2 Why We Need Model-Based Testing

202 Testing Systems with Complex State
• The IsInAcceptingState property returns true if the current model state is an
accepting state. A test run is allowed to finish only in an accepting state.
• The Reset action restores the model to its initial state. After reset CurrentState
is the initial state of the model. This does not mean that the full tester state is
restored to the initial state. Typically, the strategy keeps history of prior test runs
that affects what action is selected next.
• The IsActionEnabled method returns true if the given action is enabled in the
current model state. It returns false otherwise, and outputs a reason why the
action is not enabled.
• The SelectAction method returns an action that is enabled in the current model
state and whose action symbol is in the given set of action symbols. The given
set of action symbols may be a proper subset of ActionSymbols. The method
returns null if no action can be selected.
• The DoAction method executes the given action in the model so that CurrentState
gets the target state of this transition. A precondition of this method is that the
given action is enabled in the current state. If the action is not enabled, the
behavior of the model program is unpredictable.
The strategy is constructed for a given model program and holds a reference to it.
The strategy stores the current state of the model and it knows, by using the model
program, which actions are currently enabled, that is, which actions are enabled in
the current state. The strategy can use additional state to record history, for example,
previously taken actions, in order to implement a particular algorithm for selecting
the next action to be executed.
12.3.1 Default strategy
A default or basic strategy for a given model program is provided by the Strategy
class. The basic strategy has no additional tester state, it implements a so-called
memoryless action selection strategy; a call to SelectAction computes, in the current
state, the set of all enabled actions with the given action symbols and returns a
randomly chosen action from among those, or null if the set is empty.
We can use the conformance tester ct to run the restricted bag model program
against the bag stepper. As it happens, the very first test fails with a trace consisting
of a single action Lookup Start(null) with a failure reason that null cannot be a
key in the dictionary that is used to implement the bag. We can fix that bug easily
by counting null values separately and not trying to insert null or to look it up in
the dictionary. For now let us assume that the set of parameters is restricted to ""
and "b"; that is, we change the definition of E in Figure 12.3 to
readonly static Set E = new Set("","b");
more free ebooks download links at:
http://www.ebook-x.com