Override compareTo and sort using two strings

Question

Lets say i've a Album class. How do i code the compareTo method so that when i've a ArrayList of Album object and i call

Collections.sort()

it will sort them in by title follow by singer in ascending order.

Public class Album implements Comparable{

private String title;
private String singer;
private double price;

.....

public int compareTo(){

// How to

}

}

Answer 1

I would write it like

public int compareTo(Album other) {
    int primary = title.compareTo(other.title);
    return primary != 0 ? primary
                        : singer.compareTo(other.singer);
}

Personally however, I'd say it's arguable whether or not albums have a "natural ordering". Consider for instance if you, in the future, want to sort the albums on rating, or year.

If I were you I would probably create a Comparator for this purpose:

class TitleSingerComparator implements Comparator<Album> {
    public int compare(Album a, Album b) {
        int primary = a.title.compareTo(b.title);
        return primary != 0 ? primary
                            : a.singer.compareTo(b.singer);
    }
}

and sort the collection using

Collections.sort(albums, new TitleSingerComparator());

Answer 2

public class Album implements Comparable<Album> {
   ...
   public int compareTo(Album other) {
     int cmp = title.compareTo(other.title);
     if (cmp == 0) {
       cmp = singer.compareTo(other.singer);
     } 
     return cmp;
   }
}

Answer 3

I would write it like

public class Album implements Comparable<Album>{   
  ...   
  public int compareTo(Album alb){
  int out  = this.singer.compareTo(alb.singer);     
  return out;
  }
}

variable " out " will store either positive value( any number greater than 0) or negative value( any number less than 0) or 0 in case when Strings are equal(ie this.singer == alb.singer).