public String getExchangeRate(String key, String exchange)
  {
    if (this.exchangeRates != null && this.exchangeRates.containsKey(key))
    {
      Map<String,String> exchanges = this.exchangeRates.get(key);
      if (exchanges.containsKey(exchange))
      {
        return exchanges.get(exchange);
      }
    }
    return null;
  }