Java 8 streams - flatMap to join list of lists

We can use flatMap method to join list of lists, as flatMap helps to join streams.

let’s say we have a List<List<T>>, and we want to join all the List<T> inside the parent list together as a single list as a List<T>.

example:

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;

public class FlatMapJoinLists {
    public static void main(String[] args) {
        List<Room> roomList = new ArrayList<>();
        roomList.add(new Room("A"));
        roomList.add(new Room("B"));
        List<Room> roomList1 = new ArrayList<>();
        roomList1.add(new Room("C"));
        roomList1.add(new Room("D"));
List<List<Room>> list = new ArrayList<>();
        list.add(roomList);
        list.add(roomList1);

        List<Room> roomListMerged = list.stream().flatMap(Collection::stream).collect(Collectors.toList());

        System.out.println(roomListMerged);
/* output is the merged list
[Room@2133c8f8, Room@43a25848, Room@3ac3fd8b, Room@5594a1b5] 
*/
   }
}

class Room {
    String name;
    Room(String name) {
        this.name = name;
    }
}