Problem with method references

[Rémi Forax]

Hi Tomasz,
jdk7b123 comes with a new version of package java.dyn.
so the source of the prototype compiler and the source of jdk7
are not in sync.

For the record, Methodhandle.invoke should be MethodHandle.invokeGeneric
and SAM interface conversion should use MethodHandles.asInstance().

Rémi

On 01/02/2011 08:28 PM, Tomasz Kowalczewski wrote:
> Hi All,
>
> I got latest code for lambda prototype and started to play with it a
> little. I have encountered problems when using method references.
>
> I checked out the code using:
>
> hg clone http://hg.openjdk.java.net/lambda/lambda/langtools
>
> and compiled it against build 1.7.0-ea-fastdebug-b123.
>
> I have following problems when compiling/running my sample code:
>
> 1) This example does not compile.
>
> package org.tkowalcz.lambda.extractor;
>
> public class Lambda {
>
>    public static void main( String[] args ) {
>      Extractor x = (Extractor) Lambda#getArgCount;
>      x.extract( new Lambda() );
>    }
>
>    public int getArgCount() {
>      return 0;
>    }
> }
>
> interface Extractor {
>
>    int extract( Lambda lambda );
> }
>
> Compiler output (from within ant):
>
>       [exec] src\org\tkowalcz\lambda\extractor\Lambda.java:6: cannot find symbol
>       [exec]     Extractor x = Lambda#getArgCount;
>       [exec]                   ^
>       [exec]   symbol:   method<int>invoke(Lambda)
>       [exec]   location: class MethodHandle
>       [exec] Fatal Error: Unable to find method invoke
>
> 2) After adding a cast to Extractor it will compile but will throw
> ClassCastException when run:
>
> public class Lambda {
>
>    public static void main( String[] args ) {
>      Extractor x = (Extractor) Lambda#getArgCount;
>      x.extract( new Lambda() );
>    }
>
>    public int getArgCount() {
>      return 0;
>    }
> }
>
> abstract class Extractor {
>
>    public abstract int extract( Lambda lambda );
> }
>
>       [exec] Exception in thread "main" java.lang.ClassCastException:
> sun.dyn.DirectMethodHandle cannot be cast to
> org.tkowalcz.lambda.extractor.Extractor
>       [exec] 	at org.tkowalcz.lambda.extractor.Lambda.main(Lambda.java:6)
>       [exec] VM option '+UnlockExperimentalVMOptions'
>       [exec] VM option '+EnableInvokeDynamic'
>
> 3) However, when I change abstract class Extractor to interface then
> removing the cast will let my code compile and run:
>
> public class Lambda {
>
>    public static void main( String[] args ) {
>      Extractor x = Lambda#getArgCount;
>      x.extract( new Lambda() );
>    }
>
>    public int getArgCount() {
>      return 0;
>    }
> }
>
> interface Extractor {
>
>    int extract( Lambda lambda );
> }
>
> I hope that 1) and 2) is a correct usage and I am not abusing the
> prototype too much.
>