🎯 Assertion

📖 What is an Assertion?

Assertion is verifying the expected results in tests. With Kotest, you can perform more readable and powerful validations!

💡 Kotest Setup

build.gradle.kts

dependencies {
    testImplementation("io.kotest:kotest-runner-junit5:5.5.0")
    testImplementation("io.kotest:kotest-assertions-core:5.5.0")
}

🎯 Basic Assertions

shouldBe

import io.kotest.matchers.shouldBe
import io.kotest.core.spec.style.StringSpec

class BasicAssertionTest : StringSpec({
    "숫자 비교" {
        val result = 2 + 3
        result shouldBe 5
    }

    "문자열 비교" {
        val name = "Kotlin"
        name shouldBe "Kotlin"
    }

    "불린 검증" {
        val isValid = true
        isValid shouldBe true
    }
})

Various Matchers

import io.kotest.matchers.*
import io.kotest.matchers.string.*

class MatcherTest : StringSpec({
    "다양한 비교" {
        // 동등성
        5 shouldBe 5
        5 shouldNotBe 6

        // 크기 비교
        10 shouldBeGreaterThan 5
        3 shouldBeLessThan 10
        5 shouldBeInRange 1..10

        // 문자열
        "hello" shouldStartWith "he"
        "world" shouldEndWith "ld"
        "kotlin" shouldContain "otl"
        "test" shouldHaveLength 4
    }
})

🎨 Collection Assertions

List Validation

import io.kotest.matchers.collections.*

class CollectionTest : StringSpec({
    "리스트 검증" {
        val numbers = listOf(1, 2, 3, 4, 5)

        // 크기
        numbers shouldHaveSize 5
        numbers.shouldNotBeEmpty()

        // 포함
        numbers shouldContain 3
        numbers shouldContainAll listOf(1, 3, 5)

        // 순서
        numbers.shouldBeSorted()

        // 조건
        numbers.forAll { it shouldBeGreaterThan 0 }
        numbers.forAtLeastOne { it shouldBe 3 }
    }
})

Map Validation

class MapTest : StringSpec({
    "Map 검증" {
        val map = mapOf("name" to "홍길동", "age" to "25")

        // 키/값 존재
        map shouldContainKey "name"
        map shouldContainValue "홍길동"

        // 항목 검증
        map shouldContain ("name" to "홍길동")

        // 크기
        map shouldHaveSize 2
    }
})

🔥 Practical Examples

User Validation

data class User(
    val name: String,
    val email: String,
    val age: Int
)

class UserTest : StringSpec({
    "사용자 정보 검증" {
        val user = User("홍길동", "hong@example.com", 25)

        user.name shouldBe "홍길동"
        user.email shouldContain "@"
        user.age shouldBeInRange 1..100
    }

    "이메일 형식 검증" {
        val user = User("김철수", "kim@test.com", 30)

        user.email shouldStartWith "kim"
        user.email shouldEndWith ".com"
        user.email shouldMatch Regex("\\w+@\\w+\\.\\w+")
    }
})

Order Validation

data class Order(
    val id: String,
    val items: List<Item>,
    val total: Double
)

data class Item(val name: String, val price: Double)

class OrderTest : StringSpec({
    "주문 검증" {
        val order = Order(
            id = "ORDER-001",
            items = listOf(
                Item("사과", 1000.0),
                Item("바나나", 1500.0)
            ),
            total = 2500.0
        )

        // ID 검증
        order.id shouldStartWith "ORDER-"

        // 아이템 검증
        order.items shouldHaveSize 2
        order.items.forAll { it.price shouldBeGreaterThan 0.0 }

        // 합계 검증
        order.total shouldBe 2500.0
    }
})

🛡️ Exception Assertions

Validating Exceptions

import io.kotest.assertions.throwables.*

class ExceptionTest : StringSpec({
    "예외 발생 검증" {
        shouldThrow<IllegalArgumentException> {
            validateAge(-1)
        }
    }

