Polymorphism

Syntax

ParentType variable = new ChildType();

Examples

class Animal {
    public void makeSound() {
        System.out.println("Some generic sound");
    }
}

class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Woof!");
    }
}

class Cat extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Meow!");
    }
}

Animal[] animals = { new Dog(), new Cat(), new Animal() };
for (Animal a : animals) {
    a.makeSound();  // calls the correct version for each actual object type
}
// Woof!
// Meow!
// Some generic sound

interface Shape {
    double area();
}

class Circle implements Shape {
    private double radius;
    public Circle(double radius) { this.radius = radius; }
    public double area() { return Math.PI * radius * radius; }
}

class Square implements Shape {
    private double side;
    public Square(double side) { this.side = side; }
    public double area() { return side * side; }
}

Shape[] shapes = { new Circle(3), new Square(4) };
for (Shape s : shapes) {
    System.out.printf("Area: %.2f%n", s.area());
}

class Employee {
    protected String name;
    public Employee(String name) { this.name = name; }
    public double calculatePay() { return 0; }
}

class FullTime extends Employee {
    public FullTime(String name) { super(name); }
    @Override
    public double calculatePay() { return 5000; }
}

class Contractor extends Employee {
    public Contractor(String name) { super(name); }
    @Override
    public double calculatePay() { return 3000; }
}

public class Payroll {
    public static void printPay(Employee e) {  // works with ANY Employee subclass
        System.out.println(e.name + ": $" + e.calculatePay());
    }

    public static void main(String[] args) {
        printPay(new FullTime("Fola"));
        printPay(new Contractor("Zain"));
    }
}

Best practices

• Declare variables, parameters, and return types using the parent type or interface when you want to write flexible, extensible code
• Rely on polymorphism instead of type-checking with instanceof and manual casting where possible - let overridden methods handle the differences
• Remember polymorphism applies to overridden instance methods, not fields or static methods, which are resolved based on the declared type instead
• Design parent classes/interfaces around behavior that genuinely varies by subclass, so polymorphism adds real value rather than just ceremony

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation