import java.lang.Math;

// Here's a basic interface for geometric shapes.
interface Geometry {
    double area();
    double perim();
}

// For our example we'll implement this interface on Rectangle and Circle classes.
class Rectangle implements Geometry {
    double width, height;

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

    // To implement an interface in Java, we need to provide implementations
    // for all the methods declared in the interface.
    @Override
    public double area() {
        return width * height;
    }

    @Override
    public double perim() {
        return 2 * width + 2 * height;
    }

    @Override
    public String toString() {
        return "Rectangle{width=" + width + ", height=" + height + "}";
    }
}

class Circle implements Geometry {
    double radius;

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

    // The implementation for Circle.
    @Override
    public double area() {
        return Math.PI * radius * radius;
    }

    @Override
    public double perim() {
        return 2 * Math.PI * radius;
    }

    @Override
    public String toString() {
        return "Circle{radius=" + radius + "}";
    }
}

public class Interfaces {
    // If a parameter has an interface type, then we can pass any object
    // that implements that interface. Here's a generic measure method
    // taking advantage of this to work on any Geometry.
    public static void measure(Geometry g) {
        System.out.println(g);
        System.out.println(g.area());
        System.out.println(g.perim());
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle(3, 4);
        Circle c = new Circle(5);

        // The Rectangle and Circle classes both implement the Geometry
        // interface, so we can use instances of these classes as arguments to measure.
        measure(r);
        measure(c);
    }
}