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Exception Handling (Chapter 8)
CS 180 Recitation - February 29, 2008
Department of Computer Science
Purdue University
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Introduction
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Exceptions are cases that change a
program’s normal flow of control.
Every program should handle possible
exceptions.
Assuming nothing will go wrong through
out the program execution is wrong. After
all; anything that can go wrong, will go
wrong.
Introduction
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To handle exceptions, Java’s exception
handling facilities should be used.
2 sections of a program:
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Code segments that handle normal flow of
control.
• Code segments that handle the exceptional
cases that might occur through out the
execution.
Test Class
Consider this simple program:
import java.util.InputMismatchException;
import java.util.Scanner;
public class Test
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter the number of eggs:");
int numberOfEggs = keyboard.nextInt();
System.out.println(numberOfEggs);
}
}
Test Class
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Problem: What if the user types “4” instead
of 4.
Please enter the number of eggs:
“4”
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 Test.main(Test.java:11)
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Unacceptable if the user doesn’t know
Java
Test Class (Right Way)
import java.util.Scanner;
public class Test
{
public static int DEFAULT_EGGS = 5;
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter the number of eggs:");
int numberOfEggs;
try{
numberOfEggs = keyboard.nextInt();
}
catch(Exception e){
numberOfEggs = DEFAULT_EGGS;
System.out.println("Bad input!!!\nSetting the default value, "
+ numberOfEggs);
}
System.out.println(numberOfEggs);
}
}
Test Class (Handling multiple exceptions)
import java.util.Scanner;
public class Test
{
public static int DEFAULT_EGGS = 5;
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("Please enter the number of eggs:");
int numberOfEggs = 0;
int numberOfBasket = 0;
int eggsPerBasket = 0;
try{
numberOfEggs = keyboard.nextInt();
System.out.println("Please enter the number of baskets:");
numberOfBasket = keyboard.nextInt();
eggsPerBasket = numberOfEggs/numberOfBasket;
}
catch(Exception e){
//Opps, now what to catch? Bad user input or division by zero?
}
System.out.println(numberOfEggs);
}
}
Test Class (Handling multiple exceptions)
import java.util.InputMismatchException;
import java.util.Scanner;
public class Test
{
…………….
…………….
try{
numberOfEggs = keyboard.nextInt();
System.out.println("Please enter the number of baskets:");
numberOfBasket = keyboard.nextInt();
eggsPerBasket = numberOfEggs/numberOfBasket;
}
catch(InputMismatchException e){
numberOfEggs = DEFAULT_EGGS;
System.out.println("Bad input!!!\nSetting the default value, "
+ numberOfEggs);
}
catch(ArithmeticException e){
System.out.println(“Division by zero”);
numberOfBasket = 1;
eggsPerBasket = numberOfEggs;
}
System.out.println(numberOfEggs);
}
}
Handling multiple exceptions
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Although it catches multiple exceptions, this
piece of code doesn’t execute all the statements
if an exception occurs at the first line of try block.
If there is a way to handle exception without a
try-catch block, use it. (i.e. instead of using
ArithmeticException an if statement that checks if
numberOfBasket<1 would suffice)
For each statement, form a different try-catch
block and catch every possible exception.
Handling multiple exceptions
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While catching exceptions, it is crucial that order goes
from specific exception (sub-class) to general exception
(parent class) since the first matching catch is executed.
Compile
Error
Scanner keyboard = new Scanner(System.in);
try{
int tmp = 10/keyboard.nextInt() - keyboard.nextInt();
} catch (Exception e){
Parent Class
e.printStackTrace();
} catch (InputMismatchException e){
Sub-class of Exception
System.out.println(e.getMessage());
}
The flow of control will never go into the last catch
block since all of the exception will be caught by the
first catch block. This will cause a compile error.
So, these 2 catch blocks should be swapped.
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.
These two approaches can be mixed in a
method, catching some exceptions and
declaring others in a throws clause.
Accounting for Exceptions (1st option)
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Catching an exception inside of a method
import java.util.InputMismatchException;
import java.util.Scanner;
public class Test
{
public static void main(String[] args)
{
int numberOfEggs = getInput("Please enter the number of eggs:", 1);
int numberOfBasket = getInput("Please enter the number of baskets:", 1);
if (numberOfBasket < 1)
numberOfBasket = 1;
int eggsPerBasket = numberOfEggs/numberOfBasket;
System.out.println(numberOfEggs+"-"+numberOfBasket+"-"+eggsPerBasket);
}
public static int getInput(String prompt, int numOfTry){
Scanner keyboard = new Scanner(System.in);
System.out.println(prompt);
int k = 0;
try{
k = keyboard.nextInt();
}
catch(InputMismatchException e){
System.out.println("Bad input!!!(try number: "+numOfTry+")");
k = getInput(prompt, numOfTry+1);
}
return k;
}
}
Accounting for Exceptions (2nd option)
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Catching an exception where the method is called
public static void main(String[] args)
{
int numberOfEggs = 0;
try{
numberOfEggs = getInput("Please enter the number of eggs:");
} catch (InputMismatchException e){
numberOfEggs = 5;
}
int numberOfBasket = 0;
try{
numberOfBasket = getInput("Please enter the number of baskets:");
}catch (InputMismatchException e){
numberOfEggs = -1;
numberOfBasket = -1;
}
…
}
public static int getInput(String prompt) throws InputMismatchException{
Scanner keyboard = new Scanner(System.in);
System.out.println(prompt);
int k = keyboard.nextInt();
return k;
}
Accounting for Exceptions
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Some method in the calling hierarchy
should handle the exception (i.e. either
getInput or main method).
If an exception is thrown, but never
caught, either the program terminates or
its behavior becomes unreliable.
Keep It Simple
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If the way the exception is handled
depends on the calling method, let the
calling method handle the exception (i.e.
2nd option, in the second try-catch the first
variable is changed used in the first trycatch).
1st option is more specific and is less likely
to be used through out the program, so it
is a good practice to implement 2nd option.
Implementing New Exception
Classes
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Don’t forget catch handlers won’t be part
of user-defined exception classes
In other words you should explicitly
indicate in your program when to throw
your own exception
Test class example continued:
Test Class (Cont.)
import java.util.InputMismatchException;
import java.util.Scanner;
public class Test
{
public static void main(String[] args)
{
int numberOfEggs = getInput("Please enter the number of eggs:", 1);
int numberOfBasket = getInput("Please enter the number of baskets:", 1);
try{
if (numberOfBasket < 1)
throw new DivideByZeroException();
} catch (DivideByZeroException e) {
System.out.println(e.getMessage());
numberOfBasket = 1;
}
int eggsPerBasket = numberOfEggs/numberOfBasket;
System.out.println(numberOfEggs+"-"+numberOfBasket+"-"+eggsPerBasket);
}
public static int getInput(String prompt, int numOfTry){…}
}
class DivideByZeroException extends Exception
{
public DivideByZeroException()
{
super("Dividing by Zero!");
}
public DivideByZeroException(String message)
{
super(message);
}
}
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 method
actionPerformed must be caught in method
actionPerformed.
Similarly, if method windowClosing is redefined
in a window listener class, you may not add a
throws clause to method windowClosing.
Quiz
Scanner keyboard = new Scanner(System.in);
int tmp = 0;
try{
tmp = 10/keyboard.nextInt() - keyboard.nextInt();
} catch (InputMismatchException e){
System.out.println(“InputMismatchException”);
tmp = -1
} catch (Exception e){
System.out.println(“Exception”);
tmp = -2;
}
Assume a user tries to enter 0 first, then 5. What is the value of tmp at the end?