// Page 101
abstract class Shape {
    abstract double area();
}

class Circle extends Shape {
    final double radius;

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

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

class Rectangle extends Shape {
    final double length;
    final double width;

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

    double area() { return length * width; }
}

// Page 102
class Square extends Rectangle {
    Square(double side) {
        super(side, side);
    }

    double side() {
        return length; // or equivalently, width
    }
}