jls overridding description

[konstantin barzilovich]

Hello,

Thank you for the answer.
As I understand, you said that jls-8.4.8.1-100-B can be false while 
jls-8.4.8.1-100-C is true.
But is it possible that jls-8.4.8.1-100-C is false while 
jls-8.4.8.1-100-B is true? (the opposite situation)
I think, that the only reason to C doesn't inherit ma is that there is 
another method mc with subsignature of ma.
Speaking in short I think that jls-8.4.8.1-100-B is more general, then 
jls-8.4.8.1-100-C.

Thanks,
Konstantin.


On 20.05.2015 0:27, Dan Smith wrote:
>> On May 6, 2015, at 9:27 AM, konstantin barzilovich <konstantin.barzilovich at oracle.com> wrote:
>>
>> Hello,
>>
>> In Chapter 8 of jls there is assertion jls-8.4.8.1-100
>> "An instance method mc, declared in or inherited by class C, overrides from C another method ma, declared in class A, iff all of the following are true:"
>>   which describe conditions of overriding of classes methods. As I understand, one of jls-8.4.8.1-100-B
>> "C doesn't inherit ma"
>>   and jls-8.4.8.1-100-C
>> "The signature of mc is a subsignature of the signature of ma"
>>   is redundant because if we omit, for example, jls-8.4.8.1-100-C,
>> no cases will be missed.
> You're appealing to this (8.4.8):
>
> "A class C inherits from its direct superclass all concrete methods m (both static and instance) of the superclass for which ... no method declared in C has a signature that is a subsignature (§8.4.2) of the signature of m."
>
> and
>
> "A class C inherits from its direct superclass and direct superinterfaces all abstract and default (§9.4) methods m for which ... 	no method declared in C has a signature that is a subsignature (§8.4.2) of the signature of m, [and] no concrete method inherited by C from its direct superclass has a signature that is a subsignature of the signature of m"
>
> And, additionally, this (8.4.2):
>
> "It is a compile-time error to declare two methods with override-equivalent signatures in a class."
>
> So, how can 'mA' be inherited by C, yet 'mC' have a subsignature of the signature of 'mA'?  First, 'mC' must not be declared in C.  Second, the declaration of 'mC' must not be override equivalent with 'mA'.
>
> Typically, you get there with generics:
>
> abstract class A<T> {
>      abstract void m1(T arg);
>      abstract void m2(String arg);
> }
>
> abstract class C extends A<String> {}
>
> Now m1 and m2 are both inherited by C and have the same signature (in C).  But we do not want to claim that one overrides the other -- they just coexist in the same class (see also 8.4.8.4 and 15.12.2.5).  You could also get there with two concrete methods, although that would be an error.
>
> (BTW, sorry for not seeing this sooner.  I don't always read every post to compiler-dev, so feel free to ping me if there's something like this you're waiting for a response on.)
>
> —Dan