LinkedList源码

参考博客: https://blog.csdn.net/eson_15/article/details/51135944
LinkedList与collection的关系:
　　LinkedList是一个继承与AbatractSequentiaList的双向链表,他也可以被当作堆栈,队列或双端队列进行操作.
　　LinkedList实现了List接口,所以能对它进行队列操作.
　　LinkedList实现了Deque接口,能将LinkedList当作双端队列使用.
　　LinkedList实现了Serializable接口,所以它也支持序列化,能通过序列化去传输.
　　LinkedList是线程不安全的
　　JDK1.7
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/*双向链表*/ 
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; //链表的头结点  
    transient Node<E> last; //链表的尾节点  
   
    public LinkedList() { //默认构造函数，创建一个空链表  
    }  
   
    //按照c中的元素生成一个LinkedList  
    public LinkedList(Collection<? extends E> c) {  
        this();  
        addAll(c); //将c中的元素添加到空链表的尾部  
    }  
   
    /***************************** 添加头结点 ********************************/ 
    public void addFirst(E e) {  
        linkFirst(e);  
    }  
   
    private void linkFirst(E e) {  
        final Node<E> f = first; //f指向头结点  
        //生成一个新结点e，其前向指针为null，后向指针为f    
        final Node<E> newNode = new Node<>(null, e, f);  
        first = newNode; //first指向新生成的结点，f保存着老的头结点信息    
        if (f == null)  
            last = newNode; //如果f为null，则表示整个链表目前是空的，则尾结点也指向新结点  
        else 
            f.prev = newNode;  
        size++;  
        modCount++; //修改次数+1  
    }  
　　  　(我自己有点懵) 所以画了个图,偏于理解
    /****************** 添加尾节点，与上面添加头结点原理一样 ******************/ 
    public void addLast(E e) {  
        linkLast(e);  
    }  
   
    void linkLast(E e) {  
        final Node<E> l = last;  
        final Node<E> newNode = new Node<>(l, e, null);  
        last = newNode;  
        if (l == null)  
            first = newNode;  
        else 
            l.next = newNode;  
        size++;  
        modCount++;  
    }  
   
    /****************** 在非空节点succ之前插入新节点e ************************/ 
    void linkBefore(E e, Node<E> succ) {  
        // 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;  
         //生成一个新结点e,其前向指针指向pred，后向指针指向succ    
        final Node<E> newNode = new Node<>(pred, e, succ);  
        succ.prev = newNode; //succ的前向指针指向newNode    
        if (pred == null)  
            //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   
            first = newNode;  
        else 
            //pred的后向指针指向新生成的结点，此时已经完成了结点的插入操作  
            pred.next = newNode;  
        size++;  
        modCount++;  
    }  
   
    /*********************** 删除头结点,并返回头结点的值 *********************/ 
    public E removeFirst() {  
        final Node<E> f = first;  
        if (f == null)  
            throw new NoSuchElementException();  
        return unlinkFirst(f); //private方法  
    }  
       
    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;  
        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 
            //修改next的前向指针，因为first结点的前向指针为null   
            next.prev = null;  
        size--;  
        modCount++;  
        return element;  
    }  
   
    /********************** 删除尾节点，并返回尾节点的值 ********************/ 
    public E removeLast() {  
        final Node<E> l = last;  
        if (l == null)  
            throw new NoSuchElementException();  
        return unlinkLast(l); //private方法  
    }  
   
    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;  
        last = prev;  
        if (prev == null)  
            first = null;  
        else 
            prev.next = null;  
        size--;  
        modCount++;  
        return element;  
    }  
   
    /******************** 删除为空节点x，并返回该节点的值 ******************/ 
    E unlink(Node<E> x) {  
        // 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的后向结点）  
            first = next;  
        } else {  
            prev.next = next; //x的前向结点的后向指针指向x的后向结点    
            x.prev = null; //释放x的前向指针    
        }  
   
        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的前向结点)  
            last = prev;  
        } else {  
            next.prev = prev; //x的后向结点的前向指针指向x的前向结点  
            x.next = null; //释放x的后向指针    
        }  
   
        x.item = null; //释放x的值节点,此时x节点可以完全被GC回收    
        size--;  
        modCount++;  
        return element;  
    }  
   
    /********************** 获得头结点的值 ********************/ 
    public E getFirst() {  
        final Node<E> f = first;  
        if (f == null)  
            throw new NoSuchElementException();  
        return f.item;  
    }  
   
