Kotlin Data Type With Exclamation Point

Question

I'm learning Kotlin in Android, and I'm seeing some things return a type with a !, for example, WifiInfo.getSSID() returns a string that is of type String!. What does the ! do?

Bonus question, why can't I find this in the Kotlin documentation?

Platform types: [Single exclamation mark in Kotlin](https://stackoverflow.com/q/43826699) — Ryan M, Aug 11 '20 at 23:08
Documentation: https://kotlinlang.org/docs/reference/java-interop.html#notation-for-platform-types — Tenfour04, Aug 12 '20 at 01:03

score 2 · Accepted Answer · answered Aug 11 '20 at 23:48

? says "this can be null".

! says "we do not know if this can be null or not, but we will assume that it cannot".

In Android and other Kotlin/JVM environments, you get ! from "platform types" — basically, types returned from libraries (e.g., the Android SDK) where the return value was not annotated with @Nullable or @NotNull.

Kotlin Data Type With Exclamation Point

1 Answers1