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    3    *
    4    * This code is free software; you can redistribute it and/or modify it
    5    * under the terms of the GNU General Public License version 2 only, as
    6    * published by the Free Software Foundation.  Oracle designates this
    7    * particular file as subject to the "Classpath" exception as provided
    8    * by Oracle in the LICENSE file that accompanied this code.
    9    *
   10    * This code is distributed in the hope that it will be useful, but WITHOUT
   11    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   12    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   13    * version 2 for more details (a copy is included in the LICENSE file that
   14    * accompanied this code).
   15    *
   16    * You should have received a copy of the GNU General Public License version
   17    * 2 along with this work; if not, write to the Free Software Foundation,
   18    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   19    *
   20    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   21    * or visit www.oracle.com if you need additional information or have any
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   24   
   25   /*
   26    * This file is available under and governed by the GNU General Public
   27    * License version 2 only, as published by the Free Software Foundation.
   28    * However, the following notice accompanied the original version of this
   29    * file:
   30    *
   31    * Written by Josh Bloch of Google Inc. and released to the public domain,
   32    * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
   33    */
   34   
   35   package java.util;
   36   import java.io;
   37   
   38   /**
   39    * Resizable-array implementation of the {@link Deque} interface.  Array
   40    * deques have no capacity restrictions; they grow as necessary to support
   41    * usage.  They are not thread-safe; in the absence of external
   42    * synchronization, they do not support concurrent access by multiple threads.
   43    * Null elements are prohibited.  This class is likely to be faster than
   44    * {@link Stack} when used as a stack, and faster than {@link LinkedList}
   45    * when used as a queue.
   46    *
   47    * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time.
   48    * Exceptions include {@link #remove(Object) remove}, {@link
   49    * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence
   50    * removeLastOccurrence}, {@link #contains contains}, {@link #iterator
   51    * iterator.remove()}, and the bulk operations, all of which run in linear
   52    * time.
   53    *
   54    * <p>The iterators returned by this class's <tt>iterator</tt> method are
   55    * <i>fail-fast</i>: If the deque is modified at any time after the iterator
   56    * is created, in any way except through the iterator's own <tt>remove</tt>
   57    * method, the iterator will generally throw a {@link
   58    * ConcurrentModificationException}.  Thus, in the face of concurrent
   59    * modification, the iterator fails quickly and cleanly, rather than risking
   60    * arbitrary, non-deterministic behavior at an undetermined time in the
   61    * future.
   62    *
   63    * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
   64    * as it is, generally speaking, impossible to make any hard guarantees in the
   65    * presence of unsynchronized concurrent modification.  Fail-fast iterators
   66    * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
   67    * Therefore, it would be wrong to write a program that depended on this
   68    * exception for its correctness: <i>the fail-fast behavior of iterators
   69    * should be used only to detect bugs.</i>
   70    *
   71    * <p>This class and its iterator implement all of the
   72    * <em>optional</em> methods of the {@link Collection} and {@link
   73    * Iterator} interfaces.
   74    *
   75    * <p>This class is a member of the
   76    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   77    * Java Collections Framework</a>.
   78    *
   79    * @author  Josh Bloch and Doug Lea
   80    * @since   1.6
   81    * @param <E> the type of elements held in this collection
   82    */
   83   public class ArrayDeque<E> extends AbstractCollection<E>
   84                              implements Deque<E>, Cloneable, Serializable
   85   {
   86       /**
   87        * The array in which the elements of the deque are stored.
   88        * The capacity of the deque is the length of this array, which is
   89        * always a power of two. The array is never allowed to become
   90        * full, except transiently within an addX method where it is
   91        * resized (see doubleCapacity) immediately upon becoming full,
   92        * thus avoiding head and tail wrapping around to equal each
   93        * other.  We also guarantee that all array cells not holding
   94        * deque elements are always null.
   95        */
   96       private transient E[] elements;
   97   
   98       /**
   99        * The index of the element at the head of the deque (which is the
  100        * element that would be removed by remove() or pop()); or an
  101        * arbitrary number equal to tail if the deque is empty.
  102        */
  103       private transient int head;
  104   
  105       /**
  106        * The index at which the next element would be added to the tail
  107        * of the deque (via addLast(E), add(E), or push(E)).
