How to sort Arraylist of objects

Question

I have ArrayList, which containst football teams (class Team). Teams have points and i want to sort them by number of points.

 public class Team {
     priva         


        

Answer 1

It is not actually necessary to define a custom Comparator like this.

Instead, you can easily define one, when you want to sort your ArrayList.

Since JAVA 8 using lamda

    // Sort version.
    Iteams.sort(Comparator.comparing(Team::getPoints));

    // Complete version.
    Iteams.sort((o1, o2) -> o1.getPoints().compareTo(o2.getPoints()));

Also there are options for second comparator, if objects are equals on the first:

    // e.g. if same points, then compare their names.
    Iteams.sort(Comparator.comparing(Team::getPoints).thenComparing(Team::getName));

Also note that the default sort option is ascending, but you can set it to descending using:

    // e.g. Sort by points descending.
    Iteams.sort(Comparator.comparing(Team::getPoints).reversed());

That way, you can sort your ArrayList in different ways whenever you want, just by adding the method you want.

Answer 2

Source : Here

You can use Collections.sort with a custom Comparator<Team>.

    class Team {
        public final int points;
        // ...
    };

    List<Team> players = // ...

    Collections.sort(players, new Comparator<Team>() {
        @Override public int compare(Team p1, Team p2) {
            return p1.points- p2.points;
        }

    });

Alternatively, you can make Team implementsComparable<Team>. This defines the natural ordering for all Team objects. Using a Comparator is more flexible in that different implementations can order by name, age, etc.

See also

• Java: What is the difference between implementing Comparable and Comparator?

---

For completeness, I should caution that the return o1.f - o2.f comparison-by-subtraction shortcut must be used with extreme caution due to possible overflows (read: Effective Java 2nd Edition: Item 12: Consider implementing Comparable). Presumably hockey isn't a sport where a player can score goals in the amount that would cause problems =)

See also

• Java Integer: what is faster comparison or subtraction?

Answer 3

public class Team {
   private int points;
   private String name;

public Team(String n, int p) {
    name = n;
    points = p;
}

public int getPoints() {
    return points;
}

public String getName() {
    return name;
}

public static void main(String[] args) {
    List<Team> lteams = new ArrayList<Team>();

    lteams.add(new Team("FC Barcelona", 0));
    lteams.add(new Team("Arsenal FC", 2));
    lteams.add(new Team("Chelsea", 3));

    Collections.sort(lteams, new MyComparator());

    for (Team lteam : lteams) {
        System.out.println(lteam.name + ": " + lteam.points + " points");
    }
}

}

class MyComparator implements Comparator<Team> {
@Override
public int compare(Team o1, Team o2) {
    if (o1.getPoints() > o2.getPoints()) {
        return -1;
    } else if (o1.getPoints() < o2.getPoints()) {
        return 1;
    }
    return 0;
}}

Output:
Chelsea: 3 points
Arsenal FC: 2 points
FC Barcelona: 0 points