How to Implement Nested Grouping in Java Using Collectors.groupingBy in Java 8?

Nested (Multi Level) Group By in Java 8

The need often arises to perform multi-level grouping of objects based on multiple keys. In this case, we have a set of classes representing data involving a list of items with a key and a list of subitems, also with a key. The goal is to group the items by their key and for each aggregation, further group the subitems by their key, resulting in a nested map.

Using Java 8's Collectors.groupingBy, we can achieve this by employing a combination of flatMap and a custom flat-mapping collector. We start by creating a temporary pair using Stream.flatMap to hold the combinations of items and subitems.

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

Alternatively, we can avoid creating temporary objects by using a flat-mapping collector, which unfortunately will not be available until Java 9. With this approach, the code simplifies to:

<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>

For users who may not want to wait for Java 9, a custom flat-mapping collector can be implemented:

<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>

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