Mocking with MockK

Mock a boundary, not the calculation under test

MockK can supply controlled behavior for a dependency and verify important interactions. Add a compatible MockK JVM test dependency in the learning project; keep the production code independent from the mocking library.

Copy codeimport io.mockk.*
import kotlin.test.*

interface Titles { fun find(id: String): String? }
class Labels(private val titles: Titles) {
    fun label(id: String) = titles.find(id) ?: "Unknown"
}

class LabelsTest {
    @Test fun usesFoundTitle() {
        val titles = mockk<Titles>()
        every { titles.find("k1") } returns "Kotlin"
        assertEquals("Kotlin", Labels(titles).label("k1"))
        verify(exactly = 1) { titles.find("k1") }
    }
}

Use coEvery and coVerify for suspending members. Avoid verifying every incidental call order; doing so can make harmless refactors break tests. A small fake is often clearer for a stateful repository.

Exercise

Add a missing-title test and compare the mock with a map-backed fake.

Check: the real Labels implementation must run; mocking the class under test would only verify your stubbing.

Reference: MockK