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umlclass-diagram

How to display uml case diagram connectivity correctly

I am trying to create a system for managing vaccines against covid.

The system supports 3 different vaccines but each citizen can only get one and the system has to differentiate between the citizens who are older than 65, the AstraZeneca vaccine cannot be given to people older than that age.

Below I tried to create a basic UML class diagram. However I'm pretty sure I'm missing something since the vaccine should be also connected to the AstraZeneca class?

class diagram

Solution

  • The diagram is confusing, since it only shows associations, but regrouping them in an unexpected manner. It looks more like a decision tree than a real class diagram.

    First improvements you need to consider:

    • Pfizer BioNTech, Moderna and AstraZeneca are each a Vaccine: you should show this with a generalization from the specific vaccine to the general vaccine.

    • age 65+ seems not a good candidate for a class: in most OO languanges an object of a class keeps the class during its whole life. But citizen do not change class at 65. Age is a (derived) property of Citizen. The wording "astrazeneca vaccine cannot be given to people older than 65" moreover is an expression of a constraint.

    • Finally, if you manage vaccines, you need to manage also shots. When you write "citizen can only get one" you probably mean "one kind": the vaccines that you mention do in principle require 2 shots. And in most countries around the world, the two shots have to be of the same vaccine, which is another contraint. The remaining question is then if 65+ constraint applies to the first shot or the second?

    This would lead us to a diagram that looks as follows:

    enter image description here

    Additional thoughts:

    • You could manage the shots by making the association Vaccination an association class.

    • There is an issue in the regarding the open/closed principle: if you'd add new vaccines, you might have to add different constraints on some. Alternatives:

      • Make Vaccine an abstract class (or an interface), with some more operations that need to be implemented by the concrete classes: getRequiredMinAge(), getRecommendedMinAge(), getRecommendedMaxAge(), getrequiredMaxAge(), instead of hard-coding the constraint.
      • Use a method Vaccine::checkCompatibility(c: Citizen) transfering the constraint verification to the Vaccine class

    • One could wonder if subclassing the vaccines is really required.