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1. You are not allowed to write any production code unless it is to make a failing unit test pass.
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2. You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
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3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.
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UncleBob：http://butunclebob.com/ArticleS.UncleBob.TheThreeRulesOfTdd
中文翻译：http://blog.csdn.net/hudon/article/details/1123700
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