/*
 * Copyright (c) 1997, 2006, 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;

Linked list implementation of the List interface.  Implements all
optional list operations, and permits all elements (including
null).  In addition to implementing the List interface,
the LinkedList class provides uniformly named methods to
get, remove and insert an element at the beginning and end of the list. These operations allow linked lists to be used as a stack, queue, or 
double-ended queue. 

The class implements the Deque interface, providing
first-in-first-out queue operations for add,
poll, along with other stack and deque operations.

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.

Note that this implementation is not synchronized.
If multiple threads access a linked list concurrently, and at least
one of the threads modifies the list structurally, it must 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 Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:
  List list = Collections.synchronizedList(new LinkedList(...));
The iterators returned by this class's iterator and
listIterator methods are fail-fast: if the list is
structurally modified at any time after the iterator is created, in
any way except through the Iterator's own remove or
add methods, the iterator will throw a 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. 
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 ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness:   the fail-fast behavior of iterators
should be used only to detect bugs.
This class is a member of the

Java Collections Framework.
Author:
 Josh Bloch
Type parameters:
<E> – the type of elements held in this collection
See Also:
List
ArrayList
Vector
Since:
1.2/**
 * Linked list implementation of the <tt>List</tt> interface.  Implements all
 * optional list operations, and permits all elements (including
 * <tt>null</tt>).  In addition to implementing the <tt>List</tt> interface,
 * the <tt>LinkedList</tt> class provides uniformly named methods to
 * <tt>get</tt>, <tt>remove</tt> and <tt>insert</tt> an element at the
 * beginning and end of the list.  These operations allow linked lists to be
 * used as a stack, {@linkplain Queue queue}, or {@linkplain Deque
 * double-ended queue}. <p>
 *
 * The class implements the <tt>Deque</tt> interface, providing
 * first-in-first-out queue operations for <tt>add</tt>,
 * <tt>poll</tt>, along with other stack and deque operations.<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>
 *
 * <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 <tt>iterator</tt> and
 * <tt>listIterator</tt> 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 <tt>remove</tt> or
 * <tt>add</tt> 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 <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @see     List
 * @see     ArrayList
 * @see     Vector
 * @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
{
    private transient Entry<E> header = new Entry<E>(null, null, null);
    private transient int size = 0;

    
Constructs an empty list.
/**
     * Constructs an empty list.
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

    
Constructs a list containing the elements of the specified
collection, in the order they are returned by the collection's
iterator.
Params:
c – the collection whose elements are to be placed into this list
Throws:
NullPointerException – if the specified collection is null/**
     * 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);
    }

    
Returns the first element in this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the first element in this list/**
     * Returns the first element in this list.
     *
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getFirst() {
        if (size==0)
            throw new NoSuchElementException();

        return header.next.element;
    }

    
Returns the last element in this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the last element in this list/**
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast()  {
        if (size==0)
            throw new NoSuchElementException();

        return header.previous.element;
    }

    
Removes and returns the first element from this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the first element from this list/**
     * 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() {
        return remove(header.next);
    }

    
Removes and returns the last element from this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the last element from this list/**
     * 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() {
        return remove(header.previous);
    }

    
Inserts the specified element at the beginning of this list.
Params:
e – the element to add/**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        addBefore(e, header.next);
    }

    
Appends the specified element to the end of this list.
This method is equivalent to add. 
Params:
e – the element to add/**
     * 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) {
        addBefore(e, header);
    }

    
Returns true if this list contains the specified element.
More formally, returns true if and only if this list contains
at least one element e such that
(o==null ? e==null : o.equals(e)).
Params:
o – element whose presence in this list is to be tested
Returns:
true if this list contains the specified element/**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> 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 <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    
Returns the number of elements in this list.
Returns:
the number of elements in this list/**
     * 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.
This method is equivalent to addLast. 
Params:
e – element to be appended to this list
Returns:
true (as specified by Collection.add)/**
     * 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 <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        addBefore(e, header);
        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
i such that
(o==null ? get(i)==null : o.equals(get(i)))
(if such an element exists).  Returns true if this list
contained the specified element (or equivalently, if this list
changed as a result of the call).
Params:
o – element to be removed from this list, if present
Returns:
true if this list contained the specified element/**
     * 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
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> 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 <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o==null) {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (o.equals(e.element)) {
                    remove(e);
                    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.)
Params:
c – collection containing elements to be added to this list
Throws:
NullPointerException – if the specified collection is null
Returns:
true if this list changed as a result of the call/**
     * 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 <tt>true</tt> 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.
Params:
index – index at which to insert the first element
             from the specified collection
c – collection containing elements to be added to this list
Throws:
IndexOutOfBoundsException – {@inheritDoc}
NullPointerException – if the specified collection is null
Returns:
true if this list changed as a result of the call/**
     * 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 <tt>true</tt> 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) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew==0)
            return false;
        modCount++;

