How to Perform Nested GroupBy Operations in Java 8 for Complex Data Aggregation?

Nested (Multi-Level) GroupBy in Java 8

Multi-level grouping in Java 8 allows for complex aggregation of data objects based on multiple fields. In your case, you have a set of classes Pojo, Item, and SubItem, and you want to group items based on key1 and then subitems within that group by key2.

To achieve this, we can't simply use nested Collectors.groupingBy calls, as this would not allow for grouping by multiple keys from different objects. Instead, we resort to a combination of flatMap and grouping collectors:

<code class="java">Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()
    .flatMap(item -> item.getSubItems().stream()
        .map(sub -> new AbstractMap.SimpleImmutableEntry<>(item.getKey1(), sub)))
    .collect(Collectors.groupingBy(AbstractMap.SimpleImmutableEntry::getKey,
                Collectors.groupingBy(Map.Entry::getValue, Collectors.toList())));</code>

In this approach, we first use flatMap to create a stream of pairs containing the key1 from each Item and the corresponding SubItem. Then, we apply Collectors.groupingBy twice: once to group the pairs by key1 and again to group the SubItems by key2.

An alternative solution would be to define a custom collector that provides a flatMapping operation similar to Java 9's Collectors.flatMapping:

<code class="java">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]));
}</code>

With this custom collector, the grouping operation can be simplified:

<code class="java">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))));</code>

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