Sorting a collection of objects [duplicate]

Answer 1

The Collections.sort method can be invoked with a second argument which is the comparator to use. Create 3 comparators and use the one you want when appropriate.

Collections.sort(list , new Comparator() {
        public int compare(Object o1, Object o2) {
          ...
        }
      });

Answer 2

The Java 8 way of doing this is to use List.sort as follows:

personList.sort(Comparator.comparing(Person::getName));

To quote Stuart Marks in his answer over here.

This is the big advantage of the List.sort(cmp) extension method over Collections.sort(list, cmp). It might seem that this is merely a small syntactic advantage being able to write myList.sort(cmp) instead of Collections.sort(myList, cmp). The difference is that myList.sort(cmp), being an interface extension method, can be overridden by the specific List implementation. For example, ArrayList.sort(cmp) sorts the list in-place using Arrays.sort() whereas the default implementation implements the old copyout-sort-copyback technique.

Answer 3

Collections.sort can be called with a custom comparator. And that comparator can be implemented to allow sorting in different sort orders. Here's an example (for your Person model - with age as an Integer):

public class FlexiblePersonComparator implements Comparator<Person> {
  public enum Order {Name, Age, Country}

  private Order sortingBy = Name;

  @Override
  public int compare(Person person1, Person person2) {
    switch(sortingBy) {
      case Name: return person1.name.compareTo(person2.name);
      case Age: return person1.age.compareTo(person2.age);
      case Country: return person1.country.compareTo(person2.country);
    }
    throw new RuntimeException("Practically unreachable code, can't be thrown");
  }

  public void setSortingBy(Order sortBy) {
    this.sortingBy = sortingBy;
  }
}

And you use it like that (assuming persons is a field):

public void sortPersonsBy(FlexiblePersonComparator.Order sortingBy) {
  List<Person> persons = this.persons;  // useless line, just for clarification
  FlexiblePersonComparator comparator = new FlexiblePersonComparator();
  comparator.setSortingBy(sortingBy);
  Collections.sort(persons, comparator); // now we have a sorted list
}