code: public String extractData(XEvent event) {
                  XAttribute attribute = event.getAttributes().get(KEY_DATA);
                  if (attribute == null) {
                          return null;
                  } else {
                          return ((XAttributeLiteral) attribute).getValue();
                  }
          }
docs: Extracts the data attribute string from an event.
  
  @param event Event to be queried.
  @return Data string for the given event (may be <code>null</code> if
  not defined)