Method Overriding with Exception Handling

Method Overriding with Exception Handling in java

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Method Overriding with Exception Handling

There are few things to remember when overriding a method with exception handling. If superclass method does not declare any exception, then subclass overridden method cannot declare a checked exception but it can declare unchecked exceptions.

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Example of Subclass overridden Method declaring Checked Exception

import java.io.*;
class Super
{
 void show() { System.out.println("parent class"); }
}

public class Sub extends Super 
{
 void show() throws IOException  //Compile time error   
  { System.out.println("parent class"); } 

 public static void main( String[] args )
 {
  Super s=new Sub();
  s.show();
 }  
}

As the method show() doesn't throw any exception while in Superclass, hence its overridden version can also not throw any checked exception.

Example of Subclass overridden Method declaring Unchecked Exception

import java.io.*;
class Super
{
 void show(){ System.out.println("parent class"); }
}

public class Sub extends Super 
{
 void show() throws ArrayIndexOutOfBoundsException      //Correct
   { System.out.println("child class"); }

 public static void main(String[] args)
 {
  Super s=new Sub();
  s.show();
 }  
}

Output : child class

Because ArrayIndexOutOfBoundsException is an unchecked exception hence, overrided show() method can throw it.v

More about Overridden Methods and Exceptions

If the Super class method throws an exception, then Subclass overridden method can throw the same exception or no exception, but must not throw parent exception of the exception thrown by a Superclass method.

It means, if a Superclass method throws an object of NullPointerException class, then Subclass method can either throw the same exception or can throw no exception, but it can never throw an object of Exception class (parent of NullPointerException class).

Example of Subclass overridden method with the same Exception

import java.io.*;
class Super
{
 void show() throws Exception 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show() throws Exception  //Correct     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();}

  catch(Exception e){}
 }  
}

Output : child class

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Example of Subclass overridden method with no Exception

import java.io.*;
class Super
{
 void show() throws Exception 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show()               //Correct     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();
   }
  catch(Exception e){}
 }  
}Output : child class

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Example of Subclass overridden method with parent Exception

import java.io.*;
class Super
{
 void show() throws ArithmeticException 
  {  System.out.println("parent class");  }
}

public class Sub extends Super {
 void show() throws Exception           //Cmpile time Error     
   { System.out.println("child class"); } 

 public static void main(String[] args)
 {
  try {
   Super s=new Sub();
   s.show();
   }
  catch(Exception e){}
 }  
}