java 3D problem

[bibiki]

hey there,
I have this java 3D home assignment I need help with. I would greatly appreciate it if someone could direct me to a solution.

I need to build create a 3 D digital clock and add some key navigation to it. I managed to add the key navigation (I removed the code for key navigation to make for less code), but I can't seem to make the necessary animation going... the scene should change every second, or else it will be a frozen clock.

what I did is this:

I built two classes. One to build the clock, and another, behavior class, that I hoped would 'repaint' the scene. I know the classes are interacting, but I am getting a:

javax.media.j3d.RestrictedAccessException: Group: only BranchGroup nodes may be set

error when I try to reset the child (a 3d shape representing time) of the TransformGroup in my code. However, I get no error when I invoke the createSceneGraph() method through my behavior class, but the clock remains frozen.

here is my code:

import java.awt.*;
   import java.applet.*;
   import java.awt.event.*;
   import javax.media.j3d.*;
   import javax.vecmath.*;
   import com.sun.j3d.utils.universe.*;
   import com.sun.j3d.utils.geometry.*;
   import com.sun.j3d.utils.applet.MainFrame;
   import com.sun.j3d.utils.behaviors.keyboard.*;
   import java.util.*;

   public class Detyra3D extends Applet{
      Shape3D shape;
   	
   	
      public void init(){
         setLayout(new BorderLayout());
         Canvas3D canv = new Canvas3D(null);
         add(canv, BorderLayout.CENTER);
      	
         BranchGroup bg = createSceneGraph();
        
         SimpleUniverse su = new SimpleUniverse(canv);
         su.getViewingPlatform().setNominalViewingTransform();
         

         
        
            su.addBranchGraph(bg);
 
      
      
      	
      }
   	
      public BranchGroup createSceneGraph(){
         BranchGroup scene = new BranchGroup();
      	
      	
      	
      	
      	
         Transform3D tr = new Transform3D();
         tr.setScale(0.45);
         tr.setTranslation(new Vector3f(-0.9f, -0.2f, 0f));
      	
      	
      
      	
      	
         TransformGroup tg = new TransformGroup(tr);
			tg.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
			
			
			//this line should make possible my way of overriding
			//the processStimulus method in my Behavior class
			//to make possible re-reading of tim every half a second
			//and building a 3D shape out of it
         tg.setCapability(TransformGroup.ALLOW_CHILDREN_WRITE);
         scene.addChild(tg);
      	
      	
      
         tg.addChild(shapeClock());//-------------------------------
      	
               	
      //set light
         PointLight light = new PointLight(new Color3f(Color.red),
                                      new Point3f(1f,1f,1f),
                                       new Point3f(1f,0.1f,0f));
         BoundingSphere bounds = new BoundingSphere();
         light.setInfluencingBounds(bounds);
         scene.addChild(light);
      	
         Ndihmese ndihm = new Ndihmese(this, tg);
         ndihm.setSchedulingBounds(bounds);
         scene.addChild(ndihm);
      
         return scene;
      	
      }
   
   //I build a new shape every half a second here
      public Shape3D shapeClock(){
      	
      	
      
         Shape3D shape = new Shape3D();
         //setting appearance
         Appearance ap = new Appearance();
         ap.setMaterial(new Material());
      	
      	
      	//I read time here
         Calendar time = new GregorianCalendar();
         int hour = time.get(time.HOUR);
         int mins = time.get(time.MINUTE);
         int secs = time.get(time.SECOND);
      	
      
      //the time shall be a string, so I'm setting the font
         Font3D font = new Font3D(new Font("SansSerif", Font.PLAIN, 1),
                              new FontExtrusion());      	
      
      
      	//I build the string that represents the current time here
         String s = hour + ":" + mins + ":" + secs;
      	
      	
      	
      //setting the 3D text that shall be output
         Text3D text = new Text3D(font, s);
      //creating a 3D shape out of the text above
         shape = new Shape3D(text, ap);
      	//I System.out here to see if my behavior object
      	//is actually working
         System.out.println(s);
         return shape;
      }
      
   	
   	
   	
      public static void main(String[] args) {
         new MainFrame(new Detyra3D(), 720, 480);
      }
   
   }

//the following is my behavior class

   import java.awt.*;
   import java.applet.*;
   import java.awt.event.*;
   import javax.media.j3d.*;
   import javax.vecmath.*;
   import com.sun.j3d.utils.universe.*;
   import com.sun.j3d.utils.geometry.*;
   import com.sun.j3d.utils.applet.MainFrame;
   import com.sun.j3d.utils.behaviors.keyboard.*;
   import java.util.*;
   import java.util.*;
   import javax.media.j3d.*;

   public class Ndihmese extends Behavior {
      TransformGroup det;
      Detyra3D detyra;
      public Ndihmese(Detyra3D d3, TransformGroup trg) {
         det = trg;
         detyra = d3;
      }
   
      public void initialize() {
         wakeupOn(new WakeupOnElapsedTime(500));
      }
   
      public void processStimulus(java.util.Enumeration enumeration) {
         det.setChild(detyra.shapeClock(), 0);
         //detyra.createSceneGraph();
       /*
       	//this code rotates the scene around z-axis
      	//I tried it and it worked fine, so I know my
      	//classes are interacting  
      	int sec = Calendar.getInstance().get(Calendar.SECOND);
         Transform3D tr = new Transform3D();
         tr.rotZ(-Math.PI * sec /30.0);
         det.setTransform(tr);
         */
         wakeupOn(new WakeupOnElapsedTime(500));
      }
   }

Thank you!