java.util.ConcurrentModificationException not thrown when expected

Solution 1:

The iterator returned from ArrayList.iterator() in the implementation we're apparently both using only checks for structural modification in calls to next(), not in calls to hasNext(). The latter just looks like this (under Java 8):

public boolean hasNext() {
    return cursor != size;
}

So in your second case, the iterator "knows" that it's returned two elements, and that the list only has two elements... so hasNext() just returns false, and we never end up calling next() the third time.

I would view this as an implementation detail - basically the checking not being as strict as it could be. It would be entirely reasonable for hasNext() to perform a check and throw an exception in this case too.

Solution 2:

Note also the last paragraph of the ArrayList documentation summary:

Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

If you're worrying about forcing lists to be read-only, use the Collections.unmodifiableList method, instead of checking for ConcurrentModificationException, which as mentioned above is not guaranteed to be thrown in all relevant cases.