Inline Classes (Value Classes) trong Kotlin

1. Giới thiệu

Inline classes (từ Kotlin 1.5+ gọi là value classes) cho phép tạo type-safe wrappers mà không có overhead runtime. Compiler sẽ inline giá trị bên trong, loại bỏ wrapper object.

2. Cú pháp cơ bản


@JvmInline
value class Password(val value: String)
 
@JvmInline
value class Username(val value: String)
 
fun authenticate(username: Username, password: Password) {
    println("Authenticating ${username.value}...")
}
 
fun main() {
    val username = Username("alice")
    val password = Password("secret123")
    
    authenticate(username, password)  // OK
    // authenticate(password, username)  // Compile error!
}

Tại sao dùng Value Classes?


// ❌ Không có type safety
fun sendEmail(to: String, subject: String, body: String) { }
 
// Dễ nhầm thứ tự!
sendEmail("Hello", "alice@example.com", "Message")  // Bug!
 
// ✅ Với value classes
@JvmInline value class Email(val value: String)
@JvmInline value class Subject(val value: String)
@JvmInline value class Body(val value: String)
 
fun sendEmail(to: Email, subject: Subject, body: Body) { }
 
// Compiler catches errors
sendEmail(Email("alice@example.com"), Subject("Hello"), Body("Message"))

3. Các quy tắc


@JvmInline
value class UserId(val id: Long)  // ✅ OK
 
// ❌ Phải có exactly 1 property trong primary constructor
// value class Invalid(val a: Int, val b: Int)
 
// ❌ Không thể có init blocks với side effects
// value class Invalid(val value: Int) {
//     init { println("Created") }  // Not allowed
// }
 
// ✅ Có thể có secondary properties (computed)
@JvmInline
value class Name(val value: String) {
    val length: Int get() = value.length
    val uppercase: String get() = value.uppercase()
}

4. Methods và Properties


@JvmInline
value class Percentage(val value: Double) {
    // Computed properties
    val asDecimal: Double get() = value / 100
    
    // Functions
    fun format(): String = "$value%"
    
    // Validation trong init
    init {
        require(value in 0.0..100.0) { "Percentage must be between 0 and 100" }
    }
}
 
fun main() {
    val discount = Percentage(15.0)
    println(discount.format())      // 15.0%
    println(discount.asDecimal)     // 0.15
    
    // Percentage(150.0)  // Throws IllegalArgumentException
}

5. Implementing Interfaces


interface Printable {
    fun print()
}
 
@JvmInline
value class Message(val text: String) : Printable {
    override fun print() {
        println("Message: $text")
    }
}
 
fun main() {
    val msg = Message("Hello")
    msg.print()  // Message: Hello
    
    val printable: Printable = msg
    printable.print()  // Message: Hello
}

Note: Khi dùng interface, boxing có thể xảy ra.

6. Use Cases thực tế

Domain Primitives


@JvmInline
value class CustomerId(val value: Long) {
    init {
        require(value > 0) { "Customer ID must be positive" }
    }
}
 
@JvmInline
value class OrderId(val value: Long)
 
@JvmInline
value class Money(val cents: Long) {
    val dollars: Double get() = cents / 100.0
    
    operator fun plus(other: Money) = Money(cents + other.cents)
    operator fun minus(other: Money) = Money(cents - other.cents)
    operator fun times(multiplier: Int) = Money(cents * multiplier)
    
    fun format(): String = "$${"%.2f".format(dollars)}"
}
 
fun processOrder(customerId: CustomerId, orderId: OrderId, amount: Money) {
    println("Processing order $orderId for customer $customerId: ${amount.format()}")
}
 
fun main() {
    val customerId = CustomerId(12345)
    val orderId = OrderId(67890)
    val amount = Money(9999)  // $99.99
    
    processOrder(customerId, orderId, amount)
    // processOrder(orderId, customerId, amount)  // Compile error!
    
    val total = amount + Money(500)  // $99.99 + $5.00
    println(total.format())  // $104.99
}

Type-safe IDs


@JvmInline
value class UserId(val value: String)
 
@JvmInline
value class PostId(val value: String)
 
