Convert Map > > to list using java 8

I'm trying to convert map < string, navigablemap < long, collection < string > > into list < string > java 8

I wrote some code, but I got stuck in the middle

userTopics.values().stream().map(
    new Function<NavigableMap<Long,Collection<String>>,Collection<String>>() {
        @Override
        public Collection<String> apply(NavigableMap<Long,Collection<String>> t) {
            return null;  //TODO
        }
    }
);

Solution

Flatmap that s * * t only:

List<String> values = nestedMap.entrySet()
        .stream()
        .map(Map.Entry::getValue)
        .flatMap(m -> m.entrySet().stream())
        .map(Map.Entry::getValue)
        .flatMap(Collection::stream)
        .collect(toList());

As Holger points out, this is cleaner:

List<String> values = nestedMap.values()
        .stream()
        .flatMap(m -> m.values().stream())
        .flatMap(Collection::stream)
        .collect(toList());