Design Principles

• Identify the aspects of your application that vary and seperate them from what stays the same.
• Take what varies and "encapsulate" it so it won't affect rest of your code.
• Program to an interface, not an implementation. (Program to an interface really means program to a supertype.)
• "Has-a" can be better than "is-a"
• Strive for loosely coupled designs between objects that interact.
• classes should be open for extension, but closed for modification.