Chapter 11 - Handling Exceptions and Events

Java Programming: From Problem Analysis to Program Design, 5e

Chapter 11Handling Exceptions and Events

Java Programming: From Problem Analysis to Program Design, 5e 2

Chapter Objectives

• Learn what an exception is• See how a try/catch block is used to

handle exceptions• Become aware of the hierarchy of exception

classes• Learn about checked and unchecked

exceptions


Chapter Objectives (continued)

• Learn how to handle exceptions within a program

• Discover how to throw and rethrow an exception

• Learn how to handle events in a program


Exception

• Definition: an occurrence of an undesirable situation that can be detected during program execution

• Examples– Division by zero– Trying to open an input file that does not exist– An array index that goes out of bounds


Handling Exception within a Program

• Can use an if statement to handle an exception

• However, suppose that division by zero occurs in more than one place within the same block– In this case, using if statements may not be the

most effective way to handle the exception


Java’s Mechanism of Exception Handling

• When an exception occurs, an object of a particular exception class is created

• Java provides a number of exception classes to effectively handle certain common exceptions such as division by zero, invalid input, and file not found

• Division by zero is an arithmetic error and is handled by the class ArithmeticException


Java’s Mechanism of Exception Handling (continued)

• When a division by zero exception occurs, the program creates an object of the class ArithmeticException

• When a Scanner object is used to input data into a program, any invalid input errors are handled using the class InputMismatchException

• The class Exception (directly or indirectly) is the superclass of all the exception classes in Java


try/catch/finally Block• Statements that might generate an exception are

placed in a try block

• The try block might also contain statements that should not be executed if an exception occurs

• The try block is followed by zero or more catch blocks

• A catch block specifies the type of exception it can catch and contains an exception handler


try/catch/finally Block (continued)

• The last catch block may or may not be followed by a finally block

• Any code contained in a finally block always executes, regardless of whether an exception occurs, except when the program exits early from a try block by calling the method System.exit

• If a try block has no catch block, then it must have the finally block



•If no exception is thrown in a try block, all catch blocks associated with the try block are ignored and program execution resumes after the last catch block•If an exception is thrown in a try block, the remaining statements in the try block are ignored

- The program searches the catch blocks in the order in which they appear after the try block and looks for an appropriate exception handler



• If the type of the thrown exception matches the parameter type in one of the catch blocks, the code of that catch block executes and the remaining catch blocks after this catch block are ignored

• If there is a finally block after the last catch block, the finally block executes regardless of whether an exception occurs


Order of catch Blocks

• The heading of a catch block specifies the type of exception it handles

• A catch block can catch either all exceptions of a specific type or all types of exceptions

• A reference variable of a superclass type can point to an object of its subclass


Order of catch Blocks (continued)

• If in the heading of a catch block you declare an exception using the class Exception, then that catch block can catch all types of exceptions because the class Exception is the superclass of all exception classes

• In a sequence of catch blocks following a try block, a catch block declaring an exception of a subclass type should be placed before catch blocks declaring exceptions of a superclass type


Java Exception Hierarchy


Java Exception Hierarchy (continued)


Java’s Exception Class• class Exception

– Subclass of class Throwable – Superclass of classes designed to handle

exceptions

• Various types of exceptions– I/O exceptions– Number format exceptions– File not found exceptions– Array index out of bounds exceptions

• Various exceptions categorized into separate classes and contained in various packages


The class Exception and its Constructors


Java Exception Classes


Java Exception Classes (continued)


Checked Exceptions

• Definition: any exception that can be recognized by the compiler

• Examples– FileNotFoundExceptions


Unchecked Exceptions• Definition: exceptions that cannot be recognized

when the program compiles (must be checked for by programmer)

• Examples– Division by zero– Array index out of bounds

• Syntax

• ExceptionType1, ExceptionType2, and so on are names of exception classes


Exceptions Example Code

public static void exceptionMethod() throws InputMismatchException, FileNotFoundException{ //statements}

• The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException


The class Exception and the Operator instanceof

• A reference of a superclass type can point to objects of its subclass

• You can determine if a reference variable points to an object using the operator instanceof

