Method Overriding

Method Overriding

 Method overriding is a mechanism in which a subclass inherits the methods of superclass and sometimes the subclass modifies the implementation of a method defined in superclass. The method of superclass which gets modified in subclass has the same name and type signature. The overridden method must be called from the subclass. Consider following Java Program, in which the method(named as fun) in which a is assigned with some value is modified in the derived class. When an overridden method is called from within a subclass, it will always refer to the version of that method re-defined by the subclass. The version of the method defined by the superclass will be hidden.

Java Program[OverrideDemo.java]

class A

{

int a=0;

void fun(int i)

{

this.a=i;

}

}

class B extends A

{

int b;

void fun(int i)

{

int c;

b=20;

super.fun(i+5);

System.out.println("value of a:"+a);

System.out.println("value of b:"+b);

c=a*b;

System.out.println("The value of c= "+c);

}

}

class OverrideDemo

{

public static void main(String args[])

{

B obj_B =new B();

obj_B.fun(10);//function re-defined in derived class

}

}

Output

F:\test>javac OverrideDemo.java

F:\test>java OverrideDemo

value of a:15

value of b:20

The value of c= 300

Program Explanation

In above program, there are two class - class A and class B. The class A acts as a superclass and the class B acts as a subclass. In class A, a method fun is defined in which the variable a is assigned with some value. In the derived class B, we use the same function name fun in which, we make use of super keyword to access the variable a and then it is multiplied by b and the result of multiplication will be printed.

Rules to be followed for method overriding

1. The private data fields in superclass are not accessible to the outside class. Hence the method of superclass using the private data field cannot be overridden by the subclass.

 2. An instance method can be overridden only if it is accessible. Hence private method can not be overridden.

3. The static method can be inherited but can not be overridden.

4. Method overriding occurs only when the name of the two methods and their type signatures is same.

Difference between Method Overloading and Method Overriding

Review Questions

1. Differentiate between method overloading and method overriding. Explain both with an example.

2. What is meant by overriding method? Give example.