public GroupingOptions<G> setGroupMapper(final Function<T, G> groupMapper) {
            this.groupMapper = groupMapper;
            triggerChange(ChangeType.COMPLETE);
            return this;
        }
Sets a mapper specifying how list items (of type T) are sorted into groups (of type G)
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java