Request for review: proposal for @FunctionalInterface checking

[Sam Pullara]

Is the intent that an interface that is not functional but marked as such won't compile? 

Sam

On Dec 28, 2012, at 3:12 PM, Brian Goetz <brian.goetz at oracle.com> wrote:

> Note that this proposal does NOT intend to change the rule that functional interfaces are recognized structurally; single-method interfaces will still be recognized as SAMs.  This is more like @Override, where the user can optionally capture design intent and the compiler can warn when said design intent is violated.
> 
> I support this proposal.
> 
> On 12/28/2012 3:02 PM, Joe Darcy wrote:
>> Hello,
>> 
>> We've had some discussions internally at Oracle about adding a
>> FunctionalInterface annotation type to the platform and we'd now like to
>> get the expert group's evaluation and feedback on the proposal.
>> 
>> Just as the java.lang.Override annotation type allows compile-time
>> checking of programmer intent to override a method, the goal for the
>> FunctionalInterface annotation type is to enable analogous compile-time
>> checking of whether or not an interface type is functional.  Draft
>> specification:
>> 
>> package java.lang;
>> 
>> /**
>> Indicates that an interface type declaration is intended to be a
>> <i>functional interface</i> as defined by the Java Language
>> Specification.  Conceptually, a functional interface 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.
>> 
>> Note that instances of functional interfaces can be created with lambda
>> expressions, method references, or constructor references.
>> 
>> If a type is annotated with this annotation type, compilers are required
>> to generate an error message unless:
>> 
>> <ul>
>> <li> The type is an interface type and not an annotation type, enum, or
>> class.
>> <li> The annotated type satisfies the requirements of a functional
>> interface.
>> </ul>
>> 
>> @jls 9.8 Functional Interfaces
>> @jls 9.4.3 Interface Method Body
>> @jls 9.6.3.8 FunctionalInterface  [Interfaces in the java.lang package
>> get a corresponding JLS section]
>> @since 1.8
>> */
>> @Documented
>> @Retention(RUNTIME)
>> @Target(TYPE)
>> @interface FunctionalInterface {} // Marker annotation
>> 
>> Annotations on interfaces are *not* inherited, which is the proper
>> semantics in this case.  A subinterface of a functional interface can
>> add methods and thus not itself be functional.  There are some
>> subtleties to the definition of a functional interface, but I thought
>> that including those by reference to the JLS was sufficient and putting
>> in all the details would be more likely to confuse than clarify.
>> 
>> Please send comments by January 4, 2013; thanks,
>> 
>> -Joe
>>