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List personList=newArrayList();

Person p1=newPerson();

p1.setPersonId(1);

p1.setPersonName(“chm”);

List areaList1=new ArrayList<>();

Area a1=newArea();

a1.setAreaId(1);

a1.setAreaName(“海淀”);

a1.setCityId(1);

areaList1.add(a1);

Area a2=newArea();

a2.setAreaId(2);

a2.setAreaName(“丰台”);

a2.setCityId(1);

areaList1.add(a2);

Area a3=newArea();

a3.setAreaId(11);

a3.setAreaName(“崇明”);

a3.setCityId(2);

areaList1.add(a3);

p1.setAreaList(areaList1);

personList.add(p1);

Person p2=newPerson();

p2.setPersonId(2);

p2.setPersonName(“wt”);

List areaList2=new ArrayList<>();

Area a11=newArea();

a11.setAreaId(21);

a11.setAreaName(“临颍”);

a11.setCityId(3);

areaList2.add(a11);

Area a12=newArea();

a12.setAreaId(22);

a12.setAreaName(“郾城”);

a12.setCityId(3);

areaList2.add(a12);

Area a13=newArea();

a13.setAreaId(12);

a13.setAreaName(“浦东”);

a13.setCityId(2);

areaList2.add(a13);

p2.setAreaList(areaList2);

personList.add(p2);

List cityIdList=new ArrayList<>();for(Person person:personList){for(Area area:person.getAreaList()){if(!cityIdList.contains(area.getCityId())){

cityIdList.add(area.getCityId());

}

}

}

List> myList= personList.stream().map(i->i.getAreaList().stream().map(area->area.getCityId()).collect(Collectors.toList())).collect(Collectors.toList());

Map> basicMap =personList.stream().collect(Collectors.groupingBy(Person::getPersonId));

List personListCurrent=basicMap.get(2);