One-Way Association with Multiplicity UML

Question

I am trying to understand this certain case:

 ---------                ---------
|         |0..*      0..*|         |
|CLASS  A |------------->| CLASS B |
|         |              |         |
 ---------                ---------


 ---------                ---------
|         |1..*      0..*|         |
|CLASS  A |------------->| CLASS B |
|         |              |         |
 ---------                ---------

What I understand from the first example is: One Object from Class A has a relationship to zero or more Objects from Class B. That makes sense, because Class A knows Class B. But what exactly does the other multiplicity mean? Class B doesn't know Class A, so it doesn't seem to make sense to put a multiplicity there.

Same goes for the second example.

If someone has a good explanation, I would be very thankfull.

Greetings, john

score 1 · Accepted Answer · edited Jul 19 '20 at 17:36

1

In fact it makes sense as it expresses a constraint on your (data)model.

By setting the multiplicity to [1..*] on the A side, you state that there should always be at least 1 or more instances of A that have a relationship to B.

When writing (or generating) business code this constraint is usually not enforced, but that is only one application for a UML model.

It also serves as functional documentation, where I'm definitely interested in the multiplicities on both sides.

This type of information is also crucial when designing a database. It will determine if the FK field will be nullable or not.

edited Jul 19 '20 at 17:36

chdgaming

15
6

answered May 17 '18 at 16:49

Geert Bellekens

12,788
2
23
50

If I was to write Code for the bottom example, would there be a variable from the type of class A in my class B? – John May 23 '18 at 16:07
You wouldn't **have** to, as the UML only says you should be able to navigate easily from A to B, but it doesn't explicitly forbids it either. – Geert Bellekens May 24 '18 at 07:11

score 0 · Answer 2 · answered May 16 '18 at 17:38

The fact that B does not know about A will not hinder multiple As to connect to B. For example taking B to be an address. And A companies know a number of addresses. But then these companies hand addresses to other companies - and B does not know about that.

score 0 · Answer 3 · answered May 17 '18 at 14:25

It means that an instance of B can be targeted by many instances of A through your association.

But usually I do not set multiplicities for non-navigable roles on uml associations, since it is not used for code generation (there is usually no implementation for this multiplicity information...).

One-Way Association with Multiplicity UML

3 Answers3