this.id = id; path = Helper.getPixelsPath(id);
path = Helper.getPixelsPath(pixels.getId());
    public PixelBuffer (Integer id, Pixels pixels)    {        if (id == null)            throw new NullPointerException("Expecting a not-null id.");                this.pixels = pixels;        this.id = id;        path = Helper.getPixelsPath(id);    }