Java Abstract Classes

abstract class Vehicle {
    // Abstract method (no body)
    abstract void start();

    // Concrete method (has body)
    void stop() {
        System.out.println("Vehicle stopped.");
    }
}

abstract class Shape {
    String color;

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

    // Abstract method (Subclasses MUST implement this)
    abstract double getArea();

    // Concrete method (Subclasses inherit this)
    void display() {
        System.out.println("This is a " + color + " shape.");
    }
}

class Circle extends Shape {
    double radius;

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

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

class Rectangle extends Shape {
    double length, width;

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

    @Override
    double getArea() {
        return length * width;
    }
}

public class Main {
    public static void main(String[] args) {
        // Shape s = new Shape("Red"); // Error! Cannot instantiate abstract class

        Circle c = new Circle("Red", 5);
        c.display();
        System.out.println("Area: " + c.getArea());
    }
}