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.

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.

Review procedure

1. Find single-property wrappers, primitive-heavy APIs, and @Immutable wrappers in UI state.
2. Decide whether the single value is a real domain distinction. If not, keep the primitive or use a typealias.
3. Check whether replacing the type changes equality, serialization, Java interop, or hot-path boxing.
4. Convert only when the domain meaning is clear and the contract changes are acceptable.
5. Re-run the affected compiler/tests; for Compose performance work, re-check compiler reports or recomposition evidence.

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 beyond the wrapped value	Data class (value classes delegate to the underlying type)
Used as a generic type argument or nullable in a proven hot path	Data class or primitive

// 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 domain field
data class UserId(val value: String)

// 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 procedure

When a Compose report points at a single-field wrapper:

1. Confirm the underlying type is stable (String, primitives, or another stable type).
2. Prefer a value class over @Immutable on a wrapper whose only job is type distinction.
3. Do not change public serialization/API contracts just to silence a report.

// Before: primitive value can be mixed up with other strings
data class UiState(val userId: String)

// After: domain type is stable at the Compose boundary
@JvmInline value class UserId(val value: String)
data class UiState(val userId: UserId)

Refactor checks

Before replacing an existing wrapper, check the contract that callers observe:

Check	Action
JSON/API format matters	Verify serialization. @Serializable data class A(val value: String) encodes as an object; a value class encodes as the wrapped value.
Custom equality or hashing is required	Keep a data class. Value-class equality follows the wrapped value.
Callers use copy() or destructuring	Keep a data class or update callers deliberately. Value classes do not provide data-class conveniences.
Java or reflection-heavy framework boundary	Verify interop. Java callers see the underlying type; generic/Any use boxes.
Nullable/generic/vararg hot path	Measure before converting; those uses box.
Constructor body, lateinit, delegated properties, backing fields	Keep a data class or redesign; value classes only store the constructor value.

Packing multiple values only after evidence

Do not replace a clear multi-field data class with bit-packing unless profiling shows allocation cost on a hot path. If needed, Compose provides packFloats, packInts, and matching unpack* functions in androidx.compose.ui.util:

@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)

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 a hot path	Measure boxing cost; keep the primitive/data class if it matters
@Immutable annotation on a type that could be a value class	Replace with a value class when the underlying type is stable
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 → 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
• The replacement would silently change JSON, Java, reflection, or framework behavior

Related

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