Java: Group By then Map

Question

I have a stream of Events

public class Event {
    Location location;
    double turnout;
    //... other fields & getters
}

And a statistics class EventStatistics

public class EventStatistics {
    // Stats properties e.g. turnout standard deviation/median

    public EventStatistics(List<Event> events) {
        // Generate stats
    }
}

I need to group all the events by location & create a map of location and event statistics Map<Location, EventStatistics>

The group by is just:

Map<Location, List<Event>> byLocation = events.stream().collect(groupingBy(Event::getLocation));

I know there is an overloaded groupingBy(function, collector) collector. Can I use somehow this to generate my Map<Location, EventStatistics> in a single stream?

Answer 1

All you need is collectingAndThen:

Map<Location, EventStatistics> result = 
    events.stream()
          .collect(Collectors.groupingBy(Event::getLocation,
                                         Collectors.collectingAndThen(
                                             Collectors.toList(), 
                                             EventStatistics::new)));