public abstract class Point { // ADT interface
    // implementations and private members hidden

    public double x () {
	return r () * Math.cos (theta ());
    }

    public double y () {
	return r () * Math.sin (theta ());
    }

    public double r () {
	double x = x ();
	double y = y ();
	return Math.sqrt (x * x + y * y);
    }

    public double theta () {
	return Math.atan2 (y (), x ());
    }

    public double distance (Point p) {
	double dx = this.x () - p.x ();
	double dy = this.y () - p.y ();
	return Math.sqrt (dx * dx + dy * dy);
    }

    public String toString () {
	return "(" + x () + "," + y () + ")";
    }
}