This week's giveaway is in the Android forum.
We're giving away four copies of Android Security Essentials Live Lessons and have Godfrey Nolan on-line!
See this thread for details.
The moose likes Beginning Java and the fly likes dx = -dx; dy = -dy Big Moose Saloon
  Search | Java FAQ | Recent Topics | Flagged Topics | Hot Topics | Zero Replies
Register / Login


Win a copy of Android Security Essentials Live Lessons this week in the Android forum!
JavaRanch » Java Forums » Java » Beginning Java
Bookmark "dx = -dx; dy = -dy" Watch "dx = -dx; dy = -dy" New topic
Author

dx = -dx; dy = -dy

Jon Camilleri
Ranch Hand

Joined: Apr 25, 2008
Posts: 660

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



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


Jon
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18544
    
  40


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


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Jon Camilleri
Ranch Hand

Joined: Apr 25, 2008
Posts: 660

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
Java Cowboy
Saloon Keeper

Joined: Aug 16, 2005
Posts: 14074
    
  16

Confirmed...


Java Beginners FAQ - JavaRanch SCJP FAQ - The Java Tutorial - Java SE 7 API documentation
Scala Notes - My blog about Scala
Unnar Björnsson
Ranch Hand

Joined: Apr 30, 2005
Posts: 164
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
 
I agree. Here's the link: http://aspose.com/file-tools
 
subject: dx = -dx; dy = -dy
 
Similar Threads
C++ish Address &
New to Java. Need some proformance suggestions
exception java.lang.InterruptedException is never thrown in body of corresponding try statement
Help needed with the Code
A simple syntax question, pls help