Given a list of votes on a location, return the most recent vote for each location

GetMaxTimestampAscending.java

public List<LocationVote> getMaxTimestampAscending(List<LocationVote> locationVotesSince)
{
    // map/reduce to max timestamp for a location
    Map<LocationKey, Optional<Long>> locationToTimestampDesc = locationVotesSince
        .stream()
        .collect(
            Collectors.groupingBy(
                LocationVote::getLocationKey,
                Collectors.mapping(LocationVote::getTimestampUtcEpochMilli, Collectors.maxBy(Comparator.reverseOrder())))
                )
        ;

    // convert to list of locations that were mapped sorted by timestamp ascending
    return locationVotesSince.stream()
        .filter(vote -> {
            Optional<Long> time = locationToTimestampDesc.getOrDefault(vote.getLocationKey(), Optional.empty());
            return time.isPresent() && (time.get() == vote.getTimestampUtcEpochMilli());
        })
        .sorted(Comparator.comparingLong(LocationVote::getTimestampUtcEpochMilli))
        .collect(Collectors.toList());
}