Java使用启示比较器对列表进行排序

Question

I'm trying to sort a list of music by revelance corresponding to a list of criteria.

public class Music implements Comparable<CriteriaList> {
    private String genre, artist, album, titre, price, note; 
    // getters, setters
    public int compareTo(CriteriaList list) {
        boolean title, album, genre, artist, note;
        title = !list.getTitle().isEmpty() && this.getTitre().equals(list.getTitle());
        album = !list.getAlbum().isEmpty() && this.getAlbum().equals(list.getAlbum());
        genre = !list.getGenre().isEmpty() && this.getGenre().equals(list.getGenre());
        artist = !list.getArtist().isEmpty() && this.getArtist().equals(list.getArtist());
        note = !list.getNote().isEmpty() && (Integer.parseInt(this.getNote()) >= Integer.parseInt(list.getNote()));
        return ((title ? 1 : 0) + (album ? 1 : 0) + (genre ? 1 : 0) + (artist ? 1 : 0) + (note ? 1 : 0));
    }
}

My function compareTo return the number of fields which match to the criteria list and test if input are not empty.

public class MusicProvider extends Agent {
    public List<Music> getMusicsByCL(CriteriaList list) {
        ArrayList<Music> res = new ArrayList<Music>();
        int[] revelanceTab = new int[res.size()];
        int i = 0, revelance;
        for (Music music : musicListAvailable) {
            revelance = music.compareTo(list);
            if (revelance > 1) {
                res.add(music);
                revelanceTab[++i] = revelance;
            }
        }
        // sort res with revelanceTab
        return res;
    }
}

Here I want to retrieve musics with a minimun revelance of 1 and sort them by revelance. How can I do that ?

Answer 1

Assuming you have already created the functionality to actually calculate the relevance, I would proceed like this.

Create a simple class to hold the Music and the calculated relevance score, just pass in the Criteria and store the result of the calculation.

public class ScoredMusic {
    private int relevanceScore;
    public ScoredMusic(Music m) { ... }
    public void calculateRelevance(Criteria criteria) { ... }
    public Music getMusic() { ... }
    public int getRelevanceScore() { ... }
}

Then I would score all the Music instances you have, store them in a list, and do a very simple compareTo() implementation that simply compares the relevanceScore between each ScoredMusic instance.

Answer 2

Comparable is used to compare two instances of Music against each other. If you want to compare to an external entity, use a Comparator implementation and pass it to Collections.sort(List, Comparator) . The Comparator would need to be initialized with the CriteriaList and the compare method would return a positive number if the first element should be ranked higher, negative number if the second element should be ranked higher, or 0 if they are equivalent. In your example, you would use your compareTo method and subtract the second element's score from the first and return that.

Something like this:

import java.util.Comparator;

public class MusicComparator implements Comparator<Music> {

    private final CriteriaList criteria;

    public MusicComparator(CriteriaList criteria) {
        this.criteria = criteria;
    }
    @Override
    public int compare(Music o1, Music o2) {
        return score(o1) - score(o2);
    }

    private int score(Music music) {
        boolean title, album, genre, artist, note;
        title = criteria.getTitle().isEmpty() || criteria.getTitle().equals(music.getTitle());
        album = criteria.getAlbum().isEmpty() || criteria.getAlbum().equals(music.getAlbum());
        genre = criteria.getGenre().isEmpty() || criteria.getGenre().equals(music.getGenre());
        artist = criteria.getArtist().isEmpty() || criteria.getArtist().equals(music.getArtist());
        note = criteria.getNote().isEmpty() || (!music.getNote().isEmpty() && Integer.parseInt(music.getNote()) >= Integer.parseInt(criteria.getNote()));
        return ((title ? 1 : 0) + (album ? 1 : 0) + (genre ? 1 : 0) + (artist ? 1 : 0) + (note ? 1 : 0));
    }
}

By the way, the isEmpty() methods will not protect you from null pointer exceptions. You will need a better method of handling those if the fields are allowed to be null.

Answer 3

This is my final class,

public class ScoredMusic implements Comparable<ScoredMusic> {
    private int revelanceScore = 0;
    private Music music;

    public ScoredMusic(Music music, CriteriaList crit) {
        this.music = music;
        calculateRevelance(crit);
    }

    private void calculateRevelance(CriteriaList list) {
        boolean title, album, genre, artist, note;
        title = !list.getTitle().isEmpty() && music.getTitre().equals(list.getTitle());
        album = !list.getAlbum().isEmpty() && music.getAlbum().equals(list.getAlbum());
        genre = !list.getGenre().isEmpty() && music.getGenre().equals(list.getGenre());
        artist = !list.getArtist().isEmpty() && music.getArtist().equals(list.getArtist());
        note = !list.getNote().isEmpty() && (Integer.parseInt(music.getNote()) >= Integer.parseInt(list.getNote()));
        revelanceScore = ((title ? 1 : 0) + (album ? 1 : 0) + (genre ? 1 : 0) + (artist ? 1 : 0) + (note ? 1 : 0));
    }

    public Music getMusic() {
        return music;
    }

    public int getRevelanceScore() {
        return revelanceScore;
    }

    public int compareTo(ScoredMusic other) {
        return Integer.compare(this.getRevelanceScore(), other.getRevelanceScore());
    }
}

and in my second class

public List<ScoredMusic> getMusicsScoredByCL(CriteriaList list) {
    ArrayList<ScoredMusic> scoredMusics = new ArrayList<ScoredMusic>();
    ScoredMusic sc;
    for (Music music : musicListAvailable) {
        sc = new ScoredMusic(music, list);
        scoredMusics.add(sc);
    }
    // sort by revelance and descending order
    Collections.sort(scoredMusics, Collections.reverseOrder());
    return scoredMusics;
}