The first step is the testing of Module A. To accomplish this, stub modules representing B, C, and D must be written. Unfortunately, the production of stub modules is often misunderstood; as evidence, you may often see such statements as “a stub module need only write a message stating ‘we got this far,’ ” and “in many cases, the dummy module (stub) simply exits—without doing any work at all.” In most situations, these statements are false. Since module A calls module B, A is expecting B to perform some work; this work most likely is some result (output arguments) returned to A. If the stub simply returns control or writes an error message without returning a meaningful result, module A will fail, not because of an error in A, but because of a failure of the stub to simulate the corresponding module. Moreover, returning a “wired-in” output from a stub module is often insufficient. For instance, consider the task of writing a stub representing a square-root routine, a database table-search routine, an “obtain corresponding master-file record” routine, or the like. If the stub returns a fixed wired-in output, but, not having the particular value expected by the calling module during this invocation, the calling module may fail or produce a confusing result. Hence, the production of stubs is not a trivial task.

Another consideration is the form in which test cases are presented to the program, an important consideration that is not even mentioned in most discussions of top-down testing. In our example, the question is: How do you feed test cases to module A? Since the top module in typical programs neither receives input arguments nor performs input/output operations, the answer is not immediately obvious. The answer is that the test data are fed to the module (module A in this situation) from one or more of its stubs. To illustrate, assume that the functions of B, C, and D are as follows:

A test case for A, then, is a transaction summary returned from stub B. Stub D might contain statements to write its input data to a printer, allowing the results of each test to be examined.

In this program, another problem exists. Since module A presumably calls module B only once, the problem is how you feed more than one test case to A. One solution is to develop multiple versions of stub B, each with a different wired-in set of test data to be returned to A. To execute the test cases, the program is executed multiple times, each time with a different version of stub B. Another alternative is to place test data on external files and have stub B read the test data and return them to A. In either case, and because of the previous discussion, you should see that the development of stub modules is more difficult than it is often made out to be. Furthermore, it often is necessary, because of the characteristics of the program, to represent a test case across multiple stubs beneath the module under test (i.e., where the module receives data to be acted upon by calling multiple modules).

After A has been tested, an actual module replaces one of the stubs, and the stubs required by that module are added. For instance, Figure 3 might represent the next version of the program.

After testing the top module, numerous sequences are possible. For instance, if we are performing all the testing sequences, four examples of the many possible sequences of modules are

1. A B C D E F G H I J K L
2. A B E F J C G K D H L I
3. A D H I K L C G B F J E
4. A B F J D I E C G K H L

If parallel testing occurs, other alternatives are possible. For instance, after module A has been tested, one programmer could take module A and test the combination A-B, another programmer could test A-C, and a third could test A-D. In general, there is no best sequence, but here are two guidelines to consider:

1. If there are critical sections of the program (perhaps module G), design the sequence such that these sections are added as early as possible. A “critical section” might be a complex module, a module with a new algorithm, or a module suspected to be error prone.
2. Design the sequence such that the I/O modules are added as early as possible.