Create custom collector in java

Question

I have two classes Meeting and Guest.

The relation looks like this:

class Meeting {

private LocalDate date;
private List<Guest> guests;

}

I need to get one Meeting by date and where guests are from the USA using stream.

I have List and I have done the next one:

List<Meeting> meetings = new ArrayList<>();

public Meeting getByDay(LocalDate date) {
    List<Guest> guests = getGuestsByDate(date).stream()
            .filter(guest-> guest.getCountry().equals("USA"))
            .collect(Collectors.toList());
    return new Meeting(date, guests);
}

but now I need to refactor my code with custom Collector, which returns Meeting. I read it needs to implement the Collector interface or Collector.of() method but how it must look in my case? I would be grateful for any advice.

Answer 1

I don't recommend this approach, but if you need, you can use:

    Meeting m = Stream.of(new Guest(), new Guest())
        .collect(Collector.of(
            () -> new Meeting(date),
            Meeting::addGuest,
            (m1, m2) -> {
              ArrayList<Guest> guestsAll = new ArrayList<>(m1.getGuests());
              guestsAll.addAll(m2.getGuests());
              return new Meeting(date, guestsAll);
            }
        ));

I added couple of constructors and method to Meeting class

    Meeting(LocalDate date) {
      this.date = date;
      this.guests = new ArrayList<>();
    }

    Meeting(LocalDate date, List<Guest> guests) {
      this.date = date;
      this.guests = guests;
    }

    public void addGuest(Guest g) {
      guests.add(g);
    }