private void saveReducedGroups() {
            if (this.reducedGroupSaveId != null && this.reducedGroupIdMapper != null) {
                final ObjectNode node = JsonUtils.createObjectNode();
                JsonUtils.setCollection(node, "groups", reducedGroups, g -> JsonUtils.fromText(this.reducedGroupIdMapper.apply(g)));
                Settings.setSimpleListModelConfig(this.reducedGroupSaveId, JsonUtils.nodeToString(node));
            }
        }
Sets the  minimum number of groups on which grouping is really activated
main/src/main/java/cgeo/geocaching/ui/SimpleItemListModel.java