Expression switch exception naming

Kevin Bourrillion kevinb at
Thu Apr 19 23:51:05 UTC 2018

On Thu, Apr 19, 2018 at 2:50 PM, Brian Goetz <brian.goetz at> wrote:

I like Un{recognized,known}EnumConstantE{rror,xception}.

(Don't leave out "Unexpected", which might be the best one. It's not about
what I "know" because I "know" that value exists now. I just didn't know it
before, so I didn't *expect* it. Maybe "Unrecognized" isn't quite right
either, because it might seem like once you've seen a thing once at runtime
you should "recognize" it after that.)

When we get to sealed types, it will be the same but with something like
> s/EnumConstant/SealedTypeMember/.
> I am still having trouble squaring the Error vs Exception, but you've
> pulled me from "seems like an Exception to me" into "crap, now I don't
> know" territory :)
> I think what makes me uncomfortable is that there are some enums that are
> _intended_ to be extended, such as java.lang.annotation.ElementType.  (In
> fact, we might be adding a new member soon; RECORD_COMPONENT.)  And I would
> want clients of ElementType to be aware that they never know all the
> element types, and code accordingly.
> Which suggests that enums needs a mechanism to either mark them as sealed
> (which turns on the enhanced exhaustiveness behavior) or as non-sealed
> (which would turn it off).

In theory, clients of ElementType who are "aware that they never know all
of them" should still have just as much right to decide whether they *want
to be broken* when there are new ones to handle or not. But....

I think I do finally understand, thanks to your example, what is different
between this and the previous kinds of incompatible changes. The JDK (and
some libraries) makes strong promises not to break compatibility. Yet we
simply can't throw up our hands and refuse to add constants to enums like
ElementType. So either we need a way to mark it unsealed, or we have to do
some very fiddly messaging, like "well, it's binary-compatible, and it's
also source-compatible *except *for any breakages you *opted into* via
defaultless switch expressions." The trouble with that being that many
developers won't have actually consciously opted into it at all. But
*maybe* that
is a viable option?

Or, third alternative, could we just backpedal and issue *all* of these
messages as warnings instead of errors, and not need to make a distinction
between two kinds of enums? I worry that this may make the feature
relatively useless. I think that warnings that can be introduced nonlocally
are almost never addressed - most developers just don't page through
screenfuls of -Xlint output to handle it all. Where they are useful is when
they show up *in the code you are editing*, but this case would rarely work
that way.

This matters because if we were to go the all-warnings route then suddenly
this really isn't an IncompatibleClassChangeError at all - maybe you simply
ignored the warning. Might make it not even really an Error.

> On 4/19/2018 5:43 PM, Kevin Bourrillion wrote:
> Necromancing, since I noticed that the spec still contains a hole where
> this name would go.
> *Name:*
>    - I think something specific like UnexpectedEnumConstantE{rror,xception}
>    would seem the right way to go. (Perhaps "Unrecognized"?)
> *Hierarchy:  *
>    - It will want a common supertype it can share with the future
>    "unexpected subtype of sealed type" error/exception.
>    - As for where that supertype goes, I still maintain that this is
>    *exactly* an IncompatibleClassChangeError (argument below), and thus
>    should be a subtype of that. I also see nothing harmed by it being an Error
>    instead of Exception.
> My claim is that releasing an enum with a certain set of constants is
> qualitatively equivalent to releasing an interface with a certain set of
> abstract methods. We know that people key behavior off of enums (that's
> what enum switch is all about). That means that when we add a constant, we
> are adding new *contract*, which we (the enum owners) don't know how to
> fulfill. The call sites need to fulfill it.
> Thought experiment: I can already implement an interface in two different
> ways: the normal way, or via a dynamic proxy that throws an exception if it
> gets an unexpected method. Let's imagine that the latter way was made
> exactly as easy to express as the former. I think everyone would probably
> agree that most implementations would *still* choose the current
> behavior. (Yes?) They don't *want* anything to fail at runtime that could
> instead fail at compile-time.
> Anyway, all of this is just to support the notion that this should be an
> IncompatibleClassChangeError. Of course, the argument's been made in this
> thread that it *is* different from an incompatible class change. My
> response was that these reasons seem way too subtle to me. Or, have I been
> persistently missing something?
> On Fri, Mar 30, 2018 at 11:31 AM, Kevin Bourrillion <kevinb at>
> wrote:
>> On Fri, Mar 30, 2018 at 10:48 AM, Brian Goetz <brian.goetz at>
>> wrote:
>> Backing way up, Alex had suggested that the right exception is (a subtype
>>> of) IncompatibleClassChangeEXCEPTION, rather than Error.  I was
>>> concerned that ICC* would seem too low-level to users, though.  But you're
>>> saying ICCE and subtypes are helpful to suers, because they guide users to
>>> "blame your classpath".  SO in that case, is the ICC part a good enough
>>> trigger?
>> (Just to be clear, Remi and I have been advocating for a subtype of ICC
>> *Error* all along, in case anyone missed that.)
>> All right, I've been focusing too much on the hierarchy, but the
>> leaf-level name is more important than that (and the message text further
>> still, and since I assume we'll do a fine job of that, I can probably relax
>> a little). To answer your question, sure, the "ICC" is a pretty decent
>> signal. Have we discussed Cyrill's point on -observers that we should
>> create more specific exception types, such as UnrecognizedEnumConstantE{rror
>> ,xception}?
>> For an enum in the same class/package/module as the switch, the chance of
>>> getting the error at runtime is either zero (same class) or effectively
>>> zero (same package or module), because all sane developers build packages
>>> and modules in an atomic operation.
>>> For an enum in a different module as the switch, the chance of getting
>>> the error at runtime is nonzero, because we're linking against a JAR at
>>> runtime.
>>> So an alternative here is to tweak the language so that the "conclude
>>> exhaustiveness if all enum constants are present" behavior should be
>>> reserved for the cases where the switch and the enum are in the same
>>> module?
>>> (Just a thought.)
>> Okay, that is a sane approach, but I think it leaves too much of the
>> value on the floor. I often benefit from having my exhaustiveness validated
>> and being able to find out at compile time if things change in the future.
>> --
>> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at

Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at

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