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 {
     private int points;
     private String name;

     public Team(String n)
     {
         name = n;
     }

     public int getPoints
     {
         return points;
     }

     public void addPoints(boolean win)
 {
            if (win==true)
            {
    points = points + 3;
            }

            else if (win==false)
            {
            points = points + 1;
            }

}
 //...
 }

Main Class:

 List<Team> lteams = new ArrayList<Team>;

 lteams.add(new Team("FC Barcelona"));
 lteams.add(new Team("Arsenal FC"));
 lteams.add(new Team("Chelsea"));

 //then adding 3 points to Chelsea and 1 point to Arsenal

 lteams.get(2).addPoints(true);
 lteams.get(1).addPoints(false);

 //And want sort teams by points (first index with most points).

I did my comparator.

 public 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;    
    } 

}

now I wanna use it (in main class)

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

I want to see:

Chelsea
Arsenal
Barcelona

But it doesn't sort.

How are you adding points to the team? In the code you included the team objects are placed directly into the list, you show no separate references to them. — Perception, Jan 23 '13 at 08:50
Check this link:http://stackoverflow.com/questions/2784514/sort-arraylist-of-custom-objects-by-property — noobob, Jan 23 '13 at 08:50
@Perception no seperate references to team objects are needed because he has reference to list object and can easily identify team object based on its name. — Subin Sebastian, Jan 23 '13 at 08:52
@Perception just meant that it is possible :-) That could be the reason why his sort is not working — Subin Sebastian, Jan 23 '13 at 08:57
@OP can you post code corresponding to `//then adding 3 points to Chelsea and 2 points to Arsenal ` — Subin Sebastian, Jan 23 '13 at 08:58
You appear to be sorting on "points" which you never set so all the points are 0. — Peter Lawrey, Jan 23 '13 at 09:00
This code should work, the problem seems to be elsewhere. Just two things: your comparator should be a singleton, and you don't need to `else` if you `return`. Finally, simplify it to `return Integer.compare(o1.getPoints(), o2.getPoints());` — fge, Jan 23 '13 at 09:01
Thanks for posts guys! Im newbie so my questions could be stupid, im sorry for this. — n0hepe, Jan 23 '13 at 11:19
@Subin added code where i inserts points. Btw, its working now ! Ty. — n0hepe, Jan 23 '13 at 11:37

score 30 · Answer 1 · edited May 23 '17 at 12:18

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.

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

score 0 · Answer 3 · answered Nov 13 '17 at 00:07

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.

How to sort Arraylist of objects

3 Answers3

See also

See also

Since JAVA 8 using lamda

Linked

Related