197 Figure 12-1: Momma needs a new pair of shoes. Rephrased in English, the while statement in Listing 12-1 would say while the sum of the two dice isn’t 7 and isn’t 11 keep doing all the stuff in curly braces: { } The stuff in curly braces (the stuff that is repeated over and over again) is the code that gets two new random numbers and displays those random num- bers’ values. The statements in curly braces are repeated as long as die1 + die2 != 7 && die1 + die2 != 11 keeps being true. Each repetition of the statements in the loop is called an iteration of the loop. In Figure 12-1, the first run has two iterations, and the second run has twelve iterations. When die1 + die2 != 7 && die1 + die2 != 11 is no longer true (that is, when the sum is either 7 or 11), then the repeating of statements stops dead in its tracks. The computer marches on to the statements that come after the loop. Following the action in a loop To trace the action of the code in Listing 12-1, I’ll borrow numbers from the first run in Figure 12-1: At the start, the values of die1 and die2 are both 0. The computer gets to the top of the while statement, and checks to see if die1 + die2 != 7 && die1 + die2 != 11 is true. (See Figure 12-2.) The condition is true so the computer takes the true path in Figure 12-3.
- 216. 198 Part III: Controlling the Flow 0 not equal to 7 ? 0 not equal to 11 ? That's true. That's true. Figure 12-2: Two wrongs don’t make a right, but "true and true" two trues That makes "true." make a true. Figure 12-3: Paths through the code in Listing 12-1. The computer performs an iteration of the loop. During this iteration, the computer gets new values for die1 and die2, and prints those values on the screen. In the first run of Figure 12-1, the new values are 3 and 1. The computer returns to the top of the while statement, and checks to see if die1 + die2 != 7 && die1 + die2 != 11 is still true. The condition is true so the computer takes the true path in Figure 12-3. The computer performs another iteration of the loop. During this itera- tion, the computer gets new values for die1 and die2, and prints those values on the screen. In Figure 12-1, the new values are 4 and 3.
- 217. Chapter 12: Around and Around It Goes 199 The computer returns to the top of the while statement, and checks to see if die1 + die2 != 7 && die1 + die2 != 11 is still true. Lo and behold! This condition has become false! (See Figure 12-4.) The com- puter takes the false path in Figure 12-3. The computer leaps to the statements after the loop. The computer dis- plays Rolled 7, and ends its run of the program. 7 not equal to 7 ? 7 not equal to 11 ? That's false. That's true. Figure 12-4: "false and true" Look! I rolled That makes "false." a seven! No early bailout In Listing 12-1, when the computer finds die1 + die2 != 7 && die1 + die2 != 11 to be true, the computer marches on and executes all five statements inside the loop’s curly braces. The computer executes die1 = myRandom.nextInt(6) + 1; die2 = myRandom.nextInt(6) + 1; Maybe (just maybe), the new values of die1 and die2 add up to 7. Even so, the computer doesn’t jump out in mid-loop. The computer finishes the iteration, and executes out.print(die1); out.print(“ “); out.println(die2); one more time. The computer performs the test again (to see if die1 + die2 != 7 && die1 + die2 != 11 is still true) only after it fully executes all five statements in the loop.
- 218. 200 Part III: Controlling the Flow Statements and blocks (Plagiarizing my own sentences from Chapter 9) Java’s while statements have a lot in common { with if statements. Like an if statement, a die1 = myRandom.nextInt(6) while statement is a compound statement. + 1; That is, a while statement includes other die2 = myRandom.nextInt(6) statements within it. Also, both if statements + 1; and while statements typically include blocks out.print(die1); of statements. When you surround a bunch of out.print(“ “); statements with curly braces, you get what’s out.println(die2); called a block, and a block behaves, in all } respects, like a single statement. is a single statement. It’s a statement that has, In a typical while statement, you want the within it, five smaller statements. So this big computer to repeat several smaller statements block, (this single statement) serves as one big over and over again. To repeat several smaller statement inside the while statement in Listing statements, you combine those statements into 12-1. one big statement. To do this, you make a block That’s the story about while statements and using curly braces. blocks. To find out how this stuff applies to if In Listing 12-1, the block statements, see the “Statements and blocks” sidebar near the end of Chapter 9. Thinking about Loops (What Statements Go Where) Here’s a simplified version of the card game Twenty-One: You keep taking cards until the total is 21 or higher. Then, if the total is 21, you win. If the total is higher, you lose. (By the way, each face card counts as a 10.) To play this game, you want a program whose runs look like the runs in Figure 12-5. In most sections of this book, I put a program’s listing before the description of the program’s features. But this section is different. This section deals with strategies for composing code. So in this section, I start by brainstorming about strategies. Finding some pieces How do you write a program that plays a simplified version of Twenty-One? I start by fishing for clues in the game’s rules, spelled out in this section’s first paragraph. The big fishing expedition is illustrated in Figure 12-6.
- 219. Chapter 12: Around and Around It Goes 201 Figure 12-5: You win sum; you lose sum. "Keep doing" There's the loop's something. That means condition. Keep I need a loop. repeating as long as Keep taking cards until the total is 21 or higher. If the total is 21, you win. If the total is higher, you lose. Figure 12-6: Thinking about a I smell an statement. programm- (total==21) . . . ing problem. With the reasoning in Figure 12-6, I need a loop and an if statement: while (total < 21) { //do stuff } if (total == 21) { //You win } else { //You lose }
- 220. 202 Part III: Controlling the Flow What else do I need to make this program work? Look at the sample output in Figure 12-5. I need a heading with the words Card and Total. That’s a call to System.out.println: System.out.println(“Card Total”); I also need several lines of output — each containing two numbers. For exam- ple, in Figure 12-5, the line 6 14 displays the values of two variables. One variable stores the most recently picked card; the other variable stores the total of all cards picked so far: System.out.print(card); System.out.print(“ “); System.out.println(total); Now I have four chunks of code, but I haven’t decided how they all fit together. Well, you can go right ahead and call me crazy. But at this point in the process, I imagine those four chunks of code circling around one another, like part of a dream sequence in a low-budget movie. As you may imagine, I’m not very good at illustrating circling code in dream sequences. Even so, I handed my idea to the art department at Wiley Publishing, and they came up with the pic- ture in Figure 12-7. Figure 12-7: . . . and where they stop, nobody knows.
- 221. Chapter 12: Around and Around It Goes 203 Assembling the pieces Where should I put each piece of code? The best way to approach the prob- lem is to ask how many times each piece of code should be executed: The program displays card and total values more than once. For example, in the first run of Figure 12-5, the program displays these values four times (first 8 8, then 6 14, and so on). To get this repeated display, I put the code that creates the display inside the loop: while (total < 21) { System.out.print(card); System.out.print(“ “); System.out.println(total); } The program displays the Card Total heading only once per run. This display comes before any of the repeated number displays, so I put the heading code before the loop: System.out.println(“Card Total”); while (total < 21) { System.out.print(card); System.out.print(“ “); System.out.println(total); } The program displays You win or You lose only once per run. This message display comes after the repeated number displays. So I put the win/lose code after the loop: //Preliminary draft code - NOT ready for prime time: System.out.println(“Card Total”); while (total < 21) { System.out.print(card); System.out.print(“ “); System.out.println(total); } if (total == 21) { System.out.println(“You win :-)”); } else { System.out.println(“You lose :-(“); } Getting values for variables I almost have a working program. But if I take the code that I’ve developed for a mental test run, I face a few problems. To see what I mean, picture yourself
- 222. 204 Part III: Controlling the Flow in the computer’s shoes for a minute. (Well, a computer doesn’t have shoes. Picture yourself in the computer’s boots.) You start at the top of the code shown previously (the code that starts with the Preliminary draft comment). In the code’s first statement, you display the words Card Total. So far, so good. But then you encounter the while loop, and test the condition total < 21. Well, is total less than 21, or isn’t it? Honestly, I’m tempted to make up an answer, because I’m embarrassed about not knowing what the total variable’s value is. (I’m sure the computer is embarrassed too.) The variable total must have a known value before the computer reaches the top of the while loop. Because a player starts with no cards at all, the ini- tial total value should be 0. That settles it. I declare int total = 0 at the top of the program. But what about my friend, the card variable? Should I set card to zero also? No. There’s no zero-valued card in a deck (at least, not when I’m playing fair). Besides, card should get a new value several times during the program’s run. Wait! In the previous sentence, the phrase several times tickles a neuron in my brain. It stimulates the inside a loop reflex. So I place an assignment to the card variable inside my while loop: //This is a DRAFT - still NOT ready for prime time: int card, total = 0; System.out.println(“Card Total”); while (total < 21) { card = myRandom.nextInt(10) + 1; System.out.print(card); System.out.print(“ “); System.out.println(total); } if (total == 21) { System.out.println(“You win :-)”); } else { System.out.println(“You lose :-(“); } The code still has an error, and I can probably find the error with more com- puter role-playing. But instead, I get daring. I run this beta code to see what happens. Figure 12-8 shows part of a run. Unfortunately, the run in Figure 12-8 doesn’t stop on its own. This kind of pro- cessing is called an infinite loop. The loop runs and runs until someone trips over the computer’s extension cord.
- 223. Chapter 12: Around and Around It Goes 205 Figure 12-8: An incorrect run. You can stop a program’s run dead in its tracks. If you use JCreator, choose Tools➪Stop Tool from the main menu. With many other environments, press- ing Ctrl+C stops the execution of a runaway program. From infinity to affinity For some problems, an infinite loop is normal and desirable. Consider, for example, a real-time mission-critical application — air traffic control, or the monitoring of a heart-lung machine. In these situations, a program should run and run and run. But a game of Twenty-One should end pretty quickly. In Figure 12-8, the game doesn’t end because the total never reaches 21 or higher. In fact, the total is always zero. The problem is that my code has no statement to change the total variable’s value. I should add each card’s value to the total: total += card; Again, I ask myself where this statement belongs in the code. How many times should the computer execute this assignment statement? Once at the start of the program? Once at the end of the run? Repeatedly? The computer should repeatedly add a card’s value to the running total. In fact, the computer should add to the total each time a card gets drawn. So the assignment statement above should be inside the while loop, right along- side the statement that gets a new card value: card = myRandom.nextInt(10) + 1; total += card; With this revelation, I’m ready to see the complete program. The code is in Listing 12-2, and two runs of the code are shown in Figure 12-5.
- 224. 206 Part III: Controlling the Flow Escapism You can use a neat trick to make a program’s System.out.print(“t”); output line up correctly. In Figure 12-5, the num- System.out.println(total); bers 8 8, then 6 14 (and so on) are displayed. I In the first statement, the computer displays want these numbers to be right under the head- Card, then jumps to the next tab stop on the ing words Card and Total. Normally, you can screen, and then displays Total. In the next get perfect vertical columns by pressing the tab three statements, the computer displays a card key, but a computer program creates the output in number (like the number 6), then jumps to the next Figure 12-5. How can you get a computer program tab stop (directly under the word Total), and to press the tab key? then displays a total value (like the number 14). In Java, there’s a way. You can put t inside The t combination of characters is an example double quote marks. of an escape sequence. Another of Java’s escape sequences, the combination n, moves the cursor System.out.println(“CardtTotal to a new line. In other words, System.out. ”); print(“Hellon”) does the same thing as System.out.print(card); System.out.println(“Hello”). Listing 12-2: A Simplified Version of the Game Twenty-One import java.util.Random; class PlayTwentyOne { public static void main(String args[]) { Random myRandom = new Random(); int card, total = 0; System.out.println(“Card Total”); while (total < 21) { card = myRandom.nextInt(10) + 1; total += card; System.out.print(card); System.out.print(“ “); System.out.println(total); } if (total == 21) { System.out.println(“You win :-)”); } else { System.out.println(“You lose :-(“); } } }
- 225. Chapter 12: Around and Around It Goes 207 If you’ve read this whole section, then you’re probably exhausted. Creating a loop can be a lot of work. Fortunately, the more you practice, the easier it becomes. Thinking About Loops (Priming) I remember when I was a young boy. We lived on Front Street in Philadelphia, near where the El train turned onto Kensington Avenue. Come early morning, I’d have to go outside and get water from the well. I’d pump several times before any water would come out. Ma and Pa called it “priming the pump.” These days I don’t prime pumps. I prime while loops. Consider the case of a busy network administrator. She needs a program that extracts a username from an e-mail address. For example, the program reads John@BurdBrain.com and writes John How does the program do it? Like other examples in the chapter, this prob- lem involves repetition: Repeatedly do the following: Read a character. Write the character. The program then stops the repetition when it finds the @ sign. I take a stab at writing this program. My first attempt doesn’t work, but it’s a darn good start. It’s in Listing 12-3. Listing 12-3: Trying to Get a Username from an E-mail Address /* * This code does NOT work, but I’m not discouraged. */ import java.util.Scanner; class FirstAttempt { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); char symbol = ‘ ‘; (continued)
- 226. 208 Part III: Controlling the Flow Listing 12-3 (continued) while (symbol != ‘@’) { symbol = myScanner.findInLine(“.”).charAt(0); System.out.print(symbol); } System.out.println(); } } When you run the code in Listing 12-3, you get the output shown in Figure 12-9. The user types one character after another — the letter J, then o, then h, and so on. At first, the program in Listing 12-3 does nothing. (The computer doesn’t send any of the user’s input to the program until the user presses Enter.) After the user types a whole e-mail address and presses Enter, the program gets its first character (the J in John). Unfortunately, the program’s output isn’t what you expect. Instead of just the user name John, you get the username and the @ sign. Figure 12-9: Oops! Got the @ sign too. To find out why this happens, follow the computer’s actions as it reads the input John@BurdBrain.com: Set symbol to ‘ ‘ (a blank space). Is that blank space the same as an @ sign? No, so perform a loop iteration. Input the letter ‘J’. Print the letter ‘J’. Is that ‘J’ the same as an @ sign? No, so perform a loop iteration. Input the letter ‘o’. Print the letter ‘o’. Is that ‘o’ the same as an @ sign? No, so perform a loop iteration. Input the letter ‘h’. Print the letter ‘h’.
- 227. Chapter 12: Around and Around It Goes 209 Is that ‘h’ the same as an @ sign? No, so perform a loop iteration. Input the letter ‘n’. Print the letter ‘n’. Is that ‘n’ the same as an @ sign? //Here’s the problem. No, so perform a loop iteration. Input the @ sign. Print the @ sign. //Oops! Is that @ sign the same as an @ sign? Yes, so stop iterating. Near the end of the program’s run, the computer compares the letter n with the @ sign. Because n isn’t an @ sign, the computer dives right into the loop: The first statement in the loop reads an @ sign from the keyboard. The second statement in the loop doesn’t check to see if it’s time to stop printing. Instead, that second statement just marches ahead and displays the @ sign. After you’ve displayed the @ sign, there’s no going back. You can’t change your mind and undisplay the @ sign. So the code in Listing 12-3 doesn’t quite work. Working on the problem You learn from your mistakes. The problem with Listing 12-3 is that, between reading and writing a character, the program doesn’t check for an @ sign. Instead of doing “Test, Input, Print,” it should do “Input, Test, Print.” That is, instead of doing this: Is that ‘o’ the same as an @ sign? No, so perform a loop iteration. Input the letter ‘h’. Print the letter ‘h’. Is that ‘h’ the same as an @ sign? No, so perform a loop iteration. Input the letter ‘n’. Print the letter ‘n’. Is that ‘n’ the same as an @ sign? //Here’s the problem. No, so perform a loop iteration. Input the @ sign. Print the @ sign. //Oops!
- 228. 210 Part III: Controlling the Flow the program should do this: Input the letter ‘o’. Is that ‘o’ the same as an @ sign? No, so perform a loop iteration. Print the letter ‘o’. Input the letter ‘n’. Is that ‘n’ the same as an @ sign? No, so perform a loop iteration. Print the letter ‘n’. Input the @ sign. Is that @ sign the same as an @ sign? Yes, so stop iterating. This cycle is shown in Figure 12-10. Figure 12-10: What the program needs to do. You can try to imitate the following informal pattern: Input a character. Is that character the same as an @ sign? If not, perform a loop iteration. Print the character.
- 229. Chapter 12: Around and Around It Goes 211 The problem is, you can’t create a while loop that looks like this: //This is not correct code: { symbol = myScanner.findInLine(“.”).charAt(0); while (symbol != ‘@’) System.out.print(symbol); } You can’t sandwich a while statement’s condition between two of the state- ments that you intend to repeat. So what can you do? You need to follow the flow in Figure 12-11. Because every while loop starts with a test, that’s where you jump into the circle, First Test, then Print, and finally Input. Figure 12-11: Jumping into a loop. Listing 12-4 shows the embodiment of this “test, then print, then input” strategy. Listing 12-4: Nice Try, But . . . /* * This code almost works, but there’s one tiny error: */ import java.util.Scanner; (continued)
- 230. 212 Part III: Controlling the Flow Listing 12-4 (continued) class SecondAttempt { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); char symbol = ‘ ‘; while (symbol != ‘@’) { System.out.print(symbol); symbol = myScanner.findInLine(“.”).charAt(0); } System.out.println(); } } A run of the Listing 12-4 code is shown in Figure 12-12. The code is almost correct, but I still have a slight problem. Notice the blank space before the user’s input. The program races prematurely into the loop. The first time the computer executes the statements System.out.print(symbol); symbol = myScanner.findInLine(“.”).charAt(0); the computer displays an unwanted blank space. Then the computer gets the J in John. In some applications, an extra blank space is no big deal. But in other applications, extra output can be disastrous. Figure 12-12: The computer displays an extra blank space. Fixing the problem Disastrous or not, an unwanted blank space is the symptom of a logical flaw. The program shouldn’t display results before it has any meaningful results to display. The solution to this problem is called . . . (drumroll, please) . . . priming the loop. You pump findInLine(“.”).charAt(0) once to get some values flowing. Listing 12-5 shows you how to do it.
- 231. Chapter 12: Around and Around It Goes 213 Listing 12-5: How to Prime a Loop /* * This code works correctly! */ import java.util.Scanner; class GetUserName { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); char symbol; symbol = myScanner.findInLine(“.”).charAt(0); while (symbol != ‘@’) { System.out.print(symbol); symbol = myScanner.findInLine(“.”).charAt(0); } System.out.println(); } } Listing 12-5 follows the strategy shown in Figure 12-13. First you get a character (the letter J in John, for example), then you enter the loop. After you’re in the loop, you test the letter against the @ sign, and print the letter if it’s appropri- ate to do so. Figure 12-14 shows a beautiful run of the GetUserName program. Figure 12-13: The strategy in Listing 12-5.
- 232. 214 Part III: Controlling the Flow Figure 12-14: A run of the code in Listing 12-5. The priming of loops in an important programming technique. But it’s not the end of the story. In Chapters 14, 15, and 16, you can read about some other useful looping tricks.
- 233. Chapter 13 Piles of Files: Dealing with Information Overload In This Chapter Using data on your hard drive Writing code to access the hard drive Troubleshooting input/output behavior C onsider these scenarios: You’re a business owner with hundreds of invoices. To avoid boxes full of paper, you store invoice data in a file on your hard drive. You need customized code to sort and classify the invoices. You’re an astronomer with data from scans of the night sky. When you’re ready to analyze a chunk of data, you load the chunk onto your com- puter’s hard drive. You’re the author of a popular self-help book. Last year’s fad was called the Self Mirroring Method. This year’s craze is the Make Your Cake System. You can’t modify your manuscript without converting to the publisher’s new specifications. The trouble is, there’s no existing soft- ware to make the task bearable. Each situation calls for a new computer program, and each program reads from a large data file. On top of all that, each program creates a brand new file containing bright, shiny results. In previous chapters, the examples get input from the keyboard and send output to the screen. That’s fine for small tasks, but you can’t have the com- puter prompt you for each bit of night sky data. For big problems, you need lots of data, and the best place to store the data is on a computer’s hard drive.
- 234. 216 Part III: Controlling the Flow Running a Disk-Oriented Program To deal with volumes of data, you need tools for reading from (and writing to) disk files. At the mere mention of disk files, some peoples’ hearts start to pal- pitate with fear. After all, a disk file is elusive and invisible. It’s stored some- where inside your computer, with some magic magnetic process. The truth is, getting data from a disk is very much like getting data from the keyboard. And printing data to a disk is like printing data to the computer screen. Consider the scenario when you run the code in the previous chapters. You type some stuff on the keyboard. The program takes this stuff, and spits out some stuff of its own. The program sends this new stuff to the screen. In effect, the flow of data goes from the keyboard, to the computer’s innards, and on to the screen, as shown in Figure 13-1. Of course, the goal in this chapter is the scenario in Figure 13-2. There’s a file containing data on your hard drive. The program takes data from the disk file and spits out some brand new data. The program then sends the new data to another file on the hard drive. In effect, the flow of data goes from a disk file, to the computer’s innards, and on to another disk file. Figure 13-1: Using the keyboard and screen.
- 235. 218 Part III: Controlling the Flow Listing 13-1: Using the Keyboard and the Screen import java.util.Scanner; class ComputeTotal { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); double unitPrice, quantity, total; unitPrice = myScanner.nextDouble(); quantity = myScanner.nextInt(); total = unitPrice * quantity; System.out.println(total); } } Figure 13-3: Read from the keyboard; write to the screen. The goal is to write a program like the one in Listing 13-1. But instead of talk- ing to your keyboard and screen, this new program talks to your hard drive. The new program reads unit price and quantity from your hard drive, and writes the total back to your hard drive. Java’s API has everything you need for interacting with a hard drive. A nice example is in Listing 13-2. Listing 13-2: Using Input and Output Files import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class ReadAndWrite { public static void main(String args[]) throws FileNotFoundException {
- 236. Chapter 13: Piles of Files: Dealing with Information Overload 219 Scanner diskScanner = new Scanner(new File(“rawData.txt”)); PrintStream diskWriter = new PrintStream(“cookedData.txt”); double unitPrice, quantity, total; unitPrice = diskScanner.nextDouble(); quantity = diskScanner.nextInt(); total = unitPrice * quantity; diskWriter.println(total); } } Creating code that messes with your hard drive “I _____ (print your name)___ agree to pay $_____ each month on the ___th day of the month.” Fill in the blanks. That’s all you have to do. Reading input from a disk can work the same way. Just fill in the blanks in Listing 13-3. Listing 13-3: A Template to Read Data from a Disk File /* * Before you can compile this code, * you must fill in the blanks. */ import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; class _______________ { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“___________”)); ________ = diskScanner.nextInt(); ________ = diskScanner.nextDouble(); ________ = diskScanner.nextLine(); ________ = diskScanner.findInLine(“.”).charAt(0); (continued)
- 237. 220 Part III: Controlling the Flow Listing 13-3 (continued) // Etc. } } To use Listing 13-3, make up a name for your class. Insert that name into the first blank space. Type the name of the input file in the second space (between the quotation marks). Then, to read a whole number from the input file, call diskScanner.nextInt. To read a number that has a decimal point, call diskScanner.nextDouble. You can call any of the Scanner methods in Table 5-1 (the same methods you call when you get keystrokes from the keyboard). The stuff in Listing 13-3 isn’t a complete program. Instead, it’s a code template — a half-baked piece of code, with spaces for you to fill in. With the template in Listing 13-3, you can input data from a disk file. With a sim- ilar template, you can write output to a file. The template is in Listing 13-4. Listing 13-4: A Template to Write Data to a Disk File /* * Before you can compile this code, * you must fill in the blanks. */ import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class _______________ { public static void main(String args[]) throws FileNotFoundException { PrintStream diskWriter = new PrintStream(“___________”); diskWriter.print(_____); diskWriter.println(_____); // Etc. } } To use Listing 13-4, insert the name of your class into the first blank space. Type the name of the output file in the space between the quotation marks. Then, to write part of a line to the output file, call diskWriter.print. To write the remainder of a line to the output file, call diskWriter.println.
- 238. Chapter 13: Piles of Files: Dealing with Information Overload 221 A quick look at Java’s disk access facilities Templates like the ones in Listings 13-3 and 13-4 In Listing 13-2, the added words throws are very nice. But knowing how the templates FileNotFoundException form a work is even better. Here are a few tidbits throws clause. A throws clause is a kind of describing the inner workings of Java’s disk disclaimer. Putting a throws clause in your access code. code is like saying “I realize that this code can run into trouble.” A PrintStream is something you can use for writing output. Of course in the legal realm, you often have no choice about signing disclaimers. “If you A PrintStream is like a Scanner. The don’t sign this disclaimer then the surgeon big difference is, a Scanner is for reading won’t operate on you.” Okay, then; I’ll sign it. input and a PrintStream is for writing The same is true with a Java throws clause. output. To see what I mean, look at Listing If you put things like new PrintStream 13-2. Notice the similarity between the (“cookedData.txt”) in your code, and statements that use Scanner and the you don’t add something like throws statements that use PrintStream. FileNotFoundException, then the Java The word PrintStream is defined in the compiler refuses to compile your code. Java API. So when do you need this throws File- In Listing 13-2, the expression new NotFoundException clause, and when File(“rawData.txt”) plays the same should you do without it? Well, having certain role that System.in plays in so many things in your code — things like new other programs. PrintStream(“cookedData.txt”) — force you to create a throws clause. You can Just as System.in stands for the com- learn all about the kinds of things that puter’s keyboard, the expression new demand throws clauses. But at this point, it’s File(“rawData.txt”) stands for a file better to concentrate on other programming on your computer’s hard drive. When the issues. As a beginning Java programmer, the computer calls new File(“rawData. safest thing to do is to follow the templates in txt”), the computer creates something Listings 13-3 and 13-4. like System.in — something you can stuff inside the new Scanner( ) parentheses. The word FileNotFoundException is . . . you guessed it . . . defined in the Java API. The word File is defined in the Java API. To create this chapter’s code, I made up the A FileNotFoundException is some- names diskScanner and diskWriter. thing that may go wrong during an attempt to read input from a disk file (or an attempt The words diskScanner and disk- to write output to a disk file). Writer don’t come from the Java API. In place of diskScanner and diskWriter Disk file access is loaded with pitfalls. Even you can use any names you want. All you the best programs run into disk access trou- have to do is to use the names consistently ble occasionally. So to brace against such within each of your Java programs. pitfalls, Java insists on your adding some extra words to your code.
- 239. 222 Part III: Controlling the Flow The Pro version of JCreator has a built-in feature for creating and inserting code templates. To download a trial copy of JCreator Pro, visit www.JCreator.com. If your program gets input from one disk file and writes output to another, then combine the stuff from Listings 13-3 and 13-4. When you do, you get a program like the one in Listing 13-2. Running the sample program Testing the code in Listing 13-2 is a three-step process. Here’s an outline of the three steps: 1. Create rawData.txt file. 2. Compile and run the code in Listing 13-2. 3. View the contents of the cookedData.txt file. The next few sections cover each step in detail. Creating an input file You can use any plain old text editor to create an input file for the code in Listing 13-2. In this section, I show you how to use JCreator’s built-in editor. 1. In the File View pane, right-click the name of a project. In this example, select the project containing the Listing 13-2 code. 2. In the right-click context menu, choose Add➪New File. JCreator’s File Wizard opens to the File Path tab. 3. In the Name field, type the name of your new data file. You can type any name that your computer considers to be a valid file name. For this section’s example, I used the name rawData.txt, but other names, such as rawData.dat, rawData, or raw123.01.dataFile are fine. I try to avoid troublesome names (including short, uninforma- tive names and names containing blank spaces) but the name you choose is entirely up to you (and your computer’s operating system, and your boss’s whims, and your customer’s specifications). Always include a dot in File Path tab’s Name field. If the file’s name has no extension, add a dot at the end of the name. For example, to create a file named rawData (not rawData.txt or rawData.dat), type rawData. (that’s rawData followed by a dot.) If you don’t type your own dot any- where in the Name field, then JCreator adds a default extension to the file’s name (turning rawData into rawData.java).
- 240. Chapter 13: Piles of Files: Dealing with Information Overload 223 4. Click Finish. The file’s name appears in JCreator’s File View pane. A tab (with the new file’s name) appears in JCreator’s Editor pane. 5. Type text in the Editor pane. To create this section’s example, I typed the text 19.95 5 as shown in Figure 13-4. To create your own example, type whatever text your pro- gram needs during its run. Figure 13-4: Editing an input file. This book’s Web site has tips for readers who need to create data files with- out using JCreator. This includes instructions for Linux, Unix, and Macintosh environments. Compiling and running the code Do the same thing you do with any other Java program. Choose Build➪Compile Project, and then choose Build➪Execute Project. The result of the run (at least the part of the result that you can see in JCreator’s General Output pane) is shown in Figure 13-5. Figure 13-5: Compiling and running the code in Listing 13-2. I’m the first to admit it — the run in Figure 13-5 is duller than dirt. The total lack of any noticeable output gives some people the willies. The truth is, a pro- gram like the one in Listing 13-2 does all of its work behind the scenes. The program has no statements that read from the keyboard and has no statements that print to the screen. So if you have a very loud hard drive, you may hear a little chirping sound when you choose Build➪Execute Project, but you won’t type any program input, and you won’t see any program output. The program sends all its output to a file on your hard drive. So what do you do to see the file’s contents?
