S.O.L.I.D. Principals Cheat Sheet

I am between gigs, so I have been given opportunities to discus the SOLID Principals along with others of the classics of Object Orientation and basic Computer Science.

The SOLID Principals are somehow connected to Robert C. “Uncle Bob” Martin and his 2000 paper Design Principles and Design Patterns. The word “Solid” appears nowhere in the document. Well anyway, the SOLID Principals are:

• Single Responsibility Principle – A class should do one thing and do it well. No more epic methods or “manager” classed that do everything.
  
• Open/Closed Principle – Open for extension but closed for modification. When you publish an API, others are depending on you. Don’t change the way that a method works. Someone may even be depending on a bug in your code!
• Liskov Substitution Principle – You should be able to substitute a child for a parent and everything work. Named after Barbra Liskov of MIT; learning this kind of detail makes it easier for me to remember.
  
• Interface Segregation Principle – More smaller interfaces! Each interface should do 1 thing. Avoid creating a monster interface that does everything. (this is an extension of the Single Responsibility Principal, right?)
  
• Dependency Inversion – Don’t create an object directly. Use an abstraction to create it. Whether it is the Abstract Factory pattern, an IOC container, simply pass in the object in, or whatever.