Person.java

package com.xyzcorp;

import java.util.HashSet;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;

public class Person implements Comparable<Person>{
    private String firstName;
    private String middleName; //null by default
    private String lastName;
    private HashSet<String> interests;

    //Constructor
    public Person(String firstName, String lastName) {
        Objects.requireNonNull(firstName);
        Objects.requireNonNull(lastName);
        if (firstName.isEmpty())
            throw new IllegalArgumentException("First Name cannot be empty");
        if (lastName.isEmpty())
            throw new IllegalArgumentException("Last Name cannot be empty");
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public Person(String firstName, String middleName, String lastName) {
        this(firstName, lastName);
        Objects.requireNonNull(middleName);
        if (middleName.isEmpty())
            throw new IllegalArgumentException("Middle Name cannot be empty");
        this.middleName = middleName;
    }

    //You call these after the
    //object has been constructed
    public String getFirstName() {
        return this.firstName;
    }

    public String getLastName() {
        return this.lastName;
    }

    public Optional<String> getMiddleName() {
        return Optional.ofNullable(middleName);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return Objects.equals(firstName, person.firstName)
            && Objects.equals(lastName, person.lastName);
    }

    @Override
    public int hashCode() {
        return Objects.hash(firstName, lastName);
    }

    @Override
    public String toString() {
        return String.format("%s %s", firstName, lastName);
    }

    public void addInterest(String interest) {
        interests.add(interest);
    }

    public Set<String> getInterests() {
        return Set.copyOf(interests);
    }

    public int getNumberOfInterests() throws Exception {
        return interests.size();
    }

    @Override
    public int compareTo(Person other) {
        return this.lastName.compareTo(other.lastName);
    }
}