Robert C. Martin penned the following articles concerning the principles of object oriented programming.

The Open-Closed Principle

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

The Liskov Substitution Principle

[paraphrased] Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.

The Dependency Inversion Principle

• High level modules whould not depend upon low level modules. Both should depend upon abstractions.
• Abstractions should not depend upon details. Details should depend upon abstractions.

The Interface Segregation Principle

Clients should not be forced to depend upon interfaces that they do not use.