site = <http://dto.site|dto.site> ?: <http://this.site|this.site>!!

is this the best way to say use first if not null else use second if that also is null thow?

i would use:

site = <http://dto.site|dto.site> ?: <http://this.site|this.site> ?: throw FooEx()

fair, in this case it would be a configuration error

do you have a better exception to throw than an NPE?

that’s really the crux of it, if you can’t proceed any further past that point if the value is null, throw an exception such that a human can understand what went wrong

I’ve been growing fond of using the

check

or

checkNotNull

stdlib functions for this, to give some more info in stacktraces

site = checkNotNull(<http://dto.site|dto.site> ?: <http://this.site|this.site>) { "Site cannot be null" }

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/check-not-null.html

I like those where in Java I'd be using something like

Preconditions

from guava

Or, if 'null' is a proper and expected, but incorrect, input, you may want to properly model both good result and the possible errors. I would use something like an

Either<E,T>

as the type of this expression.

you can do

?: error("Message describing what was expected")

too, if you don't mind throwing an IllegalStateException