With the August 6 production debut of the Android 9 Pie mobile OS, Google has released an Android SDK with special capabilities for development with the Kotlin language.
The SDK has nullability annotations for frequently used APIs, preserving null-safety guarantees when Kotlin code is calling into annotated APIs in the SDK. To ensure that newly annotated APIs are compatible with existing code, an internal mechanism provided by the Kotlin compiler team marks APIs as recently annotated. These APIs result in warnings instead of errors from the Kotlin compiler. Developers need to use Kotlin 1.2.60 or later.
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