There isn't an elegant way in vanilla Java prior to Java 21.

Google Guava

The Google Guava library is great - check out their Iterables.getLast() method. This method will throw a NoSuchElementException if the list is empty, as opposed to an IndexOutOfBoundsException, as with the typical size()-1 approach - I find a NoSuchElementException much nicer, or the ability to specify a default:

lastElement = Iterables.getLast(iterableList);

You can also provide a default value if the list is empty, instead of an exception:

lastElement = Iterables.getLast(iterableList, null);

or, if you're using Options:

lastElementRaw = Iterables.getLast(iterableList, null);
lastElement = (lastElementRaw == null) ? Option.none() : Option.some(lastElementRaw);

(To complete Ankit Malpani answer) If 40 is provided by our lovely users, then you will have to restrict it to the list size:

posts.subList(posts.size()-Math.min(posts.size(),40), posts.size())

Another way to show it:

@Test
public void should_extract_last_n_entries() {
    List<String> myList = Arrays.asList("0","1","2","3","4");
    int myListSize = myList.size();
    log.info(myList.subList(myListSize,myListSize).toString());   // output : []
    log.info(myList.subList(myListSize-2,myListSize).toString()); // output : [3, 4]
    log.info(myList.subList(myListSize-5,myListSize).toString()); // output : [0, 1, 2, 3, 4]
    int lastNEntries = 50; // now use user provided int
    log.info(myList.subList(myListSize-Math.min(myListSize,lastNEntries),myListSize).toString());
                          // output : [0, 1, 2, 3, 4]
    // log.info(myList.subList(myListSize-lastNEntries,myListSize).toString());
                          // ouch IndexOutOfBoundsException: fromIndex = -45
}