JavaYDL14

Chapter 14 Exception Handling
and Text IO

Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
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Motivations
When a program runs into a runtime error, the
program terminates abnormally. How can you
handle the runtime error so that the program can
continue to run or terminate gracefully? This is the
subject we will introduce in this chapter.

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Objectives
 To get an overview of exceptions and exception handling (§14.2).
 To explore the advantages of using exception handling (§14.2).
 To distinguish exception types: Error (fatal) vs. Exception (nonfatal) and checked vs. unchecked (§14.3).
 To declare exceptions in a method header (§14.4.1).
 To throw exceptions in a method (§14.4.2).
 To write a try-catch block to handle exceptions (§14.4.3).
 To explain how an exception is propagated (§14.4.3).
 To obtain information from an exception object (§14.4.4).
 To develop applications with exception handling (§14.4.5).
 To use the finally clause in a try-catch block (§14.5).
 To use exceptions only for unexpected errors (§14.6).
 To rethrow exceptions in a catch block (§14.7).
 To create chained exceptions (§14.8).
 To define custom exception classes (§14.9).
 To discover file/directory properties, to delete and rename files/directories, and to create directories using the
File class (§14.10).
 To write data to a file using the PrintWriter class (§14.11.1).
 To read data from a file using the Scanner class (§14.11.2).
 To understand how data is read using a Scanner (§14.11.3).
 To develop a program that replaces text in a file (§14.11.4).
 To open files using a file dialog box (§14.12).
 To read data from the Web (§14.13).

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Exception-Handling Overview
Show runtime error
Quotient Run

Fix it using an if statement
QuotientWithIf Run

Introduce try-catch

QuotientWithMethod Run

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Exception Advantages

QuotientWithException Run

Now you see the advantages of using exception handling.
It enables a method to throw an exception to its caller.
Without this capability, a method must handle the
exception or terminate the program.

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Handling InputMismatchException

InputMismatchExceptionDemo Run

By handling InputMismatchException, your program will
continuously read an input until it is correct.

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Exception Types
ClassNotFoundException

ArithmeticException
IOException

Exception NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object Throwable IllegalArgumentException

Many more classes
LinkageError

Error VirtualMachineError

Many more classes

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System Errors

ArithmeticException
IOException

RuntimeException
Many more classes

Many more classes
System errors are thrown by JVM
and represented in the Error class. LinkageError
The Error class describes internal
system errors. Such errors rarely Error VirtualMachineError
occur. If one does, there is little
you can do beyond notifying the
Many more classes
user and trying to terminate the
program gracefully.

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Exceptions
Exception describes errors
caused by your program ClassNotFoundException
and external ArithmeticException
circumstances. These IOException
errors can be caught and Exception NullPointerException
handled by your program.
RuntimeException
Many more classes

Many more classes
LinkageError

Error VirtualMachineError

Many more classes

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Runtime Exceptions

ArithmeticException
IOException

RuntimeException
Many more classes

Many more classes
LinkageError
RuntimeException is caused by
programming errors, such as bad
Error VirtualMachineError casting, accessing an out-of-bounds
array, and numeric errors.
Many more classes

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Checked Exceptions vs.
Unchecked Exceptions

RuntimeException, Error and their subclasses are
known as unchecked exceptions. All other
exceptions are known as checked exceptions,
meaning that the compiler forces the programmer
to check and deal with the exceptions.

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In most cases, unchecked exceptions reflect programming
logic errors that are not recoverable. For example, a
NullPointerException is thrown if you access an object
through a reference variable before an object is assigned to
it; an IndexOutOfBoundsException is thrown if you access
an element in an array outside the bounds of the array.
These are the logic errors that should be corrected in the
program. Unchecked exceptions can occur anywhere in the
program. To avoid cumbersome overuse of try-catch
blocks, Java does not mandate you to write code to catch
unchecked exceptions.

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ArithmeticException
IOException

RuntimeException
Many more classes

Many more classes
LinkageError

Error VirtualMachineError Unchecked
exception.

