Java Review - LinkedList源码解读

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Java Review - ArrayList 源码解读


概述

从上图可知: LinkedList同时实现了List接口和Deque接口,既可以看作一个顺序容器,又可以看作一个队列(Queue),同时又可以看作一个栈(Stack) 。

当需要使用栈或者队列时,可以考虑使用LinkedList,一方面是因为Java官方已经声明不建议使用Stack类,更遗憾的是,Java里根本没有一个叫做Queue的类(它是个接口名字)。

关于栈或队列,现在的首选是ArrayDeque,它有着比LinkedList(当作栈或队列使用时)有着更好的性能。

ArrayDeque 移步 https://www.nhooo.com/java/java-arraydeque.html


LinkedList的实现方式决定了所有跟下标相关的操作都是线性时间,而在首段或者末尾删除元素只需要常数时间。

为追求效率LinkedList没有实现同步(synchronized),如果需要多个线程并发访问,可以先采用Collections.synchronizedList()方法对其进行包装。


底层数据结构-双向链表

  • LinkedList底层通过双向链表实现 。

  • 双向链表的每个节点用内部类Node表示。LinkedList通过first和last引用分别指向链表的第一个和最后一个元素。

  • 没有所谓的哑元,当链表为空的时候first和last都指向null。


其中Node是私有的内部类:


源码解析

构造函数


    /**
     * Constructs an empty list.
     */
    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
     */
    public LinkedList(Collection<? extends E> c) 
        this();
        addAll(c);
    

方法源码分析

getFirst()

获取第一个元素


    /**
     * 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;
    

getLast()

获取最后一个元素

    /**
     * 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;
    


remove相关方法

有两个版本,

  • 一个是删除跟指定元素相等的第一个元素remove(Object o)
  • 一个是删除指定下标处的元素remove(int index)


remove(e)

【boolean remove(Object o) 】

 /**
     * 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;
    

 /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) 
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null)  //删除的是第一个元素
            first = next;
         else 
            prev.next = next;
            x.prev = null;
        

        if (next == null) //删除的是最后一个元素
        
            last = prev;
         else 
            next.prev = prev;
            x.next = null;
        

        x.item = null;
        size--;
        modCount++;
        return element;
    

指的是删除第一次出现的这个元素, 如果没有这个元素,则返回false;判读的依据是equals方法, 如果equals,则直接unlink这个node;由于LinkedList可存放null元素,故也可以删除第一次出现null的元素;


remove(index)

【remove(int index)】

remove(int index)使用的是下标计数, 只需要判断该index是否有元素即可,如果有则直接unlink这个node。

    /**
     * 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));
    


removeFirst()

【删除 头部元素】

    /**
     * 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);
    


    /**
     * Unlinks non-null first node f.
     */
    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;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    


removeLast()

	/**
     * 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;
    




add()

add()方法有两个版本:

  • 一个是add(E e),该方法在LinkedList的末尾插入元素,因为有last指向链表末尾,在末尾插入元素的花费是常数时间。只需要简单修改几个相关引用即可
  • 另一个是add(int index, E element),该方法是在指定下表处插入元素,需要先通过线性查找找到具体位置,然后修改相关引用完成插入操作。
 /**
     * 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;
    

 
    /**
     * 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++;
    

add(int index, E element),

  • 当index==size时,等同于add(E e);
  • 如果不是,则分两步: 1.先根据index找到要插入的位置,即node(index)方法;2.修改引用,完成插入操作。
 /**
     * 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) 
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    
    


    /**
     * Inserts element e before non-null Node succ.
     */
    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
            pred.next = newNode;
        size++;
        modCount++;
    

上面代码中的node(int index)函数有一点小小的技巧,因为链表双向的,可以从开始往后找,也可以从结尾往前找,具体朝那个方向找取决于条件index < (size >> 1),也即是index是靠近前端还是后端。

    /**
     * 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;
        
    

从这里也可以看出,linkedList通过index检索元素的效率没有arrayList高


addAll()

addAll(index, c) 实现方式并不是直接调用add(index,e)来实现,主要是因为效率的问题,另一个是fail-fast中modCount只会增加1次;

 /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return @code true if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) 
        return addAll(size, c);
    

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c collection containing elements to be added to this list
     * @return @code true if this list changed as a result of the call
     * @throws IndexOutOfBoundsException @inheritDoc
     * @throws NullPointerException if the specified collection is null
     */
    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;
        if (index == size) 
            succ = null;
            pred = last;
         else 
            succ = node(index);
            pred = succ.prev;
        

        for (Object o : a) 
            @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;
    


clear()

为了让GC更快可以回收放置的元素,需要将node之间的引用关系赋空。

  /**
     * 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++;
    

get相关的方法

【通过index获取元素】

// Positional Access Operations

    /**
     * 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;
    
 /**
     * 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;
    

get(int index)得到指定下标处元素的引用,通过调用node(int index)方法实现


set()

将某个位置的元素重新赋值。

set(int index, E element)方法将指定下标处的元素修改成指定值,也是先通过node(int index)找到对应下表元素的引用,然后修改Node中item的值


    /**
     * 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;
        x.item = element;
        return oldVal;
    

isElementIndex()


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

isPositionIndex()

    /**
     * Tells if the argument is the index of a valid position for an
     * iterator or an add operation.
     */
    private boolean isPositionIndex(int index) 
        return index >= 0 && index <= size;
    

查找操作

查找操作的本质是查找元素的下标

indexOf

查找第一次出现的index, 如果找不到返回-1

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