Spring 2008

Mark Fontenotmfonten@engr.smu.edu

CSE 1341 Principles of Computer Science I

Note Set 10

Note Set 10 Overview

Exception Handling

exceptionException – an indication of a problem that occurs during a

program’s executionException handling allows the graceful handling of errors in

a program. Goal – allow the program to continue executing as if no

problem had been encountered

Can somewhat separate error handling code from regular processing code

The standard

Do Something (processing)Check to see if it worked (error checking)

Do something elseCheck to see if it worked

Do SomethingCheck to see if it worked

1 // Fig. 13.1: DivideByZeroNoExceptionHandling.java 2 // An application that attempts to divide by zero. 3 import java.util.Scanner; 4 5 public class DivideByZeroNoExceptionHandling 6 { 7 // demonstrates throwing an exception when a divide-by-zero occurs 8 public static int quotient( int numerator, int denominator ) 9 { 10 return numerator / denominator; // possible division by zero 11 } // end method quotient 12 13 public static void main( String args[] ) 14 { 15 Scanner scanner = new Scanner( System.in ); // scanner for input 16 17 System.out.print( "Please enter an integer numerator: " ); 18 int numerator = scanner.nextInt(); 19 System.out.print( "Please enter an integer denominator: " ); 20 int denominator = scanner.nextInt(); 21 22 int result = quotient( numerator, denominator ); 23 System.out.printf( 24 "\nResult: %d / %d = %d\n", numerator, denominator, result ); 25 } // end main 26 } // end class DivideByZeroNoExceptionHandling Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14

Attempt to divide; denominator may be zero

Read input; exception occurs if input is not a valid integer

Please enter an integer numerator: 100 Please enter an integer denominator: 7 Result: 100 / 7 = 14 Please enter an integer numerator: 100 Please enter an integer denominator: 0 Exception in thread "main" java.lang.ArithmeticException: / by zero at DivideByZeroNoExceptionHandling.quotient(DivideByZeroNoExceptionHandling.java:10) at DivideByZeroNoExceptionHandling.main(DivideByZeroNoExceptionHandling.java:22) Please enter an integer numerator: 100 Please enter an integer denominator: hello Exception in thread "main" java.util.InputMismatchException at java.util.Scanner.throwFor(Unknown Source) at java.util.Scanner.next(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) at DivideByZeroNoExceptionHandling.main(DivideByZeroNoExceptionHandling.java:20)

Stack Trace

Exception Handling Codetry { //put code here that might // throw an exception //if exception is thrown, move to catch}catch (InputMismatchException e) { //Handle the exception case here – do what is //needed to gracefully recover from the //exception }catch (Exception e) { //Can handle multiple types of exceptions}

Exceptionhandlers

Uncaught exception – exception for which there is no exception handler - exception type doesn’t match any of the catch blocks

Try Block

Exception Parameter

ExceptionTestpublic class ExceptionTest { public static void main (String [] args) { int x = 3; int y = 0; int z = x / y; System.out.println(z); } }

throws ArithmeticException

run:Exception in thread "main" java.lang.ArithmeticException: / by zero at ExceptionTest.main(ExceptionTest.java:9)Java Result: 1

ExceptionTest2public class ExceptionTest2 { public static void main (String [] args) { int x = 3; int y = 0; int z = 0; try { z = x / y; } catch (ArithmeticException e) { System.out.println("Exception Caught"); } System.out.println(z); } } run-single:

Exception Caught0

With Scannerimport java.util.*;

public class ExceptionTest3 { public static void main (String [] args) { Scanner s = new Scanner(System.in); int x = 0; int y = 0; try { x = s.nextInt(); } catch (InputMismatchException e) { e.printStackTrace(); } } }

Use Java API

JAVA API lists the exceptions thrown by each method

Termination Model of Exception Handling

try { code line 1 code line 2 code line 3} catch (Exception e) { handler 1 handler 2} code line 4code line 5

if line 2 throws exception,control transfers to exception handler then to line of code afterhandlers.

ObjectvilleAll exception classes directly or indirectly extend

java.lang.Exception.It is possible to create your own exception classes by

extending java.lang.Exception.

ObjectvilleUnchecked Exceptions

Subclass of RuntimeExceptionNot required to be handled

Checked ExceptionsSubclass of ExceptionMust be handled or explicitly

or declared w/ throws

throws

public int myFunction () throws MySpecialException {

//Code here that could throw a MySpecialException //but you don’t want to handle it here for some reason //so you have to declare that this method could also throw //MySpecialException – a checked exception

}