    /********************** 获得尾结点的值 ********************/ 
    public E getLast() {  
        final Node<E> l = last;  
        if (l == null)  
            throw new NoSuchElementException();  
        return l.item;  
    }  
   
    /*************** 判断元素（值为o）是否在链表中 *************/ 
    public boolean contains(Object o) {  
        return indexOf(o) != -1; //定位元素  
    }  
   
    //返回元素个数  
    public int size() {  
        return size;  
    }  
   
    //向链表尾部添加元素e  
    public boolean add(E e) {  
        linkLast(e);  
        return true;  
    }  
   
    /*************** 删除值为o的元素 *************/ 
    public boolean remove(Object o) {  
        if (o == null) {  
            for (Node<E> x = first; x != null; x = x.next) {  
                if (x.item == null) { //找到即返回  
                    unlink(x);  
                    return true;  
                }  
            }  
        } else {//o不为空  
            for (Node<E> x = first; x != null; x = x.next) {  
                if (o.equals(x.item)) {  
                    unlink(x);  
                    return true;  
                }  
            }  
        }  
        return false;  
    }  
   
    /*************** 将集合e中所有元素添加到链表中 *************/ 
    public boolean addAll(Collection<? extends E> c) {  
        return addAll(size, c);  
    }  
    //从index开始，向后添加的  
    public boolean addAll(int index, Collection<? extends E> c) {  
        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index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的前向节点  
        }  
   
        //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  
        for (Object o : a) {  
            @SuppressWarnings("unchecked") E e = (E) o;  
            Node<E> newNode = new Node<>(pred, e, null);  
            if (pred == null)  
                first = newNode; //如果pred为null，则succ为头结点  
            else 
                pred.next = newNode; //pred的后向指针指向新节点  
            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表示所有元素添加好之后的最后那个节点，此时pred的后向指针指向之前记录的节点，即index处的节点  
            pred.next = succ;  
            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;  
    }  
   
    /******************** 清空链表 *************************/ 
    public void clear() {  
        for (Node<E> x = first; x != null; ) {  
            Node<E> next = x.next;  
            x.item = null; //释放值结点，便于GC回收    
            x.next = null; //释放前向指针    
            x.prev = null; //释放前后指针    
            x = next; //后向遍历    
        }  
        first = last = null; //释放头尾节点  
        size = 0;  
        modCount++;  
    }  
   
   
    /******************* Positional Access Operations ***********************/ 
   
    //获得第index个节点的值  
    public E get(int index) {  
        checkElementIndex(index);  
        return node(index).item;  
    }  
   
    //设置第index元素的值  
    public E set(int index, E element) {  
        checkElementIndex(index);  
        Node<E> x = node(index);  
        E oldVal = x.item;  
        x.item = element;  
        return oldVal;  
    }  
   
    //在index个节点之前添加新的节点  
    public void add(int index, E element) {  
        checkPositionIndex(index);  
   
        if (index == size)  
            linkLast(element);  
        else 
            linkBefore(element, node(index));  
    }  
   
    //删除第index个节点  
    public E remove(int index) {  
        checkElementIndex(index);  
        return unlink(node(index));  
    }  
   
    //判断index是否为链表中的元素下标  
    private boolean isElementIndex(int index) {  
        return index >= 0 && index < size;  
    }  
   
    //判断index是否为链表中的元素下标。。。包含了size  
    private boolean isPositionIndex(int index) {  
        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));  
    }  
   
    //定位index处的节点  
    Node<E> node(int index) {  
        // assert isElementIndex(index);  
        //index<size/2时，从头开始找  
        if (index < (size >> 1)) {  
            Node<E> x = first;  
            for (int i = 0; i < index; i++)  
                x = x.next;  
            return x;  
        } else { //index>=size/2时，从尾开始找  
            Node<E> x = last;  
            for (int i = size - 1; i > index; i--)  
                x = x.prev;  
            return x;  
        }  
    }  
   
    /*************************** Search Operations *************************/ 
   
    //返回首次出现指定元素值o的节点索引  
    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; //没有则返回-1  
    }  
   
    //返回最后一次出现指定元素值o的节点索引  
    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 ***********************/ 
    //下面是与栈和队列相关的操作了  
    //实现栈的操作，返回第一个元素的值  
    public E peek() {  
        final Node<E> f = first;  
        return (f == null) ? null : f.item; //不删除  
    }  
   
