Object-oriented design principles

I recently read Head First Design Patterns, and they had a handy set of Object-Oriented design principles that were repeated throughout the book that I wanted to document for reference.

• Encapsulate what varies.
• Favor composition over inheritance.
• Program to interfaces, not implementations.
• Strive for loosely coupled designs 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.”
• “Don’t call us, we’ll call you.”
• A class should only have one reason to change.