ইন্টারফেস এবং অ্যাবস্ট্রাক্ট ক্লাস (Interfaces & Abstract Classes)

public class Main {
    // Interface — the contract
    interface Drawable {
        void draw();  // any Drawable must have this
    }

    interface Resizable {
        void resize(double factor);
    }

    // A class can implement MULTIPLE interfaces
    static class Circle implements Drawable, Resizable {
        double radius;

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

        @Override
        public void draw() {
            System.out.printf("Drawing circle with radius %.1f%n", radius);
        }

        @Override
        public void resize(double factor) {
            radius *= factor;
            System.out.printf("Resized to radius %.1f%n", radius);
        }
    }

    static class Square implements Drawable {
        double side;

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

        @Override
        public void draw() {
            System.out.printf("Drawing square with side %.1f%n", side);
        }
    }

    public static void main(String[] args) {
        // Polymorphism through interfaces
        Drawable[] art = { new Circle(5), new Square(3), new Circle(2) };

        for (Drawable d : art) {
            d.draw();  // Java picks the right version
        }

        System.out.println("---");
        Circle c = new Circle(10);
        c.resize(0.5);
        c.draw();
    }
}

Drawing circle with radius 5.0
Drawing square with side 3.0
Drawing circle with radius 2.0
---
Resized to radius 5.0
Drawing circle with radius 5.0

public class Main {
    // Interface with default method
    interface Greetable {
        String getName();

        // Default method — classes get this for free
        default void greet() {
            System.out.println("Hello! I'm " + getName());
        }
    }

    interface Farewell {
        String getName();

        default void sayBye() {
            System.out.println("Goodbye from " + getName() + "!");
        }
    }

    // Implements both interfaces
    static class Person implements Greetable, Farewell {
        private String name;

        Person(String name) {
            this.name = name;
        }

        @Override
        public String getName() {
            return name;
        }

        // Can override default methods too
        @Override
        public void greet() {
            System.out.println("Hey there! The name's " + name + ".");
        }
    }

    static class Robot implements Greetable {
        @Override
        public String getName() {
            return "RoboHelper-3000";
        }
        // Uses the default greet() — no override needed
    }

    public static void main(String[] args) {
        Person alex = new Person("Asif");
        alex.greet();    // overridden version
        alex.sayBye();   // default version

        Robot bot = new Robot();
        bot.greet();     // default version
    }
}

Hey there! The name's Asif.
Goodbye from Asif!
Hello! I'm RoboHelper-3000