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));

兩者有區別，建議第一中，第二種會有報錯，拋出異常