bounding_balls.pde

class Ball {
  int x, y, dx, dy, r;

  Ball() {
    x = (int)random(0, width);
    y = (int)random(0, height);
    dx = (int)random(-10, 10);
    dy = (int)random(-10, 10);
    r = 10;
  }

  void update() {
    int nx = x + dx;
    int ny = y + dy;

    if (nx < 0 || width <= nx) dx = -dx;
    if (ny < 0 || width <= ny) dy = -dy;
    
    x = x + dx;
    y = y + dy;
  }

  void draw() {
    fill(#ffffff);
    stroke(#ffffff);
    ellipse(x, y, r, r);
  }
}

ArrayList<Ball> list;

void setup() {
  size(400, 400);
  list = new ArrayList<Ball>();
  for (int i = 0; i < 200; ++i) {
    list.add(new Ball());
  }
}

void draw() {
  background(#000044);
  for (Ball b : list) {
    b.update();
    b.draw();
  }
}