Submitting a JEP

Mario Torre neugens.limasoftware at
Fri Oct 6 15:14:30 UTC 2017

I'm reading the proposal a bit and I don't think it's a very good idea
to introduce another keyword.

While it may be nice to have such statement, effectively you are
promoting bad programming use cases.

If we take the stack overflow link as an example, it's clear that the
developer could simply rearrange his code to be more stable by using
some of the advanced features of Java like classes and inheritance ;)
or at worse by just documenting the need to add the return after the
else branch

In all the other examples you give in the proposal, unless I'm
misreading them, it seem that they are not unreachable statements in a
sense that can the statically defined, but are simply cases that the
developer "knows" they won't happen, but they may, and so they can be
solved by an assert or by a runtime exception.

So the only use case I see is the stack overflow one, but it seems to
me this is just bad code practices, which is branching points with
multiple return statements, that is always asking for troubles.

In the short term future we will get enhanced switches with patterns,
which turns switches into expression, and it makes the stack overflow
example a lot easier to write without side effects.


2017-10-06 16:30 GMT+02:00 Daniel Trebbien <dtrebbien at>:
> On Fri, Oct 6, 2017 at 7:24 AM, David Lloyd <david.lloyd at> wrote:
>> On Thu, Oct 5, 2017 at 10:17 PM, Brian Goetz <brian.goetz at>
>> wrote:
>> > The proposal is a bad fit for the assertion facility, since the assertion
>> > facility is designed to be disabled, and what you want is something that
>> > can't be disabled.
>> I believe that in the document, Mr. Trebbien explicitly explained why
>> the assertion facility is a bad fit, and was specifically not
>> suggesting it be used, but rather using it as an example and template
>> for the new feature.
> Specifically, in the Alternatives section (
> ), I discuss why neither allowing the proposed "unreachable assertions" to
> be disabled, nor amending §14.21 Unreachable Statements to allow
> unreachable `assert false' statements, is preferred.
> The JLS specifies in §14.10 that an assertion is either enabled or
> disabled.  However, the JLS does not specify how assertions are enabled or
> disabled.
> Unless I am mistaken, there is no standard way to guarantee that assertions
> compiled into a CLASS file will be disabled at runtime.  Therefore, unless
> the distributor of Java software uses some sort of post-compilation
> assertion stripping from CLASS files, the programmers must assume that
> assertions might be enabled at runtime.
>> But here's an alternative:
>> >
>> >     throw new UnreachableAssertionException("why");
>> >
>> > This doesn't need a language feature.
>> It does, because you cannot put "throw" statements in places where the
>> compiler has statically determined that control flow cannot reach,
>> which is a key part of the justification of the feature described in
>> the document.
>> This is a feature that I for one have sorely missed for many years
>> now, especially in more complex state-oriented code where exit points
>> cannot be made visually clear.
> Yes.  §14.21 Unreachable Statements requires unreachable statements to be
> compile-time errors, so you currently cannot place a statement like `throw
> new UnreachableAssertionException("why");' at points defined to be
> unreachable.

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