Nothing

Earlier in the book, we saw the TODO() function:

fun heyThisIsNotDoneYet() {
  TODO("implement this function")
}

TODO() throws an exception with the supplied message. We use this as a more forceful alternative to simply using a TODO comment to identify incomplete work.

Contrast that with:

fun heyThisIsNotDoneYet(): Int {
  TODO("implement this function")
}

On the surface, this would seem like it could not possibly compile. We have set up heyThisIsNotDoneYet() to return an Int, yet we have no return statement or anything else to return a value. After all, if we skipped the TODO(), we definitely would get a compilation error:

fun heyThisIsNotDoneYet(): Int {
  // TODO implement this function
}

So, what is it about TODO() that allows it to somehow complete the body of a function that returns something, while not returning anything?

The answer is: nothing. Or, more accurately, Nothing.

Nothing: It’s On the Bottom

Earlier in the book, we saw Any. Any in Kotlin is analogous to Object in Java: it is the root type from which all other types inherit. In truth, Any is even more widespread in Kotlin than Object is in Java, because all Kotlin types extend Any, whereas primitives (e.g., int) and arrays (e.g., int<>) do not extend Object in Java.

Nothing has no Java analogue and is on the far other end of the inheritance spectrum. Effectively, Nothing is a sub-type of all other types. It is a sub-type of Any. It it a sub-type of String. It is a sub-type of Axolotl.

Nothing has no instances. It is impossible to create instances of Nothing, as it has a private constructor.