CODEWITHSUNDEEP
enumskotlin
lostintranslation
lostintranslation

Reputation: 24573

Adding a property to an existing enum in Kotlin

Given the following enum defined in an external api.

public enum Status {
  COMPLETE,
  RUNNING,
  WAITING
}

I would like a way to add a int flag to each enum value. I know that I can extend the enum:

fun Status.flag(): Int {
    when(this) {
        RUNNING -> return 1;
        WAITING -> return 2;
        else -> return 0;
    }
}

However I would like to define those int flag values as constants. Maybe a companion object, but I don't think I can extend an existing enum and add a companion object.

Any ideas?

Upvotes: 2

Views: 6883

Answers (2)

Alexey Romanov
Alexey Romanov

Reputation: 170723

You can add extension properties/methods to the companion object of enum/class/etc. if one exists: 

val Status.Companion.COMPLETE_INT = 0
val Status.Companion.RUNNING_INT = 1

but indeed you can't currently "create" the companion object if it doesn't. So just put the constants into your own non-companion object:

object StatusFlags {
    const val COMPLETE_INT = 0
    const val RUNNING_INT = 1
    const val WAITING_INT = 2
}

fun Status.flag(): Int {
    when(this) {
        RUNNING -> return StatusFlags.RUNNING_INT
        ...
    }
}

Upvotes: 0

A. Thom
A. Thom

Reputation: 1718

Unless you are using a field that already exists in the original enum (like ordinal), you won't be able to do what you're asking without wrapping the external enum in your own enum.

Sure you could use ordinal, but a newer version of the external API may change the order of the items in the enum, so I wouldn't recommend it. But, if you REALLY want to, you could do something like this (again, this is NOT recommended): 

val Status.flag: Int
    get() = this.ordinal

But I'd definitely recommend wrapping it. That way you guarantee that the flag integers you define won't change.

enum class MyStatus(val status: Status, val flag: Int) {
    COMPLETE(Status.COMPLETE, 0),
    RUNNING(Status.RUNNING, 1),
    WAITING(Status.WAITING, 2);

    companion object {
        private val STATUS_TO_MYSTATUS = values().associateBy { it.status }

        fun fromStatus(status: Status): MyStatus {
            return STATUS_TO_MYSTATUS[status] ?: throw Exception("No MyStatus found for status ${status.name}")
        }
    }
}

You can then convert Status to MyStatus by using MyStatus.fromStatus(...). Or you can add an extension function to Status to easily convert to MyStatus.

fun Status.toMyStatus() = MyStatus.fromStatus(this)

Upvotes: 2

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