here's an example of how to use XPath in Java:

Let's say we have the following XML file named "books.xml":

<?xml version="1.0"?>
<books>
  <book id="1">
    <title>Snow Crash</title>
    <author>Neal Stephenson</author>
    <year>1992</year>
  </book>
  <book id="2">
    <title>The Diamond Age</title>
    <author>Neal Stephenson</author>
    <year>1995</year>
  </book>
  <book id="3">
    <title>Neuromancer</title>
    <author>William Gibson</author>
    <year>1984</year>
  </book>
</books>

We can use the following Java code to select all book titles whose author is "Neal Stephenson":

import java.io.File;
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.NodeList;

public class XPathExample {
  public static void main(String[] args) throws Exception {
    // create a DOM document from the XML file
    File xmlFile = new File("books.xml");
    Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(xmlFile);

    // create an XPath object
    XPath xpath = XPathFactory.newInstance().newXPath();

    // compile the XPath expression
    String expression = "//book[author='Neal Stephenson']/title";
    NodeList nodes = (NodeList) xpath.compile(expression).evaluate(doc, XPathConstants.NODESET);

    // iterate over the result nodes and print the title
    for (int i = 0; i < nodes.getLength(); i++) {
      System.out.println(nodes.item(i).getTextContent());
    }
  }
}

This code uses the Java Document Object Model (DOM) to create a DOM document from the XML file. It then creates an XPath object and compiles the XPath expression //book[author='Neal Stephenson']/title. This expression selects all title elements that are children of book elements whose author child element has the value "Neal Stephenson".

The evaluate() method of the XPath object is then called with the DOM document and the XPath expression. The method returns a NodeList object containing the result nodes.

Finally, the code iterates over the result nodes and prints the title of each book whose author is "Neal Stephenson".

This is a simple example of how to use XPath in Java. XPath is a powerful tool for querying XML documents, and it provides a wide range of features for selecting and manipulating XML elements.