Java Polymorphism
The process of representing one form in multiple forms is known as Polymorphism.
Polymorphism is derived from 2 Greek words: poly and morphs. The word "poly" means many and "morphs" means forms. So polymorphism means many forms.
Polymorphism is not a programming concept, but it is one of the principals of OOPs. For many objects oriented programming language, polymorphism principle is common but whose implementations are varying from one objects oriented programming language to another object-oriented programming language.
Real life example of polymorphism
Suppose if you are in a classroom that time you behave like a student, when you are in the market at that time you behave like a customer, when you at your home at that time you behave like a son or daughter, Here one person present in different-different behaviors.
How to achieve Polymorphism in Java?
Polymorphism can be achieved in two of the following ways:
- Method Overloading(Compile time Polymorphism)
- Method Overriding(Run time Polymorphism)
Polymorphism principal is divided into two sub-principal they are:
- Static or Compile time polymorphism
- Dynamic or Runtime polymorphism
Static polymorphism in Java is achieved by method overloading, and Dynamic polymorphism in Java is achieved by method overriding
Note: Java programming does not support static polymorphism because of its limitations and java always supports dynamic polymorphism.
Example of Runtime Polymorphism in Java
In below example we create two class Person an Employee, Employee class extends Person class feature and override walk() method. We are calling the walk() method by the reference variable of 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. Here method invocation is determined by the JVM not compiler, So it is known as runtime polymorphism.
Example of Polymorphism in Java
class Person
{
void walk()
{
System.out.println("Can Run....");
}
}
class Employee extends Person
{
void walk()
{
System.out.println("Running Fast...");
}
public static void main(String arg[])
{
Person p=new Employee(); //upcasting
p.walk();
}
}
Output
Running fast...
Dynamic Binding
Dynamic binding always says create an object of base class but do not create the object of derived classes. Dynamic binding principal is always used for executing polymorphic applications.
The process of binding appropriate versions (overridden method) of derived classes which are inherited from the base class with the base class object is known as dynamic binding.
Advantages of dynamic binding along with polymorphism with method overriding are.
- Less memory space
- Less execution time
- More performance
Static polymorphism
The process of binding the overloaded method within the object at compile time is known as Static polymorphism due to static polymorphism utilization of resources (main memory space) is poor because for each and every overloaded method a memory space is created at compile time when it binds with an object. In C++ environment the above problem can be solved by using dynamic polymorphism by implementing with virtual and pure virtual function, so most of the C++ developer in real worlds follows only dynamic polymorphism.
Dynamic polymorphism
In dynamic polymorphism method of the program binds with an object at runtime the advantage of dynamic polymorphism is allocating the memory space for the method (either for the overloaded method or for override method) at run time.
The advantage of dynamic polymorphism is the effective utilization of the resources. So Java always uses dynamic polymorphism. Java does not support static polymorphism because of its limitation.
Form of Polymorphism
There are many forms of polymorphism,
- Polymorphism with class
- Polymorphism in methods
- Polymorphism in interface
Polymorphism with classes
Those Java classes that can pass more than one IS-A test is polymorphic in nature. All Java classes are derived from the object class so, its references can pass the instance of operator test with the object class and its own object. So, we can conclude that all Java class is polymorphic.
We can access the objects in Java through a reference variable. These reference variables can assign different objects (if reference is not declared final) also once declared, and these references type cannot be changed.
The reference variable determines which method it can invoke on the object. The reference variable can be declared as a class or interface type. A reference variable can refer to any object of the declared type or its subtype.
Example:
abstract class Animal {
public void run() {
System.out.println("Run fast...");
}
public abstract void eat();
}
class Lion extends Animal {
public void eat() {
System.out.println("Catch deer and eat");
}
}
class Deer extends Animal {
public void eat() {
System.out.println("Search grass and eat ");
}
}
class Testpolymorphism {
public static void main(String[] args) {
Animal animal1 = new Lion();
Animal animal2 = new Deer();
animal1.run();
animal2.run();
animal1.eat();
animal2.eat();
}
}
Output:
Run fast... Run fast... Catch deer and eat Search grass and eat
In method Overriding, JRE decides which method is to invoke in run time, based on the exact type of the object on which the method is called.
Polymorphism in methods (Binding of variables and methods at compile time and runtime)
Binding refers to resolving variables or methods that would be called for a reference variable. We use a reference variable of a base class to refer to an object of a derived class.
But there is a difference. Instance variables bind at compile time while the methods bind at runtime.
Example:
class Parent {
String name = "Gopal";
void displayName() {
System.out.println("Name is " + name);
}
}
class Child extends Parent {
String name = "Rohan";
void displayName() {
System.out.println("Name is " + name);
}
}
public class DynamicBinding {
public static void main(String[] args) {
Parent a = new Parent();
Parent b = new Child();
System.out.println(a.name);
System.out.println(b.name);
a.displayName();
b.displayName();
}
}
Output:
Gopal Gopal Name is Gopal Name is Rohan
System.out.println(b.name); will print Gopal since the reference variable is of type Parent. The field variable binds at the compile time.
b.displayName(); will print the name Rohan because JRE sees that the exact object is of type Child and have an overriding method displayName hence, JRE invokes the displayName of the Child Class.
Polymorphism with interfaces
We can implement polymorphism using interface also. We need a class which implements this interface.
Polymorphism with interfaces involves abstract or default methods of the interface. The interface can also define static methods, but static methods never participate in polymorphism.
Example :
class Parent implements PlayGames {
public void play() {
System.out.println("Don't play games");
}
}
class Child extends Parent implements PlayGames {
public void play() {
System.out.println("Always want to play games");
}
}
interface PlayGames {
void play();
}
public class DynamicBinding2 {
public static void main(String[] args) {
PlayGames a = new Parent();
PlayGames b = new Child();
a.play();
b.play();
}
}
Output:
Don't play games Always want to play games
JRE dynamically calls the method of the corresponding object and executes their methods.