How to Perform Nested Grouping based on Multiple Properties in Java 8

Nested Grouping in Java 8 for Complex Object Structures

In Java 8, the Collectors.groupingBy method provides a powerful way to group items based on specific properties. This allows you to aggregate and summarize data efficiently. However, what about scenarios where you have multiple levels of grouping to perform?

Consider the following class structure:

<code class="java">class Pojo {
    List<Item> items;
}

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

class SubItem {
    V key2;
    Object otherAttribute1;
}</code>

The objective is to group items based on key1 and then, for each item group, further group subitems based on key2. The desired output is a map of the form:

<code class="java">Map<T, Map<V, List<SubItem>>></code>

Using standard Collectors.groupingBy won't suffice for this scenario. The challenge lies in grouping a single item by multiple keys.

Solution: Flattening the Structure

The key to solving this problem is to flatten the structure temporarily. By doing so, you can create a stream of combinations of Item and SubItem before performing the necessary groupings.

One approach is to use the Stream.flatMap method to create a stream of Map.Entry objects, where each entry represents a pair of Item and SubItem:

<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>

Alternative Solution with Collectors.flatMapping (Java 9 )

In Java 9 and higher, a more elegant approach is available using the Collectors.flatMapping collector:

<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>

Conclusion

By leveraging Streams, Collectors, and temporary object flattening, it is possible to achieve nested grouping even for complex object structures in Java 8 and beyond.

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