obtaining a unique set with javas xpath

Given the following xml document (assuming more books then actually listed) and using the java implementation of xpath. What expression would I use to find a unique set of author names?

<inventory>
    <book year="2000">
        <title>Snow Crash</title>
        <author>Neal Stephenson</author>
        <publisher>Spectra</publisher>
        <isbn>0553380958</isbn>
        <price>14.95</price>
    </book>

    <book year="2005">开发者_如何学Go
        <title>Burning Tower</title>
        <author>Larry Niven</author>
        <author>Jerry Pournelle</author>
        <publisher>Pocket</publisher>
        <isbn>0743416910</isbn>
        <price>5.99</price>
    </book>

    <book year="1995">
        <title>Zodiac</title>
        <author>Neal Stephenson</author>
        <publisher>Spectra</publisher>
        <isbn>0553573862</isbn>
        <price>7.50</price>
    </book>

    <!-- more books... -->

</inventory>

Set<String> uniqueAuthors = new HashSet<String>();
XPathFactory factory = XPathFactory.newInstance();
XPath xPath = factory.newXPath();
XPathExpression expr = xpath.compile("//book/author/text()");
NodeList nodes = (NodeList)expr.evaluate(doc, XPathConstants.NODESET);
for (int i = 0; i < nodes.getLength(); ++i) {
    uniqueAuthors.add(nodes.item(i).getNodeValue());
}

I used the excellent article "The Java XPath API" as a reference.

XPath Version 1.0 cannot, in general, select distinct values, so I inserted the authors into a Set. At the end of the for loop, the set will contain all of the authors.