Inheritance & Polymorphism

Build class families with extends, override methods and treat different objects the same way.

🎯 What you will learn

Extend a parent class
Override methods with @Override
Use polymorphism with a parent type

👋 Let’s get started!

A Cat IS AN Animal. A Dog IS AN Animal. Inheritance lets child classes reuse a parent’s code 🧬, and polymorphism lets one line of code work for many kinds of objects.

🪜 Step by step

1. 👪 extends

The child gets the parent’s fields and methods.

class Animal {
    String name;
    void eat() { System.out.println(name + " eats"); }
}

class Dog extends Animal {
    void fetch() { System.out.println(name + " fetches!"); }
}

2. 🔁 @Override

Children can replace a parent method with their own version.

class Animal {
    String sound() { return "..."; }
}
class Cat extends Animal {
    @Override
    String sound() { return "Meow"; }
}

3. 🎭 Polymorphism

A parent-type variable can hold any child — and the child’s method runs.

Animal[] zoo = { new Cat(), new Dog() };
for (Animal a : zoo) {
    System.out.println(a.sound());
}

🧪 Try it!

Read the code, guess what it prints, then press 👀 Show output.

Shape family

abstract class Shape {
    abstract double area();
}
class Circle extends Shape {
    double r;
    Circle(double r) { this.r = r; }
    double area() { return Math.PI * r * r; }
}
class Square extends Shape {
    double s;
    Square(double s) { this.s = s; }
    double area() { return s * s; }
}
// main:
Shape[] shapes = { new Circle(1), new Square(3) };
for (Shape sh : shapes) {
    System.out.printf("%.2f%n", sh.area());
}

Override a method

class Bird extends Animal {
    @Override
    String sound() {
        return "Tweet";
    }
}

📖 Key words

Tap a card to flip it over!

Inheritance: A child class reusing a parent class 🧬
extends: Makes a class a child of another 👪
@Override: Replaces a parent method 🔁
Polymorphism: One interface, many forms 🎭

🏆 Challenge

Predict the output.

Challenge

class Vehicle { String wheels() { return "some"; } }
class Bike extends Vehicle { String wheels() { return "2"; } }
class Car extends Vehicle { String wheels() { return "4"; } }
// main:
Vehicle v = new Car();
System.out.println(v.wheels());

Knowledge Check Quiz

What does extends do?
What prints?
A Cat “is a” …