基于JDK1.8的LinkedList源码学习笔记

        LinkedList作为一种常用的List,是除了ArrayList之外最有用的List。其同样实现了List接口,但是除此之外它同样实现了Deque接口,而Deque是一个双端队列接口,其继承自Queue,所以LinkedList同样可以用来模拟队列,栈以及双端队列。

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一.基本用法

      因为LinkedList是基于链表实现的,所以注定其插入和删除操作速度要快于ArrayList,但是由于其是链表结构,所以其随机访问查找检索速度慢于基于数组的ArrayList。

         这里先主要说一下LinkedList的基本用法,以及模拟队列,模拟栈,模拟双端队列的常用方法。

1.LinkedList,List用法

List<String> myList=new LinkedList<String>();
(1)//增加元素
String s="myString"
myList.add(s);//这里等同于在链表尾端增加元素addLast(e)
myList.add(1,s);//在指定位置插入元素

2)//获取指定位置的元素
String getString=myList.get(10)//获取链表第11处元素,从头计算

3)//删除元素
myList.remove(2)//删除链表第3个元素

4)//clear清空链表
myList.clear()

(5)isEmpty(),//判断list是否为空

2.LinkedList模拟队列

Queue<String> myQueue=new LinkedList<String>();
(1)//添加元素到到队尾
   myQueue.offer(myString);
   myQueue.add(myString);

(2)检索但不删除队首元素
    String head=myQueue.peek();//若为空,返回null
    String head=myQueue.element();//若队列为空,抛出NoSuchElementException

(3)取出并且删除队首元素

   String head=myQueue.poll(); //若为空,返回null
   String head=myQueue.remove();//若队列为空,抛出NoSuchElementException

//综上,LinkedList通过在链表尾插入元素,链表首取出元素,模拟了先进先出FIFO的队列,但是
//这里的队列是单向的

3.LinkedList模拟栈Stack操作

Deque<String> stack=new LinkedList<String>();
//(1)进栈操作    
    stack.push(myString);

//(2)出栈操作,删除并且取出
    stack.pop();
//(3)若是检索不删除则还用peek
    stack.peek();
//LinkedList通过在队首插入元素,队首取出元素,模拟stack的先进后出操作

4.LinkedList模拟双端队列Deque操作

Deque<String> deque=new LinkedList<String>();
//(1)队首添加元素
  deque.offerFirst(myString);
  deque.addFirst(myString);
//(2)队尾添加元素
  deque.offerLast(myString);
  deque.addLast(myString);

//(3)检索但不删除队首元素
   String first=deque.peekFirst();
   first=deque.getFirst();
//(4)检索但不删除队尾元素
    String last=deque.peekLast();
    last=deque.getLast();

//(5)取出并删除队首元素
    deque.pollFirst();
   deque.removeFirst();
//(6)取出并删除队尾元素
   deque.pollLast();
   deque.removeLast();

//这样LinkedList通过操作链表队首队尾就实现了双端队列

5.LinkedList迭代遍历

//(1)for each 循环
List<String> list=new ArrayList<String>();
for(String s:list){
////
}

//(2)iterator迭代器
Iterator<String> it=list.iterator();
while(it.hasNext()){
   it.next();
}

//(3)同时List还提供了ListIterator接口,拥有反向正向迭代

ListIterator<String> lit=list.listIterator();
while(lit.hasNext()){
   it.next();
}//正向迭代

while(it.hasPrevious()){
   it.previous();
}//反向迭代

//值得注意的是,以前可能忽视了,listIterator迭代器同时提供了增删改的功能
//add(),在指定位置插入一个元素,当前迭代的前面插入
//set(E,e),修改当前迭代为指定元素
//remove();删除上一次迭代

二.JDK源码分析

   这里的JDK是基于JDK1.8的源码。

1.定义,LinkedList类定义

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
 // 继承了AbstractSequentialList抽象类,提供了实现List接口的基本实现
  //Deque接口, A linear collection that supports element insertion and removal at both ends.  
  //The name <i>deque</i> is short for "double ended queue" and is usually pronounced "deck"

public interface Deque<E> extends Queue<E>

//所以这里就可以知道为什么LinkedList可以模拟队列,双端队列,以及Stack栈了

2.重要属性

transient int size = 0;//记录List大小
//接下来分别是两个Node引用,分别指向链表头和链表尾
transient Node<E> first;
transient Node<E> last;

//接下就是链表中节点的定义,可以看到JDK1.8把节点都统一为Node了
 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;
        }
    }
//及其简单的定义,双向链表,向前链接,向后向后链接,元素

