BigDecimal calculateDiscount(Customer customer, LocalDate now);

class Customer {
    private final String name;
    private final LocalDate joinedAt;
    private final LocalDate dateOfBirth;
// (...)
}

@Test
void shouldCalculate10percentDiscount() {
    LocalDate now = LocalDate.of(2019, 9, 15);
    LocalDate dateOfBirth = LocalDate.of(1980, 1, 20);
    Customer customer = new Customer("Sue Smith", now.minusYears(1), dateOfBirth);

    BigDecimal discount = calculator.calculateDiscount(customer, now);

    Assertions.assertThat(discount).isEqualTo(new BigDecimal("0.1"));
}

@Test
void shouldCalculate20percentDiscount() {
    LocalDate now = LocalDate.of(2019, 9, 15);
    LocalDate dateOfBirth = LocalDate.of(1980, 1, 20);
    Customer customer = new Customer("Sue Smith", now.minusYears(2), dateOfBirth);

    BigDecimal discount = calculator.calculateDiscount(customer, now);

    Assertions.assertThat(discount).isEqualTo(new BigDecimal("0.2"));
}

@Test
void shouldCalculateBirthdayDiscount() {
    Customer customer = new Customer("Sue Smith", NOW, NOW.withYear(1980));

    BigDecimal discount = calculator.calculateDiscount(customer, NOW);

    Assertions.assertThat(discount).isEqualTo(new BigDecimal("0.3"));
}

@Test
void shouldCalculateNoDiscount() {
    Customer customer = new Customer("Sue Smith", NOW, LocalDate.of(1980, 1, 20));

    BigDecimal discount = calculator.calculateDiscount(customer, NOW);

    Assertions.assertThat(discount).isEqualTo(new BigDecimal("0"));
}

@Test
void shouldCalculate20percentDiscountFor3Years() {
    LocalDate now = LocalDate.of(2019, 9, 15);
    LocalDate dateOfBirth = LocalDate.of(1980, 1, 20);
    Customer customer = new Customer("Sue Smith", now.minusYears(3), dateOfBirth);

    BigDecimal discount = calculator.calculateDiscount(customer, now);

    Assertions.assertThat(discount).isEqualTo(new BigDecimal("0.2"));
}

BigDecimal calculateDiscount(Customer customer, LocalDate now);

class Customer {
    private final String name;
    private final LocalDate joinedAt;
    private final LocalDate dateOfBirth;
// (...)
}

@Property
boolean discountShouldNotBeOver30percent(@ForAll("validCustomer") Customer customer, @ForAll("futureDate") LocalDate now) {

    BigDecimal discount = calculator.calculateDiscount(customer, now);
    Assertions.assertThat(discount).isLessThan(new BigDecimal("0.3"));
}

                              |-------------------jqwik-------------------
tries = 374                   | # of calls to property
checks = 374                  | # of not rejected calls
generation-mode = RANDOMIZED  | parameters are randomly generated
after-failure = PREVIOUS_SEED | use the previous seed
seed = -272292185905571017    | random seed to reproduce generated values
sample = 
[Customer{name='AAA', joinedAt=2010-01-01, dateOfBirth=1980-02-29}, 2019-10-06]
original-sample = 
[Customer{name='DtQDpPfEx', joinedAt=2010-04-21, dateOfBirth=1980-02-29}, 2185-03-17]

@ForAll String sentence

@ForAll @StringLength(min = 3, max = 100) String sentence

@ForAll @AlphaChars @NumericChars @Chars({',', '.', ':', ';', '-'}) @Whitespace String sentence

@Property
boolean myProperty(@ForAll("shortStrings") String string, @ForAll("10 to 99") int number) {
    String concatenated = string + number;
    return concatenated.length() > 2 && concatenated.length() < 11;
}

@Provide
Arbitrary<String> shortStrings() {
    return Arbitraries.strings().withCharRange('a', 'z').filter('q')
        .ofMinLength(1).ofMaxLength(8);
}

@Provide("10 to 99")
Arbitrary<Integer> numbers() {
    return Arbitraries.integers().between(10, 99);
}

@Property
boolean discountShouldNotBeOver30percent(@ForAll("validCustomer") Customer customer) {
    BigDecimal discount = calculator.calculateDiscount(customer);
    Assertions.assertThat(discount).isLessThan(new BigDecimal("0.3"));
}

@Provide
Arbitrary<Customer> validCustomer() {
    Arbitrary<String> names = Arbitraries.strings().alpha().ofMinLength(3).ofMaxLength(21);
    Arbitrary<LocalDate> joined = datesGen(BUSINESS_START, LocalDate.now());
    Arbitrary<LocalDate> born = datesGen(MIN_DOB, LocalDate.now().minusYears(18));
    return Combinators.combine(names, joined, born).as(Customer::new);
}

Arbitrary<LocalDate> datesGen(LocalDate minDate, LocalDate maxDate) {
    long range = DAYS.between(minDate, maxDate);
    return Arbitraries.longs().between(0, range).map(n -> minDate.plus(n, DAYS));
}

@Property
boolean discountShouldBeProportionalToMembershipYears(@ForAll Customer older, @ForAll Customer newer) {
    Assume.that(isNotBirthday(older, now));
    Assume.that(isNotBirthday(newer, now));
    Assume.that(older.getJoinedAt().isBefore(newer.getJoinedAt()));

    BigDecimal discountForOlderCustomer = calculator.calculateDiscount(older);
    BigDecimal discountForNewerCustomer = calculator.calculateDiscount(newer);
    Assertions.assertThat(discountForOlderCustomer.isGreaterThan(discountForNewerCustomer);
}

@Property
void shouldCalculateDiscount(@ForAll("membershipYears") Short years) {
    BigDecimal discount = calculator.prepareDiscount(years);
    //(...)
}

@Provide
Arbitrary<Short> membershipYears() {
    return Arbitraries.frequency(
        Tuple.of(30, 0),
        Tuple.of(60, 1),
        Tuple.of(7, 2),
        Tuple.of(3, 3)
    );
}

@Property
boolean endOfPresentation(@ForAll Attendee attendee) {
    Assume.that(attendee.hasQuestions());

    return attendee.questionsAsked > 0;
}