Exception handling in switch (Preview)

[Clement Cherlin]

On Sat, Apr 20, 2024 at 3:01 AM Tagir Valeev <amaembo at gmail.com> wrote:

> 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
> }
>
>
There are some straightforward alternatives. To avoid long lines, wrap the
switch expression across multiple lines. To refactor the switch expression
across multiple statements for clarity, use a private method. I write this
sort of thing all the time:

private D abcd(A a) {
  var b = a.b();
  var c = b.c();
  return c.d();
}

...

switch (abcd(a)) {
  case ...
  case throws RuntimeException ex -> handle(ex);
}


> 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?
>

I agree *in part* with this sentiment. Many library methods that currently
throw exceptions would be better written as member patterns. However, there
will always be a need to handle exceptions thrown by code that works with
the outside world (databases, network, filesystem). These are often
libraries and frameworks we do not control. Having "case throws" available
would be a significant improvement for those situations.

switch (getEntityFromNetworkDatabase(id)) {
  case SomeEntity entity -> ...
  case DatabaseException ex -> ...
  case NetworkException ex -> ...
}

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
>>
>
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