Many more classes

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Declaring, Throwing, and
Catching Exceptions

method1() { declare exception
method2() throws Exception {
try {
invoke method2; if (an error occurs) {
}
catch exception catch (Exception ex) { throw new Exception(); throw exception
Process exception; }
} }
}

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Declaring Exceptions
Every method must state the types of checked
exceptions it might throw. This is known as
declaring exceptions.

public void myMethod()
throws IOException

public void myMethod()
throws IOException, OtherException

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Throwing Exceptions
When the program detects an error, the program
can create an instance of an appropriate exception
type and throw it. This is known as throwing an
exception. Here is an example,

throw new TheException();

TheException ex = new TheException();
throw ex;

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Throwing Exceptions Example
/** Set a new radius */
public void setRadius(double newRadius)
throws IllegalArgumentException {
if (newRadius >= 0)
radius = newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}

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Catching Exceptions
try {
statements; // Statements that may throw exceptions
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVar3) {
handler for exceptionN;
}

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Catching Exceptions
main method { method1 { method2 { An exception
... ... ... is thrown in
try { try { try { method3
... ... ...
invoke method1; invoke method2; invoke method3;
statement1; statement3; statement5;
} } }
catch (Exception1 ex1) { catch (Exception2 ex2) { catch (Exception3 ex3) {
Process ex1; Process ex2; Process ex3;
} } }
statement2; statement4; statement6;
} } }

Call Stack
method3

method2 method2

method1 method1 method1

main method main method main method main method

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Catch or Declare Checked Exceptions
Java forces you to deal with checked exceptions. If a method declares a
checked exception (i.e., an exception other than Error or
RuntimeException), you must invoke it in a try-catch block or declare to
throw the exception in the calling method. For example, suppose that
method p1 invokes method p2 and p2 may throw a checked exception (e.g.,
IOException), you have to write the code as shown in (a) or (b).

void p1() { void p1() throws IOException {
try {
p2(); p2();
}
catch (IOException ex) { }
...
}
}

(a) (b)

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Example: Declaring, Throwing, and
Catching Exceptions
 Objective: This example demonstrates
declaring, throwing, and catching exceptions
by modifying the setRadius method in the
Circle class defined in Chapter 8. The new
setRadius method throws an exception if
radius is negative.
TestCircleWithException CircleWithException

Run
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Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
perform operations before exits;
throw ex;
}

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The finally Clause
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}

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animation

Trace a Program Execution
Suppose no
exceptions in the
statements
try {
statements;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

The final block is
try { always executed
statements;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

Next statement in the
try { method is executed
statements;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try { Suppose an exception
statement1; of type Exception1 is
statement2; thrown in statement2
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try { The exception is
statement1; handled.
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try { The final block is
statement1; always executed.
statement2;
statement3;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try { The next statement in
statement1; the method is now
statement2; executed.
statement3;
}
handling ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try {
statement1; statement2 throws an
statement2; exception of type
statement3; Exception2.
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try {
statement1; Handling exception
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try {
statement1; Execute the final block
statement2;
statement3;
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}

Next statement;

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animation

try {
statement1; Rethrow the exception
statement2; and control is
statement3; transferred to the caller
}
handling ex;
}
handling ex;
throw ex;
}
finally {
finalStatements;
}

Next statement;

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Cautions When Using Exceptions
 Exception handling separates error-handling
code from normal programming tasks, thus
making programs easier to read and to modify.
Be aware, however, that exception handling
usually requires more time and resources
because it requires instantiating a new exception
object, rolling back the call stack, and
propagating the errors to the calling methods.

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When to Throw Exceptions
 An exception occurs in a method. If you want
the exception to be processed by its caller, you
should create an exception object and throw it.
If you can handle the exception in the method
where it occurs, there is no need to throw it.

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When to Use Exceptions
When should you use the try-catch block in the code?
You should use it to deal with unexpected error
conditions. Do not use it to deal with simple, expected
situations. For example, the following code
try {
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
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When to Use Exceptions
is better to be replaced by
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");

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Defining Custom Exception Classes
 Use the exception classes in the API whenever possible.
 Define custom exception classes if the predefined
classes are not sufficient.
 Define custom exception classes by extending
Exception or a subclass of Exception.

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Custom Exception Class Example
In Listing 13.8, the setRadius method throws an exception if the
radius is negative. Suppose you wish to pass the radius to the
handler, you have to create a custom exception class.

InvalidRadiusException

CircleWithRadiusException

TestCircleWithRadiusException Run

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Companion
Website
Assertions
An assertion is a Java statement that enables
you to assert an assumption about your
program. An assertion contains a Boolean
expression that should be true during
program execution. Assertions can be used to
assure program correctness and avoid logic
errors.

