IT1202 Fundamentals of Programming Error Handling in Java

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IT1202 - Fundamentals of ProgrammingError
Handling in Java
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Error Handling

• Programmers in any programming language wish to
  write bug free programs
• What is a Bug?
• Bug is a word which used in programming world to
  indicate some error situation in a program

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Error Handling

• In a real software program, errors can occur due
  to
• Programmers dont consider all the situations
  that problems might occur
• Situations where programmers dont test the
  written programs well
• Some situations which are out of the programmers
  control

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Error Handling

• Problems that might cause to crash a program
• Improper arithmetic operations - division by zero
  problem
• Array subscripting problem
• Bad data from users
• Corrupt files
• Network connection failures
• Hardware devices that dont respond

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Error Handling in Java

• In Java these sorts of strange events that may
  cause a program to fail are called..
• Exceptions

Exceptions
Exceptions
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Error Handling in Java
Java treatment of an exception

• If exception occurs and a handler is in effect
• Flow of control is transferred to the handler
• After handler completes flow of control continues
  with the statement following the handler
• If exception occurs and there is no handler for
  it
• The program terminates

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Error Handling in Java

• Most programming languages requires much more
  work to handle error conditions than handling
  program
• Error management can become a major problem when
  start creating larger systems
• In some other programming languages normal codes
  like if..else..if or switch are used to control
  errors

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Error Handling in Java

• Lets consider the following program

int status loadTextfile() If(status !
1)//there is a problem switch(status) case 2
//file not found case 3 // disk error case 4
// file corrupted default // other
error else // file is loaded
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Error Handling in Java

• Problems of using bad error handling mechanisms
• Errors can be manipulated Inconsistently
• Different programmers may use different special
  values for handling errors like errno or goto()
  like statements
• Code to manage these kinds of errors can often
  obscure the programs original intent making
  that code difficult to read and maintain

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Error Handling in Java

• In Java also we have if else if like syntax
  to perform some other work but not to handle
  errors
• Java language introduces a better way to deal
  with exceptional circumstances

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Error Handling in Java

• Java language introduces a better way to deal
  with exceptional circumstances with
• a special language features
• Consistency checking at compile time
• a group of classes called exceptions

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Error Handling in Java

• Using those features
• We can add a new behavior and design to our
  classes ( overall system)
• Our class definitions will describe how our
  program be behaved given the best circumstances
• We can consistently describe how the program will
  behave when errors are gernerated

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Error Handling in Java

• Java exception are actual objects and the
  instances of classes that inherit from the class
  is throwable

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Error Handling in Java

• Exceptions can be generated by
• The Java run-time system
• Manually generated by your code

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Error Handling in Java
Java Exception class hierarchy
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Error Handling in Java
Java Exception class hierarchy
Parent class of exception class hierarchy is
throwable class. It is divided into two
subclasses.
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Error Handling in Java
Java Exception class hierarchy
Instances of Error are internal errors in the
Java Runtime Environment.These errors are rare
and usually fatal. Users can do nothing about
them. But they are there so we can use them if
it is needed.
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Error Handling in Java
Java Exception class hierarchy
Exception class fall into two general groups.
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Error Handling in Java
Java Exception class hierarchy
Subclasses of RuntimeExceptions usually occur due
to errors in code. ArrayIndexOutofBounds NullPoint
erException
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Error Handling in Java
Java Exception class Hierarchy
Subclasses of IOExceptions indicate that
something strange and out of control things are
happening. 1. EOFException 2. FileNotFoundExceptio
n
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Error Handling in Java...

• Exceptions are arranged in a hierarchy like other
  classes
• Exception super classes indicate more general
  errors
• Exception sub classes indicate more specific
  errors

Understanding of the organization of these
classes are very important when we deal with
exceptions in our own code
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Error Handling in Java...

• Most of the exceptions are part of the java.lang
  package
• Throwable
• Exception
• RuntimeException
• In some other packages also some exceptions are
  defined Eg java.io package
• IOException

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Error Handling in Java...

