code: private void indexCertificate(X509Certificate cert) {
          X500Principal subject = cert.getSubjectX500Principal();
          Object oldEntry = certSubjects.put(subject, cert);
          if (oldEntry != null) { // assume this is unlikely
              if (oldEntry instanceof X509Certificate) {
                  if (cert.equals(oldEntry)) {
                      return;
                  }
                  List<X509Certificate> list = new ArrayList<>(2);
                  list.add(cert);
                  list.add((X509Certificate)oldEntry);
                  certSubjects.put(subject, list);
              } else {
                  @SuppressWarnings("unchecked") // See certSubjects javadoc.
                  List<X509Certificate> list = (List<X509Certificate>)oldEntry;
                  if (list.contains(cert) == false) {
                      list.add(cert);
                  }
                  certSubjects.put(subject, list);
              }
          }
      }
docs: Add an X509Certificate to the index.