XPath processor output as string

XPath.evaluate(expression, inputSource) seems to do what you want.

Edit: Here's some sample code:

String xml = "<foo><bar>text</bar></foo>";
Reader reader = new StringReader(xml);
InputSource inputSource = new InputSource(reader);
XPath xpath = XPathFactory.newInstance().newXPath();
System.out.println(xpath.evaluate("/foo/bar", inputSource));

Edit: this question indicates that there is no java api that can be instantly used to meet your goals. If you're testing XML, then you might want to have a look at XmlUnit

If you use

xPathExpression.evaluate(document);

where document is your DOM document, then it will return a string representation of the first node it matches. That's probably fine for the use case where you believe that your XPath is selecting a single text node. If that's not the case, you could write some toString() method for NodeList that is returned from:

xPathExpression.evaluate(document, XPathConstants.NODESET);

EDIT 1: Here's a SO article on converting a NodeList to an XML document and printing it.

EDIT 2: If you have XPath expressions that use count or logical operators that return single values, then you could catch the exception thrown and take the appropriate action:

try {
    NodeList nodeList = (NodeList) xPathExpression.evaluate(document, XPathConstants.NODESET);
    for (int i = 0; i < nodeList.getLength(); i++) {
        Node node = nodeList.item(i);
        System.out.println(node.getNodeName());
    }
} catch (XPathExpressionException e) {
    String result = xPathExpression.evaluate(document);
    System.out.println(result);
}