Just create a list and add items to it,
List<Integer> checkedList = new ArrayList<Integer>();
 for (Product p : boxAdapter.getBox()) {
   if (p.aBoolean)
       checkedList.add(p.id);
 }
I would suggest you work on your basics before you start coding, it will help you in such situations.
Hope this is the solution you were looking for.
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solved How to add numbers for arraylist