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CSEB233 Fundamentals of Software Engineering

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1 CSEB233 Fundamentals of Software Engineering
Module 4: Software Design Badariah Solemon 2011

2 Objectives To explain set of design principles, concepts, and practices. To describe four design models required for a complete design specification: data design, architectural design, user interface design, and component-level design. To introduce design support tools and evaluation. To introduce design specification document. Badariah Solemon 2011

3 What is Design? Design creates a representation or model of the software but unlike the analysis model, the design model provides detail about: software architecture, data structures, interfaces, and components that are necessary to implement the system. Why is it important? Because the model can be assessed for quality and improved before code is generated, tests are conducted, and more users are involved. Badariah Solemon 2011

4 Analysis Models  Design Model
The four design models: Each of the elements of the analysis/requirements model provides information that is necessary to create the design models required for a complete design specification. Badariah Solemon 2011

5 Characteristics of Good Design
According to McGlaughlin(1991): Must implement all of the explicit requirements contained in the analysis model, and it must accommodate all of the implicit requirements desired by the customer. Must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software. Should provide a complete picture of the software, addressing the data, functional, and behavioral domains from an implementation perspective. Badariah Solemon 2011

6 Design Principles According Davis(1995 ):
The design process should not suffer from ‘tunnel vision.’ The design should be traceable to the analysis model. The design should not reinvent the wheel. The design should “minimize the intellectual distance” [DAV95] between the software and the problem as it exists in the real world. The design should exhibit uniformity and integration. Badariah Solemon 2011

7 Design Principle: The design process should not suffer from ‘tunnel vision.’ The design should not reinvent the wheel. Reinventing the wheel is a phrase that means to duplicate a basic method that has already previously been created or optimized by others. Badariah Solemon 2011

8 Design Principles (cnt’d)
The design should be structured to accommodate change. The design should be structured to degrade gently, even when irregular data, events, or operating conditions are encountered. Design is not coding, coding is not design. The design should be assessed for quality as it is being created, not after the fact. The design should be reviewed to minimize conceptual (semantic) errors. Badariah Solemon 2011

9 Generic Task Set for Design
Refer to the “Task Set” in page 222. Badariah Solemon 2011

10 Design Concepts Fundamental design concepts that span both traditional and OO software development include: Abstraction Architecture Patterns Separation of concerns Modularity Information hiding Functional Independence Refinement Aspects Refactoring OO Design concepts Design classes Badariah Solemon 2011

11 Abstraction Designers should work to derive both procedural and data abstractions that serve the problem. Procedural abstraction – sequence of instructions that have a specific and limited function Data abstractions – a named collection of data that describes a data object Badariah Solemon 2011

12 Abstraction (cnt’d) door details of enter
algorithm implemented with a "knowledge" of the object that is associated with “enter” door manufacturer model number type swing direction weight implemented as a data structure Badariah Solemon 2011

13 Abstraction (cnt’d) Data abstraction refers to, providing only essential features by hiding its background details.   class result { int marks; float percentage; char name[20]; void input(); void output(); } main() bank b1; b1.input(); b1.output(); b1 is an object calling input and output member functions, but that code is invisible to the object b1. Badariah Solemon 2011

14 Architecture Is concerned on:
describing the fundamental organization of the system, identifying its various components and their relationships to each other, and the environment in order to meet the system's quality goals. Also, describe the overall structure of the software : organization of program modules, the manner in which these modules interact, and the structure of data used by the components. Badariah Solemon 2011

15 Patterns Online pattern searching:
Under Quick Search, try to search pattern no You will get ‘automated spell analysis’ pattern. Badariah Solemon 2011

16 Separation of Concerns
Actually, it is a rule of thumb to define how modules should be separated to each other: different or unrelated concerns should be restricted to different modules. Suggests that any complex problem can be easily handled if it is sub-divided into pieces that can be solved independently. Why? So that a problem takes less time and effort to solve. Is manifested in other design concepts: modularity, aspects, functional independence and refinement. Badariah Solemon 2011

