Question

I am using this code to convert a Set to a List:

Map<String, List<String>> mainMap = new HashMap<>();

for (int i=0; i < something.size(); i++) {
  Set<String> set = getSet(...); //returns different result each time
  List<String> listOfNames = new ArrayList<>(set);
  mainMap.put(differentKeyName, listOfNames);
}

I want to avoid creating a new list in each iteration of the loop. Is that possible?

Answer 1

You can use the List.addAll() method. It accepts a Collection as an argument, and your set is a Collection.

List<String> mainList = new ArrayList<String>();
mainList.addAll(set);

EDIT: as respond to the edit of the question.

It is easy to see that if you want to have a Map with Lists as values, in order to have k different values, you need to create k different lists.

Thus: You cannot avoid creating these lists at all, the lists will have to be created.

Possible work around:

Declare your Map as a Map<String,Set> or Map<String,Collection> instead, and just insert your set.