public void update(int elapsedTimeMilisec) {
    for (Projectile p : projectileList) {
      p.update(elapsedTimeMilisec);
    }

    // Remove inactive invaders
    for (int i = this.projectileList.size() - 1; i >= 0; i--) {
      if (this.projectileList.get(i).isRemoved()) {
        this.projectileList.remove(i);
      }
    }
    for (Wall w : this.walls) {
      w.clearPieces();
    }
    for (Animation explosion : Game.explosionList) {
      explosion.update(elapsedTimeMilisec);
    }
    for (int i = this.walls.size() - 1; i >= 0; i--) {

      if (walls.get(i).isRemoved()) {

        walls.remove(i);
      }
    }
  }
  public void render(int xOffset, int yOffset, Screen screen) {
    int x0 = xOffset;
    int y0 = yOffset;
    int x1 = x0 + screen.getWidth();
    int y1 = y0 + screen.getHeight();

    for (int y = y0; y < y1; y++) {
      for (int x = x0; x < x1; x++) {
        screen.pixels[x + y * screen.getWidth()] = pixels[x + y * width];
      }
    }
    for (Projectile p : projectileList) {
      p.render(screen);
    }
    for (Wall w : walls) {
      w.render(screen);
    }
    for (Animation explosion : Game.explosionList) {
      explosion.render(screen);
    }
  }