As well as these various stages of programming system design，the software engineer may also be required to design communication mechanisms allowing processes in the system to communicate[5].He or she may have to design file structures，and will almost certainly have to design the data structures used in his programs.He or she will have to design test cases to validate his programs.

　　There is no definitive way of establishing what is meant by a“good”design.Depending on the application and the particular project requirements，a good design might be a design which allows very efficient code to be produced，it might be a minimal design where the implementation is as compact as possible，or it might be the most maintainable design.This latter criterion is the criterion of“goodness”adopted here.A maintainable design implies that the cost of system changes is minimized and this means that the design should be understandable and that changes should be local in effect.Both of these are achieved if the software design is highly cohesive and loosely coupled[6].

　　Effective software design is best accomplished by using a consistent design methodology.There have been a vast number of design methodologies developed and used in different applications.Some of these are described by Peters(1980)and by Blank and Krijger(1983).In essence，most of these methodologies can be classified into one of three areas：

　　(1)Top-down functional design.The system is designed from a functional viewpoint，starting with a high-level view and progressively refining this into a more detailed design.This methodology is exemplified by Structured Design and stepwise refinement.

　　(2)Object-oriented design.The system is viewed as a collection of objects rather than as functions with messages passed from object to object.Each object has its own set of associated operations.Object-oriented design is based on the idea of information hiding which was first put forward by Parnas(1972)and which has been described more recently by Robson(1981)and Booch(1983).

　　(3)Data-driven design.This methodology，suggested by Jackson(1975)and Warnier(1977)suggests that the structure of a software system should reflect the structure of the data processed by that system.Therefore，the software design is derived from an analysis of the input and output system data.

　　NOTES

　　[1] straightforward：直接了當的，簡單明了的。

　　[2] ad hoc：特別的(地)。

　　[3] flowchart：流程圖。

　　[4] HIPO charts：層次輸入一處理一輸出圖。

　　[5] allowing... ：修飾communication mechanisms。

　　[6] loosely coupled：松散地耦合。

　　KEYWORDS

　　software design 軟件設計

　　object-oriented design 面向目標的設計

　　maintainable design 可維護性設計

　　data-driven design 數據驅動的設計

　　top-down functional design 自頂向下的功能設計

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