Populated nested collections

Question

Say I have two database tables, users and widgets. A user may have 0 or many widgets. So the tables look like this:
users:
userId   username
  1       One
  2       Two
  3       Threewidgets:
widgetId   userId
  1          2
  2          3
  3          3So this means user One has 0 widgets, user Two has 1 widget and user Three has 2 widgets
So I want to map this data into Java, so I have an Object like this
public class User {
    private int id;
    private String name;
    private Set widgets = new LinkedHashSet();
    //Assume getters and setters for id and name
    public void addWidget(Integer widgetId) {
        widgets.add(widgetId);
}Then I have JDBC code that executes this query:
  SELECT u.userId as id, u.username as name, w.widgetId
    FROM users u, widgets w
   WHERE u.userId = w.userId(+)
ORDER BY u.useridThis would get the widgets for a each user. You need the outer join because I still want to create a User object even if the user has no widgets. So the result from the query is:
id  name   widgetId
1  One
2  Two     1
3  Three   2
3  Three   3So then I want to have code that goes through the ResultSet and create a Set of User objects, one object for each user with the widgets Set of each User object populated with the widgetIds. The contents of the Set would be like this:
id=[1],name=[One],widgets=[]
id=[2],name=[Two],widgets=[[1]]
id=[3],name=[Three],widgets=[[2],[3]]But my question is how do you get that? If you do this:
//This will hold the Users
Set set = new LinkedHashSet();
//Assume ps is a PreparedStatement that will get the ResultSet
ResultSet rs = ps.executeQuery();
//This will hold each User
User user = null;
while(rs.next()) {
    user = new User();
    user.setId(rs.getInt("id"));
    user.setName(rs.getString("name"));
    user.addWidget(new Integer(rs.getInt("widgetId")));
    set.add(user);
}That will get 2 user objects for user 3. I could create the equals() and hashCode() methods for User, but then how can I call addWidget() on the user that is already in the set when I get to the 2nd record in the ResultSet for user 3? Should I do this? (replace the set.add(user) from above with this)
//Assuming I have the equals() and hashCode() methods for User
//set.add() will return false if the user is already in the set
if(!set.add(user)) {
    Iterator i = set.iterator();
    User tmpUser = null;
    while(i.hasNext()) {
        tmpUser = (User)i.next();
        if(user.equals(tmpUser) {
            tmpUser.addWidget(rs.getInt("widgetId"));
            break;
}Would that even work? Is there a better way?

Answer 1

OR or or or
you use join so you have unique key (u.userId) so you can do this on the server side(and this is very good idea) as you don this before
using yours query:
SELECT u.userId as id, u.username as name, w.widgetId
FROM users u, widgets w
WHERE u.userId = w.userId(+)
ORDER BY u.userid
you have always sorted data so you do not have to use hashMap , you should write simple state machine
private static User prevUser;
Integer tmpInt;
while (rs.next()) {
     user = new User();
     user.setId(rs.getInt("id"));
     user.setName(rs.getString("name"));
     // simple temporary buffer because rs.getInt("widgetId") should be readed only once  
        tmpInt = new Integer(rs.getInt("widgetId"));
      * This is your state machine
      * for the first  loop prevUser = null so
      * user.equals(prevUser) will return fals
         * you have to do the check in your equals method in User class
     if (user.equals(prevUser)) {
          prevUser.addWidget(tmpInt);
     } else {
          user.addWidget(tmpInt);
          set.add(user);
          prevUser = user;
public class User {
* equals method in User class
public boolean equals(User myUser) {
     if (myUser == null)
          return false;
     else if (this.id == myUser.getId())
          return true;
     return false;
} // end of User classgood luck