The TDD cycle looks like this:
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Write a test for the next bit of functionality you have in mind. The test should succeed only when the functionality has been implemented correctly.
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Make the test compile by creating stubs for all the missing classes and methods referenced by the test.
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Run the test. It should fail.
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Implement just enough functionality to get the test to succeed.
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Clean up the implementation as much as possible, typically by removing duplication.
這里最難的是第一點的第二句，"只有正確的程序才能通過測試"。 這幾乎是不可能的，即使可能，所耗的時間也不亞于編寫代碼的時間. 第四點的提法更有問題，程序員在編程實現(xiàn)功能的時候應該把注意力集中在所實現(xiàn)的代碼，而不是測試上。
The Test/Code Cycle in XP
- Write one test.
- Compile the test. It should fail, as you haven't implemented anything yet.
- Implement just enough to compile. (Refactor first if necessary.)
- Run the test and see it fail.
- Implement just enough to make the test pass.
- Run the test and see it pass.
- Refactor for clarity and "once and only once".
- Repeat from the top.