Flattening a collection

Question

Say I have a Map>

I can get the values of the map easily enough, and iterate over it to produce a single

Answer 1

Flatten on a function:

    private  List flatten(List list, Function> flattenFn) {
        return list
                .stream()
                .map(flattenFn)
                .flatMap(Collection::stream)
                .collect(Collectors.toUnmodifiableList());
    }