JDK7集合框架源码阅读(七) ArrayDeque

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/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * *//* * * * * * * Written by Josh Bloch of Google Inc. and released to the public domain, * as explained at http://creativecommons.org/publicdomain/zero/1.0/. */package java.util;import java.io.*;/** * Resizable-array implementation of the {@link Deque} interface.  Array * deques have no capacity restrictions; they grow as necessary to support * usage.  They are not thread-safe; in the absence of external * synchronization, they do not support concurrent access by multiple threads. * Null elements are prohibited.  This class is likely to be faster than * {@link Stack} when used as a stack, and faster than {@link LinkedList} * when used as a queue. * * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time. * Exceptions include {@link #remove(Object) remove}, {@link * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence * removeLastOccurrence}, {@link #contains contains}, {@link #iterator * iterator.remove()}, and the bulk operations, all of which run in linear * time. * * <p>The iterators returned by this class's <tt>iterator</tt> method are * <i>fail-fast</i>: If the deque is modified at any time after the iterator * is created, in any way except through the iterator's own <tt>remove</tt> * method, the iterator will generally throw a {@link * ConcurrentModificationException}.  Thus, in the face of concurrent * modification, the iterator fails quickly and cleanly, rather than risking * arbitrary, non-deterministic behavior at an undetermined time in the * future. * * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed * as it is, generally speaking, impossible to make any hard guarantees in the * presence of unsynchronized concurrent modification.  Fail-fast iterators * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>the fail-fast behavior of iterators * should be used only to detect bugs.</i> * * <p>This class and its iterator implement all of the * <em>optional</em> methods of the {@link Collection} and {@link * Iterator} interfaces. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @author  Josh Bloch and Doug Lea * @since   1.6 * @param <E> the type of elements held in this collection */public class ArrayDeque<E> extends AbstractCollection<E>                           implements Deque<E>, Cloneable, Serializable{    /**     * The array in which the elements of the deque are stored.     * The capacity of the deque is the length of this array, which is     * always a power of two. The array is never allowed to become     * full, except transiently within an addX method where it is     * resized (see doubleCapacity) immediately upon becoming full,     * thus avoiding head and tail wrapping around to equal each     * other.  We also guarantee that all array cells not holding     * deque elements are always null.     */    private transient E[] elements;    /**     * 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.     */    private 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)).     */    private 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;    // ******  Array allocation and resizing utilities ******    /**     * Allocate empty array to hold the given number of elements.     *     * @param numElements  the number of elements to hold     */    private void allocateElements(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        }        elements = (E[]) new Object[initialCapacity];    }    /**     * Double 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 = (E[])a;        head = 0;        tail = n;    }    /**     * Copies the elements from our element array into the specified array,     * in order (from first to last element in the deque).  It is assumed     * that the array is large enough to hold all elements in the deque.     *     * @return its argument     */    private <T> T[] copyElements(T[] a) {        if (head < tail) {            System.arraycopy(elements, head, a, 0, size());        } else if (head > tail) {            int headPortionLen = elements.length - head;            System.arraycopy(elements, head, a, 0, headPortionLen);            System.arraycopy(elements, 0, a, headPortionLen, tail);        }        return a;    }    /**     * Constructs an empty array deque with an initial capacity     * sufficient to hold 16 elements.     */    public ArrayDeque() {        elements = (E[]) 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);    }    // The main insertion and extraction methods are addFirst,    // addLast, pollFirst, pollLast. The other methods are defined in    // terms of these.    /**     * 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)            throw new NullPointerException();        elements[head = (head - 1) & (elements.length - 1)] = e;        if (head == tail)            doubleCapacity();    }    /**     * 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)            throw new NullPointerException();        elements[tail] = e;        if ( (tail = (tail + 1) & (elements.length - 1)) == head)            doubleCapacity();    }    /**     * Inserts the specified element at the front of this deque.     *     * @param e the element to add     * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})     * @throws NullPointerException if the specified element is null     */    public boolean offerFirst(E e) {        addFirst(e);        return true;    }    /**     * Inserts the specified element at the end of this deque.     *     * @param e the element to add     * @return <tt>true</tt> (as specified by {@link Deque#offerLast})     * @throws NullPointerException if the specified element is null     */    public boolean offerLast(E e) {        addLast(e);        return true;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E removeFirst() {        E x = pollFirst();        if (x == null)            throw new NoSuchElementException();        return x;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E removeLast() {        E x = pollLast();        if (x == null)            throw new NoSuchElementException();        return x;    }    public E pollFirst() {        int h = head;        E result = 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;    }    public E pollLast() {        int t = (tail - 1) & (elements.length - 1);        E result = elements[t];        if (result == null)            return null;        elements[t] = null;        tail = t;        return result;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E getFirst() {        E x = elements[head];        if (x == null)            throw new NoSuchElementException();        return x;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E getLast() {        E x = elements[(tail - 1) & (elements.length - 1)];        if (x == null)            throw new NoSuchElementException();        return x;    }    public E peekFirst() {        return elements[head]; // elements[head] is null if deque empty    }    public E peekLast() {        return elements[(tail - 1) & (elements.length - 1)];    }    /**     * Removes the first occurrence of the specified element in this     * deque (when traversing the deque from head to tail).     * If the deque does not contain the element, it is unchanged.     * More formally, removes the first element <tt>e</tt> such that     * <tt>o.equals(e)</tt> (if such an element exists).     * Returns <tt>true</tt> if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * @param o element to be removed from this deque, if present     * @return <tt>true</tt> if the deque contained the specified element     */    public boolean removeFirstOccurrence(Object o) {        if (o == null)            return false;        int mask = elements.length - 1;        int i = head;        E x;        while ( (x = elements[i]) != null) {            if (o.equals(x)) {                delete(i);                return true;            }            i = (i + 1) & mask;        }        return false;    }    /**     * Removes the last occurrence of the specified element in this     * deque (when traversing the deque from head to tail).     * If the deque does not contain the element, it is unchanged.     * More formally, removes the last element <tt>e</tt> such that     * <tt>o.equals(e)</tt> (if such an element exists).     * Returns <tt>true</tt> if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * @param o element to be removed from this deque, if present     * @return <tt>true</tt> if the deque contained the specified element     */    public boolean removeLastOccurrence(Object o) {        if (o == null)            return false;        int mask = elements.length - 1;        int i = (tail - 1) & mask;        E x;        while ( (x = elements[i]) != null) {            if (o.equals(x)) {                delete(i);                return true;            }            i = (i - 1) & mask;        }        return false;    }    // *** Queue methods ***    /**     * Inserts the specified element at the end of this deque.     *     * <p>This method is equivalent to {@link #addLast}.     *     * @param e the element to add     * @return <tt>true</tt> (as specified by {@link Collection#add})     * @throws NullPointerException if the specified element is null     */    public boolean add(E e) {        addLast(e);        return true;    }    /**     * Inserts the specified element at the end of this deque.     *     * <p>This method is equivalent to {@link #offerLast}.     *     * @param e the element to add     * @return <tt>true</tt> (as specified by {@link Queue#offer})     * @throws NullPointerException if the specified element is null     */    public boolean offer(E e) {        return offerLast(e);    }    /**     * Retrieves and removes the head of the queue represented by this deque.     *     * This method differs from {@link #poll poll} only in that it throws an     * exception if this deque is empty.     *     * <p>This method is equivalent to {@link #removeFirst}.     *     * @return the head of the queue represented by this deque     * @throws NoSuchElementException {@inheritDoc}     */    public E remove() {        return removeFirst();    }    /**     * Retrieves and removes the head of the queue represented by this deque     * (in other words, the first element of this deque), or returns     * <tt>null</tt> if this deque is empty.     *     * <p>This method is equivalent to {@link #pollFirst}.     *     * @return the head of the queue represented by this deque, or     *         <tt>null</tt> if this deque is empty     */    public E poll() {        return pollFirst();    }    /**     * Retrieves, but does not remove, the head of the queue represented by     * this deque.  