        Entry<E> successor = (index==size ? header : entry(index));
        Entry<E> predecessor = successor.previous;
        for (int i=0; i<numNew; i++) {
            Entry<E> e = new Entry<E>((E)a[i], successor, predecessor);
            predecessor.next = e;
            predecessor = e;
        }
        successor.previous = predecessor;

        size += numNew;
        return true;
    }

    
Removes all of the elements from this list.
/**
     * Removes all of the elements from this list.
     */
    public void clear() {
        Entry<E> e = header.next;
        while (e != header) {
            Entry<E> next = e.next;
            e.next = e.previous = null;
            e.element = null;
            e = next;
        }
        header.next = header.previous = header;
        size = 0;
        modCount++;
    }


    // Positional Access Operations

    
Returns the element at the specified position in this list.
Params:
index – index of the element to return
Throws:
IndexOutOfBoundsException – {@inheritDoc}
Returns:
the element at the specified position in this list/**
     * 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) {
        return entry(index).element;
    }

    
Replaces the element at the specified position in this list with the
specified element.
Params:
index – index of the element to replace
element – element to be stored at the specified position
Throws:
IndexOutOfBoundsException – {@inheritDoc}
Returns:
the element previously at the specified position/**
     * 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) {
        Entry<E> e = entry(index);
        E oldVal = e.element;
        e.element = 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).
Params:
index – index at which the specified element is to be inserted
element – element to be inserted
Throws:
IndexOutOfBoundsException – {@inheritDoc}/**
     * 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) {
        addBefore(element, (index==size ? header : entry(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.
Params:
index – the index of the element to be removed
Throws:
IndexOutOfBoundsException – {@inheritDoc}
Returns:
the element previously at the specified position/**
     * 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) {
        return remove(entry(index));
    }

    
Returns the indexed entry.
/**
     * Returns the indexed entry.
     */
    private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: "+index+
                                                ", Size: "+size);
        Entry<E> e = header;
        if (index < (size >> 1)) {
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else {
            for (int i = size; i > index; i--)
                e = e.previous;
        }
        return e;
    }


    // 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 i such that
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
Params:
o – element to search for
Returns:
the index of the first occurrence of the specified element in
        this list, or -1 if this list does not contain the element/**
     * 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 <tt>i</tt> 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 (Entry e = header.next; e != header; e = e.next) {
                if (e.element==null)
                    return index;
                index++;
            }
        } else {
            for (Entry e = header.next; e != header; e = e.next) {
                if (o.equals(e.element))
                    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 i such that
(o==null ? get(i)==null : o.equals(get(i))),
or -1 if there is no such index.
Params:
o – element to search for
Returns:
the index of the last occurrence of the specified element in
        this list, or -1 if this list does not contain the element/**
     * 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 <tt>i</tt> 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 (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (e.element==null)
                    return index;
            }
        } else {
            for (Entry e = header.previous; e != header; e = e.previous) {
                index--;
                if (o.equals(e.element))
                    return index;
            }
        }
        return -1;
    }

    // Queue operations.

