public Currency execute() throws IOException{
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        while (true){
            System.out.println("Introduzca el código de una divisa");
            currency = CurrencySet.getInstance().get(reader.readLine());
            if (currency != null) break;
            System.out.println();
            System.out.println("Error: no se ha introducido un código de divisa válido");
            System.out.println();
        }    
        return currency;
    }