Q:

How to convert the values of a Map of List to a single List

How is the best (efficient and simple) way to convert the values of a Map<Object1, List<Object2>> to a List<Object2>. Should I simply iterate over all entries and adAll the items to a list?

A:

Here's a third way so you don't need to create an ArrayList, which is what you actually asked.  This is the proper way to use streams because you're not changing the state of any object.
List<Object2> values = myMap.values()
                            .stream()
                            .flatMap(Collection::stream)
                            .collect(Collectors.toList());

