public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

LinkedList是一个双向链表的实现，允许所有的元素，包括null。

对于index操作，会从链头到链尾地搜索，即使它靠近一个特殊索引位置。

非线程安全的，在多线程环境下，需要外部同步或调用Collections.synchronizedList(new LinkedList(…));获得一个线程安全的链表。

迭代器是快速失败的。

3个实例变量

//链表大小
transient int size = 0;

//链头
transient Node<E> first;

//链尾
transient Node<E> last;

2个构造器

//空链表
public LinkedList() {
}

//根据c创建链表,链表的顺序取决于c的iterator返回元素的顺序
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

操作

//返回链表第一个元素
public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
}

//返回链表最后一个元素
public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
}

//删除第一个元素,返回值是第一个元素的值
public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
}

//删除最后一个元素,返回值是最后一个元素
public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
}

//插入一个元素作为链头
public void addFirst(E e) {
        linkFirst(e);
}

//插入一个元素作为链尾
public void addLast(E e) {
        linkLast(e);
}

//判断链表是否含有o
public boolean contains(Object o) {
        return indexOf(o) != -1;
}

//返回链表大小
public int size() {
        return size;
}

//与addLast的效果一样,在链尾添加一个元素
public boolean add(E e) {
        linkLast(e);
        return true;
}

//删除从链头开始遇到第一个与o等同的元素,如果返回true,表示删除成功
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c集合的元素添加到链表,插入顺序取决于c的iterator返回的元素顺序
public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
}

//在index位置开始将c集合的元素添加到链表,插入顺序取决于c的iterator返回的元素顺序
public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;

        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            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;
}

//清空链表
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; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
}

//获取在index位置的元素
public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
}

//将index位置的元素的值修改为element
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位置插入一个element元素
public void add(int index, E element) {
        checkPositionIndex(index);

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

//删除index位置的元素,返回值是被删除元素的值
public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
}

//返回从链头到链尾第一个遇到与o等同的元素所在的位置,如果不存在返回-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;
}

//返回从链尾到链头第一个遇到与o等同的元素所在的位置,如果不存在返回-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;
}

//返回链头元素值
public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
}

//返回链头元素值,如果链表为空,抛出NoSuchElementException
public E element() {
        return getFirst();
}

//返回并删除链头元素值
public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
}

//返回并删除链头元素值,如果链表为 空,抛出NoSuchElementException
public E remove() {
        return removeFirst();
}

//在链尾添加元素e
public boolean offer(E e) {
        return add(e);
}

//在链头插入元素e
public boolean offerFirst(E e) {
        addFirst(e);
        return true;
}

//在链尾插入元素e
public boolean offerLast(E e) {
        addLast(e);
        return true;
}

//返回链头元素,如果链表为空,返回null
public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
}

//返回链尾元素,如果链表为空,返回null
public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
}

//返回并删除链头元素
public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
}

//返回并删除链尾元素
public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
}

//在链头插入元素e
public void push(E e) {
        addFirst(e);
}

//删除并返回链头元素
public E pop() {
        return removeFirst();
}

//删除从链头到结尾第一个遇到与o等同的元素,如果不存在,什么也不做
public boolean removeFirstOccurrence(Object o) {
        return remove(o);
}

//删除从链尾到链头第一个遇到与o等同的元素,如果不存在,什么也不做
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;
}

//返回一个列表迭代器,可以向前或向后迭代,index指定起始位置
public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
}

//返回从链尾开始的迭代器,在ListIterator的基础上实现的
public Iterator<E> descendingIterator() {
        return new DescendingIterator();
}

支持clone

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;
}

支持序列化和反序列化

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);
}

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());
}