Java 8: merge lists with stream API

I think flatMap() is what you're looking for.

For example:

 List<AClass> allTheObjects = map.values()
         .stream()
         .flatMap(listContainer -> listContainer.lst.stream())
         .collect(Collectors.toList());

Alternative: Stream.concat()

Stream.concat(map.values().stream(), listContainer.lst.stream())
                             .collect(Collectors.toList()

In Java 8 we can use stream List1.stream().collect(Collectors.toList()).addAll(List2); Another option List1.addAll(List2)