Question

I have in code

List<HashSet<Integer>> list = new ArrayList<HashSet<Integer>>(50);
pos = 17; // just some index less than 50
list.add(pos, new HashSet<Integer>());
list.get(17).add(99);
list.get(17).add(88);

After a while I want to remove the HashSet with {99, 88} inside and create a new one, like this:

// pos is still here
list.add(pos, new HashSet<Integer>());

Is it allowed? Do you know any faster solution? Thanks.

Answer 1

list.add(pos, new HashSet<Integer>()); is going to add a new set at that position and shift the existing one to pos+1. That does not seem to be what you want.

If you want to replace the set by a new one, use set:

list.set(pos, new HashSet<Integer>()); 

In terms of performance, ArrayList#set runs in constant time so it is an efficient way of replacing the existing set in the list.