How to get a custom type instead of Integer when using Collectors.summingInt?

Question

I am currently creating a Map<String, Map<LocalDate, Integer>> like this, where the Integer represents seconds:

Map<String, Map<LocalDate, Integer>> map = stream.collect(Collectors.groupingBy(
            x -> x.getProject(),
            Collectors.groupingBy(
                x -> x.getDate(),
                Collectors.summingInt(t -> t.getDuration().toSecondOfDay())
            )
        ));

How could I instead create a Map<String, Map<LocalDate, Duration>>?

I notice that [`java.time.Duration`](https://docs.oracle.com/javase/8/docs/api/java/time/Duration.html) does not have a `toSecondOfDay()` method. Are you using another type or is this mistake for `getSeconds()`? — Didier L, May 31 '17 at 18:03
@DidierL I use LocalTime to save duration, It looks prettier in DBMS. — Alex78191, May 31 '17 at 18:18
I don't think it is a good idea to misuse a Java type for the sake of database representation. You should use a specific converter instead, to map to the DB type of your choice – if the default conversion does not work. — Didier L, May 31 '17 at 19:00

score 9 · Accepted Answer · edited May 31 '17 at 17:26

9

To change that Integer from Collectors.summingInt to a Duration, you simply need to replace that Collector with:

Collectors.collectingAndThen(
    Collectors.summingInt(t -> t.getDuration().toSecondOfDay()),
    Duration::ofSeconds
)

edited May 31 '17 at 17:26

Didier L

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answered May 31 '17 at 13:08

Eugene

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Didier L · Answer 2 · 2017-05-31T18:59:17.510

If you were using an actual Duration for getDuration() (instead of LocalTime), you could also sum directly the Duration's as follows:

Map<String, Map<LocalDate, Duration>> map = stream.collect(Collectors.groupingBy(
        MyObject::getProject,
        Collectors.groupingBy(
                MyObject::getDate,
                Collectors.mapping(MyObject::getDuration,
                        Collectors.reducing(Duration.ZERO, Duration::plus))
        )
));

With the advantage that it also sums the nanoseconds, and could be generalized to other types as well.

Note however that it creates many intermediate Duration instances which could have an impact on the performance.

How to get a custom type instead of Integer when using Collectors.summingInt?

2 Answers2

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