双链表中的previous（）和hasPrevious（）方法的实现

Question

I am writing a doubly linked list with an iterator. I managed to implement the next() and the hasNext() methods. It works fine, and I am getting the expected result after a forward traversing. But I am stuck in how to implement previous() and hasPrevious() . Here what I did:

private class ListIterator<E> implements java.util.ListIterator<E> { 
        private Node<E> current = (Node<E>) head;
        private Node<E> last = (Node<E>) tail;

        public ListIterator() {

        }

        public ListIterator(int index) {
            if (index < 0 || index > size)
                  throw new IndexOutOfBoundsException();
                for (int i = 0; i < index; i++)
                  current = current.next;
        }

        @Override
        public boolean hasNext() {
            return (current != null);
        }

        @Override
        public boolean hasPrevious() {
            return  last != null;
        }

        @Override
        public E next() {  
            E e = current.element;
            current = current.next;
            return e;
        }

        @Override
        public E previous() {
            E e = (E) last.element;
            last = last.previous;
            return e;
        }

I am getting just the last element in my List when I want a backwards traversing:

[11, 10, 9, 8, 7, 6, 5] //list
 11 10 9 8 7 6 5 //Forward traversing 
 5   //Backwards traversing

What I am doing wrong? Why can't I traverse backwards through the whole List ?

Answer 1

I don't see a reason to track the last element. Just current one. You could implement previous() using current.previous