EventBusAsync.java

public class MyDatabaseHelper{
  
  public MyDatabaseHelper(){
    EventBus.getDefault().register(this);
  }
  
  // To register an async event handler you need to define onEventAsync or onEventBackgroundThread
  // Here we will use onEventBackgroundThread, please check EventBus' docs for more details.
  public void onEventBackgroundThread(AddToCartEvent event){
    Product product = event.getProduct();
    //use this Product instance to update your databse
  }
}