Servlet produce prompt box ?

Not sure if this is a Servlet or a JSP question. I have a Data Entry Screen ( JSP) , User enters Account Number and other info. Submits form and I do Javascript validation. If all the JS validation is good. I pass data to Servlet which then forwards data to a Database class to do some validation. The 1st thing it checks - does Account already exist, if not it then checks all the rows in the database to see if the Account Number exists in a SUB-Account column. Here's my question .... If the Account Number being entered already exists in the SUB-Account column, the users want to be prompted ( alert box ) with the option to continue (add record to DB ) or cancel ( not add record to DB). Also in this box they want to know what Account Number that has the SUB Account thats currently being entered as Lead Account. How do I create an alert box ? Do I create another JSP page or ? I'm new to Servlets and JSP's so hopefully this isn't to complicated. Thanks

In that case, the response you send back to the browser should contain the JavaScript on the page to create and show the prompt box.

Remember, everything happens through the request/response cycle.