使用Java8将Map <String，NavigableMap <Long，Collection <String >>>转换为List <String>

Question

I am trying to convert Map<String, NavigableMap<Long, Collection<String>>> into List<String> Java 8.

I wrote some code but got stuck some where in mid.

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
        }
    }
);

Answer 1

Just flatMap that s**t :

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 neater:

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

Answer 2

Two flatMaps will do, flat the NavigableMap and then flat the Collection inside.

List<String> result = mapOfMaps.values()
            .stream()
            .flatMap(v -> v.values().stream())
            .flatMap(Collection::stream)
            .collect(Collectors.toList());