I mean, should I only use !! if dealing with legacy code?

Typically it is best to avoid using !! whenever possible. The main exception is testing where it is much better to discover issues early on rather than in production.

Sometimes auto-casting is not picked up, I did a not null check, further on I use the var/val my editors says it could be null. In those cases I use

!!

@Albert Think that should only occur with

var

where the variable may have been modified between the check for not null & it’s usage

@andyb I also use it for nullable

val

on a class.

example(someClass: SomeClass) {
  if (someClass.nullableField == null) {
    throw SomeSpecificException()
  }
}

lookUp(someClass.nullableField!!)

The example might be weird, but I need this for an application to throw specific exceptions when this occurs. Assigning it to a local

val

will fix this, but this is only to demonstrate

@Albert Be aware that the reason it doesn't autocast the property there is because it could be modified by another thread. For thread safety, you should use a

val

to only read the property one time. For example:

val nullableField = nullableField ?: throw SomeSpecificException()

New Kotlin contracts are a dealing with this. https://proandroiddev.com/kotlin-contracts-make-great-deals-with-the-compiler-f524e57f11c?__s=cfajh45oef5g4ge15tzo