Finding the Entities and Their Responsibilities

Finding the Entities and Their Responsibilities — Tech Epitome

Start from the brief, not from a pattern

Take the requirements and underline every noun. That gives you candidates — not a design. Most will survive as types, some will turn out to be fields on other types, and a few will be concepts the brief mentions but the system never needs to represent.

Then, for each survivor, write one sentence: "X is the only thing that knows ___." The sentence is the test. If you cannot finish it, the type has no reason to exist yet. If you need "and" to finish it, you have found two types wearing one name.

Concept

The one-sentence test

A ParkingSpace is the only thing that knows whether it is occupied and what size vehicle it takes. A Garage is the only thing that knows which spaces exist. Neither sentence needs the other, which is why they are two types.

Data without behaviour is suspicious

A type with five fields, five getters, five setters and no other methods is not really a type — it is a record with ceremony. That is fine when it genuinely is a record, like a Ticket that exists to be read. It is a problem when the behaviour that belongs to that data has drifted somewhere else, because now two places have to change together.

The tell is a method on class A that reads three fields from class B and computes something about B. That method usually belongs on B. Moving it is the single highest-value refactor in most first drafts.

Smell	What it usually means
A class with only getters and setters	Its behaviour lives elsewhere
A method that mostly reads another object's fields	Wrong home for the method
A class named ...Manager or ...Helper	A responsibility nobody has named
A class nothing else references	It may not need to exist
Two classes that always change together	They may be one class

First-draft tells, and what to look at.

Which relationships actually exist

Once you have types, the next question is who knows about whom. Three distinctions matter more than the rest, and a lot of confusion disappears when you can name them.

Relationship	Means	If the owner is destroyed
Composition	The part belongs to the whole	The part goes too
Aggregation	The whole holds the part, but does not own it	The part survives
Association	One references the other	Both survive
Dependency	One merely uses the other transiently	Nothing is held

Ownership is the axis that separates them.

A Garage composes its ParkingSpaces: the spaces exist because the garage does. A Playlist aggregates Songs: delete the playlist and the songs remain. A Ticket is associated with a Vehicle: neither owns the other. A Garage depends on a FeePolicy: it calls it and holds nothing permanent.