Exception Handling

1
Exception Handling

• Separating exception-handling blocks of code from
  normal code

2
Java Errors

• Methods that might experience an error (such as a
  device not being available during read( ) ), are
  written to throw an Exception.
• This means that, if a read error occurs, a
  special type of object (Exception object) is
  created.
• The Exception object contains information about
  the error that occurred.
• Java Requires that Exceptions be caught and
  handled by any program using this method.
• You cant ignore exceptions

3
trycatch blocks

• try some code that could produce (throw) an
  error (Example a call to System.in.read( ) ),
• then catch the error the method throws (create a
  reference to the Exception Object that contains
  information about the error)
• and handle the error, if it occurs.

4

• try // try block is normal
  code
• System.in.read()
• catch ( IOException ioe) //if error occurs
• // use the IOException object to see the
  error
• System.err.println( read error ioe )

5
catching Exceptions

• ioe is a local variable
• It is a reference to the IOException object that
  was thrown by System.in.read()
• It tells all about the exception that occurred.
• Here, we print ioe to standard error...
• catch ( IOException ioe) //if error occurs
• // use the IOException object to see error
• System.err.println( read error ioe )

6
How do You KnowWhat Exceptions to catch?

• Java methods that are written to throw an
  Exception, indicate it in the method signature
• public int read() throws IOException
• public int read(byte b) throws IOException,
• NullPointerException
• public int read(byte b, int off, int len)
• throws IOException,
• NullPointerException,
• IndexOutOfBoundsException

7
Catching multiple Exceptions

• byte b new byte10
• try
• System.in.read(b) //this throws more
• //than one Exception
• catch (IOException e1)
• System.err.println(read err e1)
• catch (NullPointerException e2)
• System.err.println(array err e2)

8
Throwing again - Instead of catching

• public class MainDriver
• public static void main(String args) throws
  IOException
• byte my_b
• System.out.println(enter a
  character)
• my_b System.in.read( )
• // System.in.read( ) throws
  IOException, and it MUST
• // either be caught or passed on
  (thrown) to the next class.
• // thats why we add the throws
  clause on our main method
• // it is a trick to keep us from
  having to catch and handle
• // this error right here. We just
  throw it on.

9
The finally blockExecutes no matter what

• try
• System.in.read()
• catch ( IOException ioe)
• System.err.println( read error ioe )
• finally
• System.out.println(we did it!)

10
Partial Java Exception Class Hierarchy

• Exception
• RuntimeException
• IndexOutOfBoundsException
• NullPointerException
• IOException
• EOFException
• FileNotFoundException
• InterruptedIOException

11
Throwing Your Own Exceptions

• public class Account
• . . .
• public void withdraw (double amount) throws
  
• 
  InsufficientFundException
• if ( balance lt amount )
• throw new InsufficientFundExceptio
  n( this )
• else
• balance balance amount
• . . .

12
Catching Your Own Exceptions

• public static void main( String args )
• Account acc new Account( 100.00)
• . . .
• //take care of some transactions
• . . .
• try
• acc.withdraw(10.00)
• catch (InsufficientFundException ex)
• System.out.println( ex )

13
Creating Your Own Exception Classes

• public class InsufficientFundException extends
  Exception
• private Account account
• public InsufficientFundException( Account a )
• super("Insufficient Funds")
• account a
• account.applyAdminCharge( 18.00 )

14
Lab

• Complete Account Exception Program

15
Project 3

• Start with the code we did in Lab 3.
  Animal.java, Dog.java, Cat.java and
  AnimalTest.java. You can get copies of it from
  the Web Site.
• For the Dog class, add a boolean attribute called
  treat. Add a setter method called setTreat(
  boolean t ). Add a getter method called
  isTreat. In the speak( ) method, add code to
  check the value of treat (use the getter method
  isTreat). If the dog does not have a treat,
  throw the UnableToSpeakException.
• For the Cat class, add a boolean attribute called
  hairball. Add a setter method called
  setHairBall( boolean h ). Add a getter method
  called isHairBall. In the speak( ) method, add
  code to check the value of hairball (use the
  getter method isHairBall). If the cat has a
  hairball throw the UnableToSpeakException.
• Write a new Exception class called
  UnableToSpeakException. Handle the
  UnableToSpeakException by printing to the screen
  that the Animal is unable to speak. If the
  object throwing the Exception is a dog, mention
  that the dog needs a treat before speaking. If
  the object throwing the Exception is a cat,
  suggest to the user that they remove the poor
  cats hairball.
• In the AnimalTest program, test your Exception
  class by causing your new Exception to be thrown
  from a Dog and a Cat.
• Turn in a hard copy of all your classes
  Animal.java, Dog.java, Cat.java, AnimalTest.java
  and UnableToSpeakException. Turn in a copy of
  your output and your disk.