how do I call the constructor of a generic type? S...

# getting-started

[Kaj Koivunen]

how do I call the constructor of a generic type? Say I have something like

class foo<T: Interface>() {
    var something: T? = null
    init {
        something = T("bar") //doesn't work
    }
}

I have an interface, the implementing classes of which have a constructor that takes one string. I'd like to make a class that can take any type that implements the interface and be able to cosntruct an instance of said class.

[Joffrey]

You can't, because you cannot guarantee that every T that implements Interface will have a constructor that takes a single string argument.

[Joffrey]

What are you trying to do, in a broader sense? This sounds like an XY problem

[Kaj Koivunen]

I have a machine translation program that can implement several translation APIs like google translate and deepl. These are implemented as children of a common interface. I'd like to also add tornadoFX fragments that can set the api key of each of these classes through a settings dialog. Right now I have a separate form for each but this duplicates code when really the only unique part of the settings form is the type of translation backend class it uses

[Kaj Koivunen]

if I made the interface an abstract class instead and gave it a constructor with the same arguments this would work then?

[Vampire]

Same problem.
Just that the abstract class has that constructor does not mean a subclass has such a constructor.

[Kaj Koivunen]

so the generics can't really be used with polymorphism then?

[Kaj Koivunen]

what's the idiomatic workaround to not being able to construct the generic type?

[Sam]

class Foo<T>(val something: T) {
    constructor(init: (String) -> T): this(init("bar"))
}

class Bar(val value: String)

val foo = Foo(::Bar)

[Joffrey]

You can pass a function to the Foo class

[Joffrey]

Ah, Sam beat me to it 😄

[Kaj Koivunen]

yeah just found on stack overflow to pass a (String) -> T to it

[Kaj Koivunen]

ty

[Joffrey]

You can also store the function itself as a property of the class, if you need to use it multiple times, or call it later instead of immediately at construction time:

class Foo<T>(private val createThing: (String) -> T) {
    fun doStuff() {
        val thing = createThing()
        // ...
    }
}