我对java测试还很陌生，想知道是否有一种方法可以测试这样的类？我特别想知道的是，如何测试像“calculatecustoms”这样的方法，因为它不接受任何输入参数。。

public class CustomsCalculator {

    private double litresWine, litresBeer;

    public void setLitresWine(double litresWine) {
        checkDoubleNotNegative(litresWine);
        this.litresWine = litresWine;
    }

    public void setLitresBeer(double litresBeer) {
        checkDoubleNotNegative(litresBeer);
        this.litresWineOverQuota = litresWineOverQuota;
    }

    public double getLitresWine() {
        return litresWine;
    }

    public double getLitresBeer() {
        return litresBeer;
    }

    public void checkDoubleNotNegative(double number) {
        if (number < 0) {
            throw new IllegalArgumentException("Negative value not allowed.");
        }
    }

    public double calculateCustoms() {
            double wineFee = 13.0*getLitresWineOverQuota();
            double beerFee = 11.0*getLitresBeerOverQuota();

            double totalFee =  wineFee + beerFee;
            return totalFee;
    }
}