ইনহেরিট্যান্স এবং পলিমরফিজম (Inheritance & Polymorphism)

public class Main {
    static class Animal {
        String name;
        int age;

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

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

        void describe() {
            System.out.println(name + " is " + age + " years old.");
        }
    }

    // Dog IS-AN Animal — it inherits everything
    static class Dog extends Animal {
        String breed;

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

        void fetch() {
            System.out.println(name + " fetches the ball!");
        }
    }

    static class Cat extends Animal {
        Cat(String name, int age) {
            super(name, age);
        }

        void purr() {
            System.out.println(name + " purrs... rrrrr");
        }
    }

    public static void main(String[] args) {
        Dog buddy = new Dog("Buddy", 4, "Labrador");
        buddy.describe();  // inherited from Animal
        buddy.eat();       // inherited from Animal
        buddy.fetch();     // Dog's own method

        Cat whiskers = new Cat("Whiskers", 7);
        whiskers.describe();
        whiskers.purr();
    }
}

Buddy is 4 years old.
Buddy is eating.
Buddy fetches the ball!
Whiskers is 7 years old.
Whiskers purrs... rrrrr

public class Main {
    static class Shape {
        String color;

        Shape(String color) {
            this.color = color;
        }

        double area() {
            return 0;  // default — shapes need to override this
        }

        void describe() {
            System.out.printf("%s shape with area %.2f%n", color, area());
        }
    }

    static class Circle extends Shape {
        double radius;

        Circle(String color, double radius) {
            super(color);
            this.radius = radius;
        }

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

    static class Rectangle extends Shape {
        double width, height;

        Rectangle(String color, double width, double height) {
            super(color);
            this.width = width;
            this.height = height;
        }

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

    public static void main(String[] args) {
        // Polymorphism: parent type, child objects
        Shape[] shapes = {
            new Circle("Red", 5),
            new Rectangle("Blue", 4, 6),
            new Circle("Green", 3)
        };

        for (Shape s : shapes) {
            s.describe();  // Java calls the RIGHT area() for each!
        }
    }
}

Red shape with area 78.54
Blue shape with area 24.00
Green shape with area 28.27

public class Main {
    // Abstract class — can't create instances directly
    static abstract class Appliance {
        String brand;

        Appliance(String brand) {
            this.brand = brand;
        }

        // Abstract method — child MUST implement this
        abstract void turnOn();

        // Regular method — child inherits this as-is
        void showBrand() {
            System.out.println("Brand: " + brand);
        }
    }

    static class Toaster extends Appliance {
        Toaster(String brand) {
            super(brand);
        }

        @Override
        void turnOn() {
            System.out.println("Toaster glows orange. Toast incoming!");
        }
    }

    static class Blender extends Appliance {
        Blender(String brand) {
            super(brand);
        }

        @Override
        void turnOn() {
            System.out.println("VRRRRRR! Blender is blending!");
        }
    }

    public static void main(String[] args) {
        // Appliance a = new Appliance("X");  // ERROR! Can't instantiate abstract

        Appliance t = new Toaster("KitchenPro");
        Appliance b = new Blender("BlendMax");

        t.showBrand();
        t.turnOn();
        b.showBrand();
        b.turnOn();
    }
}

Brand: KitchenPro
Toaster glows orange. Toast incoming!
Brand: BlendMax
VRRRRRR! Blender is blending!