  108        */
  109       private transient int tail;
  110   
  111       /**
  112        * The minimum capacity that we'll use for a newly created deque.
  113        * Must be a power of 2.
  114        */
  115       private static final int MIN_INITIAL_CAPACITY = 8;
  116   
  117       // ******  Array allocation and resizing utilities ******
  118   
  119       /**
  120        * Allocate empty array to hold the given number of elements.
  121        *
  122        * @param numElements  the number of elements to hold
  123        */
  124       private void allocateElements(int numElements) {
  125           int initialCapacity = MIN_INITIAL_CAPACITY;
  126           // Find the best power of two to hold elements.
  127           // Tests "<=" because arrays aren't kept full.
  128           if (numElements >= initialCapacity) {
  129               initialCapacity = numElements;
  130               initialCapacity |= (initialCapacity >>>  1);
  131               initialCapacity |= (initialCapacity >>>  2);
  132               initialCapacity |= (initialCapacity >>>  4);
  133               initialCapacity |= (initialCapacity >>>  8);
  134               initialCapacity |= (initialCapacity >>> 16);
  135               initialCapacity++;
  136   
  137               if (initialCapacity < 0)   // Too many elements, must back off
  138                   initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
  139           }
  140           elements = (E[]) new Object[initialCapacity];
  141       }
  142   
  143       /**
  144        * Double the capacity of this deque.  Call only when full, i.e.,
  145        * when head and tail have wrapped around to become equal.
  146        */
  147       private void doubleCapacity() {
  148           assert head == tail;
  149           int p = head;
  150           int n = elements.length;
  151           int r = n - p; // number of elements to the right of p
  152           int newCapacity = n << 1;
  153           if (newCapacity < 0)
  154               throw new IllegalStateException("Sorry, deque too big");
  155           Object[] a = new Object[newCapacity];
  156           System.arraycopy(elements, p, a, 0, r);
  157           System.arraycopy(elements, 0, a, r, p);
  158           elements = (E[])a;
  159           head = 0;
  160           tail = n;
  161       }
  162   
  163       /**
  164        * Copies the elements from our element array into the specified array,
  165        * in order (from first to last element in the deque).  It is assumed
  166        * that the array is large enough to hold all elements in the deque.
  167        *
  168        * @return its argument
  169        */
  170       private <T> T[] copyElements(T[] a) {
  171           if (head < tail) {
  172               System.arraycopy(elements, head, a, 0, size());
  173           } else if (head > tail) {
  174               int headPortionLen = elements.length - head;
  175               System.arraycopy(elements, head, a, 0, headPortionLen);
  176               System.arraycopy(elements, 0, a, headPortionLen, tail);
  177           }
  178           return a;
  179       }
  180   
  181       /**
  182        * Constructs an empty array deque with an initial capacity
  183        * sufficient to hold 16 elements.
  184        */
  185       public ArrayDeque() {
  186           elements = (E[]) new Object[16];
  187       }
  188   
  189       /**
  190        * Constructs an empty array deque with an initial capacity
  191        * sufficient to hold the specified number of elements.
  192        *
  193        * @param numElements  lower bound on initial capacity of the deque
  194        */
  195       public ArrayDeque(int numElements) {
  196           allocateElements(numElements);
  197       }
  198   
  199       /**
  200        * Constructs a deque containing the elements of the specified
  201        * collection, in the order they are returned by the collection's
  202        * iterator.  (The first element returned by the collection's
  203        * iterator becomes the first element, or <i>front</i> of the
  204        * deque.)
  205        *
  206        * @param c the collection whose elements are to be placed into the deque
  207        * @throws NullPointerException if the specified collection is null
  208        */
  209       public ArrayDeque(Collection<? extends E> c) {
  210           allocateElements(c.size());
  211           addAll(c);
  212       }
  213   
  214       // The main insertion and extraction methods are addFirst,
  215       // addLast, pollFirst, pollLast. The other methods are defined in
  216       // terms of these.
  217   
  218       /**
  219        * Inserts the specified element at the front of this deque.
