How to Group Java 8 Nested Objects by Multiple Keys Using Collectors?

Java 8 Multi-Level Grouping with Nested Collectors

Multi-level grouping, also known as nested grouping, involves aggregating data by multiple keys across different levels of a nested object hierarchy. In Java 8, you can achieve this with the help of nested Collectors.

Consider a scenario where you have classes like Pojo, Item, and SubItem, where each Pojo has a list of Items, and each Item has a list of SubItems. The goal is to group the Items by their key1 field, and for each group, further aggregate the SubItems by their key2 field.

To perform this nested grouping, you can use Collectors.groupingBy with a nested structure:

<code class="java">pojo.getItems()
    .stream()
    .collect(
        Collectors.groupingBy(Item::getKey1, **// How to group by SubItem::getKey2 here**));</code>

The question lies in the second parameter of Collectors.groupingBy - how to group the SubItems by key2 within each key1 group. The solution cannot utilize cascaded groupingBy, as it only groups by fields within the same object.

The answer involves using Stream.flatMap to create a stream of key-value pairs, where the key is Item.key1 and the value is SubItem.key2. This stream is then grouped using Collectors.groupingBy to achieve the desired nested grouping:

<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.mapping(Map.Entry::getValue,
                    Collectors.groupingBy(SubItem::getKey2))));</code>

Alternatively, with Java 9's Collectors.flatMapping, the solution 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>

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