1Z0-809 Question #229: Real Exam Question with Answer & Explanation

Given the code fragment:

final List<String> list = new CopyOnWriteArrayList<>();
final AtomicInteger ai = new AtomicInteger(0);
final CyclicBarrier barrier = new CyclicBarrier(2, new Runnable() {
 public void run() { System.out.print(list); }
});

Runnable r = new Runnable() {
 public void run() {
 try {
 Thread.sleep(1000 * ai.incrementAndGet());
 list.add("X");
 barrier.await();
 } catch (Exception ex) { }
 }
};

new Thread(r).start();
new Thread(r).start();
new Thread(r).start();
new Thread(r).start();

What is the result?