- 241. 224 Part III: Controlling the Flow Viewing the output file To see the output of the program in Listing 13-2, follow these steps: 1. In the File View pane, right-click the name of a project. In this example, select the project containing the Listing 13-2 code. 2. In the right-click context menu, choose Add➪Add Existing Files. A familiar Open dialog box appears. 3. In the Open dialog box’s Files of Type list, select All Files(*.*). When you select All Files(*.*), additional entries appear in the Open dialog’s list of filenames. 4. In the list of filenames, double-click cookedData.txt. As a result, the name cookedData.txt appears in the File View pane. 5. Double-click the cookedData.txt branch in the File View pane. The contents of cookedData.txt appear in JCreator’s editor. (See Figure 13-6.) Figure 13-6: Viewing an output file. Troubleshooting problems with disk files When you run the code in Listing 13-2, the computer executes new Scanner (new File(“rawData.txt”)). If the Java virtual machine can’t find the rawData.txt file, then you see a message like the one shown in Figure 13-7. This error message can be very frustrating. In many cases, you know darn well that there’s a rawData.txt file on your hard drive. The stupid computer simply can’t find it. Figure 13-7: The computer can’t find your file.
- 242. Chapter 13: Piles of Files: Dealing with Information Overload 225 There’s no quick, sure-fire way to fix this problem. But you should always check the following things first: Check again for a file named rawData.txt. Open My Computer and poke around for a file with that name. The filenames displayed in My Computer can be misleading. You may see the name rawData even though the file’s real name is rawData.txt. To fix this problem once and for all, read the sidebar “Those pesky file- name extensions” in Chapter 3. Check the spelling of the file’s name. Make sure that the name in your program is exactly the same as the name of the file on your hard drive. Just one misplaced letter can keep the computer from finding a file. If you use Unix or Linux, check the capitalization of the file’s name. In Unix and Linux, the difference between uppercase and lowercase can baffle the computer. Check that the file is in the correct directory. Sure, you have a file named rawData.txt. But, to find the file, don’t expect the computer to search everywhere on your hard drive. As a general rule, you should have rawData.txt and the Listing 13-2 code in the same directory on your hard drive. But file locations can be tricky, especially if you work in an unfamiliar programming environment. In some situations the general rule may not apply to you. So here’s a trick you can use: Compile and run this stripped-down ver- sion of the code in Listing 13-2: import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class JustWrite { public static void main(String args[]) throws FileNotFoundException { PrintStream diskWriter = new PrintStream(“cookedData.txt”); diskWriter.println(99.75); } } This program has no need for a stinking rawData.txt file. If you run this code and get no error messages, then search your hard drive for this pro- gram’s output (the cookedData.txt file). Note the name of the directory that contains the cookedData.txt file. When you put rawData.txt in this same directory, then any problem you had running the Listing 13-2 code should go away.
- 243. 226 Part III: Controlling the Flow Check the rawData.txt file’s content. It never hurts to peek inside the rawData.txt file, and make sure that the file contains the numbers 19.95 5. If rawData.txt no longer appears in JCreator’s editor pane, find the Listing 13-2 project in the File View tree. Double-clicking the project’s rawData.txt branch makes that file appear in JCreator’s editor pane. Writing a Disk-Oriented Program Listing 13-2 is very much like Listing 13-1. In fact, you can go from Listing 13-1 to Listing 13-2 with some simple editing. Here’s how: Add the following import declarations to the beginning of your code: import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; Add the following throws clause to the method header: throws FileNotFoundException In the call to new Scanner, replace System.in with a call to new File as follows: Scanner aVariableName = new Scanner(new File(“inputFileName”)) Create a PrintStream for writing output to a disk file. PrintStream anotherVariableName = new PrintStream(“outputFileName”); Use the Scanner variable name in calls to nextInt, nextLine, and so on. For example, to go from Listing 13-1 to Listing 13-2, I change unitPrice = myScanner.nextDouble(); quantity = myScanner.nextInt(); to unitPrice = diskScanner.nextDouble(); quantity = diskScanner.nextInt(); Use the PrintStream variable name in calls to print and println. For example, to go from Listing 13-1 to Listing 13-2, I change System.out.println(total); to diskWriter.println(total);
- 244. Chapter 13: Piles of Files: Dealing with Information Overload 227 Reading from a file All the Scanner methods can read from existing disk files. For instance, to read a word from a file named mySpeech, use code of the following kind: Scanner diskScanner = new Scanner(new File(“mySpeech”)); String oneWord = diskScanner.next(); To read a character from a file named letters.dat, and then display the character on the screen, you can do something like this: Scanner diskScanner = new Scanner(new File(“letters.dat”)); System.out.println(diskScanner.findInLine(“.”).charAt(0)); Notice how I read from a file named mySpeech, not mySpeech.txt or mySpeech.doc. Anything that you put after the dot is called a filename exten- sion, and for a file full of numbers and other data, the filename extension is optional. Sure, a Java program must be called something.java, but a data file can be named mySpeech.txt, mySpeech.reallymine.allmine, or just mySpeech. As long as the name in your new File call is the same as the file- name on your computer’s hard drive, everything is okay. Writing to a file The print and println methods can write to disk files. Here are some examples: During a run of the code in Listing 13-2, the variable total stores the number 99.75. To deposit 99.75 into the cookedData.txt file, you execute diskWriter.println(total); This println call writes to a disk file because of the following line in Listing 13-2: PrintStream diskWriter = new PrintStream(“cookedData.txt”); In another version of the program, you may decide not to use a total variable. To write 99.75 to the cookedData.txt file, you can call diskWriter.println(unitPrice * quantity); To display OK on the screen, you can make the following method call: System.out.print(“OK”);
- 245. 228 Part III: Controlling the Flow To write OK to a file named approval.txt, you can use the following code: PrintStream diskWriter = new PrintStream(“approval.txt”); diskWriter.print(“OK”); You may decide to write OK as two separate characters. To write to the screen, you can make the following calls: System.out.print(‘O’); System.out.print(‘K’); And to write OK to the approval.txt file, you can use the following code: PrintStream diskWriter = new PrintStream(“approval.txt”); diskWriter.print(‘O’); diskWriter.print(‘K’); Like their counterparts for System.out, the disk-writing print and println methods differ in their end-of-line behaviors. For example, you want to display the following text on the screen: Hankees Socks 7 3 To do this, you can make the following method calls: System.out.print(“Hankees “); System.out.println(“Socks”); System.out.print(7); System.out.print(“ “); System.out.println(3); To plant the same text into a file named scores.dat, you can use the following code: PrintStream diskWriter = new PrintStream(“scores.dat”); diskWriter.print(“Hankees “); diskWriter.println(“Socks”); diskWriter.print(7); diskWriter.print(“ “); diskWriter.println(3); When you make up a new data filename, you don’t have to use a particular three-letter extension. In fact, you don’t have to use an extension at all. Out of habit, I normally use .txt or .dat, but I could also use .text, .data, .flatworm, or I could skip the extension entirely.
- 246. Chapter 13: Piles of Files: Dealing with Information Overload 229 Name that file What if a file that contains data is not in your MyProjectsChapter13_Listing09 program’s project directory? If that’s the case, directory, and that a file named totals is in a when you call new File, the file’s name must directory named c:advertisements. (See include directory names. For example, imagine the following figure.) that your TallyBudget.java program is in JCreator’s Then, to refer to the totals file, you include Inside quotation marks, you use the double back- the directory name, the filename and (to be on slash to indicate what would usually be a single the safe side) the drive letter: backslash. So the string “c:advertise- mentstotals” stands for c:advertise- Scanner diskScanner = new Scanner(new mentstotals. That’s good, because c: File(“c:advertisementsto advertisementstotals is the way you nor- tals”)); mally refer to a file in Windows. Notice how I use double backslashes. To find Of course, if you use Unix, Linux, or a Macintosh, out why, look at the sidebar entitled “Escapism” then you’re in luck. This double backslash non- in Chapter 12. The string “totals” with a sense doesn’t apply to you. Just write single backslash stands for a tab, followed by Scanner diskScanner = otals. But in this example, the file’s name is new Scanner(new totals, not otals. With a single backslash, File(“/home/bburd/advertise- the name ...advertisementstotals” ments/totals”)); would not work correctly. or something similar that reflects your system’s directory structure.
- 247. 230 Part III: Controlling the Flow Writing, Rewriting, and Re-rewriting Given my mischievous ways, I tried a little experiment. I asked myself what would happen if I ran the same file-writing program more than once. So I cre- ated a tiny program (the program in Listing 13-5) and I ran the program twice. Then I examined the program’s output file. The output file (shown in Figure 13-8) contains only two letters. Listing 13-5: A Little Experiment import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class WriteOK { public static void main(String args[]) throws FileNotFoundException { PrintStream diskWriter = new PrintStream(new File(“approval.txt”)); diskWriter.print (‘O’); diskWriter.println(‘K’); } } Figure 13-8: Testing the waters. Here’s the sequence of events from the start to the end of the experiment: 1. Before I run the code in Listing 13-5, my computer’s hard drive has no approval.txt file. That’s okay. Every experiment has to start somewhere. 2. I run the code in Listing 13-5. The call to new PrintStream in Listing 13-5 creates a file named approval.txt. Initially, the new approval.txt file contains no charac- ters. Later in Listing 13-5, calls to print and println put characters in the file. So after running the code, the approval.txt file contains two letters — the letters OK.
- 248. Chapter 13: Piles of Files: Dealing with Information Overload 231 3. I run the code from Listing 13-5 a second time. At this point, I could imagine seeing OKOK in the approval.txt file. But that’s not what I see in Figure 13-8. After running the code twice, the approval.txt file contains just one OK. Here’s why: • The call to new PrintStream in Listing 13-5 deletes my existing approval.txt file. The call creates a new, empty approval.txt file. • After creating a new approval.txt file, the print method call drops the letter O into the new file. • The println method call adds the letter K to the same approval.txt file. So that’s the story. Each time you run the program, it trashes whatever approval.txt file is already on the hard drive. Then the program adds data to a newly created approval.txt file.
- 249. 232 Part III: Controlling the Flow
- 250. Chapter 14 Creating Loops within Loops In This Chapter Analyzing loop strategies Diagnosing loop problems Creating nested loops I f you’re an editor at Wiley Publishing, please don’t read the next few para- graphs. In the next few paragraphs, I give away an important trade secret (something you really don’t want me to do). I’m about to describe a surefire process for writing a best-selling For Dummies book. Here’s the process: Write several words to create a sentence. Do this several times to create a paragraph. Repeat the following to form a paragraph: Repeat the following to form a sentence: Write a word. Repeat the above instructions several times to make a section. Make several sections, and then make several chapters. Repeat the following to form a best-selling For Dummies book: Repeat the following to form a chapter: Repeat the following to form a section: Repeat the following to form a paragraph: Repeat the following to form a sentence: Write a word. This process involves a loop within a loop within a loop within a loop within a loop. It’s like a verbal M.C. Escher print. Is it useful, or is it frivolous?
- 251. 234 Part III: Controlling the Flow Well, in the world of computer programming, this kind of thing happens all the time. Most five-layered loops are hidden behind method calls, but two- layered loops within loops are everyday occurrences. So this chapter tells you how to compose a loop within a loop. It’s very useful stuff. By the way, if you’re a Wiley Publishing editor, you can start reading again from this point onward. Paying Your Old Code a Little Visit The program in Listing 12-5 extracts a username from an e-mail address. For example, the program reads John@BurdBrain.com from the keyboard, and writes John to the screen. Let me tell you . . . in this book, I have some pretty lame excuses for writing programs, but this simple e-mail example tops the list! Why would you want to type something on the keyboard, only to have the computer display part of what you typed? There must be a better use for code of this kind. Sure enough, there is. The BurdBrain.com network administrator has a list of 10,000 employees’ e-mail addresses. More precisely, the administrator’s hard drive has a file named email.txt. This file contains 10,000 e-mail addresses, with one address on each line, as shown in Figure 14-1. Figure 14-1: A list of e-mail addresses. The company’s e-mail software has an interesting feature. To send e-mail within the company, you don’t need to type an entire e-mail address. For example, to send e-mail to John, you can type the username John instead of John@BurdBrain.com. (This @BurdBrain.com part is called the host name.)
- 252. Chapter 14: Creating Loops within Loops 235 So the company’s network administrator wants to distill the content of the email.txt file. She wants a new file, usernames.txt, that contains user- names with no host names, as shown in Figure 14-2. Figure 14-2: Usernames extracted from the list of e-mail addresses. Reworking some existing code To solve the administrator’s problem, you need to modify the code in Listing 12-5. The new version gets an e-mail address from a disk file and writes a username to another disk file. The new version is in Listing 14-1. Listing 14-1: From One File to Another import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class ListOneUsername { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“email.txt”)); PrintStream diskWriter = new PrintStream(“usernames.txt”); char symbol; symbol = diskScanner.findInLine(“.”).charAt(0); while (symbol != ‘@’) { diskWriter.print(symbol); symbol = diskScanner.findInLine(“.”).charAt(0); } diskWriter.println(); } }
- 253. 236 Part III: Controlling the Flow Listing 14-1 does almost the same thing as its forerunner in Listing 12-5. The only difference is that the code in Listing 14-1 doesn’t interact with the user. Instead, the code in Listing 14-1 interacts with disk files. Running your code Here’s how you run the code in Listing 14-1: 1. Put the file email.txt in your project directory along with the ListOneUsername.java file (the code from Listing 14-1). In the email.txt file, put just one e-mail address. Any address will do, as long as the address contains an @ sign. 2. Compile and run the code in Listing 14-1. When you run the code, you see nothing interesting in the General Output pane. All you see is the phrase Process completed. 3. View the contents of the usernames.txt file. If your email.txt file contains John@BurdBrain.com, then the usernames.txt file contains John. For more details on any of these steps, see the discussion accompanying Listings 13-2, 13-3, and 13-4 in Chapter 13. Creating Useful Code The previous section describes a network administrator’s problem — creating a file filled with usernames from a file filled with e-mail addresses. The code in Listing 14-1 solves part of the problem — it extracts just one e-mail address. That’s a good start, but to get just one username, you don’t need a computer program. A pencil and paper does the trick. So don’t keep the network administrator waiting any longer. In this section, you develop a program that processes dozens, hundreds, and even thousands of e-mail addresses from a file on your hard drive. First you need a strategy to create the program. Take the statements in Listing 14-1 and run them over and over again. Better yet, have the state- ments run themselves over and over again. Fortunately, you already know how to do something over and over again: You use a loop. (See Chapter 12 for the basics on loops.)
- 254. Chapter 14: Creating Loops within Loops 237 So here’s the strategy: Take the statements in Listing 14-1 and enclose them in a larger loop: while (not at the end of the email.txt file) { Execute the statements in Listing 14-1 } Looking back at the code in Listing 14-1, you see that the statements in that code have a while loop of their own. So this strategy involves putting one loop inside another loop: while (not at the end of the email.txt file) { //Blah-blah while (symbol != ‘@’) { //Blah-blah-blah } //Blah-blah-blah-blah } Because one loop is inside the other, they’re called nested loops. The old loop (the symbol != ‘@’ loop) is the inner loop. The new loop (the end-of-file loop) is called the outer loop. Checking for the end of a file Now all you need is a way to test the loop’s condition. How do you know when you’re at the end of the email.txt file? The answer comes from Java’s Scanner class. This class’s hasNext method answers true or false to the following question: Does the email.txt file have anything to read in it (beyond what you’ve already read)? If the program’s findInLine calls haven’t gobbled up all the characters in the email.txt file, then the value of diskScanner.hasNext() is true. So to keep looping while you’re not at the end of the email.txt file, you do the following: while (diskScanner.hasNext()) { Execute the statements in Listing 14-1 }
- 255. 238 Part III: Controlling the Flow The first realization of this strategy is in Listing 14-2. Listing 14-2: The Mechanical Combining of Two Loops /* * This code does NOT work (but * you learn from your mistakes). */ import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class ListAllUsernames { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“email.txt”)); PrintStream diskWriter = new PrintStream(“usernames.txt”); char symbol; while (diskScanner.hasNext()) { symbol = diskScanner.findInLine(“.”).charAt(0); while (symbol != ‘@’) { diskWriter.print(symbol); symbol = diskScanner.findInLine(“.”).charAt(0); } diskWriter.println(); } } } When you run the code in Listing 14-2, you get the disappointing response shown in Figure 14-3. Figure 14-3: You goofed.
- 256. Chapter 14: Creating Loops within Loops 239 How it feels to be a computer What’s wrong with the code in Listing 14-2? To find out, I role-play the com- puter. “If I were a computer, what would I do when I execute the code in Listing 14-2?” The first several things that I’d do are pictured in Figure 14-4. I would read the J in John, then write the J in John, and then read the letter o (also in John). Figure 14-4: Role-playing the code in Listing 14-2. After a few trips through the inner loop, I’d get the @ sign in John@BurdBrain. com, as shown in Figure 14-5. Finding this @ sign would jump me out of the inner loop and back to the top of the outer loop, as shown in Figure 14-6. I’d get the B in BurdBrain, and sail back into the inner loop. But then (horror of horrors!) I’d write that B to the usernames.txt file. (See Figure 14-7.)
- 257. 240 Part III: Controlling the Flow Figure 14-5: Reaching the end of the username. Figure 14-6: Leaving the inner loop.
- 258. Chapter 14: Creating Loops within Loops 241 Figure 14-7: The error of my ways. There’s the error! You don’t want to write host names to the usernames.txt file. When the computer found the @ sign, it should have skipped past the rest of John’s e-mail address. At this point, you have a choice. You can jump straight to the corrected code in Listing 14-3, or you can read on to find out about the error message in Figure 14-3. Why the computer accidentally pushes past the end of the file Ah! You decided to read on to see why Figure 14-3 has that nasty error message. Once again, I role-play the computer. I’ve completed the steps in Figure 14-7. I shouldn’t process BurdBrain.com with the inner loop. But unfortunately, I do. I keep running and processing more e-mail addresses. When I get to the end of the last e-mail address, I grab the m in BurdBrain.com and go back to test for an @ sign, as shown in Figure 14-8.
- 259. 242 Part III: Controlling the Flow Now I’m in trouble. This last m certainly isn’t an @ sign. So I jump into the inner loop, and try to get yet another character. (See Figure 14-9.) The email.txt file has no more characters, so Java sends an error message to the computer screen. (The NullPointerException error message is back in Figure 14-3.) Figure 14-8: The journey’s last leg. Figure 14-9: Trying to read past the end of the file.
- 260. Chapter 14: Creating Loops within Loops 243 Solving the problem Listing 14-3 has the solution to the problem described with Figures 14-1 and 14-2. The code in this listing is almost identical to the code in Listing 14-2. The only difference is the added call to nextLine. When the computer reaches an @ sign, this nextLine call gobbles up the rest of the input line. (In other words, the nextLine call gobbles up the rest of the e-mail address. The idea works because each e-mail address is on its own separate line.) After chewing and swallowing @BurdBrain.com, the computer moves gracefully to the next line of input. Listing 14-3: That’s Much Better! /* * This code is correct!! */ import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import java.io.PrintStream; class ListAllUsernames { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“email.txt”)); PrintStream diskWriter = new PrintStream(“usernames.txt”); char symbol; while (diskScanner.hasNext()) { symbol = diskScanner.findInLine(“.”).charAt(0); while (symbol != ‘@’) { diskWriter.print(symbol); symbol = diskScanner.findInLine(“.”).charAt(0); } diskScanner.nextLine(); diskWriter.println(); } } } To run the code in Listing 14-3, you need an email.txt file — a file like the one shown in Figure 14-1. In the email.txt file, type several e-mail addresses. Any addresses will do, as long as each address contains an @ sign and each
- 261. 244 Part III: Controlling the Flow address is on its own separate line. Save the email.txt file in your project directory along with the ListAllUsernames.java file (the code from Listing 14-3). For more details, see the discussion accompanying Listings 13-2, 13-3, and 13-4 in Chapter 13. With Listing 14-3, you’ve reached an important milestone. You’ve analyzed a delicate programming problem and found a complete, working solution. The tools you used included thinking about strategies and role-playing the computer. As time goes on, you can use these tools to solve bigger and better problems.
- 262. Chapter 15 The Old Runaround In This Chapter More ways to create repetitive actions Creating loops within loops Insisting on a valid response from the user Looping through enumerated values I remember it distinctly — the sense of dread I would feel on the way to Aunt Edna’s house. She was a kind old woman, and her intentions were good. But visits to her house were always so agonizing. First we’d sit in the living room and talk about other relatives. That was okay, as long as I understood what people were talking about. Sometimes, the gossip would be about adult topics, and I’d become very bored. After all the family chatter, my father would help Aunt Edna with her bills. That was fun to watch, because Aunt Edna had a genetically inherited family ailment. Like me and many of my ancestors, Aunt Edna couldn’t keep track of paperwork to save her life. It was as if the paper had allergens that made Aunt Edna’s skin crawl. After ten minutes of useful bill paying, my father would find a mistake, an improper tally or something else in the ledger that needed attention. He’d ask Aunt Edna about it, and she’d shrug her shoulders. He’d become agitated trying to track down the problem, while Aunt Edna rolled her eyes and smiled with ignorant satisfaction. It was great entertainment. Then, when the bill paying was done, we’d sit down to eat dinner. That’s when I would remember why I dreaded these visits. Dinner was unbearable. Aunt Edna believed in Fletcherism — a health movement whose followers chewed each mouthful of food one hundred times. The more devoted follow- ers used a chart, with a different number for the mastication of each kind of food. The minimal number of chews for any food was 32 — one chomp for each tooth in your mouth. People who did this said they were “Fletcherizing.”
- 263. 246 Part III: Controlling the Flow Mom and Dad thought the whole Fletcher business was silly, but they respected Aunt Edna and felt that people her age should be humored, not defied. As for me, I thought I’d explode from the monotony. Each meal lasted forever. Each mouthful was an ordeal. I can still remember my mantra — the words I’d say to myself without meaning to do so: I’ve chewed 0 times so far. Have I chewed 100 times yet? If not, then Chew! Add 1 to the number of times that I’ve chewed. Go back to “Have I chewed” to find out if I’m done yet. Repeating Statements a Certain Number Times (Java for Statements) Life is filled with examples of counting loops. And computer programming mirrors life (. . . or is it the other way around?). When you tell a computer what to do, you’re often telling the computer to print three lines, process ten accounts, dial a million phone numbers, or whatever. Because counting loops are so common in programming, the people who create programming lan- guages have developed statements just for loops of this kind. In Java, the statement that repeats something a certain number of times is called a for statement. An example of a for statement is in Listing 15-1. Listing 15-1: Horace Fletcher’s Revenge import static java.lang.System.out; class AuntEdnaSettlesForTen { public static void main(String args[]) { for (int count = 0; count < 10; count++) { out.print(“I’ve chewed “); out.print(count); out.println(“ time(s).”); } out.println(“10 times! Hooray!”); out.println(“I can swallow!”); } }
- 264. Chapter 15: The Old Runaround 247 Figure 15-1 shows you what you get when you run the program of Listing 15-1: The for statement in Listing 15-1 starts by setting the count variable equal to 0. Then the for statement tests to make sure that count is less than 10 (which it certainly is). Then the for statement dives ahead and executes the printing state- ments between the curly braces. At this early stage of the game, the computer prints I’ve chewed 0 time(s). Then the for statement executes count++ — that last thing inside the for statement’s parentheses. This last action adds 1 to the value of count. Figure 15-1: Chewing ten times. This ends the first iteration of the for statement in Listing 15-1. Of course, there’s more to this loop than just one iteration: With count now equal to 1, the for statement checks again to make sure that count is less than 10. (Yes, 1 is smaller than 10.) Because the test turns out okay, the for statement marches back into the curly braced statements and prints I’ve chewed 1 time(s) on the screen. Then the for statement executes that last count++ inside its parenthe- ses. The statement adds 1 to the value of count, increasing the value of count to 2. And so on. This whole thing keeps being repeated over and over again until, after ten iterations, the value of count finally reaches 10. When this happens, the check for count being less than 10 fails, and the loop’s execution ends. The computer jumps to whatever statement comes immediately after the for statement. In Listing 15-1, the computer prints 10 times! Hooray! I can swallow! The whole process is illustrated in Figure 15-2.
- 265. 248 Part III: Controlling the Flow Figure 15-2: The action of the for loop in Listing 15-1. The anatomy of a for statement A typical for statement looks like this: for (Initialization; Condition; Update) { Statements } After the word for, you put three things in parentheses: an Initialization, a Condition, and an Update. Each of the three items in parentheses plays its own distinct role: Initialization: The initialization is executed once, when the run of your program first reaches the for statement. Condition: The condition is tested several times (at the start of each iteration). Update: The update is also evaluated several times (at the end of each iteration). If it helps, think of the loop as if its text is shifted all around:
- 266. Chapter 15: The Old Runaround 249 //This is NOT real code int count = 0 for count < 0 { out.print(“I’ve chewed “); out.print(count); out.println(“ time(s).”); count++; } Versatile looping statements If you were stuck on a desert island with only total += card; one kind of loop, what kind would you want to System.out.print(card); have? The answer is, you can get along with System.out.print(“ any kind of loop. The choice between a while “); loop and a for loop is about the code’s style System.out.println(total); } and efficiency. It’s not about necessity. Turning this loop into a for loop means Anything that you can do with a for loop, you wasting most of the stuff inside the for loop’s can do with a while loop as well. Consider, for parentheses: example, the for loop in Listing 15-1. Here’s how you can achieve the same effect with a for ( ; total < 21 ; ) { while loop: card = myRandom.nextInt(10) + 1; int count = 0; total += card; while (count < 10) { System.out.print(card); out.print(“I’ve chewed “); System.out.print(“ out.print(count); “); out.println(“ time(s).”); System.out.println(total); count++; } } The for loop above has a condition, but it has In the while loop, you have explicit statements no initialization and no update. That’s okay. to declare, initialize, and increment the count Without an initialization, nothing special hap- variable. pens when the computer first enters the for The same kind of trick works in reverse. loop. And without an update, nothing special Anything that you can do with a while loop, happens at the end of each iteration. It’s strange, you can do with a for loop as well. But turning but it works. certain while loops into for loops seems Usually, when you write a for statement, strained and unnatural. Consider a while loop you’re counting how many times to repeat from Listing 12-2: something. But, in truth, you can do just about while (total < 21) { any kind of repetition with a for statement. card = myRandom.nextInt(10) + 1;
- 267. 250 Part III: Controlling the Flow You can’t write a real for statement this way. (The compiler would throw code like this right into the garbage can.) Even so, this is the order in which the parts of the for statement are executed. The first line of a for statement (the word for followed by stuff in parenthe- ses) is not a complete statement. So you almost never put a semicolon after the stuff in parentheses. If you make a mistake and type a semicolon, you usually put the computer into an endless, do-nothing loop. The computer’s cursor just sits there and blinks until you forcibly stop the program’s run. Initializing a for loop Look at the first line of the for loop in Listing 15-1, and notice the declaration int count = 0. That’s something new. When you create a for loop, you can declare a variable (like count) as part of the loop initialization. If you declare a variable in the initialization of a for loop, you can’t use that variable outside the loop. For example, in Listing 15-1, try putting out.println(count) after the end of the loop: //This code does not compile. for (int count = 0; count < 10; count++) { out.print(“I’ve chewed “); out.print(count); out.println(“ time(s).”); } out.print(count); //The count variable doesn’t // exist here. With this extra reference to the count variable, the compiler gives you an error message. You can see the message in Figure 15-3. If you’re not experienced with for statements, the message may surprise you. “Whadaya mean ‘cannot find symbol’? There’s a count variable just four lines above that statement.” Ah, yes. But the count variable is declared in the for loop’s initialization. Outside the for loop, that count variable doesn’t exist. Figure 15-3: What count variable? I don’t see a count variable.
- 268. Chapter 15: The Old Runaround 251 To use a variable outside of a for statement, you have to declare that variable outside the for statement. You can even do this with the for statement’s counting variable. Listing 15-2 has an example. Listing 15-2: Using a Variable Declared Outside of a for Loop import static java.lang.System.out; class AuntEdnaDoesItAgain { public static void main(String args[]) { int count; for (count = 0; count < 10; count++) { out.print(“I’ve chewed “); out.print(count); out.println(“ time(s).”); } out.print(count); out.println(“ times! Hooray!”); out.println(“I can swallow!”); } } A run of the code in Listing 15-2 looks exactly like the run for Listing 15-1. The run is pictured in Figure 15-1. Unlike its predecessor, Listing 15-2 enjoys the luxury of using the count variable to display the number 10. It can do this because, in Listing 15-2, the count variable belongs to the entire main method, and not to the for loop alone. Notice the words for (count = 0 in Listing 15-2. Because count is declared above the for statement, you don’t declare count again in the for statement’s initialization. I tried declaring count twice, as in the following code: //This does NOT work: int count; for (int count = 0; count < 10; count++) { ...etc. and the compiler told me to clean up my act: count is already defined in main(java.lang.String[]) for (int count = 0; count < 10; count++) { ^
- 269. Chapter 15: The Old Runaround 253 for (each of 9 floors) for (each of 20 rooms on a floor) get a number from the file and display the number on the screen. Figure 15-5: A readable display of the data in Figure 15-4. So your program has a for loop within a for loop — a pair of nested for loops. Next, you notice how each line begins in Figure 15-5. Each line contains the word Floor, followed by the floor number. Because this Floor display occurs only nine times in Figure 15-5, the statements to print this display belong in the for each of 9 floors loop (and not in the for each of 20 rooms loop). The statements should be before the for each of 20 rooms loop, because this Floor display comes once before each line’s twenty number display: for (each of 9 floors) display “Floor” and the floor number, for (each of 20 rooms on a floor) get a number from the file and display the number on the screen. You’re almost ready to write the code. But there’s one detail that’s easy to forget. (Well, it’s a detail that I always forget.) After displaying 20 numbers, the program advances to a new line. This new-line action happens only nine times during the run of the program, and it always happens after the program displays 20 numbers: for (each of 9 floors) display “Floor” and the floor number, for (each of 20 rooms on a floor) get a number from the file and display the number on the screen, Go to the next line.
