Transcript Chapter 13 Exception Handling

Chapter 17
Exception Handling
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Objectives
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To know what is exception and what is exception
handling (§17.2).
To distinguish exception types: Error (fatal) vs.
Exception (non-fatal), and checked vs. uncheck
exceptions (§17.2).
To declare exceptions in the method header (§17.3).
To throw exceptions out of a method (§17.3).
To write a try-catch block to handle exceptions (§17.3).
To explain how an exception is propagated (§17.3).
To rethrow exceptions in a try-catch block (§17.4).
To use the finally clause in a try-catch block (§17.5).
To know when to use exceptions (§17.6).
To declare custom exception classes (§17.7 Optional).
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Syntax Errors, Runtime Errors, and
Logic Errors

Three categories of errors:
– syntax errors, runtime errors, and logic errors

Syntax errors arise because the rules of the
language have not been followed. They are
detected by the compiler

Runtime errors occur while the program is running
if the environment detects an operation that is
impossible to carry out

Logic errors occur when a program doesn't
perform the way it was intended to
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Runtime Errors
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import java.util.Scanner;
public class ExceptionDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter an integer: ");
int number = scanner.nextInt();
If an exception occurs on this
line, the rest of the lines in the
method are skipped and the
program is terminated.
// Display the result
System.out.println(
"The number entered is " + number);
}
}
Terminated.
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Catch Runtime Errors
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import java.util.*;
public class HandleExceptionDemo {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
boolean continueInput = true;
do {
try {
System.out.print("Enter an integer: ");
int number = scanner.nextInt();
If an exception occurs on this line,
the rest of lines in the try block are
skipped and the control is
transferred to the catch block.
// Display the result
System.out.println(
"The number entered is " + number);
continueInput = false;
}
catch (InputMismatchException ex) {
System.out.println("Try again. (" +
"Incorrect input: an integer is required)");
scanner.nextLine(); // discard input
}
} while (continueInput);
}
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Exception Classes
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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System Errors
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur. If one does, there is little
you can do beyond notifying the
user and trying to terminate the
program gracefully.
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
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Exceptions
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Runtime Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-bounds
array, and numeric errors.
Several 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.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Unchecked Exceptions

Unchecked exceptions reflect programming logic
errors that are not recoverable
– Example:
NullPointerException is thrown if you access an object through a
reference variable before an object is assigned to it
 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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Checked or Unchecked Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Unchecked
exception.
Several more classes
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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Declaring, Throwing, and
Catching Exceptions
method1() {
try {
invoke method2;
}
catch (Exception ex) {
Process exception;
}
catch exception
declare exception
method2() throws Exception {
if (an error occurs) {
throw new Exception();
}
throw exception
}
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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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");
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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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;
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Catching Exceptions
main method {
...
try {
...
invoke method1;
statement1;
}
catch (Exception1 ex1) {
Process ex1;
}
statement2;
}
method1 {
...
try {
...
invoke method2;
statement3;
}
catch (Exception2 ex2) {
Process ex2;
}
statement4;
}
method2 {
...
try {
...
invoke method3;
statement5;
}
catch (Exception3 ex3) {
Process ex3;
}
statement6;
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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An exception
is thrown in
method3
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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() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a)
void p1() throws IOException {
p2();
}
(b)
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
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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 6. The new
setRadius method throws an exception if
radius is negative.
TestCircleWithException
CircleWithException
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Exceptions in GUI Applications
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Methods are executed on the threads
If an exception occurs on a thread, the thread is terminated if
the exception is not handled. However, the other threads in the
application are not affected
There are several threads running to support a GUI application
A thread is launched to execute an event handler (e.g., the
actionPerformed method for the ActionEvent)
If an exception occurs during the execution of a GUI event
handler, the thread is terminated if the exception is not handled.
Interestingly, Java prints the error message on the console, but
does not terminate the application
The program goes back to its user-interface-processing loop to
run continuously.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Example: Exceptions in GUI
Applications

An error message appears on the console, but the GUI
application continues running.

Write a program that creates a user interface to perform
integer divisions. The user enters two numbers in the text
fields Number 1 and Number 2. The division of Number 1
and Number 2 is displayed in the Result field when the
Divide button is clicked.
IntegerDivision
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
perform operations before exits;
throw ex;
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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The finally Clause
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
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rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
Suppose no
exceptions in the
statements
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement in the
method is executed
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
Suppose an exception
of type Exception1 is
thrown in statement2
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The exception is
handled.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The next statement in
the method is now
executed.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
statement2 throws an
exception of type
Exception2.
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Handling exception
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Execute the final block
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
34
animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
catch(Exception2 ex) {
handling ex;
throw ex;
}
finally {
finalStatements;
}
Rethrow the exception
and control is
transferred to the caller
Next statement;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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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
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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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.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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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");
}
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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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");
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
39
Creating Custom Exception Classes

Use the exception classes in the API whenever possible.

Create custom exception classes if the predefined
classes are not sufficient.

Declare custom exception classes by extending
Exception or a subclass of Exception.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All
rights reserved. 0-13-222158-6
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Custom Exception Class Example
In Listing 17.1, 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
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rights reserved. 0-13-222158-6
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