Request for review: proposal for @FunctionalInterface checking

[Joe Darcy]

Hello Ali,

Use of @FunctionalInterface is optional and is just done to allow 
stricter compile-time checking.  If a functional interface is *not* 
annotated with @FunctionalInterface, it can still be used for lambda 
expressions, etc.

Cheers,

-Joe

On 01/08/2013 02:07 PM, Ali Ebrahimi wrote:
> Hi joe,
>
> does this mean if functional interfaces do not be annotated with 
> @FunctionalInterface annotation can not be used in lambda expressions.
>
> general question: annotating of functional interfaces is required or 
> optional?
> I ask this for this purpose that there is huge number of functional 
> interfaces in java legacy third parties code base that all are 
> candidate for use in lambda expressions and can not be retrofired to 
> annotated with this annotation.
>
> Ali
>
> On Wed, Jan 9, 2013 at 12:34 AM, Joe Darcy <joe.darcy at oracle.com 
> <mailto:joe.darcy at oracle.com>> wrote:
>
>     Hello,
>
>     While there were some reservations expressed about this proposal,
>     there were no strong objections so I'm going forward with adding
>     this annotation type to the platform, with a somewhat refined
>     specification.
>
>     Thanks,
>
>     -Joe
>
>
>     On 12/28/2012 1:38 PM, Joe Darcy wrote:
>
>         Hi Sam,
>
>         On 12/28/2012 12:28 PM, Sam Pullara wrote:
>
>             On Dec 28, 2012, at 3:02 PM, Joe Darcy
>             <joe.darcy at oracle.com <mailto:joe.darcy at oracle.com>
>             <mailto:joe.darcy at oracle.com
>             <mailto:joe.darcy at oracle.com>>> wrote:
>
>                 has exactly one abstract method.  Since default
>                 methods are not abstract, any default methods declared
>                 in an interface do not contribute to its abstract
>                 method count.  If an interface declares a method
>                 overriding one of the public methods of
>                 java.lang.Object, that also does <em>not</em> count
>                 toward the abstract method count.
>
>
>             This is pretty murky. This works:
>
>             interface Foo {
>               @Override
>               boolean equals(Object other);
>             }
>
>             but if you try this
>
>             interface Foo {
>               @Override
>               default boolean equals(Object other) {
>                 return false;
>               }
>             }
>
>             it does give an error that says I can't override:
>
>             *java: default method equals in interface spullara.Foo
>             overrides a member of java.lang.Object*
>
>             Seems like "override" is the wrong word to use and will
>             likely be confusing since we are explicitly disallowing
>             the second one.
>
>
>
>         Yes, I was contemplating whether "override" was the best
>         phrasing to use in the wording above because of this sort of
>         wrinkle. Instead "*abstract* method overriding one of the ..."
>         might help distinguish this particular case.
>
>         -Joe
>
>
>