Java 8 Stream: groupingBy with multiple Collectors

Question

I want to use a Java 8 Stream and Group by one classifier but have multiple Collector functions. So when grouping, for example the average and the sum of one field (or maybe

Answer 1

By using a map as an output type one could have a potentially infinite list of reducers each producing its own statistic and adding it to the map.

public static  Map addMap(Map map, K k, V v) {
    Map mapout = new HashMap();
    mapout.putAll(map);
    mapout.put(k, v);
    return mapout;
}

...

    List persons = new ArrayList<>();
    persons.add(new Person("Person One", 1, 18));
    persons.add(new Person("Person Two", 1, 20));
    persons.add(new Person("Person Three", 1, 30));
    persons.add(new Person("Person Four", 2, 30));
    persons.add(new Person("Person Five", 2, 29));
    persons.add(new Person("Person Six", 3, 18));

    List, Person, Map>> listOfReducers = new ArrayList<>();

    listOfReducers.add((m, p) -> addMap(m, "Count", Optional.ofNullable(m.get("Count")).orElse(0) + 1));
    listOfReducers.add((m, p) -> addMap(m, "Sum", Optional.ofNullable(m.get("Sum")).orElse(0) + p.i1));

    BiFunction, Person, Map> applyList
            = (mapin, p) -> {
                Map mapout = mapin;
                for (BiFunction, Person, Map> f : listOfReducers) {
                    mapout = f.apply(mapout, p);
                }
                return mapout;
            };
    BinaryOperator> combineMaps
            = (map1, map2) -> {
                Map mapout = new HashMap<>();
                mapout.putAll(map1);
                mapout.putAll(map2);
                return mapout;
            };
    Map map
            = persons
            .stream()
            .reduce(new HashMap(),
                    applyList, combineMaps);
    System.out.println("map = " + map);

Produces :

map = {Sum=10, Count=6}