code: public X509CRLEntry getRevokedCertificate(BigInteger serialNumber) {
          if (revokedMap.isEmpty()) {
              return null;
          }
          // assume this is a direct CRL entry (cert and CRL issuer are the same)
          X509IssuerSerial issuerSerial = new X509IssuerSerial
              (getIssuerX500Principal(), serialNumber);
          return revokedMap.get(issuerSerial);
      }
docs: Gets the CRL entry with the given serial number from this CRL.
  
  @return the entry with the given serial number, or <code>null</code> if
  no such entry exists in the CRL.
  @see X509CRLEntry