Answer

inline class (from Kotlin 1.5 — value class) allows creating wrappers around primitives without creating a separate object at runtime. This is used to enhance type safety without the overhead of memory allocation. Under the hood, such objects can be compiled to the corresponding primitive.

Example:

@JvmInline
value class UserId(val id: String)

fun printUserId(userId: UserId) {
    println(userId.id)
}

• Only one property (the primary field) is allowed.
• No-argument constructor is not possible.
• The class cannot contain state (e.g., var fields), only val.
• Cannot inherit other classes, but can implement interfaces.
• Interaction with Java code may be non-obvious — boxing/unboxing may occur.

Using inline/value classes is important for typing identifiers, money, units of measurement, etc.

Trick Question

Can a value class have more than one property?

Answer: No. A value class can contain only one property in the primary constructor.

Example of erroneous code:

@JvmInline
value class Money(val amount: Int, val currency: String) // Compilation error

Examples of real errors due to ignorance of the subtleties of the topic

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Story

In a currency exchange, an inline class was used to describe the amount and currency. They tried to add two fields to the value class, received a compilation error, and spent some time trying to work around the limitation. Eventually, they decided to create a separate data class.

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Story

When integrating with an external Java library, the inline class sometimes unexpectedly converted to an object (boxing), which affected performance. After analyzing the documentation, they replaced it with a regular value object.

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Story

In a microservices project, they used value classes as IDs in the API. One of the services returned a string directly, while another returned a value class, which led to a serialization conflict with Jackson. They fixed it by explicitly mapping between the ID and the string.