code: public void addExtension(Object extension) {
          if (extension == null)
              return;
          if (_extensions == null)
              _extensions = new ArrayList<>();
          _extensions.add(extension);
      }
docs: Adds an extension for the current entity. The configuration must have been setup to understand how this object can be read/written.
  @param extension extension (can be any object) to add; a null object will be ignored