Module 6: Inheritance and Polymorphism

Polymorphism in Action

Use one parent type to work with many child objects while Java chooses the correct overridden method at runtime.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-16

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, edited by hand, and checked against current Java 25 documentation and runnable examples.

Learning goals

Understand the idea of one type reference pointing to different child objects
See runtime method selection in practice
Write more flexible code with parent references

Polymorphism means one reference type can work with many related object types: A variable of type Animal can point to a Dog or a Cat.

Java chooses the correct overridden method at runtime: Even if the variable type is the parent class, the actual object decides which method runs.

Why this matters: Code can stay general while still getting specific behavior from each child object.

Beginner benefit: You can process many related objects in one loop without writing a separate method call for every child class.

Runnable examples

One parent type, different child behavior

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

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

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

public class Main {
    public static void main(String[] args) {
        Animal first = new Dog();
        Animal second = new Cat();

        first.makeSound();
        second.makeSound();
    }
}

Expected output

Woof
Meow

Mini exercise

Create an array of `Animal` references containing different child objects and call `makeSound()` on each one.

Summary

Polymorphism lets one parent type work with many child objects.
The actual object decides which overridden method runs.
This makes code more flexible and reusable.

Next step

Next, use abstract classes when a parent should define a template but should not be created directly.

Sources used

Polymorphism in Action

One parent type, different child behavior

Why is polymorphism useful?