Effective Enums in Kotlin with reverse lookup?

Question

First of all, the argument of fromInt() should be an Int, not an Int?. Trying to get a Type using null will obviously lead to null, and a caller shouldn't even try doing that. The Map has also no reason to be mutable. The code can be reduced to:

companion object {
    private val map = Type.values().associateBy(Type::value)
    fun fromInt(type: Int) = map[type]
}

That code is so short that, frankly, I'm not sure it's worth trying to find a reusable solution.

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 }
}

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)

Another option, that could be considered more "idiomatic", would be the following:

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

Which can then be used like Type[type].

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)

Another example implementation. This also sets the default value (here to OPEN) if no the input matches no enum option:

enum class Status(val status: Int) {
OPEN(1),
CLOSED(2);

companion object {
    @JvmStatic
    fun fromInt(status: Int): Status =
        values().find { value -> value.status == status } ?: OPEN
}

}

If you have a lot of enums, this might save a few keystrokes:

inline fun <reified T : Enum<T>, V> ((T) -> V).find(value: V): T? {
    return enumValues<T>().firstOrNull { this(it) == value }
}

Use it like this:

enum class Algorithms(val string: String) {
    Sha1("SHA-1"),
    Sha256("SHA-256"),
}

fun main() = println(
    Algorithms::string.find("SHA-256")
            ?: throw IllegalArgumentException("Bad algorithm string: SHA-256")
)

This will print Sha256