  220        *
  221        * @param e the element to add
  222        * @throws NullPointerException if the specified element is null
  223        */
  224       public void addFirst(E e) {
  225           if (e == null)
  226               throw new NullPointerException();
  227           elements[head = (head - 1) & (elements.length - 1)] = e;
  228           if (head == tail)
  229               doubleCapacity();
  230       }
  231   
  232       /**
  233        * Inserts the specified element at the end of this deque.
  234        *
  235        * <p>This method is equivalent to {@link #add}.
  236        *
  237        * @param e the element to add
  238        * @throws NullPointerException if the specified element is null
  239        */
  240       public void addLast(E e) {
  241           if (e == null)
  242               throw new NullPointerException();
  243           elements[tail] = e;
  244           if ( (tail = (tail + 1) & (elements.length - 1)) == head)
  245               doubleCapacity();
  246       }
  247   
  248       /**
  249        * Inserts the specified element at the front of this deque.
  250        *
  251        * @param e the element to add
  252        * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})
  253        * @throws NullPointerException if the specified element is null
  254        */
  255       public boolean offerFirst(E e) {
  256           addFirst(e);
  257           return true;
  258       }
  259   
  260       /**
  261        * Inserts the specified element at the end of this deque.
  262        *
  263        * @param e the element to add
  264        * @return <tt>true</tt> (as specified by {@link Deque#offerLast})
  265        * @throws NullPointerException if the specified element is null
  266        */
  267       public boolean offerLast(E e) {
  268           addLast(e);
  269           return true;
  270       }
  271   
  272       /**
  273        * @throws NoSuchElementException {@inheritDoc}
  274        */
  275       public E removeFirst() {
  276           E x = pollFirst();
  277           if (x == null)
  278               throw new NoSuchElementException();
  279           return x;
  280       }
  281   
  282       /**
  283        * @throws NoSuchElementException {@inheritDoc}
  284        */
  285       public E removeLast() {
  286           E x = pollLast();
  287           if (x == null)
  288               throw new NoSuchElementException();
  289           return x;
  290       }
  291   
  292       public E pollFirst() {
  293           int h = head;
  294           E result = elements[h]; // Element is null if deque empty
  295           if (result == null)
  296               return null;
  297           elements[h] = null;     // Must null out slot
  298           head = (h + 1) & (elements.length - 1);
  299           return result;
  300       }
  301   
  302       public E pollLast() {
  303           int t = (tail - 1) & (elements.length - 1);
  304           E result = elements[t];
  305           if (result == null)
  306               return null;
  307           elements[t] = null;
  308           tail = t;
  309           return result;
  310       }
  311   
  312       /**
  313        * @throws NoSuchElementException {@inheritDoc}
  314        */
  315       public E getFirst() {
  316           E x = elements[head];
  317           if (x == null)
  318               throw new NoSuchElementException();
  319           return x;
  320       }
  321   
  322       /**
  323        * @throws NoSuchElementException {@inheritDoc}
  324        */
  325       public E getLast() {
  326           E x = elements[(tail - 1) & (elements.length - 1)];
  327           if (x == null)
  328               throw new NoSuchElementException();
  329           return x;
  330       }
  331   
  332       public E peekFirst() {
  333           return elements[head]; // elements[head] is null if deque empty
  334       }
  335   
  336       public E peekLast() {
  337           return elements[(tail - 1) & (elements.length - 1)];
  338       }
  339   
  340       /**
  341        * Removes the first occurrence of the specified element in this
  342        * deque (when traversing the deque from head to tail).
  343        * If the deque does not contain the element, it is unchanged.
  344        * More formally, removes the first element <tt>e</tt> such that
  345        * <tt>o.equals(e)</tt> (if such an element exists).
  346        * Returns <tt>true</tt> if this deque contained the specified element
  347        * (or equivalently, if this deque changed as a result of the call).
  348        *
  349        * @param o element to be removed from this deque, if present
  350        * @return <tt>true</tt> if the deque contained the specified element
  351        */
  352       public boolean removeFirstOccurrence(Object o) {
  353           if (o == null)
  354               return false;
  355           int mask = elements.length - 1;
  356           int i = head;
  357           E x;
  358           while ( (x = elements[i]) != null) {
  359               if (o.equals(x)) {
  360                   delete(i);
  361                   return true;
  362               }
  363               i = (i + 1) & mask;
  364           }
  365           return false;
  366       }
  367   
  368       /**
  369        * Removes the last occurrence of the specified element in this
  370        * deque (when traversing the deque from head to tail).
