Service and activity communication

I am making a IM client for android and i want to know how to do the communication between a Service and Activity.

I have a Service that establish the connection to gtalk, gets the contacts, listens for the incoming messages... , and i have the activity that is the UI of my application.

In the activity i create the service and bind the activity to it. I have an aidl interface to communicate between service and the activity with this methods:

  String[] getContacts(); 
    String[] getPressence();
    int setConnection(int type);
    void logOut(); 

So far so good, i can make the connection to gtalk, i can get the contacts and the status, but now i want to do a function in the activity that listens when the service gets a message and upda开发者_开发百科tes the UI...

For this i made another aidl file with the metod void receivedMessage(String message); and whenever the service gets a message, i call that method, in the activity i display the message passed in the message String and i get an NullPointer exception.

Do i have to get the message in a handler or by some sync or async methods ? I am beginner in android and now i learn the services part so please help me .

Thank you very much!

I solved the problem, i only had to reference the listener from the activity, in order to the service to use the listener.