How to use groupingBy with reducing without getting an Optional

Pixelstix :

For this greatly simplified example of my problem, I have a Stat object with a year field and three other statistics fields. Imagine they are the results are yearly statistics for number of patients of each animal type from branches of a veterinarian chain, and I want to get sums for all branches by year.

In other words, from a list of Stat objects, I'd like to return a Map<Integer, Stat>, where the integer is the year, and the Stat object has the year and sums for each of the four fields.

public class Stat
{
    int year;
    public int getYear() { return year; }

    long cats;
    public long getCats() { return cats; }

    long dogs;
    public long getDogs() { return dogs; }

    long pigeons;
    public long getPigeons() { return pigeons; }

    public Stat(int year, long cats, long dogs, long pigeons)
    {
        this.year = year;
        this.cats = cats;
        this.dogs = dogs;
        this.pigeons = pigeons;
    }

    public Stat(Stat left, Stat right)
    {
        if (left.year != right.year)
            throw new IllegalArgumentException("Only allow combining for same year.");
        this.year = left.year;
        this.cats = left.cats + right.cats;
        this.dogs = left.dogs + right.dogs ;
        this.pigeons = left.pigeons + right.pigeons;
    }

    @Override
    public String toString()
    {
        return String.format("%d c=%d d=%d p=%d", year, cats, dogs, pigeons);
    }
}
@Test
public void testStat()
{
    List<Stat> items = Arrays.asList(
        new Stat(2017, 5, 8, 12),
        new Stat(2017, 123, 382, 15),
        new Stat(2018, 1, 2, 3)
        );
    Map<Integer, Optional<Stat>> result = items.stream()
        .collect(Collectors.groupingBy(Stat::getYear,
            Collectors.reducing(Stat::new)
        ));
    System.out.println(result);
}

The Optional is unnecessary, since groupingBy would never create a List that needs reducing if there were no elements.

Is there a way to get Map<Integer, Stat>, preferably without having to create a blank "identity" object?

If I have to resort to creating an identity creation function to reducing, the Stat object's combining constructor has to have a year (see the constructor), so how does the identity constructor get the year passed to it?

Naman :

You could rather achieve this using Collectors.toMap as :

Map<Integer, Stat> result = items.stream()
        .collect(Collectors.toMap(Stat::getYear, 
                Function.identity(), (one, another) -> sumStatsOfSameYear(one, another)));

where sumAttributes is defined as

// stat from the same year
private static Stat sumStatsOfSameYear(Stat one, Stat another) {
    new Stat(one.getYear(), one.getCats() + another.getCats(),
            one.getDogs() + another.getDogs(), one.getPigeons() + another.getPigeons()))
}