LinkedList 源代码

private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;

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

/**
* Returns the (non-null) Node at the specified element index.
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//查找第index个元素，在这有做优化的
if (index < (size >> 1)) {
//如果index在中间元素的左边，则从头结点向尾结点扫描
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//如果index在中间元素的右边，则从尾结点向头结点扫描
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

/*
* Copyright (c) 1997, 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/

package java.util;

/**
* This class provides a skeletal implementation of the <tt>List</tt>
* interface to minimize the effort required to implement this interface
* backed by a "sequential access" data store (such as a linked list).  For
* random access data (such as an array), <tt>AbstractList</tt> should be used
* in preference to this class.<p>
*
* This class is the opposite of the <tt>AbstractList</tt> class in the sense
* that it implements the "random access" methods (<tt>get(int index)</tt>,
* <tt>set(int index, E element)</tt>, <tt>add(int index, E element)</tt> and
* <tt>remove(int index)</tt>) on top of the list's list iterator, instead of
* the other way around.<p>
*
* To implement a list the programmer needs only to extend this class and
* provide implementations for the <tt>listIterator</tt> and <tt>size</tt>
* methods.  For an unmodifiable list, the programmer need only implement the
* list iterator's <tt>hasNext</tt>, <tt>next</tt>, <tt>hasPrevious</tt>,
* <tt>previous</tt> and <tt>index</tt> methods.<p>
*
* For a modifiable list the programmer should additionally implement the list
* iterator's <tt>set</tt> method.  For a variable-size list the programmer
* should additionally implement the list iterator's <tt>remove</tt> and
* <tt>add</tt> methods.<p>
*
* The programmer should generally provide a void (no argument) and collection
* constructor, as per the recommendation in the <tt>Collection</tt> interface
* specification.<p>
*
* This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author  Josh Bloch
* @author  Neal Gafter
* @see Collection
* @see List
* @see AbstractList
* @see AbstractCollection
* @since 1.2
*/
/*
此类提供了 List 接口的骨干实现，从而最大限度地减少了实现受“连续访问”数据存储（如链接列表）
支持的此接口所需的工作。对于随机访问数据（如数组），应该优先使用 AbstractList，而不是先使用此类。

从某种意义上说，此类与在列表的列表迭代器上实现“随机访问”方法（get(int index)、set(int index, E element)、
add(int index, E element) 和 remove(int index)）的 AbstractList 类相对立，而不是其他关系。

要实现一个列表，程序员只需要扩展此类，并提供 listIterator 和 size 方法的实现即可。对于不可修改的列表，
程序员只需要实现列表迭代器的 hasNext、next、hasPrevious、previous 和 index 方法即可。

对于可修改的列表，程序员应该再另外实现列表迭代器的 set 方法。对于可变大小的列表，程序员应该再另外实
现列表迭代器的 remove 和 add 方法。

按照 Collection 接口规范中的推荐，程序员通常应该提供一个 void（无参数）构造方法和 collection 构造方法。

此类是 Java Collections Framework 的成员。
*/
public abstract class AbstractSequentialList<E> extends AbstractList<E> {
/**
* Sole constructor.  (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractSequentialList() {
}

/**
* Returns the element at the specified position in this list.
*
* <p>This implementation first gets a list iterator pointing to the
* indexed element (with <tt>listIterator(index)</tt>).  Then, it gets
* the element using <tt>ListIterator.next</tt> and returns it.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
try {
//获取迭代器，取出
return listIterator(index).next();
} catch (NoSuchElementException exc) {
throw new IndexOutOfBoundsException("Index: "+index);
}
}

/**
* Replaces the element at the specified position in this list with the
* specified element (optional operation).
*
* <p>This implementation first gets a list iterator pointing to the
* indexed element (with <tt>listIterator(index)</tt>).  Then, it gets
* the current element using <tt>ListIterator.next</tt> and replaces it
* with <tt>ListIterator.set</tt>.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the list iterator does not
* implement the <tt>set</tt> operation.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
* @throws IndexOutOfBoundsException     {@inheritDoc}
*/
public E set(int index, E element) {
try {
//在迭代器中替换
ListIterator<E> e = listIterator(index);
E oldVal = e.next();
e.set(element);
return oldVal;
} catch (NoSuchElementException exc) {
throw new IndexOutOfBoundsException("Index: "+index);
}
}

