Write unit tests for a pricing function.

Given a pricing function, write focused tests with JUnit and AssertJ.

class PriceCalculator {
    BigDecimal finalPrice(BigDecimal base, int qty, boolean member) {
        if (qty <= 0) throw new IllegalArgumentException("qty");
        BigDecimal total = base.multiply(BigDecimal.valueOf(qty));
        return member ? total.multiply(new BigDecimal("0.9")) : total;
    }
}

class PriceCalculatorTest {
    private final PriceCalculator calc = new PriceCalculator();

    @Test void multipliesByQuantity() {
        assertThat(calc.finalPrice(new BigDecimal("10"), 3, false))
            .isEqualByComparingTo("30");
    }
    @Test void appliesMemberDiscount() {
        assertThat(calc.finalPrice(new BigDecimal("10"), 1, true))
            .isEqualByComparingTo("9");
    }
    @Test void rejectsNonPositiveQuantity() {
        assertThatThrownBy(() -> calc.finalPrice(BigDecimal.TEN, 0, false))
            .isInstanceOf(IllegalArgumentException.class);
    }
}

The tests cover the happy path, a rules branch and an invalid input, use names that state the behaviour, and avoid floating-point equality by comparing BigDecimal values.