The Open/Closed Principle

Inversion of Control with the Plugin Pattern | Jeremy D. Miller: "The Open/Closed Principle   “Software entities (Classes, Modules, Functions, etc.) should be open for extension but closed for modification.”    As explained by Robert Martin here,"

How to Teach Design Patterns

Erich Gamma: "how we should teach patterns. ...  You have to feel the pain of a design which has some problem. I guess you only appreciate a pattern once you have felt this design pain."

How to Use Design Patterns

Erich Gamma: "I think patterns as a whole can help people learn object-oriented thinking: how you can leverage polymorphism, design for composition, delegation, balance responsibilities, and provide pluggable behavior."