This method differs from {@link #peek peek} only in     * that it throws an exception if this deque is empty.     *     * <p>This method is equivalent to {@link #getFirst}.     *     * @return the head of the queue represented by this deque     * @throws NoSuchElementException {@inheritDoc}     */    public E element() {        return getFirst();    }    /**     * Retrieves, but does not remove, the head of the queue represented by     * this deque, or returns <tt>null</tt> if this deque is empty.     *     * <p>This method is equivalent to {@link #peekFirst}.     *     * @return the head of the queue represented by this deque, or     *         <tt>null</tt> if this deque is empty     */    public E peek() {        return peekFirst();    }    // *** Stack methods ***    /**     * Pushes an element onto the stack represented by this deque.  In other     * words, inserts the element at the front of this deque.     *     * <p>This method is equivalent to {@link #addFirst}.     *     * @param e the element to push     * @throws NullPointerException if the specified element is null     */    public void push(E e) {        addFirst(e);    }    /**     * Pops an element from the stack represented by this deque.  In other     * words, removes and returns the first element of this deque.     *     * <p>This method is equivalent to {@link #removeFirst()}.     *     * @return the element at the front of this deque (which is the top     *         of the stack represented by this deque)     * @throws NoSuchElementException {@inheritDoc}     */    public E pop() {        return removeFirst();    }    private void checkInvariants() {        assert elements[tail] == null;        assert head == tail ? elements[head] == null :            (elements[head] != null &&             elements[(tail - 1) & (elements.length - 1)] != null);        assert elements[(head - 1) & (elements.length - 1)] == null;    }    /**     * 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 E[] 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;        }    }    // *** Collection Methods ***    /**     * Returns the number of elements in this deque.     *     * @return the number of elements in this deque     */    public int size() {        return (tail - head) & (elements.length - 1);    }    /**     * Returns <tt>true</tt> if this deque contains no elements.     *     * @return <tt>true</tt> if this deque contains no elements     */    public boolean isEmpty() {        return head == tail;    }    /**     * Returns an iterator over the elements in this deque.  The elements     * will be ordered from first (head) to last (tail).  This is the same     * order that elements would be dequeued (via successive calls to     * {@link #remove} or popped (via successive calls to {@link #pop}).     *     * @return an iterator over the elements in this deque     */    public Iterator<E> iterator() {        return new DeqIterator();    }    public Iterator<E> descendingIterator() {        return new DescendingIterator();    }    private class DeqIterator implements Iterator<E> {        /**         * Index of element to be returned by subsequent call to next.         */        private int cursor = head;        /**         * Tail recorded at construction (also in remove), to stop         * iterator and also to check for comodification.         */        private int fence = tail;        /**         * Index of element returned by most recent call to next.         * Reset to -1 if element is deleted by a call to remove.         */        private int lastRet = -1;        public boolean hasNext() {            return cursor != fence;        }        public E next() {            if (cursor == fence)                throw new NoSuchElementException();            E result = elements[cursor];            // This check doesn't catch all possible comodifications,            // but does catch the ones that corrupt traversal            if (tail != fence || result == null)                throw new ConcurrentModificationException();            lastRet = cursor;            cursor = (cursor + 1) & (elements.length - 1);            return result;        }        public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            if (delete(lastRet)) { // if left-shifted, undo increment in next()                cursor = (cursor - 1) & (elements.length - 1);                fence = tail;            }            lastRet = -1;        }    }    private class DescendingIterator implements Iterator<E> {        /*         * This class is nearly a mirror-image of DeqIterator, using         * tail instead of head for initial cursor, and head instead of         * tail for fence.         */        private int cursor = tail;        private int fence = head;        private int lastRet = -1;        public boolean hasNext() {            return cursor != fence;        }        public E next() {            if (cursor == fence)                throw new NoSuchElementException();            cursor = (cursor - 1) & (elements.length - 1);            E result = elements[cursor];            if (head != fence || result == null)                throw new ConcurrentModificationException();            lastRet = cursor;            return result;        }        public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            if (!delete(lastRet)) {                cursor = (cursor + 1) & (elements.length - 1);                fence = head;            }            lastRet = -1;        }    }    /**     * Returns <tt>true</tt> if this deque contains the specified element.     * More formally, returns <tt>true</tt> if and only if this deque contains     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.     *     * @param o object to be checked for containment in this deque     * @return <tt>true</tt> if this deque contains the specified element     */    public boolean contains(Object o) {        if (o == null)            return false;        int mask = elements.length - 1;        int i = head;        E x;        while ( (x = elements[i]) != null) {            if (o.equals(x))                return true;            i = (i + 1) & mask;        }        return false;    }    /**     * Removes a single instance of the specified element from this deque.     * If the deque does not contain the element, it is unchanged.     * More formally, removes the first element <tt>e</tt> such that     * <tt>o.equals(e)</tt> (if such an element exists).     * Returns <tt>true</tt> if this deque contained the specified element     * (or equivalently, if this deque changed as a result of the call).     *     * <p>This method is equivalent to {@link #removeFirstOccurrence}.     *     * @param o element to be removed from this deque, if present     * @return <tt>true</tt> if this deque contained the specified element     */    public boolean remove(Object o) {        return removeFirstOccurrence(o);    }    /**     * 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);        }    }    /**     * 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()]);    }    /**     * Returns an array containing all of the elements in this deque in     * proper sequence (from first to last element); the runtime type of the     * returned array is that of the specified array.  If the deque fits in     * the specified array, it is returned therein.  Otherwise, a new array     * is allocated with the runtime type of the specified array and the     * size of this deque.     *     * <p>If this deque fits in the specified array with room to spare     * (i.e., the array has more elements than this deque), the element in     * the array immediately following the end of the deque is set to     * <tt>null</tt>.     *     * <p>Like the {@link #toArray()} method, this method acts as bridge between     * array-based and collection-based APIs.  Further, this method allows     * precise control over the runtime type of the output array, and may,     * under certain circumstances, be used to save allocation costs.     *     * <p>Suppose <tt>x</tt> is a deque known to contain only strings.     * The following code can be used to dump the deque into a newly     * allocated array of <tt>String</tt>:     *     * <pre>     *     String[] y = x.toArray(new String[0]);</pre>     *     * Note that <tt>toArray(new Object[0])</tt> is identical in function to     * <tt>toArray()</tt>.     *     * @param a the array into which the elements of the deque are to     *          be stored, if it is big enough; otherwise, a new array of the     *          same runtime type is allocated for this purpose     * @return an array containing all of the elements in this deque     * @throws ArrayStoreException if the runtime type of the specified array     *         is not a supertype of the runtime type of every element in     *         this deque     * @throws NullPointerException if the specified array is null     */    public <T> T[] toArray(T[] a) {        int size = size();        if (a.length < size)            a = (T[])java.lang.reflect.Array.newInstance(                    a.getClass().getComponentType(), size);        copyElements(a);        if (a.length > size)            a[size] = null;        return a;    }    // *** Object methods ***    /**     * Returns a copy of this deque.     *     * @return a copy of this deque     */    public ArrayDeque<E> clone() {        try {            ArrayDeque<E> result = (ArrayDeque<E>) super.clone();            result.elements = Arrays.copyOf(elements, elements.length);            return result;        } catch (CloneNotSupportedException e) {            throw new AssertionError();        }    }    /**     * Appease the serialization gods.     */    private static final long serialVersionUID = 2340985798034038923L;    /**     * Serialize this deque.     *     * @serialData The current size (<tt>int</tt>) of the deque,     * followed by all of its elements (each an object reference) in     * first-to-last order.     */    private void writeObject(ObjectOutputStream s) throws IOException {        s.defaultWriteObject();        // Write out size        s.writeInt(size());        // Write out elements in order.        int mask = elements.length - 1;        for (int i = head; i != tail; i = (i + 1) & mask)            s.writeObject(elements[i]);    }    /**     * Deserialize this deque.     */    private void readObject(ObjectInputStream s)            throws IOException, ClassNotFoundException {        s.defaultReadObject();        // Read in size and allocate array        int size = s.readInt();        allocateElements(size);        head = 0;        tail = size;        // Read in all elements in the proper order.        for (int i = 0; i < size; i++)            elements[i] = (E)s.readObject();    }}
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1. 接口分析

