Request for review: proposal for @FunctionalInterface checking (original) (raw)

Brian Goetz brian.goetz at oracle.com
Fri Dec 28 12:12:48 PST 2012

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 functional interface 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 not 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:

  • The type is an interface type and not an annotation type, enum, or
    •  class.
  • The annotated type satisfies the requirements of a functional
    •  interface. @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

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