Sorting a HashMap or TreeMap by value

Question

I am making a java project which records football games and displays the league table. When a game is added to the league between two teams, I need a way of storing the teams position in the league based on the number of points that the Team have.

To do this is am thinking of using a HashMap< Integer, Team > where the Integer is storing the Teams position in the league based on the number of points that a team has. Whenever a new game is added I need a way of ordering this list so that it is sorted where the Teams with the greatest number of points are closer to the number one position.

For example, if there are three teams

Team A with 8 points, Team B with 5 points, Team C with 4 point

I would like the Map to be ordered as such

1 : A, 2 : B, 3 : C

My question are:

1) Do you think I should be using a HashMap or TreeMap for this instance?

2) How would I implement the code to sort the league by points aquired?

Yes it would but then I would also like to order off of the goal difference that the two teams have. Say for instance Team A has 3 points and Team B has 3 points. If Team A's goal difference is 3 and Team B's goal difference is 5. Then Team B should have a greater position in the league. — Jack Hudson, Dec 24 '17 at 01:30
Also if goal difference is the same, then it should go off of who has the most goals. If that is the same I'm not sure it matters which order they're in. — Jack Hudson, Dec 24 '17 at 01:33
A map is not the right data structure for your use case. Use a list instead. — fps, Dec 24 '17 at 01:41

ernest_k · Accepted Answer · 2017-12-24T02:13:24.027

A List is more appropriate for your use case. It gives you indexing out of the box and hassle-free sorting. Additionally, it's more suited for ordered display of values such as in a table, etc.

I would make the Team object comparable, use an array list, then I'd be done:

public class Team implements Comparable<Team> {
    private int points;
    private int goalDifference;

    public Team(int points, int goalDifference) {
        this.points = points;
        this.goalDifference = goalDifference;
    }

    @Override
    public int compareTo(Team other) {
        int res = other.points - this.points;
        return res != 0 ? res : (other.goalDifference - this.goalDifference);
    }

    //getters and setters
}

With a comparable Team class, you can use list features:

List<Team> teams = ...
Collections.sort(teams);

If you don't want to make the Team class comparable, you can use a comparator (continuing the snippet above):

teams.sort((team1, team2) -> 
   (team2.getPoints() != team1.getPoints()) ? 
     (team2.getPoints() - team1.getPoints()) : 
     (team2.getGoalDifference() - team1.getGoalDifference())
   );

Note that implemented the descending order logic in comparables or comparators. You can choose to use the reverse comparator instead.

The option of implementing Comparator becomes more and more preferable as the number of comparison fields grows. I'd personally prefer that as it's more readable.

Thank you, would you know how to extend this answer? I also need to order by a second variable in Team - goal Difference, if two teams have the same number of points. — Jack Hudson, Dec 24 '17 at 01:55

score 0 · Answer 2 · answered Dec 24 '17 at 01:31

For number one, I would recommend using a map over storing the rank in the team object, this would make it slightly easier to verify that there are not two teams with duplicate rankings.

For number two you can write a class implementing the Comparator interface. You will have to implement the compare method where you will compare two objects (in this case Teams) and return a negative, zero, or positive number if the first object is less than, equal to, or greater than the second object. For this you would have to change the implementation of the map from HashMap to TreeMap.

Sorting a HashMap or TreeMap by value

2 Answers2