// retrieves url with HEAD. returns content length,
  // or -1 in case of error.
  public static long getURLSizeByHEAD(String url) {
    if (!url.contains("//")) url = "http://" + url;
      
    HttpURLConnection conn = null;
    try {
      conn = (HttpURLConnection) new URL(url).openConnection();
      conn.setRequestMethod("HEAD");
      conn.getInputStream();
      return conn.getContentLength();
    } catch (IOException e) {
      return -1;
    } finally {
      conn.disconnect();
    }
  }