Map<Integer, User> map = list.stream().collect(Collectors.toMap(User::getId, t -> t, (oldData, newData) -> newData));
Map<Integer, String> map2 = list.stream().collect(Collectors.toMap(User::getId, User::getName, (oldData, newData) -> newData));

Map<String, List<User>> map = Maps.newHashMap();
List<User> list = this.list();
if (CollectionUtils.isEmpty(list)) {
    return map;
}
for (User item : list) {
    map.computeIfAbsent(item.getCode(), k -> new LinkedList<>()).add(item);
}

List<User> list = map.entrySet().stream()
        .map(
                e -> User.builder().id(e.getKey()).name(e.getValue()).build()
        )
        .collect(Collectors.toList());