Formal model for defender method resolution

Rémi Forax forax at
Fri Jan 28 12:15:09 PST 2011

I agree with Sam.
If you add javadoc, you require another semantics, perhaps just slighly 
a different one but another semantics.
Why the inherited default would match the new semantics ?


On 01/28/2011 09:08 PM, Sam Pullara wrote:
> I find it unnatural that people are overriding interfaces to add Javadocs, but I guess it happens. If I was using someones library and I looked at their interface and there was no default, I wouldn't expect there to be a default. Having to go up the interface hierarchy to see if there is a default seems surprising. It wouldn't bother me though if interface B was illegal since it isn't just repeating something that was in its super interface.
> Sam
> On Jan 28, 2011, at 11:38 AM, Brian Goetz wrote:
>> Thinking more, the logical thing to do is require that a covariant
>> override of an extension method in an interface must also provide a new
>> default.  The cases where the old default will still be OK (even when it
>> is type-correct, which is already rare) under a covariant override seem
>> pretty limited.
>> However, it is a common behavior currently to override a method in an
>> interface solely to provide new Javadoc (or annotations).  Currently
>> these are a no-op from a language semantics perspective, and I believe
>> this is a behavior we should continue to respect.  Hence the current
>> implementation (and formal model) interprets
>> interface A { T m() default k }
>> interface B extends A { T m() }  // non-covariant override
>> as B inheriting A's default, rather than B overriding A's default with
>> some magic "no default".  (This is different from the "obvious" analogy
>> to concrete classes, where it is permitted to reabstract a concrete
>> method.  But such reabstraction for defaults would introduce a lot of
>> additional complexity for relatively little value.)
>> Some people find this "sub-inheritance" (inheritance of part of a
>> signature) disturbing, because there is no precedent for it in Java.
>> What do people think?  Do you find this interpretation of B (m()
>> continues to use A's default) confusing or natural (or both)?
>> On 1/24/2011 11:26 AM, Brian Goetz wrote:
>>> Dan Smith has pointed out a hole in the Featherweight Defenders
>>> document; there is an unsound disconnect introduced when we allow
>>> covariant overriding but don't require the implementation to be replaced
>>> (rules T-IntNoDef and T-ClassAbs).  This would allow the following failure:
>>>     k1 : Object
>>>     intf A { Object m() default k1 }
>>>     intf B extends A { String m() }
>>> Here, the default for A is going to produce a result that does not meet
>>> B's interface contract.  I believe what is needed is additional
>>> constraints on T-IntNoDef to include a
>>>     \Gamma(mdef(I_i))<: T
>>> constraint (for the cases where mdef(I_i) is not nil).
>>> I think this is all that is needed to plug this hole.
>>> On 1/19/2011 12:48 PM, Brian Goetz wrote:
>>>> At
>>>> I have posted a draft of a formal model for resolution of defender
>>>> methods.  This is written in the style of "Featherweight Java"
>>>> (Igarashi, Pierce, et al), in which a number of real-world language
>>>> concerns are abstracted away, in order to simplify the formalism for the
>>>> portion of the language of interest, notably the typing and resolution
>>>> of defender methods.
>>>> Hopefully this will serve as a basis for discussion of the proposal.
>>>> The T- and S- rules are implemented by the compiler and are used for
>>>> typing; the R- rules are implemented by the VM to do method selection.
>>>> (The primary computed item of interest is mres(C), which is the method
>>>> resolution for a given class -- while this is not used in any further
>>>> production (it would be used if the operational semantics were
>>>> specified), it is in fact the whole point of this exercise.)

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