Read "big talk design mode" --- open-closed Principle

Open-closed Principle

It means that software entities (classes, modules, functions, and so on) can be extended, but cannot be modified. Open for extensioning is open ), closed for Modification)

No matter how "closed" the module is, there will be some changes that cannot be closed to it. Since it is impossible to be completely closed, the designer must choose the type of change closed for the module he designed. He must first guess the most likely types of changes, and then construct an abstraction to isolate those changes, generally wait until the changes take action

To meet your needsProgramBy adding a newCodeInstead of changing the existing code.