Design Pattern

• abstraction
• encapsulation
• polymorphism
• inheritance

• Encapsulate what varies
• Favor composition over inheritance
• Program to interface, not implementations
• Strive for loosely coupled designs between objects that interact
• Classes should be open for extension but closed for modification

Stragety - defines a family of algorthms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

• reusable
• extensible
• maintainable

• show you how to buld systems with good OO design qualities
• are proven object-oriented experience
• don't give xou code, they give you general solutions to design problems. You apply them to your specific application.
• aren't invented they are discovered
• provide a shared language that can maximize the value of your communication with other developers

Observer - defines a onte-to-many dependency between objects so that when one object change state, all its dependents are notified and updated automatically

Subjects, or as we also know them, Observables, update Observers using a common interface

Observers are loosely coupled in that the Observable knows nothing about them, other than that they implement the Observer Interface