在CopyOnWriteArrayList類的set方法中有一段setArray(elements)代碼，實際上這段代碼并未對elements做任何改動，實現的volatile語意并不對CopyOnWriteArrayList實例產生任何影響，為什么還是要保留這行語句？ 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 {　　　　　　　// 就是這一段代碼 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; }