Java XML: IndexOutOfBoundsException When appending a node

Dear all,

Now that I am writing a messenger program. The following code shall store the message history.
Sadly, I've encountered an "IndexOutOfBounds" exception on a line and I've been trying for a whole day,
but still can't figure out the problem.

	public static final SimpleDateFormat SDF = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
	public static final String FILE_PATH = "/data/data/com.demo.xmppchat/";
	public static Map<Date, String> messagesMap = new TreeMap<Date, String>();
	// File should be saved as /username/buddy.xml
	public static void writeHistory(String username, String buddy, Map<Date, String> messages)
	{
		try {			
			// Check if the file exists. If the file does not exist, create and initiate one. 
			File file = new File(FILE_PATH + username);
			if (!file.exists()) {
				file.mkdirs();
			}	
			file = new File(FILE_PATH + username + "/" + buddy + ".xml");
			// file.setReadable(true); file.setWritable(true); file.setExecutable(true);
			if (!file.exists()) {
				file.createNewFile();				
				initiateHistoryFile(username, buddy);
			}
			// Now that the XML file should exists as we have created and initialized one.
			// TODO:
			// Now we modify the XML file
			DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
			DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
			Document doc = docBuilder.parse(file);
			Element root = doc.getDocumentElement();
			Element rootE = doc.getDocumentElement();	
			String s = "";
			Node n = null;
			for (Map.Entry<Date, String> entry : messages.entrySet()) {		
				// <message>
				Element messageE = doc.createElement("message");
				rootE.appendChild(messageE);
				// Set attribute to <message> element
				// messageE.setAttribute("id", "1");
				// <from>
				Element fromE = doc.createElement("from");
				fromE.appendChild(doc.createTextNode(buddy));
				messageE.appendChild(fromE);
				// <date>
				Element dateE = doc.createElement("date");
				s = SDF.format(entry.getKey());
				n = doc.createTextNode(s);
				dateE.appendChild(n);
				messageE.appendChild(dateE);
				// <text>
				Element textE = doc.createElement("text");
				s = entry.getValue();
				n = doc.createTextNode(s);
				textE.appendChild(n);
				messageE.appendChild(textE);	
				root.appendChild(messageE); // <- *** THIS IS WHERE "IndexOutOfBounds" exception OCCURS! ***
			}
			TransformerFactory transformerFactory = TransformerFactory.newInstance();
			Transformer transformer = transformerFactory.newTransformer();
			DOMSource source = new DOMSource(doc);
			StreamResult result = new StreamResult(file);
			transformer.transform(source, result);
			// Write the file
			// FileWriter fileWritter = new FileWriter(file.getName(),true);
	        // BufferedWriter bufferWritter = new BufferedWriter(fileWritter);
	        // bufferWritter.write("");
	        // bufferWritter.close();		
		} catch (Exception e) {
			ToneGenerator toneGenerator = new ToneGenerator(AudioManager.STREAM_SYSTEM, ToneGenerator.MAX_VOLUME);
			toneGenerator.startTone(ToneGenerator.TONE_PROP_BE
EP);
			e.printStackTrace();
		}
	}

Thank you so much for your help.