LinkedList（JDK1.8）源码分析

双向循环链表
双向循环链表和双向链表的不同在于，第一个节点的pre指向最后一个节点，最后一个节点的next指向第一个节点，也形成一个“环”。而LinkedList就是基于双向循环链表设计的。

LinkedList 的继承关系

LinkedList 是一个继承于AbstractSequentialList的双向循环链表。它也可以被当作堆栈、队列或双端队列进行操作。

public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable

LinkedList 实现 List 接口，能对它进行队列操作。
LinkedList 实现 Deque 接口，即能将LinkedList当作双端队列使用。
LinkedList 实现了Cloneable接口，即覆盖了函数clone()，能克隆。
LinkedList 实现java.io.Serializable接口，这意味着LinkedList支持序列化，能通过序列化去传输。
LinkedList 是非同步的。

LinkedList属性：

size:当前有多少个节点；

first：第一个节点；

last：最后一个节点；

public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
//list的元素数量
transient int size = 0;

/**
*第一个节点
*/
transient Node<E> first;

/**
* 最后一个节点
*/
transient Node<E> last;

LinkedList构造方法：

​ 空的构造方法：表示初始化的时候，size为默认值0；first和last为空；

​ 带入参的构造方法：

1. this（）调用默认无参构造方法;
2. addAll()传进去入参的集合数据;
3. 检查index索引范围 ;
4. 得到集合数据
5. 得到插入位置的前驱和后继节点
6. 遍历数据，将数据插入到指定位置

/**
* 空构造函数
*/
public LinkedList() {
}

/**
*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
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}

/**
* 将集合从指定位置开始插入
* 1. 检查index索引范围
* 2. 得到集合数据
* 3. 得到插入位置的前驱和后继节点
* 4. 遍历数据，将数据插入到指定位置
*/
public boolean addAll(int index, Collection<? extends E> c) {
//检查index范围
checkPositionIndex(index);
//得到集合的数据
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
//得到插入位置的前驱节点和后继节点
Node<E> pred, succ;
//位置为尾部，前驱节点为last，后继节点为null
if (index == size) {
succ = null;
pred = last;
} else {
//调用node()方法得到后继节点，再得到前驱节点
succ = node(index);
pred = succ.prev;
}
//遍历数据将数据插入
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
//创建新节点
Node<E> newNode = new Node<>(pred, e, null);
//前置节点为空，插入位置在链表头部
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
//如果插入位置在尾部，重置last节点
if (succ == null) {
last = pred;
} else {
//否则，将插入的链表与先前链表连接起来
pred.next = succ;
succ.prev = pred;
}

size += numNew;
modCount++;
return true;
}

新增元素操作：

/**
*  将一个元素添加至list尾部
*/
public boolean add(E e) {
linkLast(e);
return true;
}

指定位置添加元素：

1. 检查index的范围，否则抛出异常
2. 如果插入位置是链表尾部，那么调用linkLast方法
3. 如果插入位置是链表中间，那么调用linkBefore方法

/**
* 指定位置添加元素
*1. 检查index的范围，否则抛出异常
*2. 如果插入位置是链表尾部，那么调用linkLast方法
*3. 如果插入位置是链表中间，那么调用linkBefore方法
*/
public void add(int index, E element) {
//检查索引是否处于[0-size]之间
checkPositionIndex(index);
//添加在链表尾部
if (index == size)
linkLast(element);
else
//添加在链表中间
linkBefore(element, node(index));
}

linkBefore 非空节点前插入元素图示：

检索操作总结：

检索操作分为按照位置得到对象以及按照对象得到位置两种方式，其中按照对象得到位置的方法有indexOf()和lastIndexOf()；按照位置得到对象有如下方法： 

