LinkedList源碼解析

簡介

LinkedList是一個實現了List接口和Deque接口的雙端鏈表。

    //列表長度
    transient int size = 0;

    /**
     * 指向第一個節點的指針
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;

    /**
     * 指向最後一個節點的指針
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;
    
    //節點的數據結構
    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;
        }
    }

核心源碼

    /**
     * 構造一個空列表
     */
    public LinkedList() {
    }
    
    /**
     * 在末尾插入元素
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }
    
    /**
     * 將e鏈接到末尾
     */
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)  //原先的列表爲空，則新增的節點即爲頭結點也爲尾節點
            first = newNode;
        else  
            l.next = newNode;
        size++;
        modCount++;
    }
    
      
    /**
     * 在指定位置插入元素
     */
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    
    /**
     * 返回指定位置的節點
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
        
        //如果位置在前半部分，從前往後遍歷，反正從後往前遍歷
        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;
        }
    }
    
     /**
     * 在指定節點前插入元素
     */
    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++;
    }
    
     /**
     * 在頭部插入指定元素
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }
    
    /**
     * 將e作爲頭結點
     */
    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++;
    }
    
    /**
     * 檢索並且刪除頭結點
     *
     * @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);
    }
    
    /**
     * 檢索並且刪除頭結點
     */
    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  引用爲空，則在下一次垃圾回收時被清理
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }
    
    /**
     * 檢索並且刪除尾節點
     */
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }
    
    /**
     * 檢索並且刪除尾節點
     */
    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 引用爲空，則在下一次垃圾回收時被清理
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }