巧用Stream：像sql一样操作数据集合

背景：

　　从数据库中查询出的数据，最各种运算。Stream最方便。 

//转Map操作 实体中的2个字段，转为key=字段 value=字段
Map<String, Integer> collect = studentList.stream().collect(Collectors.toMap(Student::getGroupPointId, Student::getGroupStatus));

//转Map操作 key=某个字段,value=实体对象
Map<String, Student> studentDBMap = studentList.stream().collect(Collectors.toMap(Student::getGroupNo,v->v,(o,n)->n));

//分组统计，key-List
Map<Long, List<MapDic>> dicMap = mapDics.stream().collect(groupingBy(m -> m.getMapAreaId()));

//分组统计
Map<String, Long> storageCountMap = request.getStorageList().stream().collect(Collectors.groupingBy(Student::getAge, Collectors.counting()));

//多级分组 统计
Map<Long,Map<String,List<Student>>> map = points.stream().collect(groupingBy(e->e.getMapAreaId(),
groupingBy(e -> e.getLogicalCode()+"_"+e.getLogicalName())));

//转list
List<Stream> itemList = studentStreams.stream().map(this::getStreamObj).collect(Collectors.toList());
List<String> alleyNoStrList = alleyList.stream().map(Student::getAge).collect(Collectors.toList());