• Now we know about exceptions

How do we manage/handle them in our code
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Error Handling in Java

• In many cases Java run time enforces Exception
  Management when we try to use methods with
  potential exceptions circumstances
• it is necessary to handle those exceptions within
  the code, or that code will not get executed at
  all

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Error Handling in Java...
class DivideZero public static void
main(String args) int
x4,y0 int a x / y System.out.println(a
)
Code is not protected using try Block y 0
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Error Handling in Java...
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Error Handling in Java

• When you have program written with possibilities
  of generating exceptions
• You can catch an exception
• You can throw an exception

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Error Handling in Java

• To catch an exception one can do two things
• Protect the code that contains the method that
  might throw an exception inside try block
• Test for and deal with an exception inside a
  catch block

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Error Handling in Java

• Try block try to run a set of code that might
  cause an exception. if it is executed well then
  go on with the program but if it fails due to an
  error then catch the exception and deal with it

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Error Handling in Java...
Class DivideZero static int anyMethod (int x,
int y) try int a x / y return
a catch (ArithmeticException e)
System.out.println(Division by Zero )
Code is protected using try Block
Exception is caught and handled using catch block
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Error Handling in Java...

• Displaying a description of an exception
• E.g.
• catch (ArithmeticException e)
• System.out.println(Exception e)
• Exception is passed as an argument in a println()
  statement

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Error Handling in Java...
Multiple catch Clauses

• When more than one exceptions are raised by a
  single piece of code you can specify two or more
  catch clauses, each catching a different type of
  exception
• When you use multiple catch statements, then
  exception subclasses which are not belong to a
  one super class are used
• If the exceptions are belong to the same super
  class then that super class may be used instead
  of writing each and every sub class

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Error Handling in Java...
Multiple catch Clauses Example
Try //code that might generate
exceptions catch(IOException e) //here handle
IO exceptions catch(ClassNotFoundException
e2) // here handle class not found
exceptions catch(InterruptedException
e3) //handle interrupted exceptions
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Error Handling in Java...
finally clause

• If there is a piece of code that should be
  executed whether an exception occurs or not then
• include it within the optional finally clause of
  the try..catch block

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Error Handling in Java...
finally clause - Example

• try
• readTextfile()
• catch(IOException e)
• //deal with IO erros
• finally
• closeTextfile()

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Error Handling in Java...
throws clause

• used to indicate that a method might throw an
  exception
• Used after the signature of a method (before the
  opening curly bracket)
• if multiple Exceptions are thrown, include them
  in throws clause separated by commas

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Error Handling in Java...
throws clause - Usage
int myMethod (int x, int y) throws
Exception1,Exception2

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Error Handling in Java...
throws clause - Example

• public static void main(String args) throws
  IOException BufferedReader stdin new
  BufferedReader( new InputStreamReader(System.in))
  
• System.out.print("Filename ")
• String s stdin.readLine()
• BufferedReader filein new BufferedReader(
• new FileReader(s))
• int a Integer.parseInt(filein.readLine())
• int b Integer.parseInt(filein.readLine())
• System.out.println( a / b )

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Error Handling in Java...
throws clause

• throws clause provide some extra information
  about the method definition about potential
  exceptions
• By adding throws clause Java make sure the method
  is being used correctly by other people

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Error Handling in Java...
throws clause

• All the instances of exceptions are not necessary
  to list
• Implicit exception
• RuntimeException
• Error
• Implicit exceptions are thrown by Java runtime
  itself
• Explicit exceptions are potential candidates of
  our throws clause

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Error Handling in Java...
Creating your own exception

• When creating new exception it should inherit
  form some other exception from the exception
  hierarchy
• Exceptions of error hierarchy can not be
  inherited

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Error Handling in Java...
Creating your own exception
public class SunSpotException extends
Exception public SunSpotException()
public SunSpotExceotion(String msg)
super(msg)
Defining Exception
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Error Handling in Java...
When not to use exceptions

• When the exception is expected and could be
  avoided easily with a simple expression
• ArrayIndexOutofBound - instead lengh property
  could be used
• Do more testing rather than throwing exceptions

Exceptions take up a lot of processing time - use
them with care
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Some Useful URLs
Error Handling in Java...

• http//chortle.ccsu.ctstateu.edu/cs151/Notes/chap8
  1/ch81_2.html
• Exception fundamentals
• http//java.sun.com/docs/books/tutorial/essential/
  exceptions/definition.html
• A list of exception types
• http//java.sun.com/docs/books/tutorial/essential/
  exceptions/throwing.html
• throws clause

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IT1202 - Fundamentals Of ProgrammingError
Handling in Java