<java API源码初体验>2---collection集合之LinkedList原理分析

package java.util;

/**
* 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
*/
//LinkedList继承的类：AbstractSequentialList<E>
//实现接口：List<E>, Deque<E>, Cloneable, java.io.Serializable。
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0; //列表大小，默认为0

/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
*            (first.prev == null && first.item != null)
*/
transient Node<E> first; //指向第一个节点元素

/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
*            (last.next == null && last.item != null)
*/
transient Node<E> last; //指向最后一个节点元素

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

/**
* 构造一个包含指定集合c的链式列表对象。
*
* @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.
* 将包含元素e的节点作为第一个链表元素节点
*/
private void linkFirst(E e) {
final Node<E> f = first; //将第一个节点元素复制给f节点，即保留原来的首节点
final Node<E> newNode = new Node<>(null, e, f); //为元素e新建一个node对象，他的下个节点为f节点。
first = newNode; //将此新节点赋给first节点作为列表的首节点。
if (f == null) //如果列表为空，则列表中仅有newNode一个节点，所以最后一个节点也是newNode.
last = newNode;
else   //若列表有元素，则将newNode赋值给原首节点的前一节点。
f.prev = newNode;
size++; //列表元素个数+1
modCount++;
}

/**
* Links e as last element.
* 将包含元素e作为最后一个链表元素节点
*/
void linkLast(E e) {
final Node<E> l = last; //保留原来的末节点
final Node<E> newNode = new Node<>(l, e, null); //为元素e新建节点对象，并将l作为他的前一个节点。
last = newNode; //最后，再将newNode作为最后一个节点
if (l == null) //若列表为空，则将newNode也设置为第一个节点
first = newNode;
else //否则，将原末节点的下一个节点设为newNode,因为此链式列表是双向链表，所以前后都必须设置
l.next = newNode;
size++; //修改列表大小，即size+1
modCount++;
}

/**
* Inserts element e before non-null Node succ.
* 在非空节点succ之前嵌入一个元素为e的新节点
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
final Node<E> pred = succ.prev;  //先将succ的前一个节点保留下来到pred
final Node<E> newNode = new Node<>(pred, e, succ); //为元素e新建一个节点，并为其设置前后节点
succ.prev = newNode; //将newNode设置为succ的前一个结点
if (pred == null) //若pred为空，即succ前没有节点了，则将newNode设置为第一个节点
first = newNode;
else //若非空，则将pred的下一个节点设置为newNode
pred.next = newNode;
size++; //增加size=size+1;
modCount++;
}

/**
* Unlinks non-null first node f.
* 将非空的首节点从链表中去除。
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null; 断言f非空，并且是首节点
final E element = f.item; //保留首节点元素
final Node<E> next = f.next; //保留首节点的下一个节点，为将下一个节点设为首节点
f.item = null; //将原首节点元素置空
f.next = null; // help GC 将原首节点的下一个节点置空
first = next; //将下一个节点设为首节点
if (next == null) //若next为空，则说明列表有且仅有一个结点，所以将last置空
last = null;
else //若不为空，则将此节点的前一节点置空，因为现在此节点next为首节点
next.prev = null;
size--; //列表大小-1
modCount++;
return element; //返回删除的首节点的元素
}

/**
* Unlinks non-null last node l.
* 将非空的末节点从链表中去除。
* 此方法同上
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null; \\断言非空末节点
final E element = l.item;
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev;
if (prev == null)
first = null;
else
prev.next = null;
size--;
modCount++;
return element;
}

/**
* Unlinks non-null node x.
* 解除掉非空结点x
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item; //保留删除节点元素
final Node<E> next = x.next; //保留x的下一节点
final Node<E> prev = x.prev; //保留x的前一节点

if (prev == null) { //若前一节点为空，则x为首节点，将x的下一节点设为首节点
first = next;
} else { //若非空，则将next给了x的前一节点的next,并将x.prev置空
prev.next = next;
x.prev = null;
}

if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}

x.item = null;
size--;
modCount++;
return element;
}

/**
* 返回列表首元素
*
* @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) //若首元素为空，则抛出NoSuchElementException
throw new NoSuchElementException();
return f.item;
}

/**
* Returns the last element in this list.
* 返回列表末元素
* @return the last element in this list
* @throws NoSuchElementException if this list is empty
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}

/**
* Removes and returns the first element from this list.
* 去除并返回列表首元素；
* @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);  //unlinkFirst(f)为具体去除方法
}

