Can an annotation type be a functional interface?

Markus Keller markus_keller at
Tue Dec 3 05:19:33 PST 2013

[sending to lambda-spec-comment & lambda-spec-observers]

Can an annotation type (JLS7 9.6) be a functional interface (jsr335-0.7.0 

The current spec just says "A functional interface is an interface that 
[...]". JLS7 9 defines the term "interface" as applicable to normal 
interfaces as well as annotation types. Therefore, this snippet should 
compile without errors:

//javac 1.8.0-ea-b115 error: Unexpected @FunctionalInterface annotation
@interface I { }

public class Test {
    // javac error: incompatible types: I is not a functional interface
    I i= () -> I.class;
    // OK. Abstract method: Class<? extends Annotation> annotationType()
    java.lang.annotation.Annotation a= () -> I.class;

But since it hardly makes sense to use a marker annotation type as a 
functional interface, I actually agree with javac. The jsr335 spec should 
be fixed.

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