ArrayList.addAll(ArrayList) throws SOMETIMES UnsupportedOperationException

I have a code that read list from some paged string data. What I do not understand - why the UnsupportedOperationException is thrown on addAll() and why it's kind of random behaviour ?

I know creating target ArrayList and not adding to the returned one solves the issue, I'm looking for b开发者_C百科etter understanding not a fix.

List<Event> eventList = eventTable.getEvents(); // returns ArrayList
while (hasNextPage()) {
  goToNextPage();
  eventList.addAll(eventTable.getEvents());
}

List<Event> is not necessarily an ArrayList<Event>. (The opposite is true though.)

The reason you get UnsupportedOperationException sometimes, is because eventTable.getEvents() sometimes returns a list that supports addAll and sometimes it doesn't.

The implementation of getEvents could for instance look like this:

if (noEventsAvailable) {
    return Collections.emptyList();
} else {
    List<Event> toReturn = new ArrayList<Event>();
    // populate list...
    return toReturn;
}

(In your comment you write // returns ArrayList. I don't know where you've got this from, but I know one thing for sure: An ArrayList will always support the addAll operation.)

The correct way to solve it is, as you mention, to do

List<Event> eventList = new ArrayList<Event>(eventTable.getEvents());

It depends on the actual implementation of List.

e.g if the underlying list was obtained using Collections.unmodifiableList() then calling addAll() or any other modification method will throw an UnsupportedOperationException.

When it throws an exception, it should show you the exact line number and source code file - you should be able to find out exactly why it's throwing an exception.

My guess is that under certain circumstances, eventTable.getEvents() returns an immutable list, or something like that - but without knowing what eventTable is, it's hard to say for sure. If you can produce a short but complete program which demonstrates the problem, that would make it a lot easier to diagnose.