get()没有阻塞,即使有线程正在添加元素也没有阻塞。

    public E get(int index) {
        return get(getArray(), index);
    }

add()是阻塞的，

    public boolean add(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] elements = getArray();
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len + 1);
            newElements[len] = e;
            setArray(newElements);
            return true;
        } finally {
            lock.unlock();
        }
    }