Runtime Polymorphism in Java

Runtime polymorphism or Dynamic Method Dispatchis 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.

Let's first understand the upcasting before Runtime Polymorphism.

Upcasting

When reference variable of Parent class refers to the object of Child class, it is known as upcasting. For example:

1. class A{}  
2. class B extends A{}  

1. A a=new B();//upcasting  

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Example of Runtime Polymorphism

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

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

1. class Bike{  
2.   void run(){System.out.println("running");}  
3. }  
4. class Splender extends Bike{  
5.   void run(){System.out.println("running safely with 60km");}  
6.   
7.   public static void main(String args[]){  
8.     Bike b = new Splender();//upcasting  
9.     b.run();  
10.   }  
11. }  

Output:running safely with 60km.

Real example of Java Runtime Polymorphism

Consider a scenario, Bank is a class that provides method to get the rate of interest. But, rate of interest may differ according to banks. For example, SBI, ICICI and AXIS banks could provide 8%, 7% and 9% rate of interest.

Note: It is also given in method overriding but there was no upcasting.

1. class Bank{  
2. int getRateOfInterest(){return 0;}  
3. }  
4.   
5. class SBI extends Bank{  
6. int getRateOfInterest(){return 8;}  
7. }  
8.   
9. class ICICI extends Bank{  
10. int getRateOfInterest(){return 7;}  
11. }  
12. class AXIS extends Bank{  
13. int getRateOfInterest(){return 9;}  
14. }  
15.   
16. class Test{  
17. public static void main(String args[]){  
18. Bank b1=new SBI();  
19. Bank b2=new ICICI();  
20. Bank b3=new AXIS();  
21. System.out.println("SBI Rate of Interest: "+b1.getRateOfInterest());  
22. System.out.println("ICICI Rate of Interest: "+b2.getRateOfInterest());  
23. System.out.println("AXIS Rate of Interest: "+b3.getRateOfInterest());  
24. }  
25. }  

Output:
SBI Rate of Interest: 8
ICICI Rate of Interest: 7
AXIS Rate of Interest: 9

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Runtime Polymorphism with data member

Method is overridden not the datamembers, so runtime polymorphism can't be achieved by data members.

In the example given below, both the classes have a datamember speedlimit, we are accessing the datamember by the reference variable of Parent class which refers to the subclass object. Since we are accessing the datamember which is not overridden, hence it will access the datamember of Parent class always.

Rule: Runtime polymorphism can't be achieved by data members.

1. class Bike{  
2.  int speedlimit=90;  
3. }  
4. class Honda extends Bike{  
5.  int speedlimit=150;  
6.   
7.  public static void main(String args[]){  
8.   Bike obj=new Honda();  
9.   System.out.println(obj.speedlimit);//90  
10. }  

Output:90

Runtime Polymorphism with Multilevel Inheritance

Let's see the simple example of Runtime Polymorphism with multilevel inheritance.

1. class Animal{  
2. void eat(){System.out.println("eating");}  
3. }  
4.   
5. class Dog extends Animal{  
6. void eat(){System.out.println("eating fruits");}  
7. }  
8.   
9. class BabyDog extends Dog{  
10. void eat(){System.out.println("drinking milk");}  
11.   
12. public static void main(String args[]){  
13. Animal a1,a2,a3;  
14. a1=new Animal();  
15. a2=new Dog();  
16. a3=new BabyDog();  
17.   
18. a1.eat();  
19. a2.eat();  
20. a3.eat();  
21. }  
22. }  

Output: eating
        eating fruits
        drinking Milk

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Try for Output

1. class Animal{  
2. void eat(){System.out.println("animal is eating...");}  
3. }  
4.   
5. class Dog extends Animal{  
6. void eat(){System.out.println("dog is eating...");}  
7. }  
8.   
9. class BabyDog extends Dog{  
10. public static void main(String args[]){  
11. Animal a=new BabyDog();  
12. a.eat();  
13. }}  

Output: Dog is eating

Since, BabyDog is not overriding the eat() method, so eat() method of Dog class is invoked.