test-doubles.md
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-Page references to xUnit Test Patterns (XTD), especially Chapter 11 ("Test Doubles") and Practical Object-Oriented Design in Ruby (POODR), especially Chapter 9 ("Designing Cost-Effective Tests").
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+Page references to xUnit Test Patterns (XTD), especially Chapter 11 ("Test Doubles"), "Four-Phase Test" (pp. 358-361) and Practical Object-Oriented Design in Ruby (POODR), especially Chapter 9 ("Designing Cost-Effective Tests").
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+Other resources:
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+* <https://robots.thoughtbot.com/four-phase-test>
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### The thing we are testing (the SUT)
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