code: private static ZoneRulesProvider getProvider(String zoneId) {
          ZoneRulesProvider provider = ZONES.get(zoneId);
          if (provider == null) {
              if (ZONES.isEmpty()) {
                  throw new ZoneRulesException("No time-zone data files registered");
              }
              throw new ZoneRulesException("Unknown time-zone ID: " + zoneId);
          }
          return provider;
      }
docs: Gets the provider for the zone ID.
  
  @param zoneId  the zone ID as defined by {@code ZoneId}, not null
  @return the provider, not null
  @throws ZoneRulesException if the zone ID is unknown