Null Safety on IF comparison in Kotlin

Question

I have a question about how Kotlin manages NULL safety on comparison. I have this code:

imageFile.addListener { _ , oldValue : File?, newValue : File? ->
    run{
        if(oldValue?.absolutePath != newValue?.absolutePath) loadFile()
    }
}

It works fine, however if I change it to

imageFile.addListener { _ , oldValue : File?, newValue : File? ->
    run{
        if(oldValue!!.absolutePath != newValue?.absolutePath) loadFile()
    }
}

It throws a NullPointerException and that's obvious, because when the application starts oldValue is NULL.

How Kotlin manages this comparison the first time?

Thanks for your help.

Answer 1

You are using safe call which avoid NullPointerException:

option is the safe call operator, written ?.

returns null if any of the properties in it is null.

Second option not-null assertion operator throws exception

The !! Operator

the not-null assertion operator (!!) converts any value to a non-null type and throws an exception if the value is null