Possible compiler bug

[Stefan Reich]

I understand, thanks

On Tue, 22 Oct 2019 at 13:25, Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> This is what is specified in the JLS 4. (raw types):
>
> https://docs.oracle.com/javase/specs/jls/se13/html/jls-4.html#jls-4.8
>
> More specifically, the combinations of these two bullets:
>
> "
> The superclasses (respectively, superinterfaces) of a raw type are the
> erasures of the superclasses (superinterfaces) of any of the
> parameterizations of the generic type.
>
> The type of a constructor (§8.8), instance method (§8.4, §9.4), or
> non-static field (§8.3) of a raw type C that is not inherited from its
> superclasses or superinterfaces is the raw type that corresponds to the
> erasure of its type in the generic declaration corresponding to C.
> "
>
> Lead to the behavior you observed.
>
> I agree that the behavior is surprising, and this has been pointed out in
> the past. The existing rules are designed to prevent issues when a member
> of the superclass refer to some of the type variable (if the subclass is
> raw, the type parameters might be missing!). In cases such as yours, this
> rule works against you - in the sense that there's no reason to erase the
> type of 'map' since it doesn't refer to any type variable of Base. But that
> said, we looked at this issue several times in the past and come away with
> a feeling that determining in which case the compiler should erase vs.
> should _not_ erase is trickier than it seems in the general case.
>
> Maurizio
>
>
> On 22/10/2019 12:19, Stefan Reich wrote:
>
> I was confused because the map's type parameters have nothing to do with
> either the base, derived or enclosing classes.
>
> Be that as it may, your fix works. Thanks!
>
> On Tue, 22 Oct 2019 at 13:18, Stefan Reich <
> stefan.reich.maker.of.eye at googlemail.com> wrote:
>
>> Hmm... so you're saying all the fields in a base class lose their type
>> parameters when I inherit from the raw base class? That's quite surprising
>> actually.
>>
>> On Tue, 22 Oct 2019 at 13:16, Maurizio Cimadamore <
>> maurizio.cimadamore at oracle.com> wrote:
>>
>>> The problem is that Derived.Inner extends Base.Inner - which is a raw
>>> type.
>>>
>>> Because of that, Derived.Inner does not inherit 'map' with type
>>> Map<Integer, Referenced> as you expect, but instead it inherits it raw
>>> (Map). Hence the error.
>>>
>>> To fix, you need to replace the extends clause of Derived.Inner to say
>>> "extends Base<Derived.Inner>.Inner"
>>>
>>> Maurizio
>>> On 22/10/2019 12:11, Stefan Reich wrote:
>>>
>>> Hi, ok, the class relationships in this example are bit convoluted (I
>>> have them for a reason in the original source code), but I still don't see
>>> why it would fail to compile.
>>>
>>> import java.util.Map;
>>> class Referenced {
>>> }
>>> class Base<A extends Base.Inner> {
>>>
>>>     class Inner {
>>>         Map<Integer, Referenced> map;
>>>
>>>         void bla() {
>>>             Referenced r = map.get(0);
>>>         }
>>>     }
>>> }
>>> class Derived extends Base<Derived.Inner> {
>>>     class Inner extends Base.Inner {
>>>         void bla() {
>>>             Referenced r = map.get(0);
>>>         }
>>>     }
>>> }
>>>
>>> The error message is:
>>>
>>> Referenced.java:20: error: incompatible types: Object cannot be
>>> converted to Referenced
>>>             Referenced r = map.get(0);
>>>
>>> Why does the map suddenly lose its type parameters?
>>>
>>> Many greetings,
>>> Stefan
>>>
>>> --
>>> Stefan Reich
>>> BotCompany.de // Java-based operating systems
>>>
>>>
>>
>> --
>> Stefan Reich
>> BotCompany.de // Java-based operating systems
>>
>
>
> --
> Stefan Reich
> BotCompany.de // Java-based operating systems
>
>

-- 
Stefan Reich
BotCompany.de // Java-based operating systems
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