Notes on Collision Code

This applet has a lot of classes communicating even though there isn't a lot of code. It might be helpful to describe it here. I'll spare you the Booch treatment. I'll start with visible classes, then show communications, and finish with a laundry list of helper classes.

Visible Classes

An AngleCanvas has two jobs.
  1. It displays the relative position of two adjacent balls about the center of mass. Here, it represents the balls at the moment of impact. We display two AngleCanvases, but give each different information, one for lab frame, one for center of mass frame.
  2. It is a type of control where the user can drag either ball in order to change the impact parameter of the masses.

This custom scroll bar has a center position. It's value changes exponentially as you move away from the center position. We use it to change the mass ratio between mass 1 and mass 2.

This Canvas displays two balls and a center of mass marker. It is told what the ball locations are and where they will collide so it can draw the current locations and trails. Two of these are instantiated, one for the Lab Frame, and one for the Center of Mass Frame.

Collision -- a slidebar
This is the slider in-between the two CollisionGraphs. Changing the slidebar changes the time. This class is much more than a control, though. Because I figure the action of time is the nature of the collision, it is this class that decides the current location of the balls.
There are a few little classes like text fields, but these are the main ones and the others fit in sensibly.


While a lot of messages occur, the principle is simple, controls talk to calculators which talk to displays.

The two calculator classes are
Collision is active during a shot. It's Thread determines the current location of the balls for both instances of CollisionGraph. It also has static functions which will determine what happens during a collision between two balls. It uses these functions to determine where balls will be as a function of time. While this object is a listener to all the controls, it doesn't need to do any calculations until the moment the user hits the fire button.

This object has to show the actual collision of the balls at all times as a function of the mass ratio and impact parameter. It has no effect on the moving balls during a collision but controls the visible instances of AngleCanvas.

Helper Classes

Class TurnedLabel
This is simply a label on its side. You have to draw the label to a backbuffer, then apply a rotation filter (RotFilter.class) to the backbuffer, then display it.

Class RotFilter
Turns any image 90 degrees either direction. Much more efficient code than using a general affine transformation class. See TurnedLabel code to figure out how to use an image filter.

Class DoubleTextField
Simply a textfield that reads and displays double values. This implements a classic Listener interface except that the listener is a DoubleListener, shown below.

Interface DoubleListener
This interface is a promise to implement a SetValue(double,String) method where the String is like an ActionCommand to identify who is sending you the double.

Interface CollisionColors
In order to have better control over the applet colors, I put them all into an interface, kind of like a header file. That way, I can switch them in and out without rooting though the text.

Class Arrow
A Polygon that looks like an Arrow. If anyone can make one that looks like a real arrow at any angle, I'll give you ten dollars.

Class MultiLineLabel
Taken from part of an example the Nutshell Examples book. It's just what it says it is. I only typed in the features I used.