title: using interfaces - design pattern aspect
body: <p>I am looking for a good and short article+examples of how working with interfaces.
  I am not interesting the technical part, but I need the design part.
  For example, how to programming using interfaces, when and how create implementations, design patterns for regular development with interfaces.</p>
  
  <p>I have many similar classes that repeat themselves in several ways. I want to use interfaces and abstract classes in order to make things more modular - but I can't find out how to do it properly.</p>