Exceptions & exception handling
 Use sparingly.
 Things you can do with exceptions:
1. Define a new exception class.
2. Create an exception instance.
3. Throw an exception.
4. Declare that an exception may be thrown (in a
particular function).
5. Handle the possibility that an exception may be
thrown.
1. Define a new exception class
 extend Exception (or extend a subclass of Exception)
 See
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Excep
tion.html.
 This creates a new type.
 All have a ctor w/ a single String arg.
 Each has an accessor method called getMessage() that
returns the String from the ctor arg.
Define a new exception class
example
//typical sample code
public class DivisionByZeroException extends Exception {
public DivisionByZeroException ( ) {
super( “Division by zero!” );
}
public DivisionByZeroException ( String message ) {
super( message );
}
}
Define a new exception class
example (w/ more information)
public class BadNumberException extends
Exception {
private int mBadNumber;
public BadNumberException ( ) {
super( “BadNumberException” );
}
public BadNumberException ( int number ) {
super( “BadNumberException” );
mBadNumber = number;
}
public BadNumberException ( String message ) {
super( message );
}
public int getBadNumber ( ) {
return mBadNumber;
}
}
2. Create an exception instance
Ex.
new Exception( “Uh oh!” );
Exception e = new Exception( “Rats!” );
3. Throw an exception
Ex.
throw new Exception( “Invalid value.” );
Exception e = new Exception( “Invalid age.” );
throw e;
4. Declare (a method that indicates) that
an exception may be thrown
Ex.
public int f ( int x ) throws Exception {
…
}
5. Handle the possibility that an
exception may be thrown
 The try-catch blocks:
try {
…
} catch (Exception e) {
…
}
Exercises
1. What is the output produced by the following code?
int waitTime = 46;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
2. What is the output produced by the following code?
int waitTime =
12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
3. What are the throw statements (below)?
int waitTime =
12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
4. What happens when a throw statement is executed?
This is a general question. Tell what happens in
general, not simply what happens in the code below
or some other sample code.
int waitTime = 12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
5. What is the try block (below)?
int waitTime =
12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
6. What is the catch block (below)?
int waitTime =
12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
7. What is the catch block parameter (below)?
int waitTime =
12;
try {
System.out.println( “Try block entered.” );
if (waitTime>30)
throw new Exception( “Over 30.” );
else if (waitTime<30)
throw new Exception( “Under 30.” );
else
System.out.println( “No exception.” );
System.out.println( “Leaving try block.” );
} catch (Exception thrownObject) {
System.out.println( thrownObject.getMessage() );
}
System.out.println( “After catch block.” );
Exercises
 8. Is the following legal?
Exception exceptionObject = new Exception( “Oops!” );
Exercises
 9. Is the following legal?
Exception exceptionObject = new Exception( “Oops!” );
throw exceptionObject;
Try-catch blocks examples
 Recall the Integer class (see
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Inte
ger.html).
 What method do we use to convert a String into an int?
 What happens if the String does not contain an int?
 Try it.
 What happens when we walk off of the end of an
array?
 Can you avoid this behavior?
Useful exception subclasses
 ArrayIndexOutOfBoundsException
 NumberFormatException
 IOException
 NoSuchMethodException
 FileNotFoundException
Exercises
10. Define an exception class called
PowerFailureException.
 The class should have a ctor w/ no parameters. If an
exception is thrown with this zero-argument ctor,
getMessage should return “Power Failure!”
 The class should also have a ctor w/ a single parameter of
type String. If an exception is thrown w/ this ctor, then
getMessage returns the value that was used as an
argument to the ctor.
Exercises
11. Define an exception class called
TooMuchStuffException.
 The class should have a ctor w/ no parameters. If an
exception is thrown with this zero-argument ctor,
getMessage should return “Too much stuff!”
 The class should also have a ctor w/ a single parameter of
type String. If an exception is thrown w/ this ctor, then
getMessage returns the value that was used as an
argument to the ctor.
Exercises
12. Suppose the exception class ExerciseException is defined as
follows:
public class ExerciseException extends Exception {
public ExerciseException ( ) {
super("Exercise Exception thrown!");
System.out.println("Exception thrown.");
}
public ExerciseException ( String message ) {
super(message);
System.out.println( "ExerciseException invoked with an argument.");
}
}
What output would be produced by the following code (which is just an
exercise and not likely to occur in a program)?
