Why does a ArrayIndexOutOfBoundsException happen here?

Question

I got the following code in my Android-App:

Event[] events = retrieveEvents();
if (events != null && events.length>0) {
   int eventNr = getFromUserInput();
   eventNr = eventNr % events.length;
   Event event = events[eventNr];
}

retrieveEvents() gets some Events from the Internet, so this can fail an though be empty or null. The user can select which Event to display, to avoid a Exception I use the modulo operation to ensure the eventNr is within bounds. This works fine on any devices I tested on BUT:

I get error-reports from other users where the second last line (the array-access) throws an ArrayIndexOutOfBoundsException. How can this happen? What condition have I left unchecked? Where is my error?

Remember: The retrieveEvents() and the getFromUserInput() function can both return invalid data, but I think I checked every case, so where is my fault?

Answer 1

Have you ensured that eventNr is never negative? The check isn't in your quoted code. The problem being that if, for instance, eventNr is -1 and events.length is 5, -1 % 5 = -1 and of course events[-1] is out of bounds.

Answer 2

Is this a threaded application? Is it possible that retrieveEvents() is always returning a reference to the same array, but that the array is being modified in real time? If so, that could be your issue.