LinkedList 的实现原理

本文为博客园作者所写： 一寸HUI，个人博客地址：https://www.cnblogs.com/zsql/

简单的一个类就直接说了。LinkedList 的底层结构是一个带头/尾指针的双向链表，可以快速的对头/尾节点 进行操作，它允许插 入所有元素，包括 null。 相比数组（这里可以对比ArrayList源码分析进行查看），链表的特点就是在指定位置插入和删除元素的效率较高，但是查找的 效率就不如数组那么高了。如果熟悉双向链表这个数据结构，其实就很简单了，无非就是实现一些数据的添加，删除，查询，遍历等功能，双向链表的结构图如下：

每一个数据（节点）都包含3个部分，一个是数据本身item，一个是指向下一个节点的next指针，还有就是指向上一个节点的prev指针，另外，双向链表还有一个 first 指针，指向头节点，和 last 指针，指向尾节点。，在LinkedList类中通过私有的静态内部类Node作为每一个数据的封装。具体实现如下：

private static class Node<E> {  //这个类就是用来封装双向链表中的每一个数据，也是上图中的每一个框
        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;
        }
    }

接下看看LinkList类的定义：

public class LinkedList<E>
    extends AbstractSequentialList<E> //继承的类
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable //实现的各种接口
{}

接下来看看LinkedList这个类的一些属性：就三个属性，一个用来记录双向链表的大小，一个是first节点用来指向链表的头，last用来指向链表的尾

   transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;

在看看构造方法：

/**
     * Constructs an empty list.
     */
    public LinkedList() { //空参构造
    }

/**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public LinkedList(Collection<? extends E> c) { //通过已有的集合进行构造
        this();
        addAll(c); //使用addAll（）方法把集合中的数据生产LinkedList
    }

public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray(); //把集合转为数组
        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) { //对数组进行遍历，对每一个元素都封装成Node并添加到LinkedList中
            @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;
    }

接下来看看LinkedList的基本操作，添加，删除，遍历，查询等

先看添加，从双向链表的结构来看，添加元素可以在链表的头、尾、以及中间的任意位置添加新的元素。因为 LinkedList 有头指针和尾指针，所以在表头或表尾进 行插入元素只需要 O(1) 的时间，而在指定位置插入元素则需要先遍历一下链表， 所以复杂度为 O(n)。首先看看在头部添加元素：

看图可以看出，只要把first指向新的node，新的node的next指向原先firt指向的node，再把原先first指向的node的prev指向新的node就可以了。

/**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first; //使用临时node
        final Node<E> newNode = new Node<>(null, e, f); //封装新的node，并把新node的nex指向f
        first = newNode;
        if (f == null) //判断first是否为空
            last = newNode;
        else
            f.prev = newNode; //把f的prev指向新的node
        size++; //链表长度加1
        modCount++; //记录链表被修改的次数
    }

在看看在尾部添加，其实和在头部添加一样，只是把first换成了last，逻辑一样

/**
     * Links e as last element.
     */
    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++;
    }

再看看在中间的任意位置添加：

这个相对来说复杂点点，修改添加前后node的next和prev的指向，修改的相对来说多点点

/**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) { //表示在在succ节点前面添加e元素
        // assert succ != null;
        final Node<E> pred = succ.prev; //获取succ的前面节点
        final Node<E> newNode = new Node<>(pred, e, succ); //把e封装成节点，并把prev指向succ前面节点，把next指向succ节点
        succ.prev = newNode; //然后把succ的prev指向新的节点
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode; //把succ的前节点的next只想新的节点
        size++; //链表长度+1
        modCount++; //修改次数+1
    }

添加说完了，就说说删除，其实也很简单

删除也是分为从头部、尾部、中间位置删除

先看看从first位置删除

/**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item; //获取first中间的元素，用于后面的返回
        final Node<E> next = f.next; //获取f的next节点
        f.item = null;
        f.next = null; // help GC 清除
        first = next; //把first指向f的next
        if (next == null)
            last = null;
        else
            next.prev = null;  //清除
        size--; //链表长度-1
        modCount++; //修改次数+1
        return element;
    }

看了从头部删除，其实尾部删除也差不多

 /**
     * Unlinks non-null last node l.
     */
    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;
    }

在看看从指定位置删除吧

/**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item; //获取该节点的值
        final Node<E> next = x.next; //获取该节点的next节点
        final Node<E> prev = x.prev; //获取该节点的prev节点

        if (prev == null) { //把该节点的前节点的next指向该节点的next节点，并清除该节点的prev指向
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) { //把该节点的next节点的prev指向该节点的prev节点，并清除该节点的next指向
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null; //清除
        size--; //链表长度-1
        modCount++; //修改次数+1
        return element;
    }

看完增删，那就继续看查相关的方法，也有从头，尾相关的查询方法，都很简单，做判断，然后查询

/**
     * Returns the first element in this list.
     *
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    /**
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

当然还有指定index查询的

/**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);

        //判断index是在链表的前半段还是在后半段，如果在前半段就从first向后遍历，否则使用last向前遍历

        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;
        }
    }

其实基本知道了上面的方法基本对双向链表有了一定的熟悉，当然LinkedList还有很多其他的方法，不过很多都是基于上面这些方法的一些封装，例如：

/**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        linkLast(e);
    }
/**
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    /**
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }
/**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }
/**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns {@code true} if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified element
     */
    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;
    }
/**
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    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++;
    }
/**
     * Removes the element at the specified position in this list.  Shifts any
     * subsequent elements to the left (subtracts one from their indices).
     * Returns the element that was removed from the list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
/**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int indexOf(Object o) { //查找元素o是否在链表中，并返回index，没找到返回-1
        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;
    }

到这里本文就结束了了，如果想知道LinkedList的更多方法，建议去看源码