Users can add themselves to the guest book database, but they cannot change their data (for example, if their web page address changes), and they cannot remove themselves from the database. You will add those functions in this exercise. As always, you must use proper error / exception handling and show comprehensible error messages to the user of the applet.
If someone wants to change their guest book data, they will first want to retrieve their old data and make changes to that. Fortunately, in the user database, you have a key that uniquely identifies each user.
When the user fills in his/her key (possibly the e-mail address) in the guest book applet and presses the ``Retrieve information'' button, the applet should ask the server for the UserInfo object that corresponds to the given key. The applet should then fill in all text fields and other components correspondingly.
Assuming you completed the previous exercises correctly, there should be an error message when the user presses the submit button and a user with the same key is already registered. Now, you will make it possible for users to change their information.
When the user fills in all information in the guest book applet and presses the ``Update information'' button, the applet should ask the server to update the information for the user with the given key. Of course, if there is no user with the given key, an error message should be shown.
When the user fills in all information in the guest book applet and presses the ``Remove information'' button, the applet should ask the server to remove all information for the user with the given key. Of course, if there is no user with the given key, an error message should be shown.
In a real guest book, users would naturally need to use some kind of password to remove or change their information, but we ignore security in this exercise.