Non-nullable is a promise not a guarantee. It does absolutely nothing in terms of useful warnings or compiler errors within Objective-C, it only explains how Swift should consume the Objective-C API.

It's still entirely possible to get a nil from Objective-C while the API is declared as nonnullable, this even happened to me with Apple's own API's if abused to a certain limit.

Yes but if Java 10 (for example) introduced this annotation then the JVM10 could transform a null returned from a non-nullable annotated function to an exception. Compile time static checks could also be added to guarantee that such a method does not return null. But you could still use such a function from a previous java version albeit you would sacrifice the checks.

It is a way to introduce the benefits (as long as the library is developed properly) of non-nullable types without sacrificing retro compatibility.

Edit: I remember a bug in earlier version of Swift Cocoa binding where getting calendar events without titles would hang without possibility of recourse because of this problems.

Something like what they are trying for C# may work for Java too: https://blogs.msdn.microsoft.com/dotnet/2017/11/15/nullable-...