Java Abstraction

abstract class Animal {
    String name;

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

    // Abstract method — no body, must be overridden
    public abstract String sound();

    // Concrete method — shared by all subclasses
    public void breathe() {
        System.out.println(name + " is breathing.");
    }
}

class Cat extends Animal {
    public Cat(String name) { super(name); }

    @Override
    public String sound() { return "Meow"; }
}

class Cow extends Animal {
    public Cow(String name) { super(name); }

    @Override
    public String sound() { return "Moo"; }
}

public class AbstractDemo {
    public static void main(String[] args) {
        // Animal a = new Animal("X");  // ERROR: cannot instantiate abstract class

        Animal cat = new Cat("Whiskers");
        Animal cow = new Cow("Bessie");

        System.out.println(cat.name + " says: " + cat.sound());
        System.out.println(cow.name + " says: " + cow.sound());
        cat.breathe();
    }
}

interface Drawable {
    void draw();  // abstract by default

    // Default method (Java 8+) — provides a default implementation
    default void describe() {
        System.out.println("I am a drawable shape.");
    }

    // Static method in interface (Java 8+)
    static void info() {
        System.out.println("Drawable interface v1.0");
    }
}

interface Resizable {
    void resize(double factor);
}

// A class can implement multiple interfaces
class Square implements Drawable, Resizable {
    double side;

    public Square(double side) { this.side = side; }

    @Override
    public void draw() {
        System.out.println("Drawing a square with side " + side);
    }

    @Override
    public void resize(double factor) {
        side *= factor;
        System.out.println("Resized to side " + side);
    }
}

public class InterfaceDemo {
    public static void main(String[] args) {
        Square sq = new Square(5.0);
        sq.draw();       // Drawing a square with side 5.0
        sq.describe();   // I am a drawable shape. (default method)
        sq.resize(2.0);  // Resized to side 10.0

        Drawable.info(); // Drawable interface v1.0 (static method)
    }
}