Java 8 Nested (Multi level) group by

Question

I have few classes like below

class Pojo {
    List<Item> items;
}

class Item {
    T key1;
    List<SubItem> subItems;
}

class SubItem {
    V key2;
    Object otherAttribute1;
}

I want to aggregate the items based on key1 and for each aggregation, subitems should be aggregated by key2 in following way:

Map<T, Map<V, List<Subitem>>

How is this possible with Java 8 Collectors.groupingBy nesting?

I was trying something and stuck halfway at

pojo.getItems()
    .stream()
    .collect(
        Collectors.groupingBy(Item::getKey1, /* How to group by here SubItem::getKey2*/)
    );

Note: This not same as cascaded groupingBy which does multilevel aggregation based on fields in the same object as discussed here

Answer 1

You can't group a single item by multiple keys, unless you accept the item to potentially appear in multiple groups. In that case, you want to perform a kind of flatMap operation.

One way to achieve this, is to use Stream.flatMap with a temporary pair holding the combinations of Item and SubItem before collecting. Due to the absence of a standard pair type, a typical solution is to use Map.Entry for that:

Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()
    .flatMap(item -> item.subItems.stream()
        .map(sub -> new AbstractMap.SimpleImmutableEntry<>(item.getKey1(), sub)))
    .collect(Collectors.groupingBy(AbstractMap.SimpleImmutableEntry::getKey,
                Collectors.mapping(Map.Entry::getValue,
                    Collectors.groupingBy(SubItem::getKey2))));

An alternative not requiring these temporary objects would be performing the flatMap operation right in the collector, but unfortunately, flatMapping won't be there until Java 9.

With that, the solution would look like

Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()
    .collect(Collectors.groupingBy(Item::getKey1,
                Collectors.flatMapping(item -> item.getSubItems().stream(),
                    Collectors.groupingBy(SubItem::getKey2))));

and if we don't want to wait for Java 9 for that, we may add a similar collector to our code base, as it's not so hard to implement:

static <T,U,A,R> Collector<T,?,R> flatMapping(
    Function<? super T,? extends Stream<? extends U>> mapper,
    Collector<? super U,A,R> downstream) {

    BiConsumer<A, ? super U> acc = downstream.accumulator();
    return Collector.of(downstream.supplier(),
        (a, t) -> { try(Stream<? extends U> s=mapper.apply(t)) {
            if(s!=null) s.forEachOrdered(u -> acc.accept(a, u));
        }},
        downstream.combiner(), downstream.finisher(),
        downstream.characteristics().toArray(new Collector.Characteristics[0]));
}