/**
* Removes and returns the last element from this list.
* 去除并返回列表末元素；
* @return the last element from this list
* @throws NoSuchElementException if this list is empty
*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}

/**
* Inserts the specified element at the beginning of this list.
* 为列表添加元素为e的首节点，此方法对外开放。
* @param e the element to add
*/
public void addFirst(E e) {
linkFirst(e);
}

/**
* Appends the specified element to the end of this list.
*  为列表添加元素为e的末节点，此方法对外开放。
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
*/
public void addLast(E e) {
linkLast(e);
}

/**
* 检测列表是否包含对象o,有，则返回true;无，则返回false.
*
* @param o element whose presence in this list is to be tested
* @return {@code true} if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) != -1;
}

/**
* 返回列表的大小.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}

/**
* 加入元素e到链表末尾.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
linkLast(e);
return true;
}

/**
* 去除对象o的元素节点。
* 若o为null,则在遍历后若存在则删除之,并返回true；
* 若为其他元素，则遍历后若存在则删除之，并返回true;
* 若既不为空，也不存在在列表中，则返回false.
*
* @param o element to be removed from this list, if present
* @return {@code true} if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}

/**
* 往链表中加入指定集合c中的所有元素
*
* @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); //在链表size处加入集合c中的元素
}

/**
* 在链表指定index位置插入集合c
*
* @param index index at which to insert the first element
*              from the specified collection
* @param c collection containing elements to be added to this list
* @return {@code true} if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index); //检测index范围合理性

Object[] a = c.toArray(); //将c转为数组
int numNew = a.length; //得到a数组的大小
if (numNew == 0) //若c集合为空，则返回false
return false;

Node<E> pred, succ; //succ当前要插入的节点，pred插入节点的前一节点
if (index == size) { //若插入的是size处，则将succ置空，因为size处无节点，并将pred指向最后一个节点
succ = null;
pred = last;
} else { //若插入的地方为列表节点存在的位置index处，则返回当前节点给succ，并将succ前一节点赋给pred.
succ = node(index);
pred = succ.prev;
}

for (Object o : a) { //循环插入a数组中的所有元素
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode; //将新插入的节点，作为下一个即将要插入的节点的前一节点
}

if (succ == null) {// 若插入的是size处，则将最后一个插入的元素节点作为链表末节点
last = pred;
} else { //若插入在中间节点，则为其建立双向链，指定前后节点。
pred.next = succ;
succ.prev = pred;
}

size += numNew; //增加列表大小size=size+numNew;
modCount++;
return true;
}

/**
* 清除链表中的所有元素
*/
public void clear() {
// Clearing all of the links between nodes is "unnecessary", but:
// - helps a generational GC if the discarded nodes inhabit
//   more than one generation
// - is sure to free memory even if there is a reachable Iterator
for (Node<E> x = first; x != null; ) { //遍历所有列表元素，并将其元素，前后节点均置空。
Node<E> next = x.next;
x.item = null;
x.next = null;
x.prev = null;
x = next;
}
first = last = null; //置空首末节点
size = 0; //设置链表大小为0
modCount++;
}

// Positional Access Operations

/**
* 返回链表指定索引index处的元素.
*
* @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); //检测索引范围
return node(index).item; //返回指定节点元素
}

/**
* 设置指定索引index处的节点元素为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); //检测index合理性
Node<E> x = node(index); //返回index处的节点
E oldVal = x.item; //保存节点x的元素
x.item = element; //将要设置的元素放入对应index处的节点x.item中
return oldVal;
}

/**
* 在index节点处加入元素为element的新节点
*
* @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); //核对index的合理性

if (index == size) //如果index=size，则将新节点插入最后作为末节点
linkLast(element);
else //否则，将元素节点加入到指定索引index节点的前面。
linkBefore(element, node(index));
}

/**
* 从链表中去出指定索引的节点，并返回去除的元素
*
* @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); //核对index的合理性
return unlink(node(index)); //进行去除操作
}

/**
* 检测index是否存在与链表中，存在，则返回true，否则，返回false.
*
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}

/**
* 检测index是否存在与链表中，存在，则返回true，否则，返回false.
* 与isElementIndex(index)区别在于，此方法多一个位置索引size!!!
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}

/**
* 返回出现异常时的提示信息
* 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;
}

//核对指定index处的元素是否合理，不符合则抛出IndexOutOfBoundsException().
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

