content: /*
   * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
   * 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.
   */
  
  package java.util;
  
  import java.util.function.Consumer;
  
  /**
   * Doubly-linked list implementation of the {@code List} and {@code Deque}
   * interfaces.  Implements all optional list operations, and permits all
   * elements (including {@code null}).
   *
   * <p>All of the operations perform as could be expected for a doubly-linked
   * list.  Operations that index into the list will traverse the list from
   * the beginning or the end, whichever is closer to the specified index.
   *
   * <p><strong>Note that this implementation is not synchronized.</strong>
   * If multiple threads access a linked list concurrently, and at least
   * one of the threads modifies the list structurally, it <i>must</i> be
   * synchronized externally.  (A structural modification is any operation
   * that adds or deletes one or more elements; merely setting the value of
   * an element is not a structural modification.)  This is typically
   * accomplished by synchronizing on some object that naturally
   * encapsulates the list.
   *
   * If no such object exists, the list should be "wrapped" using the
   * {@link Collections#synchronizedList Collections.synchronizedList}
   * method.  This is best done at creation time, to prevent accidental
   * unsynchronized access to the list:<pre>
   *   List list = Collections.synchronizedList(new LinkedList(...));</pre>
   *
   * <p>The iterators returned by this class's {@code iterator} and
   * {@code listIterator} methods are <i>fail-fast</i>: if the list is
   * structurally modified at any time after the iterator is created, in
   * any way except through the Iterator's own {@code remove} or
   * {@code add} methods, the iterator will 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 is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @author  Josh Bloch
   * @see     List
   * @see     ArrayList
   * @since 1.2
   * @param <E> the type of elements held in this collection
   */
  
  public class LinkedList<E>
      extends AbstractSequentialList<E>
      implements List<E>, Deque<E>, Cloneable, java.io.Serializable
  {
      transient int size = 0;
  
      /**
       * Pointer to first node.
       * Invariant: (first == null && last == null) ||
       *            (first.prev == null && first.item != null)
       */
      transient Node<E> first;
  
      /**
       * Pointer to last node.
       * Invariant: (first == null && last == null) ||
       *            (last.next == null && last.item != null)
       */
      transient Node<E> last;
  
      /**
       * Constructs an empty list.
       */
      public LinkedList() {
      }
  
      /**
       * Constructs a list containing the elements of the specified
       * collection, in the order they are returned by the collection's
       * iterator.
       *
       * @param  c the collection whose elements are to be placed into this list
       * @throws NullPointerException if the specified collection is null
       */
      public LinkedList(Collection<? extends E> c) {
          this();
          addAll(c);
      }
  
      /**
       * Links e as first element.
       */
      private void linkFirst(E e) {
          final Node<E> f = first;
          final Node<E> newNode = new Node<>(null, e, f);
          first = newNode;
          if (f == null)
              last = newNode;
          else
              f.prev = newNode;
          size++;
          modCount++;
      }
  
      /**
       * Links e as last element.
       */
      void linkLast(E e) {
          final Node<E> l = last;
          final Node<E> newNode = new Node<>(l, e, null);
          last = newNode;
          if (l == null)
              first = newNode;
          else
              l.next = newNode;
          size++;
          modCount++;
      }
  
      /**
       * Inserts element e before non-null Node succ.
       */
      void linkBefore(E e, Node<E> succ) {
          // assert succ != null;
          final Node<E> pred = succ.prev;
          final Node<E> newNode = new Node<>(pred, e, succ);
          succ.prev = newNode;
          if (pred == null)
              first = newNode;
          else
              pred.next = newNode;
          size++;
          modCount++;
      }
  
      /**
       * Unlinks non-null first node f.
       */
      private E unlinkFirst(Node<E> f) {
          // assert f == first && f != null;
          final E element = f.item;
          final Node<E> next = f.next;
          f.item = null;
          f.next = null; // help GC
          first = next;
          if (next == null)
              last = null;
          else
              next.prev = null;
          size--;
          modCount++;
          return element;
      }
  
      /**
       * Unlinks non-null last node l.
       */
      private E unlinkLast(Node<E> l) {
          // assert l == last && l != null;
          final E element = l.item;
          final Node<E> prev = l.prev;
          l.item = null;
          l.prev = null; // help GC
          last = prev;
          if (prev == null)
              first = null;
          else
              prev.next = null;
          size--;
          modCount++;
          return element;
      }
  
      /**
       * Unlinks non-null node x.
       */
      E unlink(Node<E> x) {
          // assert x != null;
          final E element = x.item;
          final Node<E> next = x.next;
          final Node<E> prev = x.prev;
  
          if (prev == null) {
              first = next;
          } else {
              prev.next = next;
              x.prev = null;
          }
  
          if (next == null) {
              last = prev;
          } else {
              next.prev = prev;
              x.next = null;
          }
  
          x.item = null;
          size--;
          modCount++;
          return element;
      }
  
      /**
       * Returns the first element in this list.
       *
       * @return the first element in this list
       * @throws NoSuchElementException if this list is empty
       */
      public E getFirst() {
          final Node<E> f = first;
          if (f == null)
              throw new NoSuchElementException();
          return f.item;
      }
  
      /**
       * Returns the last element in this list.
       *
       * @return the last element in this list
       * @throws NoSuchElementException if this list is empty
       */
      public E getLast() {
          final Node<E> l = last;
          if (l == null)
              throw new NoSuchElementException();
          return l.item;
      }
  
      /**
       * Removes and returns the first element from this list.
       *
       * @return the first element from this list
       * @throws NoSuchElementException if this list is empty
       */
      public E removeFirst() {
          final Node<E> f = first;
          if (f == null)
              throw new NoSuchElementException();
          return unlinkFirst(f);
      }
  
      /**
       * Removes and returns the last element from this list.
       *
       * @return the last element from this list
       * @throws NoSuchElementException if this list is empty
       */
      public E removeLast() {
          final Node<E> l = last;
          if (l == null)
              throw new NoSuchElementException();
          return unlinkLast(l);
      }
  
      /**
       * Inserts the specified element at the beginning of this list.
       *
       * @param e the element to add
       */
      public void addFirst(E e) {
          linkFirst(e);
      }
  
      /**
       * Appends the specified element to the end of this list.
       *
       * <p>This method is equivalent to {@link #add}.
       *
       * @param e the element to add
       */
      public void addLast(E e) {
          linkLast(e);
      }
  
      /**
       * Returns {@code true} if this list contains the specified element.
       * More formally, returns {@code true} if and only if this list contains
       * at least one element {@code e} such that
       * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
       *
       * @param o element whose presence in this list is to be tested
       * @return {@code true} if this list contains the specified element
       */
      public boolean contains(Object o) {
          return indexOf(o) != -1;
      }
  
      /**
       * Returns the number of elements in this list.
       *
       * @return the number of elements in this list
       */
      public int size() {
          return size;
      }
  
      /**
       * Appends the specified element to the end of this list.
       *
       * <p>This method is equivalent to {@link #addLast}.
       *
       * @param e element to be appended to this list
       * @return {@code true} (as specified by {@link Collection#add})
       */
      public boolean add(E e) {
          linkLast(e);
          return true;
      }
  
      /**
       * Removes the first occurrence of the specified element from this list,
       * if it is present.  If this list does not contain the element, it is
       * unchanged.  More formally, removes the element with the lowest index
       * {@code i} such that
       * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
       * (if such an element exists).  Returns {@code true} if this list
       * contained the specified element (or equivalently, if this list
       * changed as a result of the call).
       *
       * @param o element to be removed from this list, if present
       * @return {@code true} if this list contained the specified element
       */
      public boolean remove(Object o) {
          if (o == null) {
              for (Node<E> x = first; x != null; x = x.next) {
                  if (x.item == null) {
                      unlink(x);
                      return true;
                  }
              }
          } else {
              for (Node<E> x = first; x != null; x = x.next) {
                  if (o.equals(x.item)) {
                      unlink(x);
                      return true;
                  }
              }
          }
          return false;
      }
  
      /**
       * Appends all of the elements in the specified collection to the end of
       * this list, in the order that they are returned by the specified
       * collection's iterator.  The behavior of this operation is undefined if
       * the specified collection is modified while the operation is in
       * progress.  (Note that this will occur if the specified collection is
       * this list, and it's nonempty.)
       *
       * @param c collection containing elements to be added to this list
       * @return {@code true} if this list changed as a result of the call
       * @throws NullPointerException if the specified collection is null
       */
      public boolean addAll(Collection<? extends E> c) {
          return addAll(size, c);
      }
  
