How can I transform a collection into a Guava Multimap grouped by the elements of a nested collection property?

Question

I have a List and want a Guava Multimap where we\'ve grouped the Foos by each tag of their Collection

Answer 1

You can use Stream.flatMap to create a stream on each tag/foo pair, and then collect it into a multimap. Newer versions of Guava provide collectors such as ImmutableSetMultimap.toImmutableSetMultimap and ImmutableListMultimap.toImmutableListMultimap for this purpose.

ImmutableSetMultimap result = list.stream()
        .flatMap(foo -> foo.getTags().stream().map(tag -> Map.entry(tag, foo)))
        .collect(ImmutableSetMultimap.toImmutableSetMultimap(Map.Entry::getKey, Map.Entry::getValue));