How to aggregate map values into set using streams

Question

Is there some easy way how to convert Map<Key, List<Value>> into Set<Value> to get a set of all unique elements nested int the map? I know this should be possible using reduce or flatmap but I am struggling with the right combination.

I know I could do this using for loop but I would like to perform this operation using streams.

Answer 1

Set<Value> set = map.values().stream()
                             .flatMap(List::stream)
                             .collect(Collectors.toSet());

Set will not add a Value that is already in it, so you have to make sure, that your Value objects have a proper equals() method.

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Edit: List::stream is equivalent to list -> list.stream() in functionality according to this post.