Hello. I have a question about kotlin value/inline classes. Not sure if this is the proper channel -...

# getting-started

[Ryan Sang]

Hello. I have a question about kotlin value/inline classes. Not sure if this is the proper channel - please direct me to a better one if appropriate!

Given a kotlin value class like

@JvmInline
value class Password private constructor(val password: String) {
    companion object {
        @JvmStatic
        @JvmName("of")
        fun of(password: String) = Password(password)
    }
}

and a corresponding java class

public class JavaClass {
    public void func(Password word) {
        System.out.println(word);
    }

    public void invoke() {
        String s = "test";
        this.func(Password.of(s));
    }
}

I get an error on the invocation of this.func claiming that the required type is Password and I have provided String. Is there any work around? Or can value classes not be used as parameters for java methods?

[ephemient]

it doesn't make sense to expose a Java parameter of type `Password`; the way Kotlin handles it, it's all String at runtime (except when boxed)

[Stephan Schröder]

generally inline classes are (mostly) optimized away at runtime, so I can see how the java compiler might "not get it".

See what happens if you make func a vararg method:

public void func(Password... words) {
        if(words.length!=1){throw new IllegalArgumentException();}
        var word = words[0];
        System.out.println(word);
    }

since arrays actually retain their generic type.

[ephemient]

this does mean that Java doesn't get type safety benefits from value classes

[ephemient]

@Stephan Schröder subject to change, perhaps? https://youtrack.jetbrains.com/issue/KT-33565/Allow-vararg-parameter-of-inline-class-type