Combine stream of Collections into one Collection - Java 8

Question

So I have a Stream<Collection<Long>> that I obtain by doing a series of transformations on another stream.

What I need to do is collect the Stream<Collection<Long>> into one Collection<Long>.

I could collect them all into a list like this:

<Stream<Collection<Long>> streamOfCollections = /* get the stream */;

List<Collection<Long>> listOfCollections = streamOfCollections.collect(Collectors.toList());

And then I could iterate through that list of collections to combine them into one.

However, I imagine there must be a simple way to combine the stream of collections into one Collection<Long> using a .map() or .collect(). I just can't think of how to do it. Any ideas?

Solution 1

This functionality can be achieved with a call to the flatMap method on the stream, which takes a Function that maps the Stream item to another Stream on which you can collect.

Here, the flatMap method converts the Stream<Collection<Long>> to a Stream<Long>, and collect collects them into a Collection<Long>.

Collection<Long> longs = streamOfCollections
    .flatMap( coll -> coll.stream())
    .collect(Collectors.toList());

Solution 2

You could do this by using collect and providing a supplier (the ArrayList::new part):

Collection<Long> longs = streamOfCollections.collect(
    ArrayList::new, 
    ArrayList::addAll,
    ArrayList::addAll
);

Solution 3

You don't need to specify classes when not needed. A better solution is:

Collection<Long> longs = streamOfCollections.collect(
    ArrayList::new,
    Collection::addAll,
    Collection::addAll
);

Say, you don't need an ArrayList but need a HashSet, then you also need to edit only one line.

Comments

[Alexis C.]

You could also use a method reference: .flatMap(Collection::stream)

[Vivin Paliath]

@Desik Most definitely - it's what I use these days. At the time I was just learning the new streaming API.

[Jordan Mackie]

@Desik why is it better performing than the accepted answer?

[Desik]

@JordanMackie Because there are less intermediate operations, and no temporary objects created. In this solution you don't call stream() on every collection.