Effective Enums in Kotlin with reverse lookup?

Question

I\'m trying to find the best way to do a \'reverse lookup\' on an enum in Kotlin. One of my takeaways from Effective Java was that you introduce a static map inside the enum

Answer 1

Based on your example, i might suggest removing the associated value and just use the ordinal which is similar to an index.

ordinal - Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero).

enum class NavInfoType {
    GreenBuoy,
    RedBuoy,
    OtherBeacon,
    Bridge,
    Unknown;

    companion object {
        private val map = values().associateBy(NavInfoType::ordinal)
        operator fun get(value: Int) = map[value] ?: Unknown
    }
}

In my case i wanted to return Unknown if the map returned null. You could also throw an illegal argument exception by replacing the get with the following:

operator fun get(value: Int) = map[value] ?: throw IllegalArgumentException()

Answer 2

It makes not much sense in this case, but here is a "logic extraction" for @JBNized's solution:

open class EnumCompanion<T, V>(private val valueMap: Map<T, V>) {
    fun fromInt(type: T) = valueMap[type]
}

enum class TT(val x: Int) {
    A(10),
    B(20),
    C(30);

    companion object : EnumCompanion<Int, TT>(TT.values().associateBy(TT::x))
}

//sorry I had to rename things for sanity

In general that's the thing about companion objects that they can be reused (unlike static members in a Java class)

Answer 3

I found myself doing the reverse lookup by custom, hand coded, value couple of times and came of up with following approach.

Make enums implement a shared interface:

interface Codified<out T : Serializable> {
    val code: T
}

enum class Alphabet(val value: Int) : Codified<Int> {
    A(1),
    B(2),
    C(3);

    override val code = value
}

This interface (however strange the name is :)) marks a certain value as the explicit code. The goal is to be able to write:

val a = Alphabet::class.decode(1) //Alphabet.A
val d = Alphabet::class.tryDecode(4) //null

Which can easily be achieved with the following code:

interface Codified<out T : Serializable> {
    val code: T

    object Enums {
        private val enumCodesByClass = ConcurrentHashMap<Class<*>, Map<Serializable, Enum<*>>>()

        inline fun <reified T, TCode : Serializable> decode(code: TCode): T where T : Codified<TCode>, T : Enum<*> {
            return decode(T::class.java, code)
        }

        fun <T, TCode : Serializable> decode(enumClass: Class<T>, code: TCode): T where T : Codified<TCode> {
            return tryDecode(enumClass, code) ?: throw IllegalArgumentException("No $enumClass value with code == $code")
        }

        inline fun <reified T, TCode : Serializable> tryDecode(code: TCode): T? where T : Codified<TCode> {
            return tryDecode(T::class.java, code)
        }

        @Suppress("UNCHECKED_CAST")
        fun <T, TCode : Serializable> tryDecode(enumClass: Class<T>, code: TCode): T? where T : Codified<TCode> {
            val valuesForEnumClass = enumCodesByClass.getOrPut(enumClass as Class<Enum<*>>, {
                enumClass.enumConstants.associateBy { (it as T).code }
            })

            return valuesForEnumClass[code] as T?
        }
    }
}

fun <T, TCode> KClass<T>.decode(code: TCode): T
        where T : Codified<TCode>, T : Enum<T>, TCode : Serializable 
        = Codified.Enums.decode(java, code)

fun <T, TCode> KClass<T>.tryDecode(code: TCode): T?
        where T : Codified<TCode>, T : Enum<T>, TCode : Serializable
        = Codified.Enums.tryDecode(java, code)

Answer 4

we can use find which Returns the first element matching the given predicate, or null if no such element was found.

companion object {
   fun valueOf(value: Int): Type? = Type.values().find { it.value == value }
}

Answer 5

A variant of some previous proposals might be the following, using ordinal field and getValue :

enum class Type {
A, B, C;

companion object {
    private val map = values().associateBy(Type::ordinal)

    fun fromInt(number: Int): Type {
        require(number in 0 until map.size) { "number out of bounds (must be positive or zero & inferior to map.size)." }
        return map.getValue(number)
    }
}

}

Answer 6

A slightly extended approach of the accepted solution with null check and invoke function

fun main(args: Array<String>) {
    val a = Type.A // find by name
    val anotherA = Type.valueOf("A") // find by name with Enums default valueOf
    val aLikeAClass = Type(3) // find by value using invoke - looks like object creation

    val againA = Type.of(3) // find by value
    val notPossible = Type.of(6) // can result in null
    val notPossibleButThrowsError = Type.ofNullSave(6) // can result in IllegalArgumentException

    // prints: A, A, 0, 3
    println("$a, ${a.name}, ${a.ordinal}, ${a.value}")
    // prints: A, A, A null, java.lang.IllegalArgumentException: No enum constant Type with value 6
    println("$anotherA, $againA, $aLikeAClass $notPossible, $notPossibleButThrowsError")
}

enum class Type(val value: Int) {
    A(3),
    B(4),
    C(5);

    companion object {
        private val map = values().associateBy(Type::value)
        operator fun invoke(type: Int) = ofNullSave(type)
        fun of(type: Int) = map[type]
        fun ofNullSave(type: Int) = map[type] ?: IllegalArgumentException("No enum constant Type with value $type")
    }
}