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

this is '!!' double-bang operator is always return not-null value and this is '?' safe call operator returns value if value is not null, and null otherwise

This is unsafe nullable type (T?) conversion to a non-nullable type (T). It will throw NullPointerException if the value is null.

It is documented here along with Kotlin means of null-safety.

ref - hotkey