继承于AbstractCollection

Deque,Cloneable,java.io.Serializable接口

2. 实现原理

循环数组存放元素,定义head与tail指针

head:队列中第一个元素指向的位置,或者说调用pop方法,队列将要被弹出元素的位置

tail:调用addLast方法,队列下一个元素将要被插入的位置

两种情况下head==tail,1. 队列为空时,2. 队列塞满时的瞬间(马上会调用扩容函数,这样head又不等于tail了)

所以只要在插入元素后检测head==tail是否成立,即可知道队列是否已满,如果成立,需要调用扩容函数

至于判定队列是否为空,只要检测head==null是否成立即可

3. 底层数组的大小必须是2的n次幂

主要原因是为了后续计算方便,底层数组如果长度为2的n次幂,很多操作可以用位运算解决,不然得用取模,相对较慢

但是这里有一些黑魔法

ArrayDeque有一个带int参数的构造函数,可以用于设置底层数组的长度,如果传入的长度不为2的n次幂,那么会向上取整到一个最接近的2的n次幂,然后新建一个对应长度的数组,对应代码如下:

    private void allocateElements(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        }        elements = (E[]) new Object[initialCapacity];    }

这一段代码非常有趣,我试着描述一下它的工作原理

假设我们传入的numElements为1024,将它转成二进制的话,就是0100,0000,0000,最高位有一个连续的1

在第一次位运算中,最高位的一个1会向左移动一位并复制,也就是得到了0110,0000,0000,现在我们高位有两个连续的1了

在第二次位运算中,最高位的两个1会向左移动两位并复制,也就是得到了0111,1000,0000,现在我们高位有四个连续的1了

。。。

连续操作几次之后,最高位的一个1,会将后面的bit全部覆盖,也就是得到0111,1111,1111

现在只要再自加1,就能得到比numElements大的最近的2的n次幂了

4. add与poll操作

    public void addLast(E e) {        if (e == null)//队列中不能加入null元素,否则会引起poll函数的错误判断            throw new NullPointerException();        elements[tail] = e;        if ( (tail = (tail + 1) & (elements.length - 1)) == head)//tail向后移动,如果越界则归0。插入元素后如果head==tail,那么说明底层数组已满            doubleCapacity();//扩容    }    public E pollFirst() {        int h = head;        E result = elements[h]; // Element is null if deque empty        if (result == null)//如果head指向的元素为null,那么队列为空            return null;        elements[h] = null;     // Must null out slot        head = (h + 1) & (elements.length - 1);//head向后移动,如果越界则归0        return result;    }

这个代码是写得非常好的,我自认写不出这么简洁的代码

5. 扩容

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);//[head,elements.length)的半段        System.arraycopy(elements, 0, a, r, p);//[0,head)的半段        elements = (E[])a;        head = 0;//重置指针        tail = n;    }

6. 不变量检测

    private void checkInvariants() {        assert elements[tail] == null;//tail指针指向的位置必须为null,虽然在队列满的瞬间tail指向的元素不为null,但是马上会进行扩容操作,然后就又为null了        assert head == tail ? elements[head] == null ://如果head==tail,那么队列必然为空,head指针指向的元素也必须为null            (elements[head] != null &&//队列不为空,那么head指向的元素也不为null             elements[(tail - 1) & (elements.length - 1)] != null);//tail指针的前一个元素也必须不为null        assert elements[(head - 1) & (elements.length - 1)] == null;//head指针的前一个元素必须为null    }


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