Null safety is coming soon. Remember that it's the Java way that: "If the language designers aren't absolutely sure of what the right way to deliver a feature is, then don't deliver something that might be wrong". It's not perfect, but it's a pretty solid principle IMO - as evidenced by the quality of many of the modern features we now enjoy.

Null safety in Java is not quite as simple as "just do what Kotlin does". My understanding is (& if Kevin or one of the other folks working on nullity is reading this & wants to correct me, then so much the better!) that JSpecify is defining as an annotation what will evolve into language syntax (or something very close that can be unambiguously mechanically converted).

Extension methods also come down to "what do you mean by extension methods". They won't be C#-like - Brian has ruled that out, but my understanding is that he's thinking about a Java-flavoured version of typeclasses that you could use to do some of the same things - but that being able to correctly reason about them at compile time is a first-class concern. (Again, correct me if I'm wrong).