Convert Set to List without creating new List
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.
Also from Guava Collect library, you can use newArrayList(Collection):
Lists.newArrayList([your_set])This would be very similar to the previous answer from amit, except that you do not need to declare (or instanciate) any list object.