How to declare a variable with two types in Kotlin, like val x: Int or String

Question

I'm gonna write a method like:

object UIBehavior {
   fun dialog(context: Context, title: Int | String, message: Int | String){
     val dialogObj = AlertDialog.Builder(context)
     dialogObj.setTitle(title)
     dialogObj.setMessage(message)
   }
}

The methods dialogObj.setTitle and dialogObj.setMessage allow two types of parameters, and how can I delare the parameter that can let the method dialog allow only two types Int and String?

Answer 1

You can't do that in Kotlin.

But you can have multiple versions of a function, e.g.

object UIBehavior {
    fun dialog(context: Context, titleId: Int, messageId: Int){
        val titleString = context.getString(titleId)
        val messageString = context.getString(messageId)
        dialog(context, titleString, messageString)
    }

    fun dialog(context: Context, title: String, message: String) {
        val dialogObj = AlertDialog.Builder(context)
        dialogObj.setTitle(title)
        dialogObj.setMessage(message)
    }
}

That way you can simply call the function with either ids or strings and it looks like you are using the same function

UIBehavior.dialog(this, R.string.title, R.string.message)
UIBehavior.dialog(this, "title", "message")

You could also use a common supertype of Int and String but that would allow a lot more and I wouldn't recommend that.

fun dialog(context: Context, title: Any, messageId: Any){
    val titleString = when (title) {
        is String -> title
        is Int -> context.getString(title)
        else -> throw IllegalArgumentException("Unsupported type")
    }
    val messageString = when ...
       ...
    dialog(context, titleString, messageString)
}

Generics don't work here either because you can't call dialogObj.setTitle(title) dynamically. It must be known at compile time whether you want to call the Int or the String overload of that function. It's also not really different from using Any.

Answer 2

1. Generics can be the solution for you.

fun <T>dialog(context: Context, title: T, message: T){
   if(title !is String || title !is Int) throw InvalidParameterException()
   val dialogObj = AlertDialog.Builder(context)
   dialogObj.setTitle(title)
   dialogObj.setMessage(message)
}

2. Also using Any can be a solution too:

fun dialog(context: Context, title: Any, message: Any){
   if(title !is String || title !is Int) throw InvalidParameterException()
   val dialogObj = AlertDialog.Builder(context)
   dialogObj.setTitle(title)
   dialogObj.setMessage(message)
}