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

list—LinkedList:

1.LinkedList源码：

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());    }}