/**
   * 考虑的比较的周全，并且包含了全部的情况，代码也不乱<b></b>
   * 
   * @param index
   *          插入的位置
   * @param c
   *          插入的元素集合
   */
  private boolean addAll(int index, Collection<? extends E> c) {
    // 缺少对于size的参数的校验
    Object[] a = c.toArray();
    int numNew = a.length;
    if (numNew == 0)
      return false;

    // 确定插入的位置和插入的前一个位置
    Node<E> pred, succ;
    if (index == size) {
      succ = null;
      pred = last;
    } else {
      succ = node(index);
      pred = succ.prev;
    }

    // 首先就是能够确认自己前面的一个节点是谁，size指的是插入的位置，后面的全部的一次
    // 向后移动
    for (Object o : a) {
      @SuppressWarnings("unchecked")
      E e = (E) o;
      Node<E> newNode = new Node<E>(pred, e, null);
      if (pred == null)
        first = newNode;
      else
        pred.next = newNode;
      pred = newNode;
    }

    // 把插入后面的元素接上
    if (succ == null) {
      last = pred;
    } else {
      pred.next = succ;
      succ.prev = pred;
    }

    size += numNew;
    return true;
  }

  /**
   * Returns the (non-null) Node at the specified element index. the alg is
   * interesting
   */
  private Node<E> node(int index) {
    if (index < (size >> 1)) {// 前半截，后半截
      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;
    }
  }

add function 代码写的还是比较的精彩的：

  public boolean add(E e) {
    linkLast(e);
    return true;
  }

  /**
   * @param e
   */
  private void linkLast(E e) {
    // 尾节点,final ？
    final Node<E> l = last;
    final Node<E> newNode = new Node<E>(l, e, null);
    last = newNode;
    // special situation
    if (l == null)
      first = newNode;
    else
      l.next = newNode;
    size++;

  }

  /**
   * 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));
  }

  private void linkBefore(E element, Node<E> succ) {
    // assert succ != null;
    final Node<E> pred = succ.prev;
    final Node<E> insert = new Node<E>(pred, element, succ);
    succ.prev = insert;

    if (pred == null)
      first = insert;
    else
      pred.next = insert;
    size++;
  }

  private boolean isPositionIndex(int index) {
    return index >= 0 && index <= size;
  }

  private void checkPositionIndex(int index) {
    if (!isPositionIndex(index))
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  }

  private String outOfBoundsMsg(int index) {
    return "Index: " + index + ", Size: " + size;
  }