2 Why We Need Model-Based Testing

62 Model Programs
reasonable work assignment. For example, a tester would choose a collection of
features that would be covered by a typical test suite. The chosen subset of features
is sometimes called a slice (of the implementation’s complete feature set).
For this example, we choose a very small subset of newsreader features: the
screens, styles, and options described in Section 5.2. This is an unusually small
collection of features for a model program that we chose to make this example brief.
5.3.3 Choose a level of abstraction
Even when we limit ourselves to a subset of features, we do not consider every
implementation detail in that subset. We must decide which details to include in
the model program; other details can be omitted or greatly simplified. This is called
abstraction; our choice of which details to include defines the level of abstraction.A
lower level shows more detail, and results in a larger, more complex model program.
A higher level shows less detail, and results in a smaller, simpler model program.
Working at a higher level of abstraction is easier because there is less information
to deal with. You should always write your model program at the highest level
of abstraction that achieves your purpose (that can answer the questions you have
formulated). Much of the skill in modeling is understanding what can be left out in
order to achieve the right level of abstraction.
We use three kinds of abstraction. Data abstraction deals with variables. A higher
level of data abstraction uses fewer variables, fewer values for those variables, and
simpler data types. Data abstraction is our primary technique for finitizing models:
we replace a potentially “infinite”collection of values with a (usually small) finite
collection. Behavioral abstraction deals with statements and methods. A higher level
of behavioral abstraction uses fewer statements or methods, where each statement or
method in the model program represents more behavior in the implementation. Environmental
abstraction deals with control structure. A higher level of environmental
abstraction removes control structure, leaving it to the tool or the environment
to choose which methods to execute at run time. In other words, environmental
abstraction replaces control structure with nondeterminism.
In this example we choose a high level of data abstraction where the variables
only represent the kind of page that is displayed and its style. We do not represent
the page contents (the topics and messages). We choose a high level of behavioral
abstraction where each action in the model program represents a large amount of
implementation behavior. Each action represents both the selection of an option that
causes a new page (or a different page style) to appear, and the appearance of that
page.
In every model program, we use the same high level of environmental abstraction.
There is no main method, nor any other control structure that determines the sequence
in which actions occur (methods are called). Instead, the model program has enabling
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