Multiple sort criteria on ArrayList

Question

I've been given a task of creating a MP3 catalogue with all the usual functions. I have a MP3_Track, MP3_UserInterface and Main classes.

public class MP3_Track implements Comparable<MP3_Track>{

private int trackNo;
private String artistName;
private String albumName;
private String trackLength;

MP3_Track(int no, String artist, String album, String length){
    this.trackNo = no;
    this.artistName = artist;
    this.albumName = album;
    this.trackLength = length;

// setters & getters

@Override
public String toString(){
        return (" Track Number : " + trackNo + "\n Artist Name  : " + artistName + "\n Album Name   : " + albumName +"\n Track Length : " + trackLength);
}

@Override
public int compareTo(MP3_Track aTrack){
    int result;
    try{
        result = artistName.compareTo(aTrack.artistName);
    }
    catch(UnsupportedOperationException e){
        throw new UnsupportedOperationException("Not supported here.");
    }
    return result;
  }
}

My main class contains:

static void deleteTrack()
static MP3_Track addTrack()
static void moveTrack()
static void exploreTracks()...etc

Main calls on the relevant methods depending what the user enters at the console.

Such as: Collections.sort(myTracks); myTracks is a collection of MP3_Tracks and at the moment mt MP3_Track overrides the compareTo() method but as you can see it only sorts it by artistName!!! I want the user to have the choice of how the tracks are sorted etc. I thought that maybe I could have a data mamber in MP3_Track which could flag what type of sort to implement then some if's in compareTo method to return the required result. This solution seems somewhat cumbersome, another possibility I thought of was maybe implementing a custom interface!!!

Any suggestions as to a slick workaround are much appreciated???

Many thanks i advance, great site BTW..

Answer 1

You need to look at the Collections.sort() method. You can pass in an implementation of the Comparator interface and it will use that to sort.

For an example see http://www.vogella.com/articles/JavaCollections/article.html#collectionssort

• Collections docs http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html

• Comparator docs http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html

Answer 2

You can pass a custom comparator to Collections.sort based on user input

For example --

if ( "artistName".equals(sortType) ) {
    Collections.sort(myList, new Comparator<MP3_Track>() {
        @Override
        public int compare(MP3_Track track1, MP3_Track track2) {
          // use artist name to compare
        }
    });
 }
 else if ("albumName".equals(sortType) {
   ...
 }