Exception Handling in JAVA

Exception Handling
By :-
SURIT DATTA

Contents
 What is an Exception ?
 Error vs. Exception
 Hierarchy of Java Exception classes
 Java Exception Handling Keywords
 Types of Exception in Java
 Internal working of java try-catch block
 Exception Handler
 Java Multi Catch block
 The Throws / Throw Keywords
 The finally block
 Common scenarios where exceptions may occur
 Sequence of Events for throw
 Checked Exceptions
 Un-Checked Exceptions
 User-defined Exceptions:

What is an exception?
 Exception is an abnormal condition.
In java, exception is an event that disrupts the normal flow of the program. It is an
object which is thrown at runtime.
 What is exception handling
Exception Handling is a mechanism to handle runtime errors such as ClassNotFound,
IO, SQL, Remote etc.
 Advantage of Exception Handling
The core advantage of exception handling is to maintain the normal flow of the
application. Exception normally disrupts the normal flow of the application
that is why we use exception handling.

Error vs Exception
 Errors: An error represents a
condition serious enough that
most reasonable applications
should not try to catch.
 Virtual Machine Error
 Out of memory
 Stack overflow
 Thread Death
 Linkage Error
 Exceptions: An error
which reasonable
applications should
catch.
 Array index out of bounds
 Arithmetic errors (divide by
zero
 Null Pointer Exception
 I/O Exceptions

Hierarchy of Java Exception classes
Object
Throwable
Exception Error
IOException
SQLException
Runtime Exception
ArithmeticException
NullPointerException
VirtualMachineError
AssertionError

Java Exception Handling Keywords
 There are 5 keywords used in java exception handling.
 Try : Java try block is used to enclose the code that might throw an exception. It must be used within
the method.
Java try block must be followed by either catch or finally block.
 Catch : Java catch block is used to handle the Exception. It must be used after the try block only.
We can use multiple catch block with a single try.
 Finally : Executes whether or not an exception is thrown in the corresponding try block or any of its
corresponding catch blocks.
Throw : You can throw an exception, either a newly instantiated one or an exception that you just
caught, by using the throw keyword.
Throws : If a method does not handle a checked exception, the method must declare it using
the throws keyword.

Types of Exception in Java
ArithmeticException
ArrayIndexOutOfBoundsException
ClassCastException
IllegalArgumentException
IndexOutOfBoundsException
NegativeArraySizeException
NullPointerException
NumberFormatException
NoSuchMethodException

Internal working of java try-catch block
int data = 10/0; Exception
object
An object of exception class is thrown
Is Handled ?
YES
NOJVM
1.Prints out exception
description
2.Prints the stack trace
3.Terminates the program
Rest of code
is executed

Exception Handler
Exception
"thrown" here
Exception
handler
Exception
handler
Thrown exception matched against
first set of exception handlers
Thrown exception matched against
first set of exception handlers
If it fails to match, it is matched against
next set of handlers, etc.
If it fails to match, it is matched against
next set of handlers, etc.
If exception matches none of handlers,
program is abandoned
If exception matches none of handlers,
program is abandoned

public class TestMultipleCatchBlock{
  public static void main(String args[]){
   try{
    int a[]=new int[5];
    a[5]=30/0;
   }
   catch(ArithmeticException e){System.out.println("task1 is completed");}
   catch(ArrayIndexOutOfBoundsException e)
{System.out.println("task 2 completed");}
   catch(Exception e){System.out.println("common task completed");}

   System.out.println("rest of the code...");
}
}
At a time only one Exception is occurred and at a time only one catch
block is executed.
All catch blocks must be ordered from most specific to most general i.e.
catch for ArithmeticException must come before catch for Exception .
Java Multi Catch block

The Throws / Throw Keywords
 If a method does not handle a checked exception, the method must
declare it using the throws keyword. The throws keyword appears at
the end of a method's signature.
 You can throw an exception, either a newly instantiated one or an
exception that you just caught, by using the throw keyword.
 ***throws is used to postpone the handling of a checked exception
and throw is used to invoke an exception explicitly.***
import java.io.*;
public class className
{ public void deposit(double amount) throws RemoteException
{ // Method implementation
throw new RemoteException();
}
//Remainder of class definition
}

The finally block
 The finally block follows a try block or a catch block. A finally block of code
always executes, irrespective of occurrence of an Exception.
 Using a finally block allows you to run any cleanup-type statements that
you want to execute, no matter what happens in the protected code.
 A finally block appears at the end of the catch blocks and has the following
syntax:
try
{ //Protected code
}catch(ExceptionType1 e1)
{ //Catch block }
catch(ExceptionType2 e2)
{ //Catch block }
finally { //The finally block always executes. }

Common scenarios where exceptions may occur
 ArithmeticException occurs
int a=50/0; //ArithmeticException
 NullPointerException occurs
String s=null;
System.out.println(s.length()); //NullPointerException
 NumberFormatException occurs
String s="abc";
int i=Integer.parseInt(s); //NumberFormatException
 ArrayIndexOutOfBoundsException occurs
int a[]=new int[5];
a[10]=50; //ArrayIndexOutOfBoundsException

Sequence of Events for throw
Preceding step
try block
throw
statement
unmatched catch
matching catch
unmatched catch
next step

Checked Exceptions
Inherit from class Exception but not from
RuntimeException
Compiler enforces catch-or-declare requirement
Compiler checks each method call and method
declaration
Checked exceptions are checked at compile-time.
e.g. IOException, SQLException etc.

Unchecked Exceptions
Inherit from class RuntimeException or class Error
Compiler does not check code to see if exception caught
or declared
If an unchecked exception occurs and not caught
- Program terminates or runs with unexpected results
Can typically be prevented by proper coding

User-defined Exceptions:
 We can create your own exceptions in Java. Keep the following
points in mind when writing your own exception classes.
 All exceptions must be a child of Throwable.
 If you want to write a checked exception that is automatically
enforced by the Handle or Declare Rule, you need to extend the
Exception class.
 If you want to write a runtime exception, you need to extend the
RuntimeException class.
 We can define our own Exception class as below:
class MyException extends Exception{ }

Exception Handling in JAVA

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Exception Handling in JAVA