list集合数据如何分组

/**

* group分组
*/
private static void group() {
　　Map<Integer, List<User>> map = list.stream().filter(obj -> obj.getAge() != null).collect(Collectors.groupingBy(User::getSex));
　　System.out.println(new Gson().toJson(map));
　　Map<Integer, Map<Integer, List<User>>> map2 = list.stream().filter(obj -> obj.getAge() != null).collect(Collectors.groupingBy(User::getSex,Collectors.groupingBy(User::getAge)));
　　System.out.println(new Gson().toJson(map2));
}