How do I write a DOM document as an XML file?

Use xerces parser from apache. You should use the XMLSerializer class to serialise dom to xml file. Here is code sniplet.

// import following package
import org.apache.xml.serialize.*;

// following code serializes
// dom to xml file

OutputFormat of = 
       new OutputFormat("XML","UTF-8",true);
XMLSerializer serializer = 
                       new XMLSerializer();
serializer.setOutputFormat(of);
serializer.setOutputstream(new              
       FileOutputstream("xmlfile.xml"));
serializer.serialize(dom);