Question on returning instances of a (not exported) derived class

Peter Levart peter.levart at
Fri Aug 26 09:21:47 UTC 2016

Hi Martin,

On 08/25/2016 02:03 PM, Martin Lehmann wrote:
> I am a bit confused here, as you mention toString() . But in case [3] 
> getName() is not related to toString() – at least not that I can see.
> InternalData.getName() overwrites Data.getName() , both returning a 
> hard-coded String as result value.
> So did you mean getName() instead of toString() ? I.e. did you mean:
> Ø...because the compiler notices the static type of the 
> ***.getName()*** target is InternalData which declares the 
> ***getName()*** method (and overrides ***Data::getName*** method). So 
> InternalData should be accessible at runtime for this invocation to 
> succeed and at compile time for compilation to succeed.
> If you did not mean this, I would be lost here... ;-)

Yes, you're right. I did mean to type "getName", but my fingers typed 

The compiler references the most specific type that declares a 
particular virtual method in the emited invokedynamic instructrion. In 
this case, it is InternalData which is not exported to the module 
performing the invocation. Hence the compile-time error.

The compiler could choose to reference the most generic type that 
declares a virtual method with no effect on the final runtime behavior 
(in this case, it would be Data), but I think the strategy to reference 
the most specific type is meant to facilitate a binary-compatible 
change: in your case, removing getName() method from Data and separate 
compiling it is a binary compatible change. If the compiler choose the 
most generic method's declaring class, it would not be, I think.

Regards, Peter

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