• 根据任意位置得到数据的get(int index)方法，当index越界会抛出异常 
• 获得头节点数据 
• getFirst()和element()方法在链表为空时会抛出NoSuchElementException 
• peek()和peekFirst()方法在链表为空时会返回null 
• 获得尾节点数据 
• getLast()在链表为空时会抛出NoSuchElementException 
• peekLast()在链表为空时会返回null

get方法：

/**
* 检索指定位置元素，索引越界，抛出异常
*/
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}

/**
* 返回指定位置的非空节点
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//如果索引位置靠链表前半部分，从头开始遍历
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//否则，从尾开始遍历
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

getFirst操作和getLast操作：

/**
* 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)
//链表为null,抛出异常
throw new NoSuchElementException();
return f.item;
}

/**
* 链表为空时，会抛出NoSuchElementException
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}

删除操作总结

删除操作由很多种方法，有： 

• 按照指定对象删除：boolean remove(Object o)，一次只会删除一个匹配的对象 
• 按照指定位置删除 
• 删除任意位置的对象：E remove(int index),当index越界时会抛出异常 
• 删除头节点位置的对象 
• 在链表为空时抛出异常：E remove()、E removeFirst()、E pop() 
• 在链表为空时返回null：E poll()、E pollFirst() 
• 删除尾节点位置的对象 
• 在链表为空时抛出异常：E removeLast() 
• 在链表为空时返回null：E pollLast()

remove（）方法：

移除第一个节点，将第一个节点置空，让下一个节点变成第一个节点，链表长度减1，修改次数加1，返回移除的第一个节点。

/**
* 在链表为空时将抛出NoSuchElementException
*/
public E remove() {
return removeFirst();
}

removeFirst()方法：

/**
* 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);
}

/**
* 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;
//第一个节点置空
f.item = null;
f.next = null; // help GC
//下一个节点变成第一个节点
first = next;
if (next == null)
last = null;
else
next.prev = null;
//链表长度减1
size--;
//修改次数加1
modCount++;
return element;
}

remove(int index)方法：

删除任意位置的元素，如果删除成功将返回true，否则返回false

• 检查index范围，属于[0,size）
• 将索引出节点删除

/**
* 删除任意位置的元素，如果删除成功将返回true，否则返回false
* 1. 检查index范围，属于[0,size）
* 2. 将索引出节点删除
*/
public E remove(int index) {
//检查index范围
checkElementIndex(index);
//将节点删除
return unlink(node(index));
}

set方法：

public E set(int index, E element) {
//检查index是否越界
checkElementIndex(index);
//索引index位置的节点
Node<E> x = node(index);
//index位置的节点内容替换为element
E oldVal = x.item;
x.item = element;
//返回原来旧的值
return oldVal;
}

clear方法：

public void clear() {
//遍历链表，置空链表元素
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;
//修改链表长度为0
size = 0;
//修改次数加一
modCount++;
}

listIterator方法：

/**
* 在ListIterator的构造器中，得到了当前位置的节点，就是变量next。next()方法返回当前节点的值并将      *next指向其后继节点，previous()方法返回当前节点的前一个节点的值并将next节点指向其前驱节点。由于      *Node是一个双端节点，所以这儿用了一个节点就可以实现从前向后迭代和从后向前迭代。另外在                *ListIterator初始时，exceptedModCount保存了当前的modCount，如果在迭代期间，有操作改变了链表      *的底层结构，那么再操作迭代器的方法时将会抛出ConcurrentModificationException。
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}

LinkedList 源码解析：

节点对象：

/**
*节点对象
*/
private static class Node<E> {
// 当前存储元素
E item;
// 下一个元素节点
Node<E> next;
// 上一个元素节点
Node<E> prev;

Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}

package java.util;

import java.util.function.Consumer;

public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
//list的元素数量
transient int size = 0;

/**
*第一个节点
*/
transient Node<E> first;

/**
* 最后一个节点
*/
transient Node<E> last;

/**
* 空构造函数
*/
public LinkedList() {
}

/**
*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
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}

