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.

getGuestsByDate(date) would be more fun to know the implementation of. — Naman
– Naman, Commented Apr 16, 2021 at 10:35
.collect(Collectors.toList()) → .collect(Collectors.collectingAndThen(Collectors.toList(), l->new Meeting(date,l))) — Holger
– Holger, Commented Apr 16, 2021 at 16:40

talex · Accepted Answer · 2021-04-16 11:05:13Z

2

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);
    }

answered Apr 16, 2021 at 11:05

talex

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Comments

Siddharth Gharge · Accepted Answer · 2021-04-16 11:11:42Z

0

Although the question is about using Collector, you can use reduce operator as well. Try the below code:

public Meeting getByDay(LocalDate date) {   
    return getGuestsByDate(date).stream()
            .filter(guest -> guest.getCountry().equals("USA"))
            .reduce(new Meeting(date, new ArrayList<>()), (meeting, guest) -> {
                meeting.guests.add(guest);
                return meeting;
            }, (meeting1, meeting2) -> {
                meeting1.guests.addAll(meeting2.guests);
                return meeting1;
            });
}

You can shift those lambda expressions into the Meeting class or some other method and use method reference or one liner lambdas.

answered Apr 16, 2021 at 11:11

Siddharth Gharge

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Comments

Harsh Mishra · Accepted Answer · 2021-04-16 10:13:14Z

-1

I know this will not the full code but you will get an idea of how you can achieve the solution

 List<Meeting> meeting = meetings.stream()
        .filter(guest-> guest.getCountry().equals("USA"))
        .map(v->{ 
          return new Meeting(something, something);
         })
        .collect(Collectors.toList());

answered Apr 16, 2021 at 10:13

Harsh Mishra

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4 Comments

talex Over a year ago

This code will return multiple meetings. Original code returns one meeting with multiple guests.

Harsh Mishra Over a year ago

Ok bro, In that case, you need to return the map. For returning a single object type you can't use a collector. This is for the Map and list related return type. your existing code is fine.

talex Over a year ago

Collector can return single object.

Harsh Mishra Over a year ago

Collector<MyData, MyResult, MyResult> myCollector = Collector.of( () -> new MyResult(), (a, e) -> { if (e.key.equals("2")) { a.color = e.value; } }, (a1, a2) -> null, // Not used, therefore not working scaffold a -> a ); MyResult result = collection.stream().collect(myCollector);

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