code: void trialHeadMethodForInstanceLength() throws IOException {
          final DownloadConnection connection = OkDownload.with().connectionFactory()
                  .create(task.getUrl());
          final DownloadListener listener = OkDownload.with().callbackDispatcher().dispatch();
          try {
              connection.setRequestMethod(METHOD_HEAD);
              final Map<String, List<String>> userHeader = task.getHeaderMapFields();
              if (userHeader != null)  Util.addUserRequestHeaderField(userHeader, connection);
  
              listener.connectTrialStart(task, connection.getRequestProperties());
              final DownloadConnection.Connected connectedForContentLength = connection.execute();
              listener.connectTrialEnd(task, connectedForContentLength.getResponseCode(),
                      connectedForContentLength.getResponseHeaderFields());
  
              this.instanceLength = Util.parseContentLength(
                      connectedForContentLength.getResponseHeaderField(CONTENT_LENGTH));
          } finally {
              connection.release();
          }
      }
docs: right one.