Problem
Simulating User Logon Tracking with an Array List
Computer systems typically track the number of users logged on to the system at any point in time. Some systems do not allow more than one simultaneous logon by a given user. Write an application to simulate user logon tracking as shown in Figure 9-52. The application should keep a list of users logged on to the system. A user name always must be entered in the User text box to log on or log off. The program also should ensure that only users not already logged on to the system can log on, and only those already logged on can log off. Keep track of the number of users logged on at any point in time, and display this number in a text box that the user cannot modify. Display appropriate messages when a user successfully logs on or off and appropriate error messages when an action fails. For Java 5.0, use a type safe Array List and any other features, such as static imports, as appropriate.
