Is it possible to create extension constructors in

问题:

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

Something like this:

// base class
class Whatever() {
    ...
}

// constructor method extension
fun Whatever.constructor(potato: String) {
    setPotato(potato)
}

fun main(args: Array<String>) {
    println(Whatever("holi"))
}

Are there any means to do this in Kotlin?

回答1:

Seems that there are not an official "function extension for constructors" but you can create a package-method that imitates a constructor

class Foo() {
    ...
}

fun Foo(stuff: Int): Foo = Foo().apply {setStuff(stuff)}

fun main(args: Array<String>){
    println(Foo(123))
}

回答2:

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)

---

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.

---

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<String>) {
    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

回答3:

You can't do this. What you can do though: Extend the companion object of a class with a factory method:

// base class
class Whatever() {
    companion object {

    }

}

// factory extension
fun Whatever.Companion.withPotato(potato: String) {
    //setPotato(potato)
}

fun main(args: Array<String>) {
    println(Whatever.withPotato("holi"))
}

The only problem: A companion object has to be existent to do this.