value class

Learn wrappers with @JvmInline value class.

value class

A value class wraps a single value and provides type safety with zero overhead. It is declared with the @JvmInline annotation and the value class keyword. Example: @JvmInline value class UserId(val id: String).

At compile time UserId is checked as a distinct type; at runtime only the inner String is stored, with no wrapper object. This gives type safety without boxing. Using UserId instead of String makes mixing it with an OrderId a compile error.

value class constraints

A value class primary constructor must contain exactly one val parameter. A second field cannot be added; if multiple fields are needed use a data class. The parameter must be marked val.

Example: @JvmInline value class UserId(val id: String) is valid. value class UserId(val id: String, val n: Int) is invalid because it has two fields. Value classes cannot be open, abstract or sealed; inheritance is closed. But they can implement interfaces and contain methods.

Exercise Define the UserId value class

Define @JvmInline value class UserId(val id: String). In main create val u = UserId("U1") and print it with println(u).

Show example solution

@JvmInline
value class UserId(val id: String)

fun main() {
    val u = UserId("U1")
    println(u)
}

Test yourself

Which annotation does a value class require?
- @Value
- @Inline
- @JvmInline
- @Data
Show answer

@JvmInline — @JvmInline is required for a value class.

Complete the value class definition

@JvmInline ____ class UserId(val id: String)

Show answer

@JvmInline value class UserId(val id: String)

Match the value class parts

@JvmInline

Annotation
value class

Wrapper
val id: String

Single field
UserId

Type

Next lesson: Type safety

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