default branch placement in switch

[Brian Goetz]

The middle is surely awful.  Though in the JDK, we have a fair number of 
uses where default is the _first_ case, which isn't unreasonable, and 
some might argue is even clearer in some cases.

The reason to tread lightly on forcing reorganization of existing 
switches is that it is allowable to fall into *and out of* the default 
case.  So if someone has:

     switch (x) {
         default:  S;   // fall through
         case COMMON: T; break;
         case UNCOMMON: U; break;
     }

then eventually getting to an error when default is not last for 
"legacy" switches (those where all labels are type-restating constants) 
means some uncomfortable refactoring just to "make the compiler happy."  
So while I agree on warnings, I'm not sure if we can ever get to error 
in all cases without picking some fights with users.



On 12/11/2017 1:16 PM, Kevin Bourrillion wrote:
> On Fri, Nov 3, 2017 at 5:25 PM, Brian Goetz <brian.goetz at oracle.com 
> <mailto:brian.goetz at oracle.com>> wrote:
>
>
>     or plan to eventually get to a place where default always comes
>     last, even for "int" switches. If we want to get to the latter, we
>     should start warning on this construct now.
>
>
> I favor starting to warn and eventually forbidding default in any 
> position but last for all constructs that have it.
>
> A switch with the default in the middle is extremely weird and 
> confusing. If I'm reading code to understand what happens when i == 3, 
> and I read as far as
>
>     switch (i) {
>       case 1:
>         justOneStuff(); break();
>       case 2:
>         justTwoStuff(); break();
>       default:
>
> ... then I immediately assume that this must be where execution is 
> continuing. Worse, even if I do notice that there are more case labels 
> to follow, and I resume searching for a `case 3:`, then when I don't 
> find one I now risk making /another/ error and forgetting to jump 
> /back/ to the default.
>
> This is kind of insane. At first I was less worried because I thought 
> "surely no one is actually doing this"... then I browsed our 
> codebase.... yikes.
>
> We should at least strongly consider this.
>
>
>
>     On 11/3/2017 5:10 PM, Tagir Valeev wrote:
>
>         Hello!
>
>         Currently the default branch can be placed in any place inside the
>         switch operator, e.g. like this:
>
>         switch(i) {
>         case 1: System.out.println("one");break;
>         default: System.out.println("other");break;
>         case 2: System.out.println("two");break;
>         }
>
>         In this case behavior does not change on the order of case blocks.
>         However in pattern matching the order of cases usually matters: if
>         some pattern matches, this means that the subsequent patterns
>         will not
>         be checked. Does this mean that with pattern matching the default
>         branch makes all the subsequent case blocks unreachable? Or
>         default
>         can still be located anywhere and is checked only after any other
>         pattern?
>
>         With best regards,
>         Tagir Valeev
>
>
>
>
>
> -- 
> Kevin Bourrillion | Java Librarian | Google, Inc. |kevinb at google.com 
> <mailto:kevinb at google.com>

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