OO Principles

Hi! Today I am reading the book Head First Object-Oriented Analysis and Design, I specially liked the OO Principles, I have already read about them in c2, but I really like the way they are summarized in this book:

• OCP: Classes should be open for extension, but closed for modification. (to avoid new requirements breaking old tested code)
  
• DRY: Don't repeat yourself, avoid duplicate code by abstracting out things that are common and placing them in a single location. (to avoid having to fix the same thing in different places, or solving the same problema again, and again and again... I believe this is releated to YAGNI balances OAOO.... i guess I could say that YAGNI balances DRY, and therefore DRY equals OAOO)
  
• SRP: Single responsability principle, every object in your system should hava a single responsibility, and all the object's services shoul be focused on carry in out that single responsability. (to avoid Big Ball of Mud, a huge object that does everything, that is hard to extend, hart to understand, and lots of stupid little object that do nothing, mmmm, this reminds me of a pricinciple I read about a long time ago about balancing the intelligence between your objects)
  
• LSP: Subclasses should be suitable for their base clases (to avoid confusing code, on which you believe you can use a member of class hierarchy, and after trying you realize thet the result is not the expected one)