code: private boolean isClass(Element element) {
      if (element.getKind() == ElementKind.CLASS) {
  
        if (element.getModifiers().contains(Modifier.ABSTRACT)) {
          ProcessorMessage.error(element,
              "Element %s is annotated with @%s but is an abstract class. "
                  + "Abstract classes can not be annotated. Annotate the concrete class "
                  + "that implements all abstract methods with @%s", element.getSimpleName(),
              ParcelablePlease.class.getSimpleName(), ParcelablePlease.class.getSimpleName());
          return false;
        }
  
        if (element.getModifiers().contains(Modifier.PRIVATE)) {
          ProcessorMessage.error(element, "The private class %s is annotated with @%s. "
                  + "Private classes are not supported because of lacking visibility.",
              element.getSimpleName(), ParcelablePlease.class.getSimpleName());
          return false;
        }
  
        // Ok, its a valid class
        return true;
      } else {
        ProcessorMessage.error(element,
            "Element %s is annotated with @%s but is not a class. Only Classes are supported",
            element.getSimpleName(), ParcelablePlease.class.getSimpleName());
        return false;
      }
    }
docs: Checks if the element is a class