Question

I’m getting started with UML after years of programming and I want to make sure that I’m properly using the symbols in my diagrams.

Does the following diagram look like a proper representation of a simple Car class?

Update:

Actually, I just realized that Make knows about model so I removed the arrowhead, but Make and Model do not know about Car so I added arrowheads:

Answer 1

I would make some adjustments to your original diagram:

Make

• The Make identifies the manufacturer, so Make will have many Models, and because no other manufacturer can take ownership of those models, the black diamond is shown to denote composition
• The relationship is bidirectionally navigable, so no arrows are shown
• Because we can navigate from Make to Model, there is a SortedSet (sorted by year) of Models shown in Make

Model

• Model holds the only direct relationship to Make; this guarantees the validity of the relationships and avoids incorrect connections that could result from allowing other classes to create relationships with Make
• Because Model may be used to navigate to Make, Model contains a reference to its Make; this ensures that any Car may navigate to its Model information and from there (and only there) navigate to its Make
• For year, the immutable type Integer was chosen, because the value will never change once it has been assigned

Car

• The relationship between Model and Car is only navigable from Car to Model, because the business domain has no need to obtain a set of all Cars that mere manufactured for any given Model (and it keeps things simpler)
• Each car is assigned a VIN, shown as an instance of UUID – Universally Unique Identifier
• The relationship between Car and Engine is only navigable from Car to Engine, so Car has an Engine member; there are use cases where parts of the system that receive a reference to a Car may want to navigate to the Car’s Engine

Engine

• Given that an Engine may start its life in one Car but then be moved into a different Car, the relationship is shown using a white diamond, denoting aggregation
• The Engine is assigned a Part Number, which is a specific identifier for that instance of Engine; therefore the immutable type String is shown
• The Engine maintains a mileage counter, which is defined to be of type: int, because the value is mutable and will change over the lifetime of the Engine

I hope this is helpful and provides some good feedback that makes your modeling exercise thought provoking. I know I had fun creating the diagram and thinking about the relationships and some of the details. Haven’t done this in awhile –