import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.*;
import org.xml.sax.*;
import org.xml.sax.helpers.DefaultHandler;

// a kind of event handler
interface DomListener
{
    public String getURI ();
    public String getLocalName ();
    public void processTree (Element tree) throws SAXException;
}

/**
 * Using SAX to Stream DOM Subtrees 
 */
public class DomFilter extends DefaultHandler
{
    private Document	factory;
    private Element	current;
    private DomListener	listener;

    public DomFilter (DomListener l)
	{ listener = l; }

    public void startDocument ()
    throws SAXException
    {
	// all this just to get an empty document;
	// we need one to use as a factory
	try {
	    factory = DocumentBuilderFactory
		.newInstance ()
		.newDocumentBuilder ()
		.newDocument ();
	} catch (Exception e) {
	    throw new SAXException ("can't get DOM factory", e);
	}
    }

    public void startElement (String uri, String local,
	String qName, Attributes atts)
    throws SAXException
    {
	// start a new subtree, or ignore
	if (current == null) {
	    if (!listener.getURI ().equals (uri)) 
		return;
	    if (!listener.getLocalName ().equals (local)) 
		return;
	    current = factory.createElementNS (uri, qName);

	// Add to current subtree, descend.
	} else {
	    Element	e;

	    if ("".equals (uri))
		e = factory.createElement (qName);
	    else
		e = factory.createElementNS (uri, qName);
	    current.appendChild (e);
	    current = e;
	}
	// NOTE:  this example discards all attributes!
	// They ought to be saved to the current element.
    }

    public void endElement (String uri, String local, String qName)
    throws SAXException
    {
	Node	parent;

	// ignore?
	if (current == null)
	    return;
	parent = current.getParentNode ();

	// end subtree?
	if (parent == null) {
	    current.normalize ();
	    listener.processTree (current);
	    current = null;

	// else climb up one level
	} else
	    current = (Element) current.getParentNode ();
    }

    // if saving, append and continue
    public void characters (char buf [], int offset, int length)
    throws SAXException
    {
	if (current != null)
	    current.appendChild (factory.createTextNode (
		new String (buf, offset, length)));
    }
}