• You can combine catch blocks using this facility


try {

System.out.print("Line 4: Enter the " + "dividend: "); dividend = console.nextInt(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextInt(); System.out.println(); quotient = dividend / divisor; System.out.println("Line 11: Quotient = " + quotient); }

catch (Exception eRef){

if (eRef instanceof ArithmeticException) System.out.println("Line 14: Exception " + eRef.toString()); else if (eRef instanceof InputMismatchException) System.out.println("Line 16: Exception " + eRef.toString()); }

The class Exception and the Operator instanceof (continued)


Rethrowing and Throwing an Exception

• When an exception occurs in a try block, control immediately passes to one of the catch blocks; typically, a catch block does one of the following:– Completely handles the exception– Partially processes the exception; in this case,

the catch block either rethrows the same exception or throws another exception for the calling environment to handle the exception

– Rethrows the same exception for the calling environment to handle the exception


Rethrowing and Throwing an Exception (continued)

• Useful when: – Catch block catches exception but is unable to

handle it– Catch block decides exception should be

handled by calling environment

• Allows programmer to provide exception-handling code in one place

• Syntaxthrow exceptionReference;


import java.util.*;public class RethrowExceptionExmp1{ static Scanner console = new Scanner(System.in); public static void main(String[] args) { int number; try { number = getNumber(); System.out.println("Line 5: number = " + number); } catch (InputMismatchException imeRef) { System.out.println("Line 7: Exception " + imeRef.toString()); } }}



public static int getNumber() throws InputMismatchException { int num; try { System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException imeRef) { throw imeRef; } }}



The Method printStackTrace

• Used to determine the order in which the methods were called and where the exception was handled


import java.io.*;public class PrintStackTraceExample1{ public static void main(String[] args) { try { methodA(); } catch (Exception e) { System.out.println(e.toString() + " caught in main"); e.printStackTrace(); } }

The Method printStackTrace (continued)


public static void methodA() throws Exception { methodB(); } public static void methodB() throws Exception { methodC(); } public static void methodC() throws Exception { throw new Exception("Exception generated " + "in method C"); }}



• Sample Runjava.lang.Exception: Exception generated in method C caught in mainjava.lang.Exception: Exception generated in method C at PrintStackTraceExample1.methodC (PrintStackTraceExample1.java:30) at PrintStackTraceExample1.methodB (PrintStackTraceExample1.java:25) at PrintStackTraceExample1.methodA (PrintStackTraceExample1.java:20) at PrintStackTraceExample1.main (PrintStackTraceExample1.java:9)



Exception-Handling Techniques

• Terminate program– Output appropriate error message upon

termination

• Fix error and continue– Repeatedly get user input– Output appropriate error message until valid

value is entered

• Log error and continue– Write error messages to file and continue with

program execution


Creating Your Own Exception Classes

• Exception class you define extends class Exception or one of its subclasses

• Syntax to throw your own exception object:

throw new ExceptionClassName(messageString);


Creating Your Own Exception Classes (continued)

public class MyDivisionByZeroException extends Exception{ public MyDivisionByZeroException() { super("Cannot divide by zero"); }

public MyDivisionByZeroException(String strMessage) { super(strMessage); }}


Event Handling

• Action events – Handled by implementing interface ActionListener

• Window events – Handled by implementing interface WindowListener

• Mouse events – Handled by implementing interface MouseListener

• Key events – Handled by implementing interface KeyListener


Event Handling (continued)

• class WindowAdapter – Implements interface WindowListener

with empty bodies to methods

• class MouseAdapter – Implements interface MouseListener

with empty bodies to methods


Registering Listeners

• Registering window listener object to GUI component– Use method addWindowListener– Window listener object being registered is passed

as parameter to method addWindowListener

• Registering mouse listener object to GUI component– Use method addMouseListener– Mouse listener object being registered is passed

as parameter to method addMouseListener


Programming Example: Calculator


Chapter Summary• Exception

– Definition

• Handling exceptions within a program– try/catch/finally block– Order of catch blocks– Using try/catch blocks in a program– The class Exception and the operator instanceof

– Rethrowing and throwing an exception

• Exception– Hierarchy – Classes


Chapter Summary (continued)

• Checked and unchecked exceptions• The method printStackTrace• Exception-handling techniques

– Terminate program– Fix error and continue– Log error and continue

• Creating your own exception classes• Event handling

Chapter 11 - Handling Exceptions and Events

Education

Transcript of Chapter 11 - Handling Exceptions and Events