Type safety

Learn type safety with value classes.

Type safety

The core purpose of a value class is type safety. UserId and OrderId both wrap a String yet they are distinct types. A declaration like fun findUser(id: UserId) makes passing an OrderId by mistake a compile error.

Using the same String type everywhere is error-prone: if a function expects both a user and an order number the compiler cannot tell which one you passed. A value class removes this risk. There is no runtime memory cost; the check happens only at compile time.

Runtime representation

A value class is represented at runtime without boxing. UserId("U1") is stored in memory simply as the String "U1"; no wrapper object is created. This gives the value class zero overhead.

Boxing can occur when a value class is assigned to an Any or added to a generic collection. In these rare cases a value class is still less overhead than a wrapper class. In most uses a value class is as fast as a String but provides type safety.

Exercise Type safety with UserId and OrderId

Define @JvmInline value class UserId(val id: String) and @JvmInline value class OrderId(val id: String). Write fun findUser(id: UserId) = println("User: ${id.id}"). In main call findUser(UserId("U1")).

Show example solution

@JvmInline
value class UserId(val id: String)
@JvmInline
value class OrderId(val id: String)

fun findUser(id: UserId) = println("User: ${id.id}")

fun main() {
    findUser(UserId("U1"))
}

Test yourself

Complete the findUser signature

fun findUser(id: ____) { /* ... */ }

Show answer

fun findUser(id: UserId) { /* ... */ }

Match the type safety concepts

UserId vs String

Distinct types
no boxing

Zero overhead
compile-time

Compile check
runtime

Underlying type
Why use a value class instead of String?
- It runs faster
- It needs less code
- It uses less memory
- It provides type safety
Show answer

It provides type safety — A value class turns type mix-ups into compile errors.

Next lesson: Methods

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