Solution: orThrow
fun <T> T?.orThrow(lazyException: () -> Throwable): T =
this ?: throw lazyException()
Example solution in playground
import org.junit.Before
import org.junit.Test
import kotlin.test.assertEquals
import kotlin.test.assertFalse
fun <T> T?.orThrow(lazyException: () -> Throwable): T =
this ?: throw lazyException()
@Suppress("RedundantNullableReturnType")
class OrThrowTest {
@Test
fun `should throw for null value`() {
val value: String? = null
val exception = RuntimeException("Value is null")
val result = runCatching { value.orThrow { exception } }
assertEquals(exception, result.exceptionOrNull())
}
@Test
fun `should return value for non-null value`() {
val value: String? = "Hello"
val result = value.orThrow { RuntimeException("Value is null") }
assertEquals("Hello", result)
}
private val value: String? = "Hello"
val result = value.orThrow { RuntimeException("Value is null") }
@Test
fun `should specify result type as non-nullable`() {
assertFalse(::result.returnType.isMarkedNullable)
}
}
class UserController(
private val userRepository: UserRepository,
private val logger: Logger,
) {
fun getUser(userId: String) = userRepository
.getUser(userId)
.orThrow { UserNotFoundException(userId) }
.also { logger.log("User with id ${it.id} found") }
.toUserJson()
}
interface UserRepository {
fun getUser(userId: String): User?
}
interface Logger {
fun log(message: String)
}
data class User(
val id: String,
val name: String,
)
data class UserJson(
val id: String,
val name: String,
)
fun User.toUserJson() = UserJson(
id = id,
name = name
)
class UserNotFoundException(userId: String) : Throwable("User $userId not found")
class FakeLogger: Logger {
var messages: List<String> = emptyList()
override fun log(message: String) {
this.messages += message
}
}
class FakeUserRepository() : UserRepository {
var users: List<User> = emptyList()
fun addUser(user: User) {
users += user
}
override fun getUser(userId: String): User? = users
.find { it.id == userId }
}
class UserControllerTest {
private val logger = FakeLogger()
private val userRepository = FakeUserRepository()
private val userController = UserController(userRepository, logger)
@Before
fun cleanup() {
logger.messages = emptyList()
userRepository.users = emptyList()
}
@Test(expected = UserNotFoundException::class)
fun `should throw for non-existing user`() {
userController.getUser("1")
}
@Test
fun `should return user dto for existing user`() {
userRepository.addUser(User("1", "John"))
val result = userController.getUser("1")
assertEquals(UserJson("1", "John"), result)
}
@Test
fun `should log for existing user`() {
userRepository.addUser(User("1", "John"))
userController.getUser("1")
assertEquals(listOf("User with id 1 found"), logger.messages)
}
}