    
Retrieves, but does not remove, the head (first element) of this list.
Returns:
the head of this list, or null if this list is empty
Since:
1.5/**
     * Retrieves, but does not remove, the head (first element) of this list.
     * @return the head of this list, or <tt>null</tt> if this list is empty
     * @since 1.5
     */
    public E peek() {
        if (size==0)
            return null;
        return getFirst();
    }

    
Retrieves, but does not remove, the head (first element) of this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the head of this list
Since:
1.5/**
     * 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
Returns:
the head of this list, or null if this list is empty
Since:
1.5/**
     * Retrieves and removes the head (first element) of this list
     * @return the head of this list, or <tt>null</tt> if this list is empty
     * @since 1.5
     */
    public E poll() {
        if (size==0)
            return null;
        return removeFirst();
    }

    
Retrieves and removes the head (first element) of this list.
Throws:
NoSuchElementException – if this list is empty
Returns:
the head of this list
Since:
1.5/**
     * 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.
Params:
e – the element to add
Returns:
true (as specified by Queue.offer)
Since:
1.5/**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return <tt>true</tt> (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.
Params:
e – the element to insert
Returns:
true (as specified by Deque.offerFirst)
Since:
1.6/**
     * Inserts the specified element at the front of this list.
     *
     * @param e the element to insert
     * @return <tt>true</tt> (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.
Params:
e – the element to insert
Returns:
true (as specified by Deque.offerLast)
Since:
1.6/**
     * Inserts the specified element at the end of this list.
     *
     * @param e the element to insert
     * @return <tt>true</tt> (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 null if this list is empty.
Returns:
the first element of this list, or null
        if this list is empty
Since:
1.6/**
     * Retrieves, but does not remove, the first element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the first element of this list, or <tt>null</tt>
     *         if this list is empty
     * @since 1.6
     */
    public E peekFirst() {
        if (size==0)
            return null;
        return getFirst();
    }

    
Retrieves, but does not remove, the last element of this list,
or returns null if this list is empty.
Returns:
the last element of this list, or null
        if this list is empty
Since:
1.6/**
     * Retrieves, but does not remove, the last element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the last element of this list, or <tt>null</tt>
     *         if this list is empty
     * @since 1.6
     */
    public E peekLast() {
        if (size==0)
            return null;
        return getLast();
    }

    
Retrieves and removes the first element of this list,
or returns null if this list is empty.
Returns:
the first element of this list, or null if
    this list is empty
Since:
1.6/**
     * Retrieves and removes the first element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the first element of this list, or <tt>null</tt> if
     *     this list is empty
     * @since 1.6
     */
    public E pollFirst() {
        if (size==0)
            return null;
        return removeFirst();
    }

    
Retrieves and removes the last element of this list,
or returns null if this list is empty.
Returns:
the last element of this list, or null if
    this list is empty
Since:
1.6/**
     * Retrieves and removes the last element of this list,
     * or returns <tt>null</tt> if this list is empty.
     *
     * @return the last element of this list, or <tt>null</tt> if
     *     this list is empty
     * @since 1.6
     */
    public E pollLast() {
        if (size==0)
            return null;
        return removeLast();
    }

    
Pushes an element onto the stack represented by this list.  In other
words, inserts the element at the front of this list.
This method is equivalent to addFirst. 
Params:
e – the element to push
Since:
1.6/**
     * 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.
This method is equivalent to removeFirst(). 
Throws:
NoSuchElementException – if this list is empty
Returns:
the element at the front of this list (which is the top
        of the stack represented by this list)
Since:
1.6/**
     * 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.
Params:
o – element to be removed from this list, if present
Returns:
true if the list contained the specified element
Since:
1.6/**
     * 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 <tt>true</tt> 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.
Params:
o – element to be removed from this list, if present
Returns:
true if the list contained the specified element
Since:
1.6/**
     * 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 <tt>true</tt> if the list contained the specified element
     * @since 1.6
     */
    public boolean removeLastOccurrence(Object o) {
        if (o==null) {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else {
            for (Entry<E> e = header.previous; e != header; e = e.previous) {
                if (o.equals(e.element)) {
                    remove(e);
                    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 List.listIterator(int).

