// Java (хороший пример)
interface Shape {
    double area();
}

class Rectangle implements Shape {
    private final double width;
    private final double height;

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

    public double area() {
        return width * height;
    }
}

class Square implements Shape {
    private final double side;

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

    public double area() {
        return side * side;
    }
}