1.   // 获取第一个元素
    2.   public E getFirst() {
    3.         final Node<E> f = first;
    4.         if (f == null)
    5.             throw new NoSuchElementException();
    6.         return f.item;
    7.     }
    1.    // 获取最后一个元素
    2.    public E getLast() {
    3.         final Node<E> l = last;
    4.         if (l == null)
    5.             throw new NoSuchElementException();
    6.         return l.item;
    7.     }
    1.     public E get(int index) {
    2.         checkElementIndex(index);
    3.         return node(index).item;
    4.     }
    6.         Node<E> node(int index) {
    7.         // assert isElementIndex(index);
    9.         if (index < (size >> 1)) {  // 先判断index在左半部分还是右半部分，然后遍历
    10.             Node<E> x = first;
    11.             for (int i = 0; i < index; i++)
    12.                 x = x.next;
    13.             return x;
    14.         } else {
    15.             Node<E> x = last;
    16.             for (int i = size - 1; i > index; i--)
    17.                 x = x.prev;
    18.             return x;
    19.         }
    20.     }