Hello, World! I have a pattern that happens a lot in our codebase: enums with a String key, and we need to get the enum value from the key, or a special “unknown” value. So what we do currently is something like:

enum class MyEnum(val key: String) {
    FOO("foo"),
    BAR("bar"),
    UNKNOWN("unknown");

    companion object {
        fun getByKey(key: String?): MyEnum {
            return MyEnum.values().firstOrNull { it.key == key } ?: MyEnum.UNKNOWN
        }
    }
}

I’m looking for a way to not repeat this companion object for all our enums. Possibly a combination of an interface and a generic extension function? Any idea? 🙂

An extension method on Enum and having enum implements an interface like

interface LookableByKey { val key: String }

?

Continuing with @dany's suggestion. You can do something like this:

interface EnumCompanion<T : LookableByKey<*>> {
    val default: T
    fun values(): Array<T>
    fun getByKey(key: String?): T = values().firstOrNull { it.key == key } ?: default
}

interface LookableByKey<T> {
    val key: T
}

enum class MyEnum(override val key: String): LookableByKey<String> {
    FOO("foo"),
    BAR("bar"),
    UNKNOWN("unknown");

    companion object : EnumCompanion<MyEnum> {
        override fun values() = MyEnum.values()
        override val default = UNKNOWN
        
    }
}

wow, I’m very sorry, I posted this in the wrong channel! Was meaning to post in #general

why not having an extension on Enum something like

fun <T> Enum<T>.getByKey() : Enum<T>

?

You will get some boilerplate per enum no matter what you do. The extension needs to know what the default-value is, and how to extract the key. And because

values()

is not part of the

Enum

class, you have to provide that function as well.

@danielm that was my first try but couldn’t achieve anything

@dany Because that will require you to have an enum value from before. Like, you can only call it with

BAR.getByKey()

. It won't allow you to do

MyEnum.getByKey()

.

there’s also

enumValueOf

, I wonder if it could be of any help

indeed marius! missed that tks

@bod Ah, right. There's also

enumValues

. That may make it simpler. 2 sec

oh well 🙂

Oh, I managed to make it work.

wow, not that’s definitely cool!

Hehe, you need it when the type parameter has more than one constraint.

even better! 👍