Update random class attribute in Kotlin

Question

I have a class with some attributes:

class DonutBox {
    var glaze: Int = 0
    var chocolate: Int = 0
    var maple: Int = 0
    var etc: Int = 0
}

fun addDonuts() {
    val omNom = DonutBox()
}

How can I increment a random attribute of the instantiated class?

For instance, if the randomly selected attribute is chocolate, then effectively:

omNom.chocolate += 1

Adam Millerchip · Accepted Answer · 2020-11-16 08:31:06Z

3

Because Kotlin's properties are statically declared, and you want to use them dynamically, most of the methods to do that will involve reflection, which can get pretty messy and difficult to understand.

When you want dynamic data, it's probably better to use a map:

val donutBox = mutableMapOf(
    "glaze" to 0,
    "chocolate" to 0,
    "maple" to 0,
    "etc" to 0,
)

val randomKey = donutBox.keys.random()
donutBox[randomKey] = donutBox.getValue(randomKey) + 1

println(donutBox)

Output:

{glaze=0, chocolate=0, maple=1, etc=0}

That said, if you really want to use reflection, you can do it like this:

data class DonutBox(
    var glaze: Int = 0,
    var chocolate: Int = 0,
    var maple: Int = 0,
    var etc: Int = 0,
)

fun addDonuts() {
    val omNom = DonutBox()
    val randomProperty = omNom::class.declaredMemberProperties.random() as KMutableProperty1<DonutBox, Int>
    val existing = randomProperty.get(omNom)
    randomProperty.set(omNom, existing + 1)
    println(omNom)
}

fun main() {
    addDonuts()
    addDonuts()
    addDonuts()
}

Output:

DonutBox(glaze=0, chocolate=1, maple=0, etc=0)
DonutBox(glaze=0, chocolate=0, maple=0, etc=1)
DonutBox(glaze=0, chocolate=1, maple=0, etc=0)

answered Nov 16, 2020 at 8:31

Adam Millerchip

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1 Comment

gidds Over a year ago

In addition to being long-winded and ugly, reflection tends to be slow, unsafe (risking runtime errors that can't be spotted at compile-time), insecure, and isn't available on the most limited platforms. There are times when you genuinely need it — but more often (like here), it's a code smell, indicating that the design could be improved.

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