// Learning Processing
  // Daniel Shiffman
  // http://www.learningprocessing.com
  
  // Example 23-2: Simple particle system with ArrayList
  
  ArrayList particles;
  
  void setup() {
    size(200,200);
    particles = new ArrayList();
    smooth();
  }
  
  void draw() {
    // A new Particle object is added to the ArrayList every cycle through draw().
    particles.add(new Particle()); 
  
    background(255);
    // Iterate through our ArrayList and get each Particle
    // The ArrayList keeps track of the total number of particles.
    for (int i = 0; i < particles.size(); i++ ) { 
      Particle p = (Particle) particles.get(i);
      p.run();
      p.gravity();
      p.display();
    }
  
    // If the ArrayList has more than 100 elements in it, we delete the first element, using remove().
    if (particles.size() > 100) {
      particles.remove(0); 
    }
  }