  371        * If the deque does not contain the element, it is unchanged.
  372        * More formally, removes the last element <tt>e</tt> such that
  373        * <tt>o.equals(e)</tt> (if such an element exists).
  374        * Returns <tt>true</tt> if this deque contained the specified element
  375        * (or equivalently, if this deque changed as a result of the call).
  376        *
  377        * @param o element to be removed from this deque, if present
  378        * @return <tt>true</tt> if the deque contained the specified element
  379        */
  380       public boolean removeLastOccurrence(Object o) {
  381           if (o == null)
  382               return false;
  383           int mask = elements.length - 1;
  384           int i = (tail - 1) & mask;
  385           E x;
  386           while ( (x = elements[i]) != null) {
  387               if (o.equals(x)) {
  388                   delete(i);
  389                   return true;
  390               }
  391               i = (i - 1) & mask;
  392           }
  393           return false;
  394       }
  395   
  396       // *** Queue methods ***
  397   
  398       /**
  399        * Inserts the specified element at the end of this deque.
  400        *
  401        * <p>This method is equivalent to {@link #addLast}.
  402        *
  403        * @param e the element to add
  404        * @return <tt>true</tt> (as specified by {@link Collection#add})
  405        * @throws NullPointerException if the specified element is null
  406        */
  407       public boolean add(E e) {
  408           addLast(e);
  409           return true;
  410       }
  411   
  412       /**
  413        * Inserts the specified element at the end of this deque.
  414        *
  415        * <p>This method is equivalent to {@link #offerLast}.
  416        *
  417        * @param e the element to add
  418        * @return <tt>true</tt> (as specified by {@link Queue#offer})
  419        * @throws NullPointerException if the specified element is null
  420        */
  421       public boolean offer(E e) {
  422           return offerLast(e);
  423       }
  424   
  425       /**
  426        * Retrieves and removes the head of the queue represented by this deque.
  427        *
  428        * This method differs from {@link #poll poll} only in that it throws an
  429        * exception if this deque is empty.
  430        *
  431        * <p>This method is equivalent to {@link #removeFirst}.
  432        *
  433        * @return the head of the queue represented by this deque
  434        * @throws NoSuchElementException {@inheritDoc}
  435        */
  436       public E remove() {
  437           return removeFirst();
  438       }
  439   
  440       /**
  441        * Retrieves and removes the head of the queue represented by this deque
  442        * (in other words, the first element of this deque), or returns
  443        * <tt>null</tt> if this deque is empty.
  444        *
  445        * <p>This method is equivalent to {@link #pollFirst}.
  446        *
  447        * @return the head of the queue represented by this deque, or
  448        *         <tt>null</tt> if this deque is empty
  449        */
  450       public E poll() {
  451           return pollFirst();
  452       }
  453   
  454       /**
  455        * Retrieves, but does not remove, the head of the queue represented by
  456        * this deque.  This method differs from {@link #peek peek} only in
  457        * that it throws an exception if this deque is empty.
  458        *
  459        * <p>This method is equivalent to {@link #getFirst}.
  460        *
  461        * @return the head of the queue represented by this deque
  462        * @throws NoSuchElementException {@inheritDoc}
  463        */
  464       public E element() {
  465           return getFirst();
  466       }
  467   
  468       /**
  469        * Retrieves, but does not remove, the head of the queue represented by
  470        * this deque, or returns <tt>null</tt> if this deque is empty.
  471        *
  472        * <p>This method is equivalent to {@link #peekFirst}.
  473        *
  474        * @return the head of the queue represented by this deque, or
  475        *         <tt>null</tt> if this deque is empty
  476        */
  477       public E peek() {
  478           return peekFirst();
  479       }
  480   
  481       // *** Stack methods ***
  482   
  483       /**
  484        * Pushes an element onto the stack represented by this deque.  In other
  485        * words, inserts the element at the front of this deque.
  486        *
  487        * <p>This method is equivalent to {@link #addFirst}.
