CODEWITHSUNDEEP
arrayskotlin
Bass
Bass

Reputation: 5348

Custom property delegation for arrays using a map

Consider I'm trying to implement delegation by storing properties in a Map instance, and one of the properties delegated is an array:

class Foo private constructor(map: Map<String, Any?>) {
    constructor(value: Array<Byte>) : this(mapOf(Foo::value.name to value))

    val value: Array<Byte> by map
}

object PropertyDelegationTest {
    @JvmStatic
    fun main(vararg args: String) {
        val foo = Foo(arrayOf(42.toByte(), 127.toByte()))
        println(foo.value[0]) // 42
        println(foo.value[1]) // 127
    }
}

The above compiles just fine and works as expected.

Now consider I want to enhance my property delegation mechanism by implementing a custom Map.getValue(thisRef: Any?, property: KProperty<*>) extension method (overriding the default extension):

import kotlin.reflect.KProperty
import kotlin.reflect.full.isSubtypeOf
import kotlin.reflect.full.starProjectedType
import kotlin.reflect.jvm.jvmName

// ...

operator fun <V, V1 : V> Map<in String, V>.getValue(thisRef: Any?, property: KProperty<*>): V1 {
    val value = this[property.name]
                ?: throw NoSuchElementException("Key ${property.name} is missing in the map.")
    val clazz = (value as Any)::class
    @Suppress("UNCHECKED_CAST")
    return when {
        clazz.starProjectedType.isSubtypeOf(property.returnType) -> value as V1
        else -> throw ClassCastException("${clazz.starProjectedType} (${clazz.jvmName}) cannot be cast to ${property.returnType}")
    }
}

This fails at run time:

Exception in thread "main" java.lang.ClassCastException: kotlin.Array<*> ([Ljava.lang.Byte;) cannot be cast to kotlin.Array<kotlin.Byte>
    at com.example.PropertyDelegationTestKt.getValue(PropertyDelegationTest.kt:30)
    at com.example.Foo.getValue(PropertyDelegationTest.kt)
    at com.example.PropertyDelegationTest.main(PropertyDelegationTest.kt:18)

Despite the effective JVM type is known ([Ljava.lang.Byte;), Kotlin-specific run time type of the value is Array<*> while Array<Byte> is required. Consistently, clazz.typeParameters[0].upperBounds[0] evaluates to kotlin.Any?, not kotlin.Byte?.

How do I implement my custom type checking which would also work correctly for arrays? Kotlin version is 1.2.71.

Upvotes: 1

Views: 230

Answers (1)

Yoni Gibbs
Yoni Gibbs

Reputation: 7036

You could remove the explicit type check and instead do a safe cast to V1 and if that fails then throw the exception, e.g.

return value as? V1 ?: throw ClassCastException(...

Upvotes: 1

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