Exceptions

1
Exceptions

• Just try me

2
Overview

• Definition of exceptions
• The history behind exceptions
• When exceptions occur
• How to handle exceptions
• Throwing exceptions
• Creating your own exceptions

3
History

• The language C has returns codes
• if (someFunction( ) 0)
• else if (someFunction( ) -1)
• Burden on programmer
• Memorize what all the return codes meant
• Had to wrap each function call in an if statement
• Sloppy coders didnt check return codes (crash)

4
Along come Exceptions

• An exception is an unusual circumstance
• Usually associated with something gone wrong
• Doesnt have to be
• Idea Some methods could possibly throw one of
  these exceptions
• You need to try to do the method
• If exception is thrown, you need to catch it

5
Introduction

• Exception
• Indication of problem during programs execution
• Although problem can occur, it occurs
  infrequently

6
Introduction

• Exception handling
• Clear, robust and more fault-tolerant programs
• Continue normal execution
• Sever problems
• Prevent normal execution
• Notification
• Termination

7
Exception Handling Overview

• Program detects error
• Throws exception (throw point)
• Caught by exception handler
• Handled
• Uncaught (no appropriate exception handler)
• Debug mode
• Ignore, continue execution
• View in debugger
• Standard execution mode
• Ignore, continue execution
• Terminate

8
Exception Handling Overview

• Try block
• Encloses code in which errors may occur

9
Exception Handling Overview

• Catch block (Catch handler)
• Appears after Try block
• Parameter included
• Handles specific exception type
• Parameterless
• Handles all exception types

10
Exception Handling Overview

• Finally block
• Appears after last Catch handler
• Optional (if one or more catch handlers exist)
• Encloses code that always executes

11
.NET Exception Hierarchy

• Class Exception
• Base class of .NET Framework exception hierarchy
• Important classes derived from Exception
• ApplicationException
• Can create exception data types specific to
  applications
• SystemException
• Runtime exceptions
• Can occur anytime during execution
• Avoid with proper coding

12
.NET Exception Hierarchy

• Benefit of hierarchy
• Inheritance
• If handling behavior same for base and derived
  classes
• Able to catch base class
• Otherwise catch derived classes individually
• Ex.
• Catch handler with parameter type Exception
• Able to catch all exceptions

13
Exception Properties

• Property Message
• Stores exception objects error message
• Default message
• Associated with exception type
• Customized message
• Passed to exception objects constructor

14
Finally Block

• Encloses code that always executes
• Code executes whether exception occurs or not
• Optional
• Not required if one or more catch handlers exist
• Why use Finally block?
• Typically releases resources acquired in Try
  block
• Helps eliminate resource leaks

15
Java Example

• There is a Thread class, with the method
• public void sleep (long milliseconds) throws
  InterruptedException
• Notice it says throws InterruptedException
• It could possibly throw one
• We must handle this method call with care
• Usage
• Thread.sleep (5000) // Ask to pause for 5
  seconds

16
Something That Wont Work

• class Driver
• public static void main (String args )
• System.out.println (I am sleeping)
• Thread.sleep (5000)
• System.out.println (I am awake)
• // end main
• // end Driver
• Compile error sleep must be placed in a
  try/catch block

17
Introducing the try/catch block

• class Driver
• public static void main (String args )
• System.out.println (I am sleeping)
• try
• Thread.sleep (5000)
• System.out.println (Yawn)
• catch (InterruptedException e)
• System.out.println (An error occurred)
• System.out.println (I am awake)
• // end main
• // end Driver

18
Introducing the try/catch block

• class Driver
• public static void main (String args )
• System.out.println (I am sleeping)
• try
• Thread.sleep (5000)
• System.out.println (Yawn)
• catch (InterruptedException e)
• System.out.println (An error occurred)
• System.out.println (I am awake)
• // end main
• // end Driver

This is what could throw the exception
19
Logic(if exception is not thrown)

• class Driver
• public static void main (String args )
• System.out.println (I am sleeping)
• try
• Thread.sleep (5000)
• System.out.println (Yawn)
• catch (InterruptedException e)
• System.out.println (An error occurred)
• System.out.println (I am awake)
• // end main
• // end Driver

If things run the way they should, output is I
am sleeping Yawn I am awake
20
Logic(if exception is thrown)

• class Driver
• public static void main (String args )
• System.out.println (I am sleeping)
• try
• Thread.sleep (5000)
• System.out.println (Yawn)
• catch (InterruptedException e)
• System.out.println (An error occurred)
• System.out.println (I am awake)
• // end main
• // end Driver

