• Encapsulate what varies.
• Favor composition over inheritance.
• Program to interfaces, not implementations.
• Strive for loosely coupled design between objects that interact.
• Classes should be open for extension but closed for modification.
• Depend on abstractions. Do not depend on concrete classes.
• Only talk to your friends.
• Dont call us, we call you.
• A class should have only one reason to change.

Aus dem Buch “Head First Design Patterns” gesammelte Object Oriented Design Principles.