public E set(int index, E element) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        E oldValue = get(elements, index);

        if (oldValue != element) {
            int len = elements.length;
            Object[] newElements = Arrays.copyOf(elements, len);
            newElements[index] = element;
            setArray(newElements);
        } else {
　　　　　　　// 就是这一段代码
            setArray(elements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}

private E get(Object[] a, int index) {
    return (E) a[index];
}

final void setArray(Object[] a) {
    array = a;
}