IBookService.java

interface IBookService {

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

  fun findUserBooks(users: List<User>, from: LocalDate, to: LocalDate): List<Tuple3<LocalDate, User, Book>> {
    val map = users.stream()
      .flatMap { user ->
        val list = findBooks(user, from, to)
        list.stream().map { t -> Tuple.tuple(t.v1, user, t.v2) }
      }.collect<TreeMap<LocalDate, Tuple2<User, Book>>, Any>(Collectors.toMap(
          Function<Tuple3<LocalDate, User, Book>, LocalDate> { it.v1() }, 
          { t -> Tuple.tuple(t.v2(), t.v3()) }, 
          { v1, v2 -> v1 }, 
          Supplier<TreeMap<LocalDate, Tuple2<User, Book>>> { TreeMap() })
        )

    return map.entries.stream()
      .map { entry -> Tuple.tuple(entry.key, entry.value.v1, entry.value.v2) }
      .collect<List<Tuple3<LocalDate, User, Book>>, Any>(Collectors.toList())

  }
}