public Invaders(int niveau, InvaderFires invaderFires, GameController gc) {
    invaders = new ArrayList<Invader>();

    if (niveau == 1) {
      for (int i = 10; i < Window.HEIGHT - 50; i += 50) {
        for (int j = Window.WIDTH - 500; j < Window.WIDTH - 50; j += 70) {
          invaders.add(new Invader(j, i, this));
        }
      }
    }
    this.invader_fires = invaderFires;
    last_shoot = 4600;
    this.gc = gc;
  }