first of all, I am a green hand, I just want to learn something.
1. why can not use the hashtable? you can create a user object whose properties is map the information of this user. use this object as the key. then, you can have another object which extends the vector , use this one as the container of the items. Can you?
2. of course to let shopping cart extends hashtable is not necessary, but I think it can also make sense.