Question

Here’s the deal :

I have Publication objets in my application.
I also have Vote objet (forOrAgainst,author,linkedPublication)

I want to sort publication by date, title… and also by number of votes.

I cant directly sort my list of publication as i dont have the number of vote in that list.
How can i sort my list of publication without adding method to the publication object.

What is the best way to link them ?

Should i return a hashmap ? a treeset ? an array ?

It’s kinda messy in my brain now…

Answer 1

Here’s an example of using Comparator to sort based on an external criteria:

import java.util.*;

class VoteComparator implements Comparator<String> {
    final Map<String, Integer> tally;
    VoteComparator(Map<String, Integer> tally) {
        this.tally = tally;
    }
    @Override public int compare(String pub1, String pub2) {
        int v1 = tally.get(pub1);
        int v2 = tally.get(pub2);
        return
           (v1 < v2) ? -1 :
           (v1 > v2) ? +1 :
           0;
    }           
};

This uses just String for publication for simplicity; you’d want to sort Publication in your application. This also uses a simple int to get the vote count, but essentially there has to be a tally service that gives you, given a Publication, what its Vote count is.

Note: English is not my first language, so perhaps “tally” isn’t the right word for it, but basically some sort of vote registrar, vote recorder, essentially a map between an object, and how many votes it gets.

Then you can sort, using, say, TreeSet.

public class SortExample {
    public static void main(String[] args) {
        Map<String, Integer> tally = new HashMap<String, Integer>();
        tally.put("foo", 42);
        tally.put("bar", 13);
        tally.put("Fizz", 3);
        tally.put("Buzz", 5);
        tally.put("FizzBuzz", 15);

        Comparator<String> voteComparator = new VoteComparator(tally);
        SortedSet<String> sortedByVote = new TreeSet<String>(voteComparator);
        sortedByVote.addAll(tally.keySet());
        for (String pub: sortedByVote) {
            System.out.println(pub + " (" + tally.get(pub) + " votes)");
        }
    }
}

This prints:

Fizz (3 votes)
Buzz (5 votes)
bar (13 votes)
FizzBuzz (15 votes)
foo (42 votes)