Why does ArrayList not throw ConcurrentModificationException when modified from multiple threads?

Question

ConcurrentModificationException : This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.<

Answer 1

The reason you are not receiving a ConcurrentModificationException is that ArrayList.remove does not throw one. You can probably get one by starting an additional thread that iterates through the array:

final List tickets = new ArrayList(100000);
for (int i = 0; i < 100000; i++) {
    tickets.add("ticket NO," + i);
}
for (int i = 0; i < 10; i++) {
    Thread salethread = new Thread() {
        public void run() {
            while (tickets.size() > 0) {
                tickets.remove(0);
                System.out.println(Thread.currentThread().getId()+"Remove 0");
            }
        }
    };
    salethread.start();
}
new Thread() {
    public void run() {
        int totalLength = 0;
        for (String s : tickets) {
            totalLength += s.length();
        }
    }
}.start();