Exception handling in switch (Preview)

[Tagir Valeev]

Dear experts,

looking into this proposal, I'm really not convinced that Java needs it. We
already have try-catch statements, and it sounds strange to provide another
way to express the same semantics. I don't see what the new construct adds,
aside from a bit of syntactic sugar. On the other hand, it creates a new
source of subtle bugs, especially when exceptions are unchecked. E.g.,
consider:

switch(a.b().c().d()) {
case ...
case throws RuntimeException ex -> handle(ex);
}

Now, one may want to refactor the code, extracting a.b(), a.b().c(), or the
whole a.b().c().d() to a separate variable for clarity, or to avoid a long
line.
This action is usually safe, and it was totally safe in switches so far
(even with patterns and case null). Now, it's not safe, as exceptions
thrown from the extracted part are not handled by the 'case throws' branch.
I don't see a good way to perform this refactoring in a semantically
equivalent way. The only possibility I see is to duplicate the exception
handler in the external catch:

try {
  var ab = a.b();
  switch(ab.c().d()) {
  case ...
  case throws RuntimeException ex -> handle(ex);
  }
}
catch(RuntimeException ex) {
  handle(ex); // duplicated code
}

As switch selector does not allow using several expressions or to declare
new variables, extract/inline refactorings can easily become very
painful, or cause subtle bugs if not performed correctly.
Note that it's not a problem inside usual try-catch statement (*), as you
can easily add or remove more statements inside the try-body.

(*) Except resource declaration, but it's rarely a problem, and in some
cases it's still possible to extract parts as separate resources, because
you can declare several of them

I think, instead of repurposing switch to be another form of try-catch we
could add more love to try-catch allowing it to be an expression with
yields in branches. The proposed JEP allows something like this:

Integer toIntOrNull(String s) {
  return switch(Integer.parseInt(s)) {
    case int i -> i;
    case throws NumberFormatException _ -> null;
  }
}

But we are still limited by a single expression in the selector. An
alternative would be
Integer toIntOrNull(String s) {
  return try { yield Integer.parseInt(s); }
    catch(NumberFormatException _) { yield null; };
}
Here, all kinds of refactorings are possible. And we actually don't need to
express pattern matching, because we essentially don't need any pattern
matching.

Also, note that some of the situations which are usually solved with
exception handling in modern Java (e.g. Pattern.compile ->
PatternSyntaxException, or UUID.fromString -> IllegalArgumentException, or
Integer.parseInt above) will be covered in future by member patterns. So
probably if we concentrate more on member patterns, people will need much
less exception handling in business logic, and such an enhancement will be
not so useful anyway? Speaking about the sample from the JEP, can we
imagine something like this in the future (sic!) Java?

switch(future) {
  case Future.cancelled() -> ...
  case Future.interrupted() -> ...
  case Future.failed(Exception ex) -> ... // no need to unwrap
ExecutionException manually
  case Future.successful(Box b) -> ...
}

One more note about the JEP text. It's unclear for me whether 'case throw'
branches could catch a residual result. More precisely, if MatchException
happens, or NullPointerException happens (selector evaluated to null, but
there's no 'case null'), can these exceptions be caught by the 'case
throws' branches in the same switch?

With best regards,
Tagir Valeev.


On Fri, Apr 19, 2024 at 3:05 PM Angelos Bimpoudis <
angelos.bimpoudis at oracle.com> wrote:

> Dear spec experts,
>
> A while ago we discussed on this list about enhancing the switch​ construct
> to
> support case​ labels that match exceptions thrown during evaluation of the
> selector expression. A draft JEP for this feature is now available at:
>
> https://bugs.openjdk.org/browse/JDK-8323658
>
> Please take a look at this new JEP and give us your feedback.
>
> Thanks,
> Aggelos
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/amber-spec-observers/attachments/20240420/dce0b490/attachment.htm>