Example demonstrating JUnit 5 and Java 8 date & Supplier with an immutable, testable class

Person.java

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;
import java.util.function.Supplier;

class Person {

    private final String givenName;
    private final String surname;
    private final LocalDate dateOfBirth;
    private final Supplier<LocalDate> currentDateSupplier;

    Person(String givenName, String surname, LocalDate dateOfBirth) {
        this(givenName, surname, dateOfBirth, LocalDate::now);
    }

    Person(String givenName, String surname, LocalDate dateOfBirth, Supplier<LocalDate> currentDateSupplier) {
        this.givenName = givenName;
        this.surname = surname;
        this.dateOfBirth = dateOfBirth;
        this.currentDateSupplier = currentDateSupplier;
    }

    String getDisplayName() {
        return surname + ", " + givenName;
    }

    long getAge() {
        return ChronoUnit.YEARS.between(dateOfBirth, currentDateSupplier.get());
    }

    long getDaysUntilBirthday() {
        LocalDate now = currentDateSupplier.get();
        LocalDate nextBirthday = getNextBirthday(now);
        return ChronoUnit.DAYS.between(now, nextBirthday);
    }

    private LocalDate getNextBirthday(LocalDate now) {
        LocalDate thisYearBirthday = dateOfBirth.withYear(now.getYear());
        if (now.isAfter(thisYearBirthday)) {
            return thisYearBirthday.plusYears(1);
        }
        else {
            return thisYearBirthday;
        }
    }

    public static void main(String... args) {
        Person person = new Person("Josh", "Hayden", LocalDate.of(2009, 1, 12));
        System.out.println(person.getDisplayName() + ": " + person.getAge() + " years old");
        System.out.println("Next birthday in " + person.getDaysUntilBirthday() + " days");
    }

}

PersonTest.java

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.function.Executable;

import java.time.LocalDate;
import java.util.function.Function;
import java.util.function.Supplier;

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.junit.jupiter.api.Assertions.*;

class PersonTest {
    private final Supplier<LocalDate> currentDateSupplier = ()-> LocalDate.parse("2015-05-02");
    private final LocalDate ageJustOver5 = LocalDate.parse("2010-05-01");
    private final LocalDate ageExactly5 = LocalDate.parse("2010-05-02");
    private final LocalDate ageAlmost5 = LocalDate.parse("2010-05-03");

    @Test
    void testGetDisplayName() {
        Person person = new Person("Josh", "Hayden", ageExactly5, currentDateSupplier);
        String displayName = person.getDisplayName();
        assertThat(displayName, equalTo("Hayden, Josh"));
    }

    @Test
    void testGetAge() {
        assertAll(
            createPersonAndAssertValue(ageAlmost5, 4, Person::getAge),
            createPersonAndAssertValue(ageExactly5, 5, Person::getAge),
            createPersonAndAssertValue(ageJustOver5, 5, Person::getAge)
        );
    }

    @Test
    void testGetDaysUntilBirthday() {
        assertAll(
            createPersonAndAssertValue(ageAlmost5, 1, Person::getDaysUntilBirthday),
            createPersonAndAssertValue(ageExactly5, 0, Person::getDaysUntilBirthday),
            createPersonAndAssertValue(ageJustOver5, 365, Person::getDaysUntilBirthday)
        );
    }

    private Executable createPersonAndAssertValue(LocalDate dateOfBirth, long expectedValue, Function<Person, Long> personLongFunction) {
        Person person = new Person("Given", "Sur", dateOfBirth, currentDateSupplier);
        long actualValue = personLongFunction.apply(person);
        return () -> assertEquals(expectedValue, actualValue);
    }
}