Kotlin Compilation Error : None of the following functions can be called with the arguments supplied

Question

I have a class whose constructor takes 2 int parameters (null values are allowed). Following is the compilation error.

None of the following functions can be called with the arguments supplied: 
public final operator fun plus(other: Byte): Int defined in kotlin.Int
public final operator fun plus(other: Double): Double defined in kotlin.Int
public final operator fun plus(other: Float): Float defined in kotlin.Int
public final operator fun plus(other: Int): Int defined in kotlin.Int
public final operator fun plus(other: Long): Long defined in kotlin.Int
public final operator fun plus(other: Short): Int defined in kotlin.Int

Here is the NumberAdder class.

class NumberAdder (num1 : Int?, num2 : Int?) {

    var first : Int? = null
    var second : Int? = null

    init{
    first = num1
    second = num2
    }

fun add() : Int?{

    if(first != null && second != null){
        return first + second
    }

    if(first == null){
        return second
    }

    if(second == null){
        return first
    }

    return null
}

}

How can I resolve this issue? I want to return null if both are null. If one of them is null, return the other, and otherwise return the sum.

Answer 1

Because first and second are vars, they will not be smart cast to non-null types when you do the if-test. Theoretically, the values can be changed by another thread after the if-test and before the +. To solve this, you can assign them to local vals before you do the if-tests.

fun add() : Int? {
    val f = first
    val s = second

    if (f != null && s != null) {
        return f + s
    }

    if (f == null) {
        return s
    }

    if (s == null) {
        return f
    }

    return null
}

Answer 2

The easiest fix for your code is to use val instead of var:

class NumberAdder (num1 : Int?, num2 : Int?) {

    val first : Int?
    val second : Int?

    init{
        first = num1
        second = num2
    }
...

I used here that Kotlin allows a val to be assigned in the constructor.

Answer 3

I experienced a similar issue with assertEquals.

My code was

assertEquals(
       expeted = 42, // notice the missing c 
       actual = foo()
)

After I've fixed the typo, my IDE said I can't use named arguments with non-Kotlin functions, so I extracted the values to variables and everything started working as it should.

 val expected = 42
 val actual = foo()
 assertEquals(expected, actual)