ArrayDeque源码详解

尊重原创，转载请标明出处   http://blog.csdn.net/abcdef314159

源码：\sources\android-25

ArrayDeque和ArrayList有区别也有相似的地方，他们都是数组存储数组，不同的是ArrayList存储是从数组位置的0开始的，而ArrayDeque有一个head和tail，head和tail可以指向数组的任何位置，head是指向数组的第一个元素，是有值的，tail是指向数组最后一个元素的下一个，是空的，如果tail大于head，那么数组大小就是tail-head，如果tail是小于head，那么数组大小为（head到数组长度+位置0到tail），代码不多，直接上代码

/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation.  Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. *//* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * 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.Serializable;import java.util.function.Consumer;// BEGIN android-note// removed link to collections framework docs// END android-note/** * 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 {@code ArrayDeque} 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 {@link #iterator() iterator} * method are <em>fail-fast</em>: If the deque is modified at any time after * the iterator is created, in any way except through the iterator's own * {@code remove} 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 {@code ConcurrentModificationException} 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. * * @author  Josh Bloch and Doug Lea * @since   1.6 * @param <E> the type of elements held in this deque */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.     */ //存储的元素，并且elements的长度必须是2的n次方，这个和之前看到HashMap很相似， //之前看的HashMap的数组长度也是2的n次方，因为要用到与运算，详细可以看 //http://blog.csdn.net/abcdef314159/article/details/51165630    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.     */ //指向队列头，这个头并不是数组的第0个元素，如果这样head就没有意义了，这个从 //下面的addFirst(E e)方法也可以看出，如果当head等于0的时候，在添加到first，那么 //会添加到数组的最后，并且head也指向数组的最后，这个下面在分析    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)).     */ //指向队列尾的下一个空间，可以这样理解，head指向的是第一个元素，tail指向的是最后 //一个元素的下一个，指的是空的。    transient int tail;    /**     * The minimum capacity that we'll use for a newly created deque.     * Must be a power of 2.     */ //最小存储空间，必须是二的n次方，    private static final int MIN_INITIAL_CAPACITY = 8;    // ******  Array allocation and resizing utilities ******    /**     * Allocates empty array to hold the given number of elements.     *     * @param numElements  the number of elements to hold     */ //分配空间，这个空间大小是比numElements大的最小的2的n次方，比如numElements //是17，则初始化大小为32，因为2的4次是16小于17,2的5次是32，才大于17，如果传入 //的正好是2的n次方，那么初始化的空间大小的2的n+1次方，    private void allocateElements(int numElements) {//根据numElements初始化空间        int initialCapacity = MIN_INITIAL_CAPACITY;        // Find the best power of two to hold elements.        // Tests "<=" because arrays aren't kept full.//如果numElements小于8，直接初始化数组elements，否则，根据numElements//的大小来计算数组的大小        if (numElements >= initialCapacity) {            initialCapacity = numElements;//下面代码很简单，其实就是移位把最左边位的1填充到后面，计算的时候你完全可以把//numElements第一个1后面的所有1都忽略，我举个例子，比如17，二进制是（总共32位，//前面全是0）10001，就可以转化为（总共32位，前面全是0）11111，就相当于把最左边//的1全部填充到后面，            initialCapacity |= (initialCapacity >>>  1);            initialCapacity |= (initialCapacity >>>  2);            initialCapacity |= (initialCapacity >>>  4);            initialCapacity |= (initialCapacity >>>  8);            initialCapacity |= (initialCapacity >>> 16);            initialCapacity++;//加1，达到2的n次方// 越界了，太大了，            if (initialCapacity < 0)    // Too many elements, must back off                initialCapacity >>>= 1; // Good luck allocating 2^30 elements        }        elements = new Object[initialCapacity];    }    /**     * Doubles the capacity of this deque.  Call only when full, i.e.,     * when head and tail have wrapped around to become equal.     */ //空间扩大一倍，仅仅当满的时候，这时候head和tail都会指向同一个元素    private void doubleCapacity() {        assert head == tail;//满了        int p = head;        int n = elements.length;//数组的长度//关键是r不好理解，举个例子，在数组中，head不一定是之前0位置的，他可以指向其他位置，比如//原来空间大小为16，head为13，也就是第14个元素（一数组是从0开始的），那么r就是16-13=3，也//就是从head往后还有多少元素，待会copy的时候也是先从最后的r个元素开始        int r = n - p; // number of elements to the right of p        int newCapacity = n << 1;//扩大一倍        if (newCapacity < 0)            throw new IllegalStateException("Sorry, deque too big");        Object[] a = new Object[newCapacity];        System.arraycopy(elements, p, a, 0, r);//先copy后面的r个        System.arraycopy(elements, 0, a, r, p);//在copy前面的p个        elements = a;//重新调整head和tail的值        head = 0;        tail = n;    }    /**     * Constructs an empty array deque with an initial capacity     * sufficient to hold 16 elements.     */    public ArrayDeque() {        elements = new Object[16];//初始化大小，默认的为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);// 分配空间    }    /**     * 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());// 分配空间，然后添加        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();//添加到head的前面，所以head-1，后面的与运算，不会出现数组越界，因为//elements.length是2的n次方，这个可以参照之前的HashMap的分析        elements[head = (head - 1) & (elements.length - 1)] = e;        if (head == tail)//判断是否满了            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();//添加到最后一个，这个方法和addFirst有很明显的不同，addFirst是添加的时候就要//计算head的位置，因为原head位置是有值的，而addLast方法是存值之后在计算tail的，//因为tail位置是没有存值的，他表示的是最右一个元素的下一个，是空，所以存值之后//要计算tail的值        elements[tail] = e;        if ( (tail = (tail + 1) & (elements.length - 1)) == head)//判断是否满            doubleCapacity();    }    /**     * Inserts the specified element at the front of this deque.     *     * @param e the element to add     * @return {@code true} (as specified by {@link Deque#offerFirst})     * @throws NullPointerException if the specified element is null     */    public boolean offerFirst(E e) {//插入到第一个        addFirst(e);        return true;    }    /**     * Inserts the specified element at the end of this deque.     *     * @param e the element to add     * @return {@code true} (as specified by {@link Deque#offerLast})     * @throws NullPointerException if the specified element is null     */    public boolean offerLast(E e) {//插入到最后一个        addLast(e);        return true;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E removeFirst() {//删除第一个        E x = pollFirst();        if (x == null)            throw new NoSuchElementException();        return x;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E removeLast() {//删除最后一个        E x = pollLast();        if (x == null)            throw new NoSuchElementException();        return x;    }    public E pollFirst() {        final Object[] elements = this.elements;        final int h = head;        @SuppressWarnings("unchecked")        E result = (E) elements[h];        // Element is null if deque empty        if (result != null) {            elements[h] = null; // Must null out slot// 删除第一个，这里的所有第一我们都认为是head所指的，不是数组的0位置，然后在//计算head的值            head = (h + 1) & (elements.length - 1);        }        return result;    }    public E pollLast() {//删除最后一个        final Object[] elements = this.elements;        final int t = (tail - 1) & (elements.length - 1);        @SuppressWarnings("unchecked")        E result = (E) elements[t];        if (result != null) {            elements[t] = null;            tail = t;        }        return result;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E getFirst() {//返回第一个        @SuppressWarnings("unchecked")        E result = (E) elements[head];        if (result == null)            throw new NoSuchElementException();        return result;    }    /**     * @throws NoSuchElementException {@inheritDoc}     */    public E getLast() {// 返回最后一个        @SuppressWarnings("unchecked")        E result = (E) elements[(tail - 1) & (elements.length - 1)];        if (result == null)            throw new NoSuchElementException();        return result;    }    @SuppressWarnings("unchecked")    public E peekFirst() {        // elements[head] is null if deque empty        return (E) elements[head];    }    @SuppressWarnings("unchecked")    public E peekLast() {        return (E) 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 {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} 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 {@code true} if the deque contained the specified element     */    public boolean removeFirstOccurrence(Object o) {//删除第一个出现的o        if (o != null) {            int mask = elements.length - 1;            int i = head;            for (Object x; (x = elements[i]) != null; i = (i + 1) & mask) {                if (o.equals(x)) {                    delete(i);                    return true;                }            }        }        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 {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} 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 {@code true} if the deque contained the specified element     */    public boolean removeLastOccurrence(Object o) {//删除最后一个出现的o        if (o != null) {            int mask = elements.length - 1;            int i = (tail - 1) & mask;            for (Object x; (x = elements[i]) != null; i = (i - 1) & mask) {                if (o.equals(x)) {                    delete(i);                    return true;                }            }        }        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 {@code true} (as specified by {@link Collection#add})     * @throws NullPointerException if the specified element is null     */    public boolean add(E e) {//添加        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 {@code true} (as specified by {@link Queue#offer})     * @throws NullPointerException if the specified element is null     */    public boolean offer(E e) {//插入到最后        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     * {@code null} if this deque is empty.     *     * <p>This method is equivalent to {@link #pollFirst}.     *     * @return the head of the queue represented by this deque, or     *         {@code null} 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 {@code null} if this deque is empty.     *     * <p>This method is equivalent to {@link #peekFirst}.     *     * @return the head of the queue represented by this deque, or     *         {@code null} 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     */ //删除数组中下标为i的元素，之前所有的操作都是操作head和tail的，但这个不一样 //他会引起head或tail的改变，    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;//i距离head的位置        final int back  = (t - i) & mask;//i距离tail的位置        // Invariant: head <= i < tail mod circularity        if (front >= ((t - h) & mask))//保证i的位置元素是存在的            throw new ConcurrentModificationException();        // Optimize for least element motion        if (front < back) {//前面的少，从前往后挪，最后head要加1            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 {//后面少，从后往前挪，后面的tail要减1            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() {//元素的size        return (tail - head) & (elements.length - 1);    }    /**     * Returns {@code true} if this deque contains no elements.     *     * @return {@code true} if this deque contains no elements     */ //是否为空，在上面添加元素的时候也可能head==tail，当添加元素之后head==tail的时候 //就认为是满了，然后扩容，重新调整head和tail的值    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() {//迭代器        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();            @SuppressWarnings("unchecked")            E result = (E) 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;        }        @Override        public void forEachRemaining(Consumer<? super E> action) {            Objects.requireNonNull(action);            Object[] a = elements;            int m = a.length - 1, f = fence, i = cursor;            cursor = f;            while (i != f) {                @SuppressWarnings("unchecked") E e = (E)a[i];                i = (i + 1) & m;                // Android-note: This uses a different heuristic for detecting                // concurrent modification exceptions than next(). As such, this is a less                // precise test.                if (e == null)                    throw new ConcurrentModificationException();                action.accept(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 class DescendingIterator implements Iterator<E> {        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);            @SuppressWarnings("unchecked")            E result = (E) 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 {@code true} if this deque contains the specified element.     * More formally, returns {@code true} if and only if this deque contains     * at least one element {@code e} such that {@code o.equals(e)}.     *     * @param o object to be checked for containment in this deque     * @return {@code true} if this deque contains the specified element     */    public boolean contains(Object o) {        if (o != null) {            int mask = elements.length - 1;            int i = head;            for (Object x; (x = elements[i]) != null; i = (i + 1) & mask) {                if (o.equals(x))                    return true;            }        }        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 {@code e} such that     * {@code o.equals(e)} (if such an element exists).     * Returns {@code true} 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(Object)}.     *     * @param o element to be removed from this deque, if present     * @return {@code true} 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() {// 清空        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() {//转化为数组        final int head = this.head;        final int tail = this.tail;        boolean wrap = (tail < head);//head有可能小于tail，尤其在addFirst方法中        int end = wrap ? tail + elements.length : tail;//如果head小于tail，直接copy从head到tail即可，如果head大于tail，再从head到tail//肯定不合适，tail需要加上数组的大小，但这里始终有一点困惑，tail + elements.length// 可能会大于数组的长度，有可能出现数组越界异常，但是看了Arrays.copyOfRange的源码之后//发现我多虑了/**    public static <T,U> T[] copyOfRange(U[] original, int from, int to, Class<? extends T[]> newType) {        int newLength = to - from;        if (newLength < 0)            throw new IllegalArgumentException(from + " > " + to);        T[] copy = ((Object)newType == (Object)Object[].class)            ? (T[]) new Object[newLength]            : (T[]) Array.newInstance(newType.getComponentType(), newLength);        System.arraycopy(original, from, copy, 0,                         Math.min(original.length - from, newLength));//这里是关键，所以不会出现越界异常        return copy;}*/        Object[] a = Arrays.copyOfRange(elements, head, end);//如果wrap为真，那么上面copy的只是从head到数组的最后，那么数组前面到tail的那部分还没copy，所以这里copy//前面的剩余部分        if (wrap)            System.arraycopy(elements, 0, a, elements.length - head, tail);        return a;    }    /**     * 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     * {@code null}.     *     * <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 {@code x} is a deque known to contain only strings.     * The following code can be used to dump the deque into a newly     * allocated array of {@code String}:     *     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>     *     * Note that {@code toArray(new Object[0])} is identical in function to     * {@code toArray()}.     *     * @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     */    @SuppressWarnings("unchecked")    public <T> T[] toArray(T[] a) {//把元素提取到数组中然后在返回        final int head = this.head;        final int tail = this.tail;        boolean wrap = (tail < head);//又是head在后面的情况        int size = (tail - head) + (wrap ? elements.length : 0);//大小//copy数组的偏移量，因为有可能先提取后部分然后在提取前部分        int firstLeg = size - (wrap ? tail : 0);        int len = a.length;        if (size > len) {            a = (T[]) Arrays.copyOfRange(elements, head, head + size,                                         a.getClass());        } else {            System.arraycopy(elements, head, a, 0, firstLeg);            if (size < len)                a[size] = null;        }        if (wrap)            System.arraycopy(elements, 0, a, firstLeg, tail);        return a;    }    // *** Object methods ***    /**     * Returns a copy of this deque.     *     * @return a copy of this deque     */    public ArrayDeque<E> clone() {//克隆        try {            @SuppressWarnings("unchecked")            ArrayDeque<E> result = (ArrayDeque<E>) super.clone();            result.elements = Arrays.copyOf(elements, elements.length);            return result;        } catch (CloneNotSupportedException e) {            throw new AssertionError();        }    }    private static final long serialVersionUID = 2340985798034038923L;    /**     * Saves this deque to a stream (that is, serializes it).     *     * @param s the stream     * @throws java.io.IOException if an I/O error occurs     * @serialData The current size ({@code int}) of the deque,     * followed by all of its elements (each an object reference) in     * first-to-last order.     */    private void writeObject(java.io.ObjectOutputStream s)            throws java.io.IOException {//序列化写        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]);    }    /**     * Reconstitutes this deque from a stream (that is, deserializes it).     * @param s the stream     * @throws ClassNotFoundException if the class of a serialized object     *         could not be found     * @throws java.io.IOException if an I/O error occurs     */    private void readObject(java.io.ObjectInputStream s)            throws java.io.IOException, ClassNotFoundException {//序列化读        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] = s.readObject();    }    /**     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>     * and <em>fail-fast</em> {@link Spliterator} over the elements in this     * deque.     *     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},     * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and     * {@link Spliterator#NONNULL}.  Overriding implementations should document     * the reporting of additional characteristic values.     *     * @return a {@code Spliterator} over the elements in this deque     * @since 1.8     */    public Spliterator<E> spliterator() {        return new DeqSpliterator<E>(this, -1, -1);    }    static final class DeqSpliterator<E> implements Spliterator<E> {        private final ArrayDeque<E> deq;        private int fence;  // -1 until first use        private int index;  // current index, modified on traverse/split        /** Creates new spliterator covering the given array and range. */        DeqSpliterator(ArrayDeque<E> deq, int origin, int fence) {            this.deq = deq;            this.index = origin;            this.fence = fence;        }        private int getFence() { // force initialization            int t;            if ((t = fence) < 0) {                t = fence = deq.tail;                index = deq.head;            }            return t;        }        public DeqSpliterator<E> trySplit() {            int t = getFence(), h = index, n = deq.elements.length;            if (h != t && ((h + 1) & (n - 1)) != t) {                if (h > t)                    t += n;                int m = ((h + t) >>> 1) & (n - 1);                return new DeqSpliterator<E>(deq, h, index = m);            }            return null;        }        public void forEachRemaining(Consumer<? super E> consumer) {            if (consumer == null)                throw new NullPointerException();            Object[] a = deq.elements;            int m = a.length - 1, f = getFence(), i = index;            index = f;            while (i != f) {                @SuppressWarnings("unchecked") E e = (E)a[i];                i = (i + 1) & m;                if (e == null)                    throw new ConcurrentModificationException();                consumer.accept(e);            }        }        public boolean tryAdvance(Consumer<? super E> consumer) {            if (consumer == null)                throw new NullPointerException();            Object[] a = deq.elements;            int m = a.length - 1, f = getFence(), i = index;            if (i != f) {                @SuppressWarnings("unchecked") E e = (E)a[i];                index = (i + 1) & m;                if (e == null)                    throw new ConcurrentModificationException();                consumer.accept(e);                return true;            }            return false;        }        public long estimateSize() {            int n = getFence() - index;            if (n < 0)                n += deq.elements.length;            return (long) n;        }        @Override        public int characteristics() {            return Spliterator.ORDERED | Spliterator.SIZED |                Spliterator.NONNULL | Spliterator.SUBSIZED;        }    }}