Classes should be open for extension but closed for modification
- The main idea of this principle is to keep existing code from breaking when you implement new features
- A class is open if you can extend it, produce a subclass and do whatever you want with it — add new methods or fields, override base behaviours etc.
- If a class is already developed, tested, reviewed, and included in some framework or otherwise used in an app, trying to mess with its code is risky. It should be closed for modification.
- A child class shouldn’t be responsible for the parent’s issues.