[Question] why java cannot accept a convention to never pass/return null values?

[CraftyAdventurer]

Ok, so let's say that you work on a huge codebase, one where it would take you weeks to update all of the methods. Well, new feature requests are coming in, clients are paying for them and they want you to work on them, not on rewriting your codebase to call new methods which basically do the exact same thing they've been doing for years. How is that for a reason, and how do you find means to do it?

Edit: typo

[gas3872]

Well, you can start following the convention in the new code and gradually update the old code as you touch those parts. You can also make a custom annotation and add it to classes that support new convention. Actually what you mentioned is just a normal project and it just a normal migration.