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 to handle the reverse lookup. Porting this over to Kotlin with a simple enum leads me to code that looks like this:

enum class Type(val value: Int) {
    A(1),
    B(2),
    C(3);

    companion object {
        val map: MutableMap = HashMap()

        init {
            for (i in Type.values()) {
                map[i.value] = i
            } 
        }

        fun fromInt(type: Int?): Type? {
            return map[type]
        }
    }
}

My question is, is this the best way to do this, or is there a better way? What if I have several enums that follow a similar pattern? Is there a way in Kotlin to make this code more re-usable across enums?

Solution

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.

This question was reposted from Stack Overflow