List<Object> (final String groupName) { synchronized (myLock) { final List<NavigatableMessageElement> children = myGroupNameToMessagesMap.get(groupName); if (children == null || children.isEmpty()) { return Collections.emptyList(); } final List<Object> result = new ArrayList<>(); for (NavigatableMessageElement child : children) { final Object data = child.getData(); if (data != null) { result.add(data); } } return result; } }