Flattening a collection

Question

Say I have a Map>

I can get the values of the map easily enough, and iterate over it to produce a single

Answer 1

When searching for "java 8 flatten" this is the only mentioning. And it's not about flattening stream either. So for great good I just leave it here

.flatMap(Collection::stream)

I'm also surprised no one has given concurrent java 8 answer to original question which is

.collect(ArrayList::new, ArrayList::addAll, ArrayList::addAll);