How to change a list of lists into a list-CodePudding

Is there a lambda expression, or something build into java to change a List of Lists into one List.

for instance -->

public List<LocalDateTime> getallsort(){

    List<LocalDateTime> elite = getElite(true);
    List<LocalDateTime> recreation = getRecreation(true);
    List<LocalDateTime> youth = getYouth(true);
    List<List<LocalDateTime>> list = Arrays.asList(elite,recreation,youth);
    list.sort((xs1, xs2) -> xs1.size() - xs2.size());
    return list. ????? 
}

Is there a fancy way of returning all lists into 1 list? I wasn't able to find this question on stack using these keywords.

CodePudding user response：

public List<LocalDateTime> getallsort(){
  List<LocalDateTime> elite = getElite(true); 
  List<LocalDateTime> recreation = getRecreation(true);
  List<LocalDateTime> youth = getYouth(true);
  List<List<LocalDateTime>> list = Arrays.asList(elite,recreation,youth);
  list.sort((xs1, xs2) -> xs1.size() - xs2.size());
  return list.stream().flatMap(List::stream).collect(Collectors.toList());
}

which answers your original question about converting a List<List<>> into a flattened List<>.

Or even

public List<LocalDateTime> getallsort(){
  return Stream.of(
    getElite(true),
    getRecreation(true),
    getYouth(true)
    )
    .sorted((xs1, xs2) -> xs1.size() - xs2.size())
    .flatMap(List::stream)
    .collect(Collectors.toList());
}

which works for your original example, but maybe does not directly answer your question.