LinkedList源码阅读

前言

我们之前已经进行过ArrayList的源码阅读，我们知道，同样继承了List接口的LinkedList，比起ArrayList而言，LinkedList在新增元素和删除元素时表显要好，那么今天我们就一起看看LinkedList源码中蕴含了什么秘密。

正文

1、LinkedList的成员变量

我们先看下LinkedList的成员变量：

    transient int size = 0;

这是LinkedList大小，初始值为0.

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

指向第一个节点的指针。

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

指向最后一个节点的指针。

    /**
     * The number of times this list has been <i>structurally modified</i>.
     * Structural modifications are those that change the size of the
     * list, or otherwise perturb it in such a fashion that iterations in
     * progress may yield incorrect results.
     *
     * <p>This field is used by the iterator and list iterator implementation
     * returned by the {@code iterator} and {@code listIterator} methods.
     * If the value of this field changes unexpectedly, the iterator (or list
     * iterator) will throw a {@code ConcurrentModificationException} in
     * response to the {@code next}, {@code remove}, {@code previous},
     * {@code set} or {@code add} operations.  This provides
     * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
     * the face of concurrent modification during iteration.
     *
     * <p><b>Use of this field by subclasses is optional.</b> If a subclass
     * wishes to provide fail-fast iterators (and list iterators), then it
     * merely has to increment this field in its {@code add(int, E)} and
     * {@code remove(int)} methods (and any other methods that it overrides
     * that result in structural modifications to the list).  A single call to
     * {@code add(int, E)} or {@code remove(int)} must add no more than
     * one to this field, or the iterators (and list iterators) will throw
     * bogus {@code ConcurrentModificationExceptions}.  If an implementation
     * does not wish to provide fail-fast iterators, this field may be
     * ignored.
     */
    protected transient int modCount = 0;

这个属性继承自LinkedList的父类AbstractList。表示这个list进行结构性修改的次数。

2、Node的内部结构

我们再来看下Node的内部结构：

    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;
        }
    }

Node里面包含了三个成员属性：
（1）E item：当前节点的数据。
（2）Node next：当前节点指向下一个节点的指针。
（3）Node prev：当前节点指向上一个节点的指针。
可以看出Node是一个链表结构，链表上的每一个节点都能沿着链表找到头节点和尾节点。
这样的结构与ArrayList不同，ArrayList是由数组来存储元素，利用数组的特性进行一系列操作。在有索引的情况下，进行get和set操作会比较快速些。而LinkedList由于采用链表结构来存储元素，导致在进行查询操作时，需要利用指针的移动来找到目标元素，相对来说性能比较差。

3、add方法

接下来，我们看看LinkedList的add方法：

    /**
     * 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;
    }

我们看到，在进行新增元素操作时，LinkedList首先调用了linkLast方法：

    /**
     * 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++;
    }

linkLast方法将目标元素作为尾节点放置在链表的末端，并将size和modCount各加1.

4、remove方法

remove方法：

    /**
     * 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;
    }

找到list中指定的第一个出现的元素，如果存在就删除它。
这里我们可以看到，LinkedList在删除元素时，是对链表进行了遍历，通过移动指针来寻找需要的元素。相比较ArrayList而言，ArrayList通过索引来定位目标元素的方式就直接的多。

5、get方法

get方法：

    /**
     * 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) {
        checkElementIndex(index);
        return node(index).item;
    }

返回list中指定位置的元素。
这里调用了checkElementIndex方法：

    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Tells if the argument is the index of an existing element.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

检查了目标位置未超出list的大小。
调用node方法：

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

返回了指定元素位置的节点对象。
这里LinkedList将链表一分为二，如果目标元素在前半段，那么LinkedList会从前半段链表从头遍历，通过指针的移动来寻找目标元素；否则LinkedList会从链表的尾节点来从后往前遍历，同样通过移动指针去寻找目标元素。
比较而言，ArrayList在进行get操作时，直接通过数组的索引来定位到目标元素，性能上要好些。