The sample test case

import java.io.StringReader;
import java.util.Iterator;
import javax.xml.namespace.NamespaceContext;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import net.sf.saxon.xpath.XPathFactoryImpl;
import org.xml.sax.InputSource;


public class Test {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws XPathExpressionException {
        XPathFactory xpathFactory = new XPathFactoryImpl();
        XPath xpath = xpathFactory.newXPath();
        
        xpath.setNamespaceContext(new NamespaceContext() {
            @Override
            public String getNamespaceURI(String prefix) {
                if (prefix.equals("xhtml"))
                {
                    return "http://www.w3.org/1999/xhtml";
                }
                else
                {
                    return null;
                }
            }

            @Override
            public String getPrefix(String string) {
                return null;
            }

            @Override
            public Iterator getPrefixes(String string) {
                return null;
            }
        });
       
        String xhtmlSample = "<title>This is a test</title>
Test¶
";
        
        InputSource source = new InputSource(new StringReader(xhtmlSample));
        
        System.out.println("Found: " + xpath.evaluate("//xhtml:title", source));
    }
    
}

compiles and runs fine against Saxon 9.6.0.8 HE while it throws a run-time exception with Saxon 9.7.0.4 HE on the evaluate call:

Exception in thread "main" java.lang.UnsupportedOperationException
	at net.sf.saxon.xpath.JAXPXPathStaticContext.iteratePrefixes(JAXPXPathStaticContext.java:165)
	at net.sf.saxon.expr.parser.RetainedStaticContext.(RetainedStaticContext.java:85)
	at net.sf.saxon.sxpath.AbstractStaticContext.makeRetainedStaticContext(AbstractStaticContext.java:109)
	at net.sf.saxon.expr.parser.XPathParser.parse(XPathParser.java:472)
	at net.sf.saxon.expr.parser.ExpressionTool.make(ExpressionTool.java:98)
	at net.sf.saxon.xpath.XPathEvaluator.compile(XPathEvaluator.java:194)
	at net.sf.saxon.xpath.XPathEvaluator.evaluate(XPathEvaluator.java:319)

I get the same exception with Saxon 9.7 when trying to run the ApplyXPathJAXP from the Saxon samples in the resources zip collection for Saxon 9.7 which also simply returns null in the getPrefixes method so I am not sure how to set up a NamespaceContext that works with 9.7.