Flatten a Map> to Map with stream and lambda

Question

I would like to flatten a Map which associates an Integer key to a list of String, without losing the key mapping. I am curious as tho

Answer 1

This should work. Please notice that you lost some keys from List.

Map> mapFrom = new HashMap<>();
Map mapTo = mapFrom.entrySet().stream()
        .flatMap(integerListEntry -> integerListEntry.getValue()
                .stream()
                .map(listItem -> new AbstractMap.SimpleEntry<>(listItem, integerListEntry.getKey())))
        .collect(Collectors.toMap(AbstractMap.SimpleEntry::getKey, AbstractMap.SimpleEntry::getValue));