Java Review - Queue和Stack 源码解读

/**
     * Constructs an empty array deque with an initial capacity
     * sufficient to hold 16 elements.
     */
    public ArrayDeque() {
        elements = new Object[16];
    }
    /**
     * Constructs an empty array deque with an initial capacity
     * sufficient to hold the specified number of elements.
     *
     * @param numElements  lower bound on initial capacity of the deque
     */
    public ArrayDeque(int numElements) {
        allocateElements(numElements);
    }
    /**
     * Constructs a deque containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.  (The first element returned by the collection's
     * iterator becomes the first element, or <i>front</i> of the
     * deque.)
     *
     * @param c the collection whose elements are to be placed into the deque
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayDeque(Collection<? extends E> c) {
        allocateElements(c.size());
        addAll(c);
    }

/**
     * Allocates empty array to hold the given number of elements.
     *
     * @param numElements  the number of elements to hold
     */
    private void allocateElements(int numElements) {
        elements = new Object[calculateSize(numElements)];
    }

/**
     * The minimum capacity that we'll use for a newly created deque.
     * Must be a power of 2.
     */
    private static final int MIN_INITIAL_CAPACITY = 8;
    // ******  Array allocation and resizing utilities ******
    private static int calculateSize(int numElements) {
        int initialCapacity = MIN_INITIAL_CAPACITY;
        // Find the best power of two to hold elements.
        // Tests "<=" because arrays aren't kept full.
        if (numElements >= initialCapacity) {
            initialCapacity = numElements;
            initialCapacity |= (initialCapacity >>>  1);
            initialCapacity |= (initialCapacity >>>  2);
            initialCapacity |= (initialCapacity >>>  4);
            initialCapacity |= (initialCapacity >>>  8);
            initialCapacity |= (initialCapacity >>> 16);
            initialCapacity++;
            if (initialCapacity < 0)   // Too many elements, must back off
                initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
        }
        return initialCapacity;
    }

/**
     * Inserts the specified element at the front of this deque.
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addFirst(E e) {
        if (e == null) //不允许放入null
            throw new NullPointerException();
        elements[head = (head - 1) & (elements.length - 1)] = e; //2.下标是否越界
        if (head == tail)//1.空间是否够用
            doubleCapacity();//扩容
    }

/**
     * Doubles the capacity of this deque.  Call only when full, i.e.,
     * when head and tail have wrapped around to become equal.
     */
    private void doubleCapacity() {
        assert head == tail;
        int p = head;
        int n = elements.length;
        int r = n - p; // number of elements to the right of p  .head右边元素的个数
        int newCapacity = n << 1; //原空间的2倍
        if (newCapacity < 0)
            throw new IllegalStateException("Sorry, deque too big");
        Object[] a = new Object[newCapacity];
        System.arraycopy(elements, p, a, 0, r);//复制右半部分，对应下图中绿色部分
        System.arraycopy(elements, 0, a, r, p);//复制左半部分，对应下图中灰色部分
        elements = a;
        head = 0;
        tail = n;
    }

/**
     * Inserts the specified element at the end of this deque.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addLast(E e) {
        if (e == null) //不允许放入null
            throw new NullPointerException();
        elements[tail] = e; //赋值
        if ( (tail = (tail + 1) & (elements.length - 1)) == head) //下标越界处理
            doubleCapacity();//扩容
    }

public E pollFirst() {
    E result = elements[head];
    if (result == null)//null值意味着deque为空
        return null;
    elements[h] = null;//let GC work
    head = (head + 1) & (elements.length - 1);//下标越界处理
    return result;
}

public E pollLast() {
    int t = (tail - 1) & (elements.length - 1);//tail的上一个位置是最后一个元素
    E result = elements[t];
    if (result == null)//null值意味着deque为空
        return null;
    elements[t] = null;//let GC work
    tail = t;
    return result;
}

public E peekFirst() {
    return elements[head]; // elements[head] is null if deque empty
}

public E peekLast() {
    return elements[(tail - 1) & (elements.length - 1)];
}