Multiple this$1 fields when mixing subclassing and inner classes

[Stefan Reich]

Hi David,

thanks for your answer.

> Even though they point to the same instance
the different typed fields may still be needed to ensure correct method
resolution for non-virtual invocations.

I think this is not a counterargument. We just change an access to a field
this$1 of type B into
an access to a field this$1 with type A followed by a cast to B. The result
will be the same in both cases, including the real and formal type. Method
invocation is not affected.

Many greetings :)


On Tue, 10 Sep 2019 at 00:39, David Holmes <david.holmes at oracle.com> wrote:

> Hi Stefan,
>
> On 10/09/2019 8:31 am, Stefan Reich wrote:
> >> I'm curious as to where there has ever been deliberation of this?
> >
> > Uh.. "whether", of course.
>
> Glad you made that correction :) I would hazard a guess that it has
> never been deliberated. AFAIK the representation of fields in the JVM is
> a mechanical translation of what is in the classfile, with no attempt to
> be clever about aliasing. Even though they point to the same instance
> the different typed fields may still be needed to ensure correct method
> resolution for non-virtual invocations.
>
> Cheers,
> David
>
> > On Tue, 10 Sep 2019 at 00:24, Stefan Reich <
> > stefan.reich.maker.of.eye at googlemail.com> wrote:
> >
> >> Consider these classes:
> >>
> >> class A {
> >>    class InnerA {}
> >> }
> >> class B extends A {
> >>    class InnerB extends InnerA {}
> >> }
> >>
> >> When I run some introspection on B.InnerB, I find HotSpot producing this
> >> object layout:
> >>
> >> Size of object B$InnerB: 24 bytes
> >>    Field A$InnerA . this$1 is at offset 12
> >>    Field B$InnerB . this$1 is at offset 16
> >>
> >> So the object has *two* outer references, one of type A and one of type
> B.
> >>
> >> That is all fair and good, but: These two references *will always point
> >> to the same object*. This follows from the syntactic rules on the Java
> >> level. (<< Crucial part of the argument, so - is this correct?)
> >>
> >> So, the question is: Wouldn't it save some RAM to just have one "this$1"
> >> field?
> >>
> >> The only downside would be that accesses to this$1 from code inside
> InnerB
> >> would require a cast, as the remaining this$1 field would formally only
> >> have type A.
> >>
> >> I'm curious as to where there has ever been deliberation of this?
> >>
> >> Best regards,
> >> Stefan
> >>
> >> --
> >> Stefan Reich
> >> BotCompany.de // Java-based operating systems
> >>
> >
> >
>


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