SE Test2 练习及参考答案

COMP223 Test 2 Review Questions

Chapter 5

1. Exercise 5.1 on Page 143

Explain why it is important to model the context of a system that is being developed. Give two examples of possible errors that could arise if software engineers do not understand the system context.

At early stage in the requirements elicitation and analysis process software engineers should decide on the boundaries of the system. This involves working with system stakeholders to distinguish what is the system and what is the s ystem’s environment. Software engineers should make these decisions early in the process to limit the system costs and the time needed for analysis.

The definition of a system boundary is not a value-free judgment.

The position of the system boundary has profound effect on the system requirements. There may be pressures to develop system boundaries that increase/decrease the influence or workload of different parts of an organization. Social and organizational concerns may mean that the position of a system boundary may be determined by non-technical factors.

Two examples:

a. Software engineers may give an incorrect system environment in

requirements documents.

b. System cost and time of system analysis may increase in the future

because the wrong system boundary was defined in the previous process.

2. Exercise 5.2 on Page 143

How might you use a model of a system that already exists? Explain why it is not always necessary for such a system model to be complete and correct. Would the same be true if you were developing a model of

a new system?

a. Using an existed model during requirements engineering will

decrease the system cost and time when setting up the corresponding system model. But it is hard to say whether the chosen model is

fulfilling to the all requirements of the current system.

b. There is no absolutely true or false for a certain system model. System modeling is the process of developing abstract models of a system, with each model presenting a different view or perspective of that system. It only helps the analyst to understand the functionality of the system and models are used to communicate with customers. System model determined by the functional and non-functional requirements, therefore different system has different system model to fulfill the system requirements and there is no complete or correct system.

c. Models of the new system are used during requirements engineering to help explain the proposed requirements to other system stakeholders. Engineers use these models to discuss design proposals and to document the system for implementation.

3. Give a short explanation of each of the following

Activity diagrams:

Use case diagrams

Sequence diagrams

Class diagrams

State diagrams

a. activity diagrams

Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. Activity diagrams can be used to describe the business and operational step-by-step workflows of components in a system. An activity diagram shows the overall flow of control.

b. use case diagrams

Use case were developed originally to support requirements elicitation and now incorporated into the UML. Each use case diagram represents a discrete task that involves external interaction with a system. Actors in a use case diagram may be people or other systems.

a Sequence diagrams:

Sequence diagrams are part of the UML and are used to model the interactions between the actors and the objects within a system. A sequence diagram shows the sequence of interactions that take place