Can you elaborate what you mean by nullable types?

You can set Object variables to null and with primitives you can do the same with their boxed alternatives (Integer) but I assume you mean something different?

Like the lack of types which can't be null? I can see how that's useful

In Kotlin/Swift by default all types do not include "bottom" or "null". You add a ? to the type to indicate you may use that value, then the compiler will validate you're checking for the existence of null before you use the type.

It's one approach... I prefer the Haskell approach with a simple Maybe/Option type.

The Kotlin page does a good job of explaining it if you aren't familiar. It's the single most important feature that makes Kotlin worth using over Java in my opinion: https://kotlinlang.org/docs/reference/null-safety.html