LinkedList源码分析

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;

  /**
  * Constructs an empty list.
  */
  public LinkedList() {
  //无参构造方法
 }

  /**
  * 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) {
  //以集合c形式，把所有元素插入链表中
  this();
  addAll(c);
  }

  /**
   * 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;
   }

/**
 * 将集合插入到链表的尾部
 */
public boolean addAll(Collection<? extends E> c) {
    return addAll(size, c);
}

public boolean addAll(int index, Collection<? extends E> c) {
    checkPositionIndex(index);

    //获取目标集合转为数组
    Object[] a = c.toArray();
    //新增元素的数量
    int numNew = a.length;
    //如果新增元素为0，则不添加，并且返回false
    if (numNew == 0)
        return false;

    //定义index节点的前置节点，后置节点
    Node<E> pred, succ;

    //判断是不是链表的尾部，如果是，那么就在链表尾部追加数据
    //尾部的后置节点一定是null，前置节点是队尾
    if (index == size) {
        succ = null;
        pred = last;
    } else {

        //如果不是在链表的末尾而是在中间位置的话，
        //取出index节点，作为后继节点
        succ = node(index);

        //index节点的前节点，作为前驱的节点
        pred = succ.prev;
    }

    //链表批量的增加，去循环遍历原数组，依次去 插入节点的操作
    for (Object o : a) {
        @SuppressWarnings("unchecked") 
        //类型转换
        E e = (E) o;
        // 前置节点为pred，后置节点为null，当前节点值为e的节点newNode
        Node<E> newNode = new Node<>(pred, e, null);
        // 如果前置节点为空， 则newNode为头节点，否则为pred的next节点
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        pred = newNode;
    }
    // 循环结束后，如果后置节点是null，说明此时是在队尾追加的
    if (succ == null) {
        last = pred;
    } else {
        //否则是在队中插入的节点 ，更新前置节点 后置节点
        pred.next = succ;
        succ.prev = pred;
    }
    // 修改数量size
    size += numNew;
    //修改modCount
    modCount++;
    return true;
}

   private static class Node<E> {
      //值
      E item;
      //后者  指向下一节点引用
      Node<E> next;
      //前者  指向前一节点引用
      Node<E> prev;

      Node(Node<E> prev, E element, Node<E> next) {
          this.item = element;
          this.next = next;
          this.prev = prev;
    }
 }

public void addFirst(E e) {
    linkFirst(e);
}

/**
 * Links e as first element.
 * 将e元素弄成链接列表的第一个元素
 */
private void linkFirst(E e) {
    final Node<E> f = first;

    //链表开头前驱为空，值为e，后继为f
    final Node<E> newNode = new Node<>(null, e, f);
    first = newNode;

    //若f为空，则表明列表中还没有元素，last也应该指向newNode
    if (f == null)
        last = newNode;
    else
        //否则，前first的前驱指向newNode
        f.prev = newNode;
    size++;
    modCount++;
}

   /**
     * Retrieves and removes the head (first element) of this list.
     * 返回链表中第一个元素，如果链表是空，则返回空
     * @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);
   }