private void endCallback(){
synchronized (callbackSync) {
    callbacks--;
    if (callbacks == 0) {
      callbackSync.notifyAll();
    }
  }
}

private void endCallback ( ) { synchronized ( callbackSync ) { callbacks -- ; if ( callbacks == 0 ) { callbackSync . notifyAll ( ) ; } } }
decrease callback counter , signals callback complete state when dropped to 0
