code: public void add(String property, UserMessage message) {
          assertArgumentNotNull("property", property);
          assertArgumentNotNull("message", message);
          assertLocked();
          final UserMessageItem item = getPropertyItem(property);
          final List<UserMessage> messageList;
          if (item == null) {
              ++itemCount;
              messageList = new ArrayList<UserMessage>();
              final String filtered = filterProperty(property);
              messageMap.put(filtered, newUserMessageItem(messageList, itemCount, filtered));
          } else {
              messageList = item.getMessageList();
          }
          messageList.add(message);
      }
docs: ===========