/**
* Inserts the specified element at the specified position in this list
* (optional operation).  Shifts the element currently at that position
* (if any) and any subsequent elements to the right (adds one to their
* indices).
*
* <p>This implementation first gets a list iterator pointing to the
* indexed element (with <tt>listIterator(index)</tt>).  Then, it
* inserts the specified element with <tt>ListIterator.add</tt>.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the list iterator does not
* implement the <tt>add</tt> operation.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
* @throws IndexOutOfBoundsException     {@inheritDoc}
*/
public void add(int index, E element) {
try {
//在迭代器添加
listIterator(index).add(element);
} catch (NoSuchElementException exc) {
throw new IndexOutOfBoundsException("Index: "+index);
}
}

/**
* Removes the element at the specified position in this list (optional
* operation).  Shifts any subsequent elements to the left (subtracts one
* from their indices).  Returns the element that was removed from the
* list.
*
* <p>This implementation first gets a list iterator pointing to the
* indexed element (with <tt>listIterator(index)</tt>).  Then, it removes
* the element with <tt>ListIterator.remove</tt>.
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the list iterator does not
* implement the <tt>remove</tt> operation.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws IndexOutOfBoundsException     {@inheritDoc}
*/
public E remove(int index) {
try {
//获取迭代器
ListIterator<E> e = listIterator(index);
//remove之前需要先调用next来获取
E outCast = e.next();
e.remove();
return outCast;
} catch (NoSuchElementException exc) {
throw new IndexOutOfBoundsException("Index: "+index);
}
}

// Bulk Operations

/**
* Inserts all of the elements in the specified collection into this
* list at the specified position (optional operation).  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 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.)
*
* <p>This implementation gets an iterator over the specified collection and
* a list iterator over this list pointing to the indexed element (with
* <tt>listIterator(index)</tt>).  Then, it iterates over the specified
* collection, inserting the elements obtained from the iterator into this
* list, one at a time, using <tt>ListIterator.add</tt> followed by
* <tt>ListIterator.next</tt> (to skip over the added element).
*
* <p>Note that this implementation will throw an
* <tt>UnsupportedOperationException</tt> if the list iterator returned by
* the <tt>listIterator</tt> method does not implement the <tt>add</tt>
* operation.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException            {@inheritDoc}
* @throws NullPointerException          {@inheritDoc}
* @throws IllegalArgumentException      {@inheritDoc}
* @throws IndexOutOfBoundsException     {@inheritDoc}
*/
public boolean addAll(int index, Collection<? extends E> c) {
try {
boolean modified = false;
ListIterator<E> e1 = listIterator(index);
Iterator<? extends E> e2 = c.iterator();
//通过迭代器添加
while (e2.hasNext()) {
e1.add(e2.next());
modified = true;
}
return modified;
} catch (NoSuchElementException exc) {
throw new IndexOutOfBoundsException("Index: "+index);
}
}

// Iterators

/**
* Returns an iterator over the elements in this list (in proper
* sequence).<p>
*
* This implementation merely returns a list iterator over the list.
*
* @return an iterator over the elements in this list (in proper sequence)
*/
public Iterator<E> iterator() {
return listIterator();
}

/**
* Returns a list iterator over the elements in this list (in proper
* sequence).
*
* @param  index index of first element to be returned from the list
*         iterator (by a call to the <code>next</code> method)
* @return a list iterator over the elements in this list (in proper
*         sequence)
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public abstract ListIterator<E> listIterator(int index);
}

/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/

package java.util;

import java.util.function.Consumer;

/**
* Doubly-linked list implementation of the {@code List} and {@code Deque}
* interfaces.  Implements all optional list operations, and permits all
* elements (including {@code null}).
*
* <p>All of the operations perform as could be expected for a doubly-linked
* list.  Operations that index into the list will traverse the list from
* the beginning or the end, whichever is closer to the specified index.
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a linked list concurrently, and at least
* one of the threads modifies the list structurally, it <i>must</i> be
* synchronized externally.  (A structural modification is any operation
* that adds or deletes one or more elements; merely setting the value of
* an element is not a structural modification.)  This is typically
* accomplished by synchronizing on some object that naturally
* encapsulates the list.
*
* If no such object exists, the list should be "wrapped" using the
* {@link Collections#synchronizedList Collections.synchronizedList}
* method.  This is best done at creation time, to prevent accidental
* unsynchronized access to the list:<pre>
*   List list = Collections.synchronizedList(new LinkedList(...));</pre>
*
* <p>The iterators returned by this class's {@code iterator} and
* {@code listIterator} methods are <i>fail-fast</i>: if the list is
* structurally modified at any time after the iterator is created, in
* any way except through the Iterator's own {@code remove} or
* {@code add} methods, the iterator will throw a {@link
* ConcurrentModificationException}.  Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than
* risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification.  Fail-fast iterators
* throw {@code ConcurrentModificationException} on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness:   <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author  Josh Bloch
* @see     List
* @see     ArrayList
* @since 1.2
* @param <E> the type of elements held in this collection
*/
/*
List 接口的链接列表实现。实现所有可选的列表操作，并且允许所有元素（包括 null）。除了实现 List 接口外，
LinkedList 类还为在列表的开头及结尾 get、remove 和 insert 元素提供了统一的命名方法。
这些操作允许将链接列表用作堆栈、队列或双端队列。

