LinkedList源码分析

本文源代码版本为1.7

LinkedList：以双向链表实现。链表无容量限制，但双向链表本身使用了更多空间，也需要额外的链表指针操作。
注意：构建的链表不带头节点。

链表的节点代码：

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;
    }
}

LinkedList类头：

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

可以看出LinkedList 继承AbstractSequentialList 抽象类，实现了List，Deque，Cloneable，Serializable 几个接口。
其中，AbstractSequentialList 继承 AbstractList，是对其中方法的再抽象，其主要作用是最大限度地减少了实现受“连续访问”数据存储（如链接列表）支持的此接口所需的工作，简单说就是，如果需要快速的添加删除数据等，用AbstractSequentialList抽象类，若是需要快速随机的访问数据等用AbstractList抽象类。
实现了Deque接口，即代表着LinkedList可以实现队列的功能。

属性：

//链表的长度
transient int size = 0;
//指向链表头
transient Node<E> first;
//指向链表尾
transient Node<E> last;
private static final long serialVersionUID = 876323262645176354L;

构造函数：

//空构造方法
public LinkedList() {
}

//将集合转换成链表
public LinkedList(Collection<? extends E> c) {
    this();
    addAll(c);
}

链表底层操作方法：

//私有函数，用户调用不了，删除链表的第一个节点（即f,f必须等于first，且不能为空），返回删除节点的值
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
    first = next;
    if (next == null)
        last = null;
    else
        next.prev = null;
    size--;
    modCount++;
    return element;
}

//私有函数，用户调用不了，删除链表的最后一个节点，（即l,l == last && l != null），返回删除节点的值
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;
}

//删除链表中的节点x，x != null
E unlink(Node<E> x) {
    final E element = x.item;
    final Node<E> next = x.next;
    final Node<E> prev = x.prev;

    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;
}

• 添加链表元素的函数：

//在链表头部节点前，插入元素e
private void linkFirst(E e) {
    final Node<E> f = first;
    final Node<E> newNode = new Node<>(null, e, f);
    first = newNode;
    if (f == null)
        last = newNode;
    else
        f.prev = newNode;
    size++;
    modCount++;
}

//在链表尾部节点后，插入元素e的节点
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++;
}

//在链表中节点succ（必须是非null）的前面插入元素是e的节点
void linkBefore(E e, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> newNode = new Node<>(pred, e, succ);
    succ.prev = newNode;
    if (pred == null)
        first = newNode;
    else
        pred.next = newNode;
    size++;
    modCount++;
}

//在链表头部节点前，添加节点e
public void addFirst(E e) {
    linkFirst(e);
}

//在链表尾部节点后，添加节点e
public void addLast(E e) {
    linkLast(e);
}

//在链表尾部节点后，添加节点e
public boolean add(E e) {
   linkLast(e);
   return true;
}

//将集合c中的所有元素，按集合中的顺序，生成节点，添加到链表的尾部
public boolean addAll(Collection<? extends E> c) {
   return addAll(size, c);
}

//将集合c中的所有元素，按集合中的顺序，生成节点，添加到链表中index位置的节点前，若index==size，则添加到节点尾部。
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;
}

//在当前链表index位置的节点前，添加元素值为element的节点，若index==size，则在链表尾部添加。
public void add(int index, E element) {
   checkPositionIndex(index);

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

• 查找链表中节点的函数：

//查找index位置的节点，并返回该节点，此函数不对传入的位置进行安全性检查，而且当index小于当前链表长度的一般时，从前向后查找，大于时，从后向前查找，提高了效率。
Node<E> node(int index) {
    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;
    }
}

//在当前链表中，从前向后查找第一个元素值为o的节点位置，找不到返回-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;
}

//在当前链表中，从前向后查找第一个元素值为o的节点位置，找不到返回-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;
}

• 获取链表中节点的元素值函数：

//获取链表的第一个节点元素值
public E getFirst() {
    final Node<E> f = first;
    if (f == null)
        throw new NoSuchElementException();
    return f.item;
}

//获取链表最后一个节点的元素值
public E getLast() {
    final Node<E> l = last;
    if (l == null)
        throw new NoSuchElementException();
    return l.item;
}

//获取链表中index位置的节点的元素值
public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}

• 删除链表中节点的函数：

//删除链表中第一个节点，返回其元素值。
public E removeFirst() {
    final Node<E> f = first;
    if (f == null)
        throw new NoSuchElementException();
    return unlinkFirst(f);
}

//删除链表中最后一个节点，返回其元素值
public E removeLast() {
    final Node<E> l = last;
    if (l == null)
        throw new NoSuchElementException();
    return unlinkLast(l);
}

//删除链表中元素值为o的节点，注意其调用了o的equals()进行元素的比较,若没有改写o的equals(),可能会出错。
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;
}

//清除链表中的所用节点
public void clear() {
    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++;
}

//删除链表中index位置的节点，返回其元素值。
public E remove(int index) {
   checkElementIndex(index);
    return unlink(node(index));
}

//删除链表中第一个元素值为o的节点，从前向后找
public boolean removeFirstOccurrence(Object o) {
    return remove(o);
}

//删除链表中第一个元素值为o的节点，从后向前找
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;
}

• 其他函数：

public boolean contains(Object o)
public E set(int index, E element)
private void writeObject(java.io.ObjectOutputStream s)
private void readObject(java.io.ObjectInputStream s)

• LinkedList实现的Deque双端队列的方法：

注意：可以用这些方法实现队列和栈
public E peek()
public E element()
public E poll()
public E remove()
public boolean offer(E e)
public boolean offerFirst(E e)
public boolean offerLast(E e)
public E peekFirst()
public E peekLast()
public E pollFirst()
public E pollLast()
public void push(E e)
public E pop()

• 几个重要函数：

克隆：就是一个浅拷贝（只是拷贝了引用）
private LinkedList<E> superClone() {
   try {
        return (LinkedList<E>) super.clone();
    } catch (CloneNotSupportedException e) {
        throw new InternalError();
    }
}

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;
}

迭代器：实现类前后都可以遍历的迭代器类
public ListIterator<E> listIterator(int index) {
   checkPositionIndex(index);
    return new ListItr(index);
}

private class ListItr implements ListIterator<E> {
    private Node<E> lastReturned = null;
    private Node<E> next;
    private int nextIndex;
    private int expectedModCount = modCount;

    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;
        next = next.next;
        nextIndex++;
        return lastReturned.item;
    }

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

    public E previous() {
        checkForComodification();
        if (!hasPrevious())
            throw new NoSuchElementException();

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

    public int nextIndex() {
        return nextIndex;
    }

    public int previousIndex() {
        return nextIndex - 1;
    }

    public void remove() {
        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) {
        if (lastReturned == null)
            throw new IllegalStateException();
        checkForComodification();
        lastReturned.item = e;
    }

    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();
    }
}

//降序迭代器类，从末尾向头节点遍历的迭代器类
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();
    }
}