when and how is compareTo() called for each element

[saurabhthard aggarwal]

//PLEASE EXPLAIN TO ME WHICH IS THE CALLED OBJECT AND WHICH IS THE CALLING OBJECT ,WHY

   
package p1;
import p1.Song2;
import java.util.*;
import java.io.*;

public class Song6
{
	ArrayList<Song2> snglist=new ArrayList<Song2>();

	public static void main(String[] args)
	{
		new Song6().go();
	}

	public void go()
	{
		getSong();
		System.out.println(snglist);
	
		Collections.sort(snglist); 
		System.out.println(snglist); 		
	}

	void getSong()
	{
		try
		{
			File f1=new File("snglist.txt");
			
			BufferedReader bf1=new BufferedReader(new FileReader(f1));
			
			String line=null;

			while((line=bf1.readLine())!=null)
			{
				addSong(line);
			}
		}

			catch(Exception e)
			{
				e.printStackTrace();
			}
		}

		void addSong(String one)
		{
			String[] tokens=one.split("/");
	
			Song2 sng = new Song2(tokens[0],tokens[1],tokens[2],tokens[3]);
			
		    snglist.add(sng);
	
		}
}

package p1;
class Song2 implements Comparable<Song2>
{
	String title;
	String name;
	String desc;
	String age;

	public int compareTo(Song2 s)
	{
		return title.compareTo(s.getTitle());
	}

	Song2(String t,String n,String d,String a)
	{
		title=t;
		name=n;
		desc=d;
		age=a;
	}

	public String getTitle()
	{
		return title;
	}

	public String getName()
	{
	    return name;	
	}

	public String getDesc()
	{
		return desc;
	}

	public String getAge()
	{
		return age;
	}

	public String toString()  
	{
		return title+" "+name+" "+desc+" "+ age +"\n";
	}

}






 

[Byju Joy]

Song2.compareTo method is not at all called in this case. Because Song2 objects are added to ArrayList, which is not 'sorted' and hence doesn't need compareTo method implementation. However if Song2 objects were being added to a sorted collection such as TreeSet, then compareTo would have been called. In that case 'caller' will be TreeSet object, and called on Song2 objects.

[Balu Sadhasivam]

Song2.compareTo method is not at all called in this case. Because Song2 objects are added to ArrayList, which is not 'sorted' and hence doesn't need compareTo method implementation. However if Song2 objects were being added to a sorted collection such as TreeSet, then compareTo would have been called. In that case 'caller' will be TreeSet object, and called on Song2 objects.

I m afraid. compareTo () method is called when Collections.sort() is called .

    ArrayList<Song2> snglist=new ArrayList<Song2>();   

Here ArrayList contains Song2 objects which implements comparable interface. Collections.sort() will actually compareTo() method to sort it .

[saurabhthard aggarwal]

According to K&B SCJP 6 Collections.sort() method will call compareTo() of comparable interface.
Input of the program is unsorted.

Output of the program is sorted.

Do you have any idea of how compareTo() is used inside sort() method.

[Balu Sadhasivam]

Did you had look at API ? Collection sort() . Check it out

[Byju Joy]

Collections.sort is not the only place which can trigger compareTo call. In the code below compareTo is called without Collections.sort. In a sorted collection while trying to insert a new item a suitable place is to determined by comparing the new item with existing items.

 package com.mypackage;

import java.util.Set;
import java.util.TreeSet;

class Item implements Comparable<Item>{
    private String name;

    Item(String name){
        this.name = name;
    }

    public String getName() {
        return name;
    }


    public int compareTo(Item o) {
        System.out.println("compareTo is called");
        return name.compareTo(o.getName());
    }

}
public class CompareToTest {
    public static void main(String[] args) {
        Set<Item> mySet = new TreeSet<Item>();
        mySet.add(new Item("A"));
        mySet.add(new Item("B"));
    }
}