#将list转为map  【注意：要对list 进行判空】

Map<Integer, User> collect = users.stream().collect(Collectors.toMap(User::getUserId, User -> User);

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#将list转为map并且key去重

Map<Integer, User> collect = users.stream().collect(Collectors.toMap(User::getUserId, User -> User,(oldValue, newValue) -> newValue));

View Code #Collectors.toMap() 多字段拼接key，并且去重

 3.1：去掉重复key 【(key1, key2) -> key2)】, key 使用多字段拼接。返回 Map<String, UserTalk> userMap1类型数据
 　　　　Map<String, UserTalk> userMap1 = userTalkList.stream().collect(Collectors.toMap(userTalk -> userTalk.getSubjectId() + "" + userTalk.getGradeId() +
         "" + userTalk.getUnitId() + "" + userTalk.getLectureType(), UserTalk -> UserTalk, (key1, key2) -> key2));

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