Association with multiplicity vs composition

[pengnam]

What happens if class A has a list of class B objects as an attribute?
Should we represent it as a composition? Is there any clear way to decide or should we use our intuition?

[pengnam]

L6 says that there is no definite way to identify composition, and if in doubt leave it out.

[damithc]

Composition is a whole-part relationship - identified based on your understanding of how two classes are related, not merely based on how the relationship is implemented in code. Just because A has a variable of type B doesn't mean it is composition. The question you should ask is, is there a whole-part relationship between the classes A and B, based on what concepts A and B represent in the application domain. For example, in some applications Car-Engine can be a whole-part relationship while in other applications it may not be (in a car dealership app vs spare parts sales app)

[aidenywl]

@pengnam From my understanding a good question to ask whether a relationship should be represented as composition is to ask: will class B objects be deleted if class A is deleted?

Your question made me think of an analogy that might help regarding class A having a list of class B objects as an attribute.

Scenario 1: modelling a simple book
A: Book
B: Page

In this scenario, Book objects have a list of Page objects. Logically, if the book is deleted, all the pages are deleted. This is a composition relationship with a multiplicity of *.

Scenario 2: modelling a class-student relationship in the academic setting.
A: Class
B: Student

In this scenario, a Class objects has a list of Student objects. In most contexts, if the Class comes to a close, expires, or closes, the students still exist. This would be modelled as a simple association with multiplicity of *, or aggregation with multiplicity of *.

Basically, relationships where B cannot exist independently without A are modelled with composition.
Relationships where B can exist independently with A are modelled with association (or aggregation in certain cases).

Multiplicity provides another dimension of information to the above.

[damithc]

Basically, relationships where B cannot exist independently without A are modelled with composition.
Relationships where B can exist independently with A are modelled with association (or aggregation in certain cases).

✔️

However, B cannot exist independently without A should be interpreted based on what each class represents, not how they are implemented. There can be situations where deleting A results in B getting deleted but that is not necessarily a composition.
In other words

1. A♦ ----B means B gets deleted when A is deleted.
2. B gets deleted when A is deleted does not necessarily mean A♦ ----B