Java 将两个对象list里面的某个字段值抽取到一个list里

- 日理万妓 2022-09-10 05:11 340阅读 0赞

## **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：

![20210905093844185.png][]

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：

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5bCP55uu5qCH6Z2S5bm0_size_11_color_FFFFFF_t_70_g_se_x_16][]

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());

[20210905093844185.png]: /images/20220829/6f40e7396c7f4cbfaf1dd05616443d19.png
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5bCP55uu5qCH6Z2S5bm0_size_11_color_FFFFFF_t_70_g_se_x_16]: /images/20220829/e03444a683024cc68ea752022934f28b.png