code: public boolean isRevoked(Certificate cert) {
          if (revokedMap.isEmpty() || (!(cert instanceof X509Certificate))) {
              return false;
          }
          X509Certificate xcert = (X509Certificate) cert;
          X509IssuerSerial issuerSerial = new X509IssuerSerial(xcert);
          return revokedMap.containsKey(issuerSerial);
      }
docs: Checks whether the given certificate is on this CRL.
  
  @param cert the certificate to check for.
  @return true if the given certificate is on this CRL,
  false otherwise.