help on Comparable

Hi,

I have tried to implement Comparable interface and the compareTo method of it for my own class Name. But the compiler says that as Name is not defined as abstract it cannot implement abstract method compareTo. I give the code I tried as follows:

import java.util.*;
import java.io.*;

class Name implements Comparable
{
 
     String firstName, lastName;
 
      public Name(String fn, String ln)
      {
            firstName = fn;
            lastName = ln;
      }
 
      public int compareTo(Name n)
      {
            return firstName.compareTo(n.getFirstName());
      }
 
      public String getFirstName()
      {
            return firstName;
      }
 
      public String getLastName()
      {
           return lastName;
      }
}

public class NameSort
{
     public ArrayList  namelist = new ArrayList();
 
     public static void main(String[] args)
     {
             new NameSort().go();
     }
 
     public void go()
     {
          getNames();
          System.out.println("Before Sorting: The Names are : " + namelist);
          Collections.sort(namelist);
          System.out.println("After Sort: The Names are: " + namelist);
     }
 
     public void getNames() throws Exception
     {
           BufferedReader br = new BufferedReader(new FileReader("c:\\balaji\\name.txt"));
           String name = br.readLine();
           while(name != null)
           {
                  namelist.add(name);
                  name=br.readLine();
           }
     }
}

Can anyone please clarify it?

Whats the Name pameter that you are giving to compareTo mathod

Hi!

with Neme as parameter it is not a regular implementation of the compareTo method.

Bu.
[ September 23, 2006: Message edited by: Burkhard Hassel ]

Hello all,

I tried the code resembles the one given in Head First Java.
The program tries to sort a name list file contains first and last names. The comparision is based on first name.

The compareTo method works fine with Strings. But to compare the custom class we need to implement Comparable interface and override compareTo method. Am I right?

Change the class declaration as:
class Name implements Comparable<Name>
and handle the exception in the go() method. Code will compile.

Here is the error message: "Name is not abstract and does not override abstract method compareTo(java.lang.Object) in java.lang.Comparable".

It tells you exactly what is wrong. As other posters (Naveen and Burkhard) have written above, the signature of your compareTo method is incorrect. You have overloaded, not overridden, the compareTo method.
[ September 23, 2006: Message edited by: Barry Gaunt ]

One other thing, after fixing the code don't expect the 'Name' class' compareTo to be called, because in the 'getNames()' method 'String' is being added to the list and not 'Name' type, therefore Collections.sort(namelist) will make use of the String object's compareTo().

If you want to test your compareTo(), you must add Name type to the list as below.

Hi,

thanks for your valuable comments. I identified what mistake I have comitted. I corrected them and my code worked well. Thanks a lot especially to Krishnan...