I am currently using the DOM, but I want to use an external DTD with my XML file. With SAX you can parse the external DTD and associate it wih the XML file. How do I do that with the DOM?

If you are using JAXP from Sun there are two things you can do.

First, use the com.sun.xml.tree.XmlDocument's setDoctype method as shown below.

XmlDocument	doc = new XmlDocument ();
...
// add a DTD, for fun
doc.setDoctype (
    null,	// no public identifier
    "http://www.example.com/xml/dtd1.dtd",
    null	// no internal subset
    );
...

Second, you can use setEntityResolver method of javax.xml.parsers.DocumentBuilder to customize how the external DTD's are resolved. Here's an example from the docs.

import org.xml.sax.EntityResolver;
 import org.xml.sax.InputSource;

 public class MyResolver implements EntityResolver {
   public InputSource resolveEntity (String publicId, String systemId)
   {
     if (systemId.equals("http://www.myhost.com/today")) {
              // return a special input source
       MyReader reader = new MyReader();
       return new InputSource(reader);
     } else {
              // use the default behaviour
       return null;
     }
   }
 }