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Companion
Website
Declaring Assertions
An assertion is declared using the new Java keyword
assert in JDK 1.4 as follows:

assert assertion; or
assert assertion : detailMessage;

where assertion is a Boolean expression and
detailMessage is a primitive-type or an Object value.

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Companion
Website
Executing Assertions
When an assertion statement is executed, Java evaluates the
assertion. If it is false, an AssertionError will be thrown. The
AssertionError class has a no-arg constructor and seven
overloaded single-argument constructors of type int, long, float,
double, boolean, char, and Object.

For the first assert statement with no detail message, the no-arg
constructor of AssertionError is used. For the second assert
statement with a detail message, an appropriate AssertionError
constructor is used to match the data type of the message. Since
AssertionError is a subclass of Error, when an assertion becomes
false, the program displays a message on the console and exits.

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Companion
Website
Executing Assertions Example
public class AssertionDemo {
public static void main(String[] args) {
int i; int sum = 0;
for (i = 0; i < 10; i++) {
sum += i;
}
assert i == 10;
assert sum > 10 && sum < 5 * 10 : "sum is " + sum;
}
}

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Companion
Website
Compiling Programs with
Assertions
Since assert is a new Java keyword introduced in
JDK 1.4, you have to compile the program using
a JDK 1.4 compiler. Furthermore, you need to
include the switch –source 1.4 in the compiler
command as follows:

javac –source 1.4 AssertionDemo.java

NOTE: If you use JDK 1.5, there is no need to
use the –source 1.4 option in the command.
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Companion
Website
Running Programs with
Assertions
By default, the assertions are disabled at runtime. To
enable it, use the switch –enableassertions, or –ea for
short, as follows:

java –ea AssertionDemo

Assertions can be selectively enabled or disabled at
class level or package level. The disable switch is –
disableassertions or –da for short. For example, the
following command enables assertions in package
package1 and disables assertions in class Class1.
java –ea:package1 –da:Class1 AssertionDemo
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Companion
Website Using Exception Handling or
Assertions
Assertion should not be used to replace exception
handling. Exception handling deals with unusual
circumstances during program execution. Assertions are
to assure the correctness of the program. Exception
handling addresses robustness and assertion addresses
correctness. Like exception handling, assertions are not
used for normal tests, but for internal consistency and
validity checks. Assertions are checked at runtime and
can be turned on or off at startup time.

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Companion
Website
Using Exception Handling or
Assertions, cont.
Do not use assertions for argument checking in public
methods. Valid arguments that may be passed to a
public method are considered to be part of the method’s
contract. The contract must always be obeyed whether
assertions are enabled or disabled. For example, the
following code should be rewritten using exception
handling as shown in Lines 28-35 in Circle.java in
Listing 13.8.
public void setRadius(double newRadius) {
assert newRadius >= 0;
radius = newRadius;
}
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Companion
Website Using Exception Handling or
Assertions, cont.
Use assertions to reaffirm assumptions. This gives you
more confidence to assure correctness of the program. A
common use of assertions is to replace assumptions with
assertions in the code.

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Companion
Website
Using Exception Handling or
Assertions, cont.
Another good use of assertions is place assertions in a
switch statement without a default case. For example,

switch (month) {
case 1: ... ; break;
...
default: assert false : "Invalid month: " + month
}

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The File Class
The File class is intended to provide an abstraction that
deals with most of the machine-dependent complexities
of files and path names in a machine-independent
fashion. The filename is a string. The File class is a
wrapper class for the file name and its directory path.

