1.     private class ListItr implements ListIterator<E> {
    2.         private Node<E> lastReturned;
    3.         private Node<E> next;  当前节点
    4.         private int nextIndex; 当前index
    5.         private int expectedModCount = modCount;
    7.         ListItr(int index) {
    8.             // assert isPositionIndex(index);
    9.             next = (index == size) ? null : node(index);
    10.             nextIndex = index;
    11.         }
    13.         public boolean hasNext() {
    14.             return nextIndex < size;
    15.         }
    17.         public E next() {
    18.             checkForComodification();
    19.             if (!hasNext())
    20.                 throw new NoSuchElementException();
    22.             lastReturned = next;
    23.             next = next.next;
    24.             nextIndex++;
    25.             return lastReturned.item;
    26.         }