public class Person {

    private final String firstName;

    private final int age;

    public Person(final String firstName, final int age) {
        this.firstName = firstName;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" + "firstName='" + this.firstName + '\'' + ", age=" + this.age + '}';
    }

    public enum Comparators implements Comparator<Person> {
        BY_FIRST_NAME {
            @Override
            public int compare(final Person o1, final Person o2) {
                return o1.firstName.compareTo(o2.firstName);
            }
        },
        BY_AGE {
            @Override
            public int compare(final Person o1, final Person o2) {
                return Integer.compare(o1.age, o2.age);
            }
        }
    }

    public static void main(final String[] args) {
        final List<Person> persons = new ArrayList<>();
        persons.add(new Person("jim", 3));
        persons.add(new Person("bob", 5));

        persons.sort(Comparators.BY_FIRST_NAME);
        System.out.println(persons);
        persons.sort(Comparators.BY_AGE);
        System.out.println(persons);
    }
}