IBookService.java

public interface IBookService {

  List<Tuple2<LocalDate , Book>> findBooks(User user , LocalDate from , LocalDate to);

  default List<Tuple3<LocalDate , User , Book>> findUserBooks(List<User> users , LocalDate from , LocalDate to) {
    Map<LocalDate , Tuple2<User , Book>> map = users.stream()
      .flatMap(user -> {
        List<Tuple2<LocalDate , Book>>  list = findBooks(user , from , to);
        return list.stream().map(t -> Tuple.tuple(t.v1 , user , t.v2));
        }
      ).collect(Collectors.toMap(Tuple3::v1, t -> Tuple.tuple(t.v2() , t.v3()) , (v1, v2) -> v1 , TreeMap::new));

    return map.entrySet().stream()
      .map(entry -> Tuple.tuple(entry.getKey() , entry.getValue().v1 , entry.getValue().v2))
      .collect(Collectors.toList());

  }
}