Chapter 6

1. Chapter 6 Software Design and design methodology

2. Software Design Deriving a solution which satisfies software requirements

3. Stages of Design • Problem understanding เข้าใจปัญหา • Look at the problem from different angles to discover the design requirements. • Identify one or more solutions หาวิธีการหนึ่งหรือมากกว่า • Evaluate possible solutions and choose the most appropriate depending on the designer's experience and available resources. • Describe solution abstractions อธิบายวิธี • Use graphical, formal or other descriptive notations to describe the components of the design. • Repeat process for each identified abstractionทำซ้ำprocessแต่ละอันuntil the design is expressed in primitive- fundamental terms.

4. The Design Process • Any design may be modelled as a directed graph made up of entities with attributes which participate in relationships. • The system should be described at several different levels of abstraction. อธิบายระบบออกมาเป็นระดับ • Design takes place in overlapping stages. It is artificial to separate it into distinct phases but some separation is usually necessary. ออกแบบลำดับแบบซ้อน

5. Phases in the Design Process

6. Design Phases Architectural design: Identify sub-systems. ระบุsub sys Abstract specification:Specify -กำหนด sub-systems. กำหนดsub sys Interface design:Describe sub-system interfaces. ออกแบบsub interface Component design:Decompose sub-systems into components. แตกระบบออกมา(SA) Data structure design: Design data structures to hold problem data. ออกแบบdata (DB) Algorithm design: Design algorithms for problem functions.

7. Procedural Abstraction • The most obvious design methods involve functional decomposition. แตกออกมาเป็นส่วนย่อยๆ • This leads to programs in which procedures represent distinct logical functions in a program. • Examples of such functions: • “Display menu” • “Get user option” • This is called procedural abstraction

8. Design • Computer systems are not monolithic -massive: they are usually composed of multiple, interacting modules. • Modularity has long been seen as a key to cheap, high quality software. • The goal of system design is to decode: • What the modules are; มีโมดุลอะไรบ้าง • What the modules should be;โมดูลทำอะไร • How the modules interact with one-anotherสามารถติต่อกับดมดูลอื่นหรือไม่

9. Modular programming • In the early days, modular programming was taken to mean constructing programs out of small pieces: “subroutines” • But modularity cannot bring benefits unless the modules are • Coherentซึ่งสอดคล้อง, ไม่ขัดแย้งในตัวเอง • Autonomous ซึ่งอยู่ได้ด้วยตนเอง, ซึ่งมีอิสระในการเลือก • Robust

10. Programs as Functions • Another view is programs as functions: input output x f  f (x) the program is viewed as a function from a set I of legal inputs to a set O of outputs. • There are programming languages (ML, Miranda, LISP) that directly support this view of programming Less well-suited to distributed, non-terminating systems - e.g., process control systems, operating systems like WinNT, ATM machines Well-suited to certain application domains - e.g., compilers

11. Object-Oriented Design • The system is viewed as a collection of interacting objects. • The system state is decentralized and each object manages its own state. • Objects may be instances of an object class and communicate by exchanging methods.

12. Five Criteria for Design Methods • We can identify five criteria to help evaluate modular design methods (Pucc-D): • Modular decomposability; • Modular composability; • Modular understandability; • Modular continuity; • Modular protection. conde (tong-kao-jai) kan pong kunCom

13. Modular Decomposability • This criterion is met by a design method if the method supports the decomposition of a problem into smaller sub-problems, which can be solved independently. • In general method will be repetitive: sub-problems will be divided still further • Top-down design methods fulfil this criterion; stepwise-ทีละชั้น refinement is an example of such method

14. Top-down Design In principle, top-down design involves starting at the uppermost components in the hierarchy and working down the hierarchy level by level. In practice, large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process.

