code: public synchronized List<String> getExtendedKeyUsage()
          throws CertificateParsingException {
          if (readOnly && extKeyUsage != null) {
              return extKeyUsage;
          } else {
              ExtendedKeyUsageExtension ext = getExtendedKeyUsageExtension();
              if (ext == null) {
                  return null;
              }
              extKeyUsage =
                  Collections.unmodifiableList(ext.getExtendedKeyUsage());
              return extKeyUsage;
          }
      }
docs: This method are the overridden implementation of
  getExtendedKeyUsage method in X509Certificate in the Sun
  provider. It is better performance-wise since it returns cached
  values.