Test doubles explained in Kotlin

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JAKUB ZALAS TEST DOUBLES EXPLAINED by in

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Jakub Zalas @jakub_zalas @jakzal mastodon.social/@jakub_zalas a.k.a. Kuba

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* Quick introduction * Behaviour & Veri fi cation * The chain premise * Collaboration tests * Contract tests PLAN

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xUnit Test Patterns Refactoring Test Code Gerard Meszaros 2007

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https://blog.thecodewhisperer.com/permalink/integrated-tests-are-a-scam Beware the Integrated Tests Scam J. B. Rainsberger / @jbrains

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DATA CALCULATIONS ACTIONS Grokking Simplicity Eric Normand 2021

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“TEST A CHAIN BY TESTING EACH LINK” MICHAEL "GEEPAW" HILL https://www.geepawhill.org/2018/01/16/underplayed-the-chain-premise-in-depth/

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WE GET TO CHOOSE WHEN AND WHERE WE BREAK THE DEPENDENCY CHAIN FOR TESTING

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COLLABORATION TESTS

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xUnit Test Patterns Refactoring Test Code Gerard Meszaros 2007

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INDIRECT INPUT

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STUB

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STUB

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STUB

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STUB A REAL OBJECT, BUT A DUMMY WOULD WORK AS WELL.

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STUB A STUB

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STUB

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STUB

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STUB

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STUB HAND-CRAFTED WITH A MOCKING FRAMEWORK

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STUB

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STUB

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STUB

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INDIRECT OUTPUT

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SPYHAND-CRAFTED

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SPYHAND-CRAFTED

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SPYHAND-CRAFTED

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SPYHAND-CRAFTED

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SPYWITH A MOCKING FRAMEWORK

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MOCK OBJECTWITH A MOCKING FRAMEWORK

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MOCK OBJECTHAND-CRAFTED

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MOCK OBJECTHAND-CRAFTED

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FAKE OBJECT

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FAKE OBJECT

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FAKE OBJECT

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CONTRACT TESTS a.k.a. "Role tests"

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UnusualSpendingSensorTest

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UnusualSpendingSensorTest PaymentRepositoryContractTest PaymentRepositoryContractTest

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SEALING THE CONTRACT

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SEALING THE CONTRACT

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SEALING THE CONTRACT

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SEALING THE CONTRACT

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COMMON PITFALLS * Value mocking * Getters mocking * Unnecessary mocking * Mock orchestration / mimicking the implementation * Mocking disproportionally more complex than the code * Adding expectations on numbers of calls * Mocking 3rd party code

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THANK YOU Verify behaviour, not implementation Mock roles, not objects Listen to your tests, let them guide your design Keep your tests as tidy as the codebase Leverage test doubles as a TDD tool @jakub_zalas @jakub_zalas @jakzal