Get the class of nullable type

Question

I am trying to match the type of the nullable String? in a Kotlin reflection exercise:

data class Test(a: String, b: String?)
val test = Test("1", "2")
val properties = test::class.declaredMemberProperties
val propertyNames = properties.joinToString(",") { 
        when (it.returnType) {
            String?::class.createType() -> "string?"
            String::class.createType() -> "string"
            else -> throw Exception()
        }
}

Alas, it is failing with the error, Type in a class literal must not be nullable, for String?::class.

A `String?` isn't a class of its own and doesn't have a class object. `String?::class` could be the class object of `String` or the class object of `null` (which doesn't make sense). This would be ambiguous. — Michael Butscher, Sep 25 '18 at 01:08

score 8 · Accepted Answer · edited Jun 03 '20 at 14:04

8

The createType function has an optional nullable parameter that seemed to work when I tested it.

import kotlin.reflect.full.*

String::class.createType(nullable = true) -> "string?"

edited Jun 03 '20 at 14:04

Raphael

9,779
5
63
94

answered Sep 25 '18 at 03:02

luminous_arbour

106
1
4

1

Alas, this answer is only valid for the jvm target. – Claude Brisson Jun 01 '22 at 05:30

Get the class of nullable type

1 Answers1