public void move(int x, int y) {
	if (this.x + x < 0)
	    throw new IllegalArgumentException(
		    "X - Coordinate must not be negative!");
	this.x = this.x + x;
	if (this.y + y < 0)
	    throw new IllegalArgumentException(
		    "Y - Coordinate must not be negative!");
	this.y = this.y + y;
    }