15. Hierarchical Design Structure

16. Modular Composability • A method satisfies this criterion if it leads to the production of modules that may be freely combined to produce new systems. • Composability is directly related to the issue of reusability • Note that composability is often at odds โอกาสที่จะเป็นไปได้with decomposability; top-down design, • for example, tends to produce modules that may not be composed in the way desired • This is because top-down design leads to modules which fulfil a specific function, rather than a general one

17. Examples • The Numerical Algorithm Group (NAG) libraries contain a wide range of routines for solving problems in linear algebra, differential equations, etc. • The Unix shell provides a facility called a pipe, written “”, whereby • the standard output of one program may be redirected to the standard input of another; this convention favours composability.

18. Modular Understandability • A design method satisfies this criterion if it encourages the development of modules which are easily understandable. • COUNTER EXAMPLE 1. Take a thousand lines program, containing no procedures; it’s just a long list of sequential statements. Divide it into twenty blocks, each fifty statements long; make each block a method. • COUNTER EXAMPLE 2. “Go to” statements.

19. Understandability • Related to several component characteristics • Can the component be understood on its own? • Are meaningful names used? • Is the design well-documented? • Are complex algorithms used? • Informally, high complexity means many relationships between different parts of the design.

20. Modular Continuity • A method satisfies this criterion if it leads to the production of software such that a small change in the problem specification leads to a change in just one (or a small number of ) modules. • EXAMPLE.Some projects enforce the rule that no numerical or textual literal should be used in programs: only symbolic constants should be used • COUNTER EXAMPLE.Static arrays (as opposed to open arrays) make this criterion harder to satisfy.

21. Modular Protection • A method satisfied this criterion if it yields architectures in which the effect of an abnormal condition at run-time only effects one (or very few) modules • EXAMPLE. Validating input at source prevents errors from propagating-แพร่พันธุ์ throughout the program. • COUNTER EXAMPLE. Using int types where subrange or short types are appropriate.

22. Five principles for Good Design • From the discussion above, we can distil -refine five principles that should be adhered -ยึดมั่น to: • Linguistic modular units; • Few interfaces;มีให้น้อย • Small interfacesมีให้เล็ก • Explicit interfaces; ชัดเจน • Information hiding. Information tee-nae-non(ชัดเจน) kiew kub Ling – tua-lek (small), mee noi mak (few) FES- ( interfaces)

23. Linguistic Modular Units • A programming language (or design language) should support the principle of linguistic modular units: • Modules must correspond to linguistic units in the language used • EXAMPLE. Java methods and classes • COUNTER EXAMPLE. Subroutines in BASIC are called by giving a line number where execution is to proceed from; there is no way of telling, just by looking at a section of code, that it is a subroutine.

24. Few Interfaces • This principle states that the overall number of communication channels between modules should be as small as possible: • Every module should communicate with as few others as possible. • So, in the system with n modules, there may be a minimum of n-1 and a maximum of links; your system should stay closer to the minimum

25. Few Interfaces

26. Small Interfaces (Loose Coupling) • This principle states: • If any two modules communicate, they should exchange as little information as possible. การแลกเปลี่ยนข้อมุลระหว่างดมดูลให้น้อย • COUNTER EXAMPLE. Declaring all instance variables as public!

27. Coupling • A measure of the strength of the inter-connections between system components. ความสัมพัน์ระหว่างโมดูล • Loose coupling means component changes are unlikely to affect other components. • Shared variables or control information exchange lead to tight coupling. • Loose coupling can be achieved by state decentralization (as in objects) and component communication via parameters or message passing.

28. Tight Coupling

29. Loose Coupling

30. Coupling and Inheritance Object-oriented systems are loosely coupled because there is no shared state and objects communicate using message passing. However, an object class is coupled to its super-classes. Changes made to the attributes or operations in a super-class propagate to all sub-classes.

31. Reusability • A major obstacle to the production of cheap quality software is the intractability of the reusability issue. • Why isn’t writing software more like producing hardware? Why do we start from scratch every time, coding similar problems time after time after time? • Obstacles: • Economic; • Organizational; • Psychological.

