Retrieve xml node value with xpath

Here is my xml file: bookstore.xml and what I am trying to do is I provide xpath as input and it should give me that node value of it. but it gives me null values.

    <?xml version="1.0" encoding="UTF-8"?>
<bookstore>
  <book category="cooking">
    <title lang="en">Everyday Italian</title>
    <author>Giada De Laurentiis</author>
    <year>2005</year>
    <price>30.00</price>
  </book>
  <book category="children">
    <title lang="en">Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <price>29.99</price>
  </book>
  <book category="web">
    <title lang="en">XQuery Kick Start</title>
    <author>James McGovern</author>
    <author>Per Bothner</author>
    <author>Kurt Cagle</author>
    <author>James Linn</author>
    <author>Vaidyanathan Nagarajan</author>
    <year>2003</year>
    <price>49.99</price>
  </book>
  <book category="web" cover="paperback">
    <title lang="en">Learning XML</title>
    <author>Erik T. Ray</author>
    <year>2003</year>
    <price>39.95</price>
  </book>
</bookstore>

here is the java code :

public class XmlParserUsingXpath {


        public void xmlParser(XPathExpression xpathexp) throws ParserConfigurationException, XPathExpressionException{


            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

            DocumentBuilder dbuilder = factory.newDocumentBuilder();

            Document doc=null;

            try {
                 doc= dbuilder.parse(new FileInputStream("D:\\Bookstore.xml"));

            } catch (SAXException e) {

                e.printStackTrace();

            } catch (IOException e) {

                e.printStackTrace();
            }

            doc.getDocumentElement().normalize();

            Object result = xpathexp.evaluate( doc, XPathConstants.NODESET );

            NodeList nodes = (NodeList) result;
            for (int i = 0; i < nodes.getLength(); i++) {
                 System.out.println(nodes.item(i));
            }

        public static void main(String[] args){

            XmlParserUsingXpath o1= new XmlParserUsingXpath();

            XPath x=XPathFactory.newInstance().newXPath();

            System.out.println("Enter the Xpath Expression : ");

            Scanner sc= new Scanner(System.in);

            String scan= sc.nextLine();

            try {

                o1.xmlParser(x.compile(scan));

            } catch (XPathExpressionException | ParserConfigurationException e) {

                e.printStackTrace();
            }

        }



}

now when i provide "//book" as input, it gives me

[book: null]
[book: null]
[book: null]
[book: null]

or "/bookstore/book[3]/author" would give

[author: null]
[author: null]
[author: null]
[author: null]
[author: null]