code: @Override
      void startListening() {
          if (!listening) {
              final IntentFilter filter = new IntentFilter();
              filter.addAction(ConnectivityManager.CONNECTIVITY_ACTION);
              Log.v(DOMAIN, "%s: startListening() registering %s with context %s", this, receiver, context);
              context.registerReceiver(receiver, filter);
              listening = true;
          }
      }
docs: NOTE: startListening() method is called from addNetworkReachabilityListener() which is
  synchronized. So this method is not necessary to be synchronized.