java 源码解析--LinkedList

LinkedList

1.看源码前应该知道的知识点

/**
 * 
 */
package com.study.collection;

import java.util.NoSuchElementException;
/**   
 * 
 * @className ：LinkedListLearn  
 * @package : com.study.collection 
 * @Description ：Linked源码学习必备知识   
 * @author：lgf   
 * @date ：2012 三月 13  10:38:11          
 * @version : 1.0
 */
public class LinkedListLearn<E> {
	// 结构
	// 本身就是一个双向链表来实现LinkedList过程
	private transient Entry<E> header = new Entry<E>(null, null, null);
    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;
    	}
    }
    // List 大小
    private transient int size = 0;
    
    /**
     * @author lgf
     * @Description: 删除指定节点的方法
     * @CreateDate 2012 三月 13 10:48:33
     * @lastModified 2012 三月 13 10:48:33
     * @version 1.0
     * @param element 要删除的几点
     * @return
     */
    @SuppressWarnings("unused")
	private E remove(Entry<E> element) {
    	// 如果e就是header未默认头节点。则抛出异常，无法删除
		if (element == header)
		    throw new NoSuchElementException();
		
		// 保存要删除节点的信息
		E result = element.element;
		// 前节点连接上后几点
		element.previous.next = element.next;
		// 后节点连接上前节点
		element.next.previous = element.previous;
		// 移除指定点设置null
	    element.next = element.previous = null;
	    element.element = null;
		size--;
		return result;
    }
    
    /**
     * @author lgf
     * @Description: 在指定节点前插入新节点
     * @CreateDate 2012 三月 13 11:21:58
     * @lastModified 2012 三月 13 11:21:58
     * @version 1.0
     * @param e 要插入的元素
     * @param entry 要插入节点的前面
     * @return
     */
    @SuppressWarnings("unused")
	private Entry<E> addBefore(E e, Entry<E> entry) {
    	// 新建一个节点,同时设置其后面和前面的节点信息
    	Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
    	// 插入到新位置
    	newEntry.previous.next = newEntry;
    	newEntry.next.previous = newEntry;
    	size++;
    	return newEntry;
    }
    
    /**
     * @author lgf
     * @Description: 对linkedList进行遍历
     * @CreateDate 2012 三月 13 11:40:11
     * @lastModified 2012 三月 13 11:40:11
     * @version 1.0
     */
    public void foreach(){
    	for (Entry<E> e = header.next; e != header; e = e.next) {
    		//TODO:
        }
    }
    
    /**
     * @author lgf
     * @Description: 获得指定下标index的值
     * @CreateDate 2012 三月 13 11:42:39
     * @lastModified 2012 三月 13 11:42:39
     * @version 1.0
     * @param index
     * @return
     */
    private Entry<E> entry(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: "+size);
        Entry<E> e = header;
        // 判断index 于 size/2 的大小关系
        if (index < (size >> 1)) {// 如果index在偏后,则从头开始完后遍历
            for (int i = 0; i <= index; i++)
                e = e.next;
        } else { // 否则index偏前,则从头向前开始遍历
            for (int i = size; i > index; i--)
            	e = e.previous;
        }
        return e;
    }
    
	public static void main(String[] args) {
		
	}
}

2.源码

package java.util;
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;

    /**
     * 空构造函数,前后互联
     */
    public LinkedList() {
        header.next = header.previous = header;
    }

    /**
     *	构造一个包含指定 collection 中的元素的列表，
	 *	这些元素按其 collection 的迭代器返回的顺序排列
     * @param  c 要将其元素放入此列表的 collection 
     * @throws NullPointerException 如果指定的 collection 为 null
     */
    public LinkedList(Collection<? extends E> c) {
		this();
		addAll(c);
    }

    /**
     * 获得第一个元素的信息
     * @return 此列表的第一个元素 
     * @throws NoSuchElementException 如果此列表为空
     */
    public E getFirst() {
		if (size==0)
			throw new NoSuchElementException();
		return header.next.element;
    }

    /**
     * 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.
     * 移除第一个
     * @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.
     * 移除最后一个
     * @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.
     * 添加第一个
     * @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.
     * 添加最后一个
     * <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 trueif and only if this list contains
     * at least one element <tt>e</tt> such that
     * (o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))
     * 判断某对象是否存在，存在则true
     * @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.
     * 大小
     * @return the number of elements in this list
     */
    public int size() {
		return size;
    }

    /**
     * Appends the specified element to the end of this list.
     * 添加一个元素，等同于addLast(E e)
     * <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
     * <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).
     * 从此列表中移除首次出现的指定元素（如果存在）。如果列表不包含该元素，则不作更改。
	 * 注意是首次出现的元素，null则查找首次出现的null
     * @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) { //元素为null
            for (Entry<E> e = header.next; e != header; e = e.next) {
                if (e.element==null) {
                    remove(e);
                    return true;
                }
            }
        } else { // 非null
            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.)
     * 加指定 collection 中的所有元素到此列表的结尾，
	 * 顺序是指定 collection 的迭代器返回这些元素的顺序 c.toArray()
     * @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.
     * 将指定 collection 中的所有元素从指定位置开始插入此列表。
	 * 移动当前在该位置上的元素（如果有），所有后续元素都向右移（增加其索引）。
	 * 新元素将按由指定 collection 的迭代器返回的顺序在列表中显示。 
     * @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++;
		
		// 要插入节点的位置
        Entry<E> successor = (index==size ? header : entry(index));
		//要插入节点位置前一个
        Entry<E> predecessor = successor.previous;
		
		// 一个个插入到列表中
		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.
     */
    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.
     *
     * @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.
     * 设置值
     * @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).
     * 在指定位置前插入值
     * @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.
     * 移除指定位置的值
     * @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.
     */
    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 <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.
     * 查找指定对象的位置下表
	 * 返回此列表中最后出现的指定元素的索引，如果此列表中不包含该元素，则返回 -1。
     * @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 <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.
     * 返回此列表中最后出现的指定元素的索引，如果此列表中不包含该元素，则返回 -1。
     * @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.
	 * 获取但不移除此列表的头（第一个元素）。 
     * @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.
     * @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
	 * 此列表的头，如果此列表为空，则返回 null
     * @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.
     *
     * @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 <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.
     *
     * @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.
     *
     * @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 <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 <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 <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 <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.
     *
     * <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 <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.
     * 从此列表中移除第一次出现的指定元素（从头部到尾部遍历列表时）。
	 * 如果列表不包含该元素，则不作更改。 
     * @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 <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
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /** 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 <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).
     *
     * <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.
     *
     * <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 <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 <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);
	}
}