public class Calculator {
    private final DecimalFormat df;

    public Calculator() {
        this.df = new DecimalFormat("#.##");
    }

    public double convert(Currency from, Currency to, double fromAmount) {
        if (!(from instanceof Currency) || (to instanceof Currency)) {
            throw new IllegalArgumentException(
                    "Object 'other' not instance of class 'Currency'");
        }
        if (fromAmount <= 0) {
            throw new IllegalArgumentException("Amount cannot be less than 0!");
        }
        if (from == to) {
            return fromAmount;
        }

        double amountInNOK = 0;
        double toAmount = 0;
        if (from.getUnit() == 100 && to.getUnit() == 100) {
            amountInNOK = fromAmount * (from.getRate() / 100);
            toAmount = amountInNOK * (to.getRate() / 100);
        } else if (from.getUnit() == 1 && to.getUnit() == 1) {
            amountInNOK = fromAmount * from.getRate();
            toAmount = amountInNOK * to.getRate();
        } else if (from.getUnit() == 1 && to.getUnit() == 100) {
            amountInNOK = fromAmount * from.getRate();
            toAmount = amountInNOK * (to.getRate() / 100);
        } else { // from.getUnit() == 100 && to.getUnit() == 1
            amountInNOK = fromAmount * (from.getRate() / 100);
            toAmount = amountInNOK * to.getRate();
        }
        return Double.parseDouble(df.format(toAmount));
    }
}