code: public ExternalFolder listFolders(String accountId, String userId, String serviceId) throws ApiException {
      return listFolders(accountId, userId, serviceId, null);
    }
docs: Retrieves a list of all the items in a specified folder from the specified cloud storage provider.
  Retrieves a list of all the items in a specified folder from the specified cloud storage provider.
  @param accountId The external account number (int) or account ID Guid. (required)
  @param userId The user ID of the user being accessed. Generally this is the user ID of the authenticated user, but if the authenticated user is an Admin on the account, this may be another user the Admin user is accessing. (required)
  @param serviceId The ID of the service to access.   Valid values are the service name (\&quot;Box\&quot;) or the numerical serviceId (\&quot;4136\&quot;). (required)
  @return ExternalFolder