Java Lambdas Multiple Summing

ecstasy :

I have a class -

public class LocationStats {

    private String state;
    private String country;
    private int latestTotalCases;
    private int diffFromPrevDay;
    private int diffFromPrev2Day;
    private int diffFromPrev3Day;
}

Top Sum-up total cases for a country I used below snippet, which is working fine.

List<LocationStats> newStats= getData(); 
    Map<String, Integer> collect = 
                    newStats
                    .stream()
                    .collect(Collectors.groupingBy(LocationStats::getCountry, Collectors.summingInt(LocationStats::getLatestTotalCases)));

I want to sum-up the other 3 attribues as well while processing the above-mentioned snippet. Is it possible ? Is there any Lambda-elegant way to achieve that ?

Eran :

You can use Collectors.toMap with a merge function.

For example:

Map<String,LocationStats> sums =
    newStats.stream()
            .collect(Collectors.toMap(LocationStats::getCountry,
                                      Function.identity(),
                                      (s1,s2)->new LocationStats(s1,s2)));

You'll have to create a LocationStats constructor that accepts two LocationStats instances and creates an instance that contains the sums of the properties of those two instances:

public LocationStats (LocationStats s1, LocationStats s2)
{
    this.state = s1.state;
    this.country = s1.country;
    this.latestTotalCases = s1.latestTotalCases + s2.latestTotalCases;
    this.diffFromPrevDay = s1.diffFromPrevDay + s2.diffFromPrevDay;
    this.diffFromPrev2Day = s1.diffFromPrev2Day + s2.diffFromPrev2Day;
    this.diffFromPrev3Day = s1.diffFromPrev3Day + s2.diffFromPrev3Day;
}