The list-iterator is fail-fast: if the list is structurally
modified at any time after the Iterator is created, in any way except
through the list-iterator's own remove or add
methods, the list-iterator will throw a
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.
Params:
index – index of the first element to be returned from the
             list-iterator (by a call to next)
Throws:
IndexOutOfBoundsException – {@inheritDoc}
See Also:
List.listIterator(int)
Returns:
a ListIterator of the elements in this list (in proper
        sequence), starting at the specified position in the list/**
     * 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 <tt>List.listIterator(int)</tt>.<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 <tt>remove</tt> or <tt>add</tt>
     * methods, the list-iterator will throw a
     * <tt>ConcurrentModificationException</tt>.  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 <tt>next</tt>)
     * @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) {
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Entry<E> lastReturned = header;
        private Entry<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            if (index < 0 || index > size)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ", Size: "+size);
            if (index < (size >> 1)) {
                next = header.next;
                for (nextIndex=0; nextIndex<index; nextIndex++)
                    next = next.next;
            } else {
                next = header;
                for (nextIndex=size; nextIndex>index; nextIndex--)
                    next = next.previous;
            }
        }

        public boolean hasNext() {
            return nextIndex != size;
        }

        public E next() {
            checkForComodification();
            if (nextIndex == size)
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.element;
        }

        public boolean hasPrevious() {
            return nextIndex != 0;
        }

        public E previous() {
            if (nextIndex == 0)
                throw new NoSuchElementException();

            lastReturned = next = next.previous;
            nextIndex--;
            checkForComodification();
            return lastReturned.element;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex-1;
        }

        public void remove() {
            checkForComodification();
            Entry<E> lastNext = lastReturned.next;
            try {
                LinkedList.this.remove(lastReturned);
            } catch (NoSuchElementException e) {
                throw new IllegalStateException();
            }
            if (next==lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = header;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == header)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.element = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = header;
            addBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private static class Entry<E> {
        E element;
        Entry<E> next;
        Entry<E> previous;

        Entry(E element, Entry<E> next, Entry<E> previous) {
            this.element = element;
            this.next = next;
            this.previous = previous;
        }
    }

    private Entry<E> addBefore(E e, Entry<E> entry) {
        Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
        newEntry.previous.next = newEntry;
        newEntry.next.previous = newEntry;
        size++;
        modCount++;
        return newEntry;
    }

    private E remove(Entry<E> e) {
        if (e == header)
            throw new NoSuchElementException();

        E result = e.element;
        e.previous.next = e.next;
        e.next.previous = e.previous;
        e.next = e.previous = null;
        e.element = null;
        size--;
        modCount++;
        return result;
    }

    
Since:
1.6/**
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    
Adapter to provide descending iterators via ListItr.previous /** Adapter to provide descending iterators via ListItr.previous */
    private class DescendingIterator implements Iterator {
        final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }

    
Returns a shallow copy of this LinkedList. (The elements
themselves are not cloned.)
Returns:
a shallow copy of this LinkedList instance/**
     * Returns a shallow copy of this <tt>LinkedList</tt>. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this <tt>LinkedList</tt> instance
     */
    public Object clone() {
        LinkedList<E> clone = null;
        try {
            clone = (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError();
        }

        // Put clone into "virgin" state
        clone.header = new Entry<E>(null, null, null);
        clone.header.next = clone.header.previous = clone.header;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Entry<E> e = header.next; e != header; e = e.next)
            clone.add(e.element);

        return clone;
    }

    
Returns an array containing all of the elements in this list
in proper sequence (from first to last element).
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.
This method acts as bridge between array-based and collection-based
APIs.
Returns:
an array containing all of the elements in this list
        in proper sequence/**
     * 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 (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;
        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.
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 null.
(This is useful in determining the length of the list only if
the caller knows that the list does not contain any null elements.)
Like the 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. 
Suppose 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 String:
    String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to
toArray().
Params:
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.
Throws:
ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in
        this list
NullPointerException – if the specified array is null
Returns:
an array containing the elements of the list/**
     * 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 <tt>null</tt>.
     * (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 <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the 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
     */
    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 (Entry<E> e = header.next; e != header; e = e.next)
            result[i++] = e.element;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    
Save the state of this LinkedList instance to a stream (that
is, serialize 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./**
     * Save the state of this <tt>LinkedList</tt> instance to a stream (that
     * is, serialize 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 (Entry e = header.next; e != header; e = e.next)
            s.writeObject(e.element);
    }

    
Reconstitute this LinkedList instance from a stream (that is
deserialize it).
/**
     * Reconstitute this <tt>LinkedList</tt> instance from a stream (that is
     * deserialize it).
     */
    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();

        // Initialize header
        header = new Entry<E>(null, null, null);
        header.next = header.previous = header;

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++)
            addBefore((E)s.readObject(), header);
    }
}