code: public boolean isSet(_Fields field) {
      if (field == null) {
        throw new IllegalArgumentException();
      }
  
      switch (field) {
      case APP:
        return isSetApp();
      case NUM_MESSAGES:
        return isSetNumMessages();
      case COMPRESSION:
        return isSetCompression();
      case CRC:
        return isSetCrc();
      case MESSAGES:
        return isSetMessages();
      }
      throw new IllegalStateException();
    }
docs: Returns true if field corresponding to fieldID is set (has been assigned a value) and false otherwise