Sales tax calculations

gistfile1.java

public class TaxRate extends AbstractValueType<Double> {

    public static TaxRate taxRate(Double value) {
        return new TaxRate(value);
    }

    private TaxRate(Double value) {
        super(value);
    }

    public TaxAmount paidOn(GrossAmount gross) {
        Double rate = asDecimal();
        Double tax = gross.value() / (1 + rate) * rate;
        return taxAmount(tax);
    }

    private Double asDecimal() {
        return value() / 100;
    }
}


public class TaxRateTest {

    @Test
    public void shouldCalculateTaxPaidOnGrossAmount() {
        assertThat(taxRate(0.0).paidOn(grossAmount(9.99)), is(taxAmount(0.0)));
        assertThat(taxRate(17.5).paidOn(grossAmount(9.99)), is(taxAmount(1.4878723404255318)));
        assertThat(taxRate(20.0).paidOn(grossAmount(9.70)), is(taxAmount(1.616666666666667)));
        assertThat(taxRate(20.0).paidOn(grossAmount(3.24)), is(taxAmount(0.54)));
        assertThat(taxRate(100.0).paidOn(grossAmount(100.0)), is(taxAmount(50.0)));
    }
}

public class TaxAmount extends AbstractValueType<Double> {

    public static TaxAmount taxAmount(Double value) {
        return new TaxAmount(value);
    }

    private TaxAmount(Double value) {
        super(value);
    }

    public TaxRate paidOn(GrossAmount gross) {
        Double amount = value();
        Double net = gross.value() - amount;
        double rate = (amount / net) * 100;
        return taxRate(rate);
    }

}

public class TaxAmountTest {

    @Test
    public void shouldWorkSucker() {
        assertThat(taxAmount(0.73).paidOn(grossAmount(4.35)), is(taxRate(20.0)));
    }
}