kotlin-types-value-class

name	kotlin-types-value-class
description	Use when writing or reviewing Kotlin type declarations to choose @JvmInline value class over data class where appropriate, including Compose stability implications. Technique-layer skill — complements the codebase-specific kotlin-expert.

Kotlin value class vs data class

Core principle

Prefer @JvmInline value class for single-field types that carry domain meaning. Data classes are for aggregating multiple fields. A value class gives you type safety (you can't mix up UserId and String) without the allocation overhead of a data class.

When to use this skill

• Writing a new Kotlin type that wraps a single value
• Reviewing a data class that has only one property
• Seeing primitive types (String, Long, Int, etc.) used where a domain type would prevent misuse
• Compose compiler reports showing unstable parameters that could be value classes

Decision flow

Situation	Prefer
Single field + domain-meaningful (UserId, EmailAddress, Percentage)	@JvmInline value class
Single field + no domain meaning (just grouping)	Type alias or keep the primitive
Multiple fields	Data class
Needs custom equals/hashCode/toString beyond the wrapped value	Data class (value classes delegate to the underlying type)
Used as a generic type argument or nullable in hot paths	Data class or primitive (autoboxing cost)

// GOOD: domain-meaningful single field
@JvmInline value class UserId(val value: String)
@JvmInline value class EmailAddress(val value: String)
@JvmInline value class Percentage(val value: Float)

// BAD: data class wrapping a single field
data class UserId(val value: String) // unnecessary allocation
data class EmailAddress(val value: String) // type safety without the overhead is available

// BAD: value class with no domain meaning
@JvmInline value class Wrapper(val value: String) // just use the String, or a type alias

// BAD: value class needing custom equality
@JvmInline value class CaseInsensitiveString(val value: String)
// value class equals delegates to String equals, which IS case-sensitive
// Use a data class if you need different equality semantics

Compose stability

@JvmInline value class is treated as Stable by the Compose compiler when its underlying type is stable (primitives, String, and other stable types). This means:

• Value classes passed as composable parameters avoid "unstable parameter" warnings
• No need for @Immutable annotations at Compose boundaries when wrapping primitives or strings
• Replacing single-field data classes with value classes at UI boundaries improves skippability

// Before: data class wrapping a single field
data class UiState(val userId: String) // works, but allocates a wrapper object

// After: value class is stable and zero-allocation at runtime
@JvmInline value class UserId(val value: String)
data class UiState(val userId: UserId)

Gotchas

• Autoboxing: Value classes are unboxed at compile time but boxed (allocated) when used as nullable (UserId?), generic type arguments (List<UserId>), or vararg parameters. In hot paths these allocations matter; in most code they don't.
• No backing fields: You cannot use init blocks, lateinit, or delegated properties like by lazy. The class body is extremely constrained — only the single constructor parameter exists.
• No data-class conveniences: No copy(), no component1() for destructuring, and no way to customize toString(). If you need any of these, use a data class.
• No custom equals/hashCode/toString: These always delegate to the underlying type. Need custom equality → use a data class.
• when exhaustiveness: Sealed hierarchies of value classes work differently than data class hierarchies. Test when branches carefully.
• Serialization semantics: With kotlinx.serialization, a @Serializable data class A(val value: String) serializes as {"value":"..."}, but a @Serializable value class A(val value: String) serializes as the underlying value ("..."). Replacing a single-field data class with a value class is a breaking change for your API/JSON contract.
• Serialization: Some serialization frameworks need explicit support for value classes (e.g., kotlinx.serialization's @Serializable works, but Jackson may need configuration).
• Interoperability: From Java, value classes appear as their underlying type. Java callers bypass the type-safety wrapper.
• Reflection and runtime erasure: When passed as Any or used in generic contexts, value classes box into a synthetic wrapper class. Java reflection sees mangled method signatures, and frameworks that rely on raw runtime types (some ORMs, DI containers, or serializers) may see the underlying type rather than the value class.

Packing multiple values

A value class can only declare one field, but Compose provides packFloats, packInts, and matching unpack* functions in androidx.compose.ui.util to store multiple primitives in a single Long. This lets you represent composite values (e.g., a 2D point, size, or padding) as a zero-allocation value class instead of a multi-field data class.

@JvmInline value class Offset(val packedValue: Long)

fun Offset(x: Float, y: Float): Offset = Offset(packFloats(x, y))
val Offset.x: Float get() = unpackFloat1(packedValue)
val Offset.y: Float get() = unpackFloat2(packedValue)

• Only use this in performance-critical paths — manual bit-packing is error-prone. A data class is simpler and safer for most UI types.
• Available in androidx.compose.ui.util — packFloats, packInts, unpackFloat1, unpackFloat2, unpackInt1, unpackInt2.

Common mistakes

Mistake	Fix
Data class wrapping a single domain field	Replace with @JvmInline value class
Value class with no domain meaning (just a wrapper)	Use a type alias or the primitive directly
Value class needing custom equality	Use a data class instead
Value class as generic type argument in hot path	Accept autoboxing cost or use the primitive
@Immutable annotation on a type that could be a value class	Replace with value class — it's Stable by default
Forgetting @JvmInline annotation	Always pair value class with @JvmInline for single-field classes

Red flags during review

• A data class with exactly one property
• A String, Long, or Int used where different values should not be interchangeable (e.g., fun transfer(from: String, to: String, amount: Long))
• An @Immutable annotation on a single-field wrapper
• A type alias used for domain distinction where value-class semantics are needed (type aliases are type-erased, no runtime protection)

When NOT to apply

• The type needs multiple fields → data class
• The type needs custom equals/hashCode/toString → data class
• The type is used heavily as a nullable or generic in performance-critical code → measure autoboxing cost first
• The project does not need the type-safety distinction → a type alias or primitive is sufficient

Related

• compose-stability-diagnostics — diagnose unstable Compose parameters; value classes are one fix