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Source: StackOverflow, jlelse, JetBrains

Question: Since Kotlin extension functions and properties are areally static methods and properties, is using them bad practice?

Answer:

It depends on how you wrote extension function/property. If they doesn't edit or access shared state i.e. if property and function is clear function: it absolutely not bad practice.

Example 1:

fun String.countSpaces(): Int {
    return this.count { c -> c == ' ' }
}

This functions works perfectly in multi-threaded environment, since String is immutable.

Example 2:

data class MutablePerson(val name: String, var speech: String)

fun MutablePerson.count(nextNumber: Int) {
    this.speech = "${this.speech} ${nextNumber}"
}

This function mutates speech property of MutablePerson object and assign operation is not atomic. If count will be called on one object from different threads - inconsistent state possible.

Example:

fun main(args: Array<String>) {
    val person = MutablePerson("Ruslan", "I'm starting count from 0 to 10:")

    (1..10).forEach { it ->
        Thread({
            person.count(it)
            println(person.speech)
        }).start()
    }

    Thread.sleep(1000)

    println(person.speech)
}

Possible output:

I'm starting count from 0 to 10: 1
I'm starting count from 0 to 10: 1 3
I'm starting count from 0 to 10: 1 3 4
I'm starting count from 0 to 10: 1 3 4 2
I'm starting count from 0 to 10: 1 3 4 2 5
I'm starting count from 0 to 10: 1 3 4 2 5 8
I'm starting count from 0 to 10: 1 3 4 2 5 6
I'm starting count from 0 to 10: 1 3 4 2 5 6 7
I'm starting count from 0 to 10: 1 3 4 2 5 6 7 9
I'm starting count from 0 to 10: 1 3 4 2 5 6 7 9 10
I'm starting count from 0 to 10: 1 3 4 2 5 6 7 9 10

So extension functions and extensions properties not bad practice, they are just like properties and method in classes: depending on how you wrote they thread safe or not.

Read the second link for further detail on another shortage of extension functions and properties, but in short, as of Kotlin 1.1.60, extension function and property are not applicable for Java and Android classes.