Concurrent threads adding to ArrayList at same time - what happens?

Question

We have multiple threads calling add(obj) on an ArrayList.

My theory is that when add is called concurrently by two threads,

Answer 1

You could also get a null, an ArrayOutOfBoundsException, or something left up to the implementation. HashMaps have been observed to go into an infinite loop in production systems. You don't really need to know what might go wrong, just don't do it.

You could use Vector, but it tends to work out the interface is not rich enough. You will probably find that you want a different data structure in most cases.