Java dom inserting node hierarchy problem

[Hamsterking]

hi there i have an XML file wich contain some music information in a node and i would like to insert more i would like to add more node on demeand when i do i receive this error HIERARCHY_REQUEST_ERR: An attempt was made to insert a node where it is not permitted. here is my XML structure

?xml version="1.0" encoding="UTF-8" standalone="no"?>
<playlist>
    <trackList>
        <track id="1">
            <location>C:\Music\subfoder\title.mp3</location>
            <creator>2Pac</creator>
            <title>Young Black Male</title>
            <album>2Pacalypse Now</album>
        </track>
        <--I WOULD LIE TO INSERT another node here track id ="2" -->
        </trackList>
        <--IN MANY THINGS I TESTED SOME INSERTED NODE HERE AND IT'S NOT WHAT I WANT-->
</playlist>

i use this code and i cant see what is the problem i have searcher all over internet couldn't any similar problem on forums or stack overflows.

public void addTrack(String playlistName) {
    String filePath = playlistName;
    File xmlFile = new File(filePath);
    DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();//nouvelle instace de Document DocumentBuilderFactory
    DocumentBuilder dBuilder;
    try{dBuilder = dbFactory.newDocumentBuilder();
    Document doc = dBuilder.parse(xmlFile);
    Node root = doc.getFirstChild();
    Node subroot =root.getFirstChild();
    int id = doc.getElementsByTagName("track").getLength()+1;
    subroot.appendChild(createMusicElement(doc, id,playlist.get(key).getLocation(), playlist.get(key).getTrack(), playlist.get(key).getAlbum(),playlist.get(key).getArtist()));
    TransformerFactory transformerFactory = TransformerFactory.newInstance();
    Transformer transformer = transformerFactory.newTransformer();
    transformer.setOutputProperty(OutputKeys.INDENT, "yes");
    DOMSource source = new DOMSource(doc);
    StreamResult file = new StreamResult(filePath);
    transformer.transform(source, file);
    }catch (Exception e) {
        e.printStackTrace();
    }
}

thanks for your times i really apreciate!