Request for review: proposal for @FunctionalInterface checking

[Kevin Bourrillion]

I see one important difference from @Override.  @Override catches errors
that might otherwise go completely uncaught.  With a type intended to be a
functional interface, the moment anyone ever tries to use it as such,
there's your compilation error.

So I don't see what sets @FunctionalInterface apart from the whole host of
static-analysis annotations that we've relegated to the now-abandoned JSR
305.


On Fri, Dec 28, 2012 at 12:17 PM, Brian Goetz <brian.goetz at oracle.com>wrote:

> 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
>>>>
>>>>
>>


-- 
Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at google.com