在 Java 中使用 Kotlin Value 类

Question

We have two projects, and the kotlin one publishes a package that's imported by java.

In kotlin, is a value class like

@JvmInline
value class CountryId(private val id: UUID) {
    override fun toString(): String = id.toString()
    companion object { fun empty(): CountryId = CountryId(EMPTY_UUID) }
}

In java, we can't see a constructor, or actually instantiate this class. I have also tried creating a factory in Kotlin to create them

class IdentifierFactory 
{
    companion object {
        fun buildString(): String {
            return "hello"
        }

        fun buildCountry(): CountryId {
            return CountryId.empty()
        }
    }
}

In java, I can call IdentifierFactory.Companion.buildString() and it will work, but IdentifierFactory.Companion.buildCountry() doesn't even exist.

Is Java really this awful with Value classes?

ps. I've attempted with @JvmStatic as well, with no success

Answer 1

It is doubtful you can use IdentifierFactory.buildString() from this code. You would need to mark it with @JvmStatic or you would have to access it as: IdentifierFactory.Companion.buildString() . buildCountry() is accessible in exactly the same way

Anyway, the main goal of value/inline classes is to inline them whenever possible. That means that in the bytecode we don't operate on CountryId , but directly on the UUID object. buildCountry() actually returns UUID and for Java interop, in most cases you just need to use UUID .