code: @Override
      public void init(boolean forward) throws CertPathValidatorException {
          if (!forward) {
              prevPubKey = trustedPubKey;
              if (PKIX.isDSAPublicKeyWithoutParams(prevPubKey)) {
                  // If TrustAnchor is a DSA public key and it has no params, it
                  // cannot be used to verify the signature of the first cert,
                  // so throw exception
                  throw new CertPathValidatorException("Key parameters missing");
              }
              prevSubject = caName;
          } else {
              throw new
                  CertPathValidatorException("forward checking not supported");
          }
      }
docs: Initializes the internal state of the checker from parameters
  specified in the constructor.