introduction.md

Introduction

Inheritance lets one class build on top of another, reusing its fields and methods without rewriting them. The class being extended is called the parent (or superclass); the class doing the extending is the child (or subclass). The relationship is described as IS-A: a Dog IS-A Animal.

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

Use the extends keyword to inherit from a class:

public class Dog extends Animal {

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

The `@Override` annotation tells the compiler that you intend to replace a parent method.
It's not required, but it's considered good practice — the compiler will warn you if the method name doesn't actually match anything in the parent.

Because Dog extends Animal, a Dog instance can be stored in an Animal variable. The actual type at runtime decides which method runs — this is called polymorphism:

Animal animal = new Dog();
animal.makeSound(); // prints "Woof!", not "..."

The super keyword

Inside a child class, super refers to the parent. You can use it to call the parent's constructor or a method you've overridden:

public class Dog extends Animal {
    private String name;

    public Dog(String name) {
        super(); // calls Animal's constructor
        this.name = name;
    }

    @Override
    public void makeSound() {
        super.makeSound(); // calls Animal's version first
        System.out.println(name + " says: Woof!");
    }
}

Abstract classes

If it doesn't make sense to instantiate the parent directly, mark it abstract. An abstract class can declare methods without providing a body — each subclass is then required to implement them:

public abstract class Shape {
    public abstract double area(); // subclasses must implement this
}

public class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

To read more about inheritance in Java, see the official documentation.