<div dir="auto">Could your initial example be directly addressed by 8.4.8.4?</div><div dir="auto"><br><div class="gmail_quote gmail_quote_container" dir="auto"><div dir="ltr" class="gmail_attr">On Mon 10 Feb 2025 at 09:44, Attila Szegedi <<a href="mailto:szegedia@gmail.com">szegedia@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div dir="ltr">
Hi both Chen and Pavel,</div><div dir="ltr"><br></div><div dir="ltr">Thank you both for answering! I was not specifically asking Brian, of course, that’s why there’s a wink next to it :-)</div>
<div dir="ltr">I now scoured JLS 23, specifically:</div><div dir="ltr"><br></div><div dir="ltr"><div dir="ltr">----</div><div dir="ltr">8.1.1.1. abstract Classes</div><div><br></div><div><div>A class C has abstract methods if either of the following is true:</div><div><br></div><div>Any of the member methods (§8.2) of C - either declared or inherited - is abstract.</div><div><br></div><div>Any of C's superclasses has an abstract method declared with package access, and there exists no method that overrides the abstract method from C or from a superclass of C.</div></div><div><br></div><div><div>8.2. Class Members</div><div><br></div><div>The members of a class are all of the following:</div><div><br></div><div>Members inherited from its direct superclass type (§8.1.4), except in the class Object, which has no direct superclass type</div><div><br></div><div>Members inherited from any direct superinterface types (§8.1.5)</div><div><br></div><div>Members declared in the body of the class (§8.1.7)</div></div><div><br></div><div><div>8.1.5. Superinterfaces:</div><div><br></div><div>Each default method (§9.4.3) of a superinterface of the class may optionally be overridden by a method in the class; if not, the default method is typically inherited and its behavior is as specified by its default body.<br></div></div><div>-----<br></div><div dir="ltr"><br></div><div dir="ltr">And nothing in there tells me unambiguously that my example should’t work (the last quoted sentence from 8.1.1.1 is weird, but adding “public” to declaration of “m” circumvents it and it still doesn’t compile.) There are few phrases that seem open to interpretation though: "default method is _typically_ inherited” (typically is doing heavy lifting here: sometimes it isn’t?) and “_any of the member methods_ (§8.2) of C - either declared or inherited - is abstract" (are A.m() and F.m() treated as separate member methods at this stage despite identical signatures?) </div><div dir="ltr"><br></div><div dir="ltr">Again, I’m not arguing current javac behavior is wrong, just trying to understand why is it right. I actually hit this in an IRL refactoring – had several classes that both extended an abstract class (A) and implemented an interface (F), and all these classes had the same implementation for an abstract method (m) that was actually semantically related to F and it felt like a good example of DRY to implement the method only once as a default in F. (Yes, I could create an interim class AF and move the implementation there, but this felt like it would’ve been a good use of an interface as a mixin.)</div><div dir="ltr"><br></div><div dir="ltr">I wonder if I recreated the example in bytecode directly whether the JVM would load the class successfully (I might need to attempt to also call the method) or would I get some kind of IncompatibleClassChangeError.</div></div></div><div><div dir="ltr"><div dir="ltr"><br></div><div dir="ltr">Attila.</div><div dir="ltr"><br></div></div>
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<div dir="ltr" class="gmail_attr">On 2025. Feb 10. at 1:58:32, Chen Liang <<a href="mailto:liangchenblue@gmail.com" target="_blank">liangchenblue@gmail.com</a>> wrote:<br></div>
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<div dir="ltr">Hello guys, amber is for new language features. Questions with java compilation or JLS/JVMS belong to the compiler group.<div><br></div><div>That said, I believe this behavior is most likely a preservation of method resolution, that for instance methods, methods from superclass have higher priority than methods from interfaces. For example, before Java 8, static methods could not be defined in interfaces; and when they were added in 8, resolution rule states that they aren't shadowed to child types, so classes cannot "inherit" interface static methods, unlike interface static fields.</div><div><br></div><div>Another fun fact is that superinterface fields take precedence in resolution over superclass fields. Also a legacy from old Java, but I don't think that is going to be changed to make things "consistent".</div><div><br></div><div>Regards,</div><div>Chen Liang</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Feb 9, 2025 at 4:26 PM Pavel Rappo <<a href="mailto:pavel.rappo@gmail.com" target="_blank">pavel.rappo@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Attila,<br>
<br>
Come to think of it, we should use some other, more narrowly focused,<br>
mailing list. Apologies to subscribers of this list. Since lambda-dev<br>
is defunct now, perhaps we could use compiler-dev, amber-dev, or<br>
core-libs-dev instead. I'm voting for amber-dev, where Brian mostly<br>
resides these days.<br>
<br>
-Pavel<br>
<br>
On Sun, Feb 9, 2025 at 10:12 PM Pavel Rappo <<a href="mailto:pavel.rappo@gmail.com" target="_blank">pavel.rappo@gmail.com</a>> wrote:<br>
><br>
> You are not specifically asking Brian, are you? If so, sorry for replying.<br>
><br>
> My humble take is that the default method is something to use when<br>
> there's nothing else there. When there's something, even if it's<br>
> abstract, we use that. So, a default method is a minimally interfering<br>
> fallback/extension scenario, if you will.<br>
><br>
> That abstract A.m() effectively replaces default F.m() in C as if<br>
> F.m() were never there. If F.m() were allowed to provide<br>
> implementation for A.m() in C, then some uncompliable code could<br>
> suddenly become compilable and operational at runtime.<br>
><br>
> -Pavel.<br>
><br>
> On Sun, Feb 9, 2025 at 9:04 PM Attila Szegedi <<a href="mailto:szegedia@gmail.com" target="_blank">szegedia@gmail.com</a>> wrote:<br>
> ><br>
> > Hey folks,<br>
> ><br>
> > I found a somewhat puzzling behavior: default interface methods aren't considered abstract method implementations. Trying to compile this minimal reproducer:<br>
> ><br>
> > ------ C.java ------<br>
> > abstract class A {<br>
> > abstract void m();<br>
> > }<br>
> ><br>
> > interface F {<br>
> > default void m() {}<br>
> > }<br>
> ><br>
> > class C extends A implements F {}<br>
> > ------ C.java ------<br>
> ><br>
> > $ javac C.java<br>
> > C.java:9: error: C is not abstract and does not override abstract method m() in A<br>
> > class C extends A implements F {<br>
> > ^<br>
> > 1 error<br>
> ><br>
> > I can accept this being valid according to JLS today (I tried with 11, 21, and 23.) I admit this is one of rare occasions when I didn't go scouring the JLS to figure out what exactly prevents the compiler from accepting F.m() as implementation of A.m() in C. I'm wondering though if this isn't something that could be improved upon in the future. (I'm sure there's a gotcha somewhere. Right, Brian? ;-) )<br>
> ><br>
> > Have a great day,<br>
> > Attila.<br>
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