public String getSingleValue() {
        final List<String> defaultList = getDefaultList();
        return size() == 1 && defaultList.size() == 1 ? defaultList.get(0) : null;
    }
IF this config has only one single String value then this value is returned. Otherwise null is returned
main/src/main/java/cgeo/geocaching/utils/config/LegacyFilterConfig.java