kotlin-testing

Kotlin Testing Patterns

JUnit 5 Fundamentals

Use @Test for simple test cases. Prefer @DisplayName for readable test names.

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.DisplayName
import kotlin.test.assertEquals
import kotlin.test.assertFailsWith

class UserServiceTest {

    @Test
    @DisplayName("should return user by ID when user exists")
    fun findUserById() {
        val service = UserService(FakeUserRepository())
        val user = service.findById(1L)
        assertEquals("Alice", user.name)
    }

    @Test
    fun `should throw when user not found`() {
        val service = UserService(FakeUserRepository())
        assertFailsWith<UserNotFoundException> {
            service.findById(999L)
        }
    }
}

Parameterized Tests

Use @ParameterizedTest with @MethodSource for complex inputs or @CsvSource for simple value pairs.

import org.junit.jupiter.params.ParameterizedTest
import org.junit.jupiter.params.provider.CsvSource
import org.junit.jupiter.params.provider.MethodSource
import org.junit.jupiter.params.provider.Arguments
import java.util.stream.Stream

class ValidatorTest {

    @ParameterizedTest