Collapse List<List<String>> into List<String> using a lambda?

Question

Say I have a list of Lists..

List<List<String>> lists = new ArrayList<>();

Is there a clever lambda way to collapse this into a List of all the contents ?

Answer 1

That's what flatMap is for :

List<String> list = inputList.stream() // create a Stream<List<String>>
                             .flatMap(l -> l.stream()) // create a Stream<String>
                                                       // of all the Strings in
                                                       // all the internal lists
                             .collect(Collectors.toList());

Answer 2

You can do

List<String> result = lists.stream()
    .flatMap(l -> l.stream())
    .collect(Collectors.toList());