Defender Extension Methods -- Resolution and Invoccation

[Brian Goetz]

There's currently no way to specify an extension method with no default.  But 
you could easily simulate that with a default that throws UOE.

Resolution is fully dynamic.  Let's say you have the following:

interface I {
     public extension void foo() default Moo.foo();
}

class C implements I { }

class Client {
     public void yada() {
         C c = new C();
         c.foo();
     }
}

At compile time, no non-default implementation exists.  But if C were 
recompiled to provide an implementation (or I is recompiled to provide a 
different default), at runtime that would be reflected in the behavior.

On 8/4/2010 5:49 PM, Mike Duigou wrote:
>> From Maurizio's Sytax samples Doc:
>
>> 3. Defender Methods
>> -------------------
>>
>> Defender methods are declared using the 'extension' keyword, as follows:
>>
>> extension List<T>  map(Mapper<T>  r) default Collections.<T>listMapper;
>>
>> A defender method declaration is structured in two parts, (i) a method signature, declaring formal arguments, formal type-arguments and return-type and (ii) a default implementation, that is, the implementation the method should default to if none is found in the class implementing the extended interface.
>
> Have you considered extensions with no default? I understand that the defender is primarily intended to support interface evolution but providing a default implementation isn't always practical. I'd prefer to see a "missing implementation of xyz" compile error to a usage pattern which involves default methods who's only purpose is to generate a runtime exception for the unsupported operation.
>
> Is the resolution of the extension method intended to occur at compile time or at invocation time? ie. will compiled code which refers to the extension method include a call to the named method through the interface or a call to the static default method. For greatest flexibility in interface evolution it would be much more useful if the resolution happens at the time the method is invoked.
>
> Mike
>