- 270. 254 Part III: Controlling the Flow That does it. That’s all you need. The code to create the display of Figure 15-5 is in Listing 15-3. Listing 15-3: Hey! Is This a For-by-For? import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import static java.lang.System.out; class DisplayHotelData { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“hotelData”)); for (int floor = 1; floor <= 9; floor++) { out.print(“Floor “); out.print(floor); out.print(“: “); for (int roomNum = 1; roomNum <= 20; roomNum++) { out.print(diskScanner.nextInt()); out.print(‘ ‘); } out.println(); } } } The code in Listing 15-3 has the variable floor going from 1 to 9, and has the variable roomNum going from 1 to 20. Because the roomNum loop is inside the floor loop, the writing of twenty numbers happens 9 times. That’s good. It’s exactly what I want. Repeating Until You Get What You Need (Java do Statements) I introduce Java’s while loop in Chapter 12. When you create a while loop, you write the loop’s condition first. After the condition, you write the code that gets repeatedly executed.
- 271. Chapter 15: The Old Runaround 255 while (Condition) { Code that gets repeatedly executed } This way of writing a while statement is no accident. The look of the state- ment emphasizes an important point — that the computer always checks the condition before executing any of the repeated code. If the loop’s condition is never true, then the stuff inside the loop is never executed — not even once. In fact, you can easily cook up a while loop whose statements are never executed (although I can’t think of a reason why you would ever want to do it): //This code doesn’t print anything: int twoPlusTwo = 2 + 2; while (twoPlusTwo == 5) { System.out.println(“Are you kidding?”); System.out.println(“2+2 doesn’t equal 5.”); System.out.print (“Everyone knows that”); System.out.println(“ 2+2 equals 3.”); } In spite of this silly twoPlusTwo example, the while statement turns out to be the most useful of Java’s looping constructs. In particular, the while loop is good for situations in which you must look before you leap. For example: “While money is in my account, write a mortgage check every month.” When you first encounter this statement, if your account has a zero balance, you don’t want to write a mortgage check — not even one check. But at times (not many), you want to leap before you look. In a situation when you’re asking the user for a response, maybe the user’s response makes sense, but maybe it doesn’t. Maybe the user’s finger slipped, or perhaps the user didn’t understand the question. In many situations, it’s important to correctly interpret the user’s response. If the user’s response doesn’t make sense, you must ask again. Getting a trustworthy response Consider a program that deletes several files. Before deleting anything, the program asks for confirmation from the user. If the user types Y, then delete; if the user types N, then don’t delete. Of course, deleting files is serious stuff. Mistaking a bad keystroke for a “yes” answer can delete the company’s records. (And mistaking a bad keystroke for a “no” answer can preserve the company’s incriminating evidence.) So if there’s any doubt about the user’s response, the program should ask the user to respond again.
- 272. 256 Part III: Controlling the Flow Pause a moment to think about the flow of actions — what should and shouldn’t happen when the computer executes the loop. A loop of this kind doesn’t need to check anything before getting the user’s first response. Indeed, before the user gives the first response, the loop has nothing to check. The loop shouldn’t start with “as long as the user’s response is invalid, get another response from the user.” Instead, the loop should just leap ahead, get a response from the user, and then check the response to see if it made sense. The code to do all this is in Listing 15-4. Listing 15-4: Repeat Before You Delete /* * DISCLAIMER: Neither the author nor Wiley Publishing, Inc., * nor anyone else even remotely connected with the * creation of this book, assumes any responsibility * for any damage of any kind due to the use of this code, * or the use of any work derived from this code, including * any work created partially or in full by the reader. * * Sign here:_______________________________ */ import java.util.Scanner; import java.io.IOException; class IHopeYouKnowWhatYoureDoing { public static void main(String args[]) throws IOException { Scanner myScanner = new Scanner(System.in); char reply; do { System.out.print(“Reply with Y or N...”); System.out.print(“ Delete all .keep files? “); reply = myScanner.findInLine(“.”).charAt(0); } while (reply != ‘Y’ && reply != ‘N’); if (reply == ‘Y’) { Runtime.getRuntime().exec(“cmd /c del *.keep”); } } } The code in Listing 15-4 works on all the industrial-strength versions of Microsoft Windows, including Windows NT, 2000, and XP. To get the same effect in Windows 95, 98, or Me, you have to change the last line of code as follows: Runtime.getRuntime().exec(“start command /c del *.keep”);
- 273. Chapter 15: The Old Runaround 257 To work the same magic in Unix or Linux, you can use the following command: Runtime.getRuntime().exec (new String[] {“/bin/sh”, “-c”, “rm -f *.keep”}); One way or another, the call to Runtime.getRuntime...yada-yada deletes all files whose names end with .keep. In Listing 15-4, the call to Runtime.getRuntime().exec enables the Java program to execute an operating system command. This Runtime business can be tricky to use, so don’t fret over the details. Just take my word for it — the call to Runtime.getRuntime().exec in Listing 15-4 deletes files. The Runtime.getRuntime().exec method is one of those “you need a throws clause” methods that I introduce in Chapter 13. Unlike the methods in Chapter 13, the exec method forces you to throw an IOException. And with a throws IOException clause comes an import java.io.IOException declaration. At this point you may be wondering how I know that the exec method needs a throws IOException clause. How many other Java API methods require throws clauses, and how do you find out about all these things? The answer is, you can find this information in Java’s API documentation. For details, see the Appendix on this book’s Web site. Deleting files A run of the Listing 15-4 program is shown in Figure 15-6. Before deleting a bunch of files, the program asks the user if it’s okay to do the deletion. If the user gives one of the two expected answers (Y or N) then the program pro- ceeds according to the user’s wishes. But if the user enters any other letter (or any digit, punctuation symbol, or whatever), then the program asks the user for another response. Figure 15-6: No! Don’t do it! In Figure 15-6, the user hems and haws for a while, first with the letter U, then the digit 8, and then with lowercase letters. Finally, the user enters Y, and the program deletes the .keep files. If you compare the files on your hard drive (before and after the run of the program), you’ll see that the program trashes files with names ending in .keep.
- 274. 258 Part III: Controlling the Flow If you use JCreator, here’s how you can tell that files are being deleted: 1. Create a project containing the code in Listing 15-4. If you downloaded the special edition of JCreator from this book’s Web site, you can skip this create-a-project step and use the existing Chapter15_Listing04 project. 2. In the File View pane, right-click the name of the project. A context menu appears. 3. In the context menu, choose Add➪New File. JCreator’s File Wizard opens to the File Path tab. 4. In the Name field, type the name of your new data file. Type irreplaceableInfo.keep, or something like that. 5. Click Finish. The file’s name appears in JCreator’s File View pane. For this experiment, you don’t have to add any text to the file. The file exists only to be deleted. 6. Repeat Steps 2 through 5 a few more times. Create files named somethingOrOther.keep and files that don’t have .keep in their names. 7. Compile and run the program. When the program runs, type Y to delete the .keep files. (The program deletes .keep files only in this program’s project directory. The program’s run has no effect on any files outside of the project directory.) After running the program, you want to check to make sure that the pro- gram deleted the .keep files. 8. In the File View pane, right-click the name of the project. A context menu appears. 9. In the context menu, select Refresh From Local. JCreator takes another look at the project directory, and lists the direc- tory’s files in the File View’s tree. Assuming that the program did its job correctly, files with names ending in .keep no longer appear in the tree. Using Java’s do statement To write the program in Listing 15-4, you need a loop — a loop that repeatedly asks the user if the .keep files should be deleted. The loop continues to ask until the user gives a meaningful response. The loop tests its condition at the end of each iteration, after each of the user’s responses.
- 275. Chapter 15: The Old Runaround 259 That’s why the program in Listing 15-4 has a do loop (also known as a do . . . while loop). With a do loop, the program jumps right in, executes some state- ments, and then checks a condition. If the condition is true, then the program goes back to the top of the loop for another go-around. If the condition is false, then the computer leaves the loop (and jumps to whatever code comes imme- diately after the loop). The action of the loop in Listing 15-4 is illustrated in Figure 15-7. Figure 15-7: Here we go loop, do loop. A closer look at the do statement The format of a do loop is do { Statements } while (Condition) Writing the Condition at the end of the loop reminds me that the computer executes the Statement inside the loop first. After the computer executes the Statement, the computer goes on to check the Condition. If the Condition is true, the computer goes back for another iteration of the Statement.
- 276. 260 Part III: Controlling the Flow With a do loop, the computer always executes the statements inside the loop at least once: //This code prints something: int twoPlusTwo = 2 + 2; do { System.out.println(“Are you kidding?”); System.out.println(“2+2 doesn’t equal 5.”); System.out.print (“Everyone knows that”); System.out.println(“ 2+2 equals 3.”); } while (twoPlusTwo == 5); This code displays Are you kidding? 2+2 doesn’t equal 5 . . . and so on, and then tests the condition twoPlusTwo == 5. Because twoPlusTwo == 5 is false, the computer doesn’t go back for another iteration. Instead, the computer jumps to whatever code comes immediately after the loop. Repeating with Predetermined Values (Java’s Enhanced for Statement) Most people say that they “never win anything.” Other people win raffles, drawings, and contests, but they don’t win things. Well, I have news for these people — other people don’t win things either. Nobody wins things. That’s how the laws of probability work. Your chance of winning one of the popular U.S. lottery jackpots is roughly 1 in 135,000,000. If you sell your quarter-million dollar house and use all the money to buy lottery tickets, your chance of win- ning is still only 1 in 540. If you play every day of the month (selling a house each day), your chance of winning the jackpot is still less than 1 in 15. Of course, nothing in the previous paragraph applies to me. I don’t buy lot- tery tickets, but I often win things. My winning streak started a few years ago. I won some expensive Java software at the end of an online seminar. Later that month, I won a microchip-enabled pinky ring (a memento from a 1998 Java conference). The following year I won a wireless PDA. Just last week I won a fancy business-class printer. I never spend money to enter any contests. All these winnings are freebies. When the national computer science educators’ conference met in Reno, Nevada, my colleagues convinced me to try the slot machines. I lost $23, and then I won back $18. At that point, I stopped playing. I wanted to quit while I was only $5 behind.
- 277. Chapter 15: The Old Runaround 261 That’s why my writing a Java program about slot machines is such a strange occurrence. A typical slot machine has three reels, with each reel having about twenty symbols. But to illustrate this section’s ideas, I don’t need twenty symbols. Instead I use four symbols — a cherry, a lemon, a kumquat, and a rutabaga. Creating an enhanced for loop When you play my simplified slot machine you can spin any one of over 60 combinations — cherry+cherry+kumquat, rutabaga+rutabaga+rutabaga, or whatever. This chapter’s goal is to list all possible combinations. But first, I show you another kind of loop. Listing 15-5 defines an enum type for a slot machine’s symbols, and Listing 15-6 displays a list of the symbols. Listing 15-5: Slot Machine Symbols enum Symbol {cherry, lemon, kumquat, rutabaga} Listing 15-6: Listing the Symbols import static java.lang.System.out; class ListSymbols { public static void main(String args[]) { for (Symbol leftReel : Symbol.values()) { out.println(leftReel); } } } Listing 15-6 uses Java’s enhanced for loop. The word “enhanced” means “enhanced compared with the loops in earlier versions of Java.” The enhanced for loop is new to Java version 5.0. If you run Java version 1.4.2 (or something like that) then you can’t use an enhanced for loop. Here’s the format of the enhanced for loop: for (TypeName variableName : RangeOfValues) { Statements }
- 278. 262 Part III: Controlling the Flow Here’s how the loop in Listing 15-6 follows the format: In Listing 15-6, the word Symbol is the name of a type. The int type describes values like -1, 0, 1, and 2. The boolean type describes the values true and false. And (because of the code in Listing 15-5) the Symbol type describes the values cherry, lemon, kumquat, and rutabaga. For more information on enum types like Symbol, see Chapter 10. In Listing 15-6, the word leftReel is the name of a variable. The loop in Listing 15-1 defines count to be an int variable. Similarly, the loop in Listing 15-6 defines leftReel to be a Symbol variable. So in theory, the variable leftReel can take on any of the four Symbol values. By the way, I call this variable leftReel because the code lists all the symbols that can appear on the leftmost of the slot machine’s three reels. Because all three of the slot machine’s reels have the same symbols, I may also have named this variable middleReel or rightReel. But on second thought, I’ll save the names middleReel and rightReel for a later example. In Listing 15-6, the expression Symbol.values() stands for the four values in Listing 15-5. To quote myself in the previous bullet, “in theory, the variable leftReel can take on any of the four Symbol values.” Well, the RangeOfValues part of the for statement turns theory into practice. This third item inside the parentheses says “Have as many loop iterations are there are Symbol values, and have the leftReel variable take on a different Symbol value during each of the loop’s iterations.” So the loop in Listing 15-6 undergoes four iterations — an iteration in which leftReel has value cherry, another iteration in which leftReel has value lemon, a third iteration in which leftReel has value kumquat, and a fourth iteration in which leftReel has value rutabaga. During each iteration, the program prints the leftReel variable’s value. The result is in Figure 15-8. Figure 15-8: The out- put of the code in Listing 15-6.
- 279. Chapter 15: The Old Runaround 263 In general, a someEnumTypeName.values() expression stands for the set of values that a particular enum type’s variable can have. For example, in Listings 10-7 and 10-8 you may use the expression WhoWins.values() to refer to the home, visitor, and neither values. The difference between a type’s name (like Symbol) and the type’s values (as in Symbol.values()) is really subtle. Fortunately, you don’t have to worry about the difference. As a beginning programmer, you can just use the .values() suffix in an enhanced loop’s RangeOfValues part. Nesting the enhanced for loops Listing 15-6 solves a simple problem in a very elegant way. So after reading about Listing 15-6, you ask about more complicated problems. “Can I list all possible three-reel combinations of the slot machine’s four symbols?” Yes, you can. Listing 15-7 shows you how to do it. Listing 15-7: Listing the Combinations import static java.lang.System.out; class ListCombinations { public static void main(String args[]) { for (Symbol leftReel : Symbol.values()) { for (Symbol middleReel : Symbol.values()) { for (Symbol rightReel : Symbol.values()) { out.print(leftReel); out.print(“ “); out.print(middleReel); out.print(“ “); out.println(rightReel); } } } } } When you run the program in Listing 15-7, you get 64 lines of output. Some of those lines are shown in Figure 15-9.
- 280. 264 Part III: Controlling the Flow Figure 15-9: The first several lines of output from the code in Listing 15-7. Like the code in Listing 15-3, the program in Listing 15-7 contains a loop within a loop. In fact Listing 15-7 has a loop within a loop within a loop. Here’s the strategy in Listing 15-7: for (each of the 4 symbols that can appear on the left reel), for (each of the 4 symbols that can appear on the middle reel), for (each of the 4 symbols that can appear on the right reel), display the three reels’ symbols. So you start the outer loop with the cherry symbol. Then you march on to the middle loop and begin that loop with the cherry symbol. Then you pro- ceed to the inner loop and pick the cherry (pun intended). At last, with each loop tuned to the cherry setting, you display cherry cherry cherry com- bination. (See Figure 15-10.) After displaying cherry cherry cherry, you continue with other values of the innermost loop. That is, you change the right reel’s value from cherry to lemon. (See Figure 15-11.) Now the three reels’ values are cherry cherry lemon, so you display these values on the screen. (See the second line in Figure 15-9.) After exhausting the four values of the innermost (right reel) loop, you jump out of that innermost loop. But the jump puts you back to the top of the middle loop, where you change the value of middleReel from cherry to lemon. Now the values of leftReel and middleReel are cherry and lemon respectively. (See Figure 15-12.)
- 281. Chapter 15: The Old Runaround 265 Figure 15-10: Entering loops for the first time in the program of Listing 15-7. Figure 15-11: Changing from cherry to lemon in the inner- most loop.
- 282. 266 Part III: Controlling the Flow Figure 15-12: Changing from cherry to lemon in the middle loop. Having changed to lemon on the middle loop, you go barreling again into the innermost loop. As if you’d never seen this inner loop before, you set the loop’s variable to cherry. (See Figure 15-13.) Figure 15-13: Restarting the inner loop.
- 283. Chapter 15: The Old Runaround 267 After displaying the tasty cherry lemon cherry combination, you start changing the values of the innermost loop. (See Figure 15-14.) The loop keeps going until it displays all 64 combinations. Whew! Figure 15-14: Traveling a second time through the innermost loop.
- 284. 268 Part III: Controlling the Flow
- 285. Part IV Using Program Units
- 286. In this part . . . W ay back in the Elvis Era, people thought that com- puter programs should be big lists of instructions. Then, during the Groovy Sixties, people decided to modular- ize their programs. A typical program consisted of several methods (like the main methods in this book’s examples). Finally, during the Weighty Eighties, programmers grouped methods and other things into units called objects. Far from being the flavor of the month, object-oriented programming has become the backbone of modern computing. This part of the book tells you all about it.
- 287. Chapter 16 Using Loops and Arrays In This Chapter Using for loops to the max Storing many values in a single variable Working with groups of values T his chapter has seven illustrations. For these illustrations, the people at Wiley Publishing insist on following numbering: Figure 16-1, Figure 16-2, Figure 16-3, Figure 16-4, Figure 16-5, Figure 16-6, and Figure 16-7. But I like a different kind of numbering. I’d like to number the illustrations figure[0], figure[1], figure[2], figure[3], figure[4], figure[5], and figure[6]. Read on in this chapter and you’ll find out why. Some for Loops in Action The Java Motel, with its ten comfortable rooms, sits in a quiet place off the main highway. Aside from a small, separate office, the motel is just one long row of ground floor rooms. Each room is easily accessible from the spacious front parking lot. Oddly enough, the motel’s rooms are numbered 0 through 9. I could say that the numbering is a fluke — something to do with the builder’s original design plan. But the truth is, starting with 0 makes the examples in this chapter easier to write. You, as the Java Motel’s manager, store occupancy data in a file on your computer’s hard drive. The file has one entry for each room in the motel. For example, in Figure 16-1, Room 0 has one guest, Room 1 has four guests, Room 2 is empty, and so on.
- 288. 272 Part IV: Using Program Units Figure 16-1: Occupancy data for the Java Motel. You want a report showing the number of guests in each room. Because you know how many rooms you have, this problem begs for a for loop. The code to solve this problem is in Listing 16-1, and a run of the code is shown in Figure 16-2. Listing 16-1: A Program to Generate an Occupancy Report import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import static java.lang.System.out; class ShowOccupancy { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“occupancy”)); out.println(“RoomtGuests”); for (int roomNum = 0; roomNum < 10; roomNum++) { out.print(roomNum); out.print(“t”); out.println(diskScanner.nextInt()); } } } Figure 16-2: Running the code in Listing 16-1.
- 289. Chapter 16: Using Loops and Arrays 273 Listing 16-1 uses a for loop — a loop of the kind described in Chapter 15. As the roomNum variable’s value marches from 0 to 9, the program displays one number after another from the occupancy file. To read more about getting numbers from a disk file like my occupancy file, see Chapter 13. This example’s input file is named occupancy — not occupancy.txt. If you use JCreator’s File Wizard to make an occupancy file, you must put a dot in the wizard’s Name field. That is, you must type occupancy. (occupancy-”dot”) in the Name field. If you don’t type your own dot anywhere in the Name field, then JCreator adds a default extension to the file’s name (turning occupancy into occupancy.java). Grabbing input here and there Listing 16-2 illustrates some pithy issues sur- another. (Before running the code in Listing 16-2, rounding the input of data. For one thing, the I go to the old Listing 16-1 project in JCreator’s program gets input from both the keyboard and a File View. I right-click the occupancy file in the disk file. (The program gets a room number from Listing 16-1 project, and select Copy from the con- the keyboard. Then the program gets the number text menu. Then I right-click the new Listing 16-2 of guests in that room from the occupancy branch, and select Paste from the context menu.) file.) To make this happen, Listing 16-2 sports In real life, having several copies of a data file can two Scanner declarations — one to declare be dangerous. You can modify one copy, and then myScanner, and a second to declare accidentally read out-of-date data from a differ- diskScanner. ent copy. Sure, you should have backup copies, Later in the program, the call myScanner. but you should have only one “master” copy — nextInt reads from the keyboard, and the copy from which all programs get the same diskScanner.nextInt reads from the file. input. Within the program, you can read from the key- So in a real-life program, you don’t copy the occu- board or the disk as many times as you want. pancy file from one project to another. What do You can even intermingle the calls — reading you do instead? You put an occupancy file in once from the keyboard, then three times from one place on your hard drive, and then have the disk, then twice from the keyboard, and so each program refer to the file using the names on. All you have to do is remember to use of the file’s directories. For example, if your myScanner whenever you read from the key- occupancy file is in the c:datahotel board, and use diskScanner whenever you directory, you write read from the disk. Scanner diskScanner = Another interesting tidbit in Listing 16-2 concerns new Scanner(new the occupancy file. Many of this chapter’s File(“c:datahoteloccu examples read from an occupancy file, and I pancy”)); use the same data in each of the examples. (I use the data shown in Figure 16-1.) To run an exam- A sidebar in Chapter 13 has more details about ple, I copy the occupancy file from one project to filenames and double backslashes.
- 290. 274 Part IV: Using Program Units Deciding on a loop’s limit at runtime On occasion, you may want a more succinct report than the one in Figure 16-2. “Don’t give me a long list of rooms,” you say. “Just give me the number of guests in Room 3.” To get such a report, you need a slightly smarter program. The program is in Listing 16-2, with runs of the program shown in Figure 16-3. Listing 16-2: Report on One Room Only, Please import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import static java.lang.System.out; public class ShowOneRoomOccupancy { public static void main(String args[]) throws FileNotFoundException { Scanner myScanner = new Scanner(System.in); Scanner diskScanner = new Scanner(new File(“occupancy”)); int whichRoom; out.print(“Which room? “); whichRoom = myScanner.nextInt(); for(int roomNum = 0; roomNum < whichRoom; roomNum++){ diskScanner.nextInt(); } out.print(“Room “); out.print(whichRoom); out.print(“ has “); out.print(diskScanner.nextInt()); out.println(“ guest(s).”); } } If Listing 16-2 has a moral, it’s that the number of for loop iterations can vary from one run to another. The loop in Listing 16-2 runs on and on as long as the counting variable roomNum is less than a room number specified by the user. When the roomNum is the same as the number specified by the user (that is, when roomNum is the same as whichRoom), the computer jumps out of the loop. Then the computer grabs one more int value from the occupancy file and displays that value on the screen.
- 291. Chapter 16: Using Loops and Arrays 275 Figure 16-3: A few one-room reports. As you stare at the runs in Figure 16-3, it’s important to remember the unusual numbering of rooms. Room 3 has two guests because Room 3 is the fourth room in the occupancy file of Figure 16-1. That’s because the motel’s rooms are numbered 0 through 9. Using all kinds of conditions in a for loop Look at the run in Figure 16-3, and notice the program’s awful behavior when the user mistakenly asks about a nonexistent room. The motel has no Room 10. If you ask for the number of guests in Room 10, the program tries to read more numbers than the occupancy file contains. This unfortunate attempt causes a NoSuchElementException. Listing 16-3 fixes the end-of-file problem. Listing 16-3: A More Refined Version of the One-Room Code import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; import static java.lang.System.out; public class BetterShowOneRoom { public static void main(String args[]) throws FileNotFoundException { Scanner myScanner = new Scanner(System.in); Scanner diskScanner = new Scanner(new File(“occupancy”)); int whichRoom; (continued)
- 292. 276 Part IV: Using Program Units Listing 16-3 (continued) out.print(“Which room? “); whichRoom = myScanner.nextInt(); for (int roomNum = 0; roomNum < whichRoom && diskScanner.hasNext(); roomNum++) { diskScanner.nextInt(); } if (diskScanner.hasNext()) { out.print(“Room “); out.print(whichRoom); out.print(“ has “); out.print(diskScanner.nextInt()); out.println(“ guest(s).”); } } } The code in Listing 16-3 isn’t earth shattering. To get this code, you take the code in Listing 16-2 and add a few tests for the end of the occupancy file. You perform the diskScanner.hasNext test before each call to nextInt. That way, if the call to nextInt is doomed to failure, you catch the potential failure before it happens. A few test runs of the code in Listing 16-3 are shown in Figure 16-4. Figure 16-4: The bad room number 10 gets no response. In Listing 16-3, I want to know if the occupancy file contains any more data (any data that I haven’t read yet). So I call the Scanner class’s hasNext method. The hasNext method looks ahead to see if I can read any kind of data — an int value, a double value, a word, a boolean, or whatever. That’s okay for this section’s example, but in some situations, you need to be pickier about your input data. For example, you may want to know if you can call nextInt (as opposed to nextDouble or nextLine). Fortunately, Java has methods for your pickiest input needs. The code if (diskScanner. hasNextInt()) tests to see if you can read an int value from the disk file. Java also has methods like hasNextLine, hasNextDouble, and so on. For more information on the plain old hasNext method, see Chapter 14.
- 293. Chapter 16: Using Loops and Arrays 277 Listing 16-3 has a big fat condition to keep the for loop going: for (int roomNum = 0; roomNum < whichRoom && diskScanner.hasNext(); roomNum++) { Many for loop conditions are simple “less than” tests, but there’s no rule saying that all for loop conditions have to be so simple. In fact, any expres- sion can be a for loop’s condition, as long as the expression has value true or false. The condition in Listing 16-3 combines a “less than” with a call to the Scanner class’s hasNext method. Reader, Meet Arrays; Arrays, Meet the Reader A weary traveler steps up to the Java Motel’s front desk. “I’d like a room,” says the traveler. So the desk clerk runs a report like the one in Figure 16-2. Noticing the first vacant room in the list, the clerk suggests Room 2. “I’ll take it,” says the traveler. It’s so hard to get good help these days. How many times have you told the clerk to fill the higher numbered rooms first? The lower numbered rooms are older, and they are badly in need of repair. For example, Room 3 has an indoor pool. (The pipes leak, so the carpet is soaking wet.) Room 2 has no heat (not in wintertime, anyway). Room 1 has serious electrical problems (so, for that room, you always get payment in advance). Besides, Room 8 is vacant, and you charge more for the higher numbered rooms. Here’s where a subtle change in presentation can make a big difference. You need a program that lists vacant rooms in reverse order. That way, Room 8 catches the clerk’s eye before Room 2 does. Think about strategies for a program that displays data in reverse. With the input from Figure 16-1, the program’s output should look like the display shown in Figure 16-5. Figure 16-5: A list of vacant rooms, with higher num- bered rooms shown first.
- 294. 278 Part IV: Using Program Units Here’s the first (bad) idea for a programming strategy: Get the last value in the occupancy file. If the value is 0, print the room number. Get the next-to-last value in the occupancy file. If the value is 0, print the room number. ...And so on. With some fancy input/output programs, this may be a workable strategy. But no matter what input/output program you use, jumping directly to the end or to the middle of a file is a big pain in the boot. It’s especially bad if you plan to jump repeatedly. So go back to the drawing board and think of something better. Here’s an idea! Read all the values in the occupancy file and store each value in a variable of its own. Then you step through the variables in reverse order, displaying a room number when it’s appropriate to do so. This idea works, but the code is so ugly that I refuse to dignify it by calling it a “Listing.” No, this is just a “see the following code” kind of thing. So please, see the following ugly code: /* * Ugh! I can’t stand this ugly code! */ guestsIn0 = diskScanner.nextInt(); guestsIn1 = diskScanner.nextInt(); guestsIn2 = diskScanner.nextInt(); guestsIn3 = diskScanner.nextInt(); guestsIn4 = diskScanner.nextInt(); guestsIn5 = diskScanner.nextInt(); guestsIn6 = diskScanner.nextInt(); guestsIn7 = diskScanner.nextInt(); guestsIn8 = diskScanner.nextInt(); guestsIn9 = diskScanner.nextInt(); if (guestsIn9 == 0) { System.out.println(9); } if (guestsIn8 == 0) { System.out.println(8); } if (guestsIn7 == 0) { System.out.println(7); } if (guestsIn6 == 0) { // ... And so on.
- 295. Chapter 16: Using Loops and Arrays 279 What you’re lacking is a uniform way of naming ten variables. That is, it would be nice to write /* * Nice idea, but this is not real Java code: */ for (int roomNum = 0; roomNum < 10; roomNum++) { guestsInroomNum = diskScanner.nextInt(); //Read forwards } for (int roomNum = 9; roomNum >= 0; roomNum--) { if (guestsInroomNum == 0) { System.out.println(roomNum); //Write backwards } } Well, there is a way to write loops of this kind. All you need is some square brackets. When you add square brackets to the idea shown in the preceding code, you get what’s called an array. An array is a row of values, like the row of rooms in a one-floor motel. To picture the array, just picture the Java Motel: First, picture the rooms, lined up next to one another. Next, picture the same rooms with their front walls missing. Inside each room you can see a certain number of guests. If you can, forget that the two guests in Room 9 are putting piles of bills into a big briefcase. Ignore the fact that the guest in Room 5 hasn’t moved away from the TV set in a day and a half. Instead of all these details, just see numbers. In each room, see a number representing the count of guests in that room. (If freeform visualization isn’t your strong point, then take a look at Figure 16-6.) In the lingo of Java programming, the entire row of rooms is called an array. Each room in the array is called a component of the array (also known as an array element). Each component has two numbers associated with it: Index: In the case of the Java Motel array, the index is the room number (a number from 0 to 9). Value: In the Java Motel array, the value is the number of guests in a given room (a number stored in a component of the array). Using an array saves you from having to declare ten separate variables: guestsIn0, guestsIn1, guestsIn2, and so on. To declare an array with ten values in it, you can write two fairly short lines of code: int guestsIn[]; guestsIn = new int[10];
- 296. 280 Part IV: Using Program Units A component whose index is 6, and whose value is 4 The value 4 1 4 0 2 2 1 4 3 0 2 Figure 16-6: An abstract 0 1 2 3 4 5 6 7 8 9 snapshot of rooms in the Java Motel. The index 6 You can even squish these two lines into one longer line: int guestsIn[] = new int[10]; In either of these code snippets, notice the use of the number 10. This number tells the computer to make the guestsIn array have ten components. Each component of the array has a name of its own. The starting component is named guestsIn[0], the next is named guestsIn[1], and so on. The last of the ten components is named guestsIn[9]. In creating an array, you always specify the number of components. The array’s indices always start with 0 and end with the number that’s one less than the total number of components. For example, if your array has ten components (and you declare the array with new int[10]), then the array’s indices go from 0 to 9. Storing values in an array After you’ve created an array, you can put values into the array’s components. For example, the guests in Room 6 are fed up with all those mint candies that you put on peoples’ beds. So they check out and Room 6 becomes vacant. You should put the value 0 into the 6 component. You can do it with this assignment statement:
- 297. Chapter 16: Using Loops and Arrays 281 guestsIn[6] = 0; On one weekday, business is awful. No one’s staying at the motel. But then you get a lucky break. A big bus pulls up to the motel. The side of the bus has a sign that says “Loners’ Convention.” Out of the bus come 25 people, each walking to the motel’s small office, none paying attention to the others who were on the bus. Each person wants a private room. Only 10 of them can stay at the Java Motel, but that’s okay, because you can send the other 15 loners down the road to the old C-Side Resort and Motor Lodge. Anyway, to register 10 of the loners into the Java Motel, you put one guest in each of your 10 rooms. Having created an array, you can take advantage of the array’s indexing and write a for loop, like this: for (int roomNum = 0; roomNum < 10; roomNum++) { guestsIn[roomNum] = 1; } This loop takes the place of ten assignment statements, because the com- puter executes the statement guestsIn[roomNum] = 1 ten times. The first time around, the value of roomNum is 0, so in effect, the computer executes guestsIn[0] = 1; In the next loop iteration, the value of roomNum is 1, so the computer exe- cutes the equivalent of the following statement: guestsIn[1] = 1; During the next iteration, the computer behaves as if it’s executing guestsIn[2] = 1; And so on. When roomNum gets to be 9, the computer executes the equivalent of the following statement: guestsIn[9] = 1; Notice how the loop’s counter goes from 0 to 9. Compare this with Figure 16-6, and remember that the indices of an array go from 0 to one less than the number of components in the array. Looping with room numbers from 0 to 9 covers all the rooms in the Java Motel.
- 298. 282 Part IV: Using Program Units When you work with an array, and you step through the array’s components using a for loop, you normally start the loop’s counter variable at 0. To form the condition that tests for another iteration, you often write an expression like roomNum < arraySize, where arraySize is the number of components in the array. Creating a report The code to create the report in Figure 16-5 is shown in Listing 16-4. This new program uses the idea in the world’s ugliest code (the code from several pages back with variables guestsIn0, guestsIn1, and so on). But instead of having ten separate variables, Listing 16-4 uses an array. Listing 16-4: Traveling through Data Both Forwards and Backwards import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; class VacanciesInReverse { public static void main(String args[]) throws FileNotFoundException { Scanner diskScanner = new Scanner(new File(“occupancy”)); int guestsIn[]; guestsIn = new int[10]; for (int roomNum = 0; roomNum < 10; roomNum++) { guestsIn[roomNum] = diskScanner.nextInt(); } for (int roomNum = 9; roomNum >= 0; roomNum--) { if (guestsIn[roomNum] == 0) { System.out.print(“Room “); System.out.print(roomNum); System.out.println(“ is vacant.”); } } } }
- 299. Chapter 16: Using Loops and Arrays 283 Notice the stuff in parentheses in the VacanciesInReverse program’s second for loop. It’s easy to get these things wrong. You’re aiming for a loop that checks Room 9, then Room 8, and so on. if (guestsIn[9] == 0) { System.out.print(roomNum); } if (guestsIn[8] == 0) { System.out.print(roomNum); } if (guestsIn[7] == 0) { System.out.print(roomNum); } ...And so on, until you get to... if (guestsIn[0] == 0) { System.out.print(roomNum); } Some observations about the code: The loop’s counter must start at 9: for (int roomNum = 9; roomNum >= 0; roomNum--) Each time through the loop, the counter goes down by one: for (int roomNum = 9; roomNum >= 0; roomNum--) The loop keeps going as long as the counter is greater than or equal to 0: for (int roomNum = 9; roomNum >= 0; roomNum--) Think through each of these three items, and you’ll write a perfect for loop. Working with Arrays Earlier in this chapter, a busload of loners showed up at your motel. When they finally left, you were glad to get rid them, even if it meant having all your rooms empty for a while. But now, another bus pulls into the parking lot. This bus has a sign that says “Gregarian Club.” Out of the bus come 50 people, each more gregarious than the next. Now everybody in your parking lot is clamoring to meet everyone else. While they meet and greet, they’re all frolicking toward the front desk, singing the club’s theme song. (Oh no! It’s the Gregarian Chant!)
- 300. 284 Part IV: Using Program Units The first five Gregarians all want Room 7. It’s a tight squeeze, but you were never big on fire codes anyway. Next comes a group of three with a yen for Room 0. (They’re computer programmers, and they think the room number is cute.) Then there’s a pack of four Gregarians who want Room 3. (The in-room pool sounds attractive to them.) Looping in style Listing 15-7 uses an enhanced for loop to step Enhanced for loops are nice and concise. But through a bunch of values. In that program, the don’t be too anxious to use enhanced loops with values belong to an enum type. Well, this chap- arrays. This feature has some nasty limitations. ter also deals with a bunch of values; namely, For example, my new howMany loop doesn’t the values in an array. So you’re probably not display room numbers. I avoid room numbers surprised if I show you an enhanced for loop because the room numbers in my guestsIn that steps through an array’s values. array are the indices 0 through 9. Unfortunately, an enhanced loop doesn’t provide easy access To see such a loop, start with the code in Listing to an array’s indices. 16-5. The last loop in that program looks some- thing like this: And here’s another unpleasant surprise. Start for (int roomNum = 0; roomNum < with the following loop from Listing 16-4: 10; roomNum++) { for (int roomNum = 0; roomNum < 10; roomNum++) { out.println(guestsIn[roomNu guestsIn[roomNum] = m]); diskScanner.nextInt(); } } To turn this into an enhanced for loop, you Turn this traditional for loop into an enhanced make up a new variable name. (What about the for loop, and you get the following misleading name howMany? I like that name.) Whatever code: name you choose, the new variable ranges over for (int howMany : guestsIn) { the values in the guestsIn array. howMany = for (int howMany : guestsIn) { diskScanner.nextInt(); out.println(howMany); //Don’t do this } } This enhanced loop uses the same format as The new enhanced loop doesn’t do what you the loop in Chapter 15. want it to do. This loop reads values from an input file and then dumps these values into the for (TypeName variableName : RangeOfValues) { garbage can. In the end, the array’s values Statements remain unchanged. } It’s sad but true. To make full use of an array, you In Chapter 15, the RangeOfValues belongs to have to fall back on Java’s plain old for loop. an enum type. But in this sidebar’s example, the RangeOfValues belongs to an array.
- 301. Chapter 16: Using Loops and Arrays 285 With all this traffic, you better switch on your computer. You start a program that enables you to enter new occupancy data. The program has five parts: Create an array, and then put 0 in each of the array’s components. When the Loners’ Club members left, the motel was suddenly empty. (Heck, even before the Loners’ Club members left, the motel seemed empty.) To declare an array and fill the array with zeros, you execute code of the following kind: int guestsIn[]; guestsIn = new int[10]; for (int roomNum = 0; roomNum < 10; roomNum++) { guestsIn[roomNum] = 0; } Get a room number, and then get the number of guests who will be staying in that room. Reading numbers typed by the user is pretty humdrum stuff. Do a little prompting and a little nextInt calling, and you’re all set: out.print(“Room number: “); whichRoom = myScanner.nextInt(); out.print(“How many guests? “); numGuests = myScanner.nextInt(); Use the room number and the number of guests to change a value in the array. Earlier in this chapter, to put one guest in Room 2, you executed guestsIn[2] = 1; So now, you have two variables — numGuests and whichRoom. Maybe numGuests is 5, and whichRoom is 7. To put numGuests in whichRoom (that is, to put 5 guests in Room 7), you can execute guestsIn[whichRoom] = numGuests; That’s the crucial step in the design of your new program. Ask the user if the program should keep going. Are there more guests to put in rooms? To find out, execute this code: out.print(“Do another? “); } while (myScanner.findInLine(“.”).charAt(0) == ‘Y’); Display the number of guests in each room. No problem! You already did this. You can steal the code (almost verba- tim) from Listing 16-1:
- 302. 286 Part IV: Using Program Units out.println(“RoomtGuests”); for (int roomNum = 0; roomNum < 10; roomNum++) { out.print(roomNum); out.print(“t”); out.println(guestsIn[roomNum]); } The only difference between this latest code snippet and the stuff in Listing 16-1 is that this new code uses the guestsIn array. The first time through this loop, the code does out.println(guestsIn[0]); displaying the number of guests in Room 0. The next time through the loop, the code does out.println(guestsIn[1]); displaying the number of guests in Room 1. The last time through the loop, the code does out.println(guestsIn[9]); That’s perfect. The complete program (with these five pieces put together) is in Listing 16-5. A run of the program is shown in Figure 16-7. Listing 16-5: Storing Occupancy Data in an Array import java.util.Scanner; import static java.lang.System.out; class AddGuests { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); int whichRoom, numGuests; int guestsIn[]; guestsIn = new int[10]; for (int roomNum = 0; roomNum < 10; roomNum++) { guestsIn[roomNum] = 0; } do { out.print(“Room number: “); whichRoom = myScanner.nextInt(); out.print(“How many guests? “); numGuests = myScanner.nextInt(); guestsIn[whichRoom] = numGuests;
- 303. Chapter 16: Using Loops and Arrays 287 out.println(); out.print(“Do another? “); } while (myScanner.findInLine(“.”).charAt(0) == ‘Y’); out.println(); out.println(“RoomtGuests”); for (int roomNum = 0; roomNum < 10; roomNum++) { out.print(roomNum); out.print(“t”); out.println(guestsIn[roomNum]); } } } Figure 16-7: Running the code in Listing 16-5. Hey! The program in Listing 16-5 is pretty big! It may be the biggest program so far in this book. But big doesn’t necessarily mean difficult. If each piece of the program makes sense, you can create each piece on its own, and then put all the pieces together. Voilà! The code is manageable.
- 304. 288 Part IV: Using Program Units
- 305. Chapter 17 Programming with Objects and Classes In This Chapter Creating classes Making objects from classes Joining the exclusive “I understand classes and objects” society C hapters 6, 7, and 8 introduce Java’s primitive types — things like int, double, char, and boolean. That’s great, but how often does a real-world problem deal exclusively with such simple values? Consider an exchange between a merchant and a customer. The customer makes a purchase, which can involve item names, model numbers, credit card info, sales tax rates, and lots of other stuff. In older computer programming languages, you treat an entire purchase like a big pile of unbundled laundry. Imagine a mound of socks, shirts, and other pieces of clothing. You have no basket, so you grab as much as you can handle. As you walk to the washer, you drop a few things — a sock here and a wash- cloth there. This is like the older way of storing the values in a purchase. In older languages, there’s no purchase. There are only double values, char values, and other loose items. You put the purchase amount in one variable, the customer’s name in another, and the sales tax data somewhere else. But that’s awful. You tend to drop things on your way to the compiler. With small errors in a program, you can easily drop an amount here and a customer’s name there. So with laundry and computer programming, you’re better off if you have a basket. The newer programming languages, like Java, allow you to combine values and make new, more useful kinds of values. For example, in Java you can combine double values, boolean values, and other kinds of values to create something that you call a Purchase. Because your purchase info is all in one big bundle, it’s easier to keep track of the purchase’s pieces. That’s the start of an important computer programming concept — the notion of object- oriented programming.
- 306. 290 Part IV: Using Program Units Creating a Class I start with a “traditional” example. The program in Listing 17-1 processes simple purchase data. Two runs of the program are shown in Figure 17-1. Listing 17-1: Doing It the Old-Fashioned Way import java.util.Scanner; class ProcessData { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); double amount; boolean taxable; double total; System.out.print(“Amount: “); amount = myScanner.nextDouble(); System.out.print(“Taxable? (true/false) “); taxable = myScanner.nextBoolean(); if (taxable) { total = amount * 1.05; } else { total = amount; } System.out.print(“Total: “); System.out.println(total); } } If the output in Figure 17-1 looks funny, it’s because I do nothing in the code to control the number of digits beyond the decimal point. So in the output, the value $20.00 looks like 20.0. That’s okay. I show you how to fix the prob- lem in Chapter 18. Figure 17-1: Processing a customer’s purchase.
- 307. Chapter 17: Programming with Objects and Classes 291 Reference types and Java classes The code in Listing 17-1 involves a few simple values — amount, taxable, and total. So here’s the main point of this chapter: By combining several simple values, you can get a single, more useful value. That’s the way it works. You take some of Java’s primitive types, whip them together to make a primitive type stew, and what do you get? You get a more useful type called a reference type. Listing 17-2 has an example. Listing 17-2: What It Means to Be a Purchase class Purchase { double amount; boolean taxable; double total; } The code in Listing 17-2 has no main method, so you can compile the code, but you can’t run it. When you choose Build➪Compile Project in JCreator’s main menu, you get a nice Process completed message. But then choose Build➪Execute Project, and the computer balks. (You get the message box shown in Figure 17-2.) Because Listing 17-2 has no main method, there’s no place to start the executing. (In fact, the code in Listing 17-2 has no state- ments at all. There’s nothing to execute.) Figure 17-2: The code in Listing 17-2 has no main method. Using a newly defined class To do something useful with the code in Listing 17-2, you need a main method. You can put the main method in a separate file. Listing 17-3 shows you such a file.
- 308. 292 Part IV: Using Program Units Listing 17-3: Making Use of Your Purchase Class import java.util.Scanner; class ProcessPurchase { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); Purchase onePurchase = new Purchase(); System.out.print(“Amount: “); onePurchase.amount = myScanner.nextDouble(); System.out.print(“Taxable? (true/false) “); onePurchase.taxable = myScanner.nextBoolean(); if (onePurchase.taxable) { onePurchase.total = onePurchase.amount * 1.05; } else { onePurchase.total = onePurchase.amount; } System.out.print(“Total: “); System.out.println(onePurchase.total); } } The best way to understand the code in Listing 17-3 is to compare it, line by line, with the code in Listing 17-1. In fact, there’s a mechanical formula for turning the code in Listing 17-1 into the code in Listing 17-3. Table 17-1 describes the formula. Table 17-1 Converting Your Code to Use a Class In Listing 17-1 In Listing 17-3 double amount; Purchase onePurchase = new Purchase(); boolean taxable; double total; amount onePurchase.amount taxable onePurchase.taxable total onePurchase.total
- 309. Chapter 17: Programming with Objects and Classes 293 The two programs (in Listings 17-1 and 17-3) do essentially the same thing, but one uses primitive variables, and the other leans on the Purchase code from Listing 17-2. Both programs have runs like the ones shown back in Figure 17-1. Running code that straddles two separate files From JCreator’s point of view, a project that contains two Java source files is no big deal. You create two classes in the same project, and then you choose Build➪Compile Project. Finally, you choose Build➪Execute Project. Everything works the way you expect it to work. The only time things become tricky is when you have two main methods in the one project. This section’s example (Listings 17-2 and 17-3) doesn’t suffer from that malady. But as you experiment with your code, you can easily add classes with additional main methods. You may also create a large application with several starting points. When a project has more than one main method, JCreator may prompt you and ask which class’s main method you want to run. But sometimes JCreator doesn’t prompt you. Instead, JCreator arbitrarily picks one of the main meth- ods and ignores all the others. This can be very confusing. You add a println call to the wrong main method, and nothing appears in the General Output pane. Hey, what gives? You can fix the problem by following these steps: 1. Right-click the project’s branch in the File View tree. 2. In the resulting context menu, select Properties. A Project Properties dialog box appears. The dialog box contains a list box labeled Run. This list box contains the names of all the classes in the project that contain main methods. 3. In the Run list box, select the class whose main method you want to run. 4. Click OK. The Project Properties dialog box disappears. 5. In JCreator’s main menu, choose Build➪Execute Project.
- 310. 294 Part IV: Using Program Units You cannot execute a project that has no main method. If you try, you get a message box like the one shown earlier in Figure 17-2. Why bother? On the surface, the code in Listing 17-3 is longer, more complicated, and harder to read. But think about a big pile of laundry. It may take time to find a basket, and to shovel socks into the basket. But when you have clothes in the basket, the clothes are much easier to carry. It’s the same way with the code in Listing 17-3. When you have your data in a Purchase basket, it’s much easier to do complicated things with purchases. From Classes Come Objects The code in Listing 17-2 defines a class. A class is a design plan; it describes the way in which you intend to combine and use pieces of data. For example, the code in Listing 17-2 announces your intention to combine double, boolean, and double values to make new Purchase values. Classes are central to all Java programming. But Java is called an object- oriented language. Java isn’t called a class-oriented language. In fact, no one uses the term class-oriented language. Why not? Well, you can’t put your arms around a class. A class isn’t real. A class with- out an object is like a day without chocolate. If you’re sitting in a room right now, glance at all the chairs in the room. How many chairs are in the room? Two? Five? Twenty? In a room with five chairs, you have five chair objects. Each chair (each object) is something real, something you can use, some- thing you can sit on. A language like Java has classes and objects. So what’s the difference between a class and an object? An object is a thing. A class is a design plan for things of that kind. For example, how would you describe what a chair is? Well, a chair has a seat, a back, and legs. In Java, you may write the stuff in Listing 17-4.
- 311. Chapter 17: Programming with Objects and Classes 295 Listing 17-4: What It Means to Be a Chair /* * This is real Java code, but this code cannot be compiled * on its own: */ class Chair { FlatHorizonalPanel seat; FlatVerticalPanel back; LongSkinnyVerticalRods legs; } The code above is a design plan for chairs. The code tells you that each chair has three things. The code names the things (seat, back, and legs), and tells you a little bit about each thing. (For example, a seat is a FlatHorizontal Panel.) In the same way, the code in Listing 17-2 tells you that each purchase has three things. The code names the things (amount, taxable, and total), and tells you the primitive type of each thing. So imagine some grand factory at the edge of the universe. While you sleep each night, this factory stamps out tangible objects — objects that you’ll encounter during the next waking day. Tomorrow you’ll go for an interview at the Sloshy Shoes Company. So tonight, the factory builds chairs for the com- pany’s offices. The factory builds chair objects, as shown in Figure 17-3, from the almost-real code in Listing 17-4. In Listing 17-3, the line Purchase onePurchase = new Purchase(); behaves like that grand factory at the edge of the universe. Instead of creat- ing chair objects, that line in Listing 17-3 creates a purchase object, as shown in Figure 17-4. That Listing 17-3 line is a declaration with an initialization. Just as the line int count = 0; declares the count variable and sets count to 0, the line in Listing 17-3 declares the onePurchase variable, and makes onePurchase point to a brand new object. That new object contains three parts: an amount part, a taxable part, and a total part.
- 312. 296 Part IV: Using Program Units Figure 17-3: Chairs objects from the Chair class. Figure 17-4: An object created from the Purchase class.
- 313. Chapter 17: Programming with Objects and Classes 297 If you want to be picky, there’s a difference between the stuff in Figure 17-4 and the action of the big bold statement in Listing 17-3. Figure 17-4 shows an object with the values 20.00, true, and 21.00 stored in it. The statement in Listing 17-3 creates a new object, but it doesn’t fill the object with useful values. Getting values comes later in Listing 17-3. Understanding (or ignoring) the subtleties Sometimes, when you refer to a particular object, you want to emphasize which class the object came from. Well, subtle differences in emphasis call for big differences in terminology. So here’s how Java programmers use the terminology: The bold line in Listing 17-3 creates a new object. The bold line in Listing 17-3 creates a new instance of the Purchase class. The words object and instance are almost synonymous, but Java program- mers never say “object of the Purchase class” (or if they do, they feel funny). By the way, if you mess up this terminology and say something like “object of the Purchase class,” then no one jumps down your throat. Everyone under- stands what you mean, and life goes on as usual. In fact, I often use a phrase like “Purchase object” to describe an instance of the Purchase class. The difference between object and instance isn’t terribly important. But it’s very important to remember that the words object and instance have the same meaning. (Okay! They have nearly the same meaning.) Making reference to an object’s parts After you’ve created an object, you use dots to refer to the object’s parts. For example, in Listing 17-3, I put a value into the onePurchase object’s amount part with the following code: onePurchase.amount = myScanner.nextDouble(); Later in Listing 17-3, I get the amount part’s value with the following code: onePurchase.total = onePurchase.amount * 1.05; This dot business may look cumbersome, but it really helps programmers when they’re trying to organize the code. In Listing 17-1, each variable is a separate entity. But in Listing 17-3, each use of the word amount is inextrica- bly linked to the notion of a purchase. That’s good.
- 314. 298 Part IV: Using Program Units Creating several objects After you’ve created a Purchase class, you can create as many purchase objects as you want. For example, in Listing 17-5, I create three purchase objects. Listing 17-5: Processing Purchases import java.util.Scanner; class ProcessPurchasesss { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); Purchase aPurchase; for (int count = 0; count < 3; count++) { aPurchase = new Purchase(); System.out.print(“Amount: “); aPurchase.amount = myScanner.nextDouble(); System.out.print(“Taxable? (true/false) “); aPurchase.taxable = myScanner.nextBoolean(); if (aPurchase.taxable) { aPurchase.total = aPurchase.amount * 1.05; } else { aPurchase.total = aPurchase.amount; } System.out.print(“Total: “); System.out.println(aPurchase.total); System.out.println(); } } } Figure 17-5 has a run of the code in Listing 17-5, and Figure 17-6 illustrates the concept. Figure 17-5: Running the code in Listing 17-5.
- 315. Chapter 17: Programming with Objects and Classes 299 Figure 17-6: From one class come three objects. To compile the code in Listing 17-5 you must have a copy of the Purchase class in the same project. (The Purchase class is in Listing 17-2.) To copy a class’s code from one project to another, see Chapter 16. (One of that chap- ter’s sidebars describes the copy-and-paste routine.) Listing 17-5 has only one variable that refers to purchase objects. (The vari- able’s name is aPurchase.) The program has three purchase objects because the assignment statement aPurchase = new Purchase(); is executed three times (once for each iteration of the for loop). Just as you can separate an int variable’s assignment from the variable’s declaration int count; count = 0; you can also separate a Purchase variable’s assignment from the variable’s declaration: Purchase aPurchase; for (int count = 0; count < 3; count++) { aPurchase = new Purchase();
- 316. 300 Part IV: Using Program Units In fact, after you’ve created the code in Listing 17-2, the word Purchase stands for a brand new type — a reference type. Java has eight built-in primitive types, and has as many reference types as people can define during your lifetime. In Listing 17-2, I define the Purchase reference type, and you can define refer- ence types too. Table 17-2 has a brief comparison of primitive types and reference types. Table 17-2 Java Types Primitive Type Reference Type How it’s created Built into the language Defined as a Java class How many are there Eight Indefinitely many Sample variable int count; Purchase aPurchase; declaration Sample assignment count = 0; aPurchase = new Purchase(); Assigning a value to (Not applicable) aPurchase.amount = 20.00; one of its parts Another Way to Think About Classes When you start learning object-oriented programming, you may think this class idea is a big hoax. Some geeks in Silicon Valley had nothing better to do, so they went to a bar and made up some confusing gibberish about classes. They don’t know what it means, but they have fun watching people struggle to understand it. Well, that’s not what classes are all about. Classes are serious stuff. What’s more, classes are useful. Many reputable studies have shown that classes and object-oriented programming save time and money. Even so, the notion of a class can be very elusive. Even experienced programmers — the ones who are new to object-oriented programming — have trouble understanding how an object differs from a class.
- 317. Chapter 17: Programming with Objects and Classes 301 Classes, objects, and tables Because classes can be so mysterious, I’ll expand your understanding with another analogy. Figure 17-7 has a table of three purchases. The table’s title consists of one word (the word “Purchase”) and the table has three column headings — the words “amount,” “taxable,” and “total.” Well, the code in Listing 17-2 has the same stuff — Purchase, amount, taxable, and total. So in Figure 17-7, think of the top part of the table (the title and column headings) as a class. Like the code in Listing 17-2, this top part of the table tells us what it means to be a Purchase. (It means having an amount value, a taxable value, and a total value.) A class is like the top part of a table. And what about an object? Well, an object is like a row of a table. For example, with the code in Listing 17-5 and the input in Figure 17-5, I create three objects (three instances of the Purchase class). The first object has amount value 20.00, taxable value true, and total value 21.00. In the table, the first row has these three values — 20.00, true, and 21.00, as shown in Figure 17-8. Figure 17-7: A table of purchases.
- 318. 302 Part IV: Using Program Units Figure 17-8: A purchase corre- sponds to a row of the table. Some questions and answers Here’s the world’s briefest object-oriented programming FAQ: Can I have an object without having a class? No, you can’t. In Java, every object is an instance of a class. Can I have a class without having an object? Yes, you can. In fact, almost every program in this book creates a class without an object. Take Listing 17-5, for example. The code in Listing 17-5 defines a class named ProcessPurchasesss. And nowhere in Listing 17-5 (or anywhere else) do I create an instance of the ProcessPurchasesss class. I have a class with no objects. That’s just fine. It’s business as usual. After I’ve created a class and its instances, can I add more instances to the class? Yes, you can. In Listing 17-5, I create one instance, then another, and then a third. If I went one additional time around the for loop, I’d have a fourth instance, and I’d put a fourth row in the table of Figure 17-8. With no objects, three objects, four objects, or more objects, I still have the same old Purchase class. Can an object come from more than one class? Bite your tongue! Maybe other object-oriented languages allow this nasty class cross-breeding, but in Java, it’s strictly forbidden. That’s one of the things that distinguishes Java from some of the languages that preceded it. Java is cleaner, more uniform, and easier to understand.
- 319. Chapter 18 Using Methods and Variables from a Java Class In This Chapter Using Java’s String class Calling methods Understanding static and non-static methods and variables Making numbers look good I hope you didn’t read Chapter 17, because I tell a big lie in the beginning of the chapter. Actually, it’s not a lie. It’s an exaggeration. Actually, it’s not an exaggeration. It’s a careful choice of wording. In Chapter 17, I write that the gathering of data into a class is the start of object-oriented programming. Well, that’s true. Except that many programming languages had data-gathering features before object-oriented programming became popular. Pascal had records. C had structs. To be painfully precise, the grouping of data into usable blobs is only a prereq- uisite to object-oriented programming. You’re not really doing object-oriented programming until you combine both data and methods in your classes. This chapter starts the “data and methods” ball rolling, and Chapter 19 rounds out the picture. The String Class The String class is declared in the Java API. This means that, somewhere in the stuff you download from java.sun.com is a file named String.java. If you hunt down this String.java file and peek at the file’s code, you find some very familiar-looking stuff: class String { ...And so on.
