import java.io.File;
import java.io.FileInputStream;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

public class FindElementsByAbsoluteLocationWithXPath {

    public static void main(String[] args) throws Exception {

        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        dbf.setValidating(false);
        DocumentBuilder db = dbf.newDocumentBuilder();

        Document doc = db.parse(new FileInputStream(new File("in.xml")));

        XPathFactory factory = XPathFactory.newInstance();

        XPath xpath = factory.newXPath();

        String expression;
        Node node;
        NodeList nodeList;

        // 1. root element
        expression = "/*";
        node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);
        System.out.println("1. " + node.getNodeName());

        // 2. root element (by name)
        expression = "/rss";
        node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);
        System.out.println("2. " + node.getNodeName());

        // 3. element under rss
        expression = "/rss/channel";
        node = (Node) xpath.evaluate(expression, doc, XPathConstants.NODE);
        System.out.println("3. " + node.getNodeName());

        // 4. all elements under rss/channel
        expression = "/rss/channel/*";
        nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
        System.out.print("4. ");
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.print(nodeList.item(i).getNodeName() + " ");
        }
        System.out.println();

        // 5. all title elements in the document
        expression = "//title";
        nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
        System.out.print("5. ");
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.print(nodeList.item(i).getNodeName() + " ");
        }
        System.out.println();

        // 6. all elements in the document except title
        expression = "//*[name() != 'title']";
        nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
        System.out.print("6. ");
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.print(nodeList.item(i).getNodeName() + " ");
        }
        System.out.println();

        // 7. all elements with at least one child element
        expression = "//*[*]";
        nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
        System.out.print("7. ");
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.print(nodeList.item(i).getNodeName() + " ");
        }
        System.out.println();

        // 8. all level-5 elements (the root being at level 1)
        expression = "/*/*/*/*";
        nodeList = (NodeList) xpath.evaluate(expression, doc, XPathConstants.NODESET);
        System.out.print("8. ");
        for (int i = 0; i < nodeList.getLength(); i++) {
            System.out.print(nodeList.item(i).getNodeName() + " ");
        }
        System.out.println();

    }

}

Input:

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0">
    <channel>
        <title>Java Tutorials and Examples 2</title>
        <language>en-us</language>
        <item>
            <title><![CDATA[Java Tutorials 2]]></title>
            <link>http://www.javacodegeeks.com/</link>
        </item>
        <item>
            <title><![CDATA[Java Examples 2]]></title>
            <link>http://examples.javacodegeeks.com/</link>
        </item>
    </channel>
</rss>

输出：

1. rss
2. rss
3. channel
4. title language item item 
5. title title title 
6. rss channel language item link item link 
7. rss channel item item 
8. title link title link