Bottom-up Testing

The next step is to examine the bottom-up incremental testing strategy. For the most part, bottom-up testing is the opposite of top-down testing; the advantages of top-down testing become the disadvantages of bottom-up testing, and the disadvantages of topdown testing become the advantages of bottom-up testing. Because of this, the discussion of bottom-up testing is shorter.

Returning to Figure 2 at page 4, the first step is to test some or all of modules E, J, G, K, L, and I, either serially or in parallel. To do so, each module needs a special driver module: a module that contains wiredin test inputs, calls the module being tested, and displays the outputs (or compares the actual outputs with the expected outputs). Unlike the situation with stubs, multiple versions of a driver are not needed, since the driver module can iteratively call the module being tested. In most cases, driver modules are easier to produce than stub modules.

As was the case earlier, a factor influencing the sequence of testing is the critical nature of the modules. If we decide that modules D and F are most critical, an intermediate state of the bottom-up incremental test might be that of Figure 4. The next steps might be to test E and then test B, combining B with the previously tested modules E, F, and J.

The problems associated with the impossibility or difficulty of creating all test situations in the top-down approach do not exist here. If you think of a driver module as a test probe, the probe is being placed directly on the module being tested; there are no intervening modules to worry about. Examining other problems associated with the top-down approach, you can’t make the unwise decision to overlap design and testing, since the bottom-up test cannot begin until the bottom of the program has been designed. Also, the problem of not completing the test of a module before starting another, because of the difficulty of encoding test data in versions of a stub, does not exist when using bottom-up testing.