Flattening an Iterable<Iterable<T>> in Guava

Question

Is there a flatten method in Guava - or an easy way to convert an Iterable<Iterable<T>> to an Iterable<T>?

I have a Multimap<K, V> [sourceMultimap] and I want to return all values where the key matches some predicate [keyPredicate]. So at the moment I have:

Iterable<Collection<V>> vals = Maps.filterKeys(sourceMultimap.asMap(), keyPredicate).values();

Collection<V> retColl = ...;
for (Collection<V> vs : vals) retColl.addAll(vs);
return retColl;

I've looked through the Guava docs, but nothing jumped out. I am just checking I've not missed anything. Otherwise, I'll extract my three lines into a short flatten generic method and leave it as that.

Answer 1

The Iterables.concat method satisfies that requirement:

public static <T> Iterable<T> concat(Iterable<? extends Iterable<? extends T>> inputs)

Answer 2

As of Java 8, you can do this without Guava. It's a bit clunky because Iterable doesn't directly provide streams, requiring the use of StreamSupport, but it doesn't require creating a new collection like the code in the question.

private static <T> Iterable<T> concat(Iterable<? extends Iterable<T>> foo) {
    return () -> StreamSupport.stream(foo.spliterator(), false)
        .flatMap(i -> StreamSupport.stream(i.spliterator(), false))
        .iterator();
}