How to prove arraylist is not thread safe with a test?

Question

In our application we got an ArrayIndexOutOfBounds exception on the ArrayList.add(Object o) operation. The most obvious explanation is thread safety, but I wasn't able to recreate the events. I've tried creating two threads. In one I'm adding elements, in the other I'm removing them(or clearing the array), but I didn't get the exception for the second time. I mean it's obvious it can happen by looking at the source of ArrayList, but it would be nice to be able to demonstrate it.

I've been running this test for quite some time without any exception:

public class Test {
static ArrayList a = new ArrayList();

public static void main(String[] args) throws Exception {
    Thread t1 = new Thread() {
        public void run() {
            while (true) {
                if (a.size() > 0)
                    a.remove(0);
            }
        }
    };

    Thread t2 = new Thread() {
        public void run() {
            while (true) {
                a.add(new Object());
            }
        }
    };

    t2.start();
    Thread.sleep(100);
    t1.start();
}
}

Answer 1

Thanks to comment from isnot2bad I found a problem in my assumptions. The problem is with concurrent adds, not add/remove. I was able to create a failing test:

static ArrayList a = new ArrayList(1);

public static void main(String[] args) throws Exception {
    Thread t1 = new Thread() {
        public void run() {
            while (true) {
                a.add(new Object());
            }
        }
    };

    Thread t2 = new Thread() {
        public void run() {
            while (true) {
                a = new ArrayList(1);
                a.add(new Object());
                a.add(new Object());
            }
        }
    };

    t2.start();
    Thread.sleep(100);
    t1.start();
}

On the line with the add at the first thread, I'm getting this:

Exception in thread "Thread-0" java.lang.ArrayIndexOutOfBoundsException: 2 

:)