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Handling Exceptions

Advanced Programming using Java

By Nora Alaqeel

Java Programming: From Problem Analysis to Program Design, 4e

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Exception

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

Examples Division by zero Invalid input errors An array index that goes out of bounds

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Handling Exception within a Program

Can use an if statement to handle an exceptionHowever, 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

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Java’s Mechanism of Exception Handling

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

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

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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 is the superclass of all the exception classes in Java

Java’s Mechanism of Exception Handling

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Java Exception Hierarchy

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Java Exception Hierarchy

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Java Exception Hierarchy

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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

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try/catch/finally Block

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

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try/catch/finally Block

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try/catch/finally Block (continued)

•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

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try/catch/finally Block

• 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

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Order of catch Blocks

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

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

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Exception-Handling Techniques

Terminate program Output appropriate error message upon termination.

Log error and continue Display error messages and continue with program execution

Fix error and continue Repeatedly get user input Output appropriate error message until valid value is entered

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Unchecked Exceptions

Syntax:

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

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Exceptions Example Code

public static void exceptionMethod() throws InputMismatchException,FileNotFoundException

{ //statements}

The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException

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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. Rethrows the same exception for the calling environment to

handle 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

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Rethrowing and Throwing an Exception

Syntaxthrow exceptionReference;

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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 ex)

{ System.out.println("Line 7: Exception ” + ex.toString());

} } }

Rethrowing and Throwing an Exception

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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 e){

throw e ;} } }

Rethrowing and Throwing an Exception

Trace

By Nora Alaqeel