Understating of Polymorphism in Java

This tutorial talks about Understating of Polymorphism in Java.This is one of the important Oops feature in object oriented programming language.

Polymorphism is one of the four fundamental Oops concepts. The other three are-

Polymorphism concept

Polymorphism is a Greek word, where poly means many and morph means change, refers to the ability of an object taking many forms.
The concept of polymorphism can be expressed as “One interface, multiple methods”. Where one general class can have the generic method and the derived classes (classes which extend the general class) may add class specific implementation to that method. At runtime or execution time “One interface” i.e. the general class will take “many forms” i.e. references of the derived classes.
You should Note that Polymorphism is extensively used with inheritance where parent class methods are overridden by the child classes.

Polymorphism in Java

There are basically two types of polymorphism in java –

Compile time polymorphism (or Static polymorphism)
Runtime polymorphism (or Dynamic Polymorphism)

We can perform polymorphism in java by:

Method overloading (Compile time polymorphism)– In method overloading it is known at compile time itself which of the overloaded method will be called.

Method overriding (Runtime polymorphism)– In method overriding it is resolved at run time which of the overridden method would be called. The decision is made based on the object reference the reference variable is pointing to.

Let’s see an example of run time polymorphism in Java to understand the concept of polymorphism.

package com.kkjavatutorials.model;

public class Circle extends Shape {
	//Constructor to initialize length
	public Circle(double radius) {
		super(radius); //calling the super class constructor
	}

	//Overriding the area() method
	public void area() {
		System.out.println("In area method of circle");
		System.out.println("Area of cirlce - " + 22/7 * getLength() * getLength());
	}
}

package com.kkjavatutorials.model;

public class Circle extends Shape {

//Constructor to initialize length

public Circle(double radius) {

super(radius); //calling the super class constructor

}

//Overriding the area() method

public void area() {

System.out.println("In area method of circle");

System.out.println("Area of cirlce - " + 22/7 * getLength() * getLength());

}

package com.kkjavatutorials.model;

public class Shape {
	private double length;

	public Shape(double length) {
		this.length = length;
	}

	public double getLength() {
		return length;
	}
	public void area() {

	}
}

package com.kkjavatutorials.model;

public class Shape {

private double length;

public Shape(double length) {

this.length = length;

}

public double getLength() {

return length;

}

public void area() {

}

package com.kkjavatutorials.model;

public class Square extends Shape {
	// Constructor to initialize length of super class variable
	public Square(double side) {
		super(side); // calling the super class constructor
	}

	// Overriding the area() method
	public void area() {
		System.out.println("Inside of area method of square");
		System.out.println("Area of square:" + getLength() * getLength());
	}
}

package com.kkjavatutorials.model;

public class Square extends Shape {

// Constructor to initialize length of super class variable

public Square(double side) {

super(side); // calling the super class constructor

}

// Overriding the area() method

public void area() {

System.out.println("Inside of area method of square");

System.out.println("Area of square:" + getLength() * getLength());

}

package com.kkjavatutorials.client;

import com.kkjavatutorials.model.Circle;
import com.kkjavatutorials.model.Shape;
import com.kkjavatutorials.model.Square;

public class PolymorphicClientTest {
	public static void main(String[] args) {
		Shape shape;
		Square square = new Square(15.0);
		Circle circle = new Circle(15.0);
		// shape dynamically bound to the Square object referenced by square
		shape = square;
		// area method of the square will be called
		shape.area();
		// shape dynamically bound to the Circle object referenced by circle
		shape = circle;
		// area method of the circle will be called
		shape.area();
	}
}

package com.kkjavatutorials.client;

import com.kkjavatutorials.model.Circle;

import com.kkjavatutorials.model.Shape;

import com.kkjavatutorials.model.Square;

public class PolymorphicClientTest {

public static void main(String[] args) {

Shape shape;

Square square = new Square(15.0);

Circle circle = new Circle(15.0);

// shape dynamically bound to the Square object referenced by square

shape = square;

// area method of the square will be called

shape.area();

// shape dynamically bound to the Circle object referenced by circle

shape = circle;

// area method of the circle will be called

shape.area();

}

Here we have a parent class Shape with a method area(), Shape class has two child classes Square and Circle.
In Square class, area() method is overridden and it provides the implementation to calculate area of a square.
Same way area() method is overridden in Circle class to provide implementation to calculate area of a circle.
Notice that in PolymorphicClientTest class’ main method object shape of class Shape is declared and later that shape object takes the reference of square and then circle and calls the area method of the respective classes.

Output after running this program would be –

Inside of area method of square
Area of square:225.0
In area method of circle
Area of cirlce - 675.0

Inside of area method of square

Area of square:225.0

In area method of circle

Area of cirlce - 675.0

So, it can be seen that the shape object holds the reference of the square object initially thus the area method of the Square class is called. Later the shape object holds the reference of the Circle object thus the area method of the Circle class is called.

Some of the key points about Polymorphism –

As we know that Polymorphism is a Greek word, where poly means many and morph means many forms thus it refers to the ability of an object taking many forms.
Many classes can implement an interface and each class is free to provide their own way of implementation. That’s how java uses interfaces, Java fully utilizes “one interface, multiple methods” aspect of polymorphism.
There are mainly two types of polymorphism in Java, compile time or static polymorphism and run time or Dynamic polymorphism.
Method overloading is an example of compile time or static polymorphism in Java.
Method overriding is an example of run time polymorphism in Java.

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That’s all about Understating of Polymorphism in Java.

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