此类实现 Deque 接口，为 add、poll 提供先进先出队列操作，以及其他堆栈和双端队列操作。

所有操作都是按照双重链接列表的需要执行的。在列表中编索引的操作将从开头或结尾遍历列表（从靠近指定索引的一端）。

注意，此实现不是同步的。如果多个线程同时访问一个链接列表，而其中至少一个线程从结构上修改了该列表，
则它必须 保持外部同步。（结构修改指添加或删除一个或多个元素的任何操作；仅设置元素的值不是结构修改。）
这一般通过对自然封装该列表的对象进行同步操作来完成。如果不存在这样的对象，则应该使用
Collections.synchronizedList 方法来“包装”该列表。最好在创建时完成这一操作，以防止对列表进行
意外的不同步访问，如下所示：

List list = Collections.synchronizedList(new LinkedList(...));此类的 iterator 和
listIterator 方法返回的迭代器是快速失败 的：在迭代器创建之后，如果从结构上对列表进行修改，
除非通过迭代器自身的 remove 或 add 方法，其他任何时间任何方式的修改，迭代器都将抛出
ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，
而不冒将来不确定的时间任意发生不确定行为的风险。

注意，迭代器的快速失败行为不能得到保证，一般来说，存在不同步的并发修改时，不可能作出任何硬性保证。
快速失败迭代器尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的，
正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。

此类是 Java Collections Framework 的成员。

*/
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
//链表大小
transient int size = 0;

/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
*            (first.prev == null && first.item != null)
*/
//链表第一个结点
transient Node<E> first;

/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
*            (last.next == null && last.item != null)
*/
//链表最后一个结点
transient Node<E> last;

/**
* Constructs an empty list.
*/
public LinkedList() {
}

/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param  c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}

/**
* Links e as first element.
*/
//将e设为第一个结点
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
//说明链接e元素前链表为空
last = newNode;
else
//双链表，需要设置f的前驱
f.prev = newNode;
//容量加一
size++;
//每次修改动作都需要modCount++
modCount++;
}

/**
* Links e as last element.
*/
//将e设为最后一个结点
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
//链接e前为空链表
first = newNode;
else
//需要设置l的后驱，因为是双链表
l.next = newNode;
//容量加一
size++;
modCount++;
}

/**
* Inserts element e before non-null Node succ.
*/
//在succ前插入结点e
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
//获取succ前驱结点
final Node<E> pred = succ.prev;
//插入新结点
final Node<E> newNode = new Node<>(pred, e, succ);
//修改succ的前驱结点为newNode
succ.prev = newNode;
if (pred == null)
//如果succ前驱结点为空，说明newNode应该为头结点
first = newNode;
else
//修改前驱结点的后驱结点为newNode
pred.next = newNode;
//容量加一
size++;
modCount++;
}

/**
* Unlinks non-null first node f.
*/
//解除头结点f
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next;
//将各项设为null
f.item = null;
f.next = null; // help GC
//头指针指向后驱结点
first = next;
if (next == null)
//后驱结点为空，说明链表为空了
last = null;
else
//修改后驱结点的前驱为null
next.prev = null;
//容量减一
size--;
modCount++;
return element;
}

/**
* Unlinks non-null last node l.
*/
//解除尾结点l
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
//
final E element = l.item;
final Node<E> prev = l.prev;
//将结点的值跟前驱设为null
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
//prev==null说明链表为空了
first = null;
else
//修改prev的后驱为null
prev.next = null;
//容量减一
size--;
modCount++;
return element;
}

/**
* Unlinks non-null node x.
*/
//取消链接x结点
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;

if (prev == null) {
//说明结点x为头结点，重新指头结点
first = next;
} else {
//将x前驱结点的next指向x的后驱结点
prev.next = next;
//x的前驱结点设为null
x.prev = null;
}

if (next == null) {
//说明结点x为尾结点，重新指尾结点
last = prev;
} else {
//将x的前驱指向x的前驱结点
next.prev = prev;
//将x的后驱结点设为null
x.next = null;
}

