Filling tab of an XML document created by DOM

Question

have a problem with filling tab of an XML file created by DOM
I used this code to create the document:
public class DOMCreation {
public void createDOM( String a, String b ) {
          Document doc = new DocumentImpl( null );
          // Il codice che segue � indipendente dal particolare parser utilizzato
          // Creazione dell'elemento root del documento, identificato dal tag <DOCUMENT>
          Element root = doc.createElement("DOCUMENT");
          doc.appendChild(root);
          // Creazione ed inserimento di un nodo
          Element element1 = doc.createElement("USERNAME");
          root.appendChild( element1 );
          // Creazione ed inserimento di un secondo nodo discendente di <DOCUMENT>
          root.appendChild( doc.createElement("PIN") );
System.out.println( a );
System.out.println( b );
          try {
               serializeDocument( doc, new FileOutputStream("d:/doc.xml"));
          } catch (FileNotFoundException e) {}
private static void serializeDocument(Document doc, OutputStream output) {
          try {
               DOMSerializer ser = new XMLSerializer( output, null);
               ser.serialize(doc);
          } catch (IOException e) {
               System.err.println("I/O exception while serializing document: " + e.getMessage());
}

Answer 1

The code lines:
System.out.println( a );
System.out.println( b );
had the purpose to check if these two parameters were passed by another procedure. These two parameters, however, represent the content to be filled in the tag of XML document created by DOM.
About the other question on "what parser do you use", I'm using Apache's Xerces. However that's the "import" that I include in this procedure:
import org.apache.xerces.dom.DocumentImpl;
import org.apache.xerces.dom.NodeIteratorImpl;
import org.apache.xml.serialize.*;
import org.w3c.dom.*;
import org.w3c.dom.traversal.*;
import java.io.*;