Cup 12: Exceptions

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Cup 12 Exceptions

• Special Topics Java

Dr. Tim Margush Department of Mathematics and
Computer Science The University of Akron
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What is an Exception

• An Object (of class Exception) created when a
  situation occurs disrupting the normal flow of
  execution
• Instance variables store information about the
  exception
• Exceptions are thrown to a block of code designed
  to catch them

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What Causes an Exception

• Runtime errors in your program
• Invalid type cast
• Array index out of bounds
• Division by zero
• Throw statement
• Allows program to create an exception

• Standard Method Exceptions
• IOException
• MalformedURLException
• StringIndexOutOfBoundsException
• Java Errors
• Out of memory
• Missing class definition
• Bytecode error

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Class Hierarchy

• Throwable extends Object
• Error extends Throwable
• You generally do not catch these objects
• Exception extends Throwable
• RuntimeException
• Arithmetic, cast, array index, illegal arguments,
  null pointer, security
• IOException
• EOF, MalformedURL, InterruptedIO
• Many, many others!!!

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Checked Exceptions

• Runtime exceptions are considered too costly to
  require catching
• It is your responsibility to ensure Runtime
  Exceptions will not occur
• You may catch Runtime Exceptions
• Other Exceptions are called Checked Exceptions
• You are required to deal with these in your code

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Dealing With Exceptions

• Catch It!
• try
• statements that might throw one or more
  exceptions
• catch (SomeException e)
• code to handle the exception
• catch (OtherException e)
• code to handle the other exception

• Pass the Buck!
• int X()throws IOException
• statements that might throw an IOException
• This makes the Exception part of the method's
  interface
• The calling method must then deal with the
  Exception

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Try, Catch, and Finally

• Each try block is followed by one or more catch
  blocks, and an optional finally block
• finally
• code to be executed after the try, regardless of
  the try block's outcome - even if a return is
  executed inside the try or catch blocks!

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Re-Throwing an Exception

• try
• ...
• catch (Exception e)
• do some things
• throw e
• This will cause the caught exception to be
  rethrown
• This method will need to declare that it might
  throw the Exception
• The calling method will also have to deal with it.

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Creating Your Own Exceptions

• You will need to define your own Exception class
  to extent Throwable
• public class YourException extends Throwable
• public YourException()
• public YourException(String s)
• super(s)
• //other methods???

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Throwable?

• class Throwable contains 4 public methods
• getMessage()
• Includes name of exception class and message
  stored in exception object
• printStackTrace()
• printStackTrace(PrintStream s)
• displays message and traceback through stack
• toString()

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Throwing Your Own Exceptions

• throw new YourException("Custom")
• This is a checked exception, so it will either
  have to be caught, or declared in the throws
  clause of the method