DataProvider.java

public class DataProvider {

    public static List<Person> getOldPersonList(){
        List<Person> persons = new ArrayList<>();
        persons.add(new Person(1, 20, "John"));
        persons.add(new Person(2, 12, "Jack"));
        persons.add(new Person(3, 8, "Michael"));
        persons.add(new Person(4, 19, "Rafael"));
        return persons;
    }

    public static List<Person> sortByAge(List<Person> oldList){
        Collections.sort(oldList, new Comparator<Person>() {
            @Override
            public int compare(Person person, Person person2) {
                return person.age - person2.age;
            }
        });
        return oldList;
    }
}