PImage back;
ArrayList particles;

void setup() {
  
  size(300,300);
  back = loadImage("back_arraylist.gif");
  framerate(40);
  stroke(1);
  colorMode(RGB,200,200,200,90);
  particles = new ArrayList();
  smooth();

  }
  
  void draw() {
    
    particles.add(new Particle());
    
   image(back,0,0);

    // iterate through our arraylist adn get each particle
    for (int i = 0; i < particles.size(); i++) {
      // when something comes out of the arraylist via "get" we have to
      // remind ourselves what type it is. In this case, it's an instance of "Particle"
      Particle p = (Particle) particles.get(i++);
      p.run();
      p.gravity();
      p.render();

      }

      // Remove the first particle when the list gets over 100.
      if (particles.size() > 100) {
        particles.remove(0);
        }

    }
    

    class Particle {
      float x;
      float y;
      float xspeed;
      float yspeed;
      
      Particle() {
        x = mouseX;
        y = mouseY;
        xspeed = random(-1,5);
        yspeed = random(-5,1);
        }
        
    void run() {
      x = x + xspeed;
      y = y + yspeed;
      }    
      
    void gravity() {
      yspeed += 0.5;
      
      }
    
    void render() {
      fill(200,40);
      noStroke();
      ellipse(x,y,random(60),random(30));
      }  
      }