Testing Kotlin Code

// src/main/kotlin/Cart.kt
class Cart {
    private val prices = mutableListOf<Double>()

    fun add(price: Double) {
        require(price >= 0) { "Price can't be negative" }
        prices.add(price)
    }

    val count get() = prices.size
    val total get() = prices.sum()
}

// src/test/kotlin/CartTest.kt
import kotlin.test.*

class CartTest {
    private lateinit var cart: Cart

    @BeforeTest
    fun setUp() {
        cart = Cart()
    }

    @Test
    fun `empty cart has zero total`() {
        assertEquals(0.0, cart.total)
    }

    @Test
    fun `total adds every price`() {
        cart.add(2.50)
        cart.add(3.25)
        assertEquals(2, cart.count)
        assertEquals(5.75, cart.total, absoluteTolerance = 0.001)
    }

    @Test
    fun `negative price is rejected`() {
        val e = assertFailsWith<IllegalArgumentException> { cart.add(-1.0) }
        assertEquals("Price can't be negative", e.message)
    }
}

interface UserRepository {
    fun findName(id: Int): String?
}

class Greeter(private val repo: UserRepository) {
    fun greet(id: Int): String = repo.findName(id)?.let { "Welcome back, $it!" } ?: "Welcome, guest!"
}

class FakeUserRepository : UserRepository {
    private val users = mapOf(1 to "Ada", 2 to "Linus")
    override fun findName(id: Int) = users[id]
}

class GreeterTest {
    private val greeter = Greeter(FakeUserRepository())

    @Test
    fun `known user is greeted by name`() {
        assertEquals("Welcome back, Ada!", greeter.greet(1))
    }

    @Test
    fun `unknown user is greeted as guest`() {
        assertEquals("Welcome, guest!", greeter.greet(99))
    }
}

import kotlinx.coroutines.delay
import kotlinx.coroutines.test.runTest
import kotlin.test.*

suspend fun fetchPrice(item: String): Double {
    delay(5_000)
    return if (item == "tea") 2.5 else error("Unknown item: $item")
}

class PriceTest {
    @Test
    fun `tea costs 2 point 5`() = runTest {
        assertEquals(2.5, fetchPrice("tea"))
    }

    @Test
    fun `unknown items fail`() = runTest {
        assertFailsWith<IllegalStateException> { fetchPrice("pizza") }
    }
}