There are three possible ends for an association line to specify navigability between two classes:
- no arrow head: navigability is unspecified, and it is unkown if the class knows the other one
- with arrow head: one class knows the other one (navigable)
- cross (X): it is forbidden, difficult or impossible for the class to interact with the other one (non-navigable)
So, 2. and 3. make actual statements about navigability, while 1. just shows an association, but does not make a statement about navigability.
In my own class diagram, I would prefer to be able to show explicit navigability on both ends, because it makes it clear that both classes know each other and can interact with each other, e.g. by calling each other's methods.
If there are no arrow heads on either side of the association, it is possible that both ends are navigable, but it is not explicit.
I have not come across this situation in my case so far, but, typing out this answer, I realized that I haven't found an option for making the ends non-navigable in StarUML, because I can only either check or un-check the navigable checkbox. Is there maybe another way to do this that I've missed?
It does seem like a lot of people just don't put arrow heads on either end, but it would be nice if there was the option to display this explicitly as per UML specification. If I show uni-directional navigability in other parts of the diagram, it is confusing and inconsistent if in some cases association's navigability is simply left unspecified, especially if it's not possible to set explicit non-navigability either.
Maybe this would be something to implement in the future? Just a suggestion, but maybe by default the association lines would have no arrows or crosses (as is now), and in the properties there would be a checkbox for "navigable" (as is now) and another checkbox for "non-navigable" (new). That way, all options would be there, I think that would be pretty cool and helpful, and it would cover all use cases for your customers regarding navigability.
Thanks for the discussion!