General design principles

1. Identify the aspects of your application that vary and separate them from what stays the same.
2. Program to an interface, not an implementation.
3. Depend upon abstractions. Do not depend upon concrete classes.
4. Favor composition over inheritance.
5. Strive for loosely coupled designs between objects that interact.
6. Classes should be open for extension, but closed for modification.
7. Principle of the Least Knowledge: talk only to your immediate friends. (the same principle: Law of Demeter)
8. The Hollywood Principle: don't call us, we'll call you.
9. A class should have only one reason to change