java - Flatmap vs two forEach

Question

Let's say we have a map of lists and each list contains only one value:

    Map<String, List<String>> map = new HashMap<String, List<String>>();
    
    ArrayList<String> list1 = new ArrayList<String>();
    list1.add("A");
    ArrayList<String> list2 = new ArrayList<String>();
    list2.add("B");
    ArrayList<String> list3 = new ArrayList<String>();
    list3.add("B");
    ...
 // listN.add("N");
    
    map.put("List1", list1);
    map.put("List2", list2);
    map.put("List3", list3);
    ...
//  map.put("ListN", listN);

There is a need to remove duplicates values from all lists via conversion map -> set. So, there are two ways how to do it.

The first one:

Set<String> set = map.values().stream()
            .flatMap(Collection::stream)
            .collect(Collectors.toSet());

The second one:

Set<String> set = new HashSet<>();
map.forEach((k, l)->l.forEach(v->set.add(v)));

The questios are:

1. What would be better performance?

2. What would be better performance if each list contains more than one value? As instance:

    Map<String, List<String>> map = new HashMap<String, List<String>>();
   
    ArrayList<String> list1 = new ArrayList<String>();
    list1.add("A");
    list1.add("B");
    list1.add("C"); 
    ArrayList<String> list2 = new ArrayList<String>();
    list2.add("B");
    list2.add("C");
    ArrayList<String> list3 = new ArrayList<String>();
    list3.add("C");
    list3.add("A");
    // and so on
   

From my point of view:

1. Should be the same performance.
2. It depends on how flatMap() works under the hood. Can't find any detail source of it. I assume flatMap provide some overhead and should be more slowly.

Answer 1

等待大神解答