State based testing(state charts) & transition sequences

Question

Hello

I am really stuck with some state based testing concepts...

I am trying to calculate some checking sequences that will cover all transitions from each state and i have the answers but i dont understand them:

Now the answers i have are :

I dont understand it at all. For example say we want to check transition a/x from s1, wouldnt we do ab only? As we are already in s1, we do a/x to test the transition to s2, then b to check we are in the previous right state(s1)? I cant understand why it is aba or even bb for s1...

Can anyone talk me through it?

Thanks

Answer 1

There are 2 events available in each of 4 states, giving 8 transitions, which the author has decided to test in 8 separate test sequences. Each sequence (except the S1 sequences - apparently the machine's initial state is S1) needs to drive the machine to the target state and then perform either event a or event b.

The sequences he has chosen are sufficient, in that each transition is covered. However, they are not unique, and - as you have observed - not minimal.

A more obvious choice would be:

a b ab aa aaa aab ba bb

I don't understand the author's purpose in adding superfluous transitions at the end of each sequence. The system is a Mealy machine - the behaviour of the machine is uniquely determined by the current state and event. There is no memory of the path leading to the current state; therefore the author's extra transitions give no additional coverage and serve only to confuse.

You are also correct that you could cover the all transitions with a shorter set of paths through the graph. However, I would be disinclined to do that. Clarity is more important than optimization for test code.