      /**
       * Inserts all of the elements in the specified collection into this
       * list, starting at the specified position.  Shifts the element
       * currently at that position (if any) and any subsequent elements to
       * the right (increases their indices).  The new elements will appear
       * in the list in the order that they are returned by the
       * specified collection's iterator.
       *
       * @param index index at which to insert the first element
       *              from the specified collection
       * @param c collection containing elements to be added to this list
       * @return {@code true} if this list changed as a result of the call
       * @throws IndexOutOfBoundsException {@inheritDoc}
       * @throws NullPointerException if the specified collection is null
       */
      public boolean addAll(int index, Collection<? extends E> c) {
          checkPositionIndex(index);
  
          Object[] a = c.toArray();
          int numNew = a.length;
          if (numNew == 0)
              return false;
  
          Node<E> pred, succ;
          if (index == size) {
              succ = null;
              pred = last;
          } else {
              succ = node(index);
              pred = succ.prev;
          }
  
          for (Object o : a) {
              @SuppressWarnings("unchecked") E e = (E) o;
              Node<E> newNode = new Node<>(pred, e, null);
              if (pred == null)
                  first = newNode;
              else
                  pred.next = newNode;
              pred = newNode;
          }
  
          if (succ == null) {
              last = pred;
          } else {
              pred.next = succ;
              succ.prev = pred;
          }
  
          size += numNew;
          modCount++;
          return true;
      }
  
      /**
       * Removes all of the elements from this list.
       * The list will be empty after this call returns.
       */
      public void clear() {
          // Clearing all of the links between nodes is "unnecessary", but:
          // - helps a generational GC if the discarded nodes inhabit
          //   more than one generation
          // - is sure to free memory even if there is a reachable Iterator
          for (Node<E> x = first; x != null; ) {
              Node<E> next = x.next;
              x.item = null;
              x.next = null;
              x.prev = null;
              x = next;
          }
          first = last = null;
          size = 0;
          modCount++;
      }
  
  
      // Positional Access Operations
  
      /**
       * Returns the element at the specified position in this list.
       *
       * @param index index of the element to return
       * @return the element at the specified position in this list
       * @throws IndexOutOfBoundsException {@inheritDoc}
       */
      public E get(int index) {
          checkElementIndex(index);
          return node(index).item;
      }
  
      /**
       * Replaces the element at the specified position in this list with the
       * specified element.
       *
       * @param index index of the element to replace
       * @param element element to be stored at the specified position
       * @return the element previously at the specified position
       * @throws IndexOutOfBoundsException {@inheritDoc}
       */
      public E set(int index, E element) {
          checkElementIndex(index);
          Node<E> x = node(index);
          E oldVal = x.item;
          x.item = element;
          return oldVal;
      }
  
      /**
       * Inserts the specified element at the specified position in this list.
       * Shifts the element currently at that position (if any) and any
       * subsequent elements to the right (adds one to their indices).
       *
       * @param index index at which the specified element is to be inserted
       * @param element element to be inserted
       * @throws IndexOutOfBoundsException {@inheritDoc}
       */
      public void add(int index, E element) {
          checkPositionIndex(index);
  
          if (index == size)
              linkLast(element);
          else
              linkBefore(element, node(index));
      }
  
      /**
       * Removes the element at the specified position in this list.  Shifts any
       * subsequent elements to the left (subtracts one from their indices).
       * Returns the element that was removed from the list.
       *
       * @param index the index of the element to be removed
       * @return the element previously at the specified position
       * @throws IndexOutOfBoundsException {@inheritDoc}
       */
      public E remove(int index) {
          checkElementIndex(index);
          return unlink(node(index));
      }
  
      /**
       * Tells if the argument is the index of an existing element.
       */
      private boolean isElementIndex(int index) {
          return index >= 0 && index < size;
      }
  
      /**
       * Tells if the argument is the index of a valid position for an
       * iterator or an add operation.
       */
      private boolean isPositionIndex(int index) {
          return index >= 0 && index <= size;
      }
  
