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Insights from Kent Beck's "The Money Example"

A very well known book on Test-Driven Development (TDD) is Kent Beck's Test-Driven Development by Example. I have just read through the first part, The Money Example, and now I am following it by writing the code i C# (most parts translate directly from Java to C#).

One of the big insights I got from following Kent's thoughts is that a lot of effort should be put into the refactoring step. Kent quickly writes a small test followed by a simple implementation to make it pass, in many cases just returning a constant value. Then the big part of the work starts, making the application code generic and clean.

Refactoring the code involves removal of duplication. Here an important insight is that there might be duplication not only of code, but also of data. For example, assume that the test expects the value 10 to be returned by the application code. Hard coding the application code to return 10 is data duplication and must be removed in the refactoring step.

I believe that the insights above are key when practicing Test-Driven Development. You need to carefully refactor the code, both application- and test code in each TDD cycle and remove both code AND data duplication.

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