public class Person {

    private String firstName = null;
    private String lastName  = null;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName  = lastName;
    }

    public String getFirstName() {
        return this.firstName;
    }

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

import java.util.Comparator;

public class PersonComparator implements Comparator<Person> {

    @Override
    public int compare(Person p1, Person p2) {
        return p1.getLastName().compareTo(p2.getLastName());
    }

    @Override
    public boolean equals(Object obj) {
        return obj.equals(this);
    }
}

import java.util.ArrayList;

public class ComparatorExample {

    public static void main(String[] args) {

        ArrayList<Person> addressBook = new ArrayList<>();

        addressBook.add(new Person("Christopher", "Nolan"));
        addressBook.add(new Person("JJ", "Abrams"));
        addressBook.add(new Person("David", "Lynch"));
        addressBook.add(new Person("Tim", "Burton"));

        for (Person p : addressBook) {
            System.out.println(p.getLastName() + ", " + p.getFirstName());
        }

        //addressBook.sort(new PersonComparator());
        addressBook.sort((Person p1, Person p2) -> p1.getLastName().compareTo(p2.getLastName()));
        System.out.println("\n-- now sorted -- \n");

        for (Person p : addressBook) {
            System.out.println(p.getLastName() + ", " + p.getFirstName());
        }
    }
}