change the vtkCallbackCommand class to java vtk

Adding an callback method on a specific event is different from C++. As you can see in some vtk Java Exemple you don't have to create a class which extends from vtkCallbackCommand to rewrite the Execute Method. To add specific behavior you have to use the Java AddObserver() method, It should be something like :

public class kbHandler 
{
    private vtkRenderWindowInteractor iren;

    public static void main(String[] args) {
        kbHandler kbh = new kbHandler();
        kbh.doit();
    }

  void callbackHandler ()
  {
     // if i'm here, a key is pressed !!
     // you can get back information from iren (which key : iren.GetKeyCode())
  }

  public void doit ()
  {
     // Do lot of things

    iren = new vtkRenderWindowInteractor();
    iren.SetRenderWindow(renWin);

    // add observer for the handler  arg1 = event to observe, arg2 object handler of the event, arg3: method to call
    iren.AddObserver("CharEvent", this, "callbackHandler");

    iren.Initialize();
    iren.Start();

  }
}