RE: Walking the DOM (was: XML APIs)

John Cowan wrote,

> Miles Sabin wrote:
> 
> > OK, I agree that this check is O(1). However, that's only
> > because the granularity of the check is so coarse: a single
> > document-level timestamp will cause a lot of unnecessary
> > invalidation ...
> 
> Agreed.  But this isn't so important if there are few active
> iterators (= iterators that will ever be resumed), which was my
> point.
> 
> > I suspect that it would make the performance
> > of modifying a document via iterators unacceptably poor.
> 
> Here I think the JDK 1.2 java.util.Iterator class is useful: it has
> a "remove" method which removes the last element iterated to
> in a safe way, or raises an exception if the underlying
> container is read-only.

Hmm ... I'm afraid I don't think this will be good enough.

Sure it solves the following problem,

  Iterator i = element.???();
  while(i.hasMore())
  {
    Node node = (Node)i.next();
    node.getParentNode().removeChild(node); // correct, but invalidates
                                            // iterator, so poor
performance
  }

but that's only one way we might want to use iterators to
modify the structure of a document. How about something like
this,

  Iterator i = elementA.???();
  Iterator j = elementB.???();
  while(i.hasMore() && j.hasMore())
  {
    Node nodeA = i.next();
    Node nodeB = j.next();

    if(nodeA.getNodeType() == Node.ELEMENT_NODE &&
       nodeB.getNodeType() == Node.ELEMENT_NODE)
    {
      Element elementA = (Element)nodeA;
      Element elementB = (Element)nodeB;

      if(elementA.getAttribute("foo").equals(elementB.getAttribute("foo"
))
        elementB.getParentNode().removeChild(elementB);
  }

(run through the iterators and remove corresponding elements
 with duplicated foo attributes ... OK, a bit artificial I
 know, but I'm sure someone can come up with a more
 realistic case).

Even with a remove() operation on the iterator class, this
will still cause serious problems, because i will be
invalidated every time a node is removed via j.

Cheers,


Miles

-- 
Miles Sabin                          Cromwell Media
Internet Systems Architect           5/6 Glenthorne Mews
+44 (0)181 410 2230                  London, W6 0LJ
msabin@cromwellmedia.co.uk           England

Received on Friday, 13 November 1998 11:48:13 UTC