code: private static String constructMessage(String property, 
                                           String newVal, String oldVal) {
      String message = "Could not change property " + property;
      if (oldVal != null) {
        message += " from \'" + oldVal;
      }
      if (newVal != null) {
        message += "\' to \'" + newVal + "\'";
      }
      return message;
    }
docs: Construct the exception message.