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[Java OOP] Discussion of Polymorphism


Polymorphism is to use common interface instead of concrete implementation while coding.

The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Quick Summary:


  1. A subclass instance can be assigned to a superclass' reference.

  2. Once substituted, we can invoke methods defined in super-class; we CANNOT invoke methods from sub-class.

  3. if sub-class overrides inherited methods from the super-class, the overridden versions will be invoked.

  4. Reference cannot point to superclass.


  1. Simpler programs

    eg. superRef[] = new Circle[2]; superRef[0] = new Cylinder(); superRef[1] = new Circle();

    Just 1 single array to represent different objects

  2. Better re-usability

A practical example

Cylinder is a subclass of Circle. We gonna do this:

Circle c1 = new Cylinder();

The code is below:

public class PolymorphismSubstitutabilityDemo {

    public static void main(String[] args) {
        Circle c1 = new Cylinder(1, "white", 10);

class Circle {
    int radius;
    String color;

    public Circle(int a, String b) {
        this.radius = a;
        this.color = b;

    public int getRadius() {
        return radius;

    public double getArea() {
        return 3.14159 * Math.pow(radius, 2);

class Cylinder extends Circle {
    int height;

    public Cylinder(int a, String b, int c) {
        super(a, b);
        this.height = c;

    public double getArea() {
        return super.getArea() * height;

The output of execution:

class Cylinder