Java 8 lambda Stream list to Map key 重复 value合并到Collection

描述： 取list集合中两个字段，且将两个字段作为key ，map，利用steam流转为map集合，且满足key相同时，将value转为List集合

查询到资料 转自https://my.oschina.net/u/3725073/blog/1807970/

List<User> userList = new ArrayList<>();
        userList.add(new User(1L, "aaa"));
        userList.add(new User(2L, "bbb"));
        userList.add(new User(3L, "ccc"));
        userList.add(new User(2L, "ddd"));
        userList.add(new User(3L, "eee"));

　资料给出的解决方案

userList.stream().collect(Collectors.toMap(User::getId,
                e -> Arrays.asList(e.getUsername()),
                (List<String> oldList, List<String> newList) -> {
                    oldList.addAll(newList);
                    return oldList;
                }));

实际操作后

报空指针错误

解决方式

userList.stream().collect(Collectors.toMap(User::getId,
                e -> new ArrayList<>(Arrays.asList(e.getUsername())),
                (List<String> oldList, List<String> newList) -> {
                    oldList.addAll(newList);
                    return oldList;
                }));