What's the correct way to iterate through properties of a singleton in Kotlin?

Question

As the title suggests, I wanted to iterate through properties of a singleton object. I tried using the kotlin-reflect as there was no other way I found currently.

object Test {
    const val a = "String"
    const val b = "Another String"
}

Test::class.declaredMemberProperties.forEach {
    println("${it.name}: ${it.get(Test)}")
}

But unfortunately this results in the following exception:

Exception in thread "main" java.lang.IllegalArgumentException: Callable expects 0 arguments, but 1 were provided.
...
at com.example.MainKt.main(main.kt:25)  // same line as println statement
...

It seems like get() function has problem (name property resolves just fine). Is there a better approach (maybe without reflection) or some solution to access those pre-compiled constants in the singleton?

Answer 1

for const val getters, you don't need to pass the receiver object as they are compile-time constants, not runtime constants. that's why the compiler gives you this error: Callable expects 0 arguments. use this code it works fine for both const and non-const:

object Test {
    const val a = "String"
    val b = "Another String"
}

fun main() {
    Test::class.memberProperties.forEach {
        if (it.isConst) {
            println("const ${it.name}: ${it.getter.call()}")
        } else {
            println("${it.name}: ${it.getter.call(Test)}")
        }
    }
}

Answer 2

Looks like an issue in reflection library.

You may remove const modifier, or use the following workaround:

Test::class.members.filterIsInstance<KProperty<*>>().forEach {
    println("${it.name}: ${it.call()}")
}

Answer 3

You may not need kotlin-reflect at all for this use case.

object Test {
    const val a = "String"
    const val b = "Another String"
}

fun main() {
    Test::class.java
        .declaredFields
        .filter { it.name != "INSTANCE" }
        .map { "${it.name} = \"${it.get(Test)}\"" }
        .forEach(::println)
}

a = "String"
b = "Another String"

Link to Kotlin Playground : https://pl.kotl.in/bEgfrZk9k