Mark a fun / property for use in Kotlin / Java only

Question

I'm currently writing a KOTLIN class were I like to use the possibility of DSL but be backwards compatible for JAVA developers. When using a var the compiler automatically creates getter and setter for JAVA, but those can't be used Builder style since they are not returning this .

So basically what I like to do is something like this:

class MyClass {
    // for use in KOTLIN only DSL style e.g. MyClass() { offset = 1 }
    var offset: Int? = null

    // for use in JAVA only BUILDER style e.g. new MyClass().withOffset(1)
    fun withOffset(value: Int) = apply { offset = value }
}

In Kotlin I like to use, but don't want to have access to the withOffset fun:

val myClass = MyClass() { offset = 1 }

In JAVA I like to use, but don't want to have access to the auto created setOffset and getOffset :

MyClass myClass = new MyClass().withOffset(1)

Renaming the getter and setter is possible via @JvmName annotation already, but is there a annotation for hiding a public property for JAVA completly and optional of course vice versa?

Answer 1

You cannot hide a constructor, while you can make a so called fake-constructor using operator fun invoke in which you can use a @JvmSyntheic annotation to hide it from the java.

And to hide a function from kotlin you can use @Deprecated with DeprecationLevel.HIDDEN .

Note: The @JvmField will instruct the compiler not to generate the default getter and setter for your var

class MyClass {
    @JvmField
    var offset: Int? = null

    @kotlin.Deprecated(message = "JUST FOR JAVA", level = DeprecationLevel.HIDDEN)
    fun withOffset(value: Int) = apply { offset = value }

    companion object {
        @JvmSynthetic
        operator fun invoke(init: MyClass.() -> Unit) = MyClass().apply(init)
    }
}

Usage in kotlin:

MyClass() { offset = 1 }
// or
MyClass { offset = 1 }

Usage in java:

MyClass().withOffset(1)

Resources:

• Hide static Java methods from Kotlin
• What's the intended use of @JvmSynthetic in Kotlin?

Answer 2

It's not possible to do something like this in Kotlin

val myClass = MyClass() { offset = 1 }

but I would suggest you do do this instead, it looks much nicer in my opinion.

// It's a inline function so there's no runtime overhead.
inline fun buildMyClass(builder: MyClass.() -> Unit): MyClass {
    return MyClass().apply(builder)
}

class MyClass {

    @JvmSynthetic
    var offset: Int? = null
        private set

    fun withOffset(value: Int) = apply { offset = value }
}

So you can call it like below

val myClass = buildMyClass {
   withOffset(0)
}

In Java, it looks like this:

final MyClass myClass = new MyClass().withOffset(0);