Convert Map>> to List using Java8

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
        }
    }
);

Boris the Spider · Answer 1 · 2016-02-28T13:09:08.453

4

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());

edited Feb 28 '16 at 13:09

answered Feb 28 '16 at 12:30

Boris the Spider

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3

A funny anti-pattern, even twice. `entrySet().stream().map(Map.Entry::getValue)` instead of streaming over the desired items in the first place: `values().stream()` – Holger Feb 28 '16 at 12:46
1

@cody123: it's the same as `.map(entry -> entry.getValue())` with `entry` being of type `Map.Entry`. When you also need the keys in an intermediate stream operation, streaming over the entries via `entrySet().stream()` and mapping to the values once a stream of values is needed, is the right solution. However, when you need the values *only*, you can right stream over the values in the first place. – Holger Feb 28 '16 at 13:24

score 3 · Answer 2 · answered Feb 28 '16 at 12:41

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());

Convert Map>> to List using Java8

2 Answers2

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