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

class Ideone {
  public static void main(String[] args) {
    final List<List<String>> listOfLists = List.of(List.of("A", "B"), List.of("C", "D"));
    final List<String> list = listOfLists.stream()
        .flatMap(List::stream)
        .collect(Collectors.toList());
    System.out.println(list);
  }
}