Default method survey results

Rémi Forax forax at
Thu Aug 16 06:15:55 PDT 2012

On 08/16/2012 11:47 AM, Maurizio Cimadamore wrote:
> On 16/08/12 10:13, Alex Blewitt wrote:
>> The point of confusion - where the method is coming from - is at use
>> site of the interface, not declaration site. What keywords you use at
>> the declaration site (probably written by someone else) will make
>> absolutely no difference to the person who implements that interface,
>> keyword or not.
> How do you get to know the set of methods that have to be implemented
> when you implement an interface? Probably you go and look at the Javadoc
> for that interface. Then you see a list of methods, some of which have
> the D-word; you know that, at the very least (i.e. not to get
> compile-time errors) you have to implement all the remaining ones (w/o
> the D-word).
> But I think we are a bit down into a syntactic rat-hole here - the
> original question was - why not allowing a third option in the survey,
> one that mandates no keywords at all to denote an 'interface method with
> body'. The motivations behind the choice were several, including:
> *) 'default' will make the interface code easier to parse
> *) a keyword will provide an handy 1-1 mapping between source code and
> Javadoc

but there will be no 1 to 1 mapping with the JVM bytecode,
there is no default modifier in the generated class file as far as I know.

And for javadoc, it's often better to present information in a different 
way than the one given by the source,
by example, you have all direct subtypes of a type even if you don't 
declare them in the source.

> *) 'default' is the dual of 'abstract' - it must be specified in
> interfaces the same way as 'abstract' must be specified in classes

see Peter Levart answer, default is not the dual of abstract.
The dual of abstract method is concrete method, which up to now, doesn't 
require any keyword.

> *) the presence of a keyword will help extending the syntax even more by
> adding new default forms

the EG has rule out any extra syntax other than the classical syntax to 
declare a method body.

> *) 'default' summarises the semantics pretty well

We already have 'abstract' that mean should be implemented,
why do you want a keyword with the meaning, non-abstract but only in the 
case where the method is defined in an interface.

Moreover, non abstract method in interface change the way the VM 
populates the class vtable by using code of method declared in interfaces,
so the semantics you're talking about, the semantics that change, is the 
semantics of 'implements',
not the semantics of the non-abstract method by itself. (cf Stephen 
Colebourne reply)

> What are the motivations behind leaving the 'default' keyword out? Note:
> it must be more than just avoiding to type 'd' 'e' 'f' 'a' 'u' 'l' 't' -
> which, as you say, probably somebody else would have typed in for you ;-)
> Maurizio


More information about the lambda-dev mailing list