public class LinkedList<E> 
extends AbstractSequentialList<E> 
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
private static class Node<E> {//静态内部类，为了方便使用，把初始化交给JVM
        E item; // 数据域
        Node<E> next; // 后继
        Node<E> prev; // 前驱
        
        // 构造函数，赋值前驱后继
        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }
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   //实现Serilizable接口时，将不需要序列化的属性前添加关键字transient，序列化对象的时候，这个属性就不会序列化到指定的目的地中。
    transient int size = 0;//实际元素个数
    //指向首节点
    transient Node<E> first;
    //指向最后一个节点
    transient Node<E> last;
    //注意，头结点、尾结点都有transient关键字修饰，这也意味着在序列化时该域是不会序列化的。
    -------------------------------------------------------------------------------------------
    //构建一个空列表
    public LinkedList() {
    }
    -------------------------------------------------------------------------------------------
    //构建一个包含集合c的列表
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }
    -------------------------------------------------------------------------------------------
   //将节点值为e的节点作为首节点
    private void linkFirst(E e) {
        final Node<E> f = first;
        //构建一个prev值为null,next为f,节点值为e的节点
        final Node<E> newNode = new Node<>(null, e, f);
        //将newNode作为首节点
        first = newNode;
        //如果newNode后面没有节点就将newNode作为最后一个节点
        if (f == null)
            last = newNode;
        //否则就将newNode赋给其prev
        else
            f.prev = newNode;
        //列表长度加一
        size++;
        modCount++;
    }
    //将节点值为e的节点作为最后一个节点
    void linkLast(E e) {
        // 保存尾结点，l为final类型，不可更改
        final Node<E> l = last;
        // 新生成结点的前驱为l,后继为null
        final Node<E> newNode = new Node<>(l, e, null);
        // 重新赋值尾结点
        last = newNode;    
        if (l == null) // l(尾节点)结点为空
            first = newNode; // 赋值头结点
        else // 尾结点不为空
            l.next = newNode; // 尾结点的后继为新生成的结点
        // 大小加1    
        size++;
        // 结构性修改加1
        modCount++;
    }
    //在非空节点succ之前插入节点e
    void linkBefore(E e, Node<E> succ) {
        final Node<E> pred = succ.prev;
        //将succ前面的节点和succ作为其prev和next
        final Node<E> newNode = new Node<>(pred, e, succ);
        //然后将newNode作为succ的prev
        succ.prev = newNode; 
        //如果原来succ是首节点，则将newNode设置为首节点
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }
     //释放非空的首节点f
    private E unlinkFirst(Node<E> f) {
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        //将first节点后面的节点设为首节点
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }
     //释放非空的尾节点l
       private E unlinkLast(Node<E> l) {
        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;
    }
    //删除非空节点x
    E unlink(Node<E> x) 
    {
        final E element = x.item;
        final Node<E> next = x.next;    //x后面的节点
        final Node<E> prev = x.prev;    //x前面的节点

        if (prev == null) {
            first = next;
        } else {
            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;
    }
    //返回列表首节点元素值
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }
    //返列表尾节点元素值
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }
    //移除首节点
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }
   //删除尾节点
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }
  //在列表首部插入节点e
    public void addFirst(E e) {
        linkFirst(e);
    }
    //在列表尾部插入节点e
    public void addLast(E e) {
        linkLast(e);
    }
   //判断列表中是否包含有元素o
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }
    //返回列表长度大小
    public int size() {
        return size;
    }
    //在列表尾部插入元素
    public boolean add(E e) {
        linkLast(e);
        return true;
    }
    //删除元素为o的节点
    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;
    }
   //将集合c中所有元素添加到列表的尾部
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
   //从指定的位置index开始，将集合c中的元素插入到列表中
    public boolean addAll(int index, Collection<? extends E> c) {
        //首先判断插入位置的合法性
        checkPositionIndex(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;
        Node<E> pred, succ;
        if (index == size) {//说明在列表尾部插入集合元素
            succ = null;
            pred = last;
        } 
        else {  //否则，找到index所在的节点
            succ = node(index);
            pred = succ.prev;
        }
        for (Object o : a) {
            @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;
        }
        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }
        size += numNew;
        modCount++;
        return true;
    }
    //删除列表中所有节点
    public void clear() {
        for (Node<E> x = first; x != null; )  
        {
            Node<E> next = x.next;//当前指针后挪
            x.item = null;//清楚前一个节点
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }
    //获取指定索引位置节点的元素值
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    //替换指定索引位置节点的元素值
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }
    //在指定索引位置之前插入元素e
    public void add(int index, E element) {
        checkPositionIndex(index);   
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    //删除指定位置的元素
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
    //判断指定索引位置的元素是否存在
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }
    //构建 IndexOutOfBoundsException详细信息
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }
    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    //返回指定索引位置的节点
    Node<E> node(int index) {
        //此处是一个小技巧，当index<size/2时，从列表前半部分开始，否则从后半部分开始
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }//返回列表中第一次出现o的位置，若不存在则返回-1
    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;
    }
    //逆向搜索，返回第一出现o的位置，不存在则返回-1
    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;
    }
   //获取列表首节点元素值
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    //获取列表首节点元素值，若为空则抛异常
    public E element() {
        return getFirst();
    }
   //检索首节点，若空则返回null,不为空则返回其元素值并删除首节点
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }
    //检索首节点，若空则抛异常,不为空则返回其元素值并删除首节点
    public E remove() {
        return removeFirst();
    }
   //在列表尾部增加节点e
    public boolean offer(E e) {
        return add(e);
    }
   //在列表首部插入节点e
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }
  //在列表尾部插入节点e
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }
  //获取列表尾节点元素值
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }
   //入栈
    public void push(E e)
    {
        addFirst(e);
    }
    //出栈
    public E pop() {
        return removeFirst();
    }
    //删除列表中第一出现o的节点
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }
    //逆向搜索，删除第一次出现o的节点
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }