code: @Override
      public boolean isFileExtensionRegistered(String fileTypeExtension) throws RegistryException, OSException {
          if (!isApplicable()) {
              return false;
          }
  
          // sanitize file extension
          fileTypeExtension = sanitizeFileExtension(fileTypeExtension);
  
          String[] hives = {Hive.HKEY_CURRENT_USER + SOFTWARE_CLASSES_PATH, Hive.HKEY_LOCAL_MACHINE + SOFTWARE_CLASSES_PATH, Hive.HKEY_CLASSES_ROOT + "\\"};
  
          for (String h : hives) {
              List<String> subkeys = WindowsRegistry.readSubkeys(h);
              if (subkeys.contains(fileTypeExtension)) {
                  return true;
              }
          }
          return false;
      }
docs: Checks if the given extension is already registered.
  
  @param fileTypeExtension File extension with leading dot, e.g.
  <code>.bar</code>.
  @return <code>true</code> if extension is registered, <code>false</code>
  otherwise.
  @throws RegistryException