ExerciseException e = new ExerciseException( “Do be do” );
System.out.println( e.getMessage() );
Exercises
14. Suppose the exception class MyException is defined as follows:
public class MyException extends Exception {
public MyException ( ) {
super("My Exception thrown!");
}
public MyException ( String message ) {
super("MyException: " + message);
}
}
What output would be produced by the following code?
int number;
try {
System.out.println( “try block entered” );
number = 42;
if (number>0)
throw new MyException( “Hi Mom!” );
System.out.println( “leaving try block” );
} catch (MyException exceptionObject) {
System.out.println( exceptionObject.getMessage() );
}
System.out.println( “end of example.” );
Exercises
15. Suppose the exception class MyException is defined as follows:
public class MyException extends Exception {
public MyException ( ) {
super("My Exception thrown!");
}
public MyException ( String message ) {
super("MyException: " + message);
}
}
What output would be produced by the following code?
int number;
try {
System.out.println( “try block entered” );
number = 42;
if (number>0)
throw new MyException( “Hi Mom!” );
System.out.println( “leaving try block” );
} catch (Exception exceptionObject) { //was MyException
System.out.println( exceptionObject.getMessage() );
}
System.out.println( “end of example.” );
Exercises
16. Suppose the exception class MyException is defined as follows:
public class MyException extends Exception {
public MyException ( ) {
super("My Exception thrown!");
}
public MyException ( String message ) {
super("MyException: " + message);
}
}
What output would be produced by the following code?
int number;
try {
System.out.println( “try block entered” );
number = -58; //was 42
if (number>0)
throw new MyException( “Hi Mom!” );
System.out.println( “leaving try block” );
} catch (MyException exceptionObject) {
System.out.println( exceptionObject.getMessage() );
}
System.out.println( “end of example.” );
Multiple catch blocks
 More general form of try-catch blocks:
try {
…
} catch (NegativeNumberException e) {
…
} catch (DivisionByZeroException e) {
…
}
The order of catch blocks is important as they are
evaluated in sequence. So put most specific first.
Exercises
19. What output will be produced by the following
code?
public class NegativeNumberException
extends Exception
{
public NegativeNumberException ( )
{
super( "Negative Number Exception!“ );
}
}
int n;
try {
n = 42;
if (n > 0)
throw new Exception();
else if (n < 0)
public NegativeNumberException ( String message )
throw new NegativeNumberException();
{
else
super( message );
System.out.println( “bingo!” );
}
} catch (NegativeNumberException e) {
System.out.println( “first catch” );
} catch (Exception e) {
System.out.println( “second catch” );
}
System.out.println( “end of exercise” );
Exercises
20. What output will be produced by the following
code?
public class NegativeNumberException
extends Exception
{
public NegativeNumberException ( )
{
super( "Negative Number Exception!“ );
}
public NegativeNumberException ( String message )
{
super( message );
}
}
int n;
try {
n = -42; //was 42
if (n > 0)
throw new Exception();
else if (n < 0)
throw new NegativeNumberException();
else
System.out.println( “bingo!” );
} catch (NegativeNumberException e) {
System.out.println( “first catch” );
} catch (Exception e) {
System.out.println( “second catch” );
}
System.out.println( “end of exercise” );
Exercises
21. What output will be produced by the following
code?
public class NegativeNumberException
extends Exception
{
public NegativeNumberException ( )
{
super( "Negative Number Exception!“ );
}
public NegativeNumberException ( String message )
{
super( message );
}
}
int n;
try {
n = 0; //was 42
if (n > 0)
throw new Exception();
else if (n < 0)
throw new NegativeNumberException();
else
System.out.println( “bingo!” );
} catch (NegativeNumberException e) {
System.out.println( “first catch” );
} catch (Exception e) {
System.out.println( “second catch” );
}
System.out.println( “end of exercise” );
Exercises
What output will be produced by the following code?
public class NegativeNumberException
extends Exception
{
public NegativeNumberException ( )
{
super( "Negative Number Exception!“ );
}
public NegativeNumberException ( String message )
{
super( message );
}
}
int n;
try {
n = -42; //was 42
if (n > 0)
throw new Exception();
else if (n < 0)
throw new NegativeNumberException();
else
System.out.println( “bingo!” );
} catch (Exception e) {
System.out.println( “first catch” );
} catch (NegativeNumberException e) {
System.out.println( “second catch” );
}
System.out.println( “end of exercise” );
Throwing exceptions in
methods

Methods can



Catch or declare rule: Such a method must then



throw exceptions, and/or
call methods that throw exceptions.
contain a try-catch block (already discussed), and/or
the function heading must specify that the method
may throw an exception.
The catch or declare rule is not always enforced.
Checked exceptions (descendents of Exception)
2. Unchecked exceptions (descendents of
RuntimeException)
1.
Declare (a method that indicates) that an
exception may be thrown
 Ex.
public int f ( int x ) throws Exception {
…
}
public boolean g ( )
throws DivideByZeroException,
SomeOtherException
{
…
}
Useful methods that may throw
exceptions
 What method can be used to convert strings to
integers?