//核对指定index处的索引本身是否合理，index=size也符合标准，不符合则抛出IndexOutOfBoundsException().
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* Returns the (non-null) Node at the specified element index.
* 返回指定index节点处的元素节点
*/
Node<E> node(int index) {
// assert isElementIndex(index);

if (index < (size >> 1)) { //size << 1 即size / 2; 在前一半遍历节约时间，提高效率
Node<E> x = first;
for (int i = 0; i < index; i++) //遍历到index-1，x=node(index).
x = x.next;
return x;
} else { //若index位于后一半，在后一半遍历更省时间，直至遍历到index+1，返回index处的节点
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

// Search Operations

/**
* 返回指定元素对应的第一次出现时的索引，若不存在，返回-1。
* 若x为空null，则用==直接比较；否则，用equals进行比较。
* index是另外定义的一个变量，与链表相独立！
*
* @param o element to search for
* @return the index of the first occurrence of the specified element in
*         this list, or -1 if this list does not contain the element
*/
public int indexOf(Object o) {
int index = 0;
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}

/**
* 返回指定元素对应的最后一次出现时的索引，若不存在，返回-1。
* 若x为空null，则用==直接比较；否则，用equals进行比较。
* index是另外定义的一个变量，与链表相独立！
*
* @param o element to search for
* @return the index of the last occurrence of the specified element in
*         this list, or -1 if this list does not contain the element
*/
public int lastIndexOf(Object o) {
int index = size;
if (o == null) {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (o.equals(x.item))
return index;
}
}
return -1;
}

// Queue operations.

/**
* 检测首节点是空的null，还是有元素存在！
*
* @return the head of this list, or {@code null} if this list is empty
* @since 1.5
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* 返回列表的头结点
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E element() {
return getFirst();
}

/**
* 检测头节点是否为空，为空则返回null，不为空则从链表中去出该头结点！
*
* @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);
}

/**
* 去除链表头结点.
*
* @return the head of this list
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E remove() {
return removeFirst();
}

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

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

/**
* Inserts the specified element at the end of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerLast})
* @since 1.6
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}

/**
* Retrieves, but does not remove, the first element of this list,
* or returns {@code null} if this list is empty.
*
* @return the first element of this list, or {@code null}
*         if this list is empty
* @since 1.6
*/
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* Retrieves, but does not remove, the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null}
*         if this list is empty
* @since 1.6
*/
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}

/**
* Retrieves and removes the first element of this list,
* or returns {@code null} if this list is empty.
*
* @return the first element of this list, or {@code null} if
*     this list is empty
* @since 1.6
*/
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* Retrieves and removes the last element of this list,
* or returns {@code null} if this list is empty.
*
* @return the last element of this list, or {@code null} if
*     this list is empty
* @since 1.6
*/
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}

/**
* Pushes an element onto the stack represented by this list.  In other
* words, inserts the element at the front of this list.
*
* <p>This method is equivalent to {@link #addFirst}.
*
* @param e the element to push
* @since 1.6
*/
public void push(E e) {
addFirst(e);
}

/**
* Pops an element from the stack represented by this list.  In other
* words, removes and returns the first element of this list.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return the element at the front of this list (which is the top
*         of the stack represented by this list)
* @throws NoSuchElementException if this list is empty
* @since 1.6
*/
public E pop() {
return removeFirst();
}

/**
* Removes the first occurrence of the specified element in this
* list (when traversing the list from head to tail).  If the list
* does not contain the element, it is unchanged.
*
* @param o element to be removed from this list, if present
* @return {@code true} if the list contained the specified element
* @since 1.6
*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}

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

/**
* 构造一个链表迭代器，增删改查链表元素
* Returns a list-iterator of the elements in this list (in proper
* sequence), starting at the specified position in the list.
* Obeys the general contract of {@code List.listIterator(int)}.<p>
*
* The list-iterator is <i>fail-fast</i>: if the list is structurally
* modified at any time after the Iterator is created, in any way except
* through the list-iterator's own {@code remove} or {@code add}
* methods, the list-iterator will throw a
* {@code ConcurrentModificationException}.  Thus, in the face of
* concurrent modification, the iterator fails quickly and cleanly, rather
* than risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
* @param index index of the first element to be returned from the
*              list-iterator (by a call to {@code next})
* @return a ListIterator of the elements in this list (in proper
*         sequence), starting at the specified position in the list
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see List#listIterator(int)
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index); //先核对index合理性
return new ListItr(index); //返回一个新建的由index开始迭代的ListItr对象
}