3.构造器

//(1)无参构造器
  public LinkedList() {
    }
  //(2)带有集合的构造器
  public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }
//调用addAll将现有集合内所有元素放到LinkedList中
  public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
  //将整个集合c中的元素加入链表中   
  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;
        //插入到结尾
        if (index == size) {
            succ = null;
            pred = last;
        } else {//插入到中间
    //这里succ则为原来在index位置的节点
            succ = node(index);
            pred = succ.prev;
        }

        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            //创建新的Node节点,其中newNode的前向节点为pred,后向节点没有定义
            Node<E> newNode = new Node<>(pred, e, null);
            //pred==null,则此节点为首节点
            if (pred == null)
                first = newNode;
            else
                //当节点不是首节点时,定义前向节点的后向节点为当前节点
                pred.next = newNode;
            pred = newNode;
        }

        if (succ == null) {
            last = pred;
        } else {
            //将原来的链表加入
            pred.next = succ;
            succ.prev = pred;
        }

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

4.常用方法源码分析

(1). add(E e)

//默认add方法,将节点放入链表尾部,同offer方法
  public boolean add(E e) {
        linkLast(e);
        return true;
    }

    //将节点放入链表尾部
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        //同样要判断当前节点是不是头节点
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }
//将元素链接放到指定位置
public void add(int index, E element) {
          //该方法主要是查看index是否合法,在范围内,否则抛出异常
        checkPositionIndex(index);
        //当index是末尾时,直接链接到结尾
        if (index == size)
            linkLast(element);
        else
           //否则找到index位置的原来节点,插入到其前面
            linkBefore(element, node(index));
    }

   //取出index位置的node节点
    Node<E> node(int index) {
        // assert isElementIndex(index);
        //这里有一处非常值得注意
        //size>>1表示的是向右移位1,该方法其实相当于除以2,去得一半的值
        //当index<size/2时,表明index在前半部分,则正序找
        //否则在后半部分,则倒序查找,节省了时间
        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;
        }
    }

//linkBefore 方法
    //这个方法是将节点插入到succ节点的前面,
//由于是在指定位置插入节点,所以要将原来的节点链接到新节点后面
    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
            //这里一定要注意,双向链表,一定要将pred节点的next节点定义为当前节点
            pred.next = newNode;
        size++;
        modCount++;
    }

  (2).addLast(),addFirst()方法

addLast()等同于add()方法,addFirst是在链表头插入节点

//将新节点放入到链表尾部
 public void addLast(E e) {
        linkLast(e);
    }

//在链表头插入节点
   public void addFirst(E e) {
        linkFirst(e);
    }

    //将新节点设置为首节点
  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++;
    }

(3). getFirst(),getLast()获取头节点和尾节点

/**
     * Returns the first element in this list.
     *
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty为空会抛出异常
     */
    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;
    }

     (4). removeFirst(),removeLast()方法

/**
     * 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);
    }

//unlinkFirst()即解开并返回头节点
   private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;//及时清除
        f.next = null; // help GC
        first = next;

        if (next == null)
            last = null;//此时链表为空
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }
/**
     * 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);
    }

    /**
     * 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;
    }

(5). contains(Object o)
    查看链表中是否存有某个元素

public boolean contains(Object o) {
        return indexOf(o) != -1;
    }
    //indexOf()这个方法返回对象O在链表中的位置
   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 {
            //同样调用的也是equals方法判断两个值是否相等
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;//没有找到时返回-1
    }

(6). get(int index)

获取指定index位置的元素

/**
     * 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;
    }

  (7).set(int index,E element)

set修改指定位置的元素

//主要还是定位获取节点之后再修改
   public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

(8).搜索元素所在位置indexOf(Object o),lastIndexOf(Object o)

分为正向indexOf(),即第1次插入时匹配的元素位置和反向lastIndexOf(),即最后一次插入匹配的位置

//indexOf()这个方法返回对象O在链表中的位置
    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 {
            //同样调用的也是equals方法判断两个值是否相等
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

    //反向查找
    //有index的时候,必然会有lastIndexOf
    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) {
                //这里值得注意的是,index先--,因为你是从size位置开始的,所以要先--
                index--;
                if (o.equals(x.item))
                    return index;
            }
        }
        return -1;
    }

5.模拟Queue操作源码分析

再次强调一次这里queue先进先出,在队尾入队,队首出队

(1).首先是检索队首,但不出队的操作,peek(),element()

