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As to your question, the answer would seem to be authentication. If this is part of a web app, then you can add basic or form authentication to protect the page. The Tomcat FAQ page has some pointers on how to set up web app security.
Have a login page. Every time before you display your calculator page, check your HttpSession object to make sure that the user has logged in. when the user logout of your site, invalidate your session. You can store the usernames and passwords in DataBase, LDAP server or flat files.
google for form-based authentication.
-- Basic/Digest authentication: Browser specific and password is encoded using Base-64 encoding. Digest is similar to basic but protects the password through encryption. This is a simple challenge-response scheme where the client is challenged for a user id and password. The Internet is divided into realms.
-- Form-based authentication: Most Web applications use the form-based authentication since it allows applications to customise the authentication interface. Uses base64 encoding which can expose username and password unless all connections are over SSL. (Since this is the most common let us look at in greater detail together ie authentication & authorisation under Authorisation).
-- Certificate based authentication: Uses PKI and SSL. This is by far the most secured authentication method.