java - 将集转换为列表而不创建新列表(Convert Set to List without creating new List)

Question

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

(我正在使用此代码将Set转换为List ：)

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

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

I want to avoid creating a new list in each iteration of the loop.

(我想避免在循环的每次迭代中创建一个新列表。)

Is that possible?

(那可能吗？)

  ask by Muhammad Imran Tariq translate from so

Answer 1

You can use the List.addAll() method.

(您可以使用List.addAll（）方法。)

It accepts a Collection as an argument, and your set is a Collection.

(它接受Collection作为参数，而您的集合就是Collection。)

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

EDIT: as respond to the edit of the question.

(编辑：作为对问题的编辑的回应。)
It is easy to see that if you want to have a Map with List s as values, in order to have k different values, you need to create k different lists.

(很容易看出，如果要使用一个List为List的Map作为值，为了拥有k个不同的值，您需要创建k个不同的列表。)
Thus: You cannot avoid creating these lists at all, the lists will have to be created.

(因此：您完全无法避免创建这些列表，必须创建列表。)

Possible work around:

(可能的解决方法：)
Declare your Map as a Map<String,Set> or Map<String,Collection> instead, and just insert your set.

(将Map声明为Map<String,Set>或Map<String,Collection> ，然后插入您的地图集。)