Download - 2003 Prentice Hall, Inc. All rights reserved. Chapter 15 – Exception Handling Outline 15.1 Introduction 15.2 Exception-Handling Overview 15.3 Exception-Handling.

2003 Prentice Hall, Inc. All rights reserved.

Chapter 15 – Exception Handling Outline

15.1 Introduction15.2 Exception-Handling Overview15.3 Exception-Handling Example: Divide by Zero15.4 Java Exception Hierarchy15.5 Rethrowing an Exception15.6 finally Clause15.7 Stack Unwinding15.8 printStackTrace, getStackTrace and getMessage15.9 Chained Exceptions15.10 Declaring New Exception Types15.11 Constructors and Exception Handling


15.1 Introduction

• Exception handling– Exception

• Indication of problem during execution

– E.g., divide by zero

– Chained exceptions


15.2 Exception-Handling Overview

• Uses of exception handling– Process exceptions from program components

– Handle exceptions in a uniform manner in large projects

– Remove error-handling code from “main line” of execution

• A method detects an error and throws an exception– Exception handler processes the error

– Uncaught exceptions yield adverse effects • Might terminate program execution


15.2 Exception-Handling Overview

• Code that could generate errors put in try blocks – Code for error handling enclosed in a catch clause

– The finally clause always executes

• Termination model of exception handling– The block in which the exception occurs expires

• throws clause specifies exceptions method throws


15.3 Exception-Handling Example: Divide by Zero

• Common programming mistake• Throws ArithmeticException

2003 Prentice Hall, Inc.All rights reserved.

Outline

DivideByZeroTest.java

1 // Fig. 15.1: DivideByZeroTest.java2 // An exception-handling example that checks for divide-by-zero.3 import java.awt.*;4 import java.awt.event.*;5 import javax.swing.*;6

7 public class DivideByZeroTest extends JFrame8 implements ActionListener {9

10 private JTextField inputField1, inputField2, outputField;11 private int number1, number2, result;12

13 // set up GUI14 public DivideByZeroTest()15 {16 super( "Demonstrating Exceptions" );17

18 // get content pane and set its layout19 Container container = getContentPane();20 container.setLayout( new GridLayout( 3, 2 ) );21

22 // set up label and inputField123 container.add(24 new JLabel( "Enter numerator ", SwingConstants.RIGHT ) );25 inputField1 = new JTextField();26 container.add( inputField1 );


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28 // set up label and inputField2; register listener29 container.add( new JLabel( "Enter denominator and press Enter ", 30 SwingConstants.RIGHT ) );31 inputField2 = new JTextField();32 container.add( inputField2 );33 inputField2.addActionListener( this );34

35 // set up label and outputField36 container.add( new JLabel( "RESULT ", SwingConstants.RIGHT ) );37 outputField = new JTextField();38 container.add( outputField );39

40 setSize( 425, 100 );41 setVisible( true );42

43 } // end DivideByZeroTest constructor44

45 // process GUI events46 public void actionPerformed( ActionEvent event )47 {48 outputField.setText( "" ); // clear outputField49

50 // read two numbers and calculate quotient51 try { 52 number1 = Integer.parseInt( inputField1.getText() );53 number2 = Integer.parseInt( inputField2.getText() );

The try block

Read integers from JTextFields


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54 55 result = quotient( number1, number2 ); 56 outputField.setText( String.valueOf( result ) ); 57 } 58 59 // process improperly formatted input 60 catch ( NumberFormatException numberFormatException ) { 61 JOptionPane.showMessageDialog( this, 62 "You must enter two integers", "Invalid Number Format",63 JOptionPane.ERROR_MESSAGE ); 64 } 65 66 // process attempts to divide by zero 67 catch ( ArithmeticException arithmeticException ) { 68 JOptionPane.showMessageDialog( this, 69 arithmeticException.toString(), "Arithmetic Exception",70 JOptionPane.ERROR_MESSAGE ); 71 } 72

73 } // end method actionPerformed74

75 // demonstrates throwing an exception when a divide-by-zero occurs76 public int quotient( int numerator, int denominator )77 throws ArithmeticException78 {79 return numerator / denominator;80 }

Method quotient attempts division

Catch NumberFormatException

Catch ArithmeticException

Method quotient throws ArithmeticException


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82 public static void main( String args[] )83 {84 DivideByZeroTest application = new DivideByZeroTest();85 application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );86 }87

