Polymorphism in Java is a concept by which we can perform a single action in different ways. Polymorphism is derived from 2 greek words: poly and morphs. The word "poly" means many and "morphs" means forms. So polymorphism means many forms.

There are two types of polymorphism in Java: compile-time polymorphism and runtime polymorphism. We can perform polymorphism in java by method overloading and method overriding.

If you overload a static method in Java, it is an example of compile-time polymorphism. Here, we will focus on runtime polymorphism in java.

Runtime Polymorphism in Java

Runtime polymorphism or Dynamic Method Dispatch is a process in which a call to an overridden method is resolved at runtime rather than compile-time.

In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.


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

class A{}  
class B extends A{}
A a=new B();//upcasting  

For upcasting, we can use the reference variable of class type or an interface type.

Example: -

interface I{}  
class A{}  
class B extends A implements I{}  

Example of Java Runtime Polymorphism

In this example, we are creating two classes Bike and Splendor. Splendor class extends Bike class and overrides its run() method. We are calling the run method by the reference variable of the Parent class. Since it refers to the subclass object and the subclass method overrides the Parent class method, the subclass method is invoked at runtime.

Since method invocation is determined by the JVM not the compiler, it is known as runtime polymorphism.

class Bike{  
  void run(){System.out.println("running");}  
class Splendor extends Bike{  
  void run(){System.out.println("running safely with 60km");}  
  public static void main(String args[]){  
    Bike b = new Splendor();//upcasting