Multiple calls to NodeList.item(n) strange behavior

Question

After adding some sys outs the implementation is gnu.xml.doc
NodeList list = doc.getElementsByTagName("foo");
// Odd behavior
for(int a = 0; a < list.getLength(); a++)
  System.out.println(list.item(a) == null);  // false
  System.out.println(list.item(a) == null);  // false
  System.out.println(list.item(a) == null);  // true
  System.out.println(list.item(a) == null);  // true
}I was iterating over the nodes in my code when the issue above happened. The actual code was something like:
if(list.item(a) != null  && list.item(a).getNodeType() != Node.COMMENT_NODE && list.item(a).getNodeName().equals("foo"))
return list.item(a);I was getting a null pointer exception. With much headache and tracking I found the issue stated. Do I not understand the functionality of list.item(a) or is this a bug?
For those interested I did the following:
NodeList list = doc.getElementsByTagName("foo");
Node n = null;
for(int a = 0; a < list.getLength(); a++)
  n = list.item(a);
  if(n != null  &&
    n.getNodeType() != Node.COMMENT_NODE &&
    n.getNodeName().equals("foo"))
  return n;
}

Answer 1

I have reproduced a related problem, which may or may not be the same...
Given a DOM Document on which I have evaluated an XPathExpression to receive a NodeList, I observed the following:
NodeList nl = xpathexp.evaluate(documentRoot, XPathConstants.NODESET);
// this expression locates 5 nodes
Node n0 = nl.item(0);
Node n1 = nl.item(1);
Node n2 = nl.item(2);
Node n3 = nl.item(3);
Node n2a = nl.item(2);
Node n4 = nl.item(4);
Node n4a = nl.item(4);
Node n5 = nl.item(4);
Node n0a = nl.item(0);
Node n0b = nl.item(0);All of n0..n4 have Node objects.
Even n2a and n4a have Node objects, so random access appears to work properly.
n5 is null, as it should be.
Once null is returned, the next call given a valid index will still return null!
So n0a is null, but n0b is NOT null.
assert null != n0;
assert null != n1;
assert null != n2;
assert null != n3;
assert null != n4;
assert null != n4a;
assert null != n2a;
// is this part of the contract? it does happen to return identical node objects...
assert n2a == n2;
assert n4a == n4;
// it returns null for index 5, as it should
assert null == n5;
// it eventually returned a non-null value for
// index 0 following the null return for index 5...
assert null != n0b;
// but this is the error: the first request (in this case index 0) following the null return
// wrongly returns null... oddly, it only does so for the first request!
assert null != n0a;