      /**
       * Constructs an IndexOutOfBoundsException detail message.
       * Of the many possible refactorings of the error handling code,
       * this "outlining" performs best with both server and client VMs.
       */
      private String outOfBoundsMsg(int index) {
          return "Index: "+index+", Size: "+size;
      }
  
      private void checkElementIndex(int index) {
          if (!isElementIndex(index))
              throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
      }
  
      private void checkPositionIndex(int index) {
          if (!isPositionIndex(index))
              throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
      }
  
      /**
       * Returns the (non-null) Node at the specified element index.
       */
      Node<E> node(int index) {
          // assert isElementIndex(index);
  
          if (index < (size >> 1)) {
              Node<E> x = first;
              for (int i = 0; i < index; i++)
                  x = x.next;
              return x;
          } else {
              Node<E> x = last;
              for (int i = size - 1; i > index; i--)
                  x = x.prev;
              return x;
          }
      }
  
      // Search Operations
  
      /**
       * Returns the index of the first occurrence of the specified element
       * in this list, or -1 if this list does not contain the element.
       * More formally, returns the lowest index {@code i} such that
       * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
       * or -1 if there is no such index.
       *
       * @param o element to search for
       * @return the index of the first occurrence of the specified element in
       *         this list, or -1 if this list does not contain the element
       */
      public int indexOf(Object o) {
          int index = 0;
          if (o == null) {
              for (Node<E> x = first; x != null; x = x.next) {
                  if (x.item == null)
                      return index;
                  index++;
              }
          } else {
              for (Node<E> x = first; x != null; x = x.next) {
                  if (o.equals(x.item))
                      return index;
                  index++;
              }
          }
          return -1;
      }
  
      /**
       * Returns the index of the last occurrence of the specified element
       * in this list, or -1 if this list does not contain the element.
       * More formally, returns the highest index {@code i} such that
       * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
       * or -1 if there is no such index.
       *
       * @param o element to search for
       * @return the index of the last occurrence of the specified element in
       *         this list, or -1 if this list does not contain the element
       */
      public int lastIndexOf(Object o) {
          int index = size;
          if (o == null) {
              for (Node<E> x = last; x != null; x = x.prev) {
                  index--;
                  if (x.item == null)
                      return index;
              }
          } else {
              for (Node<E> x = last; x != null; x = x.prev) {
                  index--;
                  if (o.equals(x.item))
                      return index;
              }
          }
          return -1;
      }
  
      // Queue operations.
  
      /**
       * Retrieves, but does not remove, the head (first element) of this list.
       *
       * @return the head of this list, or {@code null} if this list is empty
       * @since 1.5
       */
      public E peek() {
          final Node<E> f = first;
          return (f == null) ? null : f.item;
      }
  
      /**
       * Retrieves, but does not remove, the head (first element) of this list.
       *
       * @return the head of this list
       * @throws NoSuchElementException if this list is empty
       * @since 1.5
       */
      public E element() {
          return getFirst();
      }
  
      /**
       * Retrieves and removes the head (first element) of this list.
       *
       * @return the head of this list, or {@code null} if this list is empty
       * @since 1.5
       */
      public E poll() {
          final Node<E> f = first;
          return (f == null) ? null : unlinkFirst(f);
      }
  
      /**
       * Retrieves and removes the head (first element) of this list.
       *
       * @return the head of this list
       * @throws NoSuchElementException if this list is empty
       * @since 1.5
       */
      public E remove() {
          return removeFirst();
      }
  
      /**
       * Adds the specified element as the tail (last element) of this list.
       *
       * @param e the element to add
       * @return {@code true} (as specified by {@link Queue#offer})
       * @since 1.5
       */
      public boolean offer(E e) {
          return add(e);
      }
  
      // Deque operations
      /**
       * Inserts the specified element at the front of this list.
       *
       * @param e the element to insert
       * @return {@code true} (as specified by {@link Deque#offerFirst})
       * @since 1.6
       */
      public boolean offerFirst(E e) {
          addFirst(e);
          return true;
      }
  
      /**
       * Inserts the specified element at the end of this list.
       *
       * @param e the element to insert
       * @return {@code true} (as specified by {@link Deque#offerLast})
       * @since 1.6
       */
      public boolean offerLast(E e) {
          addLast(e);
          return true;
      }
  
      /**
       * Retrieves, but does not remove, the first element of this list,
       * or returns {@code null} if this list is empty.
       *
       * @return the first element of this list, or {@code null}
       *         if this list is empty
       * @since 1.6
       */
      public E peekFirst() {
          final Node<E> f = first;
          return (f == null) ? null : f.item;
       }
  
