EventBusRegisterUnregister.java

@Override
public void onCreate(Bundle savedInstanceState){
  super.onCreate(savedInstanceState);
  
  //register with the event bus
  EventBus.getDefault().register(this);
  
  // other code goes here
}

@Override
public void onDestroy(){
  super.onDestroy();
  
  //unregister from the event bus
  EventBus.getDefault().unregister(this);
}

//Subscribe to the AddToCartEvent
public void onEvent(AddToCartEvent event){
  Product product = event.getProduct();
  //use this Product instance to update your UI
}