In Java, the Java API for XML Processing (JAXP) provides a set of interfaces and classes that allow you to work with XML documents in a platform-independent manner. One important aspect of working with XML documents is indexing, which can help you quickly retrieve specific elements or attributes within a document.

The javax.xml.xpath package in JAXP provides a way to index XML documents using XPath expressions. XPath is a language for addressing parts of an XML document, and it can be used to navigate the tree of nodes created by the Document Object Model (DOM) API.

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

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document document = builder.parse(new File("example.xml"));

XPathFactory xpathFactory = XPathFactory.newInstance();
XPath xpath = xpathFactory.newXPath();
XPathExpression expression = xpath.compile("//book[@id='12345']/title");

NodeList nodes = (NodeList) expression.evaluate(document, XPathConstants.NODESET);
Element titleElement = (Element) nodes.item(0);
String title = titleElement.getTextContent();

In this example, a new Document object is created by parsing an XML document using a DocumentBuilder. A XPathFactory and a XPath object are then created, and a XPathExpression is compiled using an XPath expression that selects the title element of a book with an id attribute of 12345.

The evaluate() method is then called on the XPathExpression, passing in the Document object and a constant that indicates the desired return type (NODESET in this case). The result is a NodeList that contains all of the matching nodes in the document.

In this case, there should only be one matching title element, so the first item in the NodeList is retrieved and cast to an Element. The text content of the title element is then obtained using the getTextContent() method.

Using XPath indexing in Java can be a powerful tool for efficiently navigating and retrieving data from large XML documents. However, it can also be relatively complex, and it may not be the best approach for all use cases. Depending on the nature of your XML data and the performance requirements of your application, you may want to consider other approaches such as streaming parsers or custom indexing solutions.