java map ->文法

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 List<Person> personList=new ArrayList();        Person p1=new Person();        p1.setPersonId(1);        p1.setPersonName("chm");        List<Area> areaList1=new ArrayList<>();        Area a1=new Area();        a1.setAreaId(1);        a1.setAreaName("海澱");        a1.setCityId(1);        areaList1.add(a1);        Area a2=new Area();        a2.setAreaId(2);        a2.setAreaName("丰台");        a2.setCityId(1);        areaList1.add(a2);        Area a3=new Area();        a3.setAreaId(11);        a3.setAreaName("崇明");        a3.setCityId(2);        areaList1.add(a3);        p1.setAreaList(areaList1);        personList.add(p1);        Person p2=new Person();        p2.setPersonId(2);        p2.setPersonName("wt");        List<Area> areaList2=new ArrayList<>();        Area a11=new Area();        a11.setAreaId(21);        a11.setAreaName("臨潁");        a11.setCityId(3);        areaList2.add(a11);        Area a12=new Area();        a12.setAreaId(22);        a12.setAreaName("郾城");        a12.setCityId(3);        areaList2.add(a12);        Area a13=new Area();        a13.setAreaId(12);        a13.setAreaName("浦東");        a13.setCityId(2);        areaList2.add(a13);        p2.setAreaList(areaList2);        personList.add(p2);        List<Integer> cityIdList=new ArrayList<>();        for(Person person:personList){            for(Area area:person.getAreaList()){                if(!cityIdList.contains(area.getCityId())){                    cityIdList.add(area.getCityId());                }            }        }List<List<Integer>> myList= personList.stream().map(i->i.getAreaList().stream().map(area->area.getCityId()).collect(Collectors.toList())).collect(Collectors.toList());        Map<Integer, List<Person>> basicMap = personList.stream().collect(Collectors.groupingBy(Person::getPersonId));        List<Person>  personListCurrent=basicMap.get(2);

 

java map ->文法