Request for review: proposal for @FunctionalInterface checking

[Brian Goetz]

Yes.  If you mark an interface as functional, and it is not, the 
compiler will warn/error.  This prevents action-at-a-distance errors 
where you have a SAM, other code depends on its SAM-ness, and someone 
later decides to add another abstract method (or a method to one of its 
supertypes).  It also provide extra documentation value.

Basically, just like @Override.

On 12/28/2012 3:16 PM, Sam Pullara wrote:
> 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
>>>
>