Software Engineering for Students A Programming Approach

276 Chapter 19 ■ Testing
3. apply the test again
4. and so on until the test succeeds.
However, when you fix a bug you might introduce a new bug. Worse, this new bug
may not manifest itself with the current test. The only safe way to proceed is to apply
all the previous tests again. This is termed regression testing. Clearly this is usually a formidable
task. It can be made much easier if all the testing is carried out automatically,
rather than manually. In large developments, it is common to incorporate revised components
and reapply all the tests once a day.
Formal verification
Formal methods employ the precision and power of mathematics in attempting to verify
that a program meets its specification. They place emphasis on the precision of the
specification, which must first be rewritten in a formal mathematical notation. One such
specification language is called Z. Once the formal specification for a program has been
written, there are two alternative approaches:
1. write the program and then verify that it conforms to the specification. This
requires considerable time and skill.
2. derive the program from the specification by means of a series of transformations,
each of which preserve the correctness of the product. This is currently the favored
approach.
Formal verification is very appealing because of its potential for rigorously verifying
a program’s correctness beyond all possible doubt. However, it must be remembered
that these methods are carried out by fallible human beings, who make mistakes. So
they are not a cure-all.
Formal verification is still in its infancy and is not widely used in industry and commerce,
except in a few safety-critical applications. Further discussion of this approach is
beyond the scope of this book.
19.7 ● Unit testing
When we discussed black box and white box testing above, the programs were very
small. However, most software consists of a number of components, each the size of a
small program. How do we test each component? One answer is to create an environment
to test each component in isolation (Figure 19.2). This is termed unit testing. A
driver component makes method calls on the component under test. Any methods that
the component uses are simulated as stubs. These stubs are rudimentary replacements
for missing methods. A stub does one of the following:
■ carries out an easily written simulation of the mission of the component
■ displays a message indicating that the component has been executed
■ nothing.