C# half-fixes this with its nullable annotations. I say half-fixes, because the boundary between code that supports them and code that does not is leaky, so you can make a mistake that leaks a null into a non-nullable variable.

If you build an entire program with nullability checking on it's pretty great, though.

Java, or at least Lombok, seems to have a @NonNull annotation that does what I want— cause code not to build that fails the check, and forces propagation of the annotation.

Reality does indeed feel exactly like what you mentioned with C#, though. The annotation is going to be missing where it’s needed most unless something forces the whole project to use it.

check JSpecify (https://jspecify.dev) - it's the standardised null annotation package for Java. Intellij understands the annotations so you generally get decent null-checking across your codebase.

Even better, apply at the package level via `package-info.java` (unfortunately sub-packages need to be individually marked as well)

  @NullMarked
  package com.foo;

  import org.jspecify.annotations.NullMarked;

Kotlin fixes the null handling problem too, and with the added benefit of being able to gradually migrate Java code.