public GroupingOptions<G> setGroupDisplayMapper(final Function<ItemGroup<T, G>, TextParam> groupDisplayMapper) {
            if (groupDisplayMapper != null) {
                setGroupDisplayViewMapper(constructGroupDisplayViewMapper(groupDisplayMapper));
                triggerChange(ChangeType.COMPLETE);
            }
            return this;
        }
Sets a mapper providing the text visualization for a group item
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java