Eric,
I think the problem could be caused by 'your students' not having a firm
idea as the scope of the system.
You might try drawing a context diagram where everyone contributes and
agrees to the exact boundary to your system.
As for system clock, as an actor, the actor is 'Time' and produces an input
to your system clock.
The printing issue comes about as a result of not having system scope
defined. I.e. Is the Printer inside or outside your system? You decide. If
it's outside, the printer can be an actor, if it's inside then the actor
is/maybe the printed output or the person the output is for, depends very
much on why you're sending it to the printer.
Hope this helps,
Les.
When drawing their first use case diagrams, my students (including some
of my best) sometimes identify the system under development as an actor.
I've been telling them not to do this, since my favorite sources don't
do it, but lately it has been bothering me.
When the system is doing something like scheduling activities for
regular or delayed execution, I have seen authorities use a "system
clock" actor. Also, in some environments it is quite right to say that
the system provides services such as printing. If the system looks like
an actor, walks like an actor, and quacks like an actor, what is it?
-Eric
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