Sorting objects of a class

Question

I'm working on a Java project that asks me to implement the function obtainRanking() : void with the following description:

Sort the players list by using the class method sort (List) from the Collections class, a method to sort the objects of a collection. For that, the object's class (Player in our case), should implement the interface "Comparable" and its method compareTo.

So far, this is how I implemented the interface Comparable:

package modeloqytetet;

public interface Comparable {
    public int compareTo(Object otroJugador);
}

Inside class Player this is how I implemented the said method:

@Override
    public int compareTo(Object otherJugador) {
        int otherCapital = ((Player) otherJugador).getCapital();

        return otherCapital-getCapital();
    }

Now, the method obtainRanking() : void should be implemented in other class and I don't know how to do it. I've been trying to figure out by looking some examples around the internet but nothing seems to work.

Any help would be appreciated.

Any method called `obtainRanking` should not return void. Nevertheless, You can pass the the Comparator to `sort` method that you use in the method in other class. — Sid, Oct 21 '18 at 11:34
Maybe unrelated but I think they mean that they want you to implement [Comparable](https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html) and not your own `Comparable` interface — Mark, Oct 21 '18 at 11:34
@Sid I think that the purpose of the method is to sort an existing ArrayList of Player class so the return should be void. Anyway the naming is not right, but I didn't choose it :P — d3vcho, Oct 21 '18 at 11:35
So you can directly sort in `obtainRanking` method then, can't you? — Sid, Oct 21 '18 at 11:36
Hi d3vcho! You'll be needing this method: https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#sort-java.util.List-java.util.Comparator- - put your player instances into a collection, use that to sort it, then the ranking you want is their position in the sorted list. Hope that helps! — hugh, Oct 21 '18 at 11:37
@Mark I actually didn't think about that. They didn't explain us what an interface is so, it might be my bad. Thanks for the suggestion. — d3vcho, Oct 21 '18 at 11:40
@Sid That's right, that's what I'm asking for. I don't know if I made myself clear. — d3vcho, Oct 21 '18 at 11:41
@hugh Thanks for the suggestion, will test it up in a little bit. — d3vcho, Oct 21 '18 at 11:42
You probably should read this part of official tutorial https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html — Pshemo, Oct 21 '18 at 11:43

Sweeper · Accepted Answer · 2018-10-21T11:56:00.923

3

The instructions is telling you to implement java.lang.Comparable<T>, not your own Comparable interface.

You should do this:

class Player implements Comparable<Player> {
    @Override
    public int compareTo(Player other) {
        return Integer.compare(this.getCapital(), other.getCapital());
    }

    ...
}

For why you should not simply subtract one integer from another to compare them, see here.

Then, you can implement obtainRankings like this:

// this name is quite bad. I would call it sortPlayersCapitalInPlace or something like that
public void obtainRankings(List<Player> players) {
    Collections.sort(players);
}

edited Oct 21 '18 at 11:56

answered Oct 21 '18 at 11:44

Sweeper

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That seems good! One more thing, shouldn't I @Override the method compareTo? – d3vcho Oct 21 '18 at 11:47
@d3vcho The `@Override` annotation is optional. I didn't write it there because I was lazy... :). You should write it. – Sweeper Oct 21 '18 at 11:50
@d3vcho it's good practice, yes - but it's not actually required. In fact it was only in Java 7 that it was legal to annotate implemented `interface` methods with `@Override`. – Boris the Spider Oct 21 '18 at 11:50
1

@d3vcho You are overriding it, but if you are asking if you should add `@Override` annotation than yes, it should be used whenever your intention is to override some method to make compiler test that fact. More at [When do you use Java's @Override annotation and why?](https://stackoverflow.com/q/94361) – Pshemo Oct 21 '18 at 11:52

score 0 · Answer 2 · answered Oct 21 '18 at 12:08

You can use the following code snippets in your obtainRankings() method for customized sorting.

For Ascending order of players,

public static void obtainRankings(List<Player> list){
        Collections.sort(list, (pl1, pl2) -> {
            return ((pl1.getCapital() > pl2.getCapital()) ? 1 : -1);
        });
    }

For Descending order of players,

public static void obtainRankings(List<Player> list){
        Collections.sort(list, (pl1, pl2) -> {
            return ((pl1.getCapital() > pl2.getCapital()) ? -1 : 1);
        });
    }

Sorting objects of a class

2 Answers2