      /**
       * Retrieves, but does not remove, the last element of this list,
       * or returns {@code null} if this list is empty.
       *
       * @return the last element of this list, or {@code null}
       *         if this list is empty
       * @since 1.6
       */
      public E peekLast() {
          final Node<E> l = last;
          return (l == null) ? null : l.item;
      }
  
      /**
       * Retrieves and removes the first element of this list,
       * or returns {@code null} if this list is empty.
       *
       * @return the first element of this list, or {@code null} if
       *     this list is empty
       * @since 1.6
       */
      public E pollFirst() {
          final Node<E> f = first;
          return (f == null) ? null : unlinkFirst(f);
      }
  
      /**
       * Retrieves and removes the last element of this list,
       * or returns {@code null} if this list is empty.
       *
       * @return the last element of this list, or {@code null} if
       *     this list is empty
       * @since 1.6
       */
      public E pollLast() {
          final Node<E> l = last;
          return (l == null) ? null : unlinkLast(l);
      }
  
      /**
       * Pushes an element onto the stack represented by this list.  In other
       * words, inserts the element at the front of this list.
       *
       * <p>This method is equivalent to {@link #addFirst}.
       *
       * @param e the element to push
       * @since 1.6
       */
      public void push(E e) {
          addFirst(e);
      }
  
      /**
       * Pops an element from the stack represented by this list.  In other
       * words, removes and returns the first element of this list.
       *
       * <p>This method is equivalent to {@link #removeFirst()}.
       *
       * @return the element at the front of this list (which is the top
       *         of the stack represented by this list)
       * @throws NoSuchElementException if this list is empty
       * @since 1.6
       */
      public E pop() {
          return removeFirst();
      }
  
      /**
       * Removes the first occurrence of the specified element in this
       * list (when traversing the list from head to tail).  If the list
       * does not contain the element, it is unchanged.
       *
       * @param o element to be removed from this list, if present
       * @return {@code true} if the list contained the specified element
       * @since 1.6
       */
      public boolean removeFirstOccurrence(Object o) {
          return remove(o);
      }
  
      /**
       * Removes the last occurrence of the specified element in this
       * list (when traversing the list from head to tail).  If the list
       * does not contain the element, it is unchanged.
       *
       * @param o element to be removed from this list, if present
       * @return {@code true} if the list contained the specified element
       * @since 1.6
       */
      public boolean removeLastOccurrence(Object o) {
          if (o == null) {
              for (Node<E> x = last; x != null; x = x.prev) {
                  if (x.item == null) {
                      unlink(x);
                      return true;
                  }
              }
          } else {
              for (Node<E> x = last; x != null; x = x.prev) {
                  if (o.equals(x.item)) {
                      unlink(x);
                      return true;
                  }
              }
          }
          return false;
      }
  
      /**
       * Returns a list-iterator of the elements in this list (in proper
       * sequence), starting at the specified position in the list.
       * Obeys the general contract of {@code List.listIterator(int)}.<p>
       *
       * The list-iterator is <i>fail-fast</i>: if the list is structurally
       * modified at any time after the Iterator is created, in any way except
       * through the list-iterator's own {@code remove} or {@code add}
       * methods, the list-iterator will throw a
       * {@code 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.
       *
       * @param index index of the first element to be returned from the
       *              list-iterator (by a call to {@code next})
       * @return a ListIterator of the elements in this list (in proper
       *         sequence), starting at the specified position in the list
       * @throws IndexOutOfBoundsException {@inheritDoc}
       * @see List#listIterator(int)
       */
      public ListIterator<E> listIterator(int index) {
          checkPositionIndex(index);
          return new ListItr(index);
      }
  
      private class ListItr implements ListIterator<E> {
          private Node<E> lastReturned;
          private Node<E> next;
          private int nextIndex;
          private int expectedModCount = modCount;
  