88 } // end class DivideByZeroTest


15.4 Java Exception Hierarchy

• Superclass Throwable– Subclass Exception

• Exceptional situations

• Should be caught by program

– Subclass Error• Typically not caught by program

• Checked exceptions– Catch or declare

• Unchecked exceptions


Fig. 15.2 Inheritance hierarchy for class Throwable

Throwable

Exception Error

AWTError ThreadDeathIOExceptionRuntimeException OutOfMemoryError


15.5 Rethrowing an Exception

• Rethrow exception if catch cannot handle it


15.6 finally Clause

• Resource leak– Caused when resources are not released by a program

• The finally block– Appears after catch blocks

– Always executes

– Use to release resources


Outline

UsingExceptions.java

1 // Fig. 15.3: UsingExceptions.java2 // Demonstration of the try-catch-finally exception handling mechanism.3 public class UsingExceptions {4

5 public static void main( String args[] )6 {7 try { 8 throwException(); // call method throwException9 }10

11 // catch Exceptions thrown by method throwException12 catch ( Exception exception ) {13 System.err.println( "Exception handled in main" );14 }15

16 doesNotThrowException();17 }18

19 // demonstrate try/catch/finally20 public static void throwException() throws Exception21 {22 // throw an exception and immediately catch it23 try { 24 System.out.println( "Method throwException" );25 throw new Exception(); // generate exception26 }


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28 // catch exception thrown in try block29 catch ( Exception exception ) {30 System.err.println(31 "Exception handled in method throwException" );32 throw exception; // rethrow for further processing33

34 // any code here would not be reached35 }36

37 // this block executes regardless of what occurs in try/catch 38 finally { 39 System.err.println( "Finally executed in throwException" );40 } 41

42 // any code here would not be reached43

44 } // end method throwException45

46 // demonstrate finally when no exception occurs47 public static void doesNotThrowException()48 {49 // try block does not throw an exception50 try { 51 System.out.println( "Method doesNotThrowException" );52 }

The finally block executes, even though Exception thrown

Rethrow Exception


Outline

Method throwExceptionException handled in method throwExceptionFinally executed in throwExceptionException handled in mainMethod doesNotThrowExceptionFinally executed in doesNotThrowExceptionEnd of method doesNotThrowException


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53

54 // catch does not execute, because no exception thrown55 catch ( Exception exception ) {56 System.err.println( exception );57 }58

59 // this clause executes regardless of what occurs in try/catch60 finally { 61 System.err.println( 62 "Finally executed in doesNotThrowException" ); 63 } 64

65 System.out.println( "End of method doesNotThrowException" );66

67 } // end method doesNotThrowException68

69 } // end class UsingExceptions

The finally block always executes


15.7 Stack Unwinding

• Exception not caught in scope– Method terminates

– Stack unwinding occurs

– Another attempt to catch exception


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1 // Fig. 15.4: UsingExceptions.java2 // Demonstration of stack unwinding.3 public class UsingExceptions {4

5 public static void main( String args[] )6 {7 // call throwException to demonstrate stack unwinding8 try { 9 throwException();10 }11

12 // catch exception thrown in throwException13 catch ( Exception exception ) {14 System.err.println( "Exception handled in main" );15 }16 }17

18 // throwException throws exception that is not caught in this method19 public static void throwException() throws Exception20 {21 // throw an exception and catch it in main22 try { 23 System.out.println( "Method throwException" );24 throw new Exception(); // generate exception25 }26

Call method throwException

Catch Exception from method throwExcetion

Method declares a throws clause

Throw an Exception


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27 // catch is incorrect type, so Exception is not caught28 catch ( RuntimeException runtimeException ) {29 System.err.println( 30 "Exception handled in method throwException" );31 }32

33 // finally clause always executes 34 finally { 35 System.err.println( "Finally is always executed" );36 }37

38 } // end method throwException39

40 } // end class UsingExceptions

Method throwExceptionFinally is always executedException handled in main


15.8 printStackTrace, getStackTrace and getMessage

• Throwable class– Method printStackTrace

• Prints method call stack

– Method getStackTrace • Obtains stack-trace information

– Method getMessage• Returns descriptive string


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1 // Fig. 15.5: UsingExceptions.java2 // Demonstrating getMessage and printStackTrace from class Exception.3 public class UsingExceptions {4

