SAM identity or at least equals/hashCode

[Rémi Forax]

On 06/21/2011 02:16 PM, Peter Levart wrote:
> On 06/21/11, Rémi Forax wrote:
>>>> If you want to refer to the object instance that is the result of SAM
>>>> conversion, call that the SAM instance or the SAM object -- which is an
>>>> object.  (You can't assume much about its identity -- the compiler will
>>>> likely make sure that all instances of "stateless" lambdas are really
>>>> the same object, a la the "Flyweight" pattern.)
>>>>
>>> Only stateless? Why?
>> The idea is that stateless lambda can be constant folded by the compiler
>> + VM.
>> If lambda are stateful, you just can't.
> Can't fold it and assign it to a static final field by itself, yes. But if some meaningfull equals/hashCode is defined either on the SAM instances or somewhere private in the runtime (not exposed via SAM's equals/hashCode), then a canonicalizing cache of weakly referenced instances of SAM objects can be used by lambda expression ->  SAM conversion.
>
>>> I might have missed that and I'm sure there has been discussion already on this list about the topic, but it was a long time ago. So I thought to ask whether there are any news.
>>>
>>> The question is: Will SAM objects that lambda expressions are going to be converted to, implement any meaningfull equals/hashCode methods or are those methods going to be inherited from Object (or SAM class).
>> In the current spec, SAM is currently restricted to interface, so
>> equals/hashCode are inherited from Object.
> I see the solution as either one of:
>
> - keep equals/hashCode in SAM instances inherited from Object (i.e. identity compare) and provide a private canonicalizing cache of weakly referenced SAM instances as part of runtime.
>
> - don't burden the runtime with a canonicalizing cache of weakly referenced SAM instances, but provide meaningfull equals/hashCode methods on SAM instances that would delegate those two methods to captured state and to the identity of the code that executes lambda body.
>
> those two approaches can be combined (like in java.lang.Integer - cache only stateless lambdas on static final fields inside classes that define methods that execute lambda codes for example, and provide a meaningfull euals/hashCode too)
>
>> Rémi
> Regards, Peter

Interesting, does a computation has a meaning ?

rgds,
Rémi