          ListItr(int index) {
              // assert isPositionIndex(index);
              next = (index == size) ? null : node(index);
              nextIndex = index;
          }
  
          public boolean hasNext() {
              return nextIndex < size;
          }
  
          public E next() {
              checkForComodification();
              if (!hasNext())
                  throw new NoSuchElementException();
  
              lastReturned = next;
              next = next.next;
              nextIndex++;
              return lastReturned.item;
          }
  
          public boolean hasPrevious() {
              return nextIndex > 0;
          }
  
          public E previous() {
              checkForComodification();
              if (!hasPrevious())
                  throw new NoSuchElementException();
  
              lastReturned = next = (next == null) ? last : next.prev;
              nextIndex--;
              return lastReturned.item;
          }
  
          public int nextIndex() {
              return nextIndex;
          }
  
          public int previousIndex() {
              return nextIndex - 1;
          }
  
          public void remove() {
              checkForComodification();
              if (lastReturned == null)
                  throw new IllegalStateException();
  
              Node<E> lastNext = lastReturned.next;
              unlink(lastReturned);
              if (next == lastReturned)
                  next = lastNext;
              else
                  nextIndex--;
              lastReturned = null;
              expectedModCount++;
          }
  
          public void set(E e) {
              if (lastReturned == null)
                  throw new IllegalStateException();
              checkForComodification();
              lastReturned.item = e;
          }
  
          public void add(E e) {
              checkForComodification();
              lastReturned = null;
              if (next == null)
                  linkLast(e);
              else
                  linkBefore(e, next);
              nextIndex++;
              expectedModCount++;
          }
  
          public void forEachRemaining(Consumer<? super E> action) {
              Objects.requireNonNull(action);
              while (modCount == expectedModCount && nextIndex < size) {
                  action.accept(next.item);
                  lastReturned = next;
                  next = next.next;
                  nextIndex++;
              }
              checkForComodification();
          }
  
          final void checkForComodification() {
              if (modCount != expectedModCount)
                  throw new ConcurrentModificationException();
          }
      }
  
      private static class Node<E> {
          E item;
          Node<E> next;
          Node<E> prev;
  
          Node(Node<E> prev, E element, Node<E> next) {
              this.item = element;
              this.next = next;
              this.prev = prev;
          }
      }
  
      /**
       * @since 1.6
       */
      public Iterator<E> descendingIterator() {
          return new DescendingIterator();
      }
  
      /**
       * Adapter to provide descending iterators via ListItr.previous
       */
      private class DescendingIterator implements Iterator<E> {
          private final ListItr itr = new ListItr(size());
          public boolean hasNext() {
              return itr.hasPrevious();
          }
          public E next() {
              return itr.previous();
          }
          public void remove() {
              itr.remove();
          }
      }
  
      @SuppressWarnings("unchecked")
      private LinkedList<E> superClone() {
          try {
              return (LinkedList<E>) super.clone();
          } catch (CloneNotSupportedException e) {
              throw new InternalError(e);
          }
      }
  
      /**
       * Returns a shallow copy of this {@code LinkedList}. (The elements
       * themselves are not cloned.)
       *
       * @return a shallow copy of this {@code LinkedList} instance
       */
      public Object clone() {
          LinkedList<E> clone = superClone();
  
          // Put clone into "virgin" state
          clone.first = clone.last = null;
          clone.size = 0;
          clone.modCount = 0;
  
          // Initialize clone with our elements
          for (Node<E> x = first; x != null; x = x.next)
              clone.add(x.item);
  
          return clone;
      }
  
      /**
       * Returns an array containing all of the elements in this list
       * 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 list.  (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 list
       *         in proper sequence
       */
      public Object[] toArray() {
          Object[] result = new Object[size];
          int i = 0;
          for (Node<E> x = first; x != null; x = x.next)
              result[i++] = x.item;
          return result;
      }
  
      /**
       * Returns an array containing all of the elements in this list in
       * proper sequence (from first to last element); the runtime type of
       * the returned array is that of the specified array.  If the list 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 list.
       *
       * <p>If the list fits in the specified array with room to spare (i.e.,
       * the array has more elements than the list), the element in the array
       * immediately following the end of the list is set to {@code null}.
       * (This is useful in determining the length of the list <i>only</i> if
       * the caller knows that the list does not contain any null elements.)
       *
       * <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 list known to contain only strings.
       * The following code can be used to dump the list into a newly
       * allocated array of {@code String}:
       *
       * <pre>
       *     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 list 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 the elements of the list
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in
       *         this list
       * @throws NullPointerException if the specified array is null
       */
      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
          if (a.length < size)
              a = (T[])java.lang.reflect.Array.newInstance(
                                  a.getClass().getComponentType(), size);
          int i = 0;
          Object[] result = a;
          for (Node<E> x = first; x != null; x = x.next)
              result[i++] = x.item;
  
          if (a.length > size)
              a[size] = null;
  
          return a;
      }
  
      private static final long serialVersionUID = 876323262645176354L;
  
