Object-oriented design

Object-oriented design models responsibilities and collaborations through object interfaces and behaviour.

Subtopics

Interface

“The public interface is a contract that articulates the responsibilities of your class.” (Sandi Metz, Practical Object-Oriented Design in Ruby - Sandi Metz)

Model behaviour, not state

When modelling objects, our initial instinct is often to model state. A first draft for modelling a police officer might look like this:

wears a uniform
carries a gun
arrests people
enforces the law

Now, how might a model for an actor playing a police officer look like?

wears a uniform
carries a gun
acts out arresting someone

This approach to modelling leads to a design like this:

classDiagram
	Real <|-- PoliceOfficer
	Actor <|-- PoliceOfficer

	class PoliceOfficer{
		String name
		String uniformColour
		String gunModel
		canArrest()
	}
	class Real{
		String precinct	
	}
	class Actor{
		String playName
		String roleName
	}

However, if we look at the behaviours and relationships of these two classes, we’ll realize that they’re completely different. A police officer has several duties, is assigned to a precinct and so on. An actor acts according to a script, has different roles at different times etc.

The following model reflects this better:

classDiagram
	PoliceOfficer <|-- Profession 
	Actor <|-- Profession
	Agent <|-- Profession
    StageActor <|-- Actor
    TVActor <|-- Actor
    
	class Person{
		Profession profession
	}
	class Profession{
		String industry
	}
	class PoliceOfficer{
		String uniformColour
		String gunModel
		arrest()
	}
	class Actor{
		Person agent 
		Roles[] roles
		sign()
	}
	class Agent{
		Actor[] actors
		sign()
	}

Bookmarks

Elizabeth Gibson, “Objects — Born and Bred”, BYTE Magazine October 1990

Questions