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First off, I need an explanation rather than a solution. As you see from the code, I am trying to insert a MovieNode into a MovieList. And the method is in the MovieList class. "list" is where I keep my MovieNodes. 

public void insert(Movie movie, int index)
{
    MovieNode node = new MovieNode(movie);
    MovieNode element;
    MovieNode current = list;

    while(current.previous != null)
        current = current.previous;
    element = current; // first node in the list

    for(int n = 1; n < index; n++)
        element = element.next; // determines where to put new node

    node.next = element;
    element.previous.next = node;

}

The method above seems working. However when I change this line

element.previous.next = node;

with this one;

element = node;

There is no change in the linkedlist after insertion. It seems to me that element.previous.next is just the same thing as element itself since when we replace element with node, we also change the successor of the node that comes before the element in the list. And we point that successor to our new element which is node. I am new to the subject so I am sorry for the mistakes that I may have made.

Riemann
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2 Answers2

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There is a difference:

element.previous.next = node;

will make the previous' element next field point to node, i.e. it will change the previous element.

element = node;

will just assign the local variable element with the (new) node - so it is close to a no-op.

Eugen Rieck
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  • Thanks for the quick reply. And I have just realized that I also have to add "node.previous = element.previous;" line to my code. I mean, when I make a connection from the node that comes before the element to the new node, I also have to make the reverse connection. In the videos I watched to learn the subject, they are just saying when you insert a new node into the linkedlist, you must first point this new node's successor the the node which is at the given insertion index, then change the old node with the new one. Is there anyway to do the insertion operation in a much easier way? – Riemann May 05 '13 at 20:07
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element is a reference to a MovieNode, element.previous.next is another reference to the same MovieNode. The difference between these two is that element is a temporary reference within the scope of your function ; however the element.previous.next is a reference held by the element.previous node which is defined outside this scope.

It is important to remember that in Java saying a = b where a and b are objects, means that a and b refer to the same object. If you modify a, b will also change.

This is why you copied the MovieNode at the beginning of the function : to effectively copy the node instead of referring to it. The rest of the affectations just manipulate the previous and next references and do not handle the actual objects.

Anthony
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  • You're welcome. I just read your comment on Eugen's answer. You're right, you only connected the nodes in the forward direction. This is a doubly linked list connected in both direction, and usually with this kind of list you should keep a reference to both ends. – Anthony May 05 '13 at 20:42
  • Thanks again. Based on your answer, I just wrote the remove method and it works just fine. I actually didn't understand the references in java until I saw your explanation :). – Riemann May 05 '13 at 20:49