I have found relatively old article about the design principle of design patterns dating around June 6, 2005. Though, the article is a bit old dated ( about 2.5 years), after reading it I have definitely learn something new and refreshing about design patterns and especially about the power and flexibility that can be gained from the use of interface.

The four page article in the form of interview with the design pattern guru Erich Gamma, famous for his landmark book Design Patterns: Elements of Reusable Object-Oriented Software (Addison-Wesley, 1995) and also a member of popular GoF (Gang of Four) is certainly worth to read if you are interested in design patterns and test driven development.

After reading it, I was able to summarize following points that seem quite interesting and important to me:

• Program to an interface, not an implementation.
• Integration between current refactoring tools and XP.
• Composition over inheritance for low coupling and design flexibility in the sense that sub-class always have to make assumptions about the context of the method it overrides (my favorite).
• Something can be public but that does not mean you have published it. Quote from Martin Fowler, but I’m still a bit confused about it and still researching. I have even posted a question in the stackoverflow but have been unable to get any response clarifying my confusion.
• Interface inheritance. I have seen this when I was going through Repository Pattern in NHibernate.
• Smaller objects configuring the container and customizing the behavior of the container.
• Consistency in API design (for example: using depravation support in JAVA, .NET). Seems easy but really hard to achieve especially for beginners like me