Making Object abstract

[Brian Goetz]

I agree that we can introduce the new API point immediately.  The 17 
window hasn't even closed yet!  But we'd have to get a move on.  But 
realistically, we can expect it to be several years before we are 
comfortable erroring on the `new Object()` constructor.



> One way or another, we've got a migration problem—we want people to update their code to use a new idiom. This could look like:
>
> - With Primitive Objects, introduce an alternative API point & javac warnings. After a few versions, maybe drop support for the constructor call.
>
> - Before Primitive Objects, introduce an alternative API point & javac warnings. With Primitive Objects (final), make the constructor call illegal.
>
> There needs to be some sort of gradual pathway—can't just spring it on people when Primitive Objects are final. We did the latter with the primitive wrappers, but in that case the deprecation began in 9 (2017), and I think 'new Integer' is a more rare occurrence than 'new Object'. Not sure we could squeeze it in before Primitive Objects are final. (But if we want to, we'd better get moving on it!)
>
> I see the question of whether Object is abstract as orthogonal: unless we remove support for 'new Object' before the change, we're going to have a period where 'new Object()' is special, and not necessarily subject to the "can't instantiate an abstract class" rule.