Chapter 2 Introduction to Java Applications 91 We (Web hosting providers)
Chapter 2 Introduction to Java Applications 91 We now begin the design phase of our elevator system, which will span Chapters 2 through 13, Chapter 15 and Chapter 22, in which we gradually develop the design. Appendices G, H and I present the complete Java implementation. A system is a set of components that interact to solve a problem. In our case study, the elevator-simulator application represents the system. A system may contain subsystems, which are systems within a system. Subsystems simplify the design process by managing subsets of system responsibilities. System designers may allocate system responsibilities among the subsystems, design the subsystems, then integrate the subsystems with the overall system. Our elevator-simulation system contains three subsystems, which are defined in the problem statement: 1. the simulator model (which represents the operation of the elevator system), 2. the display of this model on screen (so that the user may view it graphically), and 3. the graphical user interface (that allows the user to control the simulation). We develop the simulator model gradually through Chapter 15 and present the implemented model in Appendix H. We discuss the GUI components allowing the user to control the model in Chapter 12 and introduce how the subsystems work together to form the system in Chapter 13. Finally, we introduce how to display the simulator model on the screen in Chapter 22 and conclude the display in Appendix I. System structure describes the system s objects and their inter-relationships. System behavior describes how the system changes as its objects interact with each other. Every system has both structure and behavior we must design both. However, there are several distinct types of system structures and behaviors. For example, the interaction among the objects in the system differs from the interaction between the user and the system, yet both are interactions that constitute the system behavior. The UML specifies nine types of diagrams for modeling systems. Each diagram models a distinct characteristic of a system s structure or behavior the first four diagrams relate to system structure; the remaining five diagrams relate to system behavior: 1. Class diagram 2. Object diagram 3. Component diagram 4. Deployment diagram 5. Activity diagram 6. Statechart diagram 7. Collaboration diagram 8. Sequence diagram 9. Use-Case diagram Class diagrams, which we explain in Thinking About Objects Section 3.8, model the classes, or building blocks, used to build a system. Each entity in the problem statement is a candidate to be a class in the system (i.e., Person, Elevator, Floor, etc.). Object diagrams, which we also explain in Section 3.8, model a snapshot of the system by modeling a system s objects and their relationships at a specific point in time. Each object represents an instance of a class from the class diagram (e.g., the elevator Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/2/01
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