将集转换为列表而不创建新列表

问题描述

我正在使用此代码将Set转换为List:

I am using this code to convert a Set to a 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);
}

我想避免在循环的每次迭代中创建一个新列表.有可能吗?

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

推荐答案

您可以使用 List.addAll()方法.它接受一个Collection作为参数，而您的集合就是一个Collection.

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);

编辑:对问题的编辑进行回复.
很容易看出，如果要使用List作为值的Map，要具有k个不同的值，则需要创建k个不同的列表.
因此:您无法避免完全创建这些列表，必须创建列表.

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.

可能的解决方法:
相反，将您的Map声明为Map<String,Set>或Map<String,Collection>，只需插入您的设置即可.

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

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