Problems with parsing and display a XML-File

Question

Hi,
I'm working on an applet to read a XML-file, display the content in a JTree, in order to modify and save it again. I found this tutorial: http://java.sun.com/webservices/jaxp/dist/1.1/docs/tutorial/dom/index.html
I took this example http://java.sun.com/webservices/jaxp/dist/1.1/docs/tutorial/dom/work/DomEcho02.java and modified it, to run it as an applet.
I changed the method main into:
public class EditorTest extends JApplet {
public void init(){
     DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
     try {
          DocumentBuilder builder = factory.newDocumentBuilder();
          File file = new File("/path/inhalt.xml");
          document = builder.parse(file);
     } catch (SAXException sxe) {
          // Error generated during parsing)
          Exception     x = sxe;
          if (sxe.getException() != null)
               x = sxe.getException();
          x.printStackTrace();
     catch (ParserConfigurationException pce) {
          // Parser with specified options can't be built
          pce.printStackTrace();
     } catch (IOException ioe) {
          // I/O error
          ioe.printStackTrace();
     makeWindow(); //equals makeFrame(); in the tutorial
} //initThe method makeWindow() looks like this:
private Container container;
private void makeWindow(){
     container = getContentPane();
     JDesktopPane desktop = new JDesktopPane();
     final EditorTest echoPanel = new EditorTest();
     JMenuBar menuBar = MakeMenuBar();  //returns a JMenuBar with a few items in it
     container.add(menuBar, BorderLayout.NORTH);
}The constructor:
public DomEcho02() {
     // Set up the tree
     JTree tree = new JTree(new DomToTreeModelAdapter());
     // Build left-side view
     JScrollPane treeView = new JScrollPane(tree);
     treeView.setPreferredSize( new Dimension( leftWidth, windowHeight ));
     // Build right-side view
     final JEditorPane htmlPane = new JEditorPane("text/html","");
     htmlPane.setEditable(true);
     JScrollPane htmlView = new JScrollPane(htmlPane);
     htmlView.setPreferredSize(new Dimension( rightWidth, windowHeight ));
     // Build split-pane view
     JSplitPane splitPane = new JSplitPane( JSplitPane.HORIZONTAL_SPLIT, treeView, htmlView );
     splitPane.setContinuousLayout( true );
     splitPane.setDividerLocation( leftWidth );
     splitPane.setPreferredSize( new Dimension( windowWidth + 10, windowHeight+10 ));
     // Add GUI components
     this.setLayout(new BorderLayout());
     this.add("Center", splitPane );
} // ConstructorI didn't change the class AdapterNode and DomToTreeModelAdapter. When I change the line "JTree tree = new JTree(new DomToTreeModelAdapter());" into "JTree tree = new JTree();", the applet starts and display a standard-JTree, as it ist generated by the constructor. If I want to display my own Tree the applet fails. When I start it with the appletviewer I get this message:
java.lang.NullPointerException
at DomEcho02$AdapterNode.childCount(DomEcho02.java:151)
at DomEcho02$DomToTreeModelAdapter.isLeaf(DomEcho02.java:167)
at javax.swing.JTree.setModel(JTree.java:704)
at javax.swing.JTree.<init>(JTree.java:511)
at DomEcho02.<init>(DomEcho02.java:51)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:494)
at java.lang.Class.newInstance0(Class.java:350)
at java.lang.Class.newInstance(Class.java:303)
at sun.applet.AppletPanel.createApplet(AppletPanel.java:721)
at sun.applet.AppletPanel.runLoader(AppletPanel.java:650)
at sun.applet.AppletPanel.run(AppletPanel.java:324)
at java.lang.Thread.run(Thread.java:595)
Is there anyone out there, who knows how to fix it?
Thanks in advance.

Answer 1

I found out, how it was going wrong.
When the applet started, it didn't start with the method init(), as it should do, it started with the constructor. Because there was no document the method childCount() throws a exception. After this, the method init() startet reading the file and started the constructor again, but the results were not shown.
After knowing this I decided to write the Applet again and now it's working fine.