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

You should use flatMap as follows:

entrySet.stream()
        .flatMap(e -> e.getValue().stream()
                       .map(s -> new SimpleImmutableEntry(e.getKey(), s)));

SimpleImmutableEntry is a nested class in AbstractMap.