- **Moral of the story: model your classes based on behaviours not on properties; model your data based on properties and not on behaviours. If it behaves like a duck, it's certainly a bird.**

         - **This strongly suggests that inheritance should never be used when the sub-class restricts the freedom implicit in the base class, but should only be used when the sub-class adds extra detail to the concept represented by the base class as in 'Monkey' is-an 'Animal'.**

     - `I`: Interface Segregation Principle

         - [Wiki](https://en.wikipedia.org/wiki/Interface_segregation_principle)

     - `D`: Dependency Inversion Principle

         - [link](http://www.oodesign.com/dependency-inversion-principle.html)