How to sum with Stream api?

Question

I'm trying to find the sum of hours between two dates in each segment.

Segment:

class Segment {
    private final LocalDateTime departureDate;

    private final LocalDateTime arrivalDate; 
    //..getter setter

Flight:

class Flight {
    private final List<Segment> segments;
   //.. getter setter

I try to do so, but it is not compile. What is problem here?

int sum = flightList.forEach(flight -> {
    flight.getSegments().stream().mapToInt(segment -> (int) ChronoUnit.HOURS.between(segment.getDepartureDate(), segment.getArrivalDate())).sum();
});

Shouldn't that `int sum = ` go _inside_ the loop? `forEach()` returns `void` and thus this can't compile. Also you probably want a sum per flight, right? — Thomas, Sep 25 '20 at 19:09
Does this answer your question? [How to sum values in a Map with a stream?](https://stackoverflow.com/questions/30089469/how-to-sum-values-in-a-map-with-a-stream) — Kars, Sep 25 '20 at 19:12

score 4 · Accepted Answer · answered Sep 25 '20 at 19:13

4

I'd stream the flightList, then flatMap it to get a list of Segments, and then map them to the hours between departure and arrival and sum. Note that ChronoUnit.between returns a long, not an int though:

long sum = 
    flightList.stream()
              .flatMap(f -> f.getSegments().stream())
              .mapToLong(s -> ChronoUnit.HOURS.between(
                                  s.getDepartureDate(), s.getArrivalDate()))
              .sum();

answered Sep 25 '20 at 19:13

Mureinik

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OP ask question _What is problem here?_ I think if you add an explanation for that then it will be more helpful, IMHO ofcourse. – Eklavya Sep 25 '20 at 19:38
@Eklavya Sorry, my English is not very good – petrov.aleksandr Sep 25 '20 at 20:44

score 1 · Answer 2 · answered Sep 25 '20 at 19:09

The .forEach() operation does not return anything, so you cannot assign it to the sum variable. What you need here is the flatMap() operation:

flightList.stream().flatMap(flight -> flight.getSegments().stream())
.mapToInt(segment -> (int) ChronoUnit.HOURS.between(segment.getDepartureDate(), segment.getArrivalDate())
.sum();

How to sum with Stream api?

2 Answers2