The problem with the previous example was that the SystemMonitor class was interacting directly with the concrete classes it was going to retrieve.
In order to achieve a design that honors the open/closed principle, we have to design toward abstractions.
A possible alternative would be to think of this class as it collaborates with the events, and then we delegate the logic for each particular type of event to its corresponding class:
Then we have to add a new (polymorphic) method to each type of event with the single responsibility of determining if it corresponds to the data being passed or ...
