Vritual extension methods

[Deepak S Patwardhan]

Brian,

I have gone through the State of Lambda, 4th edition, but don't think it
answers my questions. Let me rephrase them.

C1: public Complex subtract(Complex b) default { return
this.add(b.negate()); }
C2: public Complex divide(Complex b) default Complexes.divide;

1) Why have we broken the rule - interfaces don't have code ? Is the
construct  C1 strictly more powerful than C2 ?
2) Why are we using . (dot) in C2 instead of :: (colon colon) - the choice
for method references in lambda expressions ?

Regards,
Deepak S Patwardhan.

-----Original Message-----
From: Brian Goetz [mailto:brian.goetz at oracle.com] 
Sent: 04 July 2012 21:18
To: Deepak S Patwardhan
Cc: lambda-dev at openjdk.java.net
Subject: Re: Vritual extension methods

See the relevant section of State of the Lambda, 4th edition, which
supersedes this.

On 7/4/2012 10:37 AM, Deepak S Patwardhan wrote:
> Hello,
>
>
>
> The Defender methods v4 document (Brian Goetz, June 2011) has two 
> interesting foot notes (page 2)
>
>
>
> 5. It is a deliberate choice to not simply allow the programmer to 
> include the code of the default inline
>
> within the interface. Among other reasons, it would violate the 
> long-held rule that "interfaces don't have
>
> code".
>
>
>
> 6. The syntax of specifying the default implementation should match 
> that of specifying method references, if
>
> method references are to be added to the language.
>
>
>
> Consider the following code (which compiles successfully with build 
> 39)
>
>
>
> /* Complex.java */
>
> /** Represents Complex numbers */
>
> public interface Complex {
>
>
>
>      public Complex add(Complex b);
>
>
>
>      public Complex negate();
>
>
>
>      public Complex subtract(Complex b) default {
>
>          return this.add(b.negate());
>
>      }
>
>
>
>      public Complex multiply(Complex b);
>
>
>
>      public Complex invert();
>
>
>
>      public Complex divide(Complex b) default Complexes.divide;
>
> }
>
>
>
> /* Complexes.java */
>
> public class Complexes {
>
>
>
>      public static Complex divide(Complex a, Complex b) {
>
>          return a.multiply(b.invert());
>
>      }
>
>
>
> }
>
>
>
> First Question:
>
> As I am able to write a method body for subtract, the rule mentioned 
> in footnote 5, interfaces don't have code, seems to have been ignored. 
> Has this been finalized or will this be disallowed in a future build ?
>
>
>
> Second Question:
>
> Consider the default clause of the divide method. The syntax used for 
> the method reference is ClassName.methodName. In section 8 of State of 
> Lambda v4 (Brian Goetz, December 2011), method references have syntax 
> ClassName::methodName. It also states that this syntax is provisional.
> Example, map method of Iterable takes method references in this syntax.
>
>
>
> List<String> names = .
>
> nameLengths = names.map(String::length);
>
>
>
> Will the syntax for specifying the default method change to (only) use 
> ClassName::methodName, in a future build ? Otherwise, footnote 6 also 
> seems to be ignored for now.
>
>
>
> Regards,
>
> Deepak S Patwardhan.
>
>
>
>
>
>
>
>
>
>