Testing and Idiomatic Style

Tests and tooling

kotlin.test provides assertions and annotations that map onto JUnit 5 on the JVM and work in multiplatform tests. Function names written in backticks, such as a test called "rejects negative amounts", make test reports readable. MockK is a Kotlin-first mocking library supporting final classes, objects and suspend functions (coEvery, coVerify). kotlinx-coroutines-test provides runTest, which runs suspend code with virtual time so delay does not actually wait. For style, ktlint checks and formats code against the Kotlin coding conventions, and detekt is a static analysis tool for code smells and complexity; both run in Gradle and CI.

A coroutine test with MockK

runTest plus coEvery.

import io.mockk.coEvery
import io.mockk.mockk
import kotlinx.coroutines.test.runTest
import kotlin.test.Test
import kotlin.test.assertEquals

interface PriceApi { suspend fun price(symbol: String): Double }

class Portfolio(private val api: PriceApi) {
    suspend fun value(holdings: Map<String, Int>): Double =
        holdings.entries.sumOf { (symbol, qty) -> api.price(symbol) * qty }
}

class PortfolioTest {
    @Test
    fun `values holdings using current prices`() = runTest {
        val api = mockk<PriceApi>()
        coEvery { api.price("KT") } returns 10.0
        coEvery { api.price("JV") } returns 2.5

        val total = Portfolio(api).value(mapOf("KT" to 3, "JV" to 4))

        assertEquals(40.0, total)
    }
}

Prefer fakes for simple interfaces

A small hand-written fake implementing an interface is often clearer than a mock. Use MockK when verifying interactions or stubbing hard-to-construct dependencies.

त्वरित जाँच: What does `runTest` from kotlinx-coroutines-test provide?

• A mocking DSL for suspend functions
• A linter for coroutine code
• A test coroutine scope with virtual time, so delays are skipped
• A way to run tests on Android devices only