怎样将XML的Document 转换成 String

import java.io.StringWriter;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.apache.xerces.dom.DocumentImpl;
import org.apache.xerces.dom.DOMImplementationImpl;
import org.apache.xml.serialize.OutputFormat;
import org.apache.xml.serialize.Serializer;
import org.apache.xml.serialize.XMLSerializer;
public class HelloApache
{
  public static void main (String[] args)
  {
    try
    {
      Document doc = new DocumentImpl();
      // Create Root Element
      Element root = doc.createElement("BOOK");
      // Create 2nd level Element and attach to the Root Element
      Element item = doc.createElement("AUTHOR");
    
item.appendChild(doc.createTextNode("Bachelard.Gaston"));
      root.appendChild(item);
      // Create one more Element
      item = doc.createElement("TITLE");
      item.appendChild(doc.createTextNode
                        ("The Poetics of Reverie"));
      root.appendChild(item);
      item = doc.createElement("TRANSLATOR");
      item.appendChild(doc.createTextNode("Daniel
Russell"));
      root.appendChild(item);
      // Add the Root Element to Document
      doc.appendChild(root);
      //Serialize DOM
      OutputFormat format    = new OutputFormat (doc);
      // as a String
      StringWriter stringOut = new StringWriter (); &nb