  488        *
  489        * @param e the element to push
  490        * @throws NullPointerException if the specified element is null
  491        */
  492       public void push(E e) {
  493           addFirst(e);
  494       }
  495   
  496       /**
  497        * Pops an element from the stack represented by this deque.  In other
  498        * words, removes and returns the first element of this deque.
  499        *
  500        * <p>This method is equivalent to {@link #removeFirst()}.
  501        *
  502        * @return the element at the front of this deque (which is the top
  503        *         of the stack represented by this deque)
  504        * @throws NoSuchElementException {@inheritDoc}
  505        */
  506       public E pop() {
  507           return removeFirst();
  508       }
  509   
  510       private void checkInvariants() {
  511           assert elements[tail] == null;
  512           assert head == tail ? elements[head] == null :
  513               (elements[head] != null &&
  514                elements[(tail - 1) & (elements.length - 1)] != null);
  515           assert elements[(head - 1) & (elements.length - 1)] == null;
  516       }
  517   
  518       /**
  519        * Removes the element at the specified position in the elements array,
  520        * adjusting head and tail as necessary.  This can result in motion of
  521        * elements backwards or forwards in the array.
  522        *
  523        * <p>This method is called delete rather than remove to emphasize
  524        * that its semantics differ from those of {@link List#remove(int)}.
  525        *
  526        * @return true if elements moved backwards
  527        */
  528       private boolean delete(int i) {
  529           checkInvariants();
  530           final E[] elements = this.elements;
  531           final int mask = elements.length - 1;
  532           final int h = head;
  533           final int t = tail;
  534           final int front = (i - h) & mask;
  535           final int back  = (t - i) & mask;
  536   
  537           // Invariant: head <= i < tail mod circularity
  538           if (front >= ((t - h) & mask))
  539               throw new ConcurrentModificationException();
  540   
  541           // Optimize for least element motion
  542           if (front < back) {
  543               if (h <= i) {
  544                   System.arraycopy(elements, h, elements, h + 1, front);
  545               } else { // Wrap around
  546                   System.arraycopy(elements, 0, elements, 1, i);
  547                   elements[0] = elements[mask];
  548                   System.arraycopy(elements, h, elements, h + 1, mask - h);
  549               }
  550               elements[h] = null;
  551               head = (h + 1) & mask;
  552               return false;
  553           } else {
  554               if (i < t) { // Copy the null tail as well
  555                   System.arraycopy(elements, i + 1, elements, i, back);
  556                   tail = t - 1;
  557               } else { // Wrap around
  558                   System.arraycopy(elements, i + 1, elements, i, mask - i);
  559                   elements[mask] = elements[0];
  560                   System.arraycopy(elements, 1, elements, 0, t);
  561                   tail = (t - 1) & mask;
  562               }
  563               return true;
  564           }
  565       }
  566   
  567       // *** Collection Methods ***
  568   
  569       /**
  570        * Returns the number of elements in this deque.
  571        *
  572        * @return the number of elements in this deque
  573        */
  574       public int size() {
  575           return (tail - head) & (elements.length - 1);
  576       }
  577   
  578       /**
  579        * Returns <tt>true</tt> if this deque contains no elements.
  580        *
  581        * @return <tt>true</tt> if this deque contains no elements
  582        */
  583       public boolean isEmpty() {
  584           return head == tail;
  585       }
  586   
  587       /**
  588        * Returns an iterator over the elements in this deque.  The elements
  589        * will be ordered from first (head) to last (tail).  This is the same
  590        * order that elements would be dequeued (via successive calls to
  591        * {@link #remove} or popped (via successive calls to {@link #pop}).
  592        *
  593        * @return an iterator over the elements in this deque
  594        */
  595       public Iterator<E> iterator() {
  596           return new DeqIterator();
  597       }
  598   
  599       public Iterator<E> descendingIterator() {
  600           return new DescendingIterator();
  601       }
  602   
  603       private class DeqIterator implements Iterator<E> {
  604           /**
  605            * Index of element to be returned by subsequent call to next.
  606            */
  607           private int cursor = head;
  608   
  609           /**
  610            * Tail recorded at construction (also in remove), to stop
  611            * iterator and also to check for comodification.
  612            */
  613           private int fence = tail;
  614   
  615           /**
  616            * Index of element returned by most recent call to next.
