code: public boolean isSubscriptionPreApprovalSupported() throws NotLoggedInException {
          final XMPPConnection connection = getAuthenticatedConnectionOrThrow();
          return connection.hasFeature(SubscriptionPreApproval.ELEMENT, SubscriptionPreApproval.NAMESPACE);
      }
docs: Check for subscription pre-approval support.
  
  @return true if subscription pre-approval is supported by the server.
  @throws NotLoggedInException if not logged in.
  @since 4.2