How can I turn a List of Lists into a List in Java 8?

If I have a List<List<Object>>, how can I turn that into a List<Object> that contains all the objects in the same iteration order by using the features of Java 8?

You can use flatMap to flatten the internal lists (after converting them to Streams) into a single Stream, and then collect the result into a list:

List<List<Object>> list = ...
List<Object> flat = 
    list.stream()
        .flatMap(List::stream)
        .collect(Collectors.toList());