2 Why We Need Model-Based Testing

Systems with Finite Models 93
actions. Models are made simple by slicing and abstraction, and are often finitized
before analysis. Models are written by coding action methods with enabling conditions.
But other systems often use different vocabulary for the concepts. For example,
an action method might be called an update rule, an enabling condition is a guard or
precondition, the combination of an action method with its enabling conditions is a
guarded update rule, and a model program written in this style is a guarded update
program. Other systems also use different programming languages (or different
coding conventions in the same language) for writing the model programs.
5.9 Exercises
1. Write a simple implementation of the newsreader model program of Section 5.4.
Your implementation need not provide any user interface, just provide an API
with a method for each action. Describe how you implemented the enabling
conditions, and explain your rationale.
2. Write and execute unit tests for your implementation that execute the runs
simulated in Section 5.5. How does your implementation handle the second run,
where the last action is not enabled?
3. Create a version of your newsreader implementation that contains a defect in
(your implementation of) an enabling condition. Write a unit test that exposes
the defect.
4. Create a version of your newsreader implementation that contains a defect in
(your implementation of) an action method. Write a unit test that exposes the
defect.
5. Revise the reactive system model program of Section 5.7 to correct the design
errors. This version should have no unsafe states and no dead states. In other
words, model the implementation you wrote in Chapter 3, Exercise 3. Hint: use
the design rules given in Section 3.6.
Save the programs you wrote for these exercises to use in exercises in chapters
to come.
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