code: private static Collection<X509Certificate> getMatchingCerts
          (Collection<X509Certificate> certs, CertSelector selector) {
          // if selector not specified, all certs match
          if (selector == null) {
              return certs;
          }
          List<X509Certificate> matchedCerts = new ArrayList<>(certs.size());
          for (X509Certificate cert : certs) {
              if (selector.match(cert)) {
                  matchedCerts.add(cert);
              }
          }
          return matchedCerts;
      }
docs: Iterates over the specified Collection of X509Certificates and
  returns only those that match the criteria specified in the
  CertSelector.