public final void addCallback(StatusCallback callback){
synchronized (callbacks) {
    if (callback != null && !callbacks.contains(callback)) {
      callbacks.add(callback);
    }
  }
}

public final void addCallback ( StatusCallback callback ) { synchronized ( callbacks ) { if ( callback != null && ! callbacks . contains ( callback ) ) { callbacks . add ( callback ) ; } } }
adds a callback that will be called when the state of this session changes .
