Exception Handling
Recitation – 10/(23,24)/2008
CS 180
Department of Computer Science,
Purdue University
Announcements
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Project 6 is out
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2 week project
Milestone Due October 29, 10:00pm
Final Submission Due November 5, 10:00pm
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This is the date of Exam 2, so start early on the project!
Introduction
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An exception is an object

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Signals the occurrence of unusual event
during program execution
Every program should handle possible
exceptions.
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Throw an exception when something goes
wrong.
Catch the exception – handle the unusual
event.
Why do we need exception handling ?
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Recover from errors!
Consider this simple program:
import java.util.Scanner;
public class Test
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter a number:");
int number = keyboard.nextInt();
System.out.println(number);
}
}
What if the user types ”fifteen” ?
The program crashes with InputMismatchException
Test Class
(with appropriate exception handling)
import java.util.Scanner;
import java.util.InputMismatchException;
public class Test
{
public static int DEFAULT_NUMBER = 5;
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter a number:");
int number;
try{
//code where something could go wrong
number = keyboard.nextInt();
}
catch(InputMismatchException e){
//code executed if an InputMismatchException is thrown
number = DEFAULT_NUMBER;
System.out.println("Bad input!!! Setting the number to default value, “+ number);
}
System.out.println(number);
}
}
Test Class (Handling multiple exceptions)

A try-catch statement can have multiple catch blocks to handle
multiple possible exceptions. Example public class Test
{
public static void main(String args[]) {
int number, divisor, quotient;
try{
number = keyboard.nextInt();
System.out.println("Please enter the divisor:");
divisor = keyboard.nextInt();
//The user can enter an illegal value here
quotient = number/divisor;
//The user could have entered 0 for divisor!
}
catch (InputMismatchException e){
number = divisor = DEFAULT_NUMBER;
System.out.println("Bad input!!! Setting the default value, “ + number);
}
catch (ArithmeticException e){
System.out.println(“Division by zero”);
}
}
}
Handling multiple exceptions
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If an exception occurs at the first line of the try
block, the control transfers to the corresponding
catch block.
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This causes code to be skipped.
Solutions:
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If there is a way to handle exception without a
try-catch block, use it.
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Example – check for divisor being 0 before
dividing.
This is the best way to deal with problems.
For each statement, form a different try-catch
block and catch every possible exception.
Handling multiple exceptions
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When an exception is thrown in a try block, the catch
blocks are checked in order for the matching exception
It is crucial that order goes from specific exception (subclass) to general exception (parent class) since the first
matching catch is executed.
Compile
Error
Scanner keyboard = new Scanner(System.in);
try{
int inverse = 1/keyboard.nextInt();
} catch (Exception e){
Parent Class
e.printStackTrace();
Sub-class of
} catch (InputMismatchException e){
Exception
System.out.println(e.getMessage());
}
The flow of control will never go into the last catch block since all the
exceptions will be caught by the first catch block. This causes a
compile error.
These two catch blocks should be swapped.
Defining Your Own Exception Classes
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For handling problems of a specific type, it is better to
design a specific exception class rather than using the
Exception class.
Must be a derived class of some predefined exception
class, e.g Exception, InputMismatchException etc.
Example: a more specific exception class for division by 0.
class DivideByZeroException extends ArithmeticException
{
public DivideByZeroException() {
super("Dividing by Zero!");
}
public DivideByZeroException(String message) {
super(message);
}
}
9
Implementing New Exception Classes
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Defining your own exception class just gives
you the ability to throw and catch objects of that class.
Catch handlers are not part of the user-defined exception
classes
In other words you should explicitly indicate in your
program when to throw your own exception
Example:
try {
if (divisor == 0)
throw new DivideByZeroException(“The divisor is zero”);
}
catch(DivideByZeroException e) {
System.out.println(e.getMessage();
}
An example with multiple user-defined exception classes
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Here’s a simple program that distributes a number of eggs into some baskets equally.
public class Test
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
try{
System.out.println("Please enter the number of eggs:");
int numberOfEggs = keyboard.nextInt();
System.out.println("Please enter the number of baskets:");
int numberOfBaskets = keyboard.nextInt();
if(numberOfEggs < 0 || numberOfBaskets < 0)
throw new NegativeNumberException(“Number of eggs/baskets cannot be
negative”);
if(numberOfBaskets==0)
throw new DivideByZeroException(“No baskets?”);
…..
}
catch(DivideByZeroException e)
{
…… }
catch(NegativeNumberException e) {
…… }
}
}
Accounting for Exceptions
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An exception can be caught in a catch block within
a method definition.
Alternatively, the possibility of an exception can be
declared at the start of the method definition by
placing the exception-class name in a throws
clause.
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e.g int quotient(int dividend, int divisor) throws
DivideByZeroException { …. }
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The method quotient might throw a DivideByZeroException object
in its body but not handle it in a try-catch block.
All this tells the compiler is that any method that calls
quotient(..) is responsible for handling the thrown
exception.
Accounting for Exceptions
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Example –
int quotient(int dividend, int divisor) throws DivideByZeroException {
if(divisor==0)
throw new DivideByZeroException();
return dividend/divisor;
}
int main(String args[]) {
try {
int q = quotient(3, 0);
}
//catching exception where the method is called
catch(DivideByZeroException e) {
System.out.println(e.getMessage());
}
}
Accounting for Exceptions
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Exceptions are thrown in one method and
handled in another
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Multiple methods calling the same function can handle
errors differently.
Some method in the calling hierarchy should
handle the exception
main()
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a()
b()
c()
If method c() declares that it throws an exception, then
method b() can either catch it or pass the buck in turn.
If an exception is thrown, but never caught, the
program terminates.
Keep It Simple
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Handling exceptions in the same method where
they are thrown makes things too specific.
If the way the exception is to be handled
depends on the calling method, let the calling
method handle the exception
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This is a good programming practice.
throws Clause Not Allowed in
actionPerformed
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A throws clause cannot be added to method
actionPerformed in any action listener class.
Any exception thrown in the method
actionPerformed must be caught within.
Quiz
Scanner keyboard = new Scanner(System.in);
int number = 0;
try{
number = 10/
keyboard.nextInt()-keyboard.nextInt();
}
catch (ArithmeticException e){
number = 8;
}
catch (InputMismatchException e) {
number = 3;
}
Assume a user tries to enter 0 first, then “five”. What is the value of
number at the end?