Avoid else condition in 'When' in kotlin

Question

As per documentation of When in Kotlin, else is not mandatory if the compiler knows all the values are covered. This is very in case of emums or sealed class but how to do it in case of arrays for numbers 1 to 5 (startRating).

private fun starMapping(startRating: Int): String {

    return when (startRating) {
        1 -> "Perfect"
        2 -> "Great"
        3-> "Okay"
        4-> "Bad"
        5-> "Terrible"
        // don't want to add else as I believe it is prone to errors.
    }
}

Something similar to this

return when (AutoCompleteRowType.values()[viewType]) {
        AutoCompleteRowType.ITEM -> ItemView(
                LayoutInflater.from(parent.context).inflate(R.layout.item_venue_autocomplete_item_info, parent, false))

        AutoCompleteRowType.SECTION -> SectionView(
                LayoutInflater.from(parent.context).inflate(R.layout.item_venue_autocomplete_section, parent, false)
        )
    }

Vivianaviviane

"I believe it is prone to errors". So use else -> throw IllegalArgumentException("Illegal startRating") (this is how it's done on the official kotlin idioms page). – Herniorrhaphy

@Michael: I don't want to write the else statement altogether, similar to enums check the edit. – Vivianaviviane

Why don't you want to write the else clause? What errors are you expecting? You would have to write an enum if you want it to be obsolete... An enum seems more suitable for star ratings than Ints anyway. – Zacynthus

There is no way to guarantee that an invalid rating will never be passed to this method. You could make a typo, you could receive invalid data from the user, some external code could invoke this method reflectively, etc. The safest option is to simply throw an error, but the easiest would probably be to consider using enums. – Monopode

You'd need type of startRating to be "Int from 1 to 5", and there's no such type in Kotlin. – Paschasia

Answer 1

Using when statement it is impossible to exclude else clause in case of using ints, because compiler doesn't know what to return if startRating is not in 1..5 range. You can, for example, throw an IllegalStateException if the value is not in the required range:

private fun starMapping(startRating: Int): String {
    return when (startRating) {
        1 -> "Perfect"
        2 -> "Great"
        3-> "Okay"
        4-> "Bad"
        5 -> "Terrible"
        else -> throw IllegalStateException("Invalid rating param value")
    }
}

Or you can do something like this:

return when {
    startRating <= 1 -> "Perfect"
    startRating == 2 -> "Great"
    startRating == 3 -> "Okay"
    startRating == 4 -> "Bad"
    else -> "Terrible"
}

But else clause is required.

Bugg

If you don't want to throw an exception, the next-best option is probably to return null; that way, the caller can use e.g. the elvis or safe-call operators to invalidate whatever it's being used in. – Gramineous

Since the function is just implementing a mapping, I'd also look at storing an actual map or array of strings; the function then collapses to a simple lookup, and you can use .getOrNull() to do that safely. – Gramineous

Answer 2

You may not want to use when for this at all. Here is what I would suggest:

You could create an enum class like so:

enum class Rating(val score: Int) {
  Perfect(1),
  Great(2),
  Okay(3),
  Bad(4),
  Terrible(5)
}

And utilise it like that:

fun ratingForScore(score: Int) = Rating.values().firstOrNull {
    it.score == score
}?.toString()

ratingForScore(1) // "Perfect"

Varanasi