Is it possible to create extension constructors in Kotlin?

Question

In other languages like Swift, there is the possibility of creating a function extension that adds a new constructor.

Something like this:

// base c         


        

Answer 1

Not like in Swift, because:

Extensions are resolved statically. Extensions do not actually modify classes they extend. By defining an extension, you do not insert new members into a class, but merely make new functions callable with the dot-notation on variables of this type. (Source)

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If a companion object is defined in your target class, go with s1m0nw1's approach. The advantage is that you can call the extension function without an instance (statically) of the target class.

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If not, use a classic Factory Pattern:

class Fruit(var name: String = "") {
}

class FruitFactory {
    companion object {
        fun create(name: String): Fruit {
            return Fruit().apply {
                this.name = "Tasty $name"
            }
         }
    }
}

fun main(args: Array) {
    val orange = Fruit("Orange")
    println(orange.name)

    val apple = FruitFactory.create("Apple")
    println(apple.name)
}

You can extend the Factory as you wish with further constructors either nested or as extension functions.

Output:

Orange
Tasty Apple