Processing.js

// This is a code fragment that shows how to use an ArrayList.
// It won't compile because it's missing the Ball class.

ArrayList balls;

void setup() {
  size(200, 200);
  balls = new ArrayList();  // Create an empty ArrayList
  balls.add(new Ball(width/2, 0, 48));  // Start by adding one element
}

void draw() {
  background(255);

  // With an array, we say balls.length, with an ArrayList, 
  // we say balls.size(). The length of an ArrayList is dynamic. 
  // Notice how we are looping through the ArrayList backwards, this 
  // is because we are deleting elements from the list  
  for (int i = balls.size()-1; i >= 0; i--) { 
    // An ArrayList doesn't know what it is storing so we have 
    // to cast the object coming out
    Ball ball = (Ball) balls.get(i);
    ball.move();
    ball.display();
    if (ball.finished()) {
      // Items can be deleted with remove()
      balls.remove(i);
    }
  }  
}

void mousePressed() {
  // A new ball object is added to the ArrayList, by default to the end
  balls.add(new Ball(mouseX, mouseY, ballWidth));
}