Hands-On Design Patterns with Kotlin

Immutable collections

I think that our junior developer learned their lesson. Instead, they produced this code, which is not very efficient, but which gets rid of those variables:

data class ScoreCollector(val scores: MutableList<Int> = mutableListOf())val counter = ScoreCollector()for (i in 1..1_000) {    counter.scores += Random().nextInt(100)    println(counter.scores.sumBy { it } / counter.scores.size)}

But the maleficent thread strikes again:

thread(isDaemon= true, name="Maleficent") {    while(true) counter.scores.clear()}

We again receive ArithmeticException.

It's not enough that your data class contains only values. If its value is a collection, it must be immutable in order for the data class to be considered immutable. The same rule is applied ...