What's the difference between !! and ? in Kotlin?

Question

I am new to Kotlin. I want to know the difference between this two !! and ? in below code.

Below, there are two snippets: the first uses

Answer 1

the precedence of operators !, ?., !! is ?. > !! > !.

the !! operator will raising KotlinNullPointerException when operates on a null reference, for example:

null!!;// raise NullPointerException

the safe call ?. operator will return null when operates on a null reference, for example:

(null as? String)?.length; // return null;

the !! operator in your second approach maybe raise NullPointerException if the left side is null, for example:

mCurrentDataset?.load(..)!!
    ^-------------^
           | 
when mCurrentDataset== null || load() == null a NullPointerException raised.

you can using the elvis operator ?: instead of the !! operator in your case, for example:

!(mCurrentDataset?.load(..)?:false)