Inheritance

// Parent class
public class Animal {
    protected String name;
    protected int age;

    public Animal(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void eat() {
        System.out.println(name + " is eating.");
    }

    public String toString() {
        return name + " (age: " + age + ")";
    }
}

// Child class
public class Dog extends Animal {
    private String breed;

    public Dog(String name, int age, String breed) {
        super(name, age); // call parent constructor
        this.breed = breed;
    }

    public void bark() {
        System.out.println(name + " says Woof!");
    }
}

Dog dog = new Dog("Rex", 3, "Labrador");
dog.eat();  // inherited
dog.bark(); // own method

public class Shape {
    public double area() {
        return 0;
    }
}

public class Circle extends Shape {
    private double radius;

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

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

public class Rectangle extends Shape {
    private double width, height;

    public Rectangle(double w, double h) {
        this.width = w;
        this.height = h;
    }

    @Override
    public double area() {
        return width * height;
    }
}