Chapter Eight

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Chapter Eight

• Exception Handling

2
Objectives

• Learn about exceptions and the Exception class
• How to purposely generate a SystemException
• Learn about traditional error-handling methods
• Learn about object-oriented exception-handling
  methods

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Objectives

• How to use the Exception classs ToString()
  method and Message field
• How to catch multiple Exceptions
• How to use the finally block
• How to handle an Exception with a loop

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Objectives

• How to throw an Exception
• How to trace Exceptions through the call stack
• How to create your own Exception classes

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Understanding Exceptions

• An exception is any error condition or unexpected
  behavior in an executing program
• Certain errors are called exceptions because they
  are not usual occurrences
• The object-oriented technique used to manage
  exceptions make up the group of methods known as
  exception handling
• In C, all exceptions are objects

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Understanding Exceptions

• Most exceptions you will use derive from two
  classes that in turn derive from the Exception
  class
• The predefined Common Language Runtime exception
  classes derived from SystemException
• The user-defined application exception classes
  you derive from ApplicationException

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Purposely Generating a SystemException

• You can deliberately generate a SystemException
  exception by forcing a program to contain an
  error

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Purposely Generating a SystemException

• The DivideByZeroException object below was
  generated automatically by C
• Just because an Exception occurs when an
  Exception object is created, you dont
  necessarily have to deal with it

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Understanding Object-Oriented Exception-Handling
Methods

• In object-oriented terminology, you try a
  procedure that may not complete correctly
• A method that detects an error condition or
  Exception throws an Exception
• The block of code that processes the error
  catches the Exception

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Understanding Object-Oriented Exception-Handling
Methods

• When you write a block of code in which something
  can go wrong, you can place the code in a try
  block, consisting of
• The keyword try
• An opening curly brace
• Statements that might cause Exceptions
• A closing curly brace

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Understanding Object-Oriented Exception-Handling
Methods

• You create a catch block with the following
  elements
• The keyword catch, followed by an opening
  parenthesis, the Exception type, a name for an
  instance of the Exception type, and a closing
  parenthesis
• An opening curly brace
• Statements that take the action you want to use
  to deal with the error condition
• A closing curly brace

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Understanding Object-Oriented Exception-Handling
Methods

• General form of a trycatch pair

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Using the Exception Classs ToString() Method and
Message Field

• Any Exception generated from within a try block
  would be caught by a catch block whose argument
  is an Exception type (e.g., Exception e) there
  is no way to confirm the origin of the Exception
• You can use the ToString() method to provide a
  descriptive error message
• The Exception class also contains a field named
  Message that contains useful information about an
  Exception

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Using the Exception Classs ToString() Method and
Message Field

• Output of UsingTheException program when user
  enters 0 for second number

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Catching Multiple Exceptions

• You can place as many statements as you need
  within a try block, and you can catch as many
  different Exceptions as you want
• If you place more than one statement in a try
  block, only the first error-generating statement
  throws an Exception
• When multiple catch blocks are present, they are
  examined in sequence until a match is found for
  the Exception that occurred

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Catching Multiple Exceptions

• TwoErrors class with two catch blocks

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Catching Multiple Exceptions

• Output of TwoErrors program

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Catching Multiple Exceptions

• If you reverse the two try statements within the
  TwoErrors class, the output of the program changes

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Catching Multiple Exceptions

• When you want to execute the same code, no matter
  what type of Exception occurs, you can use only
  one catch block, which receives type Exception

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Catching Multiple Exceptions

• The Exception class is the base class for all
  Exception objects and therefore can reference all
  Exception descendants
• The catch block in the previous code accepts all
  Exception argument types
• When you list multiple catch blocks following a
  try, you must be careful that some catch blocks
  dont become unreachable

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Catching Multiple Exceptions

• Program with unreachable catch block

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Catching Multiple Exceptions

• Error message generated by UnreachableCatch
  program

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Using the finally Block

• The code within a finally block executes whether
  or not the try block identifies an Exception
• Typically, the finally block is used to perform
  clean-up tasks
• When you include a finally block, you are assured
  that the finally statements will execute before
  the program is abandoned

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Using the finally Block

• General form of a trycatch block with a finally
  block

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Handling an Exception with a Loop

• Different programs require different ways of
  handling Exceptions
• In some cases, the try-catch sequence could be
  placed in a loop that continues to execute until
  the code is successful

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Throwing Exceptions

• An advantage of using object-oriented
  exception-handling techniques is the ability to
  deal with Exceptions appropriately as you make
  conscious decisions about how to handle them
• When methods from other classes throw Exceptions,
  they dont have to catch them
• When you design your own classes that might cause
  Exceptions, you should create them to throw the
  Exception but not to handle it
• Handling an Exception should be left to the
  client (the program that uses the class)

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Throwing Exceptions

• Two executions of TrySoccerPlayer1 program

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Throwing Exceptions

• Execution of TrySoccerPlayer2 program

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Tracing Exceptions Through the Call Stack

• The memory location where the computer stores the
  list of locations to which the system must return
  (after method calls) is known as the call stack
• When a method throws an Exception, if the method
  does not catch it, then the Exception is thrown
  to the next method up the call stack
• You can print the value of the StackTrace field
  to display a list of methods in the call stack so
  you can determine the location of the Exception

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Tracing Exceptions Through the Call Stack

• The StackTrace field can be a useful debugging
  tool

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Creating Your Own Exception Classes

• You can create your own customized Exception
  class for your application
• To create your own Exception that you can throw,
  you should extend the ApplicationException class
• You should not create an excessive number of
  special Exception types for your class because it
  adds a level of complexity to your program

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Chapter Summary

• An exception is any error condition or unexpected
  behavior in an executing program
• You can purposely generate a SystemException
  exception by forcing a program to contain an
  error
• When you think an error will occur frequently, it
  is most efficient to handle it in the traditional
  way, with if statements
• In object-oriented terminology, you try a
  procedure that may not complete correctly

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Chapter Summary

• Every Exception object contains a ToString()
  method and a Message field
• You can place as many statements as you need
  within a try block, and you can catch as many
  different Exceptions as you want
• When you have actions to perform at the end of a
  trycatch sequence, you can use a finally block
• When you want to keep trying a block of code
  until some value or state within a program is
  correct, you can place a trycatch block within a
  loop

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Chapter Summary

• When methods throw Exceptions, they dont have to
  catch them instead, the program that calls a
  method that throws an Exception can catch it and
  determine what to do
• When a method throws an Exception, if the method
  does not catch it, then the Exception is thrown
  to the method that called the offending method
• To create your own Exception that you can throw,
  you should extend the ApplicationException class