queue-ArrayDeque

ArrayDeque

特点：

1.由可调整大小的数组实现，无容量限制
2.线程不安全
3.当栈用比栈块，当队列用比LinkedList块。
4.不允许存储空元素。
5.支持两头添加元素。

public class ArrayDeque<E> extends AbstractCollection<E>
                           implements Deque<E>, Cloneable, Serializable

   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;
    /**
     * 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;

注：1.发现关键字：transient，说明线程可见。如果只有单条操作，如，set，get，线程是安全的。总体上依然是线程不安全
2.最小初始化大小为2的整数倍

构造函数如下：默认elements，size为16。

 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);
    }

增

   public boolean add(E e) {
        addLast(e);
        return true;
    }

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

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

addLast将尾部的元素设置为新增的元素。
如果尾部和头部指针即将要重合，则将容量扩充两倍。
addFirst从头添加。
如果头部和尾部即将要重合，则将容量扩充两倍

删

  public E remove() {
        return removeFirst();
    }
    public E removeFirst() {
        E x = pollFirst();
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

    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;
    }

pollFirst简单粗暴的将head移动到下一个位置，并将之前的元素置null。
删除指定位置,会导致数据整体前移或后移。

  /**
     * Removes the element at the specified position in the elements array,
     * adjusting head and tail as necessary.  This can result in motion of
     * elements backwards or forwards in the array.
     *
     * <p>This method is called delete rather than remove to emphasize
     * that its semantics differ from those of {@link List#remove(int)}.
     *
     * @return true if elements moved backwards
     */
   private boolean delete(int i) {
        checkInvariants();
        final Object[] elements = this.elements;
        final int mask = elements.length - 1;
        final int h = head;
        final int t = tail;
        final int front = (i - h) & mask;
        final int back  = (t - i) & mask;

        // Invariant: head <= i < tail mod circularity
        if (front >= ((t - h) & mask))
            throw new ConcurrentModificationException();

        // Optimize for least element motion
        if (front < back) {
            if (h <= i) {
                System.arraycopy(elements, h, elements, h + 1, front);
            } else { // Wrap around
                System.arraycopy(elements, 0, elements, 1, i);
                elements[0] = elements[mask];
                System.arraycopy(elements, h, elements, h + 1, mask - h);
            }
            elements[h] = null;
            head = (h + 1) & mask;
            return false;
        } else {
            if (i < t) { // Copy the null tail as well
                System.arraycopy(elements, i + 1, elements, i, back);
                tail = t - 1;
            } else { // Wrap around
                System.arraycopy(elements, i + 1, elements, i, mask - i);
                elements[mask] = elements[0];
                System.arraycopy(elements, 1, elements, 0, t);
                tail = (t - 1) & mask;
            }
            return true;
        }
    }

清空所有数据

   /**
     * Removes all of the elements from this deque.
     * The deque will be empty after this call returns.
     */
    public void clear() {
        int h = head;
        int t = tail;
        if (h != t) { // clear all cells
            head = tail = 0;
            int i = h;
            int mask = elements.length - 1;
            do {
                elements[i] = null;
                i = (i + 1) & mask;
            } while (i != t);
        }
    }

改，无

查，遍历查询

a.用Iterator，这里是DeqIterator
参照一般的Iterator使用方法。
b.带删除查。while（!queue.isEmpty()）{queue.remove()}

  public boolean isEmpty() {
        return head == tail;
    }

其他方法

  /**
     * Returns an array containing all of the elements in this deque
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this deque.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this deque
     */
  public Object[] toArray() {
        return copyElements(new Object[size()]);
    }

其他设计思想：
如：
构造函数：elements = new Object[16];
addLast ：if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
和addFirst：
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
如果：
14&15==14
15&15==15
15&16==0

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
        int newCapacity = n << 1;
        if (newCapacity < 0)
            throw new IllegalStateException("Sorry, deque too big");
        Object[] a = new Object[newCapacity];
        System.arraycopy(elements, p, a, 0, r);
        System.arraycopy(elements, 0, a, r, p);
        elements = a;
        head = 0;
        tail = n;
    }