docs: public synchronized Map<String, String> hashURLs(URL[] urls) {
          Map<String, String> hashes = new HashMap<String, String>();
          for (URL url : urls) {
              if (url != null) {
                  String externalForm = url.toExternalForm();
                  String hash = hashURL(url, externalForm);
                  hashes.put(externalForm, hash);
              }
          }
          return hashes;
      }