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

Removing nodes while looping will cause undesirable results, e.g. either missed or duplicated results. This isn't even an issue with synchronization and thread safety, but if the nodes are modified by the loop itself. Most of Java's Iterator's will throw a ConcurrentModificationException in such a case, something that NodeList does not account for.

It can be fixed by decrementing NodeList size and by decrementing iteraror pointer at the same time. This solution can be used only if we proceed one remove action for each loop iteration.

NodeList nodes = ...;
for (int i = nodes.getLength() - 1; i >= 0; i--) {
  Element e = (Element)nodes.item(i);
   if (certain criteria involving Element e) {
    e.getParentNode().removeChild(e);
  }
}

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);
}