源码解析：Oops ！ JDK源码---集合类（二）之LinkedList源码

本文基于jdk1.8
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0x00 概述

linkedList是链表实现的集合，元素有序可以重复。

0x01 类图

由类图可知：
1、与arraylist相同的三个接口
java.util.List 接口
java.io.Serializable 接口
java.lang.Cloneable 接口
2、少于arraylist的接口
java.util.RandomAccess 接口
LinkedList 不同于 ArrayList ，使用链表实现，不支持随机访问
3、多于arraylist的接口
java.util.Deque 接口，
表示支持双端队列的功能，LinkedList 支持快速的在头尾添加元素和读取元素。
继承 java.util.AbstractSequentialList 抽象类。如下：
public abstract class AbstractSequentialList extends AbstractList，由此可知，AbstractSequentialList是AbstractList的子类，可以实现只能连续访问的数据存储的连续操作。查看类中的方法主要是如#get(int index)、#add(int index, E element)等方法。

0x02 详细解析

主要属性

	// transient关键字防止变量被持久化
    // 链表元素（节点）的个数
    transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    // 内部类 也是不可持久化
    // 指向第一节点的指针
    transient Node<E> first;

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    // 指向最后一个节点的指针
    transient Node<E> last;

主要方法

1、构造方法

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

    /**
     * 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
     */
    // 集合构造
    public LinkedList(Collection<? extends E> c) {
     
        this();
        // 添加进入链表
        addAll(c);
    }

2、添加元素

尾插法：

/**
     * Appends the specified element to the end of this list.
     *
     * 
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) {
     
   		// 尾插入链表中
        linkLast(e);
        return true;
    }
    /**
     * Links e as last element.
     */
    void linkLast(E e) {
     
    	// 记录插入的last节点
        final Node<E> l = last;
        // 创建新节点，前一个节点为l, 节点元素为e,新节点的后续节点为null
        final Node<E> newNode = new Node<>(l, e, null);
        // last指向新节点
        last = newNode;
        // 判断last节点是否为null,为null则说明first节点也为空，则将first也指向新节点
        if (l == null)
            first = newNode;
        // 不为空，则将last节点的next指向新节点
        else
            l.next = newNode;
        // 增加链表大小
        size++;
        // 增加修改次数
        modCount++;
    }

添加到指定位置：

/**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any
     * subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
     
        // 检查下标是否在范围之内
        checkPositionIndex(index);
        // 若正好等于链表的大小，直接添加至尾部
        if (index == size)
            linkLast(element);
        else
            // 添加到第index元素之前
            linkBefore(element, node(index));
    }
    ........
    /**
     * Inserts element e before non-null Node succ.
     * 在某个元素之前插入
     */
    void linkBefore(E e, Node<E> succ) {
     
        // assert succ != null;
        // 获取当前元素的前一个元素
        final Node<E> pred = succ.prev;
        // 创建新节点
        final Node<E> newNode = new Node<>(pred, e, succ);
        // 将下游节点关联上上一个
        succ.prev = newNode;
        // 若上一个为空，则链表为空，将首节点置为该新节点
        if (pred == null)
            first = newNode;
        else
            // 否则将新节点放入前一个节点与后一个之间。
            pred.next = newNode;
        // 修改链表大小，修改次数加1
        size++;
        modCount++;
    }
    ........
    /**
     * Returns the (non-null) Node at the specified element index.
     * 获取指定下表的节点
     */
    Node<E> node(int index) {
     
        // assert isElementIndex(index);
        // 若指定位置小于原链的一半，则从前往后找。否则从后往前
        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;
        }
    }

3、获取元素

获取指定位置的元素：

// LinkedList
    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
     
        // 检查下标
        checkElementIndex(index);
        // 借助node(int index) 实现节点的查找
        return node(index).item;
    }

基于Deque接口实现的方法：

// LinkedList实现了Deque接口，所以实现了peekFirst和peekLast
    /**
     * Retrieves, but does not remove, the first element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the first element of this list, or {@code null}
     *         if this list is empty
     * @since 1.6
     * 获取头部元素
     */
    public E peekFirst() {
     
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }
    /**
     * Retrieves, but does not remove, the last element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the last element of this list, or {@code null}
     *         if this list is empty
     * @since 1.6
     * 获取尾部元素
     */
    public E peekLast() {
     
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

基于Queue接口实现的方法：

// Queue operations.

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     * 返回首部元素
     */
    public E peek() {
     
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     * 获取头部元素
     */
    public E element() {
     
        return getFirst();
    }
    /**
     * 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)
           // 当元素为空，抛出NoSuchElementException异常
            throw new NoSuchElementException();
        return f.item;
    }

4、设置元素

设置指定位置的元素

    /**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
     
        // 检查下标
        checkElementIndex(index);
        // 获取原来index元素上的元素
        Node<E> x = node(index);
        E oldVal = x.item;
        // 修改对应的index节点的值
        x.item = element;
        // 返回old value
        return oldVal;
    }

5、转化数组

Object数组：

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * 
The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * 
This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list
     *         in proper sequence
     */
    public Object[] toArray() {
     
        // 创建Object数组
        Object[] result = new Object[size];
        int i = 0;
        // 遍历所有节点，挨个元素添加到数组中
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

泛型数组：

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
     
        // 若传入的数组小于size大小，则直接复制一个新的数组返回
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        // 顺序遍历链表，复制进a
        Object[] result = a;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        // 传入的数组长度大于size，将size处置为null
        if (a.length > size)
            a[size] = null;

        return a;
    }

6、序列化与反序列化

序列化：

private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
     
        // 写入非transient属性以及非静态属性
        s.defaultWriteObject();

        // 写入链表的大小
        s.writeInt(size);

        // 顺序遍历并序列化每个元素
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

反序列化：

 @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
     
        // 读取非transient属性以及非静态属性
        s.defaultReadObject();

        // 读取size
        int size = s.readInt();

        // 顺序遍历逐个反序列化添加到链表尾部
        for (int i = 0; i < size; i++)
            linkLast((E)s.readObject());
    }