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java.io.File

Obtaining file +File(pathname: String) Creates a File object for the specified pathname. The pathname may be a directory
or a file.
properties and +File(parent: String,
child: String)
Creates a File object for the child under the directory parent. The child may be a
filename or a subdirectory.
manipulating +File(parent: File,
child: String)
Creates a File object for the child under the directory parent. The parent is a File
object. In the preceding constructor, the parent is a string.
file +exists(): boolean Returns true if the file or the directory represented by the File object exists.
+canRead(): boolean Returns true if the file represented by the File object exists and can be read.
+canWrite(): boolean Returns true if the file represented by the File object exists and can be written.
+isDirectory(): boolean Returns true if the File object represents a directory.
+isFile(): boolean Returns true if the File object represents a file.
+isAbsolute(): boolean Returns true if the File object is created using an absolute path name.
+isHidden(): boolean Returns true if the file represented in the File object is hidden. The exact
definition of hidden is system-dependent. On Windows, you can mark a file
hidden in the File Properties dialog box. On UNIX systems, a file is hidden if its
name begins with a period (.) character.
+getAbsolutePath():
Returns the complete absolute file or directory name represented by the File
String
object.
+getCanonicalPath(): Returns the same as getAbsolutePath() except that it removes redundant
String names, such as "." and "..", from the pathname, resolves symbolic links (on
UNIX), and converts drive letters to standard uppercase (on Windows).
+getName(): String Returns the last name of the complete directory and file name represented by the
File object. For example, new
File("c:booktest.dat").getName() returns test.dat.
+getPath(): String Returns the complete directory and file name represented by the File object. For
example, new File("c:booktest.dat").getPath() returns
c:booktest.dat.
+getParent(): String Returns the complete parent directory of the current directory or the file represented
by the File object. For example, new
File("c:booktest.dat").getParent() returns c:book.
+lastModified(): long Returns the time that the file was last modified.
+length(): long Returns the size of the file, or 0 if it does not exist or if it is a directory.
+listFiles(): File[] Returns the files under the directory for a directory File object.
+delete(): boolean Deletes the file or directory represented by this File object. The method returns
true if the deletion succeeds.
+renameTo(dest: File): Renames the file or directory represented by this File object to the specified name
boolean represented in dest. The method returns true if the operation succeeds.
+mkdir(): boolean Creates a directory represented in this File object. Returns true if the directory is
created successfully.
+mkdirs(): boolean
Same as mkdir() except that it creates directory along with it parent directories if
Liang, Introduction to Java Programming, the parent directories do2013 Pearson Education, Inc. All
Ninth Edition, (c) not exist.
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52

Problem: Explore File Properties
Objective: Write a program that demonstrates how to
create files in a platform-independent way and use the
methods in the File class to obtain their properties. The
following figures show a sample run of the program on
Windows and on Unix.

TestFileClass
Run
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Text I/O
A File object encapsulates the properties of a file or a path,
but does not contain the methods for reading/writing data
from/to a file. In order to perform I/O, you need to create
objects using appropriate Java I/O classes. The objects
contain the methods for reading/writing data from/to a file.
This section introduces how to read/write strings and
numeric values from/to a text file using the Scanner and
PrintWriter classes.

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Writing Data Using PrintWriter
java.io.PrintWriter
+PrintWriter(filename: String) Creates a PrintWriter for the specified file.
+print(s: String): void Writes a string.
+print(c: char): void Writes a character.
+print(cArray: char[]): void Writes an array of character.
+print(i: int): void Writes an int value.
+print(l: long): void Writes a long value.
+print(f: float): void Writes a float value.
+print(d: double): void Writes a double value.
+print(b: boolean): void Writes a boolean value.
Also contains the overloaded A println method acts like a print method; additionally it
println methods. prints a line separator. The line separator string is defined
Also contains the overloaded by the system. It is rn on Windows and n on Unix.
printf methods. The printf method was introduced in §3.6, “Formatting
Console Output and Strings.”
.
WriteData Run
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Reading Data Using Scanner
java.util.Scanner
+Scanner(source: File) Creates a Scanner that produces values scanned from the specified file.
+Scanner(source: String) Creates a Scanner that produces values scanned from the specified string.
+close() Closes this scanner.
+hasNext(): boolean Returns true if this scanner has another token in its input.
+next(): String Returns next token as a string.
+nextByte(): byte Returns next token as a byte.
+nextShort(): short Returns next token as a short.
+nextInt(): int Returns next token as an int.
+nextLong(): long Returns next token as a long.
+nextFloat(): float Returns next token as a float.
+nextDouble(): double Returns next token as a double.
+useDelimiter(pattern: String): Sets this scanner’s delimiting pattern.
Scanner

ReadData Run
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Problem: Replacing Text
Write a class named ReplaceText that replaces a string in a text
file with a new string. The filename and strings are passed as
command-line arguments as follows:
java ReplaceText sourceFile targetFile oldString newString
For example, invoking
java ReplaceText FormatString.java t.txt StringBuilder StringBuffer
replaces all the occurrences of StringBuilder by StringBuffer in
FormatString.java and saves the new file in t.txt.

ReplaceText Run

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(GUI) File Dialogs

ReadFileUsingJFileChooser Run

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