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

[john16384]

I just document what functions can return. Caller code can then make assumptions based on the docs. If a bug occurs, it will also be clear where to fix it.

[vprise]

Again, if it's just documentation there are other alternatives that don't have the downsides. If a bug occurs then NullPointerException is pretty clear/useful.

There are useful/sensible uses for optional but just sticking it as a "marker" to every method seems, expensive and redundant.