    "예외 메시지 검증" {
        val exception = shouldThrow<IllegalArgumentException> {
            validateAge(-1)
        }

        exception.message shouldBe "나이는 0 이상이어야 합니다"
    }

    "예외 발생 안함" {
        shouldNotThrowAny {
            validateAge(25)
        }
    }
})

fun validateAge(age: Int) {
    require(age >= 0) { "나이는 0 이상이어야 합니다" }
}

🎯 Soft Assertions

Multiple Validations at Once

import io.kotest.assertions.assertSoftly

class SoftAssertionTest : StringSpec({
    "Soft Assertion" {
        val user = User("홍길동", "hong@example.com", 25)

        assertSoftly(user) {
            name shouldBe "홍길동"
            email shouldContain "@"
            age shouldBeInRange 20..30
        }
    }

    "모든 실패를 한번에 보기" {
        assertSoftly {
            1 shouldBe 2  // 실패해도 계속
            "hello" shouldBe "world"  // 이것도 체크
            true shouldBe false  // 이것도 체크
        }
        // 모든 실패를 한번에 보여줌
    }
})

🔧 Custom Matchers

Your Own Validation

import io.kotest.matchers.Matcher
import io.kotest.matchers.MatcherResult
import io.kotest.matchers.should

fun beEven() = Matcher<Int> { value ->
    MatcherResult(
        value % 2 == 0,
        { "$value는 짝수여야 합니다" },
        { "$value는 짝수가 아니어야 합니다" }
    )
}

fun bePositive() = Matcher<Int> { value ->
    MatcherResult(
        value > 0,
        { "$value는 양수여야 합니다" },
        { "$value는 양수가 아니어야 합니다" }
    )
}

class CustomMatcherTest : StringSpec({
    "커스텀 Matcher" {
        4 should beEven()
        10 should bePositive()
    }
})

🎨 Data Class Comparison

Partial Comparison

data class Person(
    val name: String,
    val age: Int,
    val address: String
)

class PersonTest : StringSpec({
    "데이터 클래스 비교" {
        val person = Person("홍길동", 25, "서울")

        // 전체 비교
        person shouldBe Person("홍길동", 25, "서울")

        // 부분 비교
        person.name shouldBe "홍길동"
        person.age shouldBeInRange 20..30
    }
})

🔥 Practical Patterns

Range Validation

class RangeTest : StringSpec({
    "범위 검증" {
        val score = 85

        score shouldBeInRange 0..100
        score shouldBeGreaterThan 60
        score shouldBeLessThan 90
    }

    "날짜 범위" {
        val today = "2024-12-25"

        today shouldBeGreaterThan "2024-01-01"
        today shouldBeLessThan "2025-01-01"
    }
})

Type Validation

import io.kotest.matchers.types.*

class TypeTest : StringSpec({
    "타입 검증" {
        val value: Any = "hello"

        value.shouldBeInstanceOf<String>()
        value shouldBe instanceOf<String>()
    }

    "Null 검증" {
        val nullValue: String? = null
        val nonNull: String? = "test"

        nullValue.shouldBeNull()
        nonNull.shouldNotBeNull()
    }
})

Regex Validation

class RegexTest : StringSpec({
    "정규식 매칭" {
        val email = "hong@example.com"

        email shouldMatch Regex("\\w+@\\w+\\.\\w+")
    }

    "전화번호 검증" {
        val phone = "010-1234-5678"

        phone shouldMatch Regex("\\d{3}-\\d{4}-\\d{4}")
    }
})

🤔 Frequently Asked Questions

Q1. shouldBe vs assertEquals?

A: shouldBe is more readable!

// JUnit
assertEquals(5, result)

// Kotest
result shouldBe 5  // 더 자연스러움

Q2. Customizing failure messages?

A: Use withClue!

import io.kotest.assertions.withClue

class ClueTest : StringSpec({
    "커스텀 메시지" {
        withClue("사용자 나이 검증 실패") {
            25 shouldBeInRange 20..30
        }
    }
})

Q3. Only one of multiple conditions?

A: Use anyOf!

import io.kotest.matchers.or

class AnyOfTest : StringSpec({
    "여러 조건 중 하나" {
        val value = 5

        value shouldBe (3 or 5 or 7)
    }
})

🎬 Conclusion

Reliable validation with powerful assertions!

Key Takeaways:
✅ Concise validation with shouldBe
✅ Utilize various matchers
✅ All at once with Soft Assertions
✅ Write custom matchers
✅ Type/Range/Regex validation

Next Step: Learn about test doubles in Mocking!