code: public Observable<Void> recognizeTextAsync(String url, TextRecognitionMode mode) {
          return recognizeTextWithServiceResponseAsync(url, mode).map(new Func1<ServiceResponseWithHeaders<Void, RecognizeTextHeaders>, Void>() {
              @Override
              public Void call(ServiceResponseWithHeaders<Void, RecognizeTextHeaders> response) {
                  return response.body();
              }
          });
      }
docs: Recognize Text operation. When you use the Recognize Text interface, the response contains a field called 'Operation-Location'. The 'Operation-Location' field contains the URL that you must use for your Get Recognize Text Operation Result operation.
  
  @param mode Type of text to recognize. Possible values include: 'Handwritten', 'Printed'
  @param url Publicly reachable URL of an image
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceResponseWithHeaders} object if successful.