Operator call corresponds to a dot-qualified call 'List.min().compareTo(500)' which is not allowed on a nullable receiver 'List.min()'

Question

The following functions produces error. How to use let() or similar null check functions inside a if/for statement.

Here is my code:

fun main() {
    var List = listOf<Int>(201, 53 ,5 ,556 ,70 , 9999)
    var budget: Int = 500
    if(List.min() < 500) { // this line produces the error
        println("Yes you can buy from this shop")
    }
}

And here is the error:

Operator call corresponds to a dot-qualified call 'List.min().compareTo(500)' which is not allowed on a nullable receiver 'List.min()'.

Help me with nullable types. Thank you

Answer 1

The question here is: what do you want to happen if your list is empty?

If a list has one or more items, and those items are comparable, then you can always find a minimum.  (It doesn't have to be unique.) But if the list has no items, then there is no minimum.  So what do you want to happen then?  Do you want to continue with a ‘default’ value?  Or skip that block?  Or something else?

If you want a default value, then you can use the elvis operator:

if ((list.minOrNull() ?: 0) < 500)
    println("Yes you can buy from this shop")

That substitutes the value 0 if the list is empty.  (It doesn't have to be zero; any value will do.  In fact, this can work with any type as long as it's Comparable.)

Or you could do an explicit check for the list being empty:

if (list.isEmpty()) {
    // Do something else
} else if (list.minOrNull()!! < 500)
    println("Yes you can buy from this shop")

The !! non-null assertion operator works here, but it's a code smell.  (It's easy to miss when you're changing surrounding code; it could then throw a NullPointerException.)  So it's safer to handle the null.  Perhaps the most idiomatic way is with let():

list.minOrNull().let {
    if (it == null) {
        // Do something else
    } else if (it < 500)
        println("Yes you can buy from this shop")
}

(The < check is allowed there, because by that point the compiler knows it can't be null.)

Or if you just want to avoid the check entirely, use a ?. safe-call so that let() is only called on a non-null value:

list.minOrNull()?.let {
    if (it < 500)
        println("Yes you can buy from this shop")
}