If things run the way they should, output is I
am sleeping An error occurred I am awake
21
Rules

• When the exception was thrown, we handled it in
  the catch section
• The program continues to execute
• To terminate the program, use
• System.exit (1)
• The Exception class is the parent of all
  exceptions
• You can have multiple catch blocks to catch
  specific types of exceptions
• Make sure you put most general at bottom

22
Example

• class Driver
• public static void main (String args )
• try
• // a lot of code that could throw
• // several different kinds of exceptions
• catch (NullPointerException e)
• catch (InterruptedException e)
• catch (Exception e)

23
The finally clause

• Finally works with try and catch
• Comes after the try/catch
• It executes whether an exception was thrown or
  not
• System.exit ( ) is the only way it wont execute

24
Example(Finally prints no matter what)

• class Driver
• public static void main (String args )
• try
• Thread.sleep (1000)
• System.out.println (Yawn)
• catch (Exception e)
• finally
• System.out.println (Finally!)
• // finally
• // main
• // driver

25
Example(Finally wont print if exception is
thrown)

• class Driver
• public static void main (String args )
• try
• Thread.sleep (1000)
• System.out.println (Yawn)
• catch (Exception e) System.exit (0)
• finally
• System.out.println (Finally!)
• // finally
• // main
• // driver

26
VB Example

• Private Sub Button1_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles Button1.Click
• Dim intx, inty, result As Integer
• inty 0
• intx 5
• Try
• intx TextBox1.Text
• result intx \ inty
• Catch ex As DivideByZeroException
• MessageBox.Show(ex.Message)
• Catch ex1 As Exception
• MessageBox.Show(ex1.Message)
• Finally
• MessageBox.Show("End of Proc")
• End Try
• End Sub

27
C Example

• private void button1_Click(object sender,
  System.EventArgs e)
• int a, b, c
• b 0
• try
• a Int32.Parse(textBox1.Text)
• c a / b
• catch(DivideByZeroException ex)
• MessageBox.Show(ex.Message,"Divide by Zero")
• catch(Exception ex)
• MessageBox.Show(ex.Message)

28
Creating Your Own Exception

• class SleepException extends Exception
• SleepException ( )
• super ( ) // Call the superclass constructor
• SleepException (String s)
• super ( s ) // Call the superclass
  constructor
• // Remember, Exception is the parent class of
• // exceptions

29
Getting JavaDog to sleep

• We can define a sleep function in JavaDog
• Throws our new SleepException
• When we tell JavaDog to sleep( ), it will need to
  be in a try/catch block

30
JavaDog sleeps!
Can throw any kind of Exception

• class JavaDog
• // Constructor and other methods
• void sleep (long amountOfSleep) throws
  Exception
• if (amountOfSleep lt 0)
• throw (new SleepException( ) )
• else
• Thread.sleep (amountOfSleep)
• // sleep
• // JavaDog

31
JavaDog sleeps!

• class DogDriver
• public static void main (String args )
• JavaDog j1 new JavaDog (14, bob)
• try
• j1.sleep (4000)
• catch (SleepException e)
• System.out.println (SleepException occured)
  
• catch (InterruptedException e)
• System.out.println (InterruptedException)
• finally
• System.out.println (Wake up!)
• // finally
• // main
• // DogDriver

32
VB-Create Own Exception Class

• Dim intx, inty, result As Integer
• inty 0
• intx 5
• Try
• intx TextBox1.Text
• If intx lt 0 Then
• Throw New ExceptClass("Number not
  Allowed", intx)
• End If
• If intx gt 10 Then
• Throw New ExceptClass
• End If
• Catch ex As DivideByZeroException
• MessageBox.Show(ex.Message)
• Catch ex1 As Exception

33
C-Create own Exception Class

• private void button1_Click(object sender,
  System.EventArgs e)
• int a, b, c
• b 0
• try
• a Int32.Parse(textBox1.Text)
• if( a lt 0)
• throw new ExceptionClass()
• if (a gt 10)
• throw new ExceptionClass("Out of
  Range")
• c a / b
• catch(DivideByZeroException ex)
• MessageBox.Show(ex.Message,"Divide by Zero")
• catch(Exception ex)
• MessageBox.Show(ex.Message)

34
Summary

• Exceptions are used to handle errors
• Keywords try, catch, finally, throw, throws
• You try to do a method that could throw an
  exception
• Catch will handle the exception (if thrown)
• Method are the ones who throw the exceptions
• You can easily create your own exceptions
• System.exit ( ) terminates the program