- 320. 304 Part IV: Using Program Units In your own code, you can use this String class without ever seeing what’s inside the String.java file. That’s one of the great things about object- oriented programming. A simple example A String is bunch of characters. It’s like having several char values in a row. You can declare a variable to be of type String and store several letters in the variable. Listing 18-1 has a tiny example. Listing 18-1: I’m Repeating Myself Again (Again) import java.util.Scanner; class JazzyEchoLine { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String lineIn; lineIn = myScanner.nextLine(); System.out.println(lineIn); } } A run of Listing 18-1 is shown in Figure 18-1. This run bears an uncanny resem- blance to runs of Listing 5-1 from Chapter 5. That’s because Listing 18-1 is a reprise of the effort in Listing 5-1. Figure 18-1: Running the code in Listing 18-1. The new idea in Listing 18-1 is the use of a String. In Listing 5-1, I have no variable to store the user’s input. But in Listing 18-1, I create the lineIn vari- able. This variable stores a bunch of letters, like the letters Do as I write, not as I do.
- 321. Chapter 18: Using Methods and Variables from a Java Class 305 Putting String variables to good use The program in Listing 18-1 takes the user’s input and echoes it back on the screen. This is a wonderful program, but (like many college administrators that I know) it doesn’t seem to be particularly useful. So take a look at a more useful application of Java’s String type. A nice one is in Listing 18-2. Listing 18-2: Putting a Name in a String Variable import java.util.Scanner; import static java.lang.System.out; class ProcessMoreData { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String fullName; double amount; boolean taxable; double total; out.print(“Customer’s full name: “); fullName = myScanner.nextLine(); out.print(“Amount: “); amount = myScanner.nextDouble(); out.print(“Taxable? (true/false) “); taxable = myScanner.nextBoolean(); if (taxable) { total = amount * 1.05; } else { total = amount; } out.println(); out.print(“The total for “); out.print(fullName); out.print(“ is “); out.print(total); out.println(“.”); } }
- 322. 306 Part IV: Using Program Units A run of the code in Listing 18-2 is shown in Figure 18-2. The code stores Barry A. Burd in a variable called fullName, and displays the fullName variable’s content as part of the output. To make this program work, you have to store Barry A. Burd somewhere. After all, the program follows a certain outline: Get a name. Get some other stuff. Compute the total. Display the name (along with some other stuff). Figure 18-2: Making a purchase. If you don’t have the program store the name somewhere then, by the time it’s done getting other stuff and computing the total, it forgets the name (so the program can’t display the name). Reading and writing strings To read a String value from the keyboard, you can call either next or nextLine: The method next reads up to the next blank space. For example, with the input Barry A. Burd, the statements String firstName = myScanner.next(); String middleInit = myScanner.next(); String lastName = myScanner.next(); assign Barry to firstName, A. to middleInit, and Burd to lastName. The method nextLine reads up to the end of the current line. For example, with input Barry A. Burd, the statement String fullName = myScanner.nextLine(); assigns Barry A. Burd to the variable fullName. (Hey, being an author has some hidden perks.)
- 323. Chapter 18: Using Methods and Variables from a Java Class 307 To display a String value, you can call one of your old friends, System.out. print or System.out.println. In fact, most of the programs in this book display String values. In Listing 18-2, a statement like out.print(“Customer’s full name: “); displays the String value “Customer’s full name: “. You can use print and println to write String values to a disk file. For details, see Chapter 13. Chapter 4 introduces a bunch of characters, enclosed in double quote marks: “Chocolate, royalties, sleep” In Chapter 4, I call this a literal of some kind. (It’s a literal because, unlike a variable, it looks just like the stuff that it represents.) Well, in this chapter, I can continue the story about Java’s literals: In Listing 18-2, amount and total are double variables, and 1.05 is a double literal. In Listing 18-2, fullName is a String variable, and things like “Customer’s full name: “ are String literals. In a Java program, you surround the letters in a String literal with double quote marks. Using an Object’s Methods If you’re not too concerned about classes and reference types, then the use of the type String in Listing 18-2 is no big deal. Almost everything you can do with a primitive type seems to work with the String type as well. But there’s danger around the next curve. Take a look at the code in Listing 18-3, and the run of the code shown in Figure 18-3. Figure 18-3: But I typed the correct password!
- 324. 308 Part IV: Using Program Units Listing 18-3: A Faulty Password Checker /* * This code does not work: */ import java.util.Scanner; import static java.lang.System.out; class TryToCheckPassword { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String password = “swordfish”; String userInput; out.print(“What’s the password? “); userInput = myScanner.next(); if (password == userInput) { out.println(“You’re okay!”); } else { out.println(“You’re a menace.”); } } } Here are the facts as they appear in this example: According to the code in Listing 18-3, the value of password is “swordfish”. In Figure 18-3, in response to the program’s prompt, the user types swordfish. So in the code, the value of userInput is “swordfish”. The if statement checks the condition password == userInput. Because both variables have the value “swordfish”, the condition should be true, but. . . . The condition is not true, because the program’s output is You’re a menace. What’s going on here? I try beefing up the code to see if I can find any clues. An enhanced version of the password-checking program is in Listing 18-4, with a run of the new version shown in Figure 18-4. Listing 18-4: An Attempt to Debug the Code in Listing 18-3 import java.util.Scanner; import static java.lang.System.out; class DebugCheckPassword {
- 325. Chapter 18: Using Methods and Variables from a Java Class 309 public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String password = “swordfish”; String userInput; out.print(“What’s the password? “); userInput = myScanner.next(); out.println(); out.print(“You typed “); out.println(userInput); out.print(“But the password is “); out.println(password); out.println(); if (password == userInput) { out.println(“You’re okay!”); } else { out.println(“You’re a menace.”); } } } Figure 18-4: This looks even worse. Ouch! I’m stumped this time. The run in Figure 18-4 shows that both the userInput and password variables have value swordfish. So why doesn’t the program accept the user’s input? When you compare two things with a double equal sign, reference types and primitive types don’t behave the same way. Consider, for example, int versus String: You can compare two int values with a double equal sign. When you do, things work exactly as you would expect. For example, the condition in the following code is true: int apples = 7; int oranges = 7; if (apples == oranges) System.out.println(“They’re equal.”); When you compare two String values with the double equal sign, things don’t work the way you expect. The computer doesn’t check to see if the two String values contain the same letters. Instead, the computer checks some esoteric property of the way variables are stored in memory.
- 326. 310 Part IV: Using Program Units For your purposes, the term reference type is just a fancy name for a class. Because String is defined to be a class in the Java API, I call String a refer- ence type. This terminology highlights the parallel between primitive types (such as int) and classes (that is, reference types, such as String). Comparing strings In the preceding bullets, the difference between int and String is mighty interesting. But if the double equal sign doesn’t work for String values, how do you check to see if Joe User enters the correct password? You do it with the code in Listing 18-5. Listing 18-5: Calling an Object’s Method /* * This program works! */ import java.util.Scanner; import static java.lang.System.out; class CheckPassword { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String password = “swordfish”; String userInput; out.print(“What’s the password? “); userInput = myScanner.next(); if (password.equals(userInput)) { out.println(“You’re okay!”); } else { out.println(“You’re a menace.”); } } } A run of the new password-checking code is shown in Figure 18-5 and, let me tell you, it’s a big relief! The code in Listing 18-5 actually works! When the user types swordfish, the if statement’s condition is true. Figure 18-5: At last, Joe User can log in.
- 327. Chapter 18: Using Methods and Variables from a Java Class 311 The truth about classes and methods The magic in Listing 18-5 is the use of a method named equals. I have two ways to explain the equals method — a simple way, and a more detailed way. First, here’s the simple way: The equals method compares the charac- ters in one string with the characters in another. If the characters are the same, then the condition inside the if statement is true. That’s all there is to it. Don’t use a double equal sign to compare two String objects. Instead, use one of the objects’ equals methods. For a more detailed understanding of the equals method, flip back to Chapter 17 and take a look at Figures 17-7 and 17-8. Those figures illustrate the similarities between classes, objects, and the parts of a table. In the fig- ures, each row represents a purchase, and each column represents a feature that purchases possess. You can observe the same similarities for any class, including Java’s String class. In fact, what Figure 17-7 does for purchases, Figure 18-6 does for strings. Figure 18-6: Viewing the String class and String objects as parts of a table. The stuff shown in Figure 18-6 is much simpler than the real String class story. But Figure 18-6 makes a good point. Like the purchases in Figure 17-7, each string has its own features. For example, each string has a value (the actual characters stored in the string) and each string has a count (the number of characters stored in the string). You can’t really write the following line of code: //This code does NOT work: System.out.println(password.count); but that’s because the stuff in Figure 18-6 omits a few subtle details.
- 328. 312 Part IV: Using Program Units Anyway, each row in Figure 18-6 has three items — a value, a count, and an equals method. So each row of the table contains more than just data. Each row contains an equals method, a way of doing something useful with the data. It’s as if each object (each instance of the String class) has three things: A bunch of characters (the object’s value) A number (the object’s count) A way of being compared with other strings (the object’s equals method) That’s the essence of object-oriented programming. Each string has its own personal copy of the equals method. For example, in Listing 18-5, the password string has its own equals method. When you call the password string’s equals method and put the userInput string in the method’s paren- theses, the method compares the two strings to see if those strings contain the same characters. The userInput string in Listing 18-5 has an equals method too. I could use the userInput string’s equals method to compare this string with the password string. But I don’t. In fact, in Listing 18-5, I don’t use the userInput string’s equals method at all. (To compare the userInput with the password, I had to use either the password string’s equals method or the userInput string’s equals method. So I made an arbitrary choice: I chose the password string’s method.) Calling an object’s methods Calling a string’s equals method is like getting a purchase’s total. With both equals and total, you use your old friend, the dot. For example, in Listing 17-3, you write System.out.println(onePurchase.total); and in Listing 18-5, you write if (password.equals(userInput)) A dot works the same way for an object’s variables and its methods. In either case, a dot takes the object and picks out one of the object’s parts. It works whether that part is a piece of data (as in onePurchase.total) or a method (as in password.equals).
- 329. Chapter 18: Using Methods and Variables from a Java Class 313 Combining and using data At this point in the chapter, I can finally say, “I told you so.” Here’s a quota- tion from Chapter 17: A class is a design plan. The class describes the way in which you intend to combine and use pieces of data. A class can define the way you use data. How do you use a password and a user’s input? You check to see if they’re the same. That’s why Java’s String class defines an equals method. An object can be more than just a bunch of data. With object-oriented pro- gramming, each object possesses copies of methods for using that object. Static Methods You have a fistful of checks. Each check has a number, an amount, and a payee. You print checks like these with your very own laser printer. To print the checks, you use a Java class. Each object made from the Check class has three variables (number, amount, and payee). And each object has one method (a print method). You can see all this in Figure 18-7. You’d like to print the checks in numerical order. So you need a method to sort the checks. If the checks in Figure 18-7 were sorted, the check with number 1699 would come first, and the check with number 1705 would come last. Figure 18-7: The Check class and some check objects.
- 330. 314 Part IV: Using Program Units The big question is, should each check have its own sort method? Does the check with number 1699 need to sort itself? And the answer is no. Some methods just shouldn’t belong to the objects in a class. So where do such methods belong? How can you have a sort method with- out creating a separate sort for each check? Here’s the answer. You make the sort method be static. Anything that’s static belongs to a whole class, not to any particular instance of the class. If the sort method is static, then the entire Check class has just one copy of the sort method. This copy stays with the entire Check class. No matter how many instances of the Check class you create — three, ten, or none — you have just one sort method. For an illustration of this concept, look back at Figure 18-7. The whole class has just one sort method. So the sort method is static. No matter how you call the sort method, that method uses the same values to do its work. Of course, each individual check (each object, each row of the table in Figure 18-7) still has its own number, its own amount, its own payee, and it’s own print method. When you print the first check, you get one amount, and when you print the second check get another. Because there’s a number, an amount, a payee, and a print method for each object, I call these things non-static. I call them non-static, because . . . well . . . because they’re not static. Calling static and non-static methods In this book, my first use of the word static is way back in Listing 3-1. I use static as part of every main method (and this book’s listings have lots of main methods). In Java, your main method has to be static. That’s just the way it goes. To call a static method, you use a class’s name along with a dot. This is just slightly different from the way you call a non-static method: To call an ordinary (non-static) method, you follow an object with a dot. For example, a program to process the checks in Figure 18-7 may contain code of the following kind: Check firstCheck; firstCheck.number = 1705; firstCheck.amount = 25.09; firstCheck.payee = “The Butcher”; firstCheck.print(); To call a class’s static method, you follow the class name with a dot. For example, to sort the checks in Figure 18-7, you may call Check.sort();
- 331. Chapter 18: Using Methods and Variables from a Java Class 315 Turning strings into numbers The code in Listing 18-5 introduces a non-static method named equals. To compare the password string with the userInput string, you preface .equals with either of the two string objects. In Listing 18-5, I preface .equals with the password object: if (password.equals(userInput)) Each string object has an equals method of its own, so I can achieve the same effect by writing if (userInput.equals(password)) But Java has another class named Integer, and the whole Integer class has a static method named parseInt. If someone hands you a string of characters, and you want to turn that string into an int value, you can call the Integer class’s parseInt method. Listing 18-6 has a small example. Listing 18-6: More Chips, Please import java.util.Scanner; import static java.lang.System.out; class AddChips { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); String reply; int numberOfChips; out.print(“How many chips do you have?”); out.print(“ (Type a number,”); out.print(“ or type ‘Not playing’) “); reply = myScanner.nextLine(); if (!reply.equals(“Not playing”)) { numberOfChips = Integer.parseInt(reply); numberOfChips += 10; out.print(“You now have “); out.print(numberOfChips); out.println(“ chips.”); } } } Some runs of the code in Listing 18-6 are shown in Figure 18-8. You want to give each player ten chips. But some party poopers in the room aren’t playing. So two people, each with no chips, may not get the same treatment. An empty- handed player gets ten chips, but an empty-handed party pooper gets none.
- 332. 316 Part IV: Using Program Units Figure 18-8: Running the code in Listing 18-6. So in Listing 18-6, you call the Scanner class’s nextLine method, allowing a user to enter any characters at all — not just digits. If the user types Not playing, then you don’t give the killjoy any chips. If the user types some digits, then you’re stuck holding these digits in the string variable named reply. You can’t add ten to a string like reply. So you call the Integer class’s parseInt method, which takes your string, and hands you back a nice int value. From there, you can add ten to the int value. Java has a loophole that allows you to add a number to a string. The problem is, you don’t get real addition. Adding the number 10 to the string “30” gives you “3010”, not 40. Don’t confuse Integer with int. In Java, int is the name of a primitive type (a type that I use throughout this book). But Integer is the name of a class. Java’s Integer class contains handy methods for dealing with int values. For example, in Listing 18-6, the Integer class’s parseInt method makes an int value from a string. Turning numbers into strings In Chapter 17, Listing 17-1 adds tax to the amount of a purchase. But a run of the code in Listing 17-1 has an anomaly. Look back at Figure 17-1. With five percent tax on 20 dollars, the program displays a total of 21.0. That’s peculiar. Where I come from, currency amounts aren’t normally displayed with just one digit beyond the decimal point. If you don’t choose your purchase amount carefully, the situation is even worse. For example, in Figure 18-9, I run the same program (the code in Listing 17-1) with purchase amount 19.37. The resulting display looks very nasty. With its internal zeros and ones, the computer doesn’t do arithmetic quite the way you and I are used to doing it. So how do you fix this problem?
- 333. Chapter 18: Using Methods and Variables from a Java Class 317 Figure 18-9: Do you have change for 20.33850000 0000003? The Java API has a class named NumberFormat, and the NumberFormat class has a static method named getCurrencyInstance. When you call Number Format.getCurrencyInstance() with nothing inside the parentheses, you get an object that can mold numbers into U.S. currency amounts. Listing 18-7 has an example. Listing 18-7: The Right Way to Display a Dollar Amount import java.text.NumberFormat; import java.util.Scanner; class BetterProcessData { public static void main(String args[]) { Scanner myScanner = new Scanner(System.in); double amount; boolean taxable; double total; NumberFormat currency = NumberFormat.getCurrencyInstance(); String niceTotal; System.out.print(“Amount: “); amount = myScanner.nextDouble(); System.out.print(“Taxable? (true/false) “); taxable = myScanner.nextBoolean(); if (taxable) { total = amount * 1.05; } else { total = amount; } niceTotal = currency.format(total); System.out.print(“Total: “); System.out.println(niceTotal); } }
- 334. 318 Part IV: Using Program Units For some beautiful runs of the code in Listing 18-7, see Figure 18-10. Now at last, you see a total like $20.34, not 20.338500000000003. Ah! That’s much better. Figure 18-10: See the pretty numbers. How the NumberFormat works For my current purposes, the code in Listing 18-7 contains three interesting variables: The variable total stores a number, such as 21.0. The variable currency stores an object that can mold numbers into U.S. currency amounts. The variable niceTotal is set up to store a bunch of characters. The currency object has a format method. So to get the appropriate bunch of characters into the niceTotal variable, you call the currency object’s format method. You apply this format method to the variable total. Understanding the Big Picture In this section, I answer some of the burning questions that I raise through- out the book. “What does java.util stand for?” “Why do I need the word static at certain points in the code?” “How can a degree in Horticultural Studies help you sort cancelled checks?” I also explain “static” in some unique and interesting ways. After all, static methods and variables aren’t easy to understand. It helps to read about Java’s static feature from several points of view.
- 335. Chapter 18: Using Methods and Variables from a Java Class 319 Packages and import declarations In Java, you can group a bunch of classes into something called a package. In fact, the classes in Java’s standard API are divided into about 170 packages. This book’s examples make heavy use of three packages — the packages named java.util, java.lang, and java.io. The class java.util.Scanner The package java.util contains about 50 classes, including the very useful Scanner class. Like most other classes, this Scanner class has two names — a fully qualified name and an abbreviated simple name. The class’s fully quali- fied name is java.util.Scanner, and the class’s simple name is Scanner. You get the fully qualified name by adding the package name to the class’s simple name. (That is, you add the package name java.util to the simple name Scanner. You get java.util.Scanner.) An import declaration lets you abbreviate a class’s name. With the declaration import java.util.Scanner; the Java compiler figures out where to look for the Scanner class. So instead of writing java.util.Scanner throughout your code, you can just write Scanner. The class java.lang.System The package java.lang contains about 35 classes, including the ever popu- lar System class. (The class’s fully qualified name is java.lang.System, and the class’s simple name is System.) Instead of writing java.lang.System throughout your code, you can just write System. You don’t even need an import declaration. All ye need to know I can summarize much of Java’s complexity in An object can have its own methods. An only a few sentences: object can also have its own variables. The Java API contains many packages. A class can have its own static methods. A class can also have its own static variables. A package contains classes. From a class, you can create objects.
- 336. 320 Part IV: Using Program Units Among all of Java’s packages, the java.lang package is special. With or with- out an import declaration, the compiler imports everything in the java.lang package. You can start your program with import java.lang.System. But if you don’t, the compiler adds this declaration automatically. The static System.out variable What kind of importing must you do in order to abbreviate System.out. println? How can you shorten it to out.println? An import declaration lets you abbreviate a class’s name. But in the expression System.out, the word out isn’t a class. The word out is a static variable. (The out variable refers to the place where a Java program sends text output.) So you can’t write //This code is bogus. Don’t use it: import java.lang.System.out; What do you do instead? You write import static java.lang.System.out; To find out more about the out variable’s being a static variable, read the next section. Shedding light on the static darkness I love to quote myself. When I quote my own words, I don’t need written permission. I don’t have to think about copyright infringement and I never hear from lawyers. Best of all, I can change and distort anything I say. When I paraphrase my own ideas, I can’t be misquoted. With that in mind, here’s a quote from the previous section: “Anything that’s static belongs to a whole class, not to any particular instance of the class. . . . To call a static method, you use a class’s name along with a dot.” How profound! In Listing 18-6, I introduce a static method named parseInt. Here’s the same quotation applied to the static parseInt method: The static parseInt method belongs to the whole Integer class, not to any particular instance of the Integer class. . . . To call the static parseInt method, you use the Integer class’s name along with a dot. You write something like Integer.parseInt(reply).
- 337. Chapter 18: Using Methods and Variables from a Java Class 321 That’s very nice! How about the System.out business that I introduce in Chapter 3? I can apply my quotation to that too. The static out variable belongs to the whole System class, not to any particular instance of the System class. . . . To refer to the static out variable, you use the System class’s name along with a dot. You write something like System.out.println(). If you think about what System.out means, this static business makes sense. After all, the name System.out refers to the place where a Java program sends text output. (When you use JCreator, the name System.out refers to JCreator’s General Output pane.) A typical program has only one place to send its text output. So a Java program has only one out variable. No matter how many objects you create — three, ten, or none — you have just one out variable. And when you make something static, you insure that the program has only one of those things. Alright, then! The out variable is static. To abbreviate the name of a static variable (or a static method), you don’t use an ordinary import declaration. Instead, you use a static import declara- tion. That’s why, in Chapter 9 and beyond, I use the word static to import the out variable: import static java.lang.System.out; Barry makes good on an age-old promise In Chapter 6, I pull a variable declaration outside of a main method. I go from code of the kind in Listing 18-8, to code of the kind that’s in Listing 18-9. Listing 18-8: Declaring a Variable Inside the main Method class SnitSoft { public static void main(String args[]) { double amount = 5.95; amount = amount + 25.00; System.out.println(amount); } }
- 338. 322 Part IV: Using Program Units Listing 18-9: Pulling a Variable Outside of the main Method class SnitSoft { static double amount = 5.95; public static void main(String args[]) { amount = amount + 25.00; System.out.println(amount); } } In Chapter 6, I promise to explain why Listing 18-9 needs the extra word static (in static double amount = 5.95). Well, with all the fuss about static methods in this chapter, I can finally explain everything. Look back to Figure 18-7. In that figure, you have checks and you have a sort method. Each individual check has its own number, its own amount, and its own payee. But the entire Check class has just one sort method. I don’t know about you, but to sort my cancelled checks, I hang them on my exotic Yucca Elephantipes tree. I fasten the higher numbered checks to the upper leaves, and put the lower numbered checks on the lower leaves. When I find a check whose number comes between two other checks, I select a free leaf (one that’s between the upper and lower leaves). A program to mimic my sorting method looks something like this: class Check { int number; double amount; String payee; static void sort() { Yucca tree; if (myCheck.number > 1700) { tree.attachHigh(myCheck); } // ... etc. } } Because of the word static, the Check class has only one sort method. And because I declare the tree variable inside the static sort method, this pro- gram has only one tree variable. (Indeed, I hang all my cancelled checks on just one Yucca tree.) I can move the tree variable’s declaration outside of the sort method. But if I do, I may have too many Yucca trees.
- 339. Chapter 18: Using Methods and Variables from a Java Class 323 class Check { int number; double amount; String payee; Yucca tree; //This is bad! Each check has its own tree. static void sort() { if (myCheck.number > 5000) { tree.attachHigh(myCheck); } // ... etc. } } In the nasty code above, each check has its own number, its own amount, its own payee, and its own tree. But that’s ridiculous! I don’t want to fasten each check to its own Yucca tree. Everybody knows you’re supposed to sort checks with just one Yucca tree. (That’s the way the big banks do it.) When I move the tree variable’s declaration outside of the sort method, I want to preserve the fact that I have only one tree. (To be more precise, I have only one tree for the entire Check class.) To make sure that I have only one tree, I declare the tree variable to be static. class Check { int number; double amount; String payee; static Yucca tree; //That’s better! static void sort() { if (myCheck.number > 5000) { tree.attachHigh(myCheck); } // ... etc. } } For exactly the same reason, I write static double amount when I move from Listing 18-8 to 18-9. To find out more about sorting, read UNIX For Dummies Quick Reference, 4th Edition, by Margaret Levine Young and John R. Levine. To learn more about bank checks, read Managing Your Money Online For Dummies by Kathleen Sindell. To learn more about trees, read Landscaping For Dummies by Phillip Giroux, Bob Beckstrom, and Lance Walheim.
- 340. 324 Part IV: Using Program Units
- 341. Chapter 19 Creating New Java Methods In This Chapter Creating methods that work with existing values Creating methods that modify existing values Creating methods that return new values I n Chapters 3 and 4, I introduce Java methods. I show you how to create a main method and how to call the System.out.println method. Between that chapter and this one, I make very little noise about methods. In Chapter 18, I introduce a bunch of new methods for you to call, but that’s only half of the story. This chapter completes the circle. In this chapter, you create your own Java methods — not the tired old main method that you’ve been using all along, but some new, powerful Java methods. Defining a Method within a Class In Chapter 18, Figure 18-6 introduces an interesting notion — a notion that’s at the core of object-oriented programming. Each Java string has its own equals method. That is, each string has, built within it, the functionality to compare itself with other strings. That’s an important point. When you do object-oriented programming, you bundle data and functionality into a lump called a class. Just remember Barry’s immortal words from Chapter 17: A class describes the way in which you intend to combine and use pieces of data.
- 342. 326 Part IV: Using Program Units And why are these words so important? They’re important because, in object- oriented programming, chunks of data take responsibility for themselves. With object-oriented programming, everything you have to know about a string is located in the file String.java. So if anybody has problems with the strings, they know just where to look for all the code. That’s great! So this is the deal — objects contain methods. Chapter 18 shows you how to use an object’s methods, and this chapter shows you how to create an object’s methods. Making a method Imagine a table containing the information about three accounts. (If you have trouble imagining such a thing, just look at Figure 19-1.) In the figure, each account has a last name, an identification number, and a balance. In addition (and here’s the important part), each account knows how to display itself on the screen. Each row of the table has its own copy of a display method. Figure 19-1: A table of accounts.
- 343. Chapter 19: Creating New Java Methods 327 The last names in Figure 19-1 may seem strange to you. That’s because I gen- erated the table’s data randomly. Each last name is a haphazard combination of three letters — one uppercase letter followed by two lowercase letters. Though it may seem strange, generating account values at random is common practice. When you write new code, you want to test the code to find out if it runs correctly. You can make up your own data (with values like “Smith”, 0000, and 1000.00). But to give your code a challenging workout, you should use some unexpected values. If you have values from some real-life case studies, you should use them. But if you have don’t have real data, randomly gener- ated values are easy to create. I need some code to implement the ideas in Figure 19-1. Fortunately, I have some code in Listing 19-1. Listing 19-1: An Account Class import java.text.NumberFormat; import static java.lang.System.out; class Account { String lastName; int id; double balance; void display() { NumberFormat currency = NumberFormat.getCurrencyInstance(); out.print(“The account with last name “); out.print(lastName); out.print(“ and ID number “); out.print(id); out.print(“ has balance “); out.println(currency.format(balance)); } } The Account class in Listing 19-1 defines four things — a lastName, an id, a balance, and a display. So each instance of Account class has its own lastName variable, its own id variable, its own balance variable, and its own display method. These things match up with the four columns in Figure 19-1.
- 344. 328 Part IV: Using Program Units Examining the method’s header Listing 19-1 contains the display method’s declaration. Like a main method’s declaration, the display declaration has a header and a body. (See Chapter 4.) The header has two words and some parentheses: The word void tells the computer that, when the display method is called, the display method doesn’t return anything to the place that called it. Later in this chapter, a method does return something. For now, the display method returns nothing. The word display is the method’s name. Every method must have a name. Otherwise, you don’t have a way to call the method. The parentheses contain all the things you’re going to pass to the method when you call it. When you call a method, you can pass information to that method on the fly. This display example, with its empty parentheses, looks strange. That’s because no information is passed to the display method when you call it. That’s okay. I give a meatier example later in this chapter. Examining the method’s body The display method’s body contains some print and println calls. The inter- esting thing here is that the body makes reference to the variables lastName, id, and balance. A method’s body can do that. But with each object having its own lastName, id, and balance variables, what does a variable in the display method’s body mean? Well, when I use the Account class, I create little account objects. Maybe I create an object for each row of the table in Figure 19-1. Each object has its own values for the lastName, id, and balance variables, and each object has its own copy of the display method. So take the first display method in Figure 19-1 — the method for Aju’s account. The display method for that object behaves as if it had the code in Listing 19-2.
- 345. Chapter 19: Creating New Java Methods 329 Listing 19-2: How the display Method Behaves When No One’s Looking /* * This is not real code: */ void display() { NumberFormat currency = NumberFormat.getCurrencyInstance(); out.print(“The account with last name “); out.print(“Aju”); out.print(“ and ID number “); out.print(9936); out.print(“ has balance “); out.println(currency.format(8734.00)); } In fact, each of the three display methods behaves as if its body has a slightly different code. Figure 19-2 illustrates this idea for two instances of the Account class. Figure 19-2: Two objects, each with its own display method.
