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

Here's an example of using the XMLStreamWriter to write an XML document to a file:

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

public class StAXExample {

    public static void main(String[] args) throws Exception {
        OutputStream outputStream = new FileOutputStream("example.xml");
        XMLOutputFactory factory = XMLOutputFactory.newInstance();
        XMLStreamWriter writer = factory.createXMLStreamWriter(outputStream);
        writer.writeStartDocument();
        writer.writeStartElement("root");
        writer.writeStartElement("child");
        writer.writeAttribute("attr", "value");
        writer.writeCharacters("Hello, world!");
        writer.writeEndElement();
        writer.writeEndElement();
        writer.writeEndDocument();
        writer.close();
    }
}

In this example, we create an OutputStream object from an output file using a FileOutputStream. We then create an XMLOutputFactory object using the newInstance() method, which we use to create an XMLStreamWriter object by passing in the OutputStream object to the createXMLStreamWriter() method.

We then use the XMLStreamWriter object to write out the XML document using a series of writeXXX() methods. In this case, we write a simple document with a root element, a child element with an attribute and some text content, and closing tags.

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

Note that the XMLStreamWriter interface provides a low-level, event-based API for writing 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.