private double angleTo(Point2D that){
  double dx=that.x - this.x;
  double dy=that.y - this.y;
  return Math.atan2(dy,dx);
}

private double angleTo ( Point2D that ) { double dx = that . x - this . x ; double dy = that . y - this . y ; return Math . atan2 ( dy , dx ) ; }
returns the angle between this point and that point .
