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

if (CollectionUtils.isNotEmpty(listName))

Is the same as:

if(listName != null && !listName.isEmpty())

In first approach listName can be null and null pointer exception will not be thrown. In second approach you have to check for null manually. First approach is better because it requires less work from you. Using .size() != 0 is something unnecessary at all, also i learned that it is slower than using .isEmpty()

Unless you are already using CollectionUtils I would go for List.isEmpty(), less dependencies.

Performance wise CollectionUtils will be a tad slower. Because it basically follows the same logic but has additional overhead.

So it would be readability vs. performance vs. dependencies. Not much of a big difference though.

CollectionUtils.isNotEmpty checks if your collection is not null and not empty. This is better comparing to double check but only if you have this Apache library in your project. If you don't then use:

if(list != null && !list.isEmpty())

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.