Question

This API call returns a potentially large List<String>, which is not sorted. I need to sort it, search it, and access random elements. Currently the List is implemented by an ArrayList (I checked the source), but at some unknown point in the future the API developers may choose to switch to a LinkedList implementation (without changing the interface).

Sorting, searching, accessing a potentially large LinkedList would be extremely slow and unacceptable for my program. Therefore I need to convert the List to an ArrayList to ensure the practical efficiency of my program. However, since the List is most likely an ArrayList already, it would be inefficient to needlessly create a new ArrayList copy of the List.

Given these constraints, I have come up with the following method to convert a List into an ArrayList:

private static <T> ArrayList<T> asArrayList(List<T> list) {
  if (list instanceof ArrayList) {
    return (ArrayList<T>) (list);
  } else {
    return new ArrayList<T>(list);
  }
}

My question is this: Is this the most efficient way to work with a List with an unknown implementation? Is there a better way to convert a List to an ArrayList? Is there a better option than converting the List into an ArrayList?

Answer 1

You can’t really get much simpler than what you’ve got – looks about as efficient as it could possibly be to me.

That said, this sounds very much like premature optimisation – you should only really need to worry about this if and when the author of the API you’re using changes to a LinkedList. If you worry about it now, you are likely to spend a lot of time and effort planning for a future scenario that may not even come to pass – this is time that might be better spent finding other issues to fix. Presumably the only time you’ll be changing versions of the API is between versions of your own application – handle the issue at that point, if at all.