Exception Handling

The exception handling in java is one of the powerful mechanisms to handle the runtime errors so that normal flow of the application can be maintained.

What is exception?

Dictionary Meaning: 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. Let’s take a scenario:

1. statement 1;  
2. statement 2;  
3. statement 3;  
4. statement 4;  
5. statement 5;//exception occurs  
6. statement 6;  
7. statement 7;  
8. statement 8;  
9. statement 9;  
10. statement 10;  

Types of Exception

There are mainly two types of exceptions: checked and unchecked where error is considered as unchecked exception. The sun microsystem says there are three types of exceptions:

1. Checked Exception
2. Unchecked Exception
3. Error

Difference between checked and unchecked exceptions

1) Checked Exception

The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions e.g.IOException, SQLException etc. Checked exceptions are checked at compile-time.

2) Unchecked Exception

The classes that extend RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time rather they are checked at runtime.

3) Error

Error is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.

There are given some scenarios where unchecked exceptions can occur. They are as follows:

1) Scenario where ArithmeticException occurs

If we divide any number by zero, there occurs an ArithmeticException.

1. int a=50/0;//ArithmeticException  

2) Scenario where NullPointerException occurs

If we have null value in any variable, performing any operation by the variable occurs an NullPointerException.

1. String s=null;  
2. System.out.println(s.length());//NullPointerException  

3) Scenario where NumberFormatException occurs

The wrong formatting of any value, may occur NumberFormatException. Suppose I have a string variable that have characters, converting this variable into digit will occur NumberFormatException.

1. String s=”abc”;  
2. int i=Integer.parseInt(s);//NumberFormatException  

4) Scenario where ArrayIndexOutOfBoundsException occurs

If you are inserting any value in the wrong index, it would result ArrayIndexOutOfBoundsException as shown below:

1. int a[]=new int[5];  
2. a[10]=50; //ArrayIndexOutOfBoundsException  

Java Exception Handling Keywords:

There are 5 keywords used in java exception handling.

1. try
2. catch
3. finally
4. throw
5. throws

Java try-catch

Java try block is used to enclose the code that might throw an exception. It must be used within the method.

Java catch block is used to handle the Exception. It must be used after the try block only.  You can use multiple catch block with a single try.

try-catch block

try
	{  
		//code that may throw exception  
	}
	catch(Exception_class_Name e)
	{
		//exception handling 
	}  
try-catch-finally block
	try
	{  
		//code that may throw exception  
	}
	catch(Exception_class_Name e)
	{
		//exception handling 
	}
	finally
	{
		//code which is always executed(cleanup)
	}
try-finally block
	try
{  
	//code that may throw exception  
	}
	finally
	{
		//code which is always executed(cleanup)
	}  

Problem without exception handling

1.	public class Testtrycatch1{  
2.	  public static void main(String args[]){  
3.	      int data=50/0;//may throw exception  
4.	      System.out.println("rest of the code...");  
5.	}  
6.	}  
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero

As displayed in the above example, rest of the code is not executed (in such case, rest of the code… statement is not printed).

There can be 100 lines of code after exception. So all the code after exception will not be executed.

Let’s see the solution of above problem by java try-catch block.

1.	public class Testtrycatch2{  
2.	  public static void main(String args[]){  
3.	   try{  
4.	      int data=50/0;  
5.	   }catch(ArithmeticException e){System.out.println(e);}  
6.	   System.out.println("rest of the code...");  
7.	}  
8.	}  
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero
rest of the code...

Now, as displayed in the above example, rest of the code is executed i.e. rest of the code… statement is printed.

Internal working of java try-catch block

The JVM firstly checks whether the exception is handled or not. If exception is not handled, JVM provides a default exception handler that performs the following tasks:

• Prints out exception description.
• Prints the stack trace (Hierarchy of methods where the exception occurred).
• Causes the program to terminate.

But if exception is handled by the application programmer, normal flow of the application is maintained i.e. rest of the code is executed.

Java Multi catch block

If you have to perform different tasks at the occurrence of different Exceptions, use java multi catch block.

Let’s see a simple example of java multi-catch block.

1.	public class TestMultipleCatchBlock{  
2.	  public static void main(String args[]){  
3.	   try{  
4.	    int a[]=new int[5];  
5.	    a[5]=30/0;  
6.	   }  
7.	   catch(ArithmeticException e){System.out.println("task1 is completed");}  
8.	   catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}  
9.	   catch(Exception e){System.out.println("common task completed");}  
10.	  
11.	   System.out.println("rest of the code...");  
12.	 }  
13.	}  
Output:task1 completed
       rest of the code...

Rule: At a time only one Exception is occured and at a time only one catch block is executed.

Rule: All catch blocks must be ordered from most specific to most general i.e. catch for ArithmeticException must come before catch for Exception .

1.	class TestMultipleCatchBlock1{  
2.	  public static void main(String args[]){  
3.	   try{  
4.	    int a[]=new int[5];  
5.	    a[5]=30/0;  
6.	   }  
7.	   catch(Exception e){System.out.println("common task completed");}  
8.	   catch(ArithmeticException e){System.out.println("task1 is completed");}  
9.	   catch(ArrayIndexOutOfBoundsException e){System.out.println("task 2 completed");}  
10.	   System.out.println("rest of the code...");  
11.	 }  
12.	}  
Output:
Compile-time error

Recommended Article:

• Basics of Java
• Features of Java
• Loop Statements in Java
• Control Statements in Java
• Multithreading in Java