17 Modularity When software is divided into components (modules).
Modularize a design: To ease the planning for implementation (coding, To define and deliver software increments, To easily accommodate changes, To efficiently test and debug program, and To conduct long-term maintenance without serious side effects Badariah Solemon 2011

18 Modularity : Example x With Module vs. Without Module
#include <stdio.h> void print_menu(void); main() { … print_menu(); } /* end main */ void print_menu(void) { printf(“This program draws a rectangle”); } /* end function */ vs. Without Module #include <stdio.h> main() { … printf(“This program draws a rectangle”); } Badariah Solemon 2011

19 Information Hiding Suggests that modules should be specified and designed so that information (data structures and algorithm) contain within a module is inaccessible to other modules that have no need for such information. Implies that effective modularity can be achieved by defining a set of independent modules that communicate with one another only necessary information. Badariah Solemon 2011

20 Information Hiding (cnt’d)
Serves as an effective criterion for dividing any piece of equipment, software or hardware, into modules of functionality. Provides flexibility. This flexibility allows a programmer to modify functionality of a computer program during normal evolution as the computer program is changed to better fit the needs of users. When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes. Badariah Solemon 2011

21 Information Hiding: Example
Suppose we have a Time class that counts the time of day: class Time { public: void Display(); private: int ticks; }; The Display() member function prints the current time onscreen. This member function is accessible to all. It's therefore declared public. By contrast, the data member ticks is declared private. Therefore, external users can't access it. Badariah Solemon 2011

22 Information Hiding: Example (cnt’d)
Regardless of how Display() extracts the current timestamp from ticks, users can be sure that it will "do the right thing" and display the correct time onscreen. Badariah Solemon 2011

23 Functional Independence
Achieved by developing independent modules – each module address a specific subset of requirements. Is assessed using cohesion and coupling. Cohesion - is an indication of the relative functional strength of a module. a cohesive module should (ideally) do just one thing. Coupling - is an indication of the relative interdependence among modules. depends on the interface complexity between modules, the point at which entry or reference is made to a module, and what data pass across the interface. Badariah Solemon 2011

24 Functional Independence (cnt’d)
This class lacks cohesion: public class CashRegister { public void enterPayment(int dollars, int quarters, int dimes, int nickels, int pennies) public static final double NICKEL_VALUE = 0.05; public static final double DIME_VALUE = 0.1; public static final double QUARTER_VALUE = 0.25; } Badariah Solemon 2011

25 Functional Independence (cnt’d)
The CashRegister, as described above, involves two concepts: cash register and coin Solution: Make two classes: public class Coin { public Coin(double aValue, String aName){ } public double getValue(){ } } public class CashRegister { public void enterPayment(int coinCount, Coin coinType) { } } Badariah Solemon 2011

26 Functional Independence (cnt’d)
Coupling: Badariah Solemon 2011

27 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 Badariah Solemon 2011

28 Refactoring According to Fowler (1999):
"Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure.” When software is refactored, the existing design is examined for redundancy unused design elements inefficient or unnecessary algorithms poorly constructed or inappropriate data structures or any other design failure that can be corrected to yield a better design. Badariah Solemon 2011

29 Design Model Elements Data/Class Design Architectural Design
Class diagrams transformed into the design class realization and the data structures required to implement the software. Architectural Design Provides high-level overview of the system with detailed descriptions to be given by other design elements. Defines the relationship between major structural elements of the software, the architectural styles and design patterns that can be used to achieve the requirements of the system and the constraints that affect the way in which the architecture can be implemented. Badariah Solemon 2011

30 Design Models (cnt’d) Interface Design Component-Level Design
Describes how the software communicates with systems that interoperate with it, and with human who use it. Component-Level Design Defines the data structures, algorithms, interface characteristics, and communication mechanisms allocated to each software component or module. Badariah Solemon 2011