      /**
       * Saves the state of this {@code LinkedList} instance to a stream
       * (that is, serializes it).
       *
       * @serialData The size of the list (the number of elements it
       *             contains) is emitted (int), followed by all of its
       *             elements (each an Object) in the proper order.
       */
      private void writeObject(java.io.ObjectOutputStream s)
          throws java.io.IOException {
          // Write out any hidden serialization magic
          s.defaultWriteObject();
  
          // Write out size
          s.writeInt(size);
  
          // Write out all elements in the proper order.
          for (Node<E> x = first; x != null; x = x.next)
              s.writeObject(x.item);
      }
  
      /**
       * Reconstitutes this {@code LinkedList} instance from a stream
       * (that is, deserializes it).
       */
      @SuppressWarnings("unchecked")
      private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          // Read in any hidden serialization magic
          s.defaultReadObject();
  
          // Read in size
          int size = s.readInt();
  
          // Read in all elements in the proper order.
          for (int i = 0; i < size; i++)
              linkLast((E)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
       * list.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
       * {@link Spliterator#ORDERED}.  Overriding implementations should document
       * the reporting of additional characteristic values.
       *
       * @implNote
       * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
       * and implements {@code trySplit} to permit limited parallelism..
       *
       * @return a {@code Spliterator} over the elements in this list
       * @since 1.8
       */
      @Override
      public Spliterator<E> spliterator() {
          return new LLSpliterator<E>(this, -1, 0);
      }
  
      /** A customized variant of Spliterators.IteratorSpliterator */
      static final class LLSpliterator<E> implements Spliterator<E> {
          static final int BATCH_UNIT = 1 << 10;  // batch array size increment
          static final int MAX_BATCH = 1 << 25;  // max batch array size;
          final LinkedList<E> list; // null OK unless traversed
          Node<E> current;      // current node; null until initialized
          int est;              // size estimate; -1 until first needed
          int expectedModCount; // initialized when est set
          int batch;            // batch size for splits
  
          LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
              this.list = list;
              this.est = est;
              this.expectedModCount = expectedModCount;
          }
  
          final int getEst() {
              int s; // force initialization
              final LinkedList<E> lst;
              if ((s = est) < 0) {
                  if ((lst = list) == null)
                      s = est = 0;
                  else {
                      expectedModCount = lst.modCount;
                      current = lst.first;
                      s = est = lst.size;
                  }
              }
              return s;
          }
  
          public long estimateSize() { return (long) getEst(); }
  
          public Spliterator<E> trySplit() {
              Node<E> p;
              int s = getEst();
              if (s > 1 && (p = current) != null) {
                  int n = batch + BATCH_UNIT;
                  if (n > s)
                      n = s;
                  if (n > MAX_BATCH)
                      n = MAX_BATCH;
                  Object[] a = new Object[n];
                  int j = 0;
                  do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
                  current = p;
                  batch = j;
                  est = s - j;
                  return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
              }
              return null;
          }
  
          public void forEachRemaining(Consumer<? super E> action) {
              Node<E> p; int n;
              if (action == null) throw new NullPointerException();
              if ((n = getEst()) > 0 && (p = current) != null) {
                  current = null;
                  est = 0;
                  do {
                      E e = p.item;
                      p = p.next;
                      action.accept(e);
                  } while (p != null && --n > 0);
              }
              if (list.modCount != expectedModCount)
                  throw new ConcurrentModificationException();
          }
  
          public boolean tryAdvance(Consumer<? super E> action) {
              Node<E> p;
              if (action == null) throw new NullPointerException();
              if (getEst() > 0 && (p = current) != null) {
                  --est;
                  E e = p.item;
                  current = p.next;
                  action.accept(e);
                  if (list.modCount != expectedModCount)
                      throw new ConcurrentModificationException();
                  return true;
              }
              return false;
          }
  
          public int characteristics() {
              return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
          }
      }
  
  }
  