Exception handling in switch (Preview)

[Brian Goetz]

To the question “does Java need this”, well, of course we don’t *need* it; we do have try-catch statements.  But as Dan points out, the main challenge of using methods that both return a value and throw exceptions is that we cannot handle all the results uniformly.  And as the JEP points out, backing off to try-catch is doubly painful: not only do we have to use two constructs, but we are back to statement-land, which is far more error prone and less composable than expressions.

As to “why don’t we just make try-catch an expression”, well, that’s where we started with this feature exploration.  It turns out to just be too weak to be useful.  The main constraint is that the try and catch parts have to yield the same type, but the constraint to produce a “default” value of that type from the catch arms is just too constraining.  If you try some nontrivial examples this becomes clear pretty quickly.

But there is one criticism of this feature that is I think at the root of what you are getting at, which is that having effect cases in switch produces only a shallow-ish unification (or, alternately, that it is not the primitive.)  Having effect cases lets us deal uniformly with all the consequences of evaluating the selector uniformly in one place, but like try-catch, you have to deal with them _right there_.  Whereas with a Try monad, you could capture the Try and then process it by passing it to another method, putting it on a queue and letting some other thread process it, etc.

If we were to excavate to the bottom, then we would likely want `try e` to evaluate to a Try monad, at which point the current proposal is a sugary representation of:

    switch (try e) {
        case Success(P1) -> …
        case Success(P2) -> …
        case Failure(E1) -> …
    }

We explored this point as well in the exploration, and backed off.  But we can do this in either order; if we have a `try e` primitive that evaluates to a Try, then we can retroactively redefine a switch with `case throws` clauses to be sugar for the above.

I will think further about the pattern matching connection you propose.

On Apr 20, 2024, at 10:00 AM, Tagir Valeev <amaembo at gmail.com<mailto: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
}

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<mailto: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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