public Iterator<E> iterator() {
        return new Itr();
    }

属性

        int cursor;       // 下一个要返回的元素的下标
        int lastRet = -1; // 最后一个要返回元素的下标 没有元素返回 -1
        int expectedModCount = modCount; //期望的 modCount

Itr的hasNext() 方法

        public boolean hasNext() {
            return cursor != size;
        }

Itr的next()方法

        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
        final void checkForComodification() {
            if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        }
        在迭代的时候，会校验modCount是否等于expectedModCount，
        不等于就会抛出著名的ConcurrentModificationException异常。什么时候会抛出ConcurrentModificationException？
        public static void main(String[] args) {
            ArrayList arrayList = new ArrayList();
            for (int i = 0; i < 10; i++) {
                arrayList.add(i);
            }
            remove(arrayList);
            System.out.println(arrayList);
        }
        public static void remove(ArrayList<Integer> list) {
            Iterator<Integer> iterator = list.iterator();
            while (iterator.hasNext()) {
                Integer number = iterator.next();
                if (number % 2 == 0) {
                    // 抛出ConcurrentModificationException异常
                    list.remove(number);
                }
            }
        }
        list.remove(number); 当这行代码执行后，modCount++， expectedModCount还是为原来的值，所以会发生ConcurrentModificationException异常

那怎么写才能不抛出ConcurrentModificationException？很简单，将list.remove(number);换成iterator.remove();即可。why？请看Itr的remove()源码…

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();
            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;   ！！！！！！期望的修改值被重新赋值
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }