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    1   /*
    2    * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
    3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
    4    *
    5    * This code is free software; you can redistribute it and/or modify it
    6    * under the terms of the GNU General Public License version 2 only, as
    7    * published by the Free Software Foundation.  Oracle designates this
    8    * particular file as subject to the "Classpath" exception as provided
    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
   12    * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
   16    *
   17    * You should have received a copy of the GNU General Public License version
   18    * 2 along with this work; if not, write to the Free Software Foundation,
   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package java.util;
   27   
   28   /**
   29    * Doubly-linked list implementation of the {@code List} and {@code Deque}
   30    * interfaces.  Implements all optional list operations, and permits all
   31    * elements (including {@code null}).
   32    *
   33    * <p>All of the operations perform as could be expected for a doubly-linked
   34    * list.  Operations that index into the list will traverse the list from
   35    * the beginning or the end, whichever is closer to the specified index.
   36    *
   37    * <p><strong>Note that this implementation is not synchronized.</strong>
   38    * If multiple threads access a linked list concurrently, and at least
   39    * one of the threads modifies the list structurally, it <i>must</i> be
   40    * synchronized externally.  (A structural modification is any operation
   41    * that adds or deletes one or more elements; merely setting the value of
   42    * an element is not a structural modification.)  This is typically
   43    * accomplished by synchronizing on some object that naturally
   44    * encapsulates the list.
   45    *
   46    * If no such object exists, the list should be "wrapped" using the
   47    * {@link Collections#synchronizedList Collections.synchronizedList}
   48    * method.  This is best done at creation time, to prevent accidental
   49    * unsynchronized access to the list:<pre>
   50    *   List list = Collections.synchronizedList(new LinkedList(...));</pre>
   51    *
   52    * <p>The iterators returned by this class's {@code iterator} and
   53    * {@code listIterator} methods are <i>fail-fast</i>: if the list is
   54    * structurally modified at any time after the iterator is created, in
   55    * any way except through the Iterator's own {@code remove} or
   56    * {@code add} methods, the iterator will throw a {@link
   57    * ConcurrentModificationException}.  Thus, in the face of concurrent
   58    * modification, the iterator fails quickly and cleanly, rather than
   59    * risking arbitrary, non-deterministic behavior at an undetermined
   60    * time in the future.
   61    *
   62    * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
   63    * as it is, generally speaking, impossible to make any hard guarantees in the
   64    * presence of unsynchronized concurrent modification.  Fail-fast iterators
   65    * throw {@code ConcurrentModificationException} on a best-effort basis.
   66    * Therefore, it would be wrong to write a program that depended on this
   67    * exception for its correctness:   <i>the fail-fast behavior of iterators
   68    * should be used only to detect bugs.</i>
   69    *
   70    * <p>This class is a member of the
   71    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   72    * Java Collections Framework</a>.
   73    *
   74    * @author  Josh Bloch
   75    * @see     List
   76    * @see     ArrayList
   77    * @since 1.2
   78    * @param <E> the type of elements held in this collection
   79    */
   80   
   81   public class LinkedList<E>
   82       extends AbstractSequentialList<E>
   83       implements List<E>, Deque<E>, Cloneable, java.io.Serializable
   84   {
   85       transient int size = 0;
   86   
   87       /**
   88        * Pointer to first node.
   89        * Invariant: (first == null && last == null) ||
   90        *            (first.prev == null && first.item != null)
   91        */
   92       transient Node<E> first;
   93   
   94       /**
   95        * Pointer to last node.
   96        * Invariant: (first == null && last == null) ||
   97        *            (last.next == null && last.item != null)
   98        */
   99       transient Node<E> last;
  100   
  101       /**
  102        * Constructs an empty list.
  103        */
  104       public LinkedList() {
  105       }
  106   
  107       /**
  108        * Constructs a list containing the elements of the specified
  109        * collection, in the order they are returned by the collection's
  110        * iterator.
  111        *
  112        * @param  c the collection whose elements are to be placed into this list
  113        * @throws NullPointerException if the specified collection is null
  114        */
  115       public LinkedList(Collection<? extends E> c) {
  116           this();
  117           addAll(c);
  118       }
  119   
  120       /**
  121        * Links e as first element.
  122        */
  123       private void linkFirst(E e) {
  124           final Node<E> f = first;
  125           final Node<E> newNode = new Node<>(null, e, f);
  126           first = newNode;
  127           if (f == null)
  128               last = newNode;
  129           else
  130               f.prev = newNode;
  131           size++;
  132           modCount++;
  133       }
  134   
  135       /**
  136        * Links e as last element.
  137        */
  138       void linkLast(E e) {
  139           final Node<E> l = last;
  140           final Node<E> newNode = new Node<>(l, e, null);
  141           last = newNode;
  142           if (l == null)
  143               first = newNode;
  144           else
  145               l.next = newNode;
  146           size++;
  147           modCount++;
  148       }
  149   
  150       /**
  151        * Inserts element e before non-null Node succ.
  152        */
  153       void linkBefore(E e, Node<E> succ) {
  154           // assert succ != null;
  155           final Node<E> pred = succ.prev;
  156           final Node<E> newNode = new Node<>(pred, e, succ);
  157           succ.prev = newNode;
  158           if (pred == null)
  159               first = newNode;
  160           else
  161               pred.next = newNode;
  162           size++;
  163           modCount++;
  164       }
  165   
  166       /**
  167        * Unlinks non-null first node f.
  168        */
  169       private E unlinkFirst(Node<E> f) {
  170           // assert f == first && f != null;
  171           final E element = f.item;
  172           final Node<E> next = f.next;
  173           f.item = null;
  174           f.next = null; // help GC
  175           first = next;
  176           if (next == null)
  177               last = null;
  178           else
  179               next.prev = null;
  180           size--;
  181           modCount++;
  182           return element;
  183       }
  184   
  185       /**
  186        * Unlinks non-null last node l.
  187        */
  188       private E unlinkLast(Node<E> l) {
  189           // assert l == last && l != null;
  190           final E element = l.item;
  191           final Node<E> prev = l.prev;
  192           l.item = null;
  193           l.prev = null; // help GC
  194           last = prev;
  195           if (prev == null)
  196               first = null;
  197           else
  198               prev.next = null;
  199           size--;
  200           modCount++;
  201           return element;
  202       }
  203   
  204       /**
  205        * Unlinks non-null node x.
  206        */
  207       E unlink(Node<E> x) {
  208           // assert x != null;
  209           final E element = x.item;
  210           final Node<E> next = x.next;
  211           final Node<E> prev = x.prev;
  212   
  213           if (prev == null) {
  214               first = next;
  215           } else {
  216               prev.next = next;
  217               x.prev = null;
  218           }
  219   
  220           if (next == null) {
  221               last = prev;
  222           } else {
  223               next.prev = prev;
  224               x.next = null;
  225           }
  226   
  227           x.item = null;
  228           size--;
  229           modCount++;
  230           return element;
  231       }
  232   
  233       /**
  234        * Returns the first element in this list.
  235        *
  236        * @return the first element in this list
  237        * @throws NoSuchElementException if this list is empty
  238        */
  239       public E getFirst() {
  240           final Node<E> f = first;
  241           if (f == null)
  242               throw new NoSuchElementException();
  243           return f.item;
  244       }
  245   
  246       /**
  247        * Returns the last element in this list.
  248        *
  249        * @return the last element in this list
  250        * @throws NoSuchElementException if this list is empty
  251        */
  252       public E getLast() {
  253           final Node<E> l = last;
  254           if (l == null)
  255               throw new NoSuchElementException();
  256           return l.item;
  257       }
  258   
  259       /**
  260        * Removes and returns the first element from this list.
  261        *
  262        * @return the first element from this list
  263        * @throws NoSuchElementException if this list is empty
  264        */
  265       public E removeFirst() {
  266           final Node<E> f = first;
  267           if (f == null)
  268               throw new NoSuchElementException();
  269           return unlinkFirst(f);
  270       }
  271   
  272       /**
  273        * Removes and returns the last element from this list.
  274        *
  275        * @return the last element from this list
  276        * @throws NoSuchElementException if this list is empty
  277        */
  278       public E removeLast() {
  279           final Node<E> l = last;
  280           if (l == null)
  281               throw new NoSuchElementException();
  282           return unlinkLast(l);
  283       }
  284   
  285       /**
  286        * Inserts the specified element at the beginning of this list.
  287        *
  288        * @param e the element to add
  289        */
  290       public void addFirst(E e) {
  291           linkFirst(e);
  292       }
  293   
  294       /**
  295        * Appends the specified element to the end of this list.
  296        *
  297        * <p>This method is equivalent to {@link #add}.
  298        *
  299        * @param e the element to add
  300        */
  301       public void addLast(E e) {
  302           linkLast(e);
  303       }
  304   
  305       /**
  306        * Returns {@code true} if this list contains the specified element.
  307        * More formally, returns {@code true} if and only if this list contains
  308        * at least one element {@code e} such that
  309        * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
  310        *
  311        * @param o element whose presence in this list is to be tested
  312        * @return {@code true} if this list contains the specified element
  313        */
  314       public boolean contains(Object o) {
  315           return indexOf(o) != -1;
  316       }
  317   
  318       /**
  319        * Returns the number of elements in this list.
  320        *
  321        * @return the number of elements in this list
  322        */
  323       public int size() {
  324           return size;
  325       }
  326   
  327       /**
  328        * Appends the specified element to the end of this list.
  329        *
  330        * <p>This method is equivalent to {@link #addLast}.
  331        *
  332        * @param e element to be appended to this list
  333        * @return {@code true} (as specified by {@link Collection#add})
  334        */
  335       public boolean add(E e) {
  336           linkLast(e);
  337           return true;
  338       }
  339   
  340       /**
  341        * Removes the first occurrence of the specified element from this list,
  342        * if it is present.  If this list does not contain the element, it is
  343        * unchanged.  More formally, removes the element with the lowest index
  344        * {@code i} such that
  345        * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
  346        * (if such an element exists).  Returns {@code true} if this list
  347        * contained the specified element (or equivalently, if this list
  348        * changed as a result of the call).
  349        *
  350        * @param o element to be removed from this list, if present
  351        * @return {@code true} if this list contained the specified element
  352        */
  353       public boolean remove(Object o) {
  354           if (o == null) {
  355               for (Node<E> x = first; x != null; x = x.next) {
  356                   if (x.item == null) {
  357                       unlink(x);
  358                       return true;
  359                   }
  360               }
  361           } else {
  362               for (Node<E> x = first; x != null; x = x.next) {
  363                   if (o.equals(x.item)) {
  364                       unlink(x);
  365                       return true;
  366                   }
  367               }
  368           }
  369           return false;
  370       }
  371   
  372       /**
  373        * Appends all of the elements in the specified collection to the end of
  374        * this list, in the order that they are returned by the specified
  375        * collection's iterator.  The behavior of this operation is undefined if
  376        * the specified collection is modified while the operation is in
  377        * progress.  (Note that this will occur if the specified collection is
  378        * this list, and it's nonempty.)
  379        *
  380        * @param c collection containing elements to be added to this list
  381        * @return {@code true} if this list changed as a result of the call
  382        * @throws NullPointerException if the specified collection is null
  383        */
  384       public boolean addAll(Collection<? extends E> c) {
  385           return addAll(size, c);
  386       }
  387   
  388       /**
  389        * Inserts all of the elements in the specified collection into this
  390        * list, starting at the specified position.  Shifts the element
  391        * currently at that position (if any) and any subsequent elements to
  392        * the right (increases their indices).  The new elements will appear
  393        * in the list in the order that they are returned by the
  394        * specified collection's iterator.
  395        *
  396        * @param index index at which to insert the first element
  397        *              from the specified collection
  398        * @param c collection containing elements to be added to this list
  399        * @return {@code true} if this list changed as a result of the call
  400        * @throws IndexOutOfBoundsException {@inheritDoc}
  401        * @throws NullPointerException if the specified collection is null
  402        */
  403       public boolean addAll(int index, Collection<? extends E> c) {
  404           checkPositionIndex(index);
  405   
  406           Object[] a = c.toArray();
  407           int numNew = a.length;
  408           if (numNew == 0)
  409               return false;
  410   
  411           Node<E> pred, succ;
  412           if (index == size) {
  413               succ = null;
  414               pred = last;
  415           } else {
  416               succ = node(index);
  417               pred = succ.prev;
  418           }
  419   
  420           for (Object o : a) {
  421               @SuppressWarnings("unchecked") E e = (E) o;
  422               Node<E> newNode = new Node<>(pred, e, null);
  423               if (pred == null)
  424                   first = newNode;
  425               else
  426                   pred.next = newNode;
  427               pred = newNode;
  428           }
  429   
  430           if (succ == null) {
  431               last = pred;
  432           } else {
  433               pred.next = succ;
  434               succ.prev = pred;
  435           }
  436   
  437           size += numNew;
  438           modCount++;
  439           return true;
  440       }
  441   
  442       /**
  443        * Removes all of the elements from this list.
  444        * The list will be empty after this call returns.
  445        */
  446       public void clear() {
  447           // Clearing all of the links between nodes is "unnecessary", but:
  448           // - helps a generational GC if the discarded nodes inhabit
  449           //   more than one generation
  450           // - is sure to free memory even if there is a reachable Iterator
  451           for (Node<E> x = first; x != null; ) {
  452               Node<E> next = x.next;
  453               x.item = null;
  454               x.next = null;
  455               x.prev = null;
  456               x = next;
  457           }
  458           first = last = null;
  459           size = 0;
  460           modCount++;
  461       }
  462   
  463   
  464       // Positional Access Operations
  465   
  466       /**
  467        * Returns the element at the specified position in this list.
  468        *
  469        * @param index index of the element to return
  470        * @return the element at the specified position in this list
  471        * @throws IndexOutOfBoundsException {@inheritDoc}
  472        */
  473       public E get(int index) {
  474           checkElementIndex(index);
  475           return node(index).item;
  476       }
  477   
  478       /**
  479        * Replaces the element at the specified position in this list with the
  480        * specified element.
  481        *
  482        * @param index index of the element to replace
  483        * @param element element to be stored at the specified position
  484        * @return the element previously at the specified position
  485        * @throws IndexOutOfBoundsException {@inheritDoc}
  486        */
  487       public E set(int index, E element) {
  488           checkElementIndex(index);
  489           Node<E> x = node(index);
  490           E oldVal = x.item;
  491           x.item = element;
  492           return oldVal;
  493       }
  494   
  495       /**
  496        * Inserts the specified element at the specified position in this list.
  497        * Shifts the element currently at that position (if any) and any
  498        * subsequent elements to the right (adds one to their indices).
  499        *
  500        * @param index index at which the specified element is to be inserted
  501        * @param element element to be inserted
  502        * @throws IndexOutOfBoundsException {@inheritDoc}
  503        */
  504       public void add(int index, E element) {
  505           checkPositionIndex(index);
  506   
  507           if (index == size)
  508               linkLast(element);
  509           else
  510               linkBefore(element, node(index));
  511       }
  512   
  513       /**
  514        * Removes the element at the specified position in this list.  Shifts any
  515        * subsequent elements to the left (subtracts one from their indices).
  516        * Returns the element that was removed from the list.
  517        *
  518        * @param index the index of the element to be removed
  519        * @return the element previously at the specified position
  520        * @throws IndexOutOfBoundsException {@inheritDoc}
  521        */
  522       public E remove(int index) {
  523           checkElementIndex(index);
  524           return unlink(node(index));
  525       }
  526   
  527       /**
  528        * Tells if the argument is the index of an existing element.
  529        */
  530       private boolean isElementIndex(int index) {
  531           return index >= 0 && index < size;
  532       }
  533   
  534       /**
  535        * Tells if the argument is the index of a valid position for an
  536        * iterator or an add operation.
  537        */
  538       private boolean isPositionIndex(int index) {
  539           return index >= 0 && index <= size;
  540       }
  541   
  542       /**
  543        * Constructs an IndexOutOfBoundsException detail message.
  544        * Of the many possible refactorings of the error handling code,
  545        * this "outlining" performs best with both server and client VMs.
  546        */
  547       private String outOfBoundsMsg(int index) {
  548           return "Index: "+index+", Size: "+size;
  549       }
  550   
  551       private void checkElementIndex(int index) {
  552           if (!isElementIndex(index))
  553               throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  554       }
  555   
  556       private void checkPositionIndex(int index) {
  557           if (!isPositionIndex(index))
  558               throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  559       }
  560   
  561       /**
  562        * Returns the (non-null) Node at the specified element index.
  563        */
  564       Node<E> node(int index) {
  565           // assert isElementIndex(index);
  566   
  567           if (index < (size >> 1)) {
  568               Node<E> x = first;
  569               for (int i = 0; i < index; i++)
  570                   x = x.next;
  571               return x;
  572           } else {
  573               Node<E> x = last;
  574               for (int i = size - 1; i > index; i--)
  575                   x = x.prev;
  576               return x;
  577           }
  578       }
  579   
  580       // Search Operations
  581   
  582       /**
  583        * Returns the index of the first occurrence of the specified element
  584        * in this list, or -1 if this list does not contain the element.
  585        * More formally, returns the lowest index {@code i} such that
  586        * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
  587        * or -1 if there is no such index.
  588        *
  589        * @param o element to search for
  590        * @return the index of the first occurrence of the specified element in
  591        *         this list, or -1 if this list does not contain the element
  592        */
  593       public int indexOf(Object o) {
  594           int index = 0;
  595           if (o == null) {
  596               for (Node<E> x = first; x != null; x = x.next) {
  597                   if (x.item == null)
  598                       return index;
  599                   index++;
  600               }
  601           } else {
  602               for (Node<E> x = first; x != null; x = x.next) {
  603                   if (o.equals(x.item))
  604                       return index;
  605                   index++;
  606               }
  607           }
  608           return -1;
  609       }
  610   
  611       /**
  612        * Returns the index of the last occurrence of the specified element
  613        * in this list, or -1 if this list does not contain the element.
  614        * More formally, returns the highest index {@code i} such that
  615        * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
  616        * or -1 if there is no such index.
  617        *
  618        * @param o element to search for
  619        * @return the index of the last occurrence of the specified element in
  620        *         this list, or -1 if this list does not contain the element
  621        */
  622       public int lastIndexOf(Object o) {
  623           int index = size;
  624           if (o == null) {
  625               for (Node<E> x = last; x != null; x = x.prev) {
  626                   index--;
  627                   if (x.item == null)
  628                       return index;
  629               }
  630           } else {
  631               for (Node<E> x = last; x != null; x = x.prev) {
  632                   index--;
  633                   if (o.equals(x.item))
  634                       return index;
  635               }
  636           }
  637           return -1;
  638       }
  639   
  640       // Queue operations.
  641   
  642       /**
  643        * Retrieves, but does not remove, the head (first element) of this list.
  644        *
  645        * @return the head of this list, or {@code null} if this list is empty
  646        * @since 1.5
  647        */
  648       public E peek() {
  649           final Node<E> f = first;
  650           return (f == null) ? null : f.item;
  651       }
  652   
  653       /**
  654        * Retrieves, but does not remove, the head (first element) of this list.
  655        *
  656        * @return the head of this list
  657        * @throws NoSuchElementException if this list is empty
  658        * @since 1.5
  659        */
  660       public E element() {
  661           return getFirst();
  662       }
  663   
  664       /**
  665        * Retrieves and removes the head (first element) of this list.
  666        *
  667        * @return the head of this list, or {@code null} if this list is empty
  668        * @since 1.5
  669        */
  670       public E poll() {
  671           final Node<E> f = first;
  672           return (f == null) ? null : unlinkFirst(f);
  673       }
  674   
  675       /**
  676        * Retrieves and removes the head (first element) of this list.
  677        *
  678        * @return the head of this list
  679        * @throws NoSuchElementException if this list is empty
  680        * @since 1.5
  681        */
  682       public E remove() {
  683           return removeFirst();
  684       }
  685   
  686       /**
  687        * Adds the specified element as the tail (last element) of this list.
  688        *
  689        * @param e the element to add
  690        * @return {@code true} (as specified by {@link Queue#offer})
  691        * @since 1.5
  692        */
  693       public boolean offer(E e) {
  694           return add(e);
  695       }
  696   
  697       // Deque operations
  698       /**
  699        * Inserts the specified element at the front of this list.
  700        *
  701        * @param e the element to insert
  702        * @return {@code true} (as specified by {@link Deque#offerFirst})
  703        * @since 1.6
  704        */
  705       public boolean offerFirst(E e) {
  706           addFirst(e);
  707           return true;
  708       }
  709   
  710       /**
  711        * Inserts the specified element at the end of this list.
  712        *
  713        * @param e the element to insert
  714        * @return {@code true} (as specified by {@link Deque#offerLast})
  715        * @since 1.6
  716        */
  717       public boolean offerLast(E e) {
  718           addLast(e);
  719           return true;
  720       }
  721   
  722       /**
  723        * Retrieves, but does not remove, the first element of this list,
  724        * or returns {@code null} if this list is empty.
  725        *
  726        * @return the first element of this list, or {@code null}
  727        *         if this list is empty
  728        * @since 1.6
  729        */
  730       public E peekFirst() {
  731           final Node<E> f = first;
  732           return (f == null) ? null : f.item;
  733        }
  734   
  735       /**
  736        * Retrieves, but does not remove, the last element of this list,
  737        * or returns {@code null} if this list is empty.
  738        *
  739        * @return the last element of this list, or {@code null}
  740        *         if this list is empty
  741        * @since 1.6
  742        */
  743       public E peekLast() {
  744           final Node<E> l = last;
  745           return (l == null) ? null : l.item;
  746       }
  747   
  748       /**
  749        * Retrieves and removes the first element of this list,
  750        * or returns {@code null} if this list is empty.
  751        *
  752        * @return the first element of this list, or {@code null} if
  753        *     this list is empty
  754        * @since 1.6
  755        */
  756       public E pollFirst() {
  757           final Node<E> f = first;
  758           return (f == null) ? null : unlinkFirst(f);
  759       }
  760   
  761       /**
  762        * Retrieves and removes the last element of this list,
  763        * or returns {@code null} if this list is empty.
  764        *
  765        * @return the last element of this list, or {@code null} if
  766        *     this list is empty
  767        * @since 1.6
  768        */
  769       public E pollLast() {
  770           final Node<E> l = last;
  771           return (l == null) ? null : unlinkLast(l);
  772       }
  773   
  774       /**
  775        * Pushes an element onto the stack represented by this list.  In other
  776        * words, inserts the element at the front of this list.
  777        *
  778        * <p>This method is equivalent to {@link #addFirst}.
  779        *
  780        * @param e the element to push
  781        * @since 1.6
  782        */
  783       public void push(E e) {
  784           addFirst(e);
  785       }
  786   
  787       /**
  788        * Pops an element from the stack represented by this list.  In other
  789        * words, removes and returns the first element of this list.
  790        *
  791        * <p>This method is equivalent to {@link #removeFirst()}.
  792        *
  793        * @return the element at the front of this list (which is the top
  794        *         of the stack represented by this list)
  795        * @throws NoSuchElementException if this list is empty
  796        * @since 1.6
  797        */
  798       public E pop() {
  799           return removeFirst();
  800       }
  801   
  802       /**
  803        * Removes the first occurrence of the specified element in this
  804        * list (when traversing the list from head to tail).  If the list
  805        * does not contain the element, it is unchanged.
  806        *
  807        * @param o element to be removed from this list, if present
  808        * @return {@code true} if the list contained the specified element
  809        * @since 1.6
  810        */
  811       public boolean removeFirstOccurrence(Object o) {
  812           return remove(o);
  813       }
  814   
  815       /**
  816        * Removes the last occurrence of the specified element in this
  817        * list (when traversing the list from head to tail).  If the list
  818        * does not contain the element, it is unchanged.
  819        *
  820        * @param o element to be removed from this list, if present
  821        * @return {@code true} if the list contained the specified element
  822        * @since 1.6
  823        */
  824       public boolean removeLastOccurrence(Object o) {
  825           if (o == null) {
  826               for (Node<E> x = last; x != null; x = x.prev) {
  827                   if (x.item == null) {
  828                       unlink(x);
  829                       return true;
  830                   }
  831               }
  832           } else {
  833               for (Node<E> x = last; x != null; x = x.prev) {
  834                   if (o.equals(x.item)) {
  835                       unlink(x);
  836                       return true;
  837                   }
  838               }
  839           }
  840           return false;
  841       }
  842   
  843       /**
  844        * Returns a list-iterator of the elements in this list (in proper
  845        * sequence), starting at the specified position in the list.
  846        * Obeys the general contract of {@code List.listIterator(int)}.<p>
  847        *
  848        * The list-iterator is <i>fail-fast</i>: if the list is structurally
  849        * modified at any time after the Iterator is created, in any way except
  850        * through the list-iterator's own {@code remove} or {@code add}
  851        * methods, the list-iterator will throw a
  852        * {@code ConcurrentModificationException}.  Thus, in the face of
  853        * concurrent modification, the iterator fails quickly and cleanly, rather
  854        * than risking arbitrary, non-deterministic behavior at an undetermined
  855        * time in the future.
  856        *
  857        * @param index index of the first element to be returned from the
  858        *              list-iterator (by a call to {@code next})
  859        * @return a ListIterator of the elements in this list (in proper
  860        *         sequence), starting at the specified position in the list
  861        * @throws IndexOutOfBoundsException {@inheritDoc}
  862        * @see List#listIterator(int)
  863        */
  864       public ListIterator<E> listIterator(int index) {
  865           checkPositionIndex(index);
  866           return new ListItr(index);
  867       }
  868   
  869       private class ListItr implements ListIterator<E> {
  870           private Node<E> lastReturned = null;
  871           private Node<E> next;
  872           private int nextIndex;
  873           private int expectedModCount = modCount;
  874   
  875           ListItr(int index) {
  876               // assert isPositionIndex(index);
  877               next = (index == size) ? null : node(index);
  878               nextIndex = index;
  879           }
  880   
  881           public boolean hasNext() {
  882               return nextIndex < size;
  883           }
  884   
  885           public E next() {
  886               checkForComodification();
  887               if (!hasNext())
  888                   throw new NoSuchElementException();
  889   
  890               lastReturned = next;
  891               next = next.next;
  892               nextIndex++;
  893               return lastReturned.item;
  894           }
  895   
  896           public boolean hasPrevious() {
  897               return nextIndex > 0;
  898           }
  899   
  900           public E previous() {
  901               checkForComodification();
  902               if (!hasPrevious())
  903                   throw new NoSuchElementException();
  904   
  905               lastReturned = next = (next == null) ? last : next.prev;
  906               nextIndex--;
  907               return lastReturned.item;
  908           }
  909   
  910           public int nextIndex() {
  911               return nextIndex;
  912           }
  913   
  914           public int previousIndex() {
  915               return nextIndex - 1;
  916           }
  917   
  918           public void remove() {
  919               checkForComodification();
  920               if (lastReturned == null)
  921                   throw new IllegalStateException();
  922   
  923               Node<E> lastNext = lastReturned.next;
  924               unlink(lastReturned);
  925               if (next == lastReturned)
  926                   next = lastNext;
  927               else
  928                   nextIndex--;
  929               lastReturned = null;
  930               expectedModCount++;
  931           }
  932   
  933           public void set(E e) {
  934               if (lastReturned == null)
  935                   throw new IllegalStateException();
  936               checkForComodification();
  937               lastReturned.item = e;
  938           }
  939   
  940           public void add(E e) {
  941               checkForComodification();
  942               lastReturned = null;
  943               if (next == null)
  944                   linkLast(e);
  945               else
  946                   linkBefore(e, next);
  947               nextIndex++;
  948               expectedModCount++;
  949           }
  950   
  951           final void checkForComodification() {
  952               if (modCount != expectedModCount)
  953                   throw new ConcurrentModificationException();
  954           }
  955       }
  956   
  957       private static class Node<E> {
  958           E item;
  959           Node<E> next;
  960           Node<E> prev;
  961   
  962           Node(Node<E> prev, E element, Node<E> next) {
  963               this.item = element;
  964               this.next = next;
  965               this.prev = prev;
  966           }
  967       }
  968   
  969       /**
  970        * @since 1.6
  971        */
  972       public Iterator<E> descendingIterator() {
  973           return new DescendingIterator();
  974       }
  975   
  976       /**
  977        * Adapter to provide descending iterators via ListItr.previous
  978        */
  979       private class DescendingIterator implements Iterator<E> {
  980           private final ListItr itr = new ListItr(size());
  981           public boolean hasNext() {
  982               return itr.hasPrevious();
  983           }
  984           public E next() {
  985               return itr.previous();
  986           }
  987           public void remove() {
  988               itr.remove();
  989           }
  990       }
  991   
  992       @SuppressWarnings("unchecked")
  993       private LinkedList<E> superClone() {
  994           try {
  995               return (LinkedList<E>) super.clone();
  996           } catch (CloneNotSupportedException e) {
  997               throw new InternalError();
  998           }
  999       }
 1000   
 1001       /**
 1002        * Returns a shallow copy of this {@code LinkedList}. (The elements
 1003        * themselves are not cloned.)
 1004        *
 1005        * @return a shallow copy of this {@code LinkedList} instance
 1006        */
 1007       public Object clone() {
 1008           LinkedList<E> clone = superClone();
 1009   
 1010           // Put clone into "virgin" state
 1011           clone.first = clone.last = null;
 1012           clone.size = 0;
 1013           clone.modCount = 0;
 1014   
 1015           // Initialize clone with our elements
 1016           for (Node<E> x = first; x != null; x = x.next)
 1017               clone.add(x.item);
 1018   
 1019           return clone;
 1020       }
 1021   
 1022       /**
 1023        * Returns an array containing all of the elements in this list
 1024        * in proper sequence (from first to last element).
 1025        *
 1026        * <p>The returned array will be "safe" in that no references to it are
 1027        * maintained by this list.  (In other words, this method must allocate
 1028        * a new array).  The caller is thus free to modify the returned array.
 1029        *
 1030        * <p>This method acts as bridge between array-based and collection-based
 1031        * APIs.
 1032        *
 1033        * @return an array containing all of the elements in this list
 1034        *         in proper sequence
 1035        */
 1036       public Object[] toArray() {
 1037           Object[] result = new Object[size];
 1038           int i = 0;
 1039           for (Node<E> x = first; x != null; x = x.next)
 1040               result[i++] = x.item;
 1041           return result;
 1042       }
 1043   
 1044       /**
 1045        * Returns an array containing all of the elements in this list in
 1046        * proper sequence (from first to last element); the runtime type of
 1047        * the returned array is that of the specified array.  If the list fits
 1048        * in the specified array, it is returned therein.  Otherwise, a new
 1049        * array is allocated with the runtime type of the specified array and
 1050        * the size of this list.
 1051        *
 1052        * <p>If the list fits in the specified array with room to spare (i.e.,
 1053        * the array has more elements than the list), the element in the array
 1054        * immediately following the end of the list is set to {@code null}.
 1055        * (This is useful in determining the length of the list <i>only</i> if
 1056        * the caller knows that the list does not contain any null elements.)
 1057        *
 1058        * <p>Like the {@link #toArray()} method, this method acts as bridge between
 1059        * array-based and collection-based APIs.  Further, this method allows
 1060        * precise control over the runtime type of the output array, and may,
 1061        * under certain circumstances, be used to save allocation costs.
 1062        *
 1063        * <p>Suppose {@code x} is a list known to contain only strings.
 1064        * The following code can be used to dump the list into a newly
 1065        * allocated array of {@code String}:
 1066        *
 1067        * <pre>
 1068        *     String[] y = x.toArray(new String[0]);</pre>
 1069        *
 1070        * Note that {@code toArray(new Object[0])} is identical in function to
 1071        * {@code toArray()}.
 1072        *
 1073        * @param a the array into which the elements of the list are to
 1074        *          be stored, if it is big enough; otherwise, a new array of the
 1075        *          same runtime type is allocated for this purpose.
 1076        * @return an array containing the elements of the list
 1077        * @throws ArrayStoreException if the runtime type of the specified array
 1078        *         is not a supertype of the runtime type of every element in
 1079        *         this list
 1080        * @throws NullPointerException if the specified array is null
 1081        */
 1082       @SuppressWarnings("unchecked")
 1083       public <T> T[] toArray(T[] a) {
 1084           if (a.length < size)
 1085               a = (T[])java.lang.reflect.Array.newInstance(
 1086                                   a.getClass().getComponentType(), size);
 1087           int i = 0;
 1088           Object[] result = a;
 1089           for (Node<E> x = first; x != null; x = x.next)
 1090               result[i++] = x.item;
 1091   
 1092           if (a.length > size)
 1093               a[size] = null;
 1094   
 1095           return a;
 1096       }
 1097   
 1098       private static final long serialVersionUID = 876323262645176354L;
 1099   
 1100       /**
 1101        * Saves the state of this {@code LinkedList} instance to a stream
 1102        * (that is, serializes it).
 1103        *
 1104        * @serialData The size of the list (the number of elements it
 1105        *             contains) is emitted (int), followed by all of its
 1106        *             elements (each an Object) in the proper order.
 1107        */
 1108       private void writeObject(java.io.ObjectOutputStream s)
 1109           throws java.io.IOException {
 1110           // Write out any hidden serialization magic
 1111           s.defaultWriteObject();
 1112   
 1113           // Write out size
 1114           s.writeInt(size);
 1115   
 1116           // Write out all elements in the proper order.
 1117           for (Node<E> x = first; x != null; x = x.next)
 1118               s.writeObject(x.item);
 1119       }
 1120   
 1121       /**
 1122        * Reconstitutes this {@code LinkedList} instance from a stream
 1123        * (that is, deserializes it).
 1124        */
 1125       @SuppressWarnings("unchecked")
 1126       private void readObject(java.io.ObjectInputStream s)
 1127           throws java.io.IOException, ClassNotFoundException {
 1128           // Read in any hidden serialization magic
 1129           s.defaultReadObject();
 1130   
 1131           // Read in size
 1132           int size = s.readInt();
 1133   
 1134           // Read in all elements in the proper order.
 1135           for (int i = 0; i < size; i++)
 1136               linkLast((E)s.readObject());
 1137       }
 1138   }

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