JAVA集合----LinkedList源码剖析

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;//LinkedList中元素的个数

    transient Node<E> first;//头引用   //链表的头节点

    transient Node<E> last;//尾引用   //链表的尾节点

    public LinkedList() {//默认构造函数，创建一个空链表
    }

    public LinkedList(Collection<? extends E> c) {//按照c中的元素生成一个LinkedList
        this();
        addAll(c);//将c中的元素添加到空链表的尾部
    }

    private void linkFirst(E e) {
        final Node<E> f = first;//头
        final Node<E> newNode = new Node<>(null, e, f); //创建一个新结点，其前驱指针为空，后继指针指向first所指结点（可能为null）
        first = newNode;//first指针始终指向链表的头
        if (f == null)
            last = newNode;
        else
            f.prev = newNode; //如果头插之前有结点，将其前驱指针指向新结点
        size++;
        modCount++;  //记录修改次数，迭代时检测快速失败
    }

    void linkLast(E e) {
        final Node<E> l = last;//尾
        final Node<E> newNode = new Node<>(l, e, null);   //创建一个新结点，其后继指针为空，前驱指针指向last所指结点（可能为null）
        last = newNode;
        if (l == null)  //如果LinkedList刚创建，此时新创建的结点既然是尾结点也是头结点 
            first = newNode;
        else
            l.next = newNode;  //如果尾插之前有结点，将其后继指针指向新结点
        size++;
        modCount++;
    }

    void linkBefore(E e, Node<E> succ) {//在非空节点succ之前插入新节点e
        // assert succ != null;//外界调用需保证succ不为null,否则程序会抛出空指针异常
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);//生成一个新结点e,其前驱指针指向pred，后继指针指向succ
        succ.prev = newNode;//succ的前驱指针指向newNode
        if (pred == null)
            first = newNode;//如果pred为null，则表示succ为头结点，此时头结点指向最新生成的结点newNode
        else
            pred.next = newNode;//pred的后继指针指向新生成的结点，此时已经完成了结点的插入操作
        size++;
        modCount++;
    }

    //头删法
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;//需确保f为头结点,且链表不为Null
        final E element = f.item;//获得节点的值
        final Node<E> next = f.next;//获得头结点下一个节点
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)//如果next为null，则表示f为last结点，此时链表即为空链表
            last = null;
        else
            next.prev = null;//修改next的前驱指针为NULL，因为first结点的前驱指针为null
        size--;
        modCount++;
        return element;
    }

    //尾删法
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        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;
    }

    E unlink(Node<E> x) {//删除节点x，并返回该节点的值
        // assert x != null;//需确保x不为null，否则后续操作会抛出空指针异常
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) { //如果prev为空，则x结点为first结点，此时first结点指向next结点（x的后继结点）
			
            first = next;
        } else {
            prev.next = next;//x的前驱结点的后继指针指向x的后继结点
            x.prev = null;//gc
        }

        if (next == null) {//如果next结点为空，则x结点为尾部结点，此时last结点指向prev结点(x的前驱结点)
            last = prev;
        } else {
            next.prev = prev;//x的后继结点的前驱指针指向x的前驱结点
            x.next = null;//gc
        }

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

    public E getFirst() {//获得头结点的值
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    public E getLast() {//获得尾结点的值
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    public E removeFirst() {//删除头结点
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    public E removeLast() {//删除尾结点
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    public void addFirst(E e) {//头插法
        linkFirst(e);
    }

    public void addLast(E e) {//尾插法
        linkLast(e);
    }

    public boolean contains(Object o) {//判断元素（值为o）是否在链表中
        return indexOf(o) != -1;//定位元素
    }

    public int size() {//返回元素个数
        return size;
    }

    public boolean add(E e) {//向链表尾部添加元素e
        linkLast(e);
        return true;
    }

    public boolean remove(Object o) {//删除值为o的元素
        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;
    }

    public boolean addAll(Collection<? extends E> c) {//将集合c中所有元素添加到链表中
        return addAll(size, c);
    }

    public boolean addAll(int index, Collection<? extends E> c) {//从index开始，向后添加集合c中的元素
        checkPositionIndex(index);//判断index是否越界

        Object[] a = c.toArray();//将集合c转换为数组
        int numNew = a.length;
        if (numNew == 0)
            return false;

