Java 8 Stream

Map<Long, AreaInfo> cityCollect = list.stream().collect(Collectors.toMap(AreaInfo::getCityId, po -> po, (key, key2) -> key2));

Map<Long, AreaInfo> cityCollect = list.stream().collect(Collectors.toMap(AreaInfo::getCityId, po -> po));

两者有区别，建议第一中，第二种会有报错，抛出异常