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INM312 Systems Specification; January Exam Paper; City University; 2012

c) Describe the types of events that can be represented on a state machine and illustrate each one with an example (feel free to draw from your answer to question 3a) above).

**** Ans: Something on the lines of the text below would be acceptable.

 Call events: they simply invoke an operation on the object (synchronous).

 Signal events: information sent between objects in an asynchronous manner.

 Change events: uses the keyword “when” and a Boolean expression consisting of attributes of the

class in context; event is triggered when the condition becomes true.

 Time events: use the keywords “when” or “after”; event is triggered when the timing condition

becomes true.

Any correct example will do.

****[marking criteria: Marks roughly distributed:

 12 marks for descriptions (4 for types of events [1 each] and 8 for description [2 each])

 8 marks for examples (2 each)

[20 marks]

d) Explain what is the role of the actors elements in verification and validation across models. Illustrate

your answer with an example.

**** Ans: Something on the lines of the text below would be acceptable.

Actors are introduced in use cases (they are mentioned in use case descriptions/specifications as well as reflected in the use case diagrams). Actors are also featured on sequence diagrams. Hence during V&V one must ensure that the actors mentioned in a UC spec match those depicted in a UC diag, and also ensure that actors shown on a sequence diagram match those of the UCs.