Java HTML/XML - 如何更改DOM元素值

import java.io.StringReader;
import java.io.StringWriter;

import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;
import org.w3c.dom.Text;
import org.xml.sax.InputSource;

public class Main {
  public static String getXMLData() {
    return "<a><person><email>asdf</email></person></a>";
  }

  public static void main(String[] argv) throws Exception {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    Document doc = factory.newDocumentBuilder().parse(
        new InputSource(new StringReader(getXMLData())));

    newEmail(doc, "email", "newEmail");
    
    System.out.println(documentToString(doc));

  }
  public static void newEmail(Document doc,String newname,String newemail) {
    Element root = doc.getDocumentElement();
    NodeList rootlist = root.getChildNodes();
    for(int i=0; i<rootlist.getLength(); i++) {
        Element person = (Element)rootlist.item(i);
        NodeList personlist = person.getChildNodes();
        Element name = (Element)personlist.item(0);
        NodeList namelist = name.getChildNodes();
        Text nametext = (Text)namelist.item(0);
        String oldname = nametext.getData();
        if(oldname.equals(newname)) {
            Element email = (Element)personlist.item(1);
            NodeList emaillist = email.getChildNodes();
            Text emailtext = (Text)emaillist.item(0);
            emailtext.setData(newemail);
        }
    }
  }
  public static String documentToString(Document document) {
    try {
      TransformerFactory tf = TransformerFactory.newInstance();
      Transformer trans = tf.newTransformer();
      StringWriter sw = new StringWriter();
      trans.transform(new DOMSource(document), new StreamResult(sw));
      return sw.toString();
    } catch (TransformerException tEx) {
      tEx.printStackTrace();
    }
    return null;
  }
}