Feedback on nulls in switch

[John Rose]

On Aug 10, 2020, at 4:03 PM, Stephen Colebourne <scolebourne at joda.org> wrote:
> 
> Given:
>  box instanceof Object o
> o is non-null. And given:
>  switch (box) {
>     case Box b:
>  }
> b is non-null.

That latter is not necessarily true and I think you need a better example. 

The b can be null because the case is the last case and therefore (not otherwise) may be total. When I see the last case in a switch I expect it to be a catch-all, don’t you?  Here it depends on the type of box up in the switch header. I suppose it’s a Box. So that last case catches whatever the previous zero cases did t catch. 

If Box b = box; is valid, and it sure looks that way, then that last case is indeed a catch all. And in that case, unless we enact a specific anti I’ll policy, any and all nulls will flow through that point. 

I think we all are going to have to learn to read the bottom of a switch as a probable catch-all.

Maybe we want to add a modifier to enforce the assumption, and exclude the implicit default; I’ve suggested ‘final case’ as a cite bikeshed color. There are many others. 

None of that eases your discomfort with null flows. But it does undercut your claims of illegibility.