如何使用流重写嵌套列表的查找功能

Question

Given a List<Unicorn> , with each unicorn containing a List<Rider> , return the unicorn and the latest rider info for a subset of riders.

I've written this as such:

static Optional<ImmutablePair<Unicorn, Rider>> findLatestExperiencedRiderInfo(
        final List<Unicorn> unicorns) {

    Rider latestExperiencedRider = null;
    Unicorn unicornOfLatestExperiencedRider = null;
    long latestRideTime = 0L;

    for (final Unicorn unicorn : unicorns) {
        for (final Rider rider : unicorn.getRiders()) {
            if (rider.getType() == Rider.Type.EXPERIENCED) {
                final long time = rider.getRideTime();
                if (time > latestRideTime) {
                    latestRideTime = time;
                    latestExperiencedRider = rider;
                    unicornOfLatestExperiencedRider = unicorn;
                }
            }
        }
    }

    return latestExperiencedRider == null
            ? Optional.empty()
            : Optional.of(new ImmutablePair<>(
                    unicornOfLatestExperiencedRider,
                    latestExperiencedRider));
}

I was hoping for a more succinct version, possibly using streams (or just written differently). What would be your suggestions?

Answer 1

Here's a stream version

unicorns.stream()
        .flatMap(unicorn -> unicorn.getRiders()
                .stream()
                .filter(rider -> rider.getType() == Rider.Type.EXPERIENCED)
                .map(rider -> new AbstractMap.SimpleEntry<>(unicorn, rider)))
        .max(Comparator.comparingLong(entry -> entry.getValue().getRideTime()))
        .map(entry -> new ImmutablePair<>(entry.getKey(), entry.getValue()));

The flatMap part flattens all the Riders into AbstractMap.SimpleEntry s each containing a Rider and the Unicorn it was part of. Then we get the max entry by comparing two Rider 's rideTime . If present, we create the ImmutablePair result else will be Optional.empty

Answer 2

Maybe something like this, you can add the required null checks..

// This is your rider..
final Optional<Rider> aRider = unicorns.stream()
                                       .flatMap(unicorn -> unicorn.getRiders().stream())
                                       .filter(rider -> rider.type == Rider.Type.EXPERIENCED)
                                       .max(Comparator.comparingLong(Rider::getRideTime));

 // This is your unicorn..
final Optional<Unicorn> any = unicorns.stream()
                                      .filter(unicorn -> unicorn.getRiders().contains(aRider.get()))
                                      .findAny();

You can also keep a reference to a Unicorn in the Rider I guess so you do not need to search your unicorn once you have your a rider.