@Documented @Retention(RUNTIME) @Target(TYPE) public @interface FunctionalInterface
java.lang.Object, that also does
not count toward the interface's abstract method count
since any implementation of the interface will have an
java.lang.Object or elsewhere.
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.
However, the compiler will treat any interface meeting the
definition of a functional interface as a functional interface
regardless of whether or not a
annotation is present on the interface declaration.