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