/**
* Links e as first element.
*/
private void linkFirst(E e) {
final Node<E> f = first;
//新建节点，以头节点为后继节点
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
//如果链表为空，last节点也指向该节点
if (f == null)
last = newNode;
else
//否则，将头节点的前驱指针指向新节点
f.prev = newNode;
size++;
modCount++;
}

/**
* Links e as last element.
*/
void linkLast(E e) {
//指向链表尾部
final Node<E> l = last;
//以尾部为前驱节点创建一个新节点
final Node<E> newNode = new Node<>(l, e, null);
//将链表尾部指向新节点
last = newNode;
//如果链表为空，那么该节点既是头节点也是尾节点
if (l == null)
first = newNode;
else
l.next = newNode;
//增加集合大小
size++;
modCount++;
}

/**
* 非空节点前插入元素
* 1. 创建newNode节点，将newNode的后继指针指向succ，前驱指针指向pred
* 2. 将succ的前驱指针指向newNode
* 3. 根据pred是否为null，进行不同操作。
* - 如果pred为null，说明该节点插入在头节点之前，要重置first头节点
* - 如果pred不为null，那么直接将pred的后继指针指向newNode即可
*/
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;
}

/**
* 从链表中移除元素
* 1 得到待删除节点的前驱节点和后继节点
* 2 删除前驱节点
* 3 删除后继节点
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
//后继节点
final Node<E> next = x.next;
//前驱节点
final Node<E> prev = x.prev;

//删除前驱指针
if (prev == null) {
//如果删除的节点是头节点,令头节点指向该节点的后继节点
first = next;
} else {
//将前驱节点的后继节点指向后继节点
prev.next = next;
x.prev = null;
}

//删除后继指针
if (next == null) {
//如果删除的节点是尾节点,令尾节点指向该节点的前驱节点
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)
//链表为null,抛出异常
throw new NoSuchElementException();
return f.item;
}

/**
* 链表为空时，会抛出NoSuchElementException
*/
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);
}

/**
*list头部添加指定元素
*/
public void addFirst(E e) {
linkFirst(e);
}

/**
* 将元素添加到链表尾部，与add()方法一样。所以实现也一样
*/
public void addLast(E e) {
linkLast(e);
}

/**
* 检查对象o是否存在于链表中
*/
public boolean contains(Object o) {
//返回结果不是-1，那就说明该对象存在于链表中
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;
}

/**
*  将一个元素添加至list尾部
*/
public boolean add(E e) {
linkLast(e);
return true;
}

/**
* 删除指定元素
*/
public boolean remove(Object o) {
//如果删除对象为null
if (o == null) {
//从前向后遍历
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
//从前向后遍历
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
//匹配返回true
return true;
}
}
}
return false;
}

/**
* 将集合插入到链表尾部，即开始索引位置为size
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}

/**
* 将集合从指定位置开始插入
* 1. 检查index索引范围
* 2. 得到集合数据
* 3. 得到插入位置的前驱和后继节点
*4. 遍历数据，将数据插入到指定位置
*/
public boolean addAll(int index, Collection<? extends E> c) {
//检查index范围
checkPositionIndex(index);
//得到集合的数据
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
//得到插入位置的前驱节点和后继节点
Node<E> pred, succ;
//位置为尾部，前驱节点为last，后继节点为null
if (index == size) {
succ = null;
pred = last;
} else {
//调用node()方法得到后继节点，再得到前驱节点
succ = node(index);
pred = succ.prev;
}
//遍历数据将数据插入
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
//创建新节点
Node<E> newNode = new Node<>(pred, e, null);
//前置节点为空，插入位置在链表头部
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}
//如果插入位置在尾部，重置last节点
if (succ == null) {
last = pred;
} else {
//否则，将插入的链表与先前链表连接起来
pred.next = succ;
succ.prev = pred;
}

size += numNew;
modCount++;
return true;
}

public void clear() {
//遍历链表，置空链表元素
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;
//修改链表长度为0
size = 0;
//修改次数加一
modCount++;
}

