Understanding Comparable and the compareTo( ) method

Question

I have a Contact object which lists a person's information for a "phone book" program.
I am trying to sort the Contact objects in a Vector by way of their last name fields.
I am having a hell of a time trying to use this compareTo( ) method.
I'm really struggling with understanding how to go about making the method work with a comparison. Most of the examples which I find compare "numbers' and not strings which make it more difficult to understand.
I have implimented the comparable Interface at the start of my class. To sort my Vector, I'm attempting to use the compareTo( ) method to determine which Last name field has the lower value. I keep going back and forth working through the error messages I receive and I'm just stumped trying to figure out how to get this thing to work. Can someone please point out the mistakes without writing too much code? I don't want someone to do the work for me. Here is the code:
import java.io.*;
import java.util.*;
class PhoneBook implements Comparable{
     private static Vector v = new Vector();
     private static Iterator iter = v.iterator();
     private static int num, index;
     private static Contact firstContact, secondContact, temp;  //"static" to be referenced from main
     public static void main(String arg[])throws Exception {
          Contact c;
          String str;
          BufferedReader br = new BufferedReader(
      new InputStreamReader(
           new FileInputStream(
                                       new File("contacts.txt"))));
          while((str = br.readLine()) != null){
           v.add(c= new Contact(br.readLine()));
           num = v.size();
           for(int i=0; i < num; i++){
      firstContact = (Contact)v.elementAt(i);
           if(v.elementAt(++i) == null)
                break;
           else
                secondContact = (Contact)v.elementAt(++i);
           index = compareTo(firstContact);
           if(index == 1){
                   temp = (Contact)v.elementAt(i);
                   v.setElementAt(v.elementAt(++i), i);
                   v.setElementAt(temp, ++i);
          public static int compareTo(Object person){
     int x = ((Contact)person).getLName().compareTo(secondContact.getLName());
     return x;
}yet still I get the following compiler error:
PhoneBook.java:4: compareTo(java.lang.Object) in PhoneBook cannot implement compareTo(java.lang.Object) in java.lang.Comparable; compareTo(java.lang.Object) and compareTo(java.lang.Object) are static
class PhoneBook implements Comparable{
^
1 error

Answer 1

You are pretty close here except for one incorrect basic assumption. Your compiler error is because you are trying to implement compareTo() with a static method. However, this is irrelevant because it is not the PhoneBook class that should implement Comparable, but the Contact class.
The idea is that other functions can call the compareTo() method on an instance of Contact and pass a reference to a second instance. Based on the result, the relative sort order of the two objects can be determined.
e.g.Contact c1 = new Contact("Smith", "John");
Contact c2 = new Contact("Jones", "Alan");
int x = c1.compareTo(c2);In your example, you can implement compareTo() to use the corresponding method in String. For compatibility, you should also implement equals().
public class Contact implements Comparable {
   public int compareTo(Object otherContact) {
       Contact otherC = (Contact) otherContact;  // could throw exception
       return getLName().compareTo(otherC.getLName());
   public boolean equals(Object otherContact) {
       Contact otherC = (Contact) otherContact;  // could throw exception
       return getLName().equals(otherC.getLName());
} // end of class Contact I haven't compiled or tested the above code nor does it deal with exceptions (e.g. otherContact is not an instance of Contact) - I'll leave that to you! You'll also need to change your PhoneBook sort routine to invoke firstContact.compareTo(secondContact)
FYI - I understand that this is an academic exercise and so you may need to code the sort yourself, but once Contact implements Comparable, you could sort the Vector using standard Java methods. Check out java.util.Collections - there is a method that will perform your sort in one line of code.
Good Luck.