 Open a file for reading (see
http://java.sun.com/j2se/1.5.0/docs/api/java/io/FileIn
putStream.html)
 The Scanner class (see
http://java.sun.com/j2se/1.5.0/docs/api/java/util/Scan
ner.html#nextInt())
finally clause
Most general form of try-catch-finally blocks:
try {
…
} catch (ExceptionClass1 e) {
…
} catch (ExceptionClass2 e) {
…
} catch (ExceptionClassn e) {
…
} finally {
//code executed whether or not an exception was thrown
…
}
finally clause
Three cases:
1.
no exception occurs in the try block
2.
an exception occurs in the try block and is caught
3.
an exception occurs in the try block but doesn’t match
any catch
Exercises
public class FinallyDemo {
public static void main ( String[] args ) {
try {
exerciseMethod( 42 );
} catch(Exception e) {
System.out.println( "Caught in main.“ );
}
}
29. What is the output of the following
program? What would be the output if the
argument to exerciseMethod was -42 instead
of 42. How about 0?
public static void exerciseMethod ( int n ) throws Exception {
try {
if (n > 0)
throw new Exception( );
else if (n < 0)
throw new NegativeNumberException( );
else
System.out.println( "No Exception." );
System.out.println( "Still in sampleMethod." );
} catch (NegativeNumberException e) {
System.out.println( "Caught in sampleMethod." );
} finally {
System.out.println( "In finally block." );
}
System.out.println( "After finally block." );
}
}
Exercises
22. What is the output produced by the following program?
public class Exercise {
public static void main ( String[] args ) {
try {
System.out.println( "Trying" );
sampleMethod( 98.6 );
System.out.println( "Trying after call." );
} catch(Exception e) {
System.out.println( "Catching." );
}
System.out.println( "End program." );
}
public static void sampleMethod ( double test ) throws Exception {
System.out.println( "Starting sampleMethod." );
if (test < 100)
throw new Exception( );
}
}
Exercises
23. What is the output produced by the following program?
public class Exercise {
public static void main ( String[] args ) {
try {
System.out.println( "Trying" );
sampleMethod( 212 ); //was 98.6
System.out.println( "Trying after call." );
} catch(Exception e) {
System.out.println( "Catching." );
}
System.out.println( "End program." );
}
public static void sampleMethod ( double test ) throws Exception {
System.out.println( "Starting sampleMethod." );
if (test < 100)
throw new Exception( );
}
}
Exercises
24. Correct the following method definition by adding a
suitable throws clause:
public static void doStuff ( int n ) {
if (n<0)
throw new Exception( “Negative number.” );
}
25. What happens if an exception is thrown inside a
method invocation but the exception is not caught
inside the method?
Exercises
30. What is the output of the following?
import java.util.Scanner;
import java.util.InputMismatchException;
public class InputMismatchExceptionDemo {
public static void main ( String[] args ) {
Scanner keyboard = new Scanner( System.in );
int number = 0; //to keep compiler happy
boolean done = false;
while (! done) {
try {
System.out.println( "Enter a whole number:" );
number = keyboard.nextInt();
done = true;
} catch (InputMismatchException e) {
keyboard.nextLine();
System.out.println( "Not a correctly written whole number.“ );
System.out.println( "Try again. " );
}
}
System.out.println( "You entered " + number );
}
}
Exercises
import java.util.Scanner;
import java.util.InputMismatchException;
public class InputMismatchExceptionDemo {
public static void main ( String[] args ) {
Scanner keyboard = new Scanner( System.in );
int number = 0; //to keep compiler happy
boolean done = false;
31. Give the definition for the following
method. Use code similar to this code.
/**
Precondition: keyboard is an object of
while (! done) {
the class Scanner that has been set up
try {
for keyboard input (as we have been
System.out.println( "Enter a whole number:" );
doing right along).
number = keyboard.nextInt();
}
done = true;
Returns: An int value entered at the
} catch (InputMismatchException e) {
keyboard. If the user enters an
keyboard.nextLine();
System.out.print( "Not a correctly written “);incorrectly formed input she or he is
System.out.println( “whole number.“ );
prompted to reenter the value.
System.out.println( "Try again. " );
}
*/
System.out.println( "You entered " + number );
}
}
/** Chapter 9 question 1
* Question1Average.java
*
* This program calculates the average of N numbers and throws
* an exception if a negative number is entered for N. If any
* exception occurs while entering a number, the user will be prompted
* for the number again.
*
*/
import java.util.Scanner;
public class Question1Average
{
public static void main(String[] args)
{
// Variable declarations
Scanner scan = new Scanner(System.in);
int n = 0;
int sum = 0;
double average;
boolean error;
9-43
// Loop until there is no error
do
{
try
{
error = false;
// Assume no error
System.out.println("How many numbers do you
want to enter?");
n = scan.nextInt();
if (n <= 0 ) throw new Exception("Number
must be greater than 0.");
} //end try
catch (Exception e)
// Catch any exception and print
{
// the error message
error = true;
System.out.println(e.getMessage());
} //end catch
} while (error);
9-44
// Loop through each number and calculate the average
for (int i=0; i<n; i++)
{
// Repeat input as long as there is an error
do
{
try
{
error = false;
// Assume no error
System.out.println("Enter number " + (i+1));
int num = scan.nextInt();
sum += num;
}
catch (Exception e)
{
// Set error flag if an exception occurs
error = true;
System.out.println("Error, please enter the number again.");
// Read newLine remaining from nextInt
String temp = scan.nextLine();
}
} while (error);
}// end for
average = (double) sum / n;
System.out.println("\nThe average is " + average);
} //end main
} // Question1Average9-45