code: public void setGcmId(Optional<String> gcmRegistrationId) throws IOException {
      if (gcmRegistrationId.isPresent()) {
        this.pushServiceSocket.registerGcmId(gcmRegistrationId.get());
      } else {
        this.pushServiceSocket.unregisterGcmId();
      }
    }
docs: Register/Unregister a Google Cloud Messaging registration ID.
  
  @param gcmRegistrationId The GCM id to register.  A call with an absent value will unregister.
  @throws IOException