The XMLStreamReader is an interface in Java's StAX (Streaming API for XML) API that provides a way to read XML documents in a streaming fashion. It allows you to read XML documents without having to load the entire document into memory, which can be useful for processing large or complex XML documents.

Here's an example of using the XMLStreamReader to read an XML document from a file:

import javax.xml.stream.*;
import java.io.*;

public class StAXExample {

    public static void main(String[] args) throws Exception {
        InputStream inputStream = new FileInputStream("example.xml");
        XMLInputFactory factory = XMLInputFactory.newInstance();
        XMLStreamReader reader = factory.createXMLStreamReader(inputStream);
        while (reader.hasNext()) {
            int eventType = reader.next();
            switch (eventType) {
                case XMLStreamConstants.START_ELEMENT:
                    System.out.println("Start element: " + reader.getLocalName());
                    break;
                case XMLStreamConstants.END_ELEMENT:
                    System.out.println("End element: " + reader.getLocalName());
                    break;
                case XMLStreamConstants.CHARACTERS:
                    System.out.println("Text: " + reader.getText());
                    break;
            }
        }
        reader.close();
    }
}

In this example, we create an InputStream object from an input file using a FileInputStream. We then create an XMLInputFactory object using the newInstance() method, which we use to create an XMLStreamReader object by passing in the InputStream object to the createXMLStreamReader() method.

We then use a while loop to iterate over the XML document, calling the reader's next() method to advance to the next event in the stream. We use a switch statement to handle each event type, printing out information about the start and end elements and text nodes in the document.

Finally, we close the reader using the close() method.

Note that the XMLStreamReader interface provides a low-level, event-based API for reading XML documents, which may be more difficult to use than the higher-level, object-oriented DOM (Document Object Model) API or the SAX (Simple API for XML) API. However, StAX is generally more memory-efficient than the DOM API and faster than the SAX API for large documents, making it a good choice for certain types of applications.