public GroupingOptions<G> setGroupDisplayIconMapper(final Function<ItemGroup<T, G>, ImageParam> groupDisplayIconMapper) {
            if (groupDisplayIconMapper != null) {
                this.groupDisplayIconMapper = groupDisplayIconMapper;
                triggerChange(ChangeType.COMPLETE);
            }
            return this;
        }
Sets a mapper providing an optional display icon per group
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java