  617            * Reset to -1 if element is deleted by a call to remove.
  618            */
  619           private int lastRet = -1;
  620   
  621           public boolean hasNext() {
  622               return cursor != fence;
  623           }
  624   
  625           public E next() {
  626               if (cursor == fence)
  627                   throw new NoSuchElementException();
  628               E result = elements[cursor];
  629               // This check doesn't catch all possible comodifications,
  630               // but does catch the ones that corrupt traversal
  631               if (tail != fence || result == null)
  632                   throw new ConcurrentModificationException();
  633               lastRet = cursor;
  634               cursor = (cursor + 1) & (elements.length - 1);
  635               return result;
  636           }
  637   
  638           public void remove() {
  639               if (lastRet < 0)
  640                   throw new IllegalStateException();
  641               if (delete(lastRet)) { // if left-shifted, undo increment in next()
  642                   cursor = (cursor - 1) & (elements.length - 1);
  643                   fence = tail;
  644               }
  645               lastRet = -1;
  646           }
  647       }
  648   
  649       private class DescendingIterator implements Iterator<E> {
  650           /*
  651            * This class is nearly a mirror-image of DeqIterator, using
  652            * tail instead of head for initial cursor, and head instead of
  653            * tail for fence.
  654            */
  655           private int cursor = tail;
  656           private int fence = head;
  657           private int lastRet = -1;
  658   
  659           public boolean hasNext() {
  660               return cursor != fence;
  661           }
  662   
  663           public E next() {
  664               if (cursor == fence)
  665                   throw new NoSuchElementException();
  666               cursor = (cursor - 1) & (elements.length - 1);
  667               E result = elements[cursor];
  668               if (head != fence || result == null)
  669                   throw new ConcurrentModificationException();
  670               lastRet = cursor;
  671               return result;
  672           }
  673   
  674           public void remove() {
  675               if (lastRet < 0)
  676                   throw new IllegalStateException();
  677               if (!delete(lastRet)) {
  678                   cursor = (cursor + 1) & (elements.length - 1);
  679                   fence = head;
  680               }
  681               lastRet = -1;
  682           }
  683       }
  684   
  685       /**
  686        * Returns <tt>true</tt> if this deque contains the specified element.
  687        * More formally, returns <tt>true</tt> if and only if this deque contains
  688        * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
  689        *
  690        * @param o object to be checked for containment in this deque
  691        * @return <tt>true</tt> if this deque contains the specified element
  692        */
  693       public boolean contains(Object o) {
  694           if (o == null)
  695               return false;
  696           int mask = elements.length - 1;
  697           int i = head;
  698           E x;
  699           while ( (x = elements[i]) != null) {
  700               if (o.equals(x))
  701                   return true;
  702               i = (i + 1) & mask;
  703           }
  704           return false;
  705       }
  706   
  707       /**
  708        * Removes a single instance of the specified element from this deque.
  709        * If the deque does not contain the element, it is unchanged.
  710        * More formally, removes the first element <tt>e</tt> such that
  711        * <tt>o.equals(e)</tt> (if such an element exists).
  712        * Returns <tt>true</tt> if this deque contained the specified element
  713        * (or equivalently, if this deque changed as a result of the call).
  714        *
  715        * <p>This method is equivalent to {@link #removeFirstOccurrence}.
  716        *
  717        * @param o element to be removed from this deque, if present
  718        * @return <tt>true</tt> if this deque contained the specified element
  719        */
  720       public boolean remove(Object o) {
  721           return removeFirstOccurrence(o);
  722       }
  723   
  724       /**
  725        * Removes all of the elements from this deque.
  726        * The deque will be empty after this call returns.
  727        */
  728       public void clear() {
  729           int h = head;
  730           int t = tail;
  731           if (h != t) { // clear all cells
  732               head = tail = 0;
  733               int i = h;
  734               int mask = elements.length - 1;
  735               do {
  736                   elements[i] = null;
  737                   i = (i + 1) & mask;
  738               } while (i != t);
  739           }
  740       }
  741   
  742       /**
  743        * Returns an array containing all of the elements in this deque
  744        * in proper sequence (from first to last element).
