Hands-On Design Patterns with Kotlin

Value mutation

In Maronic, we would like to count the average score over one thousand games. For that, we have the following data class:

data class AverageScore(var totalScore: Int = 0,                        var gamesPlayed: Int = 0) {    val average: Int        get() = if (gamesPlayed <= 0)                    0                else                    totalScore / gamesPlayed}

We were smart: we protected ourselves from any invalid output by checking for divisions by zero.

But what will happen when we write the following code?

val counter = AverageScore()thread(isDaemon = true) {    while(true) counter.gamesPlayed = 0}for (i in 1..1_000) {    counter.totalScore += Random().nextInt(100)    counter.gamesPlayed++    println(counter.average)}

Soon enough, you'll receive ArithmeticException anyway. Our counter somehow becomes zero.

If you want ...