TDD by Example Chapter 11: The Root of all Evil

Summary

This is a short chapter that completes the refactoring started somewhere around chapter 5. The subclasses, Dollar and Franc, are nothing but constructors. It's time to remove them. Dollar is not used anywhere and thus deleting doesn't cause any test to fail. Franc is used in the test testDifferentClassEquality() which makes sure that Money(10, "CHF") is equal to Franc(10, "CHF"). This test is unnecessary since equality is more than adequately covered in testEquality(). So, testDifferentClassEquality() is deleted and some of the assertions in testEquality() are also deleted since they are not needed to verify equality.

Commentary

This is the easy part after doing the hard work to make it easy. Delete the subclasses, but first the delete the test that knew one of the subclasses by name. He doesn't emphasize this, he doesn't emphasize most of what can be learned from the book, but Beck gives us the experience of how knowing the name of something is a form of coupling. Because the test for equality knew the name of one of the subclasses of the Money we couldn't just delete the subclass even though it was useless.

Beck also spent time cleaning up his tests after he cleaned up his code. We often focus what story the code tells, but maybe just as important is what story the test tell. A question worth asking: Do the tests express the intent of the code being tested?