//值设为null
x.item = null;
//容量减一
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
*/
//获取头结点元素
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}

/**
* Returns the last element in this list.
*
* @return the last element in this list
* @throws NoSuchElementException if this list is empty
*/
//获取尾结点元素
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
*/
//移除头结点
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
*/
//移除尾结点
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}

/**
* Inserts the specified element at the beginning of this list.
*
* @param e the element to add
*/
//添加头结点
public void addFirst(E e) {
linkFirst(e);
}

/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
*/
//添加尾结点
public void addLast(E e) {
linkLast(e);
}

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

/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
//返回容量
public int size() {
return size;
}

/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
//在链表后添加元素
linkLast(e);
return true;
}

/**
* Removes the first occurrence of the specified element from this list,
* if it is present.  If this list does not contain the element, it is
* unchanged.  More formally, removes the element with the lowest index
* {@code i} such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists).  Returns {@code true} if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return {@code true} if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
//o为null 用==比较
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}

/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the specified
* collection's iterator.  The behavior of this operation is undefined if
* the specified collection is modified while the operation is in
* progress.  (Note that this will occur if the specified collection is
* this list, and it's nonempty.)
*
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}

/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position.  Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices).  The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element
*              from the specified collection
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
//检测下标index是否越界
checkPositionIndex(index);

Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;

Node<E> pred, succ;
if (index == size) {
//在尾结点后面添加
succ = null;
pred = last;
} else {
//获得第index个结点
succ = node(index);
//第index-1个结点;
pred = succ.prev;
}

//遍历将容器c中的元素添加到链表
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
//newNode是头结点
first = newNode;
else
//将上一个结点的next指向新的结点
pred.next = newNode;
//下一次遍历，当前的结点就是上一个结点了
pred = newNode;
}

if (succ == null) {
//第index-1个结点是尾结点
last = pred;
} else {
//将原先链表index后面的元素重新接到链表
pred.next = succ;
succ.prev = pred;
}

//容量加一
size += numNew;
modCount++;
return true;
}

/**
* Removes all of the elements from this list.
* The list will be empty after this call returns.
*/
public void clear() {
// Clearing all of the links between nodes is "unnecessary", but:
// - helps a generational GC if the discarded nodes inhabit
//   more than one generation
// - is sure to free memory even if there is a reachable Iterator
//递归将node设为null
for (Node<E> x = first; x != null; ) {
Node<E> next = x.next;
x.item = null;
x.next = null;
x.prev = null;
x = next;
}
//将first、last设为null size = 0
first = last = null;
size = 0;
modCount++;
}

// Positional Access Operations

/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
checkElementIndex(index);
//返回第index个结点的元素
return node(index).item;
}

/**
* Replaces the element at the specified position in this list with the
* specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
checkElementIndex(index);
//获取第index个结点
Node<E> x = node(index);
E oldVal = x.item;
//设置新的值
x.item = element;
//将旧结点返回
return oldVal;
}

/**
* Inserts the specified element at the specified position in this list.
* Shifts the element currently at that position (if any) and any
* subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
checkPositionIndex(index);

if (index == size)
//在尾结点后面添加
linkLast(element);
else
//index前添加
linkBefore(element, node(index));
}

/**
* Removes the element at the specified position in this list.  Shifts any
* subsequent elements to the left (subtracts one from their indices).
* Returns the element that was removed from the list.
*
* @param index the index of the element to be removed
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
checkElementIndex(index);
//先找出在index的结点后在移除
return unlink(node(index));
}

/**
* Tells if the argument is the index of an existing element.
*/
//检测是否越界
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.
*/
//检测是否越界，用于迭代器和添加操作
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.
*/
//越界时抛出异常的信息
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}

//检测元素下标
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

//检测下标
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* Returns the (non-null) Node at the specified element index.
*/
//返回第index个元素
Node<E> node(int index) {
// assert isElementIndex(index);
//查找第index个元素，在这有做优化的
if (index < (size >> 1)) {
//如果index在中间元素的左边，则从头结点向尾结点扫描
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//如果index在中间元素的右边，则从尾结点向头结点扫描
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

// Search Operations

/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index {@code i} such that
* <tt>(o==null ? get(i)==null : 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) {
//从头结点扫描
int index = 0;
if (o == null) {
//用==来比较
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}

/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index {@code i} such that
* <tt>(o==null ? get(i)==null : 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) {
//从尾结点往前扫描
int index = size;
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (o.equals(x.item))
return index;
}
}
return -1;
}

// Queue operations.

