Java Runtime Polymorphism

July 29, 2019

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Polymorphism means many forms. There are two types of polymorphism in Java:

compile-time polymorphism
Runtime polymorphism

Compile-time polymorphism is achieved by overloading and Runtime polymorphism is achieved by method overriding. We will see Runtime polymorphism in this chapter.

Runtime polymorphism is also known as Dynamic Method Dispatch. Runtime polymorphism is a process in which a call to an overridden method is resolved at runtime, not at the compile-time.

An overridden method is called through the reference variable of a superclass. We use upcasting for runtime polymorphism.

Upcasting

If the reference variable of Parent class refers to the object of Child class, it is known as upcasting.

Java Runtime Polymorphism - The Coding Shala

The following code shows how to achieved runtime polymorphism using upcasting.

class Parent{}
class Child extends Parent{}
Parent p = new Child();//upcasting

Example of Java Runtime Polymorphism

This following example explains java runtime polymorphism:

//Runtime polymorphism example
//thecodingshala.com 

class Parent{
    void Display(){
        System.out.println("Inside Parent class");
    }
}

class Child extends Parent{
    void Display(){
        System.out.println("Inside Child class");
    }
}

class GrandChild extends Child{
    void Display(){
        System.out.println("Inside GrandChild class");
    }
}

class Main{
    public static void main(String[] args){
        Parent p1 = new Parent();
        p1.Display();
        
        Parent p2 = new Child(); //upcasting
        p2.Display(); //override 
        
        Parent p3 = new GrandChild();
        p3.Display(); //works on multilevel inhertance also 
    }
}

Output:

Inside Parent class
Inside Child class
Inside GrandChild class

Java Runtime Polymorphism with Data Member

In Java, we can override a method but not the data members so runtime polymorphism can not be achieved by data members. The following example explains it:

///runtime polymorphism with data member
//thecodingshala

class Parent{
    int num = 10;
}

class Child extends Parent{
    int num = 30;
}

class Main{
    public static void main(String[] args){
        Parent p = new Child();
        System.out.println(p.num);
    }

}

Output:

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