        Node<E> pred, succ;
        if (index == size) {//即a.length个节点在尾节点后面
            succ = null;
            pred = last;//pred指向尾节点
        } else {
            succ = node(index);//succ指向第index个节点
            pred = succ.prev;//pred指向succ的前驱节点
        }

        for (Object o : a) {//for循环结束后，a里面的元素都添加到当前链表里了，向后添加
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
            if (pred == null)//如果pred为null，则succ为头结点
                first = newNode;
            else
                pred.next = newNode;//pred的后继指针指向新节点
            pred = newNode;//pred指向新节点，即往后移动一个节点，用于下一次循环
        }

        if (succ == null) {//succ为null表示index为尾节点之后
            last = pred;
        } else {
            pred.next = succ;//pred表示所有元素添加好之后的最后那个节点，此时pred的后继指针指向之前记录的节点，即index处的节点
            succ.prev = pred;//之前记录的结点指向添加元素之后的最后结点
        }

        size += numNew;
        modCount++;
        return true;
    }

    public void clear() {//清空链表
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;//置空值，便于GC回收 
            x.next = null;//置空后继指针
            x.prev = null;//置空前驱指针
            x = next;
        }
        first = last = null;//释放头尾节点
        size = 0;
        modCount++;
    }

    public E get(int index) {//获得第index个节点的值
        checkElementIndex(index);
        return node(index).item;
    }

    public E set(int index, E element) {//设置第index个位置元素的值
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    public void add(int index, E element) {//在index处节点之前添加新的节点
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    public E remove(int index) {//删除第index个节点
        checkElementIndex(index);
        return unlink(node(index));
    }

    private boolean isElementIndex(int index) {//判断index是否为链表中的元素下标
        return index >= 0 && index < size;
    }

    private boolean isPositionIndex(int index) {//判断index是否为链表中的元素下标,包含size位置
        return index >= 0 && index <= size;
    }

    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));
    }

    Node<E> node(int index) {//定位index处的节点
        // assert isElementIndex(index);

        if (index < (size >> 1)) {//index<size/2时，从头节点开始找
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {//index>=size/2时，从尾节点开始找
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

    public int indexOf(Object o) {//返回首次出现指定元素值o的节点下标
        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;
    }

    public int lastIndexOf(Object o) {//返回最后一次出现指定元素值o的节点下标
        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;
    }
    
	//下面是与栈和队列相关的操作了
	//实现栈的操作，获得栈顶元素
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    public E element() {//实现队列操作，返回第一个节点
        return getFirst();
    }

    public E poll() {//实现栈的操作，删除第一个节点
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    public E remove() {//实现队列操作，删除第一个节点
        return removeFirst();
    }

    public boolean offer(E e) {//添加节点
        return add(e);//默认向链表尾部增加元素
    }

    /************************* Deque operations **********************/
	//下面都是和双端队列相关的操作了
	//头插
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    public boolean offerLast(E e) {//尾插
        addLast(e);
        return true;
    }

    public E peekFirst() {//返回头结点的值
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

    public E peekLast() {//返回尾节点的值
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    public E pollFirst() {//弹出头结点
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    public E pollLast() {//弹出尾节点
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    public void push(E e) {//栈操作,头插
        addFirst(e);
    }

    public E pop() {//栈操作,删除头结点
        return removeFirst();
    }

    public boolean removeFirstOccurrence(Object o) {//删除第一次出现o的节点
        return remove(o);
    }

    public boolean removeLastOccurrence(Object o) {//删除最后一次出现o的节点
        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;
    }


    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);//ListItr是一个双向迭代器
    }

    private class ListItr implements ListIterator<E> {//实现双向迭代器
        private Node<E> lastReturned = null;//记录当前节点信息
        private Node<E> next;//当前节点的后继节点
        private int nextIndex;//当前节点的索引
        private int expectedModCount = modCount;//修改次数

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

        public boolean hasNext() {
            return nextIndex < size;
        }

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;//记录当前节点
            next = next.next;//向后移动一个位置
            nextIndex++;//节点索引+1
            return lastReturned.item;//返回当前节点的值
        }

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        public E previous() {//返回前驱节点的值
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {//返回当前节点的索引
            return nextIndex;
        }

        public int previousIndex() {//返回当前节点的前一个索引
            return nextIndex - 1;
        }

        public void remove() {//删除当前节点
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {//设置当前节点的值
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {//在当前节点前面插入新节点信息
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

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

    private static class Node<E> {//静态内部类
        E item;//value
        Node<E> next;//后继
        Node<E> prev;//前驱

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

    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }

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

    public Object clone() {//克隆操作，执行浅拷贝，只复制引用，没有复制引用指向的内存
        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;
    }

    public Object[] toArray() {//返回LinkedList的Object[]数组
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    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);
    }

    @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());
    }
}