Handling Exceptions

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TOPIC 11

• Handling Exceptions

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• /
  
• Demonstrates an uncaught exception.
• 
  /
• public class Zero
• //-----------------------------------------------
  ----
• // Deliberately divides by zero to produce
• // an exception.
• //-----------------------------------------------
  ----
• public static void main(String args)
• int numerator 10
• int denominator 0
•   System.out.println(numerator / denominator)
• System.out.println(This text will not be
  printed.0
•  

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Exception in thread main java.lang.ArithmeticExc
eption / by zero At Zero.main(Zero.java17)

• java.lang.ArithmeticException indicates which
  exception was thrown.
• Info about the nature of the error
• / by zero
• Call stack trace ? indicates where the exception
  occurred
• At Zero.main(Zero.java17)
• Zero.main ? method that generated the error
• Zero.java17 ? line no. where exception
  occurred

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An error-handling solution

• public class Zero
• public static void main(String args)
• int numerator 10
• int denominator 0
•  
• if (denominator 0)
• System.exit(1)
•  
• System.out.println(numerator / denominator)

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• System.exit() method
• Current application ends control returns to the
  operating system.
• Return 1 if an error is causing program
  termination.
• Return 0 if the program is ending normally.

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Exception

• Definition an occurrence of an undesirable
  situation that can be detected during program
  execution
• Examples
• division by zero
• trying to open an input file that does not exist
• an array index that goes out of bounds

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Some Terminology

• Throwing an exception either Java itself or your
  code signals when something unusual happens
• Handling an exception responding to an exception
  by executing a part of the program specifically
  written for the exception
• also called catching an exception
• The normal case is handled in a try block
• The exceptional case is handled in a catch block
• The catch block takes a parameter of type
  Exception
• it is called the catch-block parameter
• e is a commonly used name for it
• If an exception is thrown execution in the try
  block ends and control passes to the catch
  block(s) after the try block

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Java Exception Hierarchy
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Constructors and Methods of the class Throwable
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The class Exception and its Subclasses from
java.lang
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The class Exception and its Subclasses from
java.util
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The class Exception and its Subclasses from
java.io
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Javas Exception Class

• class Exception
• Subclass of class Throwable
• Superclass of classes designed to handle
  exceptions
• Various types of exceptions
• I/O exceptions
• Number format exceptions
• File not found exceptions
• Array index out of bounds exceptions
• Various exceptions categorized into separate
  classes and contained in various packages

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The class Exception and its Constructors
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Java Exception Classes
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Exceptions Thrown by Methods
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Exceptions Thrown by Methods
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Handling Exceptions within a Program

• try
• //statements
• catch(ExceptionClassName1 objRef1)
• //exception handler code
• catch(ExceptionClassName2 objRef2)
• //exception handler code
• ...
• catch(ExceptionClassNameN objRefN)
• //exception handler code
• finally

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Handling Exceptions within a Program

• try/catch/finally block used to handle exceptions
  within a program
• try block
• Includes statements that may generate an
  exception
• Includes statements that should not be executed
  if an exception occurs
• If error occurs, execution transfers to
  statements in the catch block
• If no exception occurs, control transfers to
  statement following entire try-catch statement
• Followed by zero or more catch blocks
• May or may not be followed by finally block

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Handling Exceptions within a Program

• catch block
• Heading specifies type of exception it can catch
• Contains exception handler completely handles
  exception
• Can catch all exceptions of a specific type or
  all types of exceptions
• Completes without causing any new exceptions,
  then control transfers to next statement outside
  of try-catch statement
• If several catch blocks, which one is
  appropriate?
• Selects the first one with a class (ExceptionType
  ) that matches the thrown exception
• May or may not be followed by a finally block

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Handling Exceptions within a Program

• finally block
• Code contained in this block always executes
• try block with no catch block has finally block

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An Example of Exception Handling
ExceptionDemo try and catch blocks
try block
throw statement
catch block
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Chapter 8
Java an Introduction to Computer Science
Programming - Walter Savitch
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Flow of Control with Exception Handling
try System.out.println("Enter number of
donuts") donutCount SavitchIn.readLineInt()
System.out.println("Enter number of
glasses") milkCount SavitchIn.readLineInt()
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) catch(Exception e)
System.out.println(e.getMessage())
System.out.println("Go buy some
milk.") System.out.println("End of program.")
Assume user enters a positive number for number
of glasses, so no exception is thrown.
Not executed when there's no exception thrown.
Main method from Exception-Demo program
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Flow of Control with Exception Handling
try System.out.println("Enter number of
donuts") donutCount SavitchIn.readLineInt()
System.out.println("Enter number of
glasses") milkCount SavitchIn.readLineInt()
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) catch(Exception e)
System.out.println(e.getMessage())
System.out.println("Go buy some
milk.") System.out.println("End of program.")
Assume user enters zero or a negative number for
number of glasses, so an exception is thrown.
Not executed when an exception is thrown
Main method from Exception-Demo program
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• public class ErrExample1
• public static void main (Stringargs)
• int num 13, denom 0, result
• int array 22, 33, 44
• try
• result num / denom
• result arraynum
• catch (ArithmeticException error)
• System.out.println("Arithmetic error")
• catch (IndexOutOfBoundsException error)
• System.out.println("Index error")
• // end main // end class

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• int num 13, denom 2, result
• int array 22, 33, 44
• try
• result num / denom
• result arraynum
• catch (ArithmeticException error)
• System.out.println("Arithmetic error")
• catch (IndexOutOfBoundsException error)
• System.out.println("Index error")

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Order of catch Blocks

• If an exception in a try block is caught by the
  first catch block, the remaining catch blocks are
  ignored
• Must be careful about order catch blocks are
  listed

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General error handler

• try
• catch (ArithmeticException error) //define
  specific exceptions first
• System.out.println("Arithmetic error")
• catch (.. error)
• System.out.println(. )
• catch (Exception error) //define general
  exception last
• System.out.println("ERROR Notify
  Administrator")
• error.printStackTrace()
• System.exit(0)

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Generating an Exception with throw

• throw new ExceptionType(possibly_some_parameters)
• ObjectExpression
• Variable or value of a class that implements the
  Throwable interface
• i.e. must be an object of class Throwable or a
  subclass of Throwable such as Exception
• When exception is thrown, JVM looks for a catch
  statement that can handle that specific type of
  exception
• May be written within a try statement that is
  intended to catch it

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To catch an ArithmeticException (1)

• public class Zero
• public static void main(String args)
• int numerator 10, denominator 0
•   try
• if (denominator 0)
• throw new ArithmeticException()
• System.out.println(numerator / denominator)
• catch (ArithmeticException error)
• System.out.println(Attempt to divide by
  zero!)

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Using the Exception getMessage() method

• public class Zero
• public static void main(String args)
• int numerator 10, denominator 0
•   try
• System.out.println(numerator / denominator)
• catch (ArithmeticException error)
• System.out.println(The official message is
• error.getMessage())

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Do more than print error message

• public class Zero
• public static void main(String args)
• int numerator 10 denominator 0
•   try
• System.out.println(numerator / denominator)
• catch (ArithmeticException error)
• System.out.println(The official message is
• error.getMessage())
• System.out.println(Denominator corrected
  to 1)
• finally
• System.out.println(numerator / 1)