- 346. 330 Part IV: Using Program Units Calling the method To put the previous section’s ideas into action, you need more code. So the next listing (see Listing 19-3) creates instances of the Account class. Listing 19-3: Making Use of the Code in Listing 19-1 import java.util.Random; class ProcessAccounts { public static void main(String args[]) { Random myRandom = new Random(); Account anAccount; for (int i = 0; i < 3; i++) { anAccount = new Account(); anAccount.lastName = “” + (char) (myRandom.nextInt(26) + ‘A’) + (char) (myRandom.nextInt(26) + ‘a’) + (char) (myRandom.nextInt(26) + ‘a’); anAccount.id = myRandom.nextInt(10000); anAccount.balance = myRandom.nextInt(10000); anAccount.display(); } } } Here’s a summary of the action in Listing 19-3: Do the following three times: Create a new object (an instance of the Account class). Randomly generate values for the object’s lastName, id and balance. Call the object’s display method. The first of the three display calls prints the first object’s lastName, id, and balance values. The second display call prints the second object’s lastName, id, and balance values. And so on. A run of the code from Listing 19-3 is shown in Figure 19-3. Figure 19-3: Running the code in Listing 19-3.
- 347. Chapter 19: Creating New Java Methods 331 Generating words randomly Most programs don’t work correctly the first time Let’s pause for a brief summary. The expression you run them, and some programs don’t work (char) (myRandom.nextInt(26) + ‘A’) without extensive trial and error. This section’s represents a randomly generated uppercase code is a case in point. letter. In a similar way, (char) (myRandom. nextInt(26) + ‘a’) represents a randomly To write this section’s code, I needed a way to generated lowercase letter. generate three-letter words randomly. After about a dozen attempts, I got the code to work. Watch out! The next couple of steps can be But I didn’t stop there. I kept working for a few tricky. hours looking for a simple way to generate three- Java doesn’t allow you to assign a char letter words randomly. In the end, I settled on value to a string variable. the following code (in Listing 19-3): anAccount.lastName = “” + So in Listing 19-3, the following statement (char) would lead to a compiler error: (myRandom.nextInt(26) + //Bad statement: ‘A’) + anAccount.lastName = (char) (char) (myRandom.nextInt(26) + (myRandom.nextInt(26) + ‘A’); ‘a’) + (char) In Java, you can use a plus sign to add a (myRandom.nextInt(26) + char value to a string. When you do, the ‘a’); result is a string. This code isn’t simple, but it’s not nearly as bad So “” + (char) (myRandom.nextInt as my original working version. Anyway, here’s (26) + ‘A’) is string containing one ran- how the code works: domly generated uppercase character. And Each call to myRandom.nextInt(26) when you add (char) (myRandom.next Int(26) + ‘a’) onto the end of that generates a number from 0 to 25. string, you get another string — a string con- Adding ‘A’ gives you a number from 65 to 90. taining two randomly generated characters. Finally, when you add another (char) To store a letter ‘A’, the computer puts the (myRandom.nextInt(26) + ‘a’) onto number 65 in its memory. That’s why adding the end of that string, you get a string ‘A’ to 0 gives you 65, and why adding ‘A’ to 25 containing three randomly generated char- gives you 90. For more information on letters acters. So you can assign that big string to being stored as numbers, see the discussion anAccount.lastName. That’s how the of Unicode characters at the end of Chapter 8. statement in Listing 19-3 works. Applying (char) to a number turns the When you write a program like the one in number into a char value. Listing 19-3, you have to be very careful with To store the letters ‘A’ through ‘Z’, the com- numbers, char values and strings. I don’t do puter puts the numbers 65 through 90 in its this kind of programming every day of the week. memory. So applying (char) to a number So before I got this section’s example to work, from 65 to 90 turns the number into an upper- I had many false starts. That’s okay. I’m very case letter. For more information about persistent. applying things like (char), see the dis- cussion of casting in Chapter 7.
- 348. 332 Part IV: Using Program Units The flow of control Suppose that you’re running the code in Listing 19-3. The computer reaches the display method call: anAccount.display(); At that point, the computer starts running the code inside the display method. In other words, the computer jumps to the middle of the Account class’s code (the code in Listing 19-1). After executing the display method’s code (that forest of print and println calls), the computer returns to the point where it departed from Listing 19-3. That is, the computer goes back to the display method call and continues on from there. So when you run the code in Listing 19-3, the flow of action in each loop itera- tion isn’t exactly from the top to the bottom. Instead, the action goes from the for loop to the display method, and then back to the for loop. The whole business is pictured in Figure 19-4. Figure 19-4: The flow of control between Listings 19-1 and 19-3.
- 349. Chapter 19: Creating New Java Methods 333 Using punctuation In Listing 19-3, notice the use of dots. To refer to the lastName stored in the anAccount object, you write anAccount.lastName To get the anAccount object to display itself, you write anAccount.display(); That’s great! When you refer to an object’s variable, or call an object’s method, the only difference is parentheses: To refer to an object’s variable, you don’t use parentheses. To call an object’s method, you use parentheses. When you call a method, you put parentheses after the method’s name. You do this even if you have nothing to put inside the parentheses. The versatile plus sign The program in Listing 19-3 uses some cute tricks. In Java, you can do two dif- ferent things with a plus sign: You can add numbers with a plus sign. For example, you can write numberOfSheep = 2 + 5; You can concatenate strings with a plus sign. When you concatenate strings, you scrunch them together, one right after another. For example, the expression “Barry” + “ “ + “Burd” scrunches together Barry, a blank space, and Burd. The new scrunched- up string is (you guessed it) Barry Burd. In Listing 19-3, the statement anAccount.lastName = “” + (char) (myRandom.nextInt(26) + ‘A’) + (char) (myRandom.nextInt(26) + ‘a’) + (char) (myRandom.nextInt(26) + ‘a’);
- 350. 334 Part IV: Using Program Units has many plus signs, and some of the plus signs concatenate things together. The first thing is a mysterious empty string (“”). This empty string is invisible, so it never gets in the way of your seeing the second, third, and fourth things. Onto the empty string, the program concatenates a second thing. This second thing is the value of the expression (char) (myRandom.nextInt(26) + ‘A’). The expression may look complicated, but it’s really no big deal. This expression represents an uppercase letter (any uppercase letter, generated randomly). Onto the empty string and the uppercase letter, the program concatenates a third thing. This third thing is the value of the expression (char) (myRandom. nextInt(26) + ‘a’). This expression represents a lowercase letter (any lowercase letter, generated randomly). Onto all this stuff, the program concatenates another lowercase letter. So alto- gether, you have a randomly generated three-letter name. For more details, see the sidebar. In Listing 19-3, the statement anAccount.balance = myRandom.nextInt (10000) assigns an int value to balance. But balance is a double variable, not an int variable. That’s okay. In a rare case of permissiveness, Java allows you to assign an int value to a double variable. The result of the assignment is no big surprise. If you assign the int value 8734 to the double variable balance, then the value of balance becomes 8734.00. The result is shown on the first line of Figure 19-3. Let the Objects Do the Work When I was a young object, I wasn’t as smart as the objects you have nowa- days. Consider, for example, the object in Listing 19-4. Not only does this object display itself, the object can also fill itself with values. Listing 19-4: A Class with Two Methods import java.util.Random; import java.text.NumberFormat; import static java.lang.System.out; class BetterAccount { String lastName; int id; double balance;
- 351. Chapter 19: Creating New Java Methods 335 void fillWithData() { Random myRandom = new Random(); lastName = “” + (char) (myRandom.nextInt(26) + ‘A’) + (char) (myRandom.nextInt(26) + ‘a’) + (char) (myRandom.nextInt(26) + ‘a’); id = myRandom.nextInt(10000); balance = myRandom.nextInt(10000); } void display() { NumberFormat currency = NumberFormat.getCurrencyInstance(); out.print(“The account with last name “); out.print(lastName); out.print(“ and ID number “); out.print(id); out.print(“ has balance “); out.println(currency.format(balance)); } } I wrote some code to use the class in Listing 19-4. This new code is in Listing 19-5. Listing 19-5: This Is So Cool! class ProcessBetterAccounts { public static void main(String args[]) { BetterAccount anAccount; for (int i = 0; i < 3; i++) { anAccount = new BetterAccount(); anAccount.fillWithData(); anAccount.display(); } } } Listing 19-5 is pretty slick. Because the code in Listing 19-4 is so darn smart, the new code in Listing 19-5 has very little work to do. This new code just creates a BetterAccount object, and then calls the methods in Listing 19-4. When you run all this stuff, you get results like the ones in Figure 19-3.
- 352. 336 Part IV: Using Program Units Passing Values to Methods Think about sending someone to the supermarket to buy bread. When you do this, you say, “Go to the supermarket and buy some bread.” (Try it at home. You’ll have a fresh loaf of bread in no time at all!) Of course, some other time, you send that same person to the supermarket to buy bananas. You say, “Go to the supermarket and buy some bananas.” And what’s the point of all this? Well, you have a method, and you have some on-the-fly information that you pass to the method when you call it. The method is named “Go to the super- market and buy some. . . .” The on-the-fly information is either “bread” or “bananas,” depending on your culinary needs. In Java, the method calls would look like this: goToTheSupermarketAndBuySome(bread); goToTheSupermarketAndBuySome(bananas); The things in parentheses are called parameters or parameter lists. With para- meters, your methods become much more versatile. Instead of getting the same thing each time, you can send somebody to the supermarket to buy bread one time, bananas another time, and birdseed the third time. When you call your goToTheSupermarketAndBuySome method, you decide right there and then what you’re going to ask your pal to buy. These concepts are made more concrete in Listings 19-6 and 19-7. Listing 19-6: Adding Interest import java.text.NumberFormat; import static java.lang.System.out; class NiceAccount { String lastName; int id; double balance; void addInterest(double rate) { out.print(“Adding “); out.print(rate); out.println(“ percent...”); balance += balance * (rate / 100.0); }
- 353. Chapter 19: Creating New Java Methods 337 void display() { NumberFormat currency = NumberFormat.getCurrencyInstance(); out.print(“The account with last name “); out.print(lastName); out.print(“ and ID number “); out.print(id); out.print(“ has balance “); out.println(currency.format(balance)); } } Listing 19-7: Calling the addInterest Method import java.util.Random; class ProcessNiceAccounts { public static void main(String args[]) { Random myRandom = new Random(); NiceAccount anAccount; double interestRate; for (int i = 0; i < 3; i++) { anAccount = new NiceAccount(); anAccount.lastName = “” + (char) (myRandom.nextInt(26) + ‘A’) + (char) (myRandom.nextInt(26) + ‘a’) + (char) (myRandom.nextInt(26) + ‘a’); anAccount.id = myRandom.nextInt(10000); anAccount.balance = myRandom.nextInt(10000); anAccount.display(); interestRate = myRandom.nextInt(5); anAccount.addInterest(interestRate); anAccount.display(); System.out.println(); } } }
- 354. 338 Part IV: Using Program Units In Listing 19-7, the line anAccount.addInterest(interestRate); plays the same role as the line goToTheSupermarketAndBuySome(bread) in my little supermarket example. The word addInterest is a method name, and the word interestRate in parentheses is a parameter. Taken as a whole, this statement tells the code in Listing 19-6 to execute its addInterest method. This statement also tells Listing 19-6 to use a certain number (whatever value is stored in the interestRate variable) in the method’s calculations. The value of interestRate can be 1.0, 2.0, or whatever other value you get by calling myRandom.nextInt(5). In the same way, the goToTheSupermarket AndBuySome method works for bread, bananas, or whatever else you need from the market. The next section has a detailed description of addInterest and its action. In the meantime, a run of the code in Listings 19-6 and 19-7 is shown in Figure 19-5. Figure 19-5: Running the code in Listing 19-7. Java has very strict rules about the use of types. For example, you can’t assign a double value (like 3.14) to an int variable. (The compiler simply refuses to chop off the .14 part. You get an error message. So what else is new?) But Java isn’t completely unreasonable about the use of types. Java allows you to assign an int value (like myRandom.nextInt(5)) to a double variable (like interestRate). If you assign the int value 2 to the double variable interestRate, then the value of interestRate becomes 2.0. The result is shown on the second line of Figure 19-5. Handing off a value When you call a method, you can pass information to that method on the fly. This information is in the method’s parameter list. Listing 19-7 has a call to the addInterest method: anAccount.addInterest(interestRate);
- 355. Chapter 19: Creating New Java Methods 339 The first time through the loop, the value of interestRate is 2.0. (Remember, I’m using the data in Figure 19-5.) So at that point in the program’s run, the method call behaves as if it’s the following statement: anAccount.addInterest(2.0); The computer is about to run the code inside the addInterest method (a method in Listing 19-6). But first, the computer passes the value 2.0 to the parameter in the addInterest method’s header. So inside the addInterest method, the value of rate becomes 2.0. For an illustration of this idea, see Figure 19-6. Here’s something interesting. The parameter in the addInterest method’s header is rate. But, inside the ProcessNiceAccounts class, the parameter in the method call is interestRate. That’s okay. In fact, it’s standard practice. In Listings 19-6 and 19-7, the names of the parameters don’t have to be the same. The only thing that matters is that both parameters (rate and interestRate) have the same type. In Listings 19-6 and 19-7, both of these parameters are of type double. So everything is fine. Inside the addInterest method, the += assignment operator adds balance * (rate / 100.0) to the existing balance value. For some info about the += assignment operator, see Chapter 7. Figure 19-6: Passing a value to a method’s parameter.
- 356. 340 Part IV: Using Program Units Working with a method header In the next few bullets, I make some observations about the addInterest method header (in Listing 19-6): The word void tells the computer that, when the addInterest method is called, the addInterest method doesn’t send a value back to the place that called it. The next section has an example in which a method sends a value back. The word addInterest is the method’s name. That’s the name you use to call the method when you’re writing the code for the ProcessNiceAccounts class. (See Listing 19-7.) The parentheses in the header contain placeholders for all the things you’re going to pass to the method when you call it. When you call a method, you can pass information to that method on the fly. This information is the method’s parameter list. The addInterest method’s header says that the addInterest method takes one piece of information and that piece of information must be of type double: void addInterest(double rate) Sure enough, if you look at the call to addInterest (down in the ProcessNiceAccounts class’s main method), that call has the variable interestRate in it. And interestRate is of type double. When I call getInterest, I’m giving the method a value of type double. How the method uses the object’s values The addInterest method in Listing 19-6 is called three times from the main method in Listing 19-7. The actual account balances and interest rates are dif- ferent each time: In the first call of Figure 19-5, the balance is 8983.00 and the interest rate is 2.0. When this call is made, the expression balance * (rate / 100.0) stands for 8983.00 * (2.0 / 100.00). See Figure 19-7. In the second call of Figure 19-5, the balance is 3756.00 and the inter- est rate is 0.0. When the call is made, the expression balance * (rate / 100.0) stands for 3756.00 * (0.0 / 100.00). Again, see Figure 19-7.
- 357. Chapter 19: Creating New Java Methods 341 In the third call of Figure 19-5, the balance is 8474.00 and the interest rate is 3.0. When the addInterest call is made, the expression balance * (rate / 100.0) stands for 8474.00 * (3.0 / 100.00). Figure 19-7: Cbj’s account and Bry’s account. Getting a Value from a Method The last section had a story about sending a friend to buy groceries. I revisit that scenario in this section to see what treasures it holds. You make requests for groceries in the form of method calls. You issue calls such as goToTheSupermarketAndBuySome(bread); goToTheSupermarketAndBuySome(bananas); The things in parentheses are parameters. Each time you call your goToThe SupermarketAndBuySome method, you put a different value in the method’s parameter list.
- 358. 342 Part IV: Using Program Units Now what happens when your friend returns from the supermarket? “Here’s the bread you asked me to buy,” says your friend. As a result of carrying out your wishes, your friend returns something to you. You made a method call, and the method returns information (or better yet, the method returns some food). The thing returned to you is called the method’s return value, and the type of thing returned to you is called the method’s return type. An example To see how return values and a return types work in a real Java program, check out the code in Listings 19-8 and 19-9. Listing 19-8: A Method That Returns a Value import java.text.NumberFormat; import static java.lang.System.out; class GoodAccount { String lastName; int id; double balance; double getInterest(double rate) { double interest; out.print(“Adding “); out.print(rate); out.println(“ percent...”); interest = balance * (rate / 100.0); return interest; } void display() { NumberFormat currency = NumberFormat.getCurrencyInstance(); out.print(“The account with last name “); out.print(lastName); out.print(“ and ID number “); out.print(id); out.print(“ has balance “); out.println(currency.format(balance)); } }
- 359. Chapter 19: Creating New Java Methods 343 Listing 19-9: Calling the Method in Listing 19-8 import java.util.Random; import java.text.NumberFormat; class ProcessGoodAccounts { public static void main(String args[]) { Random myRandom = new Random(); NumberFormat currency = NumberFormat.getCurrencyInstance(); GoodAccount anAccount; double interestRate; double yearlyInterest; for (int i = 0; i < 3; i++) { anAccount = new GoodAccount(); anAccount.lastName = “” + (char) (myRandom.nextInt(26) + ‘A’) + (char) (myRandom.nextInt(26) + ‘a’) + (char) (myRandom.nextInt(26) + ‘a’); anAccount.id = myRandom.nextInt(10000); anAccount.balance = myRandom.nextInt(10000); anAccount.display(); interestRate = myRandom.nextInt(5); yearlyInterest = anAccount.getInterest(interestRate); System.out.print(“This year’s interest is “); System.out.println (currency.format(yearlyInterest)); System.out.println(); } } } To see a run of code from Listings 19-8 and 19-9, take a look at Figure 19-8. Figure 19-8: Running the code in Listing 19-9.
- 360. 344 Part IV: Using Program Units How return types and return values work I want to trace a piece of the action in Listings 19-8 and 19-9. For input data, I use the first set of values in Figure 19-8. Here’s what happens when getInterest is called (you can follow along in Figure 19-9): The value of balance is 4084.00, and the value of rate is 1.0. So the value of balance * (rate / 100.0) is 40.84 — forty dollars and eighty-four cents. The value 40.84 gets assigned to the interest variable, so the statement return interest; has the same effect as return 40.84; The return statement sends this value 40.84 back to the code that called the method. At that point in the process, the entire method call in Listing 19-9 — anAccount.getInterest(interestRate) — takes on the value 40.84. Finally, the value 40.84 gets assigned to the variable yearlyInterest. Figure 19-9: A method call is an expression with a value.
- 361. Chapter 19: Creating New Java Methods 345 If a method returns anything, then a call to the method is an expression with a value. That value can be printed, assigned to a variable, added to something else, or whatever. Anything you can do with any other kind of value, you can do with a method call. Working with the method header (again) When you create a method or a method call, you have to be careful to use Java’s types consistently. So make sure you check for the following: In Listing 19-8, the getInterest method’s header starts with the word double. So when the method is executed, it should send a double value back to the place that called it. Again in Listing 19-8, the last statement in the getInterest method is return interest. So the method returns whatever value is stored in the interest variable, and the interest variable has type double. So far, so good. In Listing 19-9, the value returned by the call to getInterest is assigned to a variable named yearlyInterest. Sure enough, yearlyInterest is of type double. That settles it! The use of types in the handling of method getInterest is consistent in Listings 19-8 and 19-9. I’m thrilled!
- 362. 346 Part IV: Using Program Units
- 363. Chapter 20 Oooey GUI Was a Worm In This Chapter Swinging into action Displaying an image Using buttons and textboxes T here’s a wonderful old joke about a circus acrobat jumping over mice. Unfortunately, I’d get sued for copyright infringement if I included the joke in this book. Anyway, the joke is about starting small and working your way up to bigger things. That’s what you do when you read Beginning Programming with Java For Dummies, 2nd Edition. Most of the programs in this book are text-based. A text-based program has no windows, no dialog boxes, nothing of that kind. With a text-based program, the user types characters in the command prompt window, and the program displays output in the same command prompt window. These days, very few publicly available programs are text-based. Almost all programs use a GUI — a Graphical User Interface. So if you’ve read every word of this book up to now, you’re probably saying to yourself, “When am I going to find out how to create a GUI?” I’m sorry, Skipper. A Java program with a GUI takes some muscle to write. The code itself isn’t long or ponderous. What’s difficult is understanding how the code works. So with these inspiring words of discouragement, please march ahead anyway. This chapter gives you a glimpse of the world of GUI programming in Java.
- 364. 348 Part IV: Using Program Units The Java Swing Classes Java’s Swing classes create graphical objects on a computer screen. The objects can include buttons, icons, text fields, check boxes, and other good things that make windows so useful. The name “Swing” isn’t an acronym. When the people at Sun Microsystems were first creating the code for these classes, one of the developers named it “Swing” because swing music was enjoying a nostalgic revival. And yes, in addition to String and Swing, the standard Java API has a Spring class. But that’s another story. Actually, Java’s API has several sets of windowing components. An older set is called AWT — the Abstract Windowing Toolkit. But to use some of the Swing classes, you have to call on some of the old AWT classes. Go figure! Showing an image on the screen The program in Listing 20-1 displays a window on your computer screen. To see the window, look at Figure 20-1. Listing 20-1: Creating a Window with an Image in It import javax.swing.JFrame; import javax.swing.ImageIcon; import javax.swing.JLabel; import java.awt.Container; class ShowPicture { public static void main(String args[]) { JFrame frame = new JFrame(); ImageIcon icon = new ImageIcon(“j2fd.jpg”); JLabel label = new JLabel(icon); Container contentPane = frame.getContentPane(); contentPane.add(label); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } } The code in Listing 20-1 has very little logic of its own. Instead, this code pulls together a bunch of classes from the Java API.
- 365. Chapter 20: Oooey GUI Was a Worm 349 Figure 20-1: What a nice window! Back in Listing 17-3, I create an instance of the Purchase class with the line Purchase onePurchase = new Purchase(); So in Listing 20-1, I do the same kind of thing. I create instances of the JFrame, ImageIcon, and JLabel classes with the following lines: JFrame frame = new JFrame(); ImageIcon icon = new ImageIcon(“j2fd.jpg”); JLabel label = new JLabel(icon); Here’s some gossip about each of these lines: A JFrame is like a window (except that it’s called a JFrame, not a “window”). In Listing 20-1, the line JFrame frame = new JFrame(); creates a JFrame object, but this line doesn’t display the JFrame object anywhere. (The displaying comes later in the code.) An ImageIcon object is a picture. In the directory that contains the Java code, I have a file named j2fd.jpg. That file contains the picture shown in Figure 20-1. So in Listing 20-1, the line ImageIcon icon = new ImageIcon(“j2fd.jpg”); creates an ImageIcon object — an icon containing the j2fd.jpg picture. I need a place to put the icon. I can put it on something called a JLabel. So in Listing 20-1, the line JLabel label = new JLabel(icon); creates a JLabel object and puts the j2fd.jpg icon on the new label’s face. If you read the previous bullets, you may get a false impression. The wording may suggest that the use of each component (JFrame, ImageIcon, JLabel, and so on) is a logical extension of what you already know. “Where do you put an ImageIcon? Well of course, you put it on a JLabel.” When you’ve worked long and hard with Java’s Swing components, all these things become natural to you. But until then, writing GUI code takes hours of trial and error (along with many hours of reading the API documentation).
- 366. 350 Part IV: Using Program Units You never need to memorize the names or features of Java’s API classes. Instead, you keep Java’s API documentation handy. When you need to know about a class, you look it up in the documentation. If you need a certain class often enough, you’ll remember its features. For classes that you don’t use often, you always have the docs. For tips on using Java’s API documentation, see the Appendix on this book’s web site. To find gobs of sample Java code, visit some of the Web sites listed in Chapter 21. Just another class What is a JFrame? Like any other class, a JFrame has several parts. For a sim- plified view of some of these parts, see Figure 20-2. Like the String in Figure 18-6 in Chapter 18, each object formed from the JFrame class has both data parts and method parts. The data parts include the frame’s height and width. The method parts include getContentPane, setDefaultCloseOperation, pack, and setVisible. (I can’t squeeze into setDefaultCloseOperation into Figure 20-2, but I don’t feel guilty about this. All told, the JFrame class has about 320 methods. So with or without the setDefaultCloseOperation method in Figure 20-2, you have to use your imagination.) Figure 20-2: A simplified depiction of the JFrame and Container classes.
- 367. Chapter 20: Oooey GUI Was a Worm 351 For technical reasons too burdensome for this book, you can’t use dots to refer to a frame’s height or width. But you can call many JFrame methods with those infamous dots. In Listing 20-1, I call the frame’s methods by writ- ing frame.getContentPane(), frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE), frame.pack(), and frame.setVisible(true). Here’s the scoop on the JFrame methods in Listing 20-1: You can’t put an icon directly onto a JFrame object. In fact, you can’t put a button, a text field, or anything else like that onto a JFrame object. Instead, you have to grab something called a content pane, and then put these widgets onto the content pane. In Listing 20-1, the call to frame.getContentPane grabs a content pane. Then the call contentPane.add(label) plops the label onto the pane. It seems tedious but, when you work with the Java Swing classes, you have to call all these methods. A call to frame.setDefaultCloseOperation tells Java what to do when you try to close the frame. (In Windows, you click the ‘x’ in the upper-right-hand corner by the title bar.) For a frame that’s part of a larger application, you may want the frame to disappear when you click the ‘x’, but you probably don’t want the application to stop running. But in Listing 20-1, the frame is the entire application — the whole enchi- lada. So when you click the ‘x’, you want the Java virtual machine to shut itself down. To make this happen, you call the setDefaultClose Operation method with parameter JFrame.EXIT_ON_CLOSE. The other alternatives are as follows: • JFrame.HIDE_ON_CLOSE: The frame disappears, but it still exists in the computer’s memory. • JFrame.DISPOSE_ON_CLOSE: The frame disappears and no longer exists in the computer’s memory. • JFrame.DO_NOTHING_ON_CLOSE: The frame still appears, still exists, and still does everything it did before you clicked the ‘x.’ Nothing happens when you click ‘x.’ So with this DO_NOTHING_ ON_CLOSE option, you can become very confused. If you don’t call setDefaultCloseOperation, then Java automatically chooses the HIDE_ON_CLOSE option. When you click the ‘x’, the frame disappears but the Java program keeps running. Of course, with no visible frame, the running of Listing 20-1 doesn’t do much. The only noticeable effect of the run is your development environment’s behavior. With JCreator, you don’t see the familiar Process completed message. In addition, many JCreator menu items (items like Compile Project and Execute Project) are unavailable. To end the Java program’s run and get back your precious menu items, choose Tools➪Stop Tool on JCreator’s main menu.
- 368. 352 Part IV: Using Program Units A frame’s pack method shrink-wraps the frame around whatever has been added to the frame’s content pane. Without calling pack, the frame can be much bigger or much smaller than is necessary. Unfortunately, the default is to make a frame much smaller than neces- sary. If, in Listing 20-1, you forget to call frame.pack, you get the tiny frame shown in Figure 20-3. Sure, you can enlarge the frame by dragging the frame’s edges with your mouse. But why should you have to do that? Just call frame.pack instead. Calling setVisible(true) makes the frame appear on your screen. If you forget to call setVisible(true) (and I often do), when you run the code in Listing 20-1, you’ll see nothing on your screen. It’s always so dis- concerting until you figure out what you did wrong. Figure 20-3: A frame that hasn’t been packed. What I call a “content pane” is really an instance of Java’s Container class, and each Container instance has its own add method. (Refer to Figure 20-2.) That’s why the call to contentPane.add(label) in Listing 20-1 puts the thing that holds the j2fd.jpg picture into the content pane. Keeping the User Busy (Working with Buttons and Text Fields) It takes some muscle to create a high-powered GUI program. First, you create a frame with buttons and other widgets. Then you write extra methods to respond to keystrokes, button clicks, and other such things. The next section contains some “take-my-word-for-it” code to respond to a user’s button clicks. But in this section, the example simply displays a button and a text field. The code is in Listing 20-2, and two views of the code’s frame are shown in Figures 20-4 and 20-5.
- 369. Chapter 20: Oooey GUI Was a Worm 353 Listing 20-2: Adding Components to a Frame import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.JButton; import java.awt.Container; import java.awt.FlowLayout; class MyLittleGUI { public static void main(String args[]) { JFrame frame; Container contentPane; JTextField textfield; JButton button; FlowLayout layout; String sorry; frame = new JFrame(); frame.setTitle(“Interact”); contentPane = frame.getContentPane(); textfield = new JTextField(“Type your text here.”); sorry = “This button is temporarily out of order.”; button = new JButton(sorry); contentPane.add(textfield); contentPane.add(button); layout = new FlowLayout(); contentPane.setLayout(layout); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } } Figure 20-4: The frame in Listing 20-2.
