Javaで複数のセットのデカルト積を計算するにはどうすればよいですか?

public static Set<Set<Object>> cartesianProduct(Set<?>... sets) {
    if (sets.length < 2)
        throw new IllegalArgumentException("Can't have a product of fewer than two sets (got " + sets.length + ")");

    return _cartesianProduct(0, sets);
}

private static Set<Set<Object>> _cartesianProduct(int index, Set<?>... sets) {
    Set<Set<Object>> ret = new HashSet<>();
    if (index == sets.length) {
        ret.add(new HashSet<>());
    } else {
        for (Object obj : sets[index]) {
            for (Set<Object> set : _cartesianProduct(index + 1, sets)) {
                set.add(obj);
                ret.add(set);
            }
        }
    }
    return ret;
}