Loading all the available values on the page and manipulating them with JavaScript (which I think is kapil's "option 1" ) can be a good choice if you have a small set of values, but it sounds like this would not fit your cases.
Option2 is relatively straight forward to implement, but I don't really have and code that I could pass along. When the user selects a value in the parent list you need to submit the page setting some type of flag that you can use to bypass validation and to tell the "save" action to not save the record to the database but to forward back to the "display" action. Your display action would have the logic to retrieve the child lists from the database.
AJAX is another option that can provide the best aspects of both of these options, but I have yet to integrate AJAX features into a
Struts application.
- Brent