/*
* ListItr内部类
*/
private class ListItr implements ListIterator<E> {
private Node<E> lastReturned = null; //上一个已经返回的节点
private Node<E> next; //下一个即将要返回的节点
private int nextIndex; //链表节点索引
private int expectedModCount = modCount;

ListItr(int index) {
// assert isPositionIndex(index);
next = (index == size) ? null : node(index); //返回index处的节点作为下一节点
nextIndex = index; //返回index作为下一个节点索引。
}

//判断是否存在下一节点
public boolean hasNext() {
return nextIndex < size;
}

//返回下一节点的元素
public E next() {
checkForComodification();
if (!hasNext()) //不存在下一节点，则抛出NoSuchElementException()
throw new NoSuchElementException();

lastReturned = next; //将即将要返回的节点作为这次要返回的节点，返回完成则变为上次已经返回的节点。
next = next.next; //将next赋值为下一次要返回的节点
nextIndex++; //索引值随着next()的调用，自动增加
return lastReturned.item;
}

//判断是否存在前一个元素
public boolean hasPrevious() {
return nextIndex > 0;
}

//返回前一个元素
public E previous() {
checkForComodification();
if (!hasPrevious()) //若前边没有节点了，则抛出NoSuchElementException()
throw new NoSuchElementException();
//若next节点元素不存在，则返回最后一个节点，否则，返
//回本节点的前一节点，作为这次要返回的节点。也作为下次要返回节点的后一节点
lastReturned = next = (next == null) ? last : next.prev;
nextIndex--;
return lastReturned.item;
}

/*
* 返回下一个节点索引。
*/
public int nextIndex() {
return nextIndex;
}

//返回前一节点索引
public int previousIndex() {
return nextIndex - 1;
}

/*
* 去除上一个已经返回的节点。
* 调用此方法前必须先调用next(),previous()，才会对lastReturned赋值.
*
*/
public void remove() {
checkForComodification();
if (lastReturned == null)
throw new IllegalStateException();

Node<E> lastNext = lastReturned.next; //返回lastReturned节点的下一个节点
unlink(lastReturned); //删除lastReturned节点
if (next == lastReturned) //若调用了previous()方法，则next与lastReturned相等！
next = lastNext; //删除后下一个即将要返回的节点变为lastNext节点
else //否则，下一个节点索引nextindex-1
nextIndex--;
lastReturned = null;
expectedModCount++;
}

//设置上一个返回的节点元素为e.
public void set(E e) {
if (lastReturned == null)
throw new IllegalStateException();
checkForComodification();
lastReturned.item = e;
}

//加入元素e的节点
public void add(E e) {
checkForComodification();
lastReturned = null; //置空是为了避免在其后调用remove(),set()方法.
if (next == null) //若为空，则将其置为末节点
linkLast(e);
else //否则，在next节点之前插入e元素节点
linkBefore(e, next);
nextIndex++; //自动增加索引
expectedModCount++;
}

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

//类Node对象
private static class Node<E> {
E item; //Node的元素
Node<E> next; //Node的下一节点
Node<E> prev; //Node的前一节点

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
* 向前迭代的迭代器。
*/

private class DescendingIterator implements Iterator<E> {
private final ListItr itr = new ListItr(size());//构造一个同size大小的ListItr对象
public boolean hasNext() {
return itr.hasPrevious(); //返回是否存在向前迭代的下个元素
}
public E next() {
return itr.previous(); //返回向前迭代的下个元素
}
public void remove() {
itr.remove(); //去除某一节点
}
}

@SuppressWarnings("unchecked")
//克隆方法
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}

/**
* Returns a shallow copy of this {@code LinkedList}. (The elements
* themselves are not cloned.)
* 返回一个浅拷贝
* @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).
* 将链表转换为数组。
* <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];//新建一个size大小的数组
int i = 0;
for (Node<E> x = first; x != null; x = x.next)
result[i++] = x.item; //遍历将列表元素存入数组
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) //若a数组的长度小于链表大小，则新建一个size大小的a数组元素类型的新的a数组
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
int i = 0;
Object[] result = a; //将result也指向新建的a数组。
for (Node<E> x = first; x != null; x = x.next)
result[i++] = x.item; //再将链表元素赋给result数组

if (a.length > size)
a[size] = null; //若长度大于size，则将剩余元素置空，等待gc收集。

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());
}
}