Kotlin - Factory Function for Class with Private Constructor

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()