With a name like TEST-driven development you’d expect TDD is mostly about testing. Especially when the end result is tests. In our book Design Patterns Explained we discussed how testability (how easily code can be tested) is an intrinsic property of software and is highly correlated with good design. This is why the 1st mantra of design patterns is to design to the behavior of the objects being written. These behaviors being described as tests.
One of the characteristics of good design is that it is modifiable with less effort than poor design. So testing first give us both better design of the code & increase its sustainability-enabling emergence. This is the big difference between test-FIRST and unit testing.
But we now have the added burden of the tests which often add to maintenance costs. The cause of this is insufficient focus on the quality of the tests. An ideal test will validate only one behavior. The intent is to decouple the tests so that when one behavior changes only one test needs to change. This requires attending to the coupling of tests as future enhancements are made. Coupling of tests inform coupling in design.