Checking if a collection is empty in Java: which is the best method?

Question

I have two ways of checking if a List is empty or not

if (CollectionUtils.isNotEmpty(listName)) 

and

if (listName != null && listName.size() != 0)

My arch tells me that the former is better than latter. But I think the latter is better.

Can anyone please clarify it?

Answer 1

You should absolutely use isEmpty(). Computing the size() of an arbitrary list could be expensive. Even validating whether it has any elements can be expensive, of course, but there's no optimization for size() which can't also make isEmpty() faster, whereas the reverse is not the case.

For example, suppose you had a linked list structure which didn't cache the size (whereas LinkedList<E> does). Then size() would become an O(N) operation, whereas isEmpty() would still be O(1).

Additionally of course, using isEmpty() states what you're actually interested in more clearly.