`@JvmInline value class` wraps a single value with a distinct type at compile time but no runtime overhead — at runtime it's just the underlying primitive/object. Replaces typedef tricks and prevents UserId vs PostId mix-ups.
Kotlin
@JvmInline value class UserId(val raw: Long)
@JvmInline value class PostId(val raw: Long)
@JvmInline value class Email(val raw: String) {
init { require(raw.contains("@")) { "not an email: $raw" } }
val domain: String get() = raw.substringAfter("@")
}
fun deleteUser(id: UserId) { println("deleting user ${id.raw}") }
fun main() {
val u = UserId(42L)
val p = PostId(99L)
deleteUser(u) // ✓
// deleteUser(p) // ✗ compile error — different types
// deleteUser(42L) // ✗ compile error — Long isn't UserId
val e = Email("alice@example.com")
println(e.domain) // example.com
// At runtime, UserId is just a Long — zero memory overhead vs raw Long.
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