  745        *
  746        * <p>The returned array will be "safe" in that no references to it are
  747        * maintained by this deque.  (In other words, this method must allocate
  748        * a new array).  The caller is thus free to modify the returned array.
  749        *
  750        * <p>This method acts as bridge between array-based and collection-based
  751        * APIs.
  752        *
  753        * @return an array containing all of the elements in this deque
  754        */
  755       public Object[] toArray() {
  756           return copyElements(new Object[size()]);
  757       }
  758   
  759       /**
  760        * Returns an array containing all of the elements in this deque in
  761        * proper sequence (from first to last element); the runtime type of the
  762        * returned array is that of the specified array.  If the deque fits in
  763        * the specified array, it is returned therein.  Otherwise, a new array
  764        * is allocated with the runtime type of the specified array and the
  765        * size of this deque.
  766        *
  767        * <p>If this deque fits in the specified array with room to spare
  768        * (i.e., the array has more elements than this deque), the element in
  769        * the array immediately following the end of the deque is set to
  770        * <tt>null</tt>.
  771        *
  772        * <p>Like the {@link #toArray()} method, this method acts as bridge between
  773        * array-based and collection-based APIs.  Further, this method allows
  774        * precise control over the runtime type of the output array, and may,
  775        * under certain circumstances, be used to save allocation costs.
  776        *
  777        * <p>Suppose <tt>x</tt> is a deque known to contain only strings.
  778        * The following code can be used to dump the deque into a newly
  779        * allocated array of <tt>String</tt>:
  780        *
  781        * <pre>
  782        *     String[] y = x.toArray(new String[0]);</pre>
  783        *
  784        * Note that <tt>toArray(new Object[0])</tt> is identical in function to
  785        * <tt>toArray()</tt>.
  786        *
  787        * @param a the array into which the elements of the deque are to
  788        *          be stored, if it is big enough; otherwise, a new array of the
  789        *          same runtime type is allocated for this purpose
  790        * @return an array containing all of the elements in this deque
  791        * @throws ArrayStoreException if the runtime type of the specified array
  792        *         is not a supertype of the runtime type of every element in
  793        *         this deque
  794        * @throws NullPointerException if the specified array is null
  795        */
  796       public <T> T[] toArray(T[] a) {
  797           int size = size();
  798           if (a.length < size)
  799               a = (T[])java.lang.reflect.Array.newInstance(
  800                       a.getClass().getComponentType(), size);
  801           copyElements(a);
  802           if (a.length > size)
  803               a[size] = null;
  804           return a;
  805       }
  806   
  807       // *** Object methods ***
  808   
  809       /**
  810        * Returns a copy of this deque.
  811        *
  812        * @return a copy of this deque
  813        */
  814       public ArrayDeque<E> clone() {
  815           try {
  816               ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
  817               result.elements = Arrays.copyOf(elements, elements.length);
  818               return result;
  819   
  820           } catch (CloneNotSupportedException e) {
  821               throw new AssertionError();
  822           }
  823       }
  824   
  825       /**
  826        * Appease the serialization gods.
  827        */
  828       private static final long serialVersionUID = 2340985798034038923L;
  829   
  830       /**
  831        * Serialize this deque.
  832        *
  833        * @serialData The current size (<tt>int</tt>) of the deque,
  834        * followed by all of its elements (each an object reference) in
  835        * first-to-last order.
  836        */
  837       private void writeObject(ObjectOutputStream s) throws IOException {
  838           s.defaultWriteObject();
  839   
  840           // Write out size
  841           s.writeInt(size());
  842   
  843           // Write out elements in order.
  844           int mask = elements.length - 1;
  845           for (int i = head; i != tail; i = (i + 1) & mask)
  846               s.writeObject(elements[i]);
  847       }
  848   
  849       /**
  850        * Deserialize this deque.
  851        */
  852       private void readObject(ObjectInputStream s)
  853               throws IOException, ClassNotFoundException {
  854           s.defaultReadObject();
  855   
  856           // Read in size and allocate array
  857           int size = s.readInt();
  858           allocateElements(size);
  859           head = 0;
  860           tail = size;
  861   
  862           // Read in all elements in the proper order.
  863           for (int i = 0; i < size; i++)
  864               elements[i] = (E)s.readObject();
  865       }
  866   }

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