List集合之LinkedList

starlin 272 2018-05-02

LinkedList概述

LinkedList和ArrayList一样都是实现了List接口,只是ArrayList是可变数组的实现,LinkedList是链表实现。基于链表实现方式使得LinkedList在插入和删除时更优于ArrayList,而随机访问ArrayList更有优势。

LinkedList除了实现List接口以外,还实现了Deque接口,为 add、poll 提供先进先出队列操作,以及其他堆栈和双端队列操作。

Linkedlist同样也是线程不安全的,想要让线程变的安全,可以使用如下方式:

List list = Collections.synchronized(new LinkedList(...))

本文源码若无特殊说明,均为JDK1.8

LinkedList源码解析

LinkedList定义

从下面这段代码可以看出LinkedList继承AbstractSequentialList,实现了List、Deque、Cloneable、Serializable。其中
AbstractSequentialList提供了List接口的主要实现,Deque是一个线性Collection,支持在两端插入和移动元素,也就是双端队列的操作。

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

内部结构

LinkedList内部是一个双端链表的结构,结构如下图:

从上图可以看出,LinkedList内部是一个双端链表结构,有两个变量,first指向链表头部,last指向链表尾部。

    //链表中数据
    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;

Node类是LinkedList的静态内部类

    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;
        }
    }

构造方法

LinkedList提供了2个构造方法:LinkedList()和LinkedList(Collection<? extends E> c)


    public LinkedList() {
    }

    public LinkedList(Collection<? extends E> c) {
        this();
        //添加集合中所有元素
        addAll(c);
    }
addAll(Collection<? extends E> c)方法

当使用第二个构造方法时,会调用addAll()方法将集合中的元素添加到链表中。

    //将集合插入到链表尾部,即开始索引位置为size
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
    
    //将集合从指定位置开始插入
    public boolean addAll(int index, Collection<? extends E> c) {
        //检查index范围
        checkPositionIndex(index);
        
        //得到集合的数据
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;
        
        //得到插入位置的前驱节点和后继节点
        Node<E> pred, succ;
        //如果插入位置为尾部,前驱节点为last,后继节点为null
        if (index == size) {
            succ = null;
            pred = last;
        } else { //否则,调用node()方法得到后继节点,再得到前驱节点
            succ = node(index);
            pred = succ.prev;
        }
        
        //遍历数据将数据插入
        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            //创建新节点
            Node<E> newNode = new Node<>(pred, e, null);
            //如果前节点为空,则插入位置在链表头部
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }
        //如果插入位置在尾部,重置last节点
        if (succ == null) {
            last = pred;
        } else {//否则,将插入的链表与先前链表连接起来
            pred.next = succ;
            succ.prev = pred;
        }

        size += numNew;
        modCount++;
        return true;
    }

从上面的代码可以看到,addAll方法主要分为4步:

  1. 检查index索引范围
  2. 得到集合数据
  3. 得到插入位置的前驱和后继节点
  4. 遍历数据,将数据插入到指定位置

List接口增加方法

add(E e)

add(E e)方法用于将元素添加至链表尾部,方法如下:

    /**
     * Appends the specified element to the end of this list.
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) {
        //保存原来链表尾部节点,last 是全局变量,用来表示队尾元素
        final Node<E> l = last;
        //以尾部为前驱节点创建一个新节点
        final Node<E> newNode = new Node<>(l, e, null);
        //将新节点设置为对尾
        last = newNode;
        //如果原来的队尾元素为空,那么说明原来的整个列表是空的,就把新节点赋值给头结点
        if (l == null)
            first = newNode;
        else//原来尾结点的后面为新生成的结点
            l.next = newNode;
        //节点数+1    
        size++;
        modCount++;
    }

linkLast方法中就是一个链表尾部添加一个双端节点的操作,但是需要注意对链表为空时头节点的处理。

add(int index, E element)

add(int index, E element)用于在指定位置添加元素

    /**
     * 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).
     */
    public void add(int index, E element) {
        checkPositionIndex(index);

        //添加在链表尾部
        if (index == size)
            linkLast(element);
        else//添加在链表其他位置
            linkBefore(element, node(index));
    }

主要看linkBefore()方法,其中有个node(index) ,其实现如下:

    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
        //// 如果 index 在前半段,从前往后遍历获取node
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {//如果 index 在后半段,从后往前遍历获取node
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

得到 Node 后,就会去调用 linkBefore(element, node) 方法

     /**
     * Inserts element e before non-null Node succ.
     */
    //将元素节点插入到 succ 之前的位置
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        //保存 index 节点的前节点
        final Node<E> pred = succ.prev;
        //初始化节点,并指明前驱和后继节点
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        //// 判断尾节点是否为空,为空表示当前链表还没有节点 
        if (pred == null)
            first = newNode;
        else//succ节点前驱的后继引用指向新节点    
            pred.next = newNode;
        size++;
        modCount++;
    }

从以上linkBefore方法来看,基本流程如下:

  1. 创建新节点newNode,将newNode的前驱指针指向pred和后继指针指向succ
  2. 将succ的前驱指针指向新节点newNode
  3. 根据pred是否为null,进行不同操作
  • 如果pred为null,说明该节点插入在头节点之前,要重置first头节点
  • 如果pred不为null,那么直接将pred的后继指针指向newNode即可

linkBefore()方法在第二个参数节点之前插入一个新节点。示意图如下:


Deque接口增加方法

addFirst(E e)方法

addFirst()方法用于将元素添加到链表头部,其实现如下:

