Unable to refresh the Access Token-Blackberry

I am getting error as while launching my sampe code as "facebook.FacebookException: Unable to refresh the Access Token".What does that mean?

Code that i used is

String NEXT_URL = "http://www.facebook.com/connect/login_success.html";
    String APPLICATION_ID = "140793189321103";
    String[] PERMISSIONS = Facebook.Permissions.USER_DATA_PERMISSIONS;
    ApplicationSettings as = new ApplicationSettings(NEXT_URL, APPLICATION_ID, PERMISSIONS);
    Facebook fb = Facebook.getInstance(as);

   try{
       if(fb!=null)
           try {
               User user = fb.getCurrentUser();
               System.out.println("UserClass"+user.getFirstName());
           } catch (FacebookException e) {
               System.out.println("FaceBookException"+e);
               e.printStackTrace();
           }
           else{

               System.out.println("FaceBook not connected");

           }

   }catch(Exception e){}
    开发者_如何学Go    // TODO Auto-generated catch block


}

I am getting access token as null,how can i get the particular user account initially?

Regards Rakesh Shankar.

---

instead of using User user = fb.getCurrentUser(); try this user = fb.getUser("me");

and also set all needed permission before instantiating Facebook class object

using ApplicationPermissionManager ApplicationPermission classes