Exception Handling in Java

Exception Handling in Java

The process of converting system error messages into user friendly error message is known asException handling. This is one of the powerful feature of Java to handle run time error and maintain normal flow of java application.

Exception

An Exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's Instructions.

Why use Exception Handling

Handling the exception is nothing but converting system error generated message into user friendly error message. Whenever an exception occurs in the java application, JVM will create an object of appropriate exception of sub class and generates system error message, these system generated messages are not understandable by user so need to convert it into user friendly error message. You can convert system error message into user friendly error message by using exception handling feature of java.
For Example: when you divide any number by zero then system generate / by zero so this is not understandable by user so you can convert this message into user friendly error message like Don't enter zero for denominator.

Hierarchy of Exception classes

Type of Exception

• Checked Exception
• Un-Checked Exception

Checked Exception

Checked Exception are the exception which checked at compile-time. These exception are directly sub-class of java.lang.Exception class.

Only for remember: Checked means checked by compiler so checked exception are checked at compile-time.

Un-Checked Exception

Un-Checked Exception are the exception both identifies or raised at run time. These exception are directly sub-class of java.lang.RuntimeException class.

Note: In real time application mostly we can handle un-checked exception.

Only for remember: Un-checked means not checked by compiler so un-checked exception are checked at run-time not compile time.

Difference between checked Exception and un-checked Exception

	Checked Exception	Un-Checked Exception
1	checked Exception are checked at compile time	un-checked Exception are checked at run time
3	e.g. FileNotFoundException, NumberNotFoundException etc.	e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc.

Difference between Error and Exception

	Error	Exception
1	Can't be handle.	Can be handle.
2	Example: NoSuchMethodError OutOfMemoryError	Example: ClassNotFoundException NumberFormateException

Handling the Exception

Handling the exception is nothing but converting system error generated message into user friendly error message in others word whenever an exception occurs in the java application, JVM will create an object of appropriate exception of sub class and generates system error message, these system generated messages are not understandable by user so need to convert it into user-friendly error message. You can convert system error message into user-friendly error message by using exception handling feature of java.

Use Five keywords for Handling the Exception

• try
• catch
• finally
• throws
• throw

Syntax for handling the exception

Syntax

try
{
  // statements causes problem at run time 
}
catch(type of exception-1 object-1)
{
  // statements provides user friendly error message 
}
catch(type of exception-2 object-2)
{
  // statements provides user friendly error message
}
finally
{
  // statements which will execute compulsory 
}

Example without Exception Handling

Syntax

class ExceptionDemo 
{
public static void main(String[] args) 
{
int a=10, ans=0;
ans=a/0;
System.out.println("Denominator not be zero");  
}
}

Abnormally terminate program and give a message like below, this error message is not understandable by user so we convert this error message into user friendly error message, like "denominator not be zero".

Example of Exception Handling

Example

class ExceptionDemo 
{
public static void main(String[] args) 
{
int a=10, ans=0;
try
{
ans=a/0;
}
catch (Exception e)
{
System.out.println("Denominator not be zero");
} 
}
}

Output

Denominator not be zero

Next -- try and catch block

try and catch block

try block

Inside try block we write the block of statements which causes executions at run time in other words try block always contains problematic statements.

Important points about try block

• If any exception occurs in try block then CPU controls comes out to the try block and executes appropriate catch block.
• After executing appropriate catch block, even through we use run time statement, CPU control never goes to try block to execute the rest of the statements.
• Each and every try block must be immediately followed by catch block that is no intermediate statements are allowed between try and catch block.

Syntax

try
{
  .....
}
/* Here no other statements are allowed 
between try and catch block */
catch()
{
  ....
}

• Each and every try block must contains at least one catch block. But it is highly recommended to write multiple catch blocks for generating multiple user friendly error messages.
• One try block can contains another try block that is nested or inner try block can be possible.

Syntax

try
{
.......
try
{
.......
}
}

catch block

Inside catch block we write the block of statements which will generates user friendly error messages.

catch block important points

• Catch block will execute exception occurs in try block.
• You can write multiple catch blocks for generating multiple user friendly error messages to make your application strong. You can see below example.
• At a time only one catch block will execute out of multiple catch blocks.
• in catch block you declare an object of sub class and it will be internally referenced by JVM.

Example without Exception Handling

Example

class ExceptionDemo 
{
public static void main(String[] args) 
{
int a=10, ans=0;
ans=a/0;
System.out.println("Denominator not be zero");  
}
}

Abnormally terminate program and give a message like below, this error message is not understandable by user so we convert this error message into user friendly error message, like "denominator not be zero".

Example of Exception Handling

Example

class ExceptionDemo 
{
public static void main(String[] args) 
{
int a=10, ans=0;
try
{
ans=a/0;
}
catch (Exception e)
{
System.out.println("Denominator not be zero");
} 
}
}

Output

Denominator not be zero

Multiple catch block

You can write multiple catch blocks for generating multiple user friendly error messages to make your application strong. You can see below example.

Example

import java.util.*;
class ExceptionDemo 
{
public static void main(String[] args) 
{
int a, b, ans=0;
Scanner s=new Scanner(System.in);
System.out.println("Enter any two numbers: ");
try
{
 a=s.nextInt();
 b=s.nextInt();
 ans=a/b;
 System.out.println("Result: "+ans);
}
catch(ArithmeticException ae)
{
System.out.println("Denominator not be zero");
} 
catch(Exception e)
{
System.out.println("Enter valid number");
} 
}
}

Output

Enter any two number: 5 0
Denominator not be zero

finally Block in Exception Handling

Inside finallyblock we write the block of statements which will relinquish (released or close or terminate) the resource (file or database) where data store permanently.

finally block important points

• Finally block will execute compulsory
• Writing finally block is optional.
• You can write finally block for the entire java program
• In some of the circumstances one can also write try and catch block in finally block.

Example

class ExceptionDemo 
{
public static void main(String[] args) 
{
int a=10, ans=0;
try
{
ans=a/0;
}
catch (Exception e)
{
System.out.println("Denominator not be zero");
} 
finally
{
System.out.println("I am from finally block");
}
}
}

Output

Denominator not be zero
I am from finally block

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