Would it be better (or even possible) to make an ArrayList of the ArrayList of the class?

Certainly... But I don't know why you would want to. In either case they both boil down to a dynamic number of colorful rectangles.

Or maybe  a simple class Explosion, that creates the array or Arraylist of rectangular balls. Depending on your plans it may be easier to control stuff.

Thanks tfguy44!

PS: I threw in OPENGL so it runs much faster.

1. import processing.opengl.*;
2. 
   
3. ArrayList manyRects;
4. int rectNumber = 200;
5. 
   
6. void setup() {
7.   size(900, 600, OPENGL);
8.   manyRects = new ArrayList();
9.   for (int i=0; i < rectNumber; i++) {
10.     manyRects.add(new Ball(width/2, height/2));
11.   }
12. }
13. 
    
14. void draw() {
15.   fill(255, 50);
16.   rect(0, 0, width, height);
17. 
    
18.   for (int i=0; i < manyRects.size(); i++) {
19.     Ball myRects = (Ball)manyRects.get(i);
20.     myRects.move();
21.     if (myRects.a < 0)
22.       manyRects.remove(i);
23.   }
24. }
25. 
    
26. void mousePressed() {
27.   for (int i=0; i < rectNumber; i++) {
28.     manyRects.add(new Ball(mouseX, mouseY));
29.   }
30. }
31. 
    
32. class Ball {
33.   float ballx;
34.   float bally;
35.   int ballSize = (int)random(5, 60);
36.   float ballspeed = random(2, 5);
37.   float ballxdirection = random(-1, 1);
38.   float ballydirection = random(-1, 1);
39.   int r = (int)random(254);
40.   int g = (int)random(254);
41.   int b = (int)random(254);
42.   int a = (int)random(50, 150);
43. 
    
44.   Ball(float tempballx, float tempbally) {
45.     ballx = tempballx;
46.     bally = tempbally;
47.   }
48. 
    
49.   void move() {
50.     if (ballx > width-ballSize/2 || ballx < ballSize/2)
51.       ballxdirection *= -1;
52.     if (bally > height-ballSize/2 || bally < ballSize/2)
53.       ballydirection *= -1;
54. 
    
55.     ballx += ballspeed*ballxdirection;
56.     bally += ballspeed*ballydirection;
57.     fill(r, g, b, a);
58.     noStroke();
59.     rect(ballx, bally, ballSize, ballSize);
60.     a -= 4;
61.   }
62. }

Some cosmetic improvements!  Just to use the Iterator class!  ^_^

Line #3 -> final static List<Ball> manyRects = new ArrayList();

Line #8 -> remove it!

Lines #18 - #23 :
    for ( Iterator<Ball> myRects = manyRects.iterator(); myRects.hasNext(); )
        if ( myRects.next().move() < 0 )   myRects.remove();

Line #49 -> int move() {

Line #60 -> return a -= 4;

Nice explosion of colorful rects!