Convert Set to List without creating new List

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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.

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