Cannot resolve string supplied to vararg parameter in extension function

Question

strings.xml

<string name="my_string">Showing your number: %1$s</string>

ActivityExt.kt

fun Activity.showToast(textResId: Int, vararg formatArgs: String) {
    val text = getString(textResId, formatArgs)
    Toast.makeText(this, text, Toast.LENGTH_SHORT).show()
}

MainActivity.kt

val number = 11
showToast(R.string.my_string, number.toString())

Toast with following text is showing:

Showing your number: [Ljava.lang.String;@2cfa3b]

Why this happens?

Answer 1

Use the spread operator:

fun Activity.showToast(textResId: Int, vararg formatArgs: String) {
    val text = getString(textResId, *formatArgs)
    Toast.makeText(this, text, Toast.LENGTH_SHORT).show()
}

Currently, you're passing an array as the format argument. By using the spread operator you pass the contents of the array as the format argument.

Answer 2

You should use the spread operator to pass in the varargs to the getString function:

val text = getString(textResId, *formatArgs)

This is because the type of formatArgs inside the showToast function is Array<String> (there's no vararg type or anything like that), and if you pass that in without the *, you'll only pass a single parameter, which will be the array instead of its contents.