classy song

package com.company;
import java.util.*;

class Song{
    private String title, artist;
    private SortedSet<String> uListeners;
    public Song(String title, String artist) {
        this.title = title;
        this.artist = artist;
        uListeners = new TreeSet<>();
    }
    public String getTitle() {
        return title;
    }
    public String getArtist() {
        return artist;
    }
    public int howMany(ArrayList<String> listeners) {
        int ans = 0;
        for (String listener : listeners) {
            listener = listener.toLowerCase();
            if (!uListeners.contains(listener)) {
                uListeners.add(listener);
                ans++;
            }
        }
        return ans;
    }
}

public class Main {
    public static void main(String[] args) {
        Song mountMoose = new Song("Mount Moose", "The Snazzy Moose");
        System.out.println(mountMoose.howMany(new ArrayList<String>(Arrays.asList("John", "Fred", "BOb", "carl", "RyAn"))));
        System.out.println(mountMoose.howMany(new ArrayList<String>(Arrays.asList("JoHn", "Luke", "AmAndA"))));
    }
}
//https://www.codewars.com/kata/6089c7992df556001253ba7d