java8 stream多次map_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 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 oldList, List newList) ->{

oldList.addAll(newList);returnoldList;

}));

实际操作后

00ab3ad649ff6ecccfea1531145b3032.png

报空指针错误

解决方式

userList.str

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