JLS tweaks

Archie Cobbs archie at dellroad.org
Tue Mar 11 18:10:03 UTC 2014

Just a friendly reminder... still waiting on input regarding the questions
below (which were in response to Brian's earlier email).


On Mon, Mar 3, 2014 at 2:55 PM, Archie Cobbs <archie at dellroad.org> wrote:

> Brian,
> Thanks for your comments.
> On Mon, Mar 3, 2014 at 11:45 AM, Brian Goetz <brian.goetz at oracle.com>wrote:
>> *Idea #2: Allow non-this code prior to super()/this() call in
>> constructors*
>>  This is well-traveled ground; this request has come up, and been shot
>> down, many times.  If anything, the language spec is already too loose
>> here, allowing too many unsafe operations (e.g., calling virtual methods
>> from constructors, allowing constructor to publish 'this', etc) and this
>> would make this problem dramatically worse.
> I would like to better understand what you're saying here. I am certainly
> not interested in changes that would weaken language safety, so I must be
> misunderstanding something.
> When you refer to calling virtual methods and publishing 'this', do you
> mean prior to superclass constructor invocation? If so how is that possible
> currently? Because the JVM spec seems to disallow<http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms->it..  I'm referring to the sentence "Before that method invokes another
> instance initialization method of myClass or its direct superclass on this,
> the only operation the method can perform on this is assigning fields
> declared within myClass." I take this to mean that for example that you
> couldn't pass 'this' as a parameter, assign it to a field, invoke a method
> on it, or do anything else that would allow its uninitialized escape.
> Or, if you meant calling virtual methods and publishing 'this' after
> super()/this(), then how does this proposal have any relation to those
> problems? They are on the "other side" of super()/this() line, so to speak,
> and this proposal would not make them any more permitted than they already
> are.
> It seems like the JVM spec gets it right - i.e., you can do (mostly)
> whatever you want prior to super()/this() as long as it doesn't try to make
> use of an uninitialized 'this', and no uninitialized 'this' can escape. It
> seems reasonable to want the JLS to more closely mirror this sensible
> stance.
> I must be missing something...
> *Idea #3: Allow generic type declarations wherever normal type
>> declarations are allowed*
>> Obviously "wherever normal type declarations" is broader than you mean.
>> What you're looking for is the ability to "uncapture" a captured variable,
>> giving it a name.  As you've pointed out, generic methods already provide
>> you the ability to do that, and what you're asking for is do to that
>> 'inline' in a block of code.  My recommendation is to try and take this to
>> the next step (a more detailed analysis); I think you'll find you'll run
>> into complexity at multiple levels.
> I may not be smart enough to actually reach those multiple levels of
> complexity... :) Can you give me an example?
> In case it wasn't already clear... the original simplistic thought was
> that at any variable declaration where the declared type is generic, you
> could have the option of piggybacking onto that a generic type variable
> declaration.
> So any:
>    final List<?> x = ...
> could be changed to:
>   final <T> List<T> x = ...
> and the T would be declared as a bound type variable with the same scope
> as 'x'.
> *Idea #5: Allow enum types to have generic type parameters*
>> Another frequent suggestion.  I agree it would have been better if it
>> were done this way initially, and the cost would have been low.  The cost
>> of doing this now in a binary-compatible way would be higher.
> Ah, I didn't realize there was a binary compatibility problem here. What
> specifically is the problem?
> Thanks,
> -Archie
> --
> Archie L. Cobbs

Archie L. Cobbs
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