Custom NodeList Implementation

I want my own org.w3c.dom.NodeList backed by an ArrayList, so I implemented:

public class SimNodeList extends ArrayList<Node> implements NodeList {

private static final long serialVersionUID = 1L;

public SimNodeList() {

super();

}

public SimNodeList(NodeList nodeList) {

super();

this.add(nodeList);

}

public Node item(int index) {

if (index>=size())

return null;

return get(index);

}

public int getLength() {

return size();

}

public void add(NodeList nodeList) {

if (nodeList==null)

return;

for (int i = 0; i < nodeList.getLength(); i++)

super.add(nodeList.item(i));

}

This seems to work ok until I try and use my custom NodeList as the context for another xpath.evaluate:

SimNodeList result1 = new SimNodeList();

result1.add( ..some NodeList... )

SimNodeList result2 = new SimNodeList((NodeList)xpath.evaluate("*", result 1, XPathConstants.NODESET));

If I use anything but "." as the xpath expression I get the following:

javax.xml.transform.TransformerException: Unknown error in XPath.

at com.sun.org.apache.xpath.internal.XPath.execute(Unknown Source)

at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.eval(Unknown Source)

at com.sun.org.apache.xpath.internal.jaxp.XPathImpl.evaluate(Unknown Source)

at org.rje.sim.core.ConfigHelper.getFullOrderedMessageExchangeProcess(ConfigHelper.java:74)

at org.rje.sim.test.ConfigHelperTest.main(ConfigHelperTest.java:41)

Caused by: java.lang.NullPointerException

at com.sun.org.apache.xpath.internal.axes.ChildTestIterator.setRoot(Unknown Source)

at com.sun.org.apache.xpath.internal.axes.NodeSequence.setRoot(Unknown Source)

at com.sun.org.apache.xpath.internal.axes.LocPathIterator.execute(Unknown Source)

... 5 more

java.lang.NullPointerException