Refactoring and Test Fakes: Crafting Resilient Code with Confidence

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Photo by Nubelson Fernandes on Unsplash

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ashdavies.dev

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ashdavies.dev

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Unce!ainty ashdavies.dev

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Coupling ! ashdavies.dev

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Inheritance ashdavies.dev

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Mutability Photo by Richard Horvath on Unsplash

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Unpredictability ashdavies.dev

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? ashdavies.dev

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Extreme Programming ashdavies.dev

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Pair Programming ashdavies.dev

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Test Driven Development ashdavies.dev

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O!ensive Programming < > ashdavies.dev

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ashdavies.dev

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Kotlin: Mutability ashdavies.dev

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Kotlin: Mutability fun sumAbsolute(list: MutableList): Int { for (i in list.indices) list[i] = abs(list[i]) return list.sum() } ashdavies.dev

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Kotlin: Mutability private val GROUNDHOG_DAY = TODO("java.util.Date()") fun startOfSpring(): java.util.Date = GROUNDHOG_DAY val partyDate = startOfSpring() partyDate.month = partyDate.month + 1 // Date is mutable ! ashdavies.dev

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Kotlin: Mutability Shared Mutable State ashdavies.dev

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Kotlin: Immutability Unidirectional Data Flow ashdavies.dev

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Kotlin: Immutability Collections fun List.toMutableList(): MutableList fun Map.toMutableMap(): MutableMap fun Set.toMutableSet(): MutableSet ashdavies.dev

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Kotlin: Immutability org.jetbrains.kotlinx:kotlinx-collections-immutable fun Iterable.toPersistentList(): PersistentList fun Iterable.toPersistentSet(): PersistentSet ashdavies.dev

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Kotlin: Immutability IntelliJ IDEA ashdavies.dev

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Kotlin: Immutability Final Concretions By default, Kotlin classes are !nal – they can't be inherited ashdavies.dev

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Testing Photo by DiEGO MüLLER on Unsplash

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Photos by Daniel Romero, Taylor Vick on Unsplash

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Testing ashdavies.dev

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Anti-Pa!erns and Code Smell ashdavies.dev

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Refactoring ashdavies.dev

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Seams Photo by Jackson David on Unsplash

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Refactoring: Seams internal class FitFilter { private val parser: Parser = Parser.newInstance() } ashdavies.dev

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Refactoring: Seams =================================================================== diff --git a/FitFilter.kt b/FitFilter.kt - internal class FitFilter { - private val parser: Parser = Parser.newInstance() - } - + internal fun interface FitFilter { + fun filter(input: String): String + } + + internal fun FitFilter(parser: Parser) = FitFilter { input -> + parser.parse(input) + } ashdavies.dev

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Refactoring: Seams ashdavies.dev

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Refactoring Dependency Injection ashdavies.dev

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Refactoring Dependency Injection =================================================================== diff --git a/CoffeeMaker.kt b/CoffeeMaker.kt - internal class CoffeeMaker { - private val heater: Heater = ElectricHeater() - private val pump: Pump = Thermosiphon(heater) - } + internal class CoffeeMaker( + private val heater: Heater, + private val pump: Pump, + ) ashdavies.dev

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Refactoring Dependency Injection =================================================================== diff --git a/CoffeeMaker.kt b/CoffeeMaker.kt + internal class CoffeeMaker( + private val heater: Heater, - private val thermosiphon: Thermosiphon, + private val pump: Pump, + ) + + internal interface Pump { + fun pump() + } + - internal class Thermosiphon { + internal class Thermosiphon : Pump { ashdavies.dev

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Testing: Dependencies Dependency Injection internal class CoffeeMaker( private val heater: Heater, private val pump: Pump, ) ashdavies.dev

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Testing: Dependencies Dependency Injection val heater = NuclearFusionHeater() // Expensive! val maker = CoffeeMaker( pump = Thermosiphon(heater), heater = heater, ) assertTrue(maker.brew()) ashdavies.dev

