How to get generic param class in Kotlin?

Question

I need to be able to tell the generic type of kotlin collection at runtime. How can I do it?

val list1 = listOf("my", "list")
val list2 = listOf(1, 2, 3)
val list3 = listOf<Double>()

/* ... */

when(list.genericType()) {
    is String -> handleString(list)
    is Int -> handleInt(list)
    is Double -> handleDouble(list)
}

Answer 1

Kotlin generics share Java's characteristic of being erased at compile time, so, at run time, those lists no longer carry the necessary information to do what you're asking. The exception to this is if you write an inline function, using reified types. For example this would work:

inline fun <reified T> handleList(l: List<T>) {
    when (T::class) {
        Int::class -> handleInt(l)
        Double::class -> handleDouble(l)
        String::class -> handleString(l)
    }
}

fun main() {
    handleList(mutableListOf(1,2,3))
}

Inline functions get expanded at every call site, though, and mess with your stack traces, so you should use them sparingly.

Depending on what you're trying to achieve, though, there's some alternatives. You can achieve something similar at the element level with sealed classes:

sealed class ElementType {
    class DoubleElement(val x: Double) : ElementType()
    class StringElement(val s: String) : ElementType()
    class IntElement(val i: Int) : ElementType()
}

fun handleList(l: List<ElementType>) {
    l.forEach {
        when (it) {
            is ElementType.DoubleElement -> handleDouble(it.x)
            is ElementType.StringElement -> handleString(it.s)
            is ElementType.IntElement -> handleInt(it.i)
        }
    }
}