Why after I add a object to a list, the whole list changes?

Question

I tried to call the add method of ClientList from a jsp webpage,
by looking at the log file, i found that for the 2nd time insert an Object into clientlist (say
client 1 is: IPAddress= 167.30.22.33
client 2 is IPAddress=167.30.22.44), what i get from the system log is that I have two
167.30.22.44 in the list, the first one is gone,
after I add the third item, the list just contains three identical third item, the former two are gone,
What 's wrong with my code? From the system log, I know that the parameters passed to the method clientlist.add are correct.
thank you.
My code is like this :
public class ClientList {
ArrayList<Client> listitems = null;
int numberOfItems = 0;
public ClientList()
          items = new HashMap<String, ClientListItem>();
          listitems = new ArrayList<Client>();
public synchronized void add(
Client cl) {
if (items.containsKey(cl.getIPAddress())) {
               ClientListItem clitem = (ClientListItem)items.get(cl.getIPAddress());
               clitem.incrementQuantity();
} else {
               System.out.println("clist.add called: " + cl.getIPAddress() + "\n");
               ClientListItem newItem = new ClientListItem(cl);
               items.put(cl.getIPAddress(), newItem);
               listitems.add(cl);
               for (Iterator i = listitems.iterator(); i.hasNext(); )
                    Client clt = (Client)(i.next());
                    System.out.println("after adding : " + clt.getIPAddress() + "\n");
}

Answer 1

I'm willing to bet that what is happening here goes as follows:
// Step 1:  Create a client 1 object
// Step 2:  Call add with this client object creating a string key with a client value
// Step 3:  redefine the data in the first client object with the data for the second
             // In this step you probably did not create anew client object, but simply
             // changed the data in the original
// Step 4:  Call add with the "redefined" client object comparing the string key with
             // some data from the client, determining it is not in the list adding it againWhat the above does (if this is the way you are doing it) simply places the same client
object into the hash twice with two different keys. And in step 3, by changing the original object,
you also changed the object already in the hash, because as already said, they are the same object.
Edit: And nevermind, jverd already said this. ;-)