I have an issue with kotlin on an Android application, I think this is the best channel to ask it: `...

# android

[Pablo]

I have an issue with kotlin on an Android application, I think this is the best channel to ask it:

if (parameters[i] != null) {
    if (parameters[i].isNotEmpty()) {

Why this gives error on .isNotEmpty()? I mean... it's supposed that the != null should about this problem:

Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?

[Michael Krussel]

I don't know what type parameters is, but I will assume that the compiler cannot assume that parameters[i] returns the same value every time you call it, so smart cast is not happening.

[Pablo]

is a String?

[Michael Krussel]

paramaters is a String or parameters[i] is a String?

But still the [] operator is a function call and there's no guarantee that it returns the same object every time. Now Kotlin could hard take advantage that String is immutable, but String.get cannot return null so there's no need, so I"m sure parameters is not a String.

It is probably either a List or an Array both of which are not immutable.

[Pablo]

parameters[i] contains String?

[Pablo]

is an array of String?

[Michael Krussel]

So since arrays are not immutable, the compiler cannot smart cast the returned string.

[Landry Norris]

It's possible that another thread has access to 'parameters' and can set it to null between the check and usage. It's safer to create a local variable, which the compiler can guarantee won't be modified concurrently, and will smart cast for you.

[Ciaran Sloan]

Capture the value from the array in a variable then the compiler can assess it as you're expecting:

val myValue = parameters[i]
if (myValue != null) {
    if (myValue.isNotEmpty()) {
        // .....
    }
}

Or alternatively:

val myValue = parameters[i]
if (myValue.isNullOrEmpty()) {
    // ....
}

[Hristijan]

Or use let

parameters[i]?.let { nonNullValue -> }

[Ciaran Sloan]

.let is a functional mapper. If you are not interested in the return value, then that's not its best use case. It should not be a default replacement for a standard null check