Pooling objects

Question

Hello,
I have a dilemma on how to manage object instances. Imagine having an Address Book. Now an Address Book can have more then one Contact. Each Contact has the Contact Details and Personal Information and Working Details.
Now Contact Details and Personal Information are different for each Contact, however Working Details can be shared between different Contacts.
I could simply avoid this problem by creating a new Working Details object for each contact even when these would be the same. However I really wish to avoid this. I want to like have a pool of Working details and then the Contacts would just have a reference to this WorkDetails.
AddressBook addressBook = new AddressBook();
WorkDetails wd = new WorkDetails(working details);
Contact one = new Contact(contact one details);
Contact two = new Contact(contact two details);
one.setWorkDetails(wd);
two.setWorkDetails(wd);
addressBook.addContact(one);
addressBook.addContact(two);In the above code I am creating one working details object and then share it between contact one and two.
Now my question is, should I make a pool for working details in the address book object, and then I make it only possible to add working details to a contact only if already existing in the pool of the address book.
Or else as soon as an instance of working details is created it is also added in the shared pool of the address book, and when a new object with the same working details is created it just retrieves it from this pool?
Is this a good design? Or there are other better ways how I can implement this. I have given a simple example, but if not enough I will try to come with others.
Regards,
Sim085

Answer 1

I dont think you need to go down to pen and paper ist just basic OO design, and knowing how to apply patterns here is some example code. There may be some errors just double check it. The code is just for demonstration purpose and should not be directly used in to your application.
public interface Contact{
    public String getName();
    public WDetails getWDetails();
public interface WDetails{
    public String getName();
    public String gatData();
import java.util.*;
public class ContactFactory{
    private List<WDetails> wDList = new LinkedList<WDetails>();
    private Map<Contact,WDetails> cMap = new HashMap<Contact,WDetails>();
    public Contact createContact(String cName,String wDName, String wDData){
        ContactImpl con = new ContactImpl(cName);
        //if(cMap.containsKey(contact)) throw some exception
        WDetails wd = new WDetailsImpl(wDName,wDData);
     if(wDList.contains(wd))wd = wDList.get(wDList.indexOf(wd));
     else{
         //this cast is safe
         ((WDetailsImpl)wd).unsetQuery();
         wDList.add(wd);
     cMap.put(con,wd);
     return con;
    public void removeContact(Contact con){
        WDetails wd = con.getWDetails();
     cMap.remove(wd);
     if(!cMap.containsValue(wd))wDList.remove(wd);
    private class ContactImpl implements Contact{
        String name;
        private ContactImpl(String name){
         this.name = name;
     public boolean equals(Object o){
         if(!(o instanceof Contact)) return false;
         Contact rhs = (Contact)o;
         return name.equals(rhs.getName());
     public int hashCode(){
         return name.hashCode();
        public String getName(){
         return name;
        public WDetails getWDetails(){
         if(!cMap.containsKey(this))return null;//throw some exception here
         return cMap.get(this);
    private class WDetailsImpl implements WDetails{
        String name,data;
     boolean queryMode = true;//do not check this in equals or hashCode
     private WDetailsImpl(String name,String data){
         this.name = name;
         this.data = data;
     public boolean equals(Object o){
         if(!(o instanceof WDetails))return false;
         WDetails rhs = (WDetails)o;
         return name.equals(rhs.getName());
     public int hashCode(){
         return name.hashCode();
     private boolean isQuery(){
         return queryMode;
     private void unsetQuery(){
         queryMode = false;
        public String getName(){
            return name ;
        public String gatData(){
            return data;
}