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Third, you make a little change to your functional code

Next, you run the tests

In the optional fifth step, you refactor your code to remove any duplication.

two simple rules. First, you should write new business code only when an automated test has failed. Second, you should eliminate any duplication that you find.

Your designs must consist of highly cohesive

You write your own tests because you can’t wait

the running code providing feedback between decisions

Test-Driven Development (Addison-Wesley 2003)

a Smalltalk system with a completely test-driven approach that took four years and 40 person-years of effort

there’s far more to testing than unit tests, so you’ll still need other testing techniques such as functional testing, user acceptance testing, system integration testing and so on.

you have a clear measure of success

most programmers don’t read the written documentation for a system; instead, they prefer to dig right into the code.

Your acceptance tests define exactly what your stakeholders expect of your system; therefore, they truly do determine a good portion of your requirements.

Bob Martin says it best: “The act of writing a unit test is more an act of design than of verification. It’s also more an act of documentation than of verification.

Side by Side: Comparing TDD and AMDD TDD shortens the programming feedback loop; AMDD abridges the modeling feedback loop. TDD provides detailed specification (tests), while AMDD provides traditional specifications (agile documents). TDD promotes the development of high-quality code; AMDD encourages high-quality communication between your stakeholders and other developers. TDD provides concrete evidence that your software works, whereas AMDD supports your entire team, including stakeholders, in working toward a common understanding. TDD provides finely grained, concrete feedback in minutes. However, concrete feedback requires developers to follow the practice Prove It With Code, and they may become dependent on non-AM techniques; AMDD lets you get verbal feedback in minutes. TDD ensures that your design is clean by focusing on creation of callable and testable operations; AMDD lets you think through larger design and architectural issues before you code. TDD isn’t visually oriented; AMDD is. Both techniques are new and therefore may be threatening to traditional developers. Both techniques support evolutionary development. —S. W. Ambler