How to get all combinations of keys of nested maps with Java Streams?

Question

Assuming I have a Map<Integer, Map<Class, String>> how could I get all combinations of the Integer keys of the outer map and the keys of all maps which are values of outer map in one elegant (but probably long) Java stream declaration?

My approaches:

Map<Integer, Map<Class, String>> map = new HashMap<>();
map.keySet().stream()
        .map(intKey -> map.get(intKey).keySet().stream()
                .map(classKey -> new SimpleEntry(intKey, classKey)))
        .collect(Collectors.toList());

collects a list of java.util.stream.ReferencePipeline. With

map.keySet().stream()
        .map(intKey -> map.get(intKey).keySet().stream()
                .map(classKey -> new SimpleEntry(intKey, classKey))
                .collect(Collectors.toList()))
        .collect(Collectors.toList());

I have trouble wrapping my head around the collection of already collected SimpleEntrys. Of course, the approach can be wrong completely.

My problems is that I simply don't find a way to solve this. Please note that I'm not looking for a workaround as I want to broaden my understanding.

score 2 · Accepted Answer · answered Aug 14 '18 at 21:41

You're almost there. You just need a flatMap():

map.entrySet()
        .stream()
        .flatMap(e -> e.getValue()
                .keySet()
                .stream()
                .map(c -> new SimpleEntry<>(e.getKey(), c)))
        .collect(Collectors.toList())

Just for fun, here's the StreamEx solution:

EntryStream.of(map)
        .mapValues(Map::keySet)
        .flatMapValues(Set::stream)
        .toList()

How to get all combinations of keys of nested maps with Java Streams?

1 Answers1