What is the best way to change an XML attribute with DOM4J

Question

This question is much more easy to solve using W3C DOM as keshlam suggested. Here's my solution to update the value 4 to another value in the line <parameter key="k" value="4"/>

import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
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.NamedNodeMap;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;*/


import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;



import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;



import org.w3c.dom.Document;
import org.w3c.dom.NamedNodeMap;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;

import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerConfigurationException;
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.Element;




public class XMLParser {


     public void modifyDocument(String inputXml)
     {


                 try {
                     FileInputStream file = new FileInputStream(new File(inputXml));

                     DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();

                     DocumentBuilder builder =  builderFactory.newDocumentBuilder();

                     Document xmlDocument = builder.parse(file);

                     XPath xPath =  XPathFactory.newInstance().newXPath();

                     String expression = "//operator[@class='k_medoids']/parameter[@key='k']";
                     String email = xPath.compile(expression).evaluate(xmlDocument);

                     NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);

                     System.out.println(nodeList.getLength());
                     for (int i = 0; i < nodeList.getLength(); i++) {

                         NamedNodeMap attr = nodeList.item(i).getAttributes();
                        Node nodeAttr = attr.getNamedItem("value");
                        nodeAttr.setTextContent("VALUE TO CHANGE"); 
                     }

                  // write the content into xml file
                        TransformerFactory transformerFactory = TransformerFactory.newInstance();
                        Transformer transformer = transformerFactory.newTransformer();
                        DOMSource source = new DOMSource(xmlDocument);
                        StreamResult result = new StreamResult(new File(inputXml));
                        transformer.transform(source, result);              

                 } catch (FileNotFoundException e) {
                     e.printStackTrace();
                 } catch (SAXException e) {
                     e.printStackTrace();
                 } catch (IOException e) {
                     e.printStackTrace();
                 } catch (ParserConfigurationException e) {
                     e.printStackTrace();
                 } catch (XPathExpressionException e) {
                     e.printStackTrace();
                 } catch (TransformerConfigurationException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (TransformerException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }       
             }

}

I must say that XPath is very useful.

Best Regards