ISBN : 978-81-963532-2-3 (E-Book) elaborating the drawing. In the use case diagram, each use case is represented by an ellipse with the name of the use case written inside the ellipse. All the ellipses (i.e. use cases) of a system are enclosed within a rectangle which represents the system boundary. The name of the system being modeled (such as Library Information System) appears inside the rectangle. The different users of the system are represented by using the stick person icon. Each stick person icon is normally referred to as an actor. An actor is a role played by a user with respect to the system use. It is possible that the same user may play the role of multiple actors. Each actor can participate in one or more use cases. The line connecting the actor and the use case is called the communication relationship. It indicates that the actor makes use of the functionality provided by the use case. Both the human users and the external systems can be represented by stick person icons. When a stick person icon represents an external system, it is annotated by the stereotype <<external system>>. Example 1: Tic-Tac-Toe Computer Game Tic-tac-toe is a computer game in which a human player and the computer make alternative moves on a 3×3 square. A move consists of marking previously unmarked square. The player who first places three consecutive marks along a straight line on the square (i.e. along a row, column, or diagonal) wins the game. As soon as either the human player or the computer wins, a message congratulating the winner should be displayed. If neither player manages to get three consecutive marks along a straight line, but all the squares on the board are filled up, then the game is drawn. The computer always tries to win a game. The use case model for the Tic-tac-toe problem is shown in fig. 13.1. This software has only one use case “play move”. Note that the use case “get-user- move” is not used here. The name “get-user-move” would be inappropriate because the use cases should be named from the user’s perspective. Fig.13.1:Usecasemodelfortic-tac-toegame Text Description Each ellipse on the use case diagram should be accompanied by a text description. The text description 61 Software Engineering Keerthana P, Manasa KN, Ganga D Bengal
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