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());
    }
}