RandomAccess

实现该接口，就说明支持随机访问方式遍历

public static void main(String[] args)  {
        ArrayList list = new ArrayList<>();
        if (list instanceof RandomAccess) {
            //随机访问
            for (int i = 0; i < list.size(); i++) {
                System.out.println(list.get(i));
            }
        } else {
            //迭代器访问
            Iterator iterator = list.iterator();
            while (iterator.hasNext()) {
                System.out.println(iterator.next());
            }
        }
    }