In the movie Amadeus ,decide :public void decide(int a, int b, int c, int d, int e, int f) { if (a > b || c > d || e > f) { DoOneThing(); } else { DoAnother(); } // One-letter variable names are used here only because of limited space. } // You should use better names. Do as I say, not as I do. :-)
2192 tests Too many tests. line coverage Two tests (a > b || c > d || e > f) is true; one where it is false. Not enough tests to detect most bugs decide where a == b , you might find that the sub-expression a > b was incorrect and the code should have been a >= b . And it might make sense to also run tests where a < b and a > b . So that's three tests for a compared to b . For all of the parameters, that would 3 * 3 * 3 = 27 tests. That's probably too many. decide , where the logical sub-expressions have been extracted out:public void decide(int a, int b, int c, int d, int e, int f) { if (tallerThan(a, b) || harderThan(c, d) || heavierThan(e, f)) { DoOneThing(); } else { DoAnother(); } } boolean tallerThan(int a, int b) { return a > b; } // Note “package scope” boolean harderThan(int c, int d) { return c > d; } // rather than public; JUnit boolean heavierThan(int e, int f) { return e > f; } // tests can access these.
decide . One where tallerThan is true. One where harderThan is true. One where heavierThan is true. And one where they are all false. We could test each of the extracted functions with two tests, so the total would be 4 + 2 * 3 = 10 tests just enough tests decreased encapsulation increased testability. Don't write redundant tests, and don't write too few tests. this episode of Testing on the Toilet and post it in your office.
Great example! A suggestion if I may?
ReplyDeleteThe 27 test example doesn't take advantage of the independence provided by OR. One doesn't need to test every combination of pairs, as there is no conditional logic between them. Consider:
A = B, C = D, E = F : DoAnother()
A < B, C < D, E < F : DoAnother()
A > B, C < D, E < F : DoOneThing()
A < B, C > D, E < F : DoOneThing()
A < B, C < D, E > F : DoOneThing()
That's five tests, but it tests equivalence, greater than, and less than for each pair.
This provides the same confidence as the 27 test example, with half the tests of the decreased encapsulation example.
-Ben
It is also interesting to look at the NPath complexity metric for a function or method as it gives you a pretty good estimate of how many tests you need.
ReplyDeleteGood example of equivalence class partioning.
ReplyDelete