class Ball extends Throwable {}
class P{
    P target;
    P(P target) {
        this.target = target;
    }
    void aim (Ball ball) {
        try {
            throw ball;
        }
        catch (Ball b) {
            target.aim(b);
        }
    }
    public static void main(String[] args) {
        P parent = new P(null);
        P child = new P(parent);
        parent.target = child;
        parent.aim(new Ball());
    }
}