- 370. 354 Part IV: Using Program Units Figure 20-5: The frame in Listing 20-2 with the but- ton pressed. As with other programs that use classes from Java’s API, Listing 20-2 comes with my litany of descriptions and explanations of the classes’ features. One way or another, it’s all the same story. Each object has its own data and its own methods. To refer to an object’s data or methods, use a dot. And to find out more about an object’s data or methods, use Java’s API documentation. Each frame (that is, each instance of the JFrame class) has a setTitle method. If you want, get a pencil and add a setTitle column to the JFrame table in Figure 20-2. In Listing 20-2, I make the frame’s title be the word Interact (as if inter- acting with this frame makes anything useful happen). You can see Interact in the frame’s title bar in Figures 20-4 and 20-5. The JTextField class describes those long white boxes, like the box containing the words Type your text here in Figures 20-4 and 20-5. In Listing 20-2, I create a new text field (an instance of the JTextField class), and I add this new text field to the frame’s content pane. When you run the code in Listing 20-2, you can type stuff into the text field. But, because I haven’t written any code to respond to the typing of text, nothing happens when you type. C’est la vie. The JButton class describes those clickable things, like the thing con- taining the words This button is temporarily out of order in Figures 20-4 and 20-5. In Listing 20-2, I create a new button (an instance of the JButton class), and I add this new button to the frame’s content pane. When you run the code in Listing 20-2, you can click the button all you want. Because I haven’t written any code to respond to the clicking, noth- ing happens when you click the button. For a program that responds to button clicks, see the next section. Each Java container has a setLayout method. A call to this method ensures that the doohickeys on the frame are arranged in a certain way. In Listing 20-2, I feed a FlowLayout object to the setLayout method. This FlowLayout business arranges the text field and the button one right after another (as in Figures 20-4 and 20-5). For descriptions of some other things that are going on in Listing 20-2, see the “Showing an image on the screen” section, earlier in this chapter.
- 371. Chapter 20: Oooey GUI Was a Worm 355 Taking Action The previous section’s code leaves me feeling a little empty. When you click the button, nothing happens. When you type in the text field, nothing hap- pens. What a waste! To make me feel better, I include one more program in this chapter. The pro- gram (in Listings 20-3 and 20-4) responds to a button click. When you click the frame’s button, any text in the text field becomes all uppercase. That’s very nice, but the code is quite complicated. In fact, the code has so many advanced features that I can’t fully describe them in the space that I’m allot- ted. So you may have to trust me. Listing 20-3: Capitalism in Action import javax.swing.JFrame; import javax.swing.JTextField; import javax.swing.JButton; import java.awt.Container; import java.awt.FlowLayout; class CapitalizeMe { public static void main(String args[]) { JFrame frame; Container contentPane; JTextField textfield; JButton button; FlowLayout layout; frame = new JFrame(); frame.setTitle(“Handy Capitalization Service”); contentPane = frame.getContentPane(); textfield = new JTextField(“Type your text here.”, 20); button = new JButton(“Capitalize”); button.addActionListener (new MyActionListener(textfield)); contentPane.add(textfield); contentPane.add(button); layout = new FlowLayout(); contentPane.setLayout(layout); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.pack(); frame.setVisible(true); } }
- 372. 356 Part IV: Using Program Units Listing 20-4: Responding to Button Clicks import javax.swing.JTextField; import java.awt.event.ActionListener; import java.awt.event.ActionEvent; class MyActionListener implements ActionListener { JTextField textfield; MyActionListener(JTextField textfield) { this.textfield = textfield; } public void actionPerformed(ActionEvent e) { textfield.setText (textfield.getText().toUpperCase()); } } You can run the code in Listings 20-3 and 20-4. If you do, you see something like the screen shots in Figures 20-6, 20-7, and 20-8. To get you started reading the code, I include a few hints about the code’s features: Calling new JTextField(“Type your text here.”, 20) creates a text field containing the words Type your text here. To allow more space for the user’s typing, the new text field is 20 characters wide. Java’s API has a package named java.awt.event, which includes things like ActionEvent and ActionListener. • The clicking of a button is an ActionEvent. Other ActionEvent examples include the user’s pressing Enter in a text field or the user’s double-clicking an item in a scrolling list. • An ActionListener is a piece of code that waits for an Action Event to take place. (In other words, the ActionListener “lis- tens” for an ActionEvent.) In Listing 20-3, the call to button.addActionListener tells the Java virtual machine to make an announcement whenever the user clicks the button. The JVM announces the action to the ActionListener code in Listing 20-4. The ActionListener in Listing 20-4 is supposed to do something useful in response to the ActionEvent. The JVM’s “announcement” fires up the actionPerformed method in Listing 20-4, which in turn makes a call to the toUpperCase method. That’s how the letters in the text field become uppercase letters.
- 373. Chapter 20: Oooey GUI Was a Worm 357 Figure 20-6: A brand- new frame. Figure 20-7: The user types in the text box. Figure 20-8: Clicking the button capitalizes the text in the text box. Want to read more? I have a whole chapter about it in Java 2 For Dummies, 2nd Edition (written by yours truly and published by Wiley Publishing, Inc.).
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- 375. Part V The Part of Tens
- 376. In this part . . . Y ou’re near the end of the book, and it’s time to sum it all up. This part of the book is your slam-bam two- thousand-words-or-less resource for Java. What? You didn’t read every word in the chapters before this one? That’s okay. You’ll pick up a lot of useful information in this Part of Tens.
- 377. Chapter 21 Ten Sets of Web Links In This Chapter Finding resources from Sun Microsystems Getting sample code Reading the latest Java news Moving up — jobs, certification, and more Finding out about other useful technologies and languages N o wonder the Web is so popular: It’s both useful and fun. This chap- ter has ten bundles of resources. Each bundle has Web sites for you to visit. Each Web site has resources to help you write programs more effectively. The Horse’s Mouth Sun’s official Web site for Java is java.sun.com. This site has all the latest development kits, and many of them are free. The site also has a great section with online tutorials and mini-courses. The tutorial/mini-course section’s Web address is java.sun.com/developer/onlineTraining. In addition, Sun has two special-purpose Java Web sites. Consumers of Java technology should visit www.java.com. Programmers and developers inter- ested in sharing Java technology can go to www.java.net.
- 378. 362 Part V: The Part of Tens Finding News, Reviews, and Sample Code The Web has plenty of sites devoted exclusively to Java. Many of these sites feature reviews, links to other sites, and best of all, gobs of sample Java code. They may also offer free mailing lists that keep you informed of the latest Java developments. Here’s a brief list of such sites: The JavaRanch: www.javaranch.com Developer.com/Gamelan: www.developer.com/java The Giant Java Tree: www.gjt.org The Java Boutique: javaboutique.internet.com FreewareJava.com: www.freewarejava.com Java Shareware: www.javashareware.com Improving Your Code with Tutorials To find out more about Java, you can visit Sun’s online training pages. Some other nice sets of tutorials are available at the following Web sites: Richard Baldwin’s Web site: www.dickbaldwin.com IBM developerWorks: www-106.ibm.com/developerworks/training ProgrammingTutorials.com: www.programmingtutorials.com Finding Help on Newsgroups Have a roadblock you just can’t get past? Try posting your question on an Internet newsgroup. Almost always, some friendly expert will post just the right reply. With or without Java, you should definitely start exploring newsgroups. You can find thousands of newsgroups — groups on just about every conceivable topic. (Yes, there are more newsgroups than For Dummies titles!) To get started with newsgroups, visit groups.google.com. For postings specific to Java, look for the groups whose names begin with comp.lang.java. As a novice, you’ll probably find the following three groups to be the most useful: comp.lang.java.programmer comp.lang.java.help comp.lang.java.api
- 379. Chapter 21: Ten Sets of Web Links 363 Reading Documentation with Additional Commentary When programmers write documentation, they ask themselves questions and then answer those questions as best they can. But sometimes, they don’t ask themselves all the important questions. And often, they assume that the reader already knows certain things. If you’re a reader who doesn’t already know these things, you may be plain out of luck. One way or another, all documentation omits some details. That’s why other peoples’ comments about the documentation can be so helpful. At www. jdocs.com experienced Java programmers annotate existing Java documen- tation with their own comments. The comments include tips and tricks, but they also add useful pieces of information — pieces that the documentation’s original authors omitted. If you need help with an aspect of the Java API, this is a great Web site to visit. Checking the FAQs for Useful Info Has the acronym FAQ made it to Oxford English Dictionary yet? Everybody seems to be using FAQ as an ordinary English word. In case you don’t already know, FAQ stands for Frequently Asked Questions. In reality, a FAQ should be called ATQTWTOA. This acronym stands for Answers to Questions That We’re Tired of Answering. You can find several FAQs at the official Sun Web site. You can also check www.javafaq.com — a Web site devoted to questions commonly posed by Java programmers. Opinions and Advocacy Java isn’t just techie stuff. The field has issues and opinions of all shapes and sizes. To find out more about them, visit any of these sites: blogs.sun.com www.javablogs.com www.javalobby.org In case you don’t know, a blog is a Web log — an online diary for a person’s thoughts and opinions. Someone who writes a blog is called a blogger.
- 380. 364 Part V: The Part of Tens Blogs are hot stuff these days. Business people, politicians, and others write blogs to draw attention to their ideas. And many people write blogs just for fun. When it comes to reading about Java, I have a few favorite blogs. I list them here in alphabetical order: Simon Phipps’s blog: www.webmink.net/minkblog.htm Simon is Chief Technology Evangelist at Sun Microsystems. No matter what subject he chooses, Simon always speaks his mind. Jonathan Schwartz’s blog: blogs.sun.com/jonathan Jonathan is Chief Operating Officer at Sun Microsystems. When Jonathan speaks, people listen. And when Jonathan writes, people read. Mary Smaragdis’s blog: blogs.sun.com/mary Mary is Marketing Manager at Sun Microsystems. When you read Mary’s blog, her enthusiasm gushes from the computer screen. And I’ve met her at several conferences. She’s even more lively in person. Looking for Java Jobs Are you looking for work? Would you like to have an exciting, lucrative career as a computer programmer? If so, check the SkillMarket at mshiltonj.com/sm. This site has statistics on the demand for various technology areas. The site compares languages, databases, certifications, and more. Best of all, the site is updated every day. After you’ve checked all the SkillMarket numbers, try visiting a Web site designed specially for computer job seekers. Point your Web browser to java.computerwork.com and to www.javajobs.com. Finding Out More about Other Programming Languages It’s always good to widen your view. So to find out more about some languages other than Java, visit the Éric Lévénez site: www.levenez.com/lang. This site includes a cool chart that traces the genealogy of the world’s most popular programming languages. For other language lists, visit the following Web sites:
- 381. Chapter 21: Ten Sets of Web Links 365 HyperNews: www.hypernews.org/HyperNews/get/computing/ lang-list.html Open Here!: www.openhere.com/tech1/programming/languages Steinar Knutsen’s Language list page: home.nvg.org/~sk/ lang/lang.html Finally, for quick information about anything related to computing, visit the foldoc.doc.ic.ac.uk/foldoc — the Free On-Line Dictionary of Computing. Everyone’s Favorite Sites It’s true — these two sites aren’t devoted exclusively to Java. However, no geek-worthy list of resources would be complete without Slashdot and SourceForge. Slashdot’s slogan, “News for nerds, stuff that matters,” says it all. At slashdot. org you find news, reviews, and commentary on almost anything related to computing. There’s even a new word to describe a Web site that’s reviewed or discussed on the Slashdot site. When a site becomes overwhelmed with hits from Slashdot referrals, one says that the site has been slashdotted. Although it’s not quite as high-profile, sourceforge.net is the place to look for open source software of any kind. The SourceForge repository contains over 80,000 projects. At the SourceForge site, you can download software, read about works in process, contribute to existing projects, and even start a pro- ject of your own. SourceForge is a great site for programmers and developers at all levels of experience.
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- 383. Chapter 22 Ten Useful Classes in the Java API In This Chapter Finding out more about some classes that are introduced earlier in this book Discovering some other helpful classes I ’m proud of myself. I’ve written around 400 pages about Java using less than thirty classes from the Java API. The standard API has about 3,000 classes, with at least 700 more in the very popular Enterprise Edition API. So I think I’m doing very well. Anyway, to help acquaint you with some of my favorite Java API classes, this chapter contains a brief list. Some of the classes in this list appear in examples throughout this book. Others are so darn useful that I can’t finish the book without including them. For more information on the classes in this chapter, check Java’s API documentation. Applet What Java book is complete without some mention of applets? An applet is a piece of code that runs inside a Web browser window. For example, a small currency calculator running in a little rectangle on your Web page can be a piece of code written in Java. At one time, Java applets were really hot stuff, but nowadays, people are much more interested in using Java for business processing. Anyway, if applets are your thing, then don’t be shy. Check the Applet page of Java’s API documentation.
- 384. 368 Part V: The Part of Tens ArrayList Chapter 16 introduces arrays. This is good stuff but, in any programming lan- guage, arrays have their limitations. For example, take an array of size 100. If you suddenly need to store a 101st value, then you’re plain out of luck. You can’t change an array’s size without rewriting some code. Inserting a value into an array is another problem. To squeeze “Tim” alphabetically between “Thom” and “Tom”, you may have to make room by moving thousands of “Tyler”, “Uriah”, and “Victor” names. But Java has an ArrayList class. An ArrayList is like an array, except that ArrayList objects grow and shrink as needed. You can also insert new values without pain using the ArrayList class’s add method. ArrayList objects are very useful, because they do all kinds of nice things that arrays can’t do. File Talk about your useful Java classes! The File class does a bunch of things that aren’t included in this book’s examples. Method canRead tells you whether you can read from a file or not. Method canWrite tells you if you can write to a file. Calling method setReadOnly ensures that you can’t accidentally write to a file. Method deleteOnExit erases a file, but not until your pro- gram stops running. Method exists checks to see if you have a particular file. Methods isHidden, lastModified, and length give you even more information about a file. You can even create a new directory by calling the mkdir method. Face it, this File class is powerful stuff! Integer Chapter 18 describes the Integer class and its parseInt method. The Integer class has lots of other features that come in handy when you work with int values. For example, Integer.MAX_VALUE stands for the number 2147483647. That’s the largest value that an int variable can store. (Refer to Table 7-1 in Chapter 7.) The expression Integer.MIN_VALUE stands for the number –2147483648 (the smallest value that an int variable can store). A call to Integer.toBinaryString takes an int and returns its base-2 (binary) representation. And what Integer.toBinaryString does for base 2, Integer.toHexString does for base 16 (hexadecimal).
- 385. Chapter 22: Ten Useful Classes in the Java API 369 Math Do you have any numbers to crunch? Do you use your computer to do exotic calculations? If so, try Java’s Math class. (It’s a piece of code, not a place to sit down and listen to lectures about algebra.) The Math class deals with π, e, logarithms, trig functions, square roots, and all those other mathematical things that give most people the creeps. NumberFormat Chapter 18 has a section about the NumberFormat.getCurrencyInstance method. With this method, you can turn 20.338500000000003 into $20.34. If the United States isn’t your home, or if your company sells products worldwide, you can enhance your currency instance with a Java Locale. For example, with euro = NumberFormat.getCurrencyInstance(Locale.FRANCE), a call to euro.format(3) returns 3,00 € instead of $3.00. The NumberFormat class also has methods for displaying things that aren’t currency amounts. For example, you can display a number with or without commas, with or without leading zeros, and with as many digits beyond the decimal point as you care to include. Scanner Java’s Scanner class can do more than what it does in this book’s examples. Like the NumberFormat class, the Scanner can handle numbers from various locales. For example, to input 3,5 and have it mean “three and half,” you can type myScanner.useLocale(Locale.FRANCE). You can also tell a Scanner to skip certain input strings or use numeric bases other than 10. All in all, the Scanner class is very versatile. String Chapter 18 examines Java’s String class. The chapter describes (in gory detail) a method named equals. The String class has many other useful methods. For example, with the length method, you find the number of characters in a string. With replaceAll, you can easily change the phrase “my fault” to “your fault” wherever “my fault” appears inside a string. And with compareTo, you can sort strings alphabetically.
- 386. 370 Part V: The Part of Tens StringTokenizer I often need to chop strings into pieces. For example, I have a fullName vari- able that stores my narcissistic “Barry A. Burd” string. From this fullName value, I need to create firstName, middleInitial, and lastName values. I have one big string (“Barry A. Burd”), and I need three little strings — “Barry”, “A.”, and “Burd”. Fortunately, the StringTokenizer class does this kind of grunt work. Using this class, you can separate “Barry A. Burd” or “Barry,A.,Burd” or even “Barry<tab>A.<tab>Burd” into pieces. You can also treat each separator as valuable data, or you can ignore each separator as if it were trash. To do lots of interesting processing using strings, check out Java’s StringTokenizer class. System You’re probably familiar with System.in and System.out. But what about System.getProperty? The getProperty method reveals all kinds of infor- mation about your computer. Some of the information you can find includes your operating system name, you processor’s architecture, your Java Virtual Machine version, your classpath, your username, and whether your system uses a backslash or a forward slash to separate folder names from one another. Sure, you may already know all this stuff. But does your Java code need to discover it on the fly?
- 387. Index % (percent sign) remainder operator, • Symbols • 108–111 && (and operator), 158, 161 + (plus sign) += (assignment operator), 339 add char value to string, 331 ** (asterisk, double) Javadoc comment, 57 addition operator, 108 */ (close traditional comment), 56 concatenating strings with, 333–334 == (comparing values), 133, 152, 309 ++ (plus sign, double) ?: (conditional operator), 192–194 postincrement operator, 114–117 {} (curly braces) preincrement operator, 111–114 block and, 155 ‘ (quotation mark), 122 class and, 64 “” (quotation marks, double), 53, 307 example using, 54, 55 ; (semicolon) nested statements and, 171 adding unnecessary, 78 / (division sign), 108 if statement, 145, 146 . (dot), 312, 314 omitting, 77–78 “” (double backslash inside quotation statement and, 61 marks), 229 [] (square brackets), 279 // (end-of-line comment), 56 t (tab stop), 206 = (equal sign) in assignment statement, 92 > (greater than), 133 >= (greater than or equal to), 132, 133 •A• < (less than), 133 abbreviating <= (less than or equal to), 133 class name, 319 - (minus sign), 108 code, 155–156 — (minus sign, double) name of static variable, 320–321 postdecrement operator, 117 abstract method, 80 predecrement operator, 117 Abstract Windowing Toolkit (AWT), 348 * (multiplication sign), 108 Account class n (move cursor to new line), 206 creating instances of, 330 != (not equal to), 133 description of, 327 ! (not operator), 158 display method, 327, 329 /* (open traditional comment), 56 account value, generating at random, || (or operator), 158, 159, 161 326–327 () (parentheses) ActionEvent, 356 conditions and, 168–169 ActionListener code, 356 if statement, 145, 146 actionPerformed method, 356 method and, 328 active project, 36–37 parameters and, 336 AddChips class, 315 ) (parenthesis closing) omitting, 82 AddGuests class, 286–287
- 388. 372 Beginning Programming with Java For Dummies, 2nd Edition Adding Components to Frame program, 353 •B• Adding Interest program, 336–337 Backgammon game, simulating, 157–159 addInterest method backslash, double, inside quotation marks calling, 336–338 (“”), 229 header, 340–341 BadBreaks class, 188–189 amount variable, 91, 95 Beckstrom, Bob, Landscaping For An Answer for Every Occasion program, Dummies, 323 182–183 BetterAccount class, 334–335 and operator (&&), 158, 161 BetterProcessData class, 317 AnotherAgeCheck class, 170–171 BetterShowOneRoom class, 275–276 AnswerYesOrNo program, 147–150 blank line, inserting in program output, 111 API (Application Programming Interface), blank space rule and Scanner 17–18 methods, 106 API documentation block, as statement, 155, 200 description of, 18 blogs, 363–364 downloading and installing, 26–27 body of method, 59, 328–329 reading and understanding, 125 books recommended using, 350 Java 2 For Dummies (Barry Burd), 3, 357 Web site, 363 Landscaping For Dummies (Phillip Giroux, Applet class, 367 Bob Beckstrom, and Lance Application Programming Interface (API), Walheim), 323 17–18 Managing Your Money Online For Are You Paying Too Much? program, Dummies (Kathleen Sindell), 323 160–161 UNIX For Dummies Quick Reference, 4th args identifier, 52 Edition (Margaret Levine Young and array John R. Levine), 323 description of, 277–280 boolean type, 138 enhanced for statement and, 284 boolean variable storing values in, 280–282, 286–287 description of, 131–132, 159 Traveling through Data Both Forwards George Boole Would Be Proud program, and Backwards program, 282–283 165–166 working with, 283–287 Borland JBuilder IDE, 21 ArrayList class, 368 break statement, 184, 188–190 assignment operator Build Output pane (JCreator), 37 description of, 117–118 Build➪Compile Project (JCreator), placement of, 162 37, 43, 79 +=, 339 Build➪Execute Project (JCreator), assignment statement 37, 44, 79 description of, 92–93 Burd, Barry initialization compared to, 100 e-mail address and Web site of, 6 order of, 106–107 Java 2 For Dummies, 3, 357 asterisk, double (**) Javadoc comment, 57 buttons AuntEdnaDoesItAgain class, 251 click, responding to, 355–357 AuntEdnaSettlesForTen class, 246 displaying, 352–354 AWT (Abstract Windowing Toolkit), 348 byte value, 120 bytecode, 12, 15
- 389. Index 373 Character.toUpperCase method, •C• 123–125 Calling addInterest Method program, 337 charAt method, 129–131 calling method Cheat Sheet and keywords, 51 addInterest, 340–341 Check class, 313–314, 322–324 description of, 60 CheckAgeForDiscount class, 167 display, 330 CheckPassword class, 310 equals method of string, 312 class example of, 61, 63 Account, 327, 329–330 FixTheAlternator, 58 AddChips, 315 frame.getContentPane, 351 AddGuests, 286–287 frame.setDefaultCloseOperation, 351 AnotherAgeCheck, 170–171 JFrame, 351 AnswerYesOrNo, 147 myRandom.nextInt, 150, 181–182 Applet, 367 next, 306 ArrayList, 368 nextLine, 306 AuntEdnaDoesItAgain, 251 passing information on the fly, 338–339 AuntEdnaSettlesForTen, 246 program for, 58 BadBreaks, 188–189 static and non-static, 314 BetterAccount, 334–335 System.out.print, 93 BetterProcessData, 317 System.out.println, 62–63, 93, 111 BetterShowOneRoom, 275–276 Calling Method in Listing 19-8 program, 343 CanIKeepKidsQuiet, 131 Calling Object’s Method program, 310 CapitalizeMe, 355 CanIKeepKidsQuiet class, 131 CelsiusToFahrenheit, 134 cannot find symbol error message, Chair, 295 76–77, 122 Check, 313–314, 322–324 Capitalism in Action program, 355 CheckAgeForDiscount, 167 cascading if statement, 172–175 CheckPassword, 310 case clause and switch statement Container, 350, 352, 354 function of, 183–184 creating in JCreator, 41 order of, 186–187 creating traditional way, 290 case sensitivity, 43, 51, 76 curly braces ({}) and, 64 casting, 112–113 DaysInEachMonth, 190–191 CelsiusToFahrenheit class, 134 DebugCheckPassword, 308–309 Chair class, 295 defining methods within, 325–334 char type, 138 description of, 63–64, 294, 313 char value, 331 DisplayHotelData, 254 characters EchoLine, 67 comparing, 137–138 File, 368 conditions and, 164–165 GoodAccount, 342 description of, 122–123 IHopeYouKnowWhatYoureDoing, 256 fetching from keyboard, 72 ImageIcon, 349 reading single, 129–131 import declaration and, 319 sending to computer screen, 70 Integer, 315–316, 368 variable and, 125 interface compared to, 85
- 390. 374 Beginning Programming with Java For Dummies, 2nd Edition class (continued) String, 303–307, 369 JazzyEchoLine, 304 StringTokenizer, 370 JButton, 354 Swing, 348–349 JFrame, 349, 350–352 System, 370 JLabel, 349 TheOldSwitcheroo, 182–183 JTextField, 354 TicketPrice, 160 KeepingKidsQuiet, 104 TicketPriceWithDiscount, 163–164 KeepingMoreKidsQuiet, 105 TryToCheckPassword, 308 ListCombinations, 263 TwoTeams, 153 ListSymbols, 261 using newly defined, 291–293 LowerToUpper, 122 VacanciesInReverse , 282 main method and, 63, 290–293 WinLoseOrTie, 173–174 MakeChange, 109–110 .class file Math, 369 bytecode and, 15 MyActionListener implements description of, 12 ActionListener, 356 failed compilation and, 79 MyExperiment, 124 ‘class’ or ‘interface’ expected MyFirstJavaClass, 43 error message, 83 MyLittleGUI, 353 Class with Two Methods program, 334–335 newly defined, using, 291–293 closing NiceAccount, 336–337 frame, 351 NicePrice, 165–166 traditional comment (*/), 56 NumberFormat, 316–318, 369 COBOL code, 11 object compared to, 300–302 code. See also compiler; computer object-oriented programming and, 289, program; listings; programming 303, 325–326 language; syntax ProcessAccounts, 330 abbreviating, 155–156 ProcessBetterAccounts, 335 converting, to use in class, 292 ProcessData, 290 description of, 10 ProcessGoodAccounts, 343 disk access, 221 ProcessMoreData, 305 indentation of, 54–55, 64, 87, 150, 171 ProcessNiceAccounts, 337 in other languages, comparisons of, 11 ProcessPurchase, 292 running, 13–17 ProcessPurchasesss, 298 spaces in, 87 Purchase, 291, 298, 299–300 translating, 12–13 Random, 148, 149, 150 typing and running own, 38–44 ReverseWord, 127 code template, 220, 221 Scanner, 73, 106, 191, 237–238, combining conditions 273, 276, 369 example of, 159–161 Scoreboard, 177 logical operators and, 158–159 ShowOccupancy, 272 commands ShowOneRoomOccupancy, 274 Build➪Compile Project (JCreator), ShowPicture, 348 37, 43, 79 SnitSoft, 90, 321–322 Build➪Execute Project (JCreator), Special Offer, 151 37, 44, 79
- 391. Index 375 File➪Open Workspace (JCreator), 35 curly braces ({}) issuing, 19 block and, 155 Tools➪Stop Tool (JCreator), 38, 205 class and, 64 comment, 56–57 example using, 54, 55 comparing nested statements and, 171 characters, 137–138 currency variable, 318 numbers, 134–135, 137 cursor, moving to new line, 206 strings, 310, 311–312 values (==), 133, 152, 309 comparison operator, 133–134 •D• compilation, failed, 79 DaysInEachMonth class, 190–191 compiler DebugCheckPassword class, 308–309 description of, 12–13 debugger, 136 downloading and installing, 24–26 debugging, 308–309 component of array, 279 decimal numbers compound statement, 144, 200 comparing, 134–135, 137 computer, as stupid, 65–66 definition of, 96 computer program reading from keyboard, 96–97 application programming interface, 17–18 types of, 118–120 compiler and, 12–13 decision making description of, 10 forks in road, 141–143 disk-oriented, writing, 226–228 if statement, 143–150 looking at someone else’s, 47 declaration text-based, 33, 347 dragging outside of method, 100–101 tools for creating, 12 import, 73, 155–156, 319–320 typing and running, 68–70 method, 59, 60 virtual machine, 13–17 Scanner class, 273 concatenating string, 333–334 static import, 155–156, 321 condition variable, combining, 101–102, 110–111 character and, 164–165 declaring combining, 157–161 method, 58 description of, 132–133 variable, 95–96 for statement and, 275–277 Declaring Variable Inside main Method parentheses and, 168–169 program, 321 conditional operator (?:), 192–194 default clause and switch statement, configuring JCreator LE (Lite Edition) IDE, 184–185, 187 29–31 deleting file, 257–258 Container class, 350, 352, 354 diagnosing errors. See also error messages content pane, 351, 352 case sensitivity, 76 converting code to use in class, 292 debugger and, 136 copying file, 273 disk file, 224–226 count variable, 246–248, 249, 250–251, 295 first message, relying on, 81–83 Creating Window with Image in It mortgage-calculating program, 38 program, 348 overview of, 76 Ctrl+C (stop program run), 205 in punctuation, 77–81
