public class ContainerNotSafeDemo {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 30; i++) {
            new Thread(() -> {
                list.add(UUID.randomUUID().toString().substring(0, 8));
                System.out.println(list);
            }, "线程--"+i).start();
        }
    }
}

public class IteratorNotSafeDemo {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("zzz");
        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            String next = iterator.next();
            if ("zzz".equals(next)) {
                list.remove(next);
            }
        }
    }
}