31 Data/Class Design Elements
example: High-level DFD Source: Badariah Solemon 2011

32 Data/Class Design Elements (cnt’d)
example: ERD Source: Badariah Solemon 2011

33 Data/Class Design Elements (cnt’d)
example: Class Diagram Source: Badariah Solemon 2011

34 Architectural Design Elements
The architectural model [Sha96] is derived from three sources: information about the application domain for the software to be built; specific requirements model elements such as data flow diagrams or analysis classes, their relationships and collaborations for the problem at hand, and the availability of architectural patterns (Chapter 12) and styles (Chapter 9). Badariah Solemon 2011

35 Architectural Design Elements (cnt’d)
Example: architecture diagram Source: Badariah Solemon 2011

36 Architectural Design Elements (cnt’d)
Example: Architectural styles Source: x Badariah Solemon 2011

37 Interface Design Elements
External interfaces to other systems, devices, networks or other producers or consumers of information Badariah Solemon 2011

38 Interface Design Elements (cnt’d)
The user interface (UI) Badariah Solemon 2011

39 Component Design Elements
UML Component Diagram Source: Badariah Solemon 2011

40 Design Support Tools All aspects of the software engineering can be supported by software tools: from project management software through tools for business and functional analysis, system design, code storage, compilers, translation tools, test software, and so on. However, tools that are concerned with analysis and design, and with using design information to create parts (or all) of the software product, are most frequently thought of as CASE tools. List of CASE tools: Badariah Solemon 2011

41 CASE Tool: Example IBM Rational Software Architect
Source: Badariah Solemon 2011

42 CASE Tool: Example (cnt’d)
Database Modeling: Enterprise Architect by Sparx Systems Pty Ltd. Source: Badariah Solemon 2011

43 Design Evaluation "A good software design minimizes the time required to create, modify, and maintain the software while achieving run-time performance.“ (Shore and Chromatic, 2007) According to Elssamadisy (2007): Design quality is people-sensitive. For instance, design quality is dependent upon the programmers writing and maintaining the code. Design quality is change-specific. There are two general ways to make designs of higher quality with respect to this aspect: generalizing from the tools like design patterns, and using tests and refactoring as change-enablers. Badariah Solemon 2011

44 Design Evaluation (cnt’d)
Modification and maintenance time are more important than creation time. Because: time spent in maintenance is much more than creation time of a software, and in iterative development, modification happens during the initial creation of the software. Design quality is unpredictable. Because quality is really dependant on the team developing the software. As the team changes, or evolves, then the design quality also evolves. You really only know how good a design is by it standing the test of time and modifications. Badariah Solemon 2011

45 Design Evaluation (cnt’d)
Points to ponder: to maintain the quality of the design, we must maintain the theory of the design as the programming team evolves, That design quality is tied to the people who are building and maintaining the software(Shore and Chromatic, 2007). Badariah Solemon 2011

46 Design Specification IEEE Standard for Information Technology -Systems Design - Software Design Descriptions (IEEE ) an improved version of the 1998 version. This standard specifies an organizational structure for a software design description (SDD). An SDD is a document used to specify system architecture and application design in a software related project. Provides a generic template for an SDD. Badariah Solemon 2011

47 SDD: Examples Refer to Refer to SDDSample.doc Badariah Solemon 2011

48 Summary This module has:
Introduced set of design principles, concepts, and practices. Described four design models required for a complete design specification: data design, architectural design, user interface design, and component-level design. Introduced design support tools and evaluation. Introduced design specification document (SDD). Badariah Solemon 2011

49 References Pressman, R. (2003). Software Engineering: A Practitioner's Approach. 6th & 5th edition. New York: McGraw-Hill. Somerville I. (2001). Software Engineering. 6th edition. Addison Wesley Shore, J. and Chromatic, (2007). The Art of Agile Development. O'Reilly Media. Elssamadisy, A. (2007). Defining Design Quality. Badariah Solemon 2011

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