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