public static <T> List<T> union(
            List<? extends T> list1,
            List<? extends T> list2,
            List<? extends T> list3) {
        ArrayList<T> result = new ArrayList<>(list1.size() + list2.size() + list3.size());
        result.addAll(list1);
        result.addAll(list2);
        result.addAll(list3);
        return result;
    }
Note that the method only checks the immutable classes used by Turms.
turms-server-common/src/main/java/im/turms/server/common/infra/collection/CollectionUtil.java