public class MovingPolygon extends DPolygon
{
  /*
  A MovingPolygon, is just a DPolygon with a velocity (speed, direction).  The MovingPolygon
  includes just one function, move, to move itself one step according to its velocity.  This
  class also defines the topology--a 2D torus manifold.  Polygons moving off one end of the
  screen simply reappear on the other side.
  */

  // maximum coordinates before wrapping to 0.0
  public static double X_LIMIT = 400.0, Y_LIMIT = 400.0;

  public double speed, direction; // defines a velocity

  public MovingPolygon() // allocate space for a moving polygon
  {
    super();
    speed = 0.0;
    direction = 0.0;
  }

  public MovingPolygon(MovingPolygon poly) // make a copy of a moving polygon
  {
    super(poly);
    speed = poly.speed;
    direction = poly.direction;
  }

  public MovingPolygon(double[] x, double[] y, double r, DPoint c)
  { // create a moving polygon given coordinates, a radius, and a center point
    super(x,y,r,c);
  }

  public void move() // move the polygon one step within the 2D torus manifold
  {
    translate(speed * Math.cos(direction),
              speed * Math.sin(direction));

    // check bounds on the visible area and fix if out of range
    if      (center.x < 0.0)     translate(X_LIMIT,  0.0);
    else if (center.x > X_LIMIT) translate(-X_LIMIT, 0.0);
    if      (center.y < 0.0)     translate(0.0,  Y_LIMIT);
    else if (center.y > Y_LIMIT) translate(0.0, -Y_LIMIT);
  }
}