#yyds干货盘点# Collection - LinkedList源码解析

Posted 2022-03-19 灰太狼_cxh

Collection - LinkedList源码解析

概述

LinkedList同时实现了List接口和Deque接口，也就是说它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)。这样看来，LinkedList简直就是个全能冠军。当你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。

LinkedLists实现

底层数据结构

LinkedList底层通过双向链表实现，本节将着重讲解插入和删除元素时双向链表的维护过程，也即是之间解跟List接口相关的函数，而将Queue和Stack以及Deque相关的知识放在下一节讲。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。注意这里没有所谓的哑元，当链表为空的时候first和last​都指向null。

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;

    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 collections
     * 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);
    

getFirst(), getLast()

获取第一个元素， 和获取最后一个元素:

/**
     * 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;
    

    /**
     * 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;
    

removeFirest(), removeLast(), remove(e), remove(index)

remove()方法也有两个版本，一个是删除跟指定元素相等的第一个元素remove(Object o)，另一个是删除指定下标处的元素remove(int index)。

/**
     * 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) // 第一个元素
            first = next;
         else 
            prev.next = next;
            x.prev = null;
        

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

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

    /**
     * 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));
    

    /**
     * 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;