Basic Exception Handling

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Basic Exception Handling
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Exceptions in Java

• Either the Java language itself or your code
  provides a mechanism that signals when something
  unusual happens.
• This is called throwing an exception.
• Another part of your code contains code for
  handling the exception.

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Example of Exception Handling

• import java.util.public class GotMilk
  public static void main(String args)
  int donutCount, milkCount double
  donutsPerGlass Scanner keyboard new
  Scanner(System.in) System.out.println("E
  nter number of donuts") donutCount
  keyboard.nextInt( ) System.out.println("E
  nter number of glasses of milk")
  milkCount keyboard.nextInt( ) if
  (milkCount lt 1)
  System.out.println("No Milk!")
  System.out.println("Go buy some milk.")
  else
  donutsPerGlass donutCount/(double)milkCount
  System.out.println(donutCount "
  donuts.") System.out.println(milkCoun
  t " glasses of milk.")
  System.out.println("You have " donutsPerGlass
  " donuts for each glass of
  milk.") System.out.println("End
  of program.")

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Example of Exception Handling (contd)

• import java.util.public class
  ExceptionDemo public static void
  main(String args) int donutCount,
  milkCount double donutsPerGlass
  Scanner keyboard new Scanner(System.in)
  try System.out.println("En
  ter number of donuts") donutCount
  keyboard.nextInt( )
  System.out.println("Enter number of glasses of
  milk") milkCount
  keyboard.nextInt( ) if (milkCount lt
  1) throw new Exception("Exception
  No Milk!") donutsPerGlass
  donutCount/(double)milkCount
  System.out.println(donutCount " donuts.")
  System.out.println(milkCount " glasses
  of milk.") System.out.println("You
  have " donutsPerGlass
  " donuts for each glass of milk.")
  catch(Exception e)
  System.out.println(e.getMessage( ))
  System.out.println("Go buy some milk.")
  System.out.println("End of program.")
  

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How Exception Handling Works

• Exception is a predefined class.
• The throw statement creates an object of the
  class Exception and throws it.
• When an exception is thrown, the code in the
  surrounding try block stops execution, and
  another portion of code, know as a catch block,
  begins execution.
• Execution of the catch block is called catching
  the exception.

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The try Block

• The basic way of handling exceptions in Java
  consists of the try-throw-catch threesome.
• A try block contains the code for the basic
  algorithm that tells the computer what to do when
  everything goes smoothly.
• It is called try because you are not 100 percent
  sure everything will go smoothly.

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The throw Statement

• If something goes wrong, you want to throw an
  exception.
• The throw statement creates a new object of the
  class Exception and throws it.
• When an exception is thrown, the code in the try
  block stops execution, and another portion of
  code, the catch block, begins execution.

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The throw Statement (contd)

• In the example,
• new Exception(Exception No Milk!)
• the string Exception No Milk! is an argument
  for the constructor for the class Exception.
• The Exception object, created with new, stores
  this string in an instance variable of the
  object, so that it can be recovered in the catch
  block.

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The catch block

• Although it is not a method definition it looks a
  little like one.
• It is a separate piece of code that is executed
  when a program throws an exception.
• The throw statement is similar to a method call,
  but instead of calling a method, it calls the
  catch block.

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The catch Block Parameter

• The following the word catch is the catch-block
  parameter.
• The class name preceding the catch-block
  parameter specifies what kind of exception the
  catch-block can catch.
• The catch-block parameter gives you a name for
  the exception that is caught, so that you can
  write code in the catch block in order to
  manipulate that exception object.

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The getMessage Method

• Every exception has a String instance variable
  that contains some message, which typically
  identifies the reason for the exception.
• If the object is called e, then the method call
  e.getMessage() returns this string.