Kotlin extension for next Enum value without reflection

Question

I wrote a Kotlin extension which adds next() for enum values. But is there an better way to do that?

fun <T : Enum<*>> T.next(): T {
    val values = this::class.java.getEnumConstants()
    return if (this.ordinal < values.size - 1) 
            values[this.ordinal + 1] 
        else 
            values[0]
}
enum class Color {Red, Yellow, Green}
Color.Red.next() //Yellow

1. Can I achieve that without reflections?
2. If not, how to do that with Kotlins reflection?

The Idea is to iterate from enum value to the next. But only for a specific value, not for whole enum values() list. The thing I want to prevent is to add the following to every enum type:

fun next() = if (this.ordinal == Color.values().size - 1) 
        Color.values()[0] 
    else 
        Color.values()[this.ordinal + 1]

Answer 1

You can do that without reflection in Kotlin 1.1 (currently, you can try the 1.1 RC build) using the enumValues<T>(), which does not use the reflection inside, because it is inlined at call sites instead, and the corresponding enum type is placed in the resulting code at compile-time. Example:

inline fun <reified T: Enum<T>> T.next(): T {
    val values = enumValues<T>()
    val nextOrdinal = (ordinal + 1) % values.size
    return values[nextOrdinal]
}

^{(demo of this code) | The inline and reified are required because enumValues<T>() has reified T.}

An alternative that works with Kotlin 1.0.6 (but still uses getDeclaringClass() from Java reflection):

fun <T: Enum<T>> T.next(): T {
    val values = declaringClass.enumConstants
    val nextOrdinal = (ordinal + 1) % values.size
    return values[nextOrdinal]
}