What's a simple way in java to evaluate an xpath on a string and return a result string

Here you go, the following can be done with Java SE:

import java.io.StringReader;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathFactory;
import org.xml.sax.InputSource;

public class Demo {

    public static void main(String[] args) throws Exception {
        String xml = "<car><manufacturer>toyota</manufacturer></car>";
        String xpath = "/car/manufacturer";
        XPath xPath = XPathFactory.newInstance().newXPath();
        assertEquals("toyota",xPath.evaluate(xpath, new InputSource(new StringReader(xml))));
    }

}

For this use case the XMLUnit library may be a perfect fit: http://xmlunit.sourceforge.net/userguide/html/index.html#Xpath%20Tests

It provides some additional assert methods.

For example:

assertXpathEvaluatesTo("toyota", "/car/manufacturer",
    "<car><manufacturer>toyota</manufacturer></car>");