Queue.ArrayDequeue

#ArrayDequeue

• 內部數組，雙向隊列，線程不安全，棧性能高於stack。作隊列性能高於linkedList，內部數組。
• 內部操做指針，進行操做，容量不足，自動擴容。

transient Object[] elements; // non-private to simplify nested class access
    /**
     * The index of the element at the head of the deque (which is the
     * element that would be removed by remove() or pop()); or an
     * arbitrary number equal to tail if the deque is empty.
     */
    transient int head;
    /**
     * The index at which the next element would be added to the tail
     * of the deque (via addLast(E), add(E), or push(E)).
     */
    transient int tail;

• offer方法和add方法都是經過其中的addLast方法實現，每添加一個元素，就把元素加到數組的尾部，此時，head指針沒有變化，而tail指針加一，由於指針是循環加的，因此當tail追上head,（(this.tail = this.tail + 1 & this.elements.length - 1) == this.head）時，數組容量翻一倍，繼續執行。

public void addLast(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;
        if ( (tail = (tail + 1) & (elements.length - 1)) == head)
            doubleCapacity();
    }

• remove方法和poll方法都是經過其中的pollFirst方法實現，每移除一個元素，該元素所在位置變成null，此時，tail指針沒有變化，而head指針加一，當數組中沒有數據時，返回null

public E pollFirst() {
        int h = head;
        @SuppressWarnings("unchecked")
        E result = (E) elements[h];
        // Element is null if deque empty
        if (result == null)
            return null;
        elements[h] = null;     // Must null out slot
        head = (h + 1) & (elements.length - 1);
        return result;
    }

• 擴容，先複製head右邊，再複製head左邊