- 392. 376 Beginning Programming with Java For Dummies, 2nd Edition diagnosing errors (continued) downloading run-time error message, 85–86 API documentation, 26–27 same error, different message, 84–85 compiler, 24–26 dice, rolling, 157–159 Java Development Kit (JDK), 25 directory JCreator LE (Lite Edition) IDE, 28–29 docs, 27 drag-and-drop IDE, 20 Java home, 26 dragging declaration outside of method, MyProjects, 35, 39 100–101 disk access code, 221 disk file keyboard/screen program, 217–219 •E• reading from and writing to, 216–217, EchoLine program 219–220, 222 by another programmer, 87 rewriting, 229–231 explanation of, 70–72 running sample program, 222–224 overview of, 66–68 troubleshooting, 224–226 Eclipse IDE, 20 writing String value to, 307 editor disk-oriented program, writing, 226–228 description of, 19 display method syntax coloring and, 39 calling, 330 element of array, 279 description of, 326, 328–329 else clause and if statement, 151–153 flow of control, 332 end-of-line comment (//), 56 DisplayHotelData class, 254 enhanced for statement. See also for displaying statement button and text field, 352–354 creating, 261–263 data in reverse, 277–278, 282–283 nesting, 263–267 String value, 307 stepping through array values with, 284 division sign (/), 108 enum type do statement creating, 176 description of, 258–259 description of, 175 syntax, 259–260 using, 176–179 Do You Have a Coupon? program, 163–164 equal sign (=) in assignment statement, 92 docs directory, 27 equals method, 311–312 dot (.), 312, 314 error messages double asterisk (**) Javadoc comment, 57 cannot find symbol, 76–77, 122 double backslash inside quotation marks ‘class’ or ‘interface’ (“”), 229 expected, 83 double keyword, 95–96 if statement, 146 double minus sign (—), as predecrement InputMismatchException, 131 and postdecrement operators, 117 interpreting, 76–77, 78–83 double plus sign (++) loop within loop, 241–242 postincrement operator, 114–117 missing method body, or declare preincrement operator, 113 abstract, 80 double quotation marks (“”), 53, 307 NullPointerException, 242 double value, 112–113, 130, 137 run-time, 85–86 double variable, 119, 120
- 393. Index 377 errors, diagnosing errors. See also error FileNotFoundException expression, 221 messages File➪Open Workspace (JCreator), 35 case sensitivity, 76 Finding the Number of Days in a Month debugger and, 136 program, 190–191 disk file, 224–226 findInLine method, 129–131 first message, relying on, 81–83 FixTheAlternator method, 58 mortgage-calculating program, 38 Fletcherism, 245 overview of, 76 float value, 120 in punctuation, 77–81 FlowLayout object, 354 run-time error message, 85–86 for statement. See also enhanced for same error, different message, 84–85 statement escape sequence, 206 in action, 271–273 examples in book and version of Java, conditions in, 275–277 3, 21, 23 description of, 246 expecting unexpected, 74–75 example of, 246–248 experimenting with test program, 123–125 initializing, 250–251 expression nesting, 252–254 description of, 115, 132–133 syntax, 248–250 FileNotFoundException, 221 while statement compared to, 249 new File(“rawData.txt”), 221 forks in road of decision making, 141–143 regular, 129 frame.getContentPane method, 351 frames, window •F• creating, 348–350 JFrame class and, 350–352 fall-through setTitle method, 354 break statement and, 188–190 frame.setDefaultCloseOperation taking advantage of, 190–192 method, 351 false value, 130–131 fully qualified name, 73, 319 FAQ sites, 363 Faulty Password Checker program, 308 file. See also disk file •G• .class, 12, 15, 79 General Output pane (JCreator), 33–34, copying, 273 37–38 deleting, 257–258 generating input, 222–223, 273 occupancy report, 272–277 java file, 17 random number, 148–149 javac file, 13 random word, 331 naming, 229 George Boole Would Be Proud program, output, viewing, 224 165–166 running code that straddles two, 293–294 getCurrencyInstance method, 317 source, creating project with two, getInterest method, 342–345 179–180 GetUserName program String.java, 326 fixing problem with, 212–214 File class, 368 listing, 207–209 File View pane (JCreator), 36–37, 224 working on problem with, 209–212 filename extensions, 34, 227, 228
- 394. 378 Beginning Programming with Java For Dummies, 2nd Edition Giroux, Phillip, Landscaping For identifiers Dummies, 323 with agreed upon meanings, 52–53 goals of programming, 176 compiler and, 86 GoodAccount class, 342 description of, 52 greater than (>), 133 naming, 86 greater than or equal to (>=), 132, 133 variable name as, 92 GUI (Graphical User Interface) if statement button click, responding to, 355–357 cascading, 172–175 buttons and text fields, 352–354 complete program using, 147–150 description of, 347 conditions with characters and, 164–165 frame, creating, 350–352 indenting, 150 showing image on screen, 348–349 listing, 143 mixing logical operators and, 166–168 •H• nested, 170–172 nesting within switch statement, 191 hard drive, storing data on one-statement rule and, 155 keyboard/screen program for, 217–219 packing more into, 153–154 overview of, 215 parentheses and, 168–169 reading from and writing to file, switch statement compared to, 187–188 219–220, 222 syntax, 143–146 rewriting disk file, 229–231 without else clause, 151–153 running disk-oriented program, 216–217 IHopeYouKnowWhatYoureDoing class, 256 testing code, 222–224 I’m Repeating Myself Again (Again) troubleshooting disk file, 224–226 program, 304 writing disk-oriented program, 226–228 image, showing on screen, 348–349 hasNext method, 237–238, 276 ImageIcon class, 349 header for method import declaration addInterest method, 340 description of, 319 description of, 59 examples of, 155–156 display method, 328 packages and, 319–320 getInterest method, 345 Scanner class and, 73 Hide Extensions feature (Microsoft static, 155–156, 321 Windows), 34 import java.util.Scanner line, 74 host name, 234 In Case of a Tie... program, 173–174 How display Method Behaves When No indentation of code One’s Looking program, 329 example of, 64, 87 if statement, 150 nested statements and, 171 •I• punctuation and, 54–55 I Know Everything program, 147 index, 279 IDE (integrated development infinite loop, 204 environment). See also JCreator LE initializing (Lite Edition) IDE for statement, 250–251 description of, 19–20 variable, 99–100, 161–162 Java tools with, 21 inner loop, 237 JCreator LE (Lite Edition), 21–22
- 395. Index 379 input file creating, 222–223 •J• naming, 273 Java Development Kit (JDK) InputMismatchException error description of, 28 message, 131 downloading, 25 inserting blank line in program output, 111 java file, 17, 21 installing Java home directory, 26 API documentation, 26–27 Java, overview of, 319 compiler, 24–26 Java Runtime Environment (JRE), 28 JCreator LE (Lite Edition) IDE, 28–29 Java 2 Enterprise Edition (J2EE), 25 instance, 297, 302 Java 2 For Dummies (Barry Burd), 3, 357 int value Java 2 Micro Edition (J2ME), 25 comparing, 309 Java 2 Standard Edition (J2SE), 24, 25 description of, 105 Java Virtual Machine (JVM) dividing by another int value, 108 description of, 13–17 double value and, 112–113 popularity of, 18 Integer class compared to, 316 java.awt.event package, 356 Integer class, 315–316, 368 javac file, 13 integrated development environment Javadoc comment (**), 57 (IDE). See also JCreator LE (Lite javadoc program, 57 Edition) IDE Javadocs description of, 19–20 description of, 18 Java tools with, 21 downloading and installing, 26–27 JCreator LE (Lite Edition), 21–22 reading and understanding, 125 interface, class compared to, 85 using, 350 Internet resources Web site, 363 author, 6 java.lang package, 319–320 blogs, 363–364 java.util package, 319 Eclipse IDE, 20 JazzyEchoLine class, 304 Javadocs, 363 JButton class, 354 Java-related, 362 JCreator LE (Lite Edition) IDE Jigloo graphical plug-in, 20 Build Output pane, 37 for job seekers, 364 Class Wizard, 41, 42 opinions and advocacy, 363 code templates and, 222 programming languages, 364–365 configuring, 29–31 Slashdot, 365 creating project in MyWorkspace, 39–44 SourceForge, 365 description of, 21–22 Sun Microsystems, 25, 361 downloading and installing, 28–29 tutorials, 362 error messages in, 77 Wiley, 141 File View pane, 36–37, 224 IOException clause, 257 General Output pane, 33–34, 37–38 ISETL code, 11 jdb debugger and, 136 issuing commands, 19 launching, 35 iteration of loop, 197 project, creating with two source files, 179–180
- 396. 380 Beginning Programming with Java For Dummies, 2nd Edition JCreator LE (Lite Edition) IDE (continued) language Project Wizard, 40, 41 comments and, 56–57 stopping program run in, 205 data-gathering features of, 303 Task List pane, 44 English compared to Java, 49–50 work area, 30–31, 35 identifiers, 52–53 workspaces, 36 keywords, 50–51 JCreator Setup Wizard, 29, 30 literals, 53–54 .jcw extension, 36 punctuation, 54–55 jdb debugger, 136 Web sites, 364–365 JDK (Java Development Kit) launching JCreator, 35 description of, 28 less than (<), 133 downloading, 25 less than or equal to (<=), 133 JFrame class Lévénez, Eric, Web site of, 364 description of, 349, 350–352 Levine, John R., UNIX For Dummies Quick setTitle method, 354 Reference, 4th Edition, 323 Jigloo graphical plug-in, 20 lineIn variable, 304 JLabel class, 349 Linux JRE (Java Runtime Environment), 28 IDE for, 22 JTextField class, 354 Runtime.getRuntime().exe code J2EE (Java 2 Enterprise Edition), 25 for, 257 J2ME (Java 2 Micro Edition), 25 ListCombinations class, 263 J2SE (Java 2 Standard Edition), 24, 25 listings JVM (Java Virtual Machine) Account Class, 327 description of, 13–17 Adding Components to Frame, 353 popularity of, 18 Adding Interest, 336–337 Answer for Every Occasion, 182–183 •K• Are You Paying Too Much?, 160 Aren’t You Lucky?, 151 KeepingKidsQuiet program Attempt to Debug Code in Listing 18-3, description of, 103–105 308–309 revision to, 105–106 boolean type, 131 KeepingMoreKidsQuiet class, 105 Calling addInterest Method, 337 keyboard, fetching characters from, 71 Calling Method in Listing 19-8, 343 keyboard input, program to echo, 66–68 Calling Object’s Method, 310 keyboard, reading from Capitalism in Action, 355 decimal number, 96–97 char type, 122 whole number, 105–106 Character.toUpperCase method, 124 keywords Class with Two Methods, 334–335 compiler and, 86 computer program example, 10 description of, 50–51 creating class in traditional way, 290 double, 95–96 Creating Window with Image in It, 348 Declaring Variable Inside main Method, 321 •L• displaying things, 43 Landscaping For Dummies (Phillip Do You Have a Coupon?, 163–164 Giroux, Bob Beckstrom, and Double Value from Keyboard, 97 Lance Walheim), 323 EchoLine program, 67
- 397. Index 381 enum type, 176 Right Way to Display Dollar Amount, 317 Faulty Password Checker, 308 Roll 7 or 11, 196 Finding Number of Days in Month, Slot Machine Symbols, 261 190–191 SnitSoft’s Grand Scam, 90 From One File to Another, 235 Storing Occupancy Data in Array, 286–287 generating occupancy report, 272 Template to Read Data from Disk File, George Boole Would Be Proud, 165–166 219–220 Hey! Is This a For-by-For?, 254 Template to Write Data to Disk File, Horace Fletcher’s Revenge, 246 220, 222 How display Method Behaves When No That’s Much Better!, 243 One’s Looking, 329 ThingsILike program, 48 How to Prime Loop, 213 This Is So Cool!, 335 if statement, 143 Traveling through Data Both Forwards if statement, complete program using, and Backwards, 282 147–150 Trying to Get Username from E-mail I’m Repeating Myself Again (Again), 304 Address, 207–208 In Case of Tie..., 173–174 Twenty-One card game, simplified It’s Warm and Cozy in Here, 134 version, 206 KeepingKidsQuiet program, 104 Using Input and Output Files, 218–219 Listing Combinations, 263 Using Keyboard and Screen, 218 Listing Symbols, 261 Using Variable Declared Outside for Little Experiment, 230 Loop, 251 MakeChange program, 109–110 What It Means to Be a Chair, 295 Making Use of Code in Listing 19-1, 330 What It Means to Be a Purchase, 291 Making Use of Your Purchase Class, 292 ListOneUsername program Making Word Go Backwards, 127 description of, 234–235 May Best Team Be Displayed First, 153 listing, 235 Mechanical Combining of Two Loops, 237 running, 236 Method That Returns Value, 342 ListSymbols class, 261 More Chips, Please, 315 Lite Edition (JCreator LE) IDE Nested if Statements, 170–171 Build Output pane, 37 Nice Try, But..., 211–212 Class Wizard, 41, 42 No Extra Break for Kids or Seniors, 167 code templates and, 222 Please, Gimme a Break!, 188–189 configuring, 29–31 Processing Purchases, 298 creating project in MyWorkspace, 39–44 Proud Winners and Sore Losers, 177 description of, 21–22 Pulling Variable Outside main Method downloading and installing, 28–29 program, 322 error messages in, 77 Putting Name in String Variable, 305 File View pane, 36–37, 224 Refined Version of One-Room Code, General Output pane, 33–34, 37–38 275–276 jdb debugger and, 136 Repeat Before You Delete, 255 launching, 35 Report on One Room Only, Please, 274 project, creating with two source files, Responding to Button Clicks, 356 179–180 revised KeepingKidsQuiet program, Project Wizard, 40, 41 105–106 stopping program run in, 205
- 398. 382 Beginning Programming with Java For Dummies, 2nd Edition Lite Edition (JCreator LE) IDE (continued) Making Use of Code in Listing 19-1 Task List pane, 44 program, 330 work area, 30–31, 35 Managing Your Money Online For Dummies workspaces, 36 (Kathleen Sindell), 323 literals Math class, 369 char, 122 math operator, 108 description of, 53–54, 307 Mechanical Combining of Two Loops logical operators program, 237 description of, 158 method mixing together, 166–168 abstract, 80 long value, 120 actionPerformed, 356 loop. See also specific statements addInterest, 336–338, 340–341 analyzing, 239–241 body of, 328–329 assembling pieces of code, 203 calling, 58, 61, 63, 310–311, 330 brainstorming strategies for, 200–202 Character.toUpperCase, 123–125 deciding limit at runtime, 274–275 charAt, 129–131 error message, 241–242 creating, 326–327 following action in, 197–199 declaring, 58 infinite, 204 defining within class, 325–334 from infinity to affinity, 205 description of, 57 inner, 237 display, 326, 328–329, 330, 332 iteration of, 197 dragging declaration outside of, 100–101 within loop, composing, 233–234, equals, 311–312 236–237, 243–244 findInLine, 129–131 priming, 207–209, 212–214 FixTheAlternator, 58 Repeat Before You Delete program, flow of control, 332 254–257 frame.getContentPane, 351 testing, 237–238 frame.setDefaultCloseOperation, 351 values for variables, 203–205 getCurrencyInstance, 317 working on problem with, 209–212 getInterest, 342–345 LowerToUpper class, 122 getting value from, 341–345 hasNext, 237–238, 276 •M• header for, 328, 340, 345 JFrame, 351 Macintosh main, 60, 62, 63, 290–294, 314, 321–323 IDE for, 22 myRandom.nextInt(), 148–150, 157–159 Java Web site for, 25 myScanner.nextInt(), 72 main identifier, 53 myScanner.nextLine(), 71–72 main method name of, 328 class and, 63 next method, 306 creating class and, 290–293 nextDouble, 97–99, 130 running code with multiple, 293–294 nextInt, 106, 130 static variable and, 314, 321–323 nextLine method, 306 ThingsILike program, 60, 62 non-static, calling, 314 MakeChange program, 109–110 object with two, 334–335
- 399. Index 383 pack, 352 MyLittleGUI class, 353 parentheses and, 328 MyProjects directory, 35, 39 parseInt, 316 myRandom.nextInt() method passing values to, 336–339 calling, 181–182 print, 227–228 description of, 148–150 println, 227–228 rolling dice and, 157–159 referring to, 333 myScanner.nextInt() method, 72 Runtime.getRuntime().exec, 257 myScanner.nextLine() method, 71–72 scanner, 72, 106 setDefaultCloseOperation, 351 setLayout, 354 •N• setTitle, 354 n (move cursor to new line), 206 setVisible(true), 352 name sort, 313–314, 322–324 of class, 319 static, 101, 313–314 fully qualified, 73, 319 System.out.print, 93 of method, 328 System.out.println, 62, 70, 93, 111 of variable, 91, 320–321 terminology for, 58–60 naming ThingsILike program, 62–63 conventions for, 52 toUpperCase, 356 file, 229 using, 307–310, 313 identifiers, 86 Method That Returns Value program, 342 input file, 273 Microsoft Windows nested loop, 237 Hide Extensions feature, 34 nesting JavaVirtual Machine and, 22 enhanced for statements, 263–267 JCreator and, 22 for statements, 252–254 IDE for, 22 if statements, 170–172, 192 Runtime.getRuntime().exec code new File(“rawData.txt”) for, 257 expression, 221 minus sign (-), 108 newsgroups, 75, 362 minus sign, double (—) next method, 306 postdecrement operator, 117 nextDouble method, 97–99, 130 predecrement operator, 117 nextInt method, 106, 130 missing method body, or declare nextLine method, 306 abstract error message, 79–80 NiceAccount class, 336–337 mixing logical operators, 166–168 NicePrice class, 165–166 More Chips, Please program, 315 niceTotal variable, 318 mortgage-calculating program, running, No Extra Break for Kids or Seniors 33–38 program, 167 moving cursor to new line, 206 non-numeric types, primitive, 138 multiplication sign (*), 108 non-static method, calling, 314 MyActionListener implements NoSuchElementException, 275 ActionListener class, 356 not equal to (!+), 133 MyExperiment class, 124 not operator (!), 158 MyFirstJavaClass class, 43
- 400. 384 Beginning Programming with Java For Dummies, 2nd Edition NullPointerException error on-the-fly information, 336 message, 242 OOP (object-oriented programming) NumberFormat class, 316–318, 369 classes and, 289, 303, 325–326 numbers description of, 63–64 adding, 333 essence of, 312 comparing, 134–135, 137 objects and, 302 pseudorandom, 149 opening random, generating, 148–149 Javadoc comment (/**), 57 turning into strings, 316–318 traditional comment (/*), 56 turning strings into, 315–316 operators numbers using decimal assignment, 117–118, 162, 339 comparing, 134–135, 137 comparison, 133–134 definition of, 96 conditional, 192–194 reading from keyboard, 96–97 logical, 158, 166–168 types of, 118–120 math, 108 numbers, whole postincrement, 114–117 comparing, 134 predecrement and postdecrement, 117 overview of, 103–105 preincrement, 111–114 reading from keyboard, 105–106 remainder, 108–111, 112–113 remainder operator and, 112–113 or operator (||), 158, 159, 161 types of, 118–120 out identifier, 53 numeric types, primitive, 120 out variable, 320–321 outer loop, 237 •O• output blank line, inserting in, 111 object lining up, 206 calling methods of, 310–311 wrapping, 93–94 class compared to, 300–302 output file, viewing, 224 creating multiple, 298–300 overusing punctuation, 78–81 description of, 294–297 instance compared to, 297 methods, using, 307–310, 313 •P• referring to parts of, 297 pack method, 352 with two methods, 334–335 package object code, 12–13 java.awt.event, 356 object-oriented programming (OOP) java.lang, 319–320 classes and, 289, 303, 325–326 java.util, 319 description of, 63–64 parameters essence of, 312 description of, 336 objects and, 302 example of, 338 occupancy report, generating, 272–277 JFrame methods, 351 offline installation, 25 names of, 339 omitting punctuation, 77–78 parentheses [()] one-statement rule, 155 conditions and, 168–169 online installation, 26 if statement, 145, 146
- 401. Index 385 method and, 328 punctuation, 54–55 parameters and, 336 Web sites, 364–365 parenthesis, closing [)], omitting, 82 project parseInt method, 316 active, 36–37 passing value to method, 336–339 creating in JCreator MyWorkspace, 39–44 percent sign (%) remainder operator, creating with two source files, 179–180 108–111 prompt, 98 Phipps, Simon, blog of, 364 Proud Winners and Sore Losers Please, Gimme a Break! program, 188–189 program, 177 plus sign (+) pseudorandom number, 149 add char value to string, 331 public keyword, 51 addition operator, 108 Pulling Variable Outside main Method concatenating strings with, 333–334 program, 322 plus sign double (++) punctuation. See also specific punctuation postincrement operator, 114–117 finding omitted, 77–78 preincrement operator, 111–114 if statement, 145–146 portability, 18 overview of, 54–55 price variable, 160, 161–162 parenthesis, closing [)], omitting, 82 priming while loop, 207–209, 212–214 for referring to variables and primitive types methods, 333 non-numeric, 138 using too much, 78–81 numeric, 120 Purchase class, 291, 298, 299–300 object-oriented programming and, 289 reference type compared to, 300, 309 print method, 227–228 •Q• println identifier, 53 quotation mark (‘), 122 println method, 227–228 quotation marks PrintStream, 221 double (“”), 53, 307 ProcessAccounts class, 330 double, inside quotation marks ProcessBetterAccounts class, 335 (“”), 229 ProcessData class, 290 ProcessGoodAccounts class, 343 ProcessMoreData class, 305 •R• ProcessNiceAccounts class, 337 Random class, 148, 149, 150 ProcessPurchase class, 292 random number, generating, 148–149 ProcessPurchasesss class, 298 random words, generating, 331 programmer, 10–11, 98 reading programming, goals of, 176 characters, 129–131 programming language data from disk file, 219–220, 227 comments, 56–57 decimal number from keyboard, 96–97 data-gathering features of, 303 String value from keyboard, 306 description of, 11, 49–50 whole number from keyboard, 105–106 identifiers, 52–53 reference type keywords, 50–51 description of, 310 literals, 53–54 example of, 291 primitive type compared to, 300, 309
- 402. 386 Beginning Programming with Java For Dummies, 2nd Edition regular expression, 129 setLayout method, 354 remainder operator (%), 108–111, 112–113 setTitle method, 354 Repeat Before You Delete program, setVisible(true) method, 352 254–257 short value, 120 responding to button click, 355–357 showing image on screen, 348–349 Responding to Button Clicks program, 356 ShowOccupancy class, 272 return type, 341–345 ShowOneRoomOccupancy class, 274 return value, 341–345 ShowPicture class, 348 reusing variable, 125–127 simple name, 319 reversing letters in word, 127–129 Sindell, Kathleen, Managing Your Money rewriting disk file, 229–231 Online For Dummies, 323 Right Way to Display Dollar Amount SkillMarket Web site, 364 program, 317 skinning cat, origins of phrase, 150 Roll 7 or 11 program, 196 Slashdot Web site, 365 running slot machine program, 261–267 canned program, 33–38 Smaragdis, Mary, blog of, 364 code, 13–17 SnitSoft class, 90, 321–322 code that straddles two files, 293–294 software, open source, Web site for, 365 disk-oriented program, 216–217 sorting, 321–324 own code, 38–44 source code, 12–13 run-time error message, 85–86 source files, creating project with two, Runtime.getRuntime().exec method, 179–180 256–257 SourceForge Web site, 365 spaces in code, 87 •S• SpecialOffer program, 151–153 square brackets ([]), 279 Scanner class statement. See also specific statements declarations, 273 assignment, 92–93, 100, 106–107 description of, 73, 369 block as, 155, 200 hasNext method, 237–238, 276 break statement, 184, 188–190 methods, 72, 106 compound, 144, 200 nextBoolean method, 292 description of, 61 Scanner myScanner - new expression and, 115 Scanner(System.in) line, 74 semicolon (;) and, 61 Schwartz, Jonathan, blog of, 364 static import declaration, 155–156, 321 Scoreboard class, 177 static keyword, 51 screen static method, 101, 313–314 sending characters to, 70 static variable, 320–323 showing image on, 348–349 stopping program, 38, 205 semicolon (;) storing data on hard drive adding unnecessary, 78 keyboard/screen program for, 217–219 if statement, 145, 146 overview of, 215 omitting, 77–78 reading from and writing to file, statement and, 61 219–220, 222 setDefaultCloseOperation method, 351 rewriting disk file, 229–231 running disk-oriented program, 216–217
- 403. Index 387 testing code, 222–224 System identifier, 53 troubleshooting disk file, 224–226 System.out, abbreviating, 320–321 writing disk-oriented program, 226–228 System.out.print method, 93 storing value in array, 280–282, 286–287 System.out.println method, 62, string 70, 93, 111 adding char value to, 331 comparing, 310, 311–312 concatenating, 333–334 •T• turning into number, 315–316 /t (tab stop), 206 turning number into, 316–318 tables, 301–302 String class Task List pane (JCreator), 44 description of, 303–304, 369 template, 220, 221 equals method and, 311–312 test program, writing, 123–125 example of, 304 text field, displaying, 352–354 reading and writing values, 306–307 text-based program, 33, 347 variables, 305–306 That’s Much Better! program, 243 String identifier, 53 TheOldSwitcheroo class, 182–183 String type ThingsILike program comparisons and, 137, 309 code for, 48 storing word or sentence and, 125 explanation of, 49 String.java file, 326 main method, 60, 62 StringTokenizer class, 370 methods, 62–63 style and writing code, 87 This Is So Cool! program, 335 Sun Microsystems Web site, 25, 361 throws clause, 221 support, asking for, 75 TicketPrice program, 160–162 Swing class, 348–349 TicketPriceWithDiscount class, switch statement 163–164 break statement and, 188–189 tools. See also compiler; editor; Java case clause and, 183–184, 186–187 Virtual Machine (JVM) default clause and, 184–185, 187 Application Programming Interface, 17–18 description of, 181–182 with IDE, 21 example of, 182–183 integrated development environment, fall-through and, 190–192 19–20 nesting if statement within, 191 Tools➪Stop Tool (JCreator), 38, 205 syntax, 185–188 total variable, 318 syntax toUpperCase method, 356 cascading if statement, 174 traditional comment (/* and */), 56 do statement, 259–260 translating code, 12–13 enhanced for statement, 261 tree variable, 322–324 for statement, 248–250 troubleshooting if statement, 143–146 cannot find symbol, 76–77, 122 switch statement, 185–188 case sensitivity, 76 while statement, 196, 255 ‘class’ or ‘interface’ syntax coloring in editor, 39 expected, 83 System class, 370 disk file, 224–226
- 404. 388 Beginning Programming with Java For Dummies, 2nd Edition troubleshooting (continued) first error message, relying on, 81–83 •V• if statement, 146 VacanciesInReverse class, 282 InputMismatchException, 131 value interpreting, 76–77, 78–83 array and, 279 jdb debugger and, 136 byte, 120 loop within loop, 241–242 char, 331 mortgage-calculating program, 38 combining, 289 missing method body, or declare comparing (==), 133, 152, 309 abstract, 80 description of, 91 NullPointerException, 242 double, 112–113, 130, 137 punctuation, 77–81 float, 120 run-time error message, 85–86 getting from method, 341–345 same error, different message, 84–85 int, 105, 108, 112–113, 316 true value, 130–131 long, 120 Trying to Get a Username from an E-mail passing to method, 336–339 Address repeating with predetermined fixing problem with, 212–214 return, 341–345 listing, 207–209 short, 120 working on problem with, 209–212 storing in array, 280–282, 286–287 TryToCheckPassword class, 308 String class and, 306–307 tutorial Web sites, 362 true or false, 130–131 Twenty-One card game simulation variable assembling pieces of code, 203 amount, 91, 95 brainstorming strategies for, 200–202 assignment statement and, 92–93 listing, 206 boolean, 131–132, 159, 165–166 values for variables, 203–205 char, 125 TwoTeams class, 153 combining declarations, 101–102, 110–111 type of variable, 91, 95–96, 338 count, 246–248, 249, 250–251, 295 typefaces in book, 2 creating new rather than reusing, 127–129 typing code, 38–44 currency, 318 declaring, 95–96 •U• double, 119 initializing, 99–100, 161–162 Unicode, 138 int, 112 Unix lineIn, 304 IDE for, 22 loop and, 203–205 Runtime.getRuntime().exec code name of, 91, 320–321 for, 257 niceTotal, 318 UNIX For Dummies Quick Reference, 4th out, 320–321 Edition (Margaret Levine Young and overview of, 89–90 John R. Levine), 323 price, 160, 161–162 user, 98 referring to, 333 reusing, 125–127
- 405. Index 389 static, 320–323 reading from keyboard, 105–106 String class, 305–306 remainder operator and, 112–113 total, 318 types of, 118–120 tree, 322–324 window with image in it, creating, 348–350 types of, 91, 95–96, 338 Windows (Microsoft) using, 90–92 Hide Extensions feature, 34 using outside for statement, 250–251 JavaVirtual Machine and, 22 version of Java JCreator and, 22 examples and, 3, 21, 23 IDE for, 22 numbering of, 24 Runtime.getRuntime().exec code viewing output file, 224 for, 257 Visual Basic code, 11 WinLoseOrTie class, 173–174 void keyword, 51 word generating randomly, 331 •W• reversing, 127–129 wrapping output, 93–94 Walheim, Lance, Landscaping For Dummies, Write Once, Run Anywhere model of 323 computing, 18 Web sites writing author, 6 code, style and, 87 blogs, 363–364 to disk file, 220, 222, 227–228 Eclipse IDE, 20 disk-oriented program, 226–228 Javadocs, 363 String value to disk file, 307 Java-related, 362 Jigloo graphical plug-in, 20 for job seekers, 364 •Y• opinions and advocacy, 363 Young, Margaret Levine, UNIX For programming languages, 364–365 Dummies Quick Reference, 4th Slashdot, 365 Edition, 323 SourceForge, 365 Sun Microsystems, 25, 361 tutorials, 362 •Z• Wiley, 141 zero-one sequences, 94–95 while statement as compound statement, 200 condition and, 254–255 example of, 196 following action in, 197–199 for statement compared to, 249 priming, 207–209, 212–214 syntax, 196 working on problem with, 209–212 whole numbers comparing, 134 overview of, 103–105
- 406. 390 Beginning Programming with Java For Dummies, 2nd Edition
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