How many unit tests per method? A rule of thumb

The other day, I met with Burke Cox who heads up Stelligent, a company that specializes in helping sites set up their code-quality infrastructure (build systems, test frameworks, code coverage analysis, continuous integration--the whole works, all on a fixed-price basis). One thing Stelligent does before leaving is to impart some of the best practices they've developed over the years.

A best practice Stelligent uses for determining the number of unit tests to write for a given method struck me as completely original. The number of tests is based on the method's cyclomatic complexity (aka McCabe complexity). This complexity metric starts at 1 and adds 1 for every path the code can take. Many tools today generate cyclomatic complexity counts for methods. Stelligent's rule of thumb is:

• complexity 1-3: no test (likely the method is a getter/setter)
• complexity 3-10: 1 test
• complexity 11+: number of tests = complexity / 2

I like this guide, but would change one aspect. I think a cyclomatic complexity of 10 should have more than 1 test. I'd be more inclined to go with: complexity 3-10: # of tests = complexity / 3.

Note: you have to be careful with cyclomatic complexity. I recently wrote a switch statement that had 40 cases. Technically, that's a complexity measure of 40. Obviously, it's pointless to write lots of unit tests for 40 case statements that differ trivially. But, when the complexity number derives directly from the logic of the method, I think Stelligent's rule of thumb (with my modification) is an excellent place to start.

Effectiveness of Pair-wise Tests

The more I use unit testing, the more I wander into areas that are more typically the province of true testers (rather than of developers). One area I frequently visit is the problem of combinatorial testing, which is how to test code where there is a large number of possible values it must handle. Let's say, I have a function with four parameters that are all boolean. There are, therefore, 16 possible combos. My temptation is to write 16 unit tests. But the concept of pairwise testing argues against this test-every-permutation approach. It is based on the belief that most bugs occur between the interaction of pairs of values (rather than a specific configuration of three, or four of them). So, pair-wise experts look at the 16 possible values my switches can have and choose the minimum number in which all pairs of values have been exercised. It turns out there are 5 tests that will exercise every pair of switch combinations.

The question I've wondered about is if I write those five unit tests, rather than the more ambitious 16 tests, what have I given up? The answer is: not much. At the recent Software Test & Performance Conference, I attended a session by BJ Rollison who heads up TestingMentor.com when he's not drilling Microsoft's testers in the latest techniques. He provided some interesting figures from Microsoft's analysis of pair-wise testing.

For attrib.exe (which takes a path + 6 optional args), he did minimal testing, pairwise, and comprehensive testing with the following results:

Minimal: 9 tests, 74% code coverage, 358 code blocks covered.
Pairwise: 13 tests, 77% code coverage, 370 code blocks covered
Maximal: 972 tests, 77% code coverage, 370 code blocks covered

A similar test exploration with findstr.exe (which takes a string + 19 optional args) found that pairwise testing via 136 tests covered 74% of the app, while maximal coverage consisting of 3,533 tests covered 76% of the app.

These numbers make sense. Surely, if you test a key subset of pairs possibilities, testing additional combinations is not likely to exercise completely different routines, so code coverage should not increase much for the tests that exceed pair-wise recommendations. What surprised me was that pairwise got such high-numbers to begin with. 70+ % is pretty decent coverage.

From now on, pair-wise testing will be part of my unit-testing design toolbox. For a list of tools that can find the pairs to test, see here. Rollison highly recommended Microsoft's free PICT tool (see previous link), which also provides a means to specify special relationships between the various factors in the combinations.

Update to my review of Java IDEs

My review of three leading enterprise Java IDEs appeared in later March in InfoWorld. I've received some nice comments on the piece and a few corrections. Most of the corrections come from Sun in my coverage of NetBeans 5.5. Here are the principal items:

• I complained that NetBeans does not use anti-aliased fonts. I overlooked a switch that can turn on these fonts in Java 5. On Java 6, they're on by default, if your system is set with font smoothing on. (It's hard to figure why NetBeans does not default to these fonts on Java 5, as do Eclipse, IntelliJ, and most of the other IDEs.)
  
• I recommended that potential users look at NetBeans 6.0 beta, because it adds many of the features that I complain are missing. Sun gently points out that the current version of 6.0 is not quite in beta yet, but should be in beta within the next few months. For the latest version and roadmap, go to netbeans.org
• After extensive discussions, Sun convinced me that they have greater support for Java 6 than I originally gave them credit for. I originally wrote they provided 'minimal' coverage. In retrospect, I would say 'good' support for Java 6.
  
• Finally, Sun was kind enough to point out that I give them too much credit in saying that they have deployment support for JavaDB. In fact, they provide only start/stop control from within NetBeans.
  

If there are further corrections, I'll update this post.