场景举例：
两个List（rooms1 和 rooms2）里面放的对象都是 Room ,
现在想把两个List里面的 room 的id 都抽取出来，放在一个List里面 。
实现代码：
List<Integer> idList = Stream.concat(rooms1.stream().map(Room::getId), rooms2.stream().map(Room::getId)).collect(Collectors.toList());
或
List<Integer> idList = Stream.of(rooms1, rooms2).flatMap(List::stream).map(Room::getId).collect(Collectors.toList());
例子：
import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; /** * @Author JCccc * @Description * @Date 2021/8/20 9:05 */ public class Room { private Integer id; private String name; private Short type; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Short getType() { return type; } public void setType(Short type) { this.type = type; } public static void main(String[] args) { List<Room> rooms1=new ArrayList<>(); Room roomA=new Room(); roomA.setId(1); roomA.setName("1-A"); roomA.setType((short)1); Room roomB=new Room(); roomB.setId(2); roomB.setName("2-A"); roomB.setType((short)1); rooms1.add(roomA); rooms1.add(roomB); List<Room> rooms2=new ArrayList<>(); Room roomC=new Room(); roomC.setId(3); roomC.setName("3-C"); roomC.setType((short)2); Room roomD=new Room(); roomD.setId(4); roomD.setName("4-D"); roomD.setType((short)2); rooms2.add(roomC); rooms2.add(roomD); //去重set (注意真正操作时需要对俩list做判空校验) List<Integer> idList=new ArrayList<>(Stream.concat(rooms1.stream().map(Room::getId), rooms2.stream().map(Room::getId)).collect(Collectors.toSet())); //去重set (注意真正操作时需要对俩list做判空校验) List<Integer> idList=new ArrayList<>( Stream.of(rooms1, rooms2).flatMap(List::stream).map(Room::getId).collect(Collectors.toSet())); //不去重list // List<Integer> idList = Stream.concat(rooms1.stream().map(Room::getId), rooms2.stream().map(Room::getId)).collect(Collectors.toList()); System.out.println(idList.toString()); } }
输出结果：
ps：
同样，如果来自一个List的两个值，也想抽出来放到一个list里面去：
只需要做点小改动，
List<Room> rooms1=new ArrayList<>();
Room roomA=new Room();
roomA.setId(1);
roomA.setName("1-A");
roomA.setType((short)1);
roomA.setFid(5);
Room roomB=new Room();
roomB.setId(2);
roomB.setName("2-A");
roomB.setType((short)1);
roomB.setFid(10);
rooms1.add(roomA);
rooms1.add(roomB);
List<Integer> idList = Stream.concat(rooms1.stream().map(Room::getId), rooms1.stream().map(Room::getFid)).collect(Collectors.toList());
