code: public void setUserId(String userId) {
          if (userId == null) {
              _properties.remove(AMQP_PROP_USER_ID);
              return;
          }
          
          _properties.put(AMQP_PROP_USER_ID, userId);
      }
docs: Sets the value of "userId" property. If a null value is passed
  in, it indicates that the property is not set.
  
  @param  userId    value of "userId" property