Class Diagram: Default value for unmentioned multiplicity

[qweiping31415]

I have a question regarding unmentioned multiplicities in class diagrams. From the sample paper, I assumed that unmentioned multiplicities meant that it holds a default value to 1, but I’m not sure whether that assumption affects my answer for the following questions. Could I clarify whether my assumption is true in the module and whether it affects the explanation of my answers?

A13. A Lid must be associated to a Container but it is optional for a Container to have a Lid.
The stated answer is (B). This is because the association label could be a Lid#close(Container) method, hence this is a dependency from Lid to Container instead of an association. This possibility also counters the given code scenario in A17.

[damithc]

must be associated means there should be a 1 multiplicity in the diagram, which the diagram doesn't have. Hence, B.
Unmentioned does not default to 1.

[qweiping31415]

Does unmentioned default to 0..1? I missed out the possibility of null value.

If not, what does it default to, if any?

[damithc]

It does not default to anything. It is unknown.

[jxofficial]

Am I correct in saying this:

If the arrow is like this Container → Lid it means that only Container knows about Lid. ie Container is associated with Lid. But Lid is NOT associated with container
class Container { Lid lid; }

But Lid definitely does not know about Container right? ie Lid is NOT associated with Container Because if it does, then it would be: Container — Lid without the arrow head (navigability)

[damithc]

Am I correct in saying this:

If the arrow is like this Container → Lid it means that only Container knows about Lid. ie Container is associated with Lid. But Lid is NOT associated with container
class Container { Lid lid; }

But Lid definitely does not know about Container right? ie Lid is NOT associated with Container Because if it does, then it would be: Container — Lid without the arrow head (navigability)

Don't confuse navigability with association. If A is associated with B, B is automatically associated with A, irrespective of the navigability.

• A → B
  • The A class is associated with the B class
  • The B class is associated with the A class
  • An A object has a reference to the B object it is associated with
  • A B object does not have a reference to the A object it is associated with

[jxofficial]

Thank you, that clears things up!