/**
     * 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
     */最常用操作,peek(),若为空会,返回null
    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
     *///若为空会抛出异常
    public E element() {
        return getFirst();
    }

//再回头看一眼getFirst(),
   public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();//抛出异常
        return f.item;
    }

(2).出队操作,取出队首元素,poll(),remove()

/**
     * Retrieves and removes 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 poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * Retrieves and removes 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 remove() {
        return removeFirst();
    }

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

(3).队尾插入元素offer()

/**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Queue#offer})
     * @since 1.5
     */
    public boolean offer(E e) {
        return add(e);
    }

6. 模拟双端队列Deque操作源码分析

双端队列,其实就是整条链表头尾都操作,有了前面的基础,这里应该非常简单了

(1).在队首,队尾插入元素,offerFirst(),offerLast()

其实就是分别调用addFirst(E e)和addLast(E e)方法

/**
     * Inserts the specified element at the front of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerFirst})
     * @since 1.6
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    /**
     * Inserts the specified element at the end of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerLast})
     * @since 1.6
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

(2).检索队首,队尾元素,但不出队peekFirst(),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
     */
    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
     */
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

(3). 出队操作,取出队首,队尾元素,pollFirst(),pollLast()

/**
     * Retrieves and removes 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 pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * Retrieves and removes 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 pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

7. 模拟栈Stack操作源码分析

值得注意的是Stack操作一直是对链表头进行操作,不管是进栈push还是出栈pop方法

/**
     * Pushes an element onto the stack represented by this list.  In other
     * words, inserts the element at the front of this list.
     *
     * <p>This method is equivalent to {@link #addFirst}.
     *
     * @param e the element to push
     * @since 1.6
     */
    public void push(E e) {
        addFirst(e);
    }

    /**出栈操作,若栈为空会抛出异常
     * Pops an element from the stack represented by this list.  In other
     * words, removes and returns the first element of this list.
     *
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this list (which is the top
     *         of the stack represented by this list)
     * @throws NoSuchElementException if this list is empty
     * @since 1.6
     */
    public E pop() {
        return removeFirst();
    }

8. 最后再看一下LinkedList的迭代器ListIterator

listIterator()方法,返回ListIterator迭代器 ,这个不带参数listIterator方法是 AbstractlList中的方法

public ListIterator<E> listIterator() {
        return listIterator(0);
    }

  //从第几个链表节点开始迭代
  public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

  //ListItr是其中的一个内部类,该类是一个List迭代器
  private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;//永远记录上一次迭代的节点
        private Node<E> next;
        private int nextIndex;
        //这个变量非常重要,能够查看迭代过程中是否修改了List,使得迭代过程中的数据与原List中的数据一致
        //Fail_fast原理,不一致时立马失败抛出异常
        private int expectedModCount = modCount;

        //这里给出index,则可以看成是从哪个节点开始迭代
        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

        //正向迭代,向后迭代
        public boolean hasNext() {
            return nextIndex < size;
        }

        public E next() {
            //每次迭代前都检查一下,是否修改了原List,若原List自行修改,而没有经过ListItr迭代器修改则将抛出异常
            //Fail-Fast
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

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

        //反向迭代,即向前迭代
        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;
        }

        //返回下标
        public int nextIndex() {
            return nextIndex;
        }

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

        //迭代操作时,唯一的增删改方式,值得注意的是这里的修改操作都是针对上一次的迭代
        //也就是调用next()得到元素,若要对这个变量进行修改,则可以进行修改
        //这种设计也十分合理,我只有得到元素我才知道我要对元素做什么
        
        public void remove() {
            //当迭代过程中要想删除元素,一定要用迭代器的remove方法
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;

            //由于上面调用unlink时,modCount++;
            //所以为了下一次迭代不抛出异常,这里也要进行 expectedModCount++
            expectedModCount++;
        }

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

        //增也是增在next()后的元素之后
        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }
        //// 判断expectedModCount和modCount是否一致,以确保通过ListItr的修改操作正确的反映在LinkedList中
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

三.简单总结

       LinkedList是十分常用的类,而且其方法实在太多了,而且其功能还狠多,之前老是记不住,这次掰开揉碎过一遍JDK源码,发现实现其实非常简单,但是里面有很多小技巧是值得学习的。所以阅读源码应该成为我今后学习的一个好习惯,任何框架任何技术,知其所以然才能融汇贯通。

posted @ 2015-10-24 10:43  Tobe程序源  阅读(312)  评论(0编辑  收藏  举报