The Art of Software Testing, Second Edition

5.3. Top-down versus Bottom-up Testing

Given the conclusion of the previous section—that incremental testing is superior to nonincremental testing—two incremental strategies are explored: top-down and bottom-up testing. Before discussing them, however, several misconceptions should be clarified. First, the terms "top-down testing," "bottom-down development," and "top-down design" are often used as synonyms. Top-down testing and top-down development are synonyms (they represent a strategy of ordering the coding and testing of modules), but top-down design is something quite different and independent. A program that was designed in top-down fashioncan be incrementally tested in either a top-down or a bottom-up fashion.

Second, bottom-up testing (or bottom-up development) is often mistakenly equated with nonincremental testing. The reason is that bottom-up testing begins in a manner that is identical to a nonincremental test (i.e., when the bottom, or terminal, modules are tested), but as we saw in the previous section, bottom-up testing is an incremental strategy. Finally, since both strategies are incremental, the advantages of incremental testing are not repeated here; only the differences between top-down and bottom-up testing are discussed.

5.3.1. Top-down Testing

The top-down strategy starts with the top, or initial, module in the program. After this, there is no single right procedure for selecting the next module to be incrementally tested; the only rule is that to beeligible ...