Kotlin - Factory Function for Class with Private Constructor - kotlin

Question

In Kotlin, is it possible to have a factory function that creates an instance of a class with a private constructor?
My goal is to enforce the factory function to be used and to prevent instantiation via the class's constructor.
Example:
// factory function, valid
val myInstance = myClassOf()
// class instantiation, invalid
val myInstance = MyClass()
I'm trying to mimic the behavior of some of the built-in factory functions like intArrayOf(), e.g.
// works
val myIntArray = intArrayOf()
// not possible as IntArray has a private constructor
val myIntArray = IntArray()

Answer 1

You can use companion object in this way:
class MyClass private constructor() {
companion object {
operator fun invoke() = MyClass()
}
}
val myInstance = MyClass() // Calls the factory function invoke()
Name the factory function if it has a special meaning. For example:
class MyClass private constructor(values: List<String>) {
companion object {
fun of(vararg values: String) = MyClass(values.toList())
}
}
val myInstance = MyClass.of("first", "second")

Answer 2

You can do something like this:
import MyClass.Companion.myClassOf
class MyClass private constructor() {
companion object {
fun myClassOf() = MyClass()
}
}
//val myInstance1 = MyClass() // not allowed
val myInstance2 = myClassOf()

Answer 3

Try Builder instead.
class FoodOrder private constructor(
val bread: String?,
val condiments: String?,
val meat: String?,
val fish: String?) {
data class Builder(
var bread: String? = null,
var condiments: String? = null,
var meat: String? = null,
var fish: String? = null) {
fun bread(bread: String) = apply { this.bread = bread }
fun condiments(condiments: String) = apply { this.condiments = condiments }
fun meat(meat: String) = apply { this.meat = meat }
fun fish(fish: String) = apply { this.fish = fish }
fun build() = FoodOrder(bread, condiments, meat, fish)
}
}
Reference: https://www.baeldung.com/kotlin-builder-pattern