Java: combine two map keys if the values are matching using streams

Question

I have two maps

Map 1: {123, "aaa"}, {234, "bbb"}

Map 2: {345, "ccc"}, {456, "aaa"}

Using streams, I would like to go through these maps and return {123, 456}

I've tried something like

map1.entrySet().stream()
    .filter(node ->
        map2.entrySet().stream().anyMatch(
            newNode -> node.getValue().equals(newNode.getValue()))
.collect(...);

But that only gives me the the list of the first map. Any ideas?

Answer 1

Abandon the idea that this is something that's easily done in streams, and just iterate across.

Basically - if you have a match, you need to add both keys. With the collect operator, you're doing one discrete operation and you really want to do more than one.

So...just iterate instead.

public static <K, V> Collection<K> findKeysOfEqualValues(Map<K, V> map1, Map<K, V> map2) {
    Set<K> keyList = new LinkedHashSet<>();
    for(Map.Entry<K, V> mapEntry1 : map1.entrySet()) {
        for(Map.Entry<K, V> mapEntry2 : map2.entrySet()) {
            if(Objects.equals(mapEntry1.getValue(), mapEntry2.getValue())) {
                keyList.add(mapEntry1.getKey());
                keyList.add(mapEntry2.getKey());
            }
        }
    }

    return keyList;
}