kotlin-testing

# Kotlin Testing Patterns

Comprehensive Kotlin testing patterns for writing reliable, maintainable tests following TDD methodology with Kotest and MockK.

## When to Use

- Writing new Kotlin functions or classes
- Adding test coverage to existing Kotlin code
- Implementing property-based tests
- Following TDD workflow in Kotlin projects
- Configuring Kover for code coverage

## How It Works

1. **Identify target code** — Find the function, class, or module to test
2. **Write a Kotest spec** — Choose a spec style (StringSpec, FunSpec, BehaviorSpec) matching the test scope
3. **Mock dependencies** — Use MockK to isolate the unit under test
4. **Run tests (RED)** — Verify the test fails with the expected error
5. **Implement code (GREEN)** — Write minimal code to pass the test
6. **Refactor** — Improve the implementation while keeping tests green
7. **Check coverage** — Run `./gradlew koverHtmlReport` and verify 80%+ coverage

## Examples

The following sections contain detailed, runnable examples for each testing pattern:

### Quick Reference

- **Kotest specs** — StringSpec, FunSpec, BehaviorSpec, DescribeSpec examples in [Kotest Spec Styles](#kotest-spec-styles)
- **Mocking** — MockK setup, coroutine mocking, argument capture in [MockK](#mockk)
- **TDD walkthrough** — Full RED/GREEN/REFACTOR cycle with EmailValidator in [TDD Workflow for Kotlin](#tdd-workflow-for-kotlin)
- **Coverage** — Kover configuration and commands in [Kover Coverage](#kover-coverage)
- **Ktor testing** — testApplication setup in [Ktor testApplication Testing](#ktor-testapplication-testing)

### TDD Workflow for Kotlin

#### The RED-GREEN-REFACTOR Cycle

```
RED     -> Write a failing test first
GREEN   -> Write minimal code to pass the test
REFACTOR -> Improve code while keeping tests green
REPEAT  -> Continue with next requirement
```

#### Step-by-Step TDD in Kotlin

```kotlin
// Step 1: Define the interface/signature
// EmailValidator.kt
package com.example.validator

fun validateEmail(email: String): Result<String> {
    TODO("not implemented")
}

// Step 2: Write failing test (RED)
// EmailValidatorTest.kt
package com.example.validator

import io.kotest.core.spec.style.StringSpec
import io.kotest.matchers.result.shouldBeFailure
import io.kotest.matchers.result.shouldBeSuccess

class EmailValidatorTest : StringSpec({
    "valid email returns success" {
        validateEmail("user@example.com").shouldBeSuccess("user@example.com")
    }

    "empty email returns failure" {
        validateEmail("").shouldBeFailure()
    }

    "email without @ returns failure" {
        validateEmail("userexample.com").shouldBeFailure()
    }
})

// Step 3: Run tests - verify FAIL
// $ ./gradlew test
// EmailValidatorTest > valid email returns success FAILED
//   kotlin.NotImplementedError: An operation is not implemented

// Step 4: Implement minimal code (GREEN)
fun validateEmail(email: String): Result<String> {
    if (email.isBlank()) return Result.failure(IllegalArgumentException("Email cannot be blank"))
    if ('@' !in email) return Result.failure(IllegalArgumentException("Email must contain @"))
    val regex = Regex("^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$")
    if (!regex.matches(email)) return Result.failure(IllegalArgumentException("Invalid email format"))
    return Result.success(email)
}

// Step 5: Run tests - verify PASS
// $ ./gradlew test
// EmailValidatorTest > valid email returns success PASSED
// EmailValidatorTest > empty email returns failure PASSED
// EmailValidatorTest > email without @ returns failure PASSED

// Step 6: Refactor if needed, verify tests still pass
```

### Kotest Spec Styles

#### StringSpec (Simplest)

```kotlin
class CalculatorTest : StringSpec({
    "add two positive numbers" {
        Calculator.add(2, 3) shouldBe 5
    }

    "add negative numbers" {
        Calculator.add(-1, -2) shouldBe -3
    }

    "add zero" {
        Calculator.add(0, 5) shouldBe 5
    }
})
```

#### FunSpec (JUnit-like)

```kotlin
class UserServiceTest : FunSpec({
    val repository = mockk<UserRepository>()
    val service = UserService(repository)

    test("getUser returns user when found") {
        val expected = User(id = "1", name = "Alice")
        coEvery { repository.findById("1") } returns expected

        val result = service.getUser("1")

        result shouldBe expected
    }

    test("getUser throws when not found") {
        coEvery { repository.findById("999") } returns null

        shouldThrow<UserNotFoundException> {
            service.getUser("999")
        }
    }
})
```

#### BehaviorSpec (BDD Style)

```kotlin
class OrderServiceTest : BehaviorSpec({
    val repository = mockk<OrderRepository>()
    val paymentService = mockk<PaymentService>()
    val service = OrderService(repository, paymentService)

    Given("a valid order request") {
        val request = CreateOrderRequest(
            userId = "user-1",
            items = listOf(OrderItem("product-1", quantity = 2)),
        )

        When("the order is placed") {
            coEvery { paymentService.charge(any()) } returns PaymentResult.Success
            coEvery { repository.save(any()) } answers { firstArg() }

            val result = service.placeOrder(request)

            Then("it should return a confirmed order") {
                result.status shouldBe OrderStatus.CONFIRMED
            }

            Then("it should charge payment") {
                coVerify(exactly = 1) { paymentService.charge(any()) }
            }
        }

        When("payment fails") {
            coEvery { paymentService.charge(any()) } returns PaymentResult.Declined

            Then("it should throw PaymentException") {
                shouldThrow<PaymentException> {
                    service.placeOrder(request)
                }
            }
        }
    }
})
```

#### DescribeSpec (RSpec Style)

```kotlin
class UserValidatorTest : DescribeSpec({
    describe("validateUser") {
        val validator = UserValidator()