Removing DOM nodes when traversing a NodeList

Question

I'm about to delete certain elements in an XML document, using code like the following:

NodeList nodes = ...;
for (int i = 0; i < nodes.getLength(); i++) {
  Element e = (Element)nodes.item(i);
  if (certain criteria involving Element e) {
    e.getParentNode().removeChild(e);
  }
}

Will this interfere with proper traversal of the NodeList? Any other caveats with this approach? If this is totally wrong, what's the proper way to do it?

Answer 1

According to the DOM specificaion, the result of a call to node.getElementsByTagName("...") is supposed to be "live", that is, any modification made to the DOM tree will be reflected in the NodeList object. Well, for conforming implementations, that is...

NodeList and NamedNodeMap objects in the DOM are live; that is, changes to the underlying document structure are reflected in all relevant NodeList and NamedNodeMap objects.

(DOM Specification)

So, when you modify the tree structure, a conforming implementation will change the NodeList to reflect these changes.

Answer 2

So, given that removing nodes while traversing the NodeList will cause the NodeList to be updated to reflect the new reality, I assume that my indices will become invalid and this will not work.

So, it seems the solution is to keep track of the elements to delete during the traversal, and delete them all afterward, once the NodeList is no longer used.

NodeList nodes = ...;
Set<Element> targetElements = new HashSet<Element>();
for (int i = 0; i < nodes.getLength(); i++) {
  Element e = (Element)nodes.item(i);
  if (certain criteria involving Element e) {
    targetElements.add(e);
  }
}
for (Element e: targetElements) {
  e.getParentNode().removeChild(e);
}

Answer 3

The Practical XML library now contains NodeListIterator, which wraps a NodeList and provides full Iterator support (this seemed like a better choice than posting the code that we discussed in the comments). If you don't want to use the full library, feel free to copy that one class: http://practicalxml.svn.sourceforge.net/viewvc/practicalxml/trunk/src/main/java/net/sf/practicalxml/util/NodeListIterator.java?revision=125&view=markup