SOLID principles

Single Responsibility Principle: A class or method needs to be modified for one and only one reason.

Open/Close principle: A class must be open for extension and closed for modification

Liskov's substitution principle: A class and its subclasses must be exchangeable. 

Interface segregation principle: A class must not be made to implement methods it does not need.

Dependency inversion principle: High level module must not depend on low level modules. Both must depend on abstraction. Abstraction should not depend on details. Details should depend on abstraction.

DRY - Do not repeat yourself.

KISS - Keep it simple stupid.

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