Exception Handling

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Exception Handling

• Chapter 8

2
Reminders

• Project 5 due Oct 20 _at_ 1030 pm
• Project 3 regrades due by midnight tonight
• Discussion groups now twice a week (7-9 pm M in
  CS G066, W in Phys 11)

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Exam 2

• Tuesday, October 25
• 700 800 PM
• Physics 112
• Covers chapters 5-9 (and material from 1-4 as
  well)
• Same format as exam 1
• 20 MC
• 5 programming

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Introduction

• A program can be written assuming that nothing
  unusual or incorrect will happen.
• The user will always enter an integer when
  prompted to do so.
• There will always be a nonempty list for a
  program that takes an entry from the list.
• The file containing the needed information will
  always exist.
• Unfortunately, it isnt always so.

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Exception Handling

• Either your code or Java signals when something
  unusual happens.
• The signaling process is called throwing an
  exception.
• Somewhere in your program, you can place code to
  handle the exception.

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Exceptions in Java Example, cont.

• simple example, cont.
• But what if there is no milk?
• An attempt to divide the number of donuts by the
  number of glasses of milk will result in an
  attempt to divide by zero.
• This would be an utter (udder?) disaster, known
  in Java as an exception.

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Exceptions in Java Example, cont.

• In Java, it is possible to test for this unusual
  situation using an if-else statement, for example.

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Exceptions in Java Example, cont.
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Exceptions in Java Example, cont.

• In Java, it is also possible to throw an
  exception.

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Exceptions in Java Example, cont.
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Defining Exception Classes

• You can define your own exception classes, but
  they must be derived from an existing exception
  class.
• Constructors are the most important, and often
  the only methods (except for methods inherited
  from the base class).

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Defining Exception Classes, cont.

• class DivideByZeroException

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Java Tip Preserve getMessage

• For all predefined exception classes, method
  getMessage returns either
• the string that is passed as an argument to the
  constructor or
• a default string if no argument is passed to the
  constructor.
• The behavior of method getMessage should be
  preserved in any exception class you define.

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Java Tip Preserve getMessage, cont.

• This is done by including a string parameter that
  begins with a call to super.
• public MyException(String message)
• super(message)
• More_Code_If_Appropriate

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Java Tip Preserve getMessage, cont.

• Also include a default constructor.
• public MyException()
• super(MyException thrown)
• More_Code_If_Appropriate

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Programming Tip When to Define an Exception Class

• In general, define an exception class if you are
  going to insert a throw statement in your code.
• This permits catch blocks to distinguish between
  your exceptions and exceptions thrown by
  predefined methods.

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Guidelines

• Use class Exception as the base class unless
  there is a compelling reason to do otherwise.
• Define at least two constructors. Typically, no
  other methods are needed.
• Begin each constructor definition with a call to
  the constructor of the base class.

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Declaring Exceptions (Passing the Buck)

• Sometimes is it appropriate to handle an
  exception other than in the method where the
  exception occurred.
• For example, it might be better to handle the
  exception in the method that called the method
  that called the method that threw the exception

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Declaring Exceptions, cont.

• If a method can throw an exception but does not
  catch it, it must alert the programmer to the
  possibility of an exception by including a throws
  clause.
• Example
• public void someMethod()
• throws DivideByZeroException

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Accounting for Exceptions

• 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.

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Accounting for Exceptions, cont.

• If method_A uses a throws clause instead of
  handling an exception and method_B calls
  method_A, then method_B either must handle the
  exception or must also include a throws clause.

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Accounting for Exceptions, cont.

• A throws clause can include more than one
  exception type.
• example
• public int someMethod()throws
• IOException, DivideByZeroException

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Accounting for Exceptions, cont.

• Some method in the calling hierarchy should
  handle the exception.
• If an exception is thrown, but never caught,
  either the program terminates or its behavior
  becomes unreliable.

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Exceptions That Need Not Be Caught

• Some exceptions do not need to be accounted for
  in any way.
• (Perhaps these are the exceptions that prove the
  rule about needing to account for exceptions.)
• Exceptions that do not need to be accounted for
  result from errors and usually are thrown by
  methods in predefined classes.

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throws Clauses in Derived Classes

• When a method is redefined in a derived class,
  the redefined method cannot contain any exception
  classes that are not in the throws clause of the
  same method in the base class (though the derived
  class can list fewer exceptions in its throws
  clause).
• Any exceptions thrown in the derived class must
  be caught or thrown by the base class.

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Multiple Throws and Catches

• A try block can throw any number of different
  types of exceptions.
• Each catch block can catch only one type of
  exception.
• Multiple catch blocks after a try block can catch
  multiple types of exceptions.

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Java Tip Catch the More Specific Exceptions First

• catch blocks are examined in order.
• The first matching catch block is executed.
• More specific exceptions should precede less
  specific exceptions, i.e. exceptions lower in the
  exception hierarchy should come before exceptions
  higher in the exception hierarchy.

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Keep It Simple

• Attempt to modify a program or class definition
  so that it does not need a throw statement.
• In general, use exceptions sparingly.
• If the way the exception is handled depends on
  the calling method, let the calling method handle
  the exception.
• Consider throwing the exception and catching the
  exception in separate methods.

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Keep It Simple, cont.

• public void methodB()
• ...
• try
• ...
• methodA()
• ...
• catch (MyException e)
• ...

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The finally Block

• A finally block can be added after a try block
  and its catch blocks.
• The finally block is executed
• if the try block throws no exceptions
• if the try block throws an exception which is
  caught by a catch block
• if an exception is thrown but not caught
• i.e. it is always executed.

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The finally Block, cont.

• syntax
• try
• catch Block(s)
• finally

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• try
• doMethod(test)
• catch (Exception e)
• System.out.println(Exception)
• return
• finally
• System.out.println(Done)
• return
• If doMethod() throws an exception, what is the
  output?

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Output

• Exception
• Done

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Output

• Exception
• Done
• why?

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Graphics Supplement Exceptions in GUIs

• An uncaught exception in a (non-GUI) application
  will end the program.
• An uncaught exception in a GUI program (either a
  JFrame GUI or an applet) will not end the
  program.
• However, unless the exception is caught, the GUI
  may not cope correctly with the exception or the
  user may receive insufficient instructions.

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Programming Example A JFrame Using Exceptions

• The example JFrame GUI allows the user to write
  the name of a color in a text field and to click
  the Show Color button.
• If the GUI recognizes the color, the background
  color changes to the named color.
• Otherwise, the text field displays Unknown
  Color and the background changes to gray.

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Programming Example A JFrame Using Exceptions,
cont.

• class ColorDemo

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Programming Example A JFrame Using Exceptions,
cont.

• class UnknownColorException

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Programming Example A JFrame Using Exceptions,
cont.

• class ShowColorDemo

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Programming Example A JFrame Using Exceptions,
cont.
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throws Clause Not Allowed in actionPerformed

• 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.

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Summary

• You have become familiar with the notion of
  exception handling.
• You have learned Java syntax for exception
  handling.
• You have learned to use exception handling
  effectively in classes and programs.