I am just testing a few things out to improve my understanding and I wrote the following code to produce a ConcurrentModificationExecption

public static void main(String[] args) throws InterruptedException {
    Set<String> set = new HashSet<>();

    Runnable updateList = () -> {
        while (true) {
            set.add("Hello");
        }
    };
    ExecutorService executorService1 = Executors.newSingleThreadExecutor();
    executorService1.execute(updateList);

    Runnable printList = () -> {
        while (true) {
            if (set.contains("Hello")) {
                System.out.println(set);
                set.add("Bye");
            }
        }
    };
    ExecutorService executorService2 = Executors.newSingleThreadExecutor();
    executorService2.execute(printList);

    Thread.sleep(1000);
    executorService1.shutdown();
    executorService2.shutdown();
}

However its not producing the exception like I thought it would. When I replace the HashSet with and ArrayList, I egt the exception.
Any idea why?