To create such would require what we call a requirements document. Your post supplied two sentences and as such falls flat as to requirements.
Beyond that, no one I know would write such a thing in just Java. Today you would create your requirements and design documents, get approval and then begin development. My thought is that it would more likely be another web site with the usual backend of SQL, PHP and such. Java might not be used but a safe bet is that Javascript would be seen on the frontend along with HTML.
There's a lot more such as proper security for the site, logins and data but you would call that out in your documents.