2 Why We Need Model-Based Testing

194 Testing Systems with Complex State
12.2 Implementation, model and stepper
For the example in this chapter we use an implementation of a bag, shown in
Figure 12.1, as a sample IUT throughout this section to illustrate various techniques
that can be used for on-the-fly testing. This example is small and easy to understand
while it naturally leads to a model program with complex state and infinite behavior.
It helps to illustrate on a smaller scale many of the properties of more complex
model programs that use state variables with unbounded data types.
12.2.1 Implementation
12.2.2 Model
A bag is an unordered collection of elements. An element can be added to the bag,
deleted from the bag, and looked up in the bag. The multiplicity of an element is the
number of times it occurs in the bag. Looking up an element in the bag returns the
multiplicity of that element. An element is not in the bag if and only if its multiplicity
is zero. You can also count the total number of elements in the bag. Unlike the bag
data type in the modeling library, the bag implementation is not a value type.
The bag model has an action vocabulary that corresponds to the methods of the bag
implementation. The model program is shown in Figure 12.2. The following action
trace is an example of a correct behavior of the bag:
Add("elem1")
Add("elem1")
Count_Start()
Count_Finish(2)
Lookup_Start("elem1")
Lookup_Finish(2)
Lookup_Start("elem2")
Lookup_Finish(0)
Delete("elem1")
Lookup_Start("elem1")
Lookup_Finish(1)
The content variable is the only state variable of the model program. In the
initial state, the value of content is the empty bag. The bag model program defines
an infinite state machine because the input parameter domains of the actions are
unbounded and the size of the bag is unbounded. One way to constrain the set of
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