32. Stepwise Refinement • The simplest realistic design method, widely used in practice. • Not appropriate for large-scale, distributed systems: mainly applicable to the design of methods. • Basic idea is: • Start with a high-level spec of what a method is to achieve; • Break this down into a small number of problems (usually no more than 10) • For each of these problems do the same; • Repeat until the sub-problems may be solved immediately.

33. Explicit Interfaces • If two modules must communicate, they must do it so that we can see it: • If modules A and B communicate, this must be obvious from the text of A or B or both. • Why? If we change a module, we need to see what other modules may be affected by these changes.

34. Information Hiding • This principle states: ข้อมุลซ่อน • All information about a module, (and particularly how the module does what it does) should be private to the module unless it is specifically declared otherwise. • Thus each module should have some interface, which is how the world sees it anything beyond that interface should be hidden. • The default Java rule: • Make everything private

35. Cohesion A measure of how well a component “fits together”. • A component should implement a single logical entity or function. • Cohesion is a desirable design component attribute as when a change has to be made, it is localized in a single cohesive component. • Various levels of cohesion have been identified. จำนวนกิจกรรมภายในโมดูล

36. Cohesion Levels • Coincidental cohesion (weak) • Parts of a component are simply bundled together. • Logical association (weak) • Components which perform similar functions are grouped. • Temporal cohesion (weak) • Components which are activated at the same time are grouped.

37. Cohesion Levels • Communicational cohesion (medium) • All the elements of a component operate on the same input or produce the same output. • Sequential cohesion (medium) • The output for one part of a component is the input to another part. • Functional cohesion (strong) • Each part of a component is necessary for the execution of a single function. • Object cohesion (strong) • Each operation provides functionality which allows object attributes to be modified or inspected.

38. Cohesion as a Design Attribute • Not well-defined. Often difficult to classify cohesion. • Inheriting attributes from super-classes weakens cohesion. • To understand a component, the super-classes as well as the component class must be examined. • Object class browsers assist with this process.

39. Assignment • 2 students in the Group Project work out for each Design • 4 designs have to be done (explain next) • Report (Final Revision from 4 Designs) - abstract (why design for each work) - conclusion and further work (mistake, development, suggestion) • Presentation

41. Assignment in Design Methodology • Procedural Design • Interface design • Architecture Design • Data Design

42. 1. Procedural Design • Procedural Abstraction +Stepwise Refinement (pseudo code) • Modularity (Modular Design) • Software Procedure

43. Software Procedural Design เน้นรายละเอียดในการประมวลผลแต่ละโมดูล ลำดับเหตุการณ์ จุดตัดสินใจ การทำงานซ้ำ โครงสร้างข้อมูล

44. Module A

45. Module A

46. Procedural Abstraction เป็นการคิดแยกรายละเอียดของปัญหา ออกเป็นระดับที่ชัดเจน

47. Procedural Abstraction “Enter” 1. เดินไปที่ประตู 2. ยื่นมือไปที่ลูกบิด 3. หมุนลูกบิด 4. ดึงประตู 5. เดินเข้าประตู

48. ซอฟต์แวร์คำนวณหาพื้นที่ของสามเหลี่ยม ซอฟต์แวร์คำนวณหาพื้นที่ของสามเหลี่ยม Abstraction ระดับที่ 2 • งานรับข้อมูลส่วนสูง, ฐาน • งานตรวจเช็คข้อมูล เท่ากับศูนย์ หรือติดลบหรือไม่ • งานคำนวณตามสูตร • งานแสดงผล

49. Stepwise Refinement • ออกแบบในลักษณะ Top-down • ขยายรายละเอียดเป็นลำดับขั้น (Hierarchy)

50. Stepwise Refinement open walk to door; reach for knob; open door; repeat until door opens turn knob clockwise; walk through; if knob doesn't turn, then close door. take key out; find correct key; insert in lock; endif pull/push door move out of way; end repeat