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

    /**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        //新建节点,以头节点为后继节点
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        //如果链表为空,last节点也指向该节点
        if (f == null)
            last = newNode;
        else//否则,将头节点的前驱指针指向新节点
            f.prev = newNode;
        size++;
        modCount++;
    }

从上面的代码看到,实现就是在头节点插入一个节点使新节点成为新节点,但是和linkLast一样需要注意当链表为空时,对last节点的设置。

addLast(E e)方法

addLast()方法用于将元素添加到链表尾部,与add()方法一样。所以实现也一样,如下:

    /**
     * Appends the specified element to the end of this list.
     */
    public void addLast(E e) {
        linkLast(e);
    }

     /**
     * 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++;
    }
offer(E e)方法

offer(E e)方法用于将数据添加到链表尾部,其内部调用了add(E e)方法,如下:

    /**
     * Adds the specified element as the tail (last element) of this list.
     */
    public boolean offer(E e) {
        return add(e);
    }
offerFirst(E e)方法

offerFirst(E e)将数据插入链表头部,与addFirst的区别在于该方法可以返回特定的返回值,而addFirst的返回值为void

  public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }
offerLast(E e)方法

offerLast()与addLast()的区别和offerFirst()和addFirst()的区别一样

    /**
     * Inserts the specified element at the end of this list.
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

查找方法

get(int index)方法

get(int index)方法根据指定索引返回数据,在内部调用了node(index),前面已经分析过了

    /**
     * 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) {
        //检查边界
        checkElementIndex(index);
        return node(index).item;
    }
getFirst()、element()、peek()、peekFirst()方法,获得位置为0的头节点数据

getFirst()、element()、peek()、peekFirst()方法获取位置为0的头节点数据,区别在于对链表为空时的处理,是抛出异常还是返回null。
其中getFirst()和element()方法将会在链表为空时,抛出异常

    /**
     * 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;
    }
    
     /**
     * 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
     */
    public E element() {
        return getFirst();
    }

其中peek()和peekFirst(),当链表为空的时候方法返回null,其实现如下:

    /**
     * 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
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * 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
     */
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }
getLast()和peekLast()方法,获得位置为size-1的尾节点数据

getLast()和peekLast()为获得为节点数据的方法:

    /**
     * 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;
    }

    /**
     * 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
     */
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

可以看到,getLast()方法在链表为空时,会抛出NoSuchElementException,而peekLast()则不会,只是会返回null。

idnexOf()方法

idnexOf()方法实现的在于一个从前往后遍历,其方法实现如下:

    /**
     * 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
     */
    //返回第一个匹配的索引,不包含次元素则返回-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;
    }
lastIndexOf(Object o)方法

lastIndexOf()方法返回最后一个匹配的索引,实现为从后往前遍历,其方法实现如下:

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest 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 last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    //返回此列表中最后出现的指定元素的索引,如果此列表中不包含该元素,则返回 -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;
    }

删除方法

删除操作分为按照位置删除和按照对象删除,其中按照位置删除的方法又有区别,有的只是返回是否删除成功的标志,有的还需要返回被删除的元素

remove(Object o)方法,删除指定对象

当删除指定对象时,只需调用remove(Object o)即可,不过该方法一次只会删除一个匹配的对象,如果删除了匹配对象,返回true,否则false。


    /**
     * 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;
    }

从以上代码,可以看出主要是由unlink()方法来实现删除操作,代码如下:

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        //得到后继节点
        final Node<E> next = x.next;
        //得到前驱节点
        final Node<E> prev = x.prev;
        
        //删除前驱节点,那么设置头节点为下一个节点
        if (prev == null) {
            first = next;
        } else {//设置该节点的前节点的 next 为该节点的 next
            prev.next = next;
            x.prev = null;
        }

        //删除后继节点,那么设置尾节点为上一个节点
        if (next == null) {
            last = prev;
        } else {//设置该节点的下一个节点的 prev 为该节点的 prev
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }
remove(int index)方法,按照位置删除对象

remove(int index)方法用于删除任意位置的元素,如果删除成功将返回true,否则返回false,实现如下:

    /**
     * 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));
    }
remove()、removeFirst()、pop()、poll()、pollFirst()删除头结点的对象

删除头节点的对象的方法有很多,包括remove()、removeFirst()、pop()、poll()、pollFirst(),其中前三个方法在链表为空时将抛出NoSuchElementException,后两个方法在链表为空时将返回null。
remove()、pop()、removeFirst()的实现如下:

    public E remove() {
        return removeFirst();
    }

    public E pop() {
        return removeFirst();
    }

    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

poll()和pollFirst()的实现:

    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }
removeLast()和pollLast()删除尾节点的对象

删除尾节点的对象的方法有removeLast()和pollLast()
removeLast的实现如下:

    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

pollLast()的实现如下:

    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

set方法

取代在指定位置上的元素,并返回旧值,其实现如下:

   /**
     * 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) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        // 设置 x 节点的值为新值,然后返回旧值
        x.item = element;
        return oldVal;
    }

clear方法

删除该链表中的全部元素,其实现如下:

    /**
     * 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
        // 遍历链表,然后一一删除置空
        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++;
    }

总结

LinkedList是基于双端链表的List,其内部的实现源于对链表的操作,所以适用于频繁增加、删除的情况;该类不是线程安全的;另外,由于LinkedList实现了Queue接口,所以LinkedList不止有队列的接口,还有栈的接口,可以使用LinkedList作为队列和栈的实现。


# java集合