5 public static void main( String args[] )6 {7 try { 8 method1(); // call method1 9 }10

11 // catch Exceptions thrown from method112 catch ( Exception exception ) { 13 System.err.println( exception.getMessage() + "\n" );14 exception.printStackTrace();15

16 // obtain the stack-trace information17 StackTraceElement[] traceElements = exception.getStackTrace();18 19 System.out.println( "\nStack trace from getStackTrace:" );20 System.out.println( "Class\t\tFile\t\t\tLine\tMethod" );21

22 // loop through traceElements to get exception description23 for ( int i = 0; i < traceElements.length; i++ ) {24 StackTraceElement currentElement = traceElements[ i ];25 System.out.print( currentElement.getClassName() + "\t" );26 System.out.print( currentElement.getFileName() + "\t" );

Print information generated by getMessage and printStackTrace

Call method1

Print StackTraceElements


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27 System.out.print( currentElement.getLineNumber() + "\t" );28 System.out.print( currentElement.getMethodName() + "\n" );29 30 } // end for statement31

32 } // end catch33

34 } // end method main35

36 // call method2; throw exceptions back to main37 public static void method1() throws Exception38 {39 method2();40 }41

42 // call method3; throw exceptions back to method143 public static void method2() throws Exception44 {45 method3();46 }47

48 // throw Exception back to method249 public static void method3() throws Exception50 {51 throw new Exception( "Exception thrown in method3" );52 } Throw an

Exception that propagates back to

main

method1 declares a throw clause

Call method2


Call method3


Print StackTraceElements


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53

54 } // end class Using Exceptions

Exception thrown in method3

java.lang.Exception: Exception thrown in method3 at UsingExceptions.method3(UsingExceptions.java:51) at UsingExceptions.method2(UsingExceptions.java:45) at UsingExceptions.method1(UsingExceptions.java:39) at UsingExceptions.main(UsingExceptions.java:8)

Stack trace from getStackTrace:Class File Line MethodUsingExceptions UsingExceptions.java 51 method3UsingExceptions UsingExceptions.java 45 method2UsingExceptions UsingExceptions.java 39 method1UsingExceptions UsingExceptions.java 8 main


15.9 Chained Exceptions

• Wraps existing exception in a new exception– enables exception to maintain complete stack-trace


Outline

UsingChainedExceptions.java

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1 // Fig. 15.6: UsingChainedExceptions.java2 // Demonstrating chained exceptions.3 public class UsingChainedExceptions {4

5 public static void main( String args[] )6 {7 try { 8 method1(); // call method19 }10

11 // catch Exceptions thrown from method112 catch ( Exception exception ) { 13 exception.printStackTrace();14 }15 }16

17 // call method2; throw exceptions back to main18 public static void method1() throws Exception19 {20 try { 21 method2(); // call method222 }23

24 // catch Exception thrown from method225 catch ( Exception exception ) {26 throw new Exception( "Exception thrown in method1", exception );

Call method1

Catch Exception and print stack trace


Call method2

An existing Exception is chained to another


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27 }28 }29

30 // call method3; throw exceptions back to method131 public static void method2() throws Exception32 {33 try { 34 method3(); // call method335 }36

37 // catch Exception thrown from method338 catch ( Exception exception ) {39 throw new Exception( "Exception thrown in method2", exception );40 }41 }42

43 // throw Exception back to method244 public static void method3() throws Exception45 {46 throw new Exception( "Exception thrown in method3" );47 }48

49 } // end class Using Exceptions

An existing Exception is chained to another


Call method3

Throw a new Exception


Outline


java.lang.Exception: Exception thrown in method1 at UsingChainedExceptions.method1(UsingChainedExceptions.java:26) at UsingChainedExceptions.main(UsingChainedExceptions.java:8)Caused by: java.lang.Exception: Exception thrown in method2 at UsingChainedExceptions.method2(UsingChainedExceptions.java:39) at UsingChainedExceptions.method1(UsingChainedExceptions.java:21) ... 1 moreCaused by: java.lang.Exception: Exception thrown in method3 at UsingChainedExceptions.method3(UsingChainedExceptions.java:46) at UsingChainedExceptions.method2(UsingChainedExceptions.java:34) ... 2 more


15.10 Declaring New Exception Types

• Extend existing exception class


15.11 Constructors and Exception Handling

• Throw exception if constructor causes error