Get enum value based on it's ordinal in a generic class

Question

Consider the below code:

class SegmentController<E : Enum<E>>() {

    fun getEnumForOrdinal(ordinal: Int) : E {
        //values is undefined?
        //how can I return the enum for the specified ordinal value?
        return E.values()[ordinal]
    }

    fun getOrdinalForEnum(enum: E): Int {
        //no problem getting the ordinal from the supplied E enum
        return enum.ordinal
    }

}

Its a generic class, taking any Enum as a template.

How can I return an Enum value based on on ordinal in getEnumForOrdinal()?
I would expect the E.values() to be available, but its not inside the generic class?

I did my research but cannot find any solution based on the ordinal, links I found:

Marek · Accepted Answer · 2020-05-16T10:17:46.467

You could pass the enum's class as a constructor argument and access the enum values via enumConstants:

class SegmentController<E : Enum<E>>(private val clazz: Class<E>) {

    fun getEnumForOrdinal(ordinal: Int): E {
        return clazz.enumConstants[ordinal]
    }

    fun getOrdinalForEnum(e: Enum<E>): Int {
        return e.ordinal
    }
}

enum class SEGMENTS {A, B, C}

fun main() {
    val controller = SegmentController(SEGMENTS::class.java)
    println(controller.getOrdinalForEnum(SEGMENTS.B))
    println(controller.getEnumForOrdinal(1))
}

Alternatively, you could define getEnumForOrdinal as an inline function with a reified type parameter E and use enumValues to access the enum's values:

class SegmentController<E : Enum<E>>() {

    inline fun <reified E : Enum<E>> getEnumForOrdinal(ordinal: Int): E {
        return enumValues<E>()[ordinal]
    }

    fun getOrdinalForEnum(e: Enum<E>): Int {
        return e.ordinal
    }
}

enum class SEGMENTS {A, B, C}

fun main() {
    val controller = SegmentController<SEGMENTS>()
    println(controller.getOrdinalForEnum(SEGMENTS.B))
    println(controller.getEnumForOrdinal<SEGMENTS>(1))
}

See also: https://kotlinlang.org/docs/reference/enum-classes.html#working-with-enum-constants

A superb explanation of the reified keyword is given by this answer.

I would expect the E.values() to be available, but its not inside the generic class?

The values() method is not available in the Enum class.

Get enum value based on it's ordinal in a generic class

1 Answers1