importNode throws org.w3c.dom.DOMException: NOT_SUPPORTED_ERR

I am trying to merge two XML documents both with standard Oracle JDK 7 and also with Saxon HE but I keep getting an:

org.w3c.dom.DOMException: NOT_SUPPORTED_ERR: The implementation does not support the requested type of object or operation.`

On the line with the importNode below (same thing happens with adoptNode BTW):

import java.io.*;
import org.w3c.dom.*;
import javax.xml.parsers.*;

public class FooMain {

    private static Document slurp(String s) throws Exception {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        return factory.newDocumentBuilder().parse(new ByteArrayInputStream(s.getBytes("UTF-8")));
    }

    public static void main(String args[]) throws Exception {
        Document doc  = slurp("<a></a>");
        Document doc2 = slurp("<b></b>");
        Node not_used = doc.importNode(doc2, true);
    }
}

I’ve tried both with Saxon-HE-9.4.jar on my classpath or not, but I still get the same error.

update

Based on comments I received, when I try with:

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance("net.sf.saxon.dom.DocumentBuilderFactoryImpl", null);

I get instead:

java.lang.UnsupportedOperationException: The Saxon DOM implementation cannot be updated
  at net.sf.saxon.dom.NodeOverNodeInfo.disallowUpdate(NodeOverNodeInfo.java:719)
  at net.sf.saxon.dom.DocumentOverNodeInfo.importNode(DocumentOverNodeInfo.java:211)
  at FooMain.main(FooMain.java:16)

After googling I see old messages to the effect that Saxon apparently creates a read-only DOM which is weird since I thought that one of the comparison points of DOM vs SAX is that DOM is read-write whereas SAX is read-only.

Answer

To merge two documents (I’m not sure exactly what you mean by this, but I’ll guess) using Saxon, you don’t want to be fiddling about with a DOM. Just run this XQuery:

<doc>{doc('a.xml'), doc('b.xml')}</doc>