Are Kotlin data types built off primitive or non-primitive Java data types?

I am new to Kotlin and was playing around with the data types. I took an Int type and then tried to cast it as a Double by saying num as Double, a call that is valid in java (non syntactically but you get the point). However, this failed, saying that Int cannot be cast to Double. I am assuming this is because it is built off the Integer class rather than the raw int data type. Am I correct, and what is the most efficient way to cast values? There is a .toDouble() function, but this seems inefficient and unwieldy.