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Testing: Dependencies Dependency Injection val heater = DiskCachedHeater() // Stateful! val maker = CoffeeMaker( pump = Thermosiphon(heater), heater = heater, ) assertTrue(maker.brew()) ashdavies.dev

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Testing: Dependencies Dependency Injection val heater = UnbalancedHeater() // Error prone! val maker = CoffeeMaker( pump = Thermosiphon(heater), heater = heater, ) assertTrue(maker.brew()) ashdavies.dev

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Testing: Dependencies Test Doubles ashdavies.dev

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Test Doubles Mocks ashdavies.dev

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Test Doubles: Mocks val heater = mock() val pump = mock() val maker = CoffeeMaker( heater = heater, pump = pump, ) assertTrue(maker.brew()) ashdavies.dev

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Test Doubles: Mocks val heater = mock() val pump = mock() val maker = CoffeeMaker( heater = heater, pump = pump, ) assertTrue(maker.brew()) // ⚠ Fails! ashdavies.dev

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Test Doubles: Mocks val heater = mock { on { isHeating } doAnswer { true } } val pump = mock { on { pump() } doAnswer { true } } val maker = CoffeeMaker( heater = heater, pump = pump, ) assertTrue(maker.brew()) ashdavies.dev

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Test Doubles: Mocks Footguns ashdavies.dev

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Test Doubles: Mocks Accidental Invocation spy(emptyList()) { on { get(0) } doAnswer { "foo" } // throws IndexOutOfBoundsException } ashdavies.dev

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Test Doubles: Mocks API Sensitivity internal interface Heater { val isHeating: Boolean } val heater = mock { on { isHeating } doAnswer { true } } ashdavies.dev

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Test Doubles: Mocks API Sensitivity internal interface Heater { + fun heat(body: () -> T): T val isHeating: Boolean } val heater = mock { on { isHeating } doAnswer { true } } ashdavies.dev

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Test Doubles: Mocks Data Classes ashdavies.dev

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Test Doubles: Mocks Data Classes Just Don't. ashdavies.dev

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Test Doubles: Mocks Default Answers =================================================================== diff --git a/PumpTest.kt b/PumpTest.kt - val heater: Heater = mock() + val heater: Heater = mock(defaultAnswer = RETURNS_SMART_NULLS) + + val isHeating: Boolean = heater.isHeating // false ashdavies.dev

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Test Doubles: Mocks Pe!ormance Expensive real implementations replaced by expensive mocks. • Runtime code generation • Bytecode manipulation • Re!ection ! ashdavies.dev

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Test Doubles: Mocks Dynamic Mutability internal class CoffeeMakerTest { private lateinit var heater: Heater @Before fun setUp() { heater = mock { on { isHeating } doAnswer { true } } } @Test fun `should brew coffee`() { // heater already has state! } } ashdavies.dev

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Test Doubles: Mocks Dynamic Mutability Framework generated mocks introduce a shared, mutable, dynamic, runtime declaration. ashdavies.dev

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Unpredictability Costs ashdavies.dev

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Unpredictability: Costs Learning Curve ashdavies.dev

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Unpredictability: Costs Peer Review ashdavies.dev

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Unpredictability: Costs Risk of Bugs ashdavies.dev

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Unpredictability: Costs Slowed Feature Delivery ashdavies.dev

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Unpredictability Victims • Junior developers • New team members • Future you ashdavies.dev

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Test Doubles: Mocks Don't Mock Classes You Don't Own testing.googleblog.com/2020/07/testing-on-toilet-dont-mock-types-you.html ashdavies.dev

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You Don't Own Your Code! Your code belongs to your team. Be considerate. ashdavies.dev

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Test Doubles: Mocks Interaction Veri!cation ashdavies.dev

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Test Doubles Mocks ashdavies.dev