    //实现队列操作，返回第一个节点  
    public E element() {  
        return getFirst();  
    }  
   
    //实现栈的操作，弹出第一个节点  
    public E poll() {  
        final Node<E> f = first;  
        return (f == null) ? null : unlinkFirst(f); //删除  
    }  
   
    //实现队列操作，删除节点  
    public E remove() {  
        return removeFirst();  
    }  
   
    //添加节点  
    public boolean offer(E e) {  
        return add(e);  
    }  
   
    /************************* Deque operations **********************/ 
    //下面都是和双端队列相关的操作了  
    //添加头结点  
    public boolean offerFirst(E e) {  
        addFirst(e);  
        return true;  
    }  
   
    //添加尾节点  
    public boolean offerLast(E e) {  
        addLast(e);  
        return true;  
    }  
   
    //返回头结点的值  
    public E peekFirst() {  
        final Node<E> f = first;  
        return (f == null) ? null : f.item;  
     }  
   
    //返回尾节点的值  
    public E peekLast() {  
        final Node<E> l = last;  
        return (l == null) ? null : l.item;  
    }  
   
    //弹出头结点  
    public E pollFirst() {  
        final Node<E> f = first;  
        return (f == null) ? null : unlinkFirst(f); //删除  
    }  
   
    //弹出尾节点  
    public E pollLast() {  
        final Node<E> l = last;  
        return (l == null) ? null : unlinkLast(l); //删除  
    }  
   
    //栈操作，添加头结点  
    public void push(E e) {  
        addFirst(e);  
    }  
   
    //栈操作，删除头结点  
    public E pop() {  
        return removeFirst();  
    }  
   
    //删除第一次出现o的节点  
    public boolean removeFirstOccurrence(Object o) {  
        return remove(o);  
    }  
   
    //删除最后一次出现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;  
    }  
   
    /************************* ListIterator ***********************/ 
       
    public ListIterator<E> listIterator(int index) {  
        checkPositionIndex(index);  
        return new ListItr(index); //ListItr是一个双向迭代器  
    }  
   
    //实现双向迭代器  
    private class ListItr implements ListIterator<E> {  
        private Node<E> lastReturned = null;//记录当前节点信息  
        private Node<E> next; //当前节点的后向节点  
        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;  
        }  
   
        //返回前向节点的值  
        public E previous() {  
            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;  
        }  
   
        //在当前节点前面插入新节点信息  
        public void add(E e) {  
            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> {  
        E item;  
        Node<E> next;  
        Node<E> prev;  
   
        Node(Node<E> prev, E element, Node<E> next) {  
            this.item = element;  
            this.next = next;  
            this.prev = prev;  
        }  
    }  
   
    //返回前向迭代器  
    public Iterator<E> descendingIterator() {  
        return new DescendingIterator();  
    }  
   
    //通过ListItr.previous来提供前向迭代器，方向与原来相反  
    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();  
        }  
    }  
   
    //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  
    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;  
    }  
   
    /*************************** toArray ****************************/ 
    //返回LinkedList的Object[]数组  
    public Object[] toArray() {  
        Object[] result = new Object[size];  
        int i = 0;  
        for (Node<E> x = first; x != null; x = x.next)  
            result[i++] = x.item;  
        return result;  
    }  
   
    //返回LinkedList的模板数组，存储在a中  
    @SuppressWarnings("unchecked")  
    public <T> T[] toArray(T[] a) {  
        if (a.length < size)  
            //如果a的大小 < LinkedList的元素个数，意味着数组a不能容纳LinkedList的全部元素  
            //则新建一个T[]数组，T[]的大小为LinkedList大小，并将T[]赋给a  
            a = (T[])java.lang.reflect.Array.newInstance(  
                                a.getClass().getComponentType(), size);  
        //如果a大小够容纳LinkedList的全部元素  
        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;  
   
    /************************* Serializable **************************/ 
    //java.io.Serializable的写入函数  
    //将LinkedList的“容量，所有元素值”写入到输出流中  
    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); //写入容量  
   
        // Write out all elements in the proper order.  
        for (Node<E> x = first; x != null; x = x.next) //写入所有数据  
            s.writeObject(x.item);  
    }  
   
    //java.io.Serializable的读取函数：根据写入方式反向读出  
    //先将LinkedList的“容量”读出，然后将“所有元素值”读出  
    @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(); //读出容量  
   
        // Read in all elements in the proper order.  
        for (int i = 0; i < size; i++) //读出所有元素值  
            linkLast((E)s.readObject());  
    }  
}  