Context inside default method body

[Dan Smith]

This is closely tied to the discussion about default methods overriding the methods of java.lang.Object.  (See the recent "Defaults for Objects' methods" thread).

As it stands right now, the members of interfaces include certain methods that look like public Object methods (see JLS 9.2), but interfaces do not actually inherit anything from Object.  You can see this by trying to invoke 'clone' on a variable of an interface type, like 'Runnable'.  The error message says that member doesn't exist.  (Of course, if it did exist, you'd get an error anyway, due to accessibility constraints.  So in 7 it's mostly possible to imagine that interfaces _do_ inherit everything from Object, unless you're writing code in the java.lang package.)

Among other things, a problem with inheriting things from 'Object' is that it allows interfaces to have non-public members.  That opens a small can of worms.

Anyway, thanks for highlighting the behavior.  It's on the radar, and it's useful at least to know that somebody noticed this and cares about it.

—Dan

On Jul 20, 2012, at 3:00 AM, Maurizio Cimadamore wrote:

> Nice catch - the accessibility rules for 'protected' in the compiler are 
> oblivious to extension methods. I don't see any change in the spec draft 
> regarding this - but it feels like something that should be 
> special-cased for the sake of consistency.
> 
> Maurizio
> 
> On 20/07/12 06:43, Deepak S Patwardhan wrote:
>> Hi all,
>> 
>> The following doesn't compile with the latest build of lambda :
>> 
>> public interface Islander {
>> 
>>     public void replaceLiver() default {
>>         try {
>>             Object myClone = this.clone(); //.. Compile error
>>             //.. get the replacement from myClone
>>         } catch (CloneNotSupportedException cnse) {
>>             throw new UnsupportedOperationException();
>>         }
>>     }
>> }
>> 
>> Basically, I cannot access clone(), which is protected in Object, in spite
>> of having the *this* reference.
>> 
>> I think that's ok and not a big deal since Object has only two protected
>> methods, and the other method (finalize) shouldn't be invoked by anyone,
>> except the GC.  But, it just feels like a minor anomaly that default bodies
>> can do everything that normal (ie Class) methods can do, except that they
>> cannot clone their callees. (Don't hold limitations of Interfaces against
>> them, like access to instance variables)
>> 
>> Regards,
>> Deepak S Patwardhan.
>> 
>> 
> 
> 
>