@JvmInline
value class CommentId(val value: String)
 
class PostRepository {
    fun getPost(id: PostId): Post? = TODO()
    fun getComments(postId: PostId): List<Comment> = TODO()
}
 
class UserRepository {
    fun getUser(id: UserId): User? = TODO()
}
 
// Compiler prevents mixing up IDs
fun loadPostWithAuthor(postId: PostId, userId: UserId) {
    val repo = PostRepository()
    // repo.getPost(userId)  // Compile error! Type mismatch
    val post = repo.getPost(postId)  // OK
}

Units of Measurement


@JvmInline
value class Meters(val value: Double) {
    fun toKilometers() = Kilometers(value / 1000)
    fun toFeet() = Feet(value * 3.28084)
}
 
@JvmInline
value class Kilometers(val value: Double) {
    fun toMeters() = Meters(value * 1000)
}
 
@JvmInline
value class Feet(val value: Double) {
    fun toMeters() = Meters(value / 3.28084)
}
 
// Extension functions for nice syntax
val Int.meters get() = Meters(this.toDouble())
val Int.km get() = Kilometers(this.toDouble())
val Double.meters get() = Meters(this)
 
fun main() {
    val distance = 5.km
    println("${distance.value} km = ${distance.toMeters().value} meters")
    
    val height = 100.meters
    println("${height.value} m = ${height.toFeet().value} feet")
}

Duration (before Kotlin Duration)


@JvmInline
value class Milliseconds(val value: Long) {
    val seconds: Long get() = value / 1000
    val minutes: Long get() = value / 60_000
    
    operator fun plus(other: Milliseconds) = Milliseconds(value + other.value)
    operator fun compareTo(other: Milliseconds) = value.compareTo(other.value)
}
 
val Int.ms get() = Milliseconds(this.toLong())
val Int.seconds get() = Milliseconds(this * 1000L)
val Int.minutes get() = Milliseconds(this * 60_000L)
 
fun main() {
    val timeout = 30.seconds
    val delay = 500.ms
    
    println("${timeout.value} ms")  // 30000 ms
    println("${(timeout + delay).seconds} seconds")  // 30 seconds
}

7. Boxing và Performance


@JvmInline
value class Id(val value: Long)
 
fun directUse(id: Id) {
    // No boxing - id được inline thành Long
    println(id.value)
}
 
fun nullableUse(id: Id?) {
    // Boxing occurs - cần object để represent null
    println(id?.value)
}
 
fun genericUse(item: Any) {
    // Boxing occurs - cần object cho Any
    println(item)
}
 
fun main() {
    val id = Id(123)
    
    directUse(id)      // No boxing
    nullableUse(id)    // Boxing
    genericUse(id)     // Boxing
}

8. So sánh với Type Alias


// Type alias - chỉ là tên khác, KHÔNG type-safe
typealias UserId = Long
typealias OrderId = Long
 
fun processWithAlias(userId: UserId, orderId: OrderId) {}
 
// Value class - type-safe, có runtime cost minimal
@JvmInline value class SafeUserId(val value: Long)
@JvmInline value class SafeOrderId(val value: Long)
 
fun processWithValue(userId: SafeUserId, orderId: SafeOrderId) {}
 
fun main() {
    val userId: UserId = 1L
    val orderId: OrderId = 2L
    
    // ❌ Type alias không ngăn lỗi
    processWithAlias(orderId, userId)  // Compiles! Bug!
    
    val safeUserId = SafeUserId(1L)
    val safeOrderId = SafeOrderId(2L)
    
    // ✅ Value class catches errors
    // processWithValue(safeOrderId, safeUserId)  // Compile error!
    processWithValue(safeUserId, safeOrderId)  // OK
}

📝 Tóm tắt

Feature	Description
Syntax	`@JvmInline value class Name(val value: Type)`
Properties	Only 1 primary property, can have computed properties
Methods	Can have methods và implement interfaces
Performance	Inlined at runtime (no boxing in most cases)
Type safety	Prevents mixing up similar types

Khi nào dùng value classes:

Wrapper primitives với meaning khác nhau (IDs, units)
Domain primitives với validation
Type-safe API design
Khi cần type safety mà không muốn overhead

Khi KHÔNG nên dùng:

Khi cần multiple properties
Khi cần inheritance
Khi thường xuyên dùng với generics/nullables (boxing)