2 Why We Need Model-Based Testing

198 Testing Systems with Complex State
model program as arguments. Notice that the ElementRestriction feature has a
void Lookup Start method as opposed to a Lookup method that returns an integer.
This is because the feature does not need to restrict the return values. Using features
to restrict parameter domains is one possible use of features. We discuss more
about the use of features in forthcoming sections. The scenario model produced by
CreateScenario is still infinite because the size of the bag is unbounded. However,
unlike the Contract model program, the model program created by CreateScenario
is explorable because all parameter domains are finite.
Often it is useful to identify a particular set of states of a model program or
a restricted model program as accepting states. A legal run must terminate in an
accepting state. What states are considered as accepting states may depend on what
features are considered. The feature in Figure 12.3 defines that the accepting state
condition for this feature is that the bag is empty; thus, the initial state is the only
accepting state in this case. If no accepting state condition is provided then all
states are considered as accepting states. If multiple features are considered then
the accepting state condition is the conjunction of all the accepting state conditions
present in the included features.
12.2.3 Stepping the implementation
Recall that an IUT is exposed through a stepper (Section 8.3). The stepper “steps”
through the IUT one action at a time. It has a current state that corresponds to the
current state of the IUT. Initially, the stepper’s current state corresponds to the IUT’s
initial state.
When the stepper performs an action, it calls the IUT with the corresponding input
arguments. The inputs encoded in the action are abstract and are not necessarily the
concrete values required by the IUT. However, there must be a well-defined mapping
from abstract input actions to concrete input values. This mapping is an integral part
of the stepper.
Performing an action in the stepper may cause the stepper to return an output
action as a return value. This action either corresponds to the immediate return value
of a method or represents an abstraction of the resulting state of the IUT. There must
be a well-defined mapping from the concrete return values of the IUT to the abstract
actions returned by the stepper.
Steppers are implementations of the modeling library’s IStepper interface.
public interface IStepper
{
Action DoAction(Action action);
void Reset();
}
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