Design Principles

Encapsulate what varies.

Identify the aspects of your application that vary and separate them from what stays the same. Take what varies and “encapsulate” it so it won’t affect the rest of your code.

Dependency Inversion Principle

Depend upon abstractions. Do not depend upon concrete classes

• No variable should hold a reference to a concrete class.
• No class should derive from a concrete class.
• No method should override an implemented method of any of its based classes.

Least Knowledge (Law of Demeter)

Talk only to your immediate friends.

Only invoke methods that belongs to:

• The object itself
• Objects passed in as a parameter to the method
• Any object the method creates or instantiates
• Any components of the object

The Hollywood

Don’t call us, we’ll call you

Single Responsibility

A class should have only one reason to change

Favor composition over inheritance.

Program to interfaces, not implementation.

Strive for loosely coupled designs between objects that interact.

Classes open for extension, but closed for modification.