java8 先groupingBy 后map

Map<Integer,List<String>> mapBanJI_UserNameList=list.stream()
.collect(Collectors.groupingBy(UserInfo::getBanJi,Collectors.mapping(UserInfo::getUserName,Collectors.toList())));

Map<Integer, Integer> userMap = userInfoList.stream().collect(Collectors.toMap(UserInfo::getUserId, UserInfo::getUserName));