TreeMap - Comparator how to get it to work

Question

Hi All,
I am a newbie java programmer. Ik trying to understand de TreeMap en Compator and objects. I got some questions maybe that someone could help me.
I have a class Customer:
import java.util.TreeMap;
import java.util.Set;
import java.util.Iterator;
import java.util.Collection;
import java.util.Map;
public class Customer
     private TreeMap customertable = new TreeMap();
     public void put(Object key, String value)
          // Record subscriber
          customertable.put(key,value);
     public Abonnee get(String key) // Abonnee in english means subscriber
          return (Abonnee)customertable.get(key);
     public void remove(String key)
          customertable.remove(key);
and a comparator class like:
import java.util.*;
class Vergelijk implements Comparator
     public int compare(Object obj1, Object obj2)
     String str = (String)obj1;
     return str.compareTo((String)obj2);
public boolean equal(Object obj)
     return obj == this;
How do i get the Customer class working with the Vergelijk class (comparator) which statements do i have to add and in which class?
My program can add a object but when i add a second object i get a errors like:
java.lang.ClassCastException: Abonnee
at java.util.TreeMap.compare(TreeMap.java:1081)
at java.util.TreeMap.put(TreeMap.java:459)
at Klanten.put(Klanten.java:33)
at GuiVb.toeVoegen(GuiVb.java:84)
at GuiVb.actionPerformed(GuiVb.java:152)
at java.awt.Button.processActionEvent(Button.java:381)
at java.awt.Button.processEvent(Button.java:350)
at java.awt.Component.dispatchEventImpl(Component.java:3598)
at java.awt.Component.dispatchEvent(Component.java:3439)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:450)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchTh
read.java:197)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:144)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:136)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:99)
hope get some good hints.

Answer 1

Maestro, you may want to change you the put method in the Klanten class to public void put(String key, value)
    klantentabel.put(key, value);
}and then replace any occurances of
klant.put(p, p.getNaam());with
klant.put(p.getNaam(), p);Why?
Quote from http://java.sun.com/j2se/1.4.1/docs/api/java/util/TreeMap.html
"This class guarantees that the map will be in ascending key order, ..."
So, if use Strings as keys, you don't need to write your own comparator, since Strings are already comparable.
You did use your Abonne object as key and its name as value when putting data into the map, but on retrieval and deletion you did use Abonne's name as key => obviously can't work (i.e. no matching entry will ever be found).
If you do not understand the concepts of Maps, refer to
http://java.sun.com/j2se/1.4.1/docs/api/java/util/Map.html