code: public static void assertVoidReturnType(final Method method, Class<? extends Annotation> annotation) 
     {
        if ((!method.getReturnType().equals(Void.class)) && (!method.getReturnType().equals(Void.TYPE)))
        {
           throw annotation == null ? MESSAGES.methodHasToReturnVoid(method) : MESSAGES.methodHasToReturnVoid2(method, annotation);
        }
     }
docs: Asserts method return void.
  
  @param method to validate
  @param annotation annotation to propagate in exception message