Defaut methods are not visible if -source 1.7 is set

[Joe Darcy]

On 10/29/2013 10:48 AM, Brian Goetz wrote:
> We discussed this extensively at some point; its not as simple as 
> this, though I admit the details are paging in slowly.
>
> IIRC, the root problem is this: you have a class
>
> class Foo extends List { }
>
> that you want to compile with the 1.8 javac but with -source 1.7. What 
> could happen?
>
> 1.  You could fail to compile, because the supertype uses language/VM 
> features you don't support.  This would be not very nice.
>
> 2.  You could force the user to implement the default methods, since 
> we can't assume they're inherited.  This is also not very nice.
>
> 3.  We could ignore them.
>
> The root problem is that -source 1.7 still exposes 1.8 libraries to 
> the compilation, which is just wrong.  What should happen is we should 
> be compiling with the fictitious -platform 1.7, which not only 
> enforces the 1.7 language level, but also puts the 1.7 JDK classes on 
> the bootclasspath.
>
> Eventually, modularity was supposed help here.  At the time, (3) 
> seemed the least bad alternative.  But this is indeed a problem for 
> methods that acquire defaults.
>
>> spec shoud be changed to say that adding a default implementation
>> is not a compatible change
>
> There's no way we're doing this to work around a tooling bug.

IMO, the current behavior of javac is the least-bad option.

(The issue before was how AnnotatedElement.isAnnotationPresent should be 
handled. It was a Java SE 5 method on AnnotatedElement that was turned 
into a default. The behavior of javac meant that the method 
"disappeared" from the implementations of the interface, like 
java.lang.Class, for code being compiled under earlier source versions. 
We addressed this by adding back concrete implementations in the 
interface implementations that called back to the default method. That 
means the method appears present in all source versions and we still get 
some sharing of the implementation code.)

There is no specification that governs how a compiler should present 
new-in-8 language features when compiling under a older-than-8 source 
level. More generally, the -source and -target options are not required 
parts of the platform specification, just a convenience to developers.

-Joe