Unable to implement Comparable interface on Kotlin Enum

Question

I would like my Enum values to be comparable with other types, for instance, String. I am not entirely sure why it complains and what the error means.

enum class Fruits(val value: String): Comparable<String>{
    Apple("apple"),
    Orange("orange");

    override fun compareTo(other: String): Int {
        return compareValuesBy(this.value, other)
    }
}

val test: Boolean = Fruits.Apple == "apple"

Error says:

Type parameter T of 'Comparable' has inconsistent values: Fruits, String

There are two issues, the major one is that compareTo is final in Enum so you may have to rethink your approach. The other one is that you need Comparable<Fruits> rather then Comparable<String> — David Soroko
– David Soroko, Commented Sep 19, 2022 at 14:59
Also, implementing Comparable doesn't modify the implementation of ==. That's managed by the equals function. But the contract for equals is that it must be reflexive. Since you cannot modify the String.equals() function, you cannot safely do this. — Tenfour04
– Tenfour04, Commented Sep 19, 2022 at 15:11
@Tenfour04 oh yeah, you are completely correct. I think I had a brain-fart. — Arturs Vancans
– Arturs Vancans, Commented Sep 19, 2022 at 15:44

David Soroko · Accepted Answer · 2023-08-02 15:01:17Z

4

Perhaps sealed classes are better suited to implement comparable enum, something like:

sealed class Fruit(val name: String) : Comparable<Fruit> {
    override fun compareTo(other: Fruit): Int = this.name.compareTo(other.name)
}

data object Apple    : Fruit("apple")
data object Orange   : Fruit("orange")
data object Zucchini : Fruit("zucchini")

fun main() {
    println(Apple < Orange)    // true
    println(Orange < Zucchini) // true
}

edited Aug 2, 2023 at 15:01

answered Sep 19, 2022 at 15:11

David Soroko

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2 Comments

Arturs Vancans Over a year ago

I think I was more looking for equality of different data types. ie. equality between an enum and a string

David Soroko Over a year ago

I guess that is slightly different, you can start with Comparable<Any> / compareTo(other: Any) and see how it goes.

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