I'm trying to migrate some code from Java to Kotli...

# announcements

[Daniele Segato]

I'm trying to migrate some code from Java to Kotlin, it has an annotation where I used constants inside the annotation (in Java):

@IntDef(
            flag = false,
            value = {
                    AspectRatioMode.AUTO,
                    AspectRatioMode.BY_WIDTH,
                    AspectRatioMode.BY_HEIGHT,
            }
    )
    public @interface AspectRatioMode {
        int AUTO = 0;
        int BY_WIDTH = 1;
        int BY_HEIGHT = 2;
    }

@IntDef is an Android annotation allowing to define an annotation for Integer constants on Integers:

@AspectRatioMode
int myMode = AspectRatioMode.AUTO; // can only be one of AUTO / BY_WIDTH / BY_HEIGHT

While in kotlin the conversion would be:

@IntDef(flag = false, value =[
        AspectRatioMode.AUTO,
        AspectRatioMode.BY_WIDTH,
        AspectRatioMode.BY_HEIGHT
    ])
    annotation class AspectRatioMode {
        companion object {
            val AUTO = 0
            val BY_WIDTH = 1
            val BY_HEIGHT = 2
        }
    }

but I get an error on `companion object`: "Members are not allowed in annotation class"

I can move those constants out:

object AspectRatioModeConst {
        const val AUTO = 0
        const val BY_WIDTH = 1
        const val BY_HEIGHT = 2
    }

    @IntDef(flag = false, value = [
        AspectRatioModeConst.AUTO,
        AspectRatioModeConst.BY_WIDTH,
        AspectRatioModeConst.BY_HEIGHT
    ])
    annotation class AspectRatioMode

but that is a lot uglier then Java where I could more cleanly use the annotation directly for constants.

@AspectRatioMode
val myGoodMode : Int = AspectRatioMode.AUTO; // I want this
val myBadMode : Int = AspectRatioModeConst.AUTO; // not this

Is there any other way I can obtain the same result of Java with Kotlin?

[cbruegg]

I'm wondering if it would be infeasible to simply migrate to enums instead. See also: https://www.reddit.com/r/androiddev/comments/7so7ne/you_should_strictly_avoid_using_enums_on_android/dt6aqki/

[Daniele Segato]

@cbruegg According to this article enum are bad for performance in Android: https://android.jlelse.eu/android-performance-avoid-using-enum-on-android-326be0794dc3

I'm not sure if this is still the case, but this was the main reason I went for Int in that code

[cbruegg]

It's a micro-optimization that may make sense for the Android framework, but for most applications, it probably won't make a difference. Generally, before making such optimizations, I'd advise to benchmark first.

[cbruegg]

One more thing, the Reddit comment I've linked to is by Jake Wharton, who actually now works in the Android Framework Team at Google.

[Daniele Segato]

Yeah I know Jake 🙂
And still have to read the link

[Daniele Segato]

Ah, apparently they changed their mind:
https://stackoverflow.com/questions/5143256/why-was-avoid-enums-where-you-only-need-ints-removed-from-androids-performanc

[Daniele Segato]

I just trusted them about that

[Daniele Segato]

I guess I shouldn't have

[gildor]

This will be possilbe in Kotlin 1.3
https://blog.jetbrains.com/kotlin/2018/07/see-whats-coming-in-kotlin-1-3-m1/#Nested%20declarations%20in%20annotation%20classes

[gildor]

Also you can keep your annotation in Java for now to avoid API changes and wait 1.3, looks exactly the same for Kotlin and Java