/**
* Retrieves, but does not remove, the head (first element) of this list.
*
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
//取出头结点元素，但是没有删除头结点，如果没有的话不会抛出异常，返回null
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* Retrieves, but does not remove, the head (first element) of this list.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
//取出头结点，如果没有的话会抛出异常
public E element() {
return getFirst();
}

/**
* Retrieves and removes the head (first element) of this list.
*
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* Retrieves and removes the head (first element) of this list.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
//移除头结点
public E remove() {
return removeFirst();
}

/**
* Adds the specified element as the tail (last element) of this list.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Queue#offer})
* @since 1.5
*/
//在尾结点后面插入元素
public boolean offer(E e) {
return add(e);
}

// Deque operations
/**
* Inserts the specified element at the front of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerFirst})
* @since 1.6
*/
//从头结点前面插入元素
public boolean offerFirst(E e) {
addFirst(e);
return true;
}

/**
* Inserts the specified element at the end of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerLast})
* @since 1.6
*/
//从尾结点后面插入元素
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
*/
//返回头结点，不删除
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* Retrieves, but does not remove, the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null}
*         if this list is empty
* @since 1.6
*/
//返回尾结点，不删除
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
*/
//返回头结点，删除
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* Retrieves and removes the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null} if
*     this list is empty
* @since 1.6
*/
//返回尾结点，删除
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
*/
//从头结点前面插入元素
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
*/
//返回头结点并移除
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
*/
//移除第一个在链表中出现的o
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}

/**
* Removes the last occurrence of the specified element in this
* list (when traversing the list from head to tail).  If the list
* does not contain the element, it is unchanged.
*
* @param o element to be removed from this list, if present
* @return {@code true} if the list contained the specified element
* @since 1.6
*/
//移除最后一个出现o的结点
public boolean removeLastOccurrence(Object o) {
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}

/**
* Returns a list-iterator of the elements in this list (in proper
* sequence), starting at the specified position in the list.
* Obeys the general contract of {@code List.listIterator(int)}.<p>
*
* The list-iterator is <i>fail-fast</i>: if the list is structurally
* modified at any time after the Iterator is created, in any way except
* through the list-iterator's own {@code remove} or {@code add}
* methods, the list-iterator will throw a
* {@code ConcurrentModificationException}.  Thus, in the face of
* concurrent modification, the iterator fails quickly and cleanly, rather
* than risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
* @param index index of the first element to be returned from the
*              list-iterator (by a call to {@code next})
* @return a ListIterator of the elements in this list (in proper
*         sequence), starting at the specified position in the list
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see List#listIterator(int)
*/
//返回list迭代器
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}

private class ListItr implements ListIterator<E> {
//上次调用的结点
private Node<E> lastReturned = null;
//将要调用next方法的结点
private Node<E> next;
//将要调用next方法的结点的下标
private int nextIndex;
private int expectedModCount = modCount;

ListItr(int index) {
// assert isPositionIndex(index);
next = (index == size) ? null : node(index);
nextIndex = index;
}

//是否有后驱结点
public boolean hasNext() {
return nextIndex < size;
}

//获取后驱结点
public E next() {
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();

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

//是否有前驱结点
public boolean hasPrevious() {
return nextIndex > 0;
}

//获取前驱结点
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;
}

//移除掉上次调用的元素
public void remove() {
checkForComodification();
if (lastReturned == null)
throw new IllegalStateException();

Node<E> lastNext = lastReturned.next;
unlink(lastReturned);

if (next == lastReturned)
//上次调用是previous，则需要再设置next
next = lastNext;
else
nextIndex--;
//释放掉
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)
//从尾结点后面添加
linkLast(e);
else
//从next的前面插入结点
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();
}
}

//链表的类 双向链表
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;

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

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

/**
* Adapter to provide descending iterators via ListItr.previous
*/
//反序列迭代器
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() {
//克隆父类
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
//初始化每个元素
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;
//遍历每个元素
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
//默认方法
s.defaultWriteObject();

// Write out size
//先将size写入
s.writeInt(size);

// Write out all elements in the proper order.
//将每个结点的元素写入
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
//先读取size
int size = s.readInt();

// Read in all elements in the proper order.
//读取每个元素，调用linkLast来插入组成链表
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;
}
}

}