It causes runtime errors, rather than compile time ones. It also can move the place an error is detected to elsewhere (since passing null around and sticking it places works fine, right up until you try and use the value you expect), which can be a bit of a pain to trace back.

In the particular context of Java, null solves some of the problems of the language, like what happens when a constructor invokes a derived method which reaches up and accesses a not-yet-initialized variable in the superclass. I do think some of these problems could be solved in a different way, but it's not completely straightforward.

Disallowing method calls before all variables have been initialised in the constructor seems like a relatively simple mostly-fix. For any non-final values, it'd work fine to just set them to some explicit dummy value, and final variables should probably be explicitly assigned anyway. (I can't think of many cases where having computation implicitly operate on undefined values is a good thing...)

Also, to adwn: thanks -- I figured Rust probably did something like that, but haven't really played with it much and hadn't bothered to go check.