code: @Override
      public Comparable<?> getCompareValue(final AbstractUIField _uiField)
          throws EFapsException
      {
          Comparable<?> ret = (Comparable<?>) _uiField.getValue().getReadOnlyValue(_uiField.getParent().getMode());
          if (ret instanceof String && StringUtils.isEmpty((String) ret)) {
              ret = null;
          }
          return ret;
      }
docs: {@inheritDoc}