Coding Pattern: Classes and Objects

Classes

• Classes: How do you decide what a class is and what it is not?
  • What is the domain or the problem space or the problem domain?
  • Look for Nouns!
  • Remember SRP: Single Responsibility Principle (see Single Responsibility Principle)
  • Ask yourself: “What does this class represent/do?”

Objects

• Classes and objects are different!
• Instance variables
• Constructor (no destructor in Ruby!)
• Methods: private/public

Polymorphism

• When the same interface can be used with objects of different classes
  • Example: In ruby, message #to_s can be sent to objects of different classes
  • Why is it useful?
• Ways to achieve this:
  • Inheritance
  • “Duck” Typing
• “Duck” typing.
  • Don’t rely on the class/type of a thing to know what it can do
  • Reverse it: based on what it can do, you can tell the type of a thing.
  • How is duck typing is an alternative/complement to inheritence?
  • And what about Java interfaces?
  • Consider a method like #talk in a gaming application

Inheritance and Delegation

• This is pretty subtle!
• Inheritence: rarely is this the best solution
• Consider Composition and Delegation: often a better solution
• Look at this sample: Ruby Inheritance Demo Code