Problems with comparable interface

[ayan biswas]

I got this code form "Head First Java".

class Song implements Comparable<Song>{  
String art;  
String tit;  
Song(String a,String t)  
{  
tit=t;  
art=a;  
}  
public int compareTo(Song s)  
{  
return title.compareTo(s.getTitle());  
}  
String getTitile()  
{}  
String getAuthor()  
{}  
}  
class Songsort{  
Arraylist <Song> sl=new Arraylist<Song>();  
public static void main(String arg[])  
{new Songsort.go();  
}  
void go(){  
getSong();  
Collections.sort(sl);}  
}  
void getSong(){  
//..i/o code  
}  
}  

Here we are adding the song objects in the arraylist.Now ,while sorting we are calling sort() method where as a parameter we are passing the entire arraylist of song objects.But how does sort() call compareTo() on a particular song object?And how does
return title.compareTo(s.getTitle());
work? which song objects title are we comparing with s.getTtitle();??

[Dheeraj Joshi]

From Java Doc

The sorting of object arrays is a little more involved, as the compiler doesn't check everything for you. If the object in the array implements Comparable, then you can just sort the array directly, in whole or in part. Otherwise, you must provide a Comparator to do the sorting for you. You can also provide a Comparator implementation if you don't like the default ordering.

When you do

Collections.sort(Object[])

Is calls the default compareTo method. But in your case you have override the method in song class. So override method will be called during sorting.

Please refer comparable interface document.

Regards
Dheeraj Joshi

[ayan biswas]

@ Dheeraj thanks for the reply..But I still have confusions..When I call sort() using arraylist of song objects ..the control transfers to compareTo()..now, the comparison ->this.title.compareTo(s.getTitle())..in this line is it that each song object is picked up one by one from the list and compared?If yes,then how does the first comparison take place?the first song object is compared to which object?
Sorry for asking stupid questions..

[Dheeraj Joshi]

Yes each object must be compared with rest of the object.

Java's internal sort algorithm is modified merge sort.

Regards
Dheeraj Joshi