Kotlin annotation with optional parameters

Question

Custom annotation is defined below:

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)    
annotation class Custom(val a: String, 
        val b: String = "[null]", 
        val c: String = "[null]")

Desired usage:

1.

@Custom(a = "Testa", b = "Testb", c = "Testc")
fun xyz(){ ... }

2.

@Custom(a = "Testa")
fun pqr(){ ... }

When I am trying desired usage #2, it throws No values passed for parameter "b".

How can it be achieved to have optional parameters in kotlin cusotm annotations?

Answer 1

Your code works as is, which can be verified by

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)    
annotation class Custom(val a: String, 
        val b: String = "[null]", 
        val c: String = "[null]")

object W {
    @Custom(a = "Testa")
    fun pqr(){}
}

fun main(args: Array<String>) {
    println(W::class.java.getMethod("pqr").getAnnotations()[0])
}

printing @Custom(b=[null], c=[null], a=Testa), so b and c got their default values. (You could also write W::pqr.annotations[0], but that doesn't work on the playground.)