dx = -dx; dy = -dy

[Jon Camilleri]

Can you explain how variables dx and dy are used to reverse the direction of a bouncing ball?

import java.awt.geom.*;

/**
   A ball that moves and bounces off the edges of a 
   rectangle
 * @version 1.33 2007-05-17
 * @author Cay Horstmann
*/
public class Ball
{
   /**
      Moves the ball to the next position, reversing direction
      if it hits one of the edges
   */
   public void move(Rectangle2D bounds)
   {
      x += dx;
      y += dy;
      if (x < bounds.getMinX())
      { 
         x = bounds.getMinX();
         dx = -dx;
      }
      if (x + XSIZE >= bounds.getMaxX())
      {
         x = bounds.getMaxX() - XSIZE; 
         dx = -dx; 
      }
      if (y < bounds.getMinY())
      {
         y = bounds.getMinY(); 
         dy = -dy;
      }
      if (y + YSIZE >= bounds.getMaxY())
      {
         y = bounds.getMaxY() - YSIZE;
         dy = -dy; 
      }
   }

   /**
      Gets the shape of the ball at its current position.
   */
   public Ellipse2D getShape()
   {
      return new Ellipse2D.Double(x, y, XSIZE, YSIZE);
   }

   private static final int XSIZE = 15;
   private static final int YSIZE = 15;
   private double x = 0;
   private double y = 0;
   private double dx = 1;
   private double dy = 1;
}

Source Core Java Vol.I (8th Ed), Ch.14

[Henry Wong]

It isn't very hard code to figure out -- in fact, it is incredibly easy. And figuring out another's code is a common task all developers do. Care to take a shot at it, and tell us how you think it works first?

Henry

[Jon Camilleri]

Henry Wong wrote:
It isn't very hard code to figure out -- in fact, it is incredibly easy. And figuring out another's code is a common task all developers do. Care to take a shot at it, and tell us how you think it works first?

Henry

I figured out that by default the ball object moves by (1,1), however, when the ball reaches the enclosing bounds (of type Rectangle2D), the sign is reversed so that the sign of x and y are changed. This is done so when the dx or dy exceeds the bounds the ball switches to the opposing bound. For example, if x is becoming -5 then it becomes +5, so that the ball position starts from (5, y). I just needed a confirmation.

[Jesper de Jong]

Confirmed...

[Unnar Björnsson]

Jon Camilleri wrote:
I figured out that by default the ball object moves by (1,1), however, when the ball reaches the enclosing bounds (of type Rectangle2D), the sign is reversed so that the sign of x and y are changed. This is done so when the dx or dy exceeds the bounds the ball switches to the opposing bound. For example, if x is becoming -5 then it becomes +5, so that the ball position starts from (5, y). I just needed a confirmation.

No the sign of x and y is never changed, when the ball's position crosses any of the four boundaries, the ball is set to the boundary wall it exceeded and the sign of either dx or dy is changed f.ex when x becomes less than bounds.getMinX() x is set to bounds.getMinX() and dx becomes -dx