getFunctionalMethod
Get the functional method defined by the type, if any.
Per JLS §9.8, a functional interface is always an interface, which is a reference type. Primitive types, array types, and void can therefore never be functional interfaces. Passing such a type returns empty immediately without throwing an exception.
Note: bounded type variables (<T extends Runnable>) and intersection types (Runnable & Serializable) are not handled here and also return empty. Supporting those cases is a separate, more complex enhancement (see JLS §15.27.3).
Return
the single abstract method of the functional interface, or empty if the type is not a functional interface or is not a reference type at all
Parameters
the resolved type to inspect — may be any ResolvedType subclass
See also
Get the functional method defined by the type, if any.