UML: Object Diagram

Question

I am beginner in UML. The following is a UML object-model diagram that shows a single object called Timer, which has attributes minutes and seconds of type integer, as well as public operations tick() and reset().

The correspondig C Structure is:

struct Timer_t {
   int mins;    /*## attribute mins */
   int secs;    /*## attribute secs */
};

/* Operations */
/*## operation reset() */
void Timer_reset();
/*## operation tick() */
void Timer_tick();

With respect to this diagram I would like to understand the physical interpretation of the following:

1- 1 at the top left (Is it no. of instances that will be instantiated during the lifetime of the application?)

2- <<Singleton>> (If it is stereotype to highlight that the object is an instance of a singleton class, then why do we also mention 1 at the top left?)

source : UML for C Programmers

Answer 1

First, the above is not an object(/instance) but a class. An object does not show attributes/methods in compartments and the name would be underlined.

Second, the <<Singleton>> stereotype is placed wrongly. It needs to appear under the name of the class.

Third, the 1 top left signaling multiplicity needs to be enclosed in brackets like {1}. Not sure about the position, I just know right below the class name.

Now regarding your question. The multiplicity tells that there must be only one instance of this class in a system. The stereotype <<Singleton>> doesn't tell anything else (it's a tautology). However, the name Singleton is well known and you can find implementation patterns for any language via Google - which will fail for the key word 1;-)