/**
* 检索指定位置元素，索引越界，抛出异常
*/
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}

public E set(int index, E element) {
//检查index是否越界
checkElementIndex(index);
//索引index位置的节点
Node<E> x = node(index);
//index位置的节点内容替换为element
E oldVal = x.item;
x.item = element;
//返回原来旧的值
return oldVal;
}

/**
* 指定位置添加元素
*1. 检查index的范围，否则抛出异常
*2. 如果插入位置是链表尾部，那么调用linkLast方法
*3. 如果插入位置是链表中间，那么调用linkBefore方法
*/
public void add(int index, E element) {
//检查索引是否处于[0-size]之间
checkPositionIndex(index);
//添加在链表尾部
if (index == size)
linkLast(element);
else
//添加在链表中间
linkBefore(element, node(index));
}

/**
* 删除任意位置的元素，如果删除成功将返回true，否则返回false
* 1. 检查index范围，属于[0,size）
* 2. 将索引出节点删除
*/
public E remove(int index) {
//检查index范围
checkElementIndex(index);
//将节点删除
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));
}

/**
* 返回指定位置的非空节点
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//如果索引位置靠链表前半部分，从头开始遍历
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//否则，从尾开始遍历
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

// Search Operations

/**
*返回第一个匹配的索引
*/
public int indexOf(Object o) {
int index = 0;
if (o == null) {
//从头往后遍历,元素为空的时候的检索，此处可以观察到LinkedList是支持空元素的
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
//从头往后遍历，元素不为空的时候的检索
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}

/**
*返回最后一个匹配的索引
*/
public int lastIndexOf(Object o) {
int index = size;
if (o == null) {
//从后向前遍历,元素为空，返回
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
//从后向前遍历,元素不为空，匹配的情况下返回
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;
//不会抛出异常，但是会返回null
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();
}

/**
* 在链表为空时将返回null
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* 在链表为空时将抛出NoSuchElementException
*/
public E remove() {
return removeFirst();
}

/**
* 将数据添加到链表尾部，其内部调用了add(E e)方法
*/
public boolean offer(E e) {
return add(e);
}

/**
*将数据插入链表头部
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
}

/**
* 数据添加到链表尾部
*/
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;
}

/**
* 为空，返回null，不会抛异常
*/
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);
}

/**
* 链表为空时会返回null，而不是抛出异常
*/
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);
}

/**
* 在链表为空时将抛出NoSuchElementException
*/
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);
}

/**
* 链表为空时将抛出NoSuchElementException
*/
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;
}

/**
* 在ListIterator的构造器中，得到了当前位置的节点，就是变量next。next()方法返回当前节点的值并将      *next指向其后继节点，previous()方法返回当前节点的前一个节点的值并将next节点指向其前驱节点。由于      *Node是一个双端节点，所以这儿用了一个节点就可以实现从前向后迭代和从后向前迭代。另外在                *ListIterator初始时，exceptedModCount保存了当前的modCount，如果在迭代期间，有操作改变了链表      *的底层结构，那么再操作迭代器的方法时将会抛出ConcurrentModificationException。
*/
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;
//保存当前modCount，确保fail-fast机制
private int expectedModCount = modCount;

ListItr(int index) {
// assert isPositionIndex(index);
//得到当前索引指向的next节点
next = (index == size) ? null : node(index);
nextIndex = index;
}

public boolean hasNext() {
return nextIndex < size;
}
/**
*获取下一个节点
*/
public E next() {
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();

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

public boolean hasPrevious() {
return nextIndex > 0;
}

/**
*获取前一个节点，将next节点向前移
*/
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();
}
}
/**
*节点对象
*/
private static class Node<E> {
// 当前存储元素
E item;
// 下一个元素节点
Node<E> next;
// 上一个元素节点
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
;
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;
}
}

}