Flattening an Iterable<Iterable<T>> in Guava

java collections guava flatten
Andy Whitfield picture Andy Whitfield · May 10, 2011 · Viewed 14.9k times · Source

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

Sean Parsons picture Sean Parsons · May 10, 2011

The Iterables.concat method satisfies that requirement:

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

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