Combine multiple lists in Java

Question

If I want to make two lists into one in Java, I can use ListUtils.union(List list1,List list2). But what if I want to combine multiple lists?

This works

Answer 1

Adding other alternatives:

OPTION 1:

List joinedList = joinLists(list1, list2, list3, list4);

public static  List joinLists(List... lists) {
        return Arrays.stream(lists).flatMap(Collection::stream).collect(Collectors.toList()); 
}

OPTION 2:

List joinedList = new ArrayList<>();
Stream.of(list1, list2, list3, list4).forEach(joinedList::addAll);