lamda List转map拼接key

1、List转Map，key由多个字段拼接

 

Map<String, User> map= userList.stream()
        .collect(Collectors.toMap(k -> k.getUserId() + k.getUserName(), Function.identity()));

2、单个key

Map map =userList.stream().collect(Collectors.toMap(User::getUserName,Function.identity()));