Testing Kotlin: JUnit 5 and MockK

kotlin.test Basics

import kotlin.test.*
class DiscountTest {
    @Test fun tenPercentOff() {
        assertEquals(450, discount(500, 10))
        assertTrue(isAdult(21))
        assertFailsWith<IllegalArgumentException> { discount(-5, 10) }
    }
}

JUnit 5 Power

• @ParameterizedTest with @ValueSource/@CsvSource - one test, many rows.
• @Nested inner classes - group by behaviour with readable names.
• assertThrows - exception paths covered.

MockK for Mocks

val repo = mockk<BookRepository>()
every { repo.findBySlug("x") } returns book
verify { repo.findBySlug("x") }

MockK understands Kotlin (object mocks, coEvery for suspend functions) better than plain Mockito.

Testing Coroutines

@Test fun loadsTotal() = runTest {
    assertEquals(998, fetchTotal())
}

runTest executes suspend code deterministically with virtual time - no sleeps, no flakes.

Key Points

• kotlin.test plus JUnit 5 covers units cleanly.
• MockK for Kotlin-idiomatic mocks, coEvery for suspend.
• runTest makes coroutine tests fast and stable.