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Test Doubles: Stubs ashdavies.dev

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Test Doubles: Stubs Simple internal interface Pump { fun pump(): Boolean } internal object StubPump : Pump { override fun pump(): Boolean = true } ashdavies.dev

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Test Doubles: Stubs Idiomatic internal fun interface Pump { fun pump(): Boolean } val stub = Pump { true } ashdavies.dev

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Testing: Stubs API Sensitive + private const val DEFAULT_AMOUNT = 250 // ml + - internal fun interface Pump { + internal interface Pump { - fun pump(): Boolean + fun pump(amount: Int = DEFAULT_AMOUNT): Boolean + } ashdavies.dev

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Testing: Stubs API Sensitive private const val DEFAULT_AMOUNT = 250 // ml internal interface Pump { fun pump(amount: Int = DEFAULT_AMOUNT): Boolean } val stub = Pump { true } // Compilation failure... ashdavies.dev

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Testing: Fakes ashdavies.dev

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Testing: Fakes public class FakePump(private val onPump: (Boolean) -> Boolean) : Pump { public val pumped = mutableListOf>() override fun pump(full: Boolean): Boolean = onPump(full).also { pumped += full to it } } ashdavies.dev

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Testing: Fakes Additional Behaviour class DelegatingHeater(private val delegate: Heater) : Heater by delegate { private val _drinks = mutableListOf() val drinks: List by ::_drinks override fun heat(body: () -> T): T { return delegate.heat(body).also { _drinks += it } } } ashdavies.dev

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Testing: Fakes Authoring ashdavies.dev

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Testing: Fakes Quali!cations Those who wrote the code are the most uniquely quali!ed to write the tests. ashdavies.dev

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Testing: In Memory internal fun interface CoffeeStore { fun has(type: CoffeeType): Boolean } internal enum class CoffeeType { CAPPUCCINO, ESPRESSO, LATTE, } ashdavies.dev

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Testing: In Memory internal class InMemoryCoffeeStore : CoffeeStore { private val _stock = mutableMapOf() val stock: Map by ::_stock override fun has(type: CoffeeType): Boolean { return (_stock[type] ?: 0) > 0 } fun add(type: CoffeeType, amount: Int = 1) { _stock[type] = (_stock[type] ?: 0) + amount } } ashdavies.dev

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Be Be!er ashdavies.dev

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Inte!ace Segregation No code should be forced to depend on methods it does not use. Image: dribbble.com/shots/3251806-Inte!ace-Segregation-Principle

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Testing: Android Legacy Inheritance ashdavies.dev

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android.content.Context ashdavies.dev

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Recap Extreme Programming ashdavies.dev

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Recap Shared Mutable State ashdavies.dev

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Recap Test Doubles ashdavies.dev

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Thanks! ashdavies.dev

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Fu!her Reading • Ma!in Flower: Mocks Aren't Stubs ma!infowler.com/a!icles/mocksArentStubs.html • Ma!in Fowler: Practical Test Pyramid ma!infowler.com/a!icles/practical-test-pyramid.html • Images: Monkey User monkeyuser.com • Michael Feathers: Working E"ectively with Legacy Code ISBN: 978-0-13117-705-5 • Sam Edwards: Wrapping Mockito Mocks for Reusability handstandsam.com/2020/06/08/wrapping-mockito-mocks-for-reusability • Steve Freeman, Nat Pryce: Growing Object-Oriented So#ware, Guided by Tests ISBN: 978-0-32150-362-6 • Testing on the Toilet: Don't mock Types You Don't Own testing.googleblog.com/2020/07/testing-on-toilet-dont-mock-types-you.html • Testing on the Toilet: Know Your Test Doubles testing.googleblog.com/2013/07/testing-on-toilet-know-your-test-doubles.html • Marcello Galhardo: No Mocks Allowed marcellogalhardo.dev/posts/no-mocks-allowed ashdavies.dev