Lecture 6:Lecture 6: Software Design Software Design (Part I)(Part I)

Dr Valentina PlekhanovaDr Valentina Plekhanova

University of Sunderland, UKUniversity of Sunderland, UK

http://www.cet.sunderland.ac.uk/~cs0vpl/SE-Com185.htmhttp://www.cet.sunderland.ac.uk/~cs0vpl/SE-Com185.htm

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Design Process: DefinitionDesign Process: Definition The next step after requirements analysis and

specification is software designsoftware design. “DesignDesign is the creative process of transforming

the problem into a solution; the description of a solution is also called designdesign.” [Pfleeger, 1998]

DesignDesign is a process of constructing preliminary is a process of constructing preliminary models for producing an expected models for producing an expected product/software/program. product/software/program.

Design processDesign process defines several models of the systems at different levels of abstraction.

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Principles of Software Design: Principles of Software Design: Abstraction and DecompositionAbstraction and Decomposition AbstractionAbstraction is an expression that shows the

most important information of the program without telling its implementation details.

DecompositionDecomposition is a way to divide a big task into small tasks, and to develop details of an abstraction. The benefit of decomposition is to reduce the complexity of programs.

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Principles of Software Design: Principles of Software Design: Decomposition and ModularityDecomposition and Modularity Every design method involves some kind of

decomposition: starting with a high levelhigh level depiction of the system’s key elements and creating lower levellower level looks at how the system’s features and functions will fit together [Pfleeger, 1998].

ModularityModularity is a property that a program consists of many independent modules. A modulemodule is a modest-sized component that performs independently specific functions.

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Principles of Software Design: Principles of Software Design: Encapsulation & Information HidingEncapsulation & Information Hiding EncapsulationEncapsulation is the grouping of related

ideas into one unit, which can thereafter be referred to by a single name.

Information hidingInformation hiding is the use of encapsulation to restrict from external visibility certain information or implementation decisions that are internal to the encapsulation structure.

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Design Levels & Design MethodsDesign Levels & Design Methods Design -> Abstract Design & Detailed DesignDesign -> Abstract Design & Detailed Design Abstract DesignAbstract Design: different methods can be

used, e.g. top down design, modular design, structured design, etc.

Detailed DesignDetailed Design, e.g. high level design, low level design.

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Conceptual Design & Technical DesignConceptual Design & Technical Design This stage is concerned with planning how to

build the software itself. Conceptual designConceptual design tells the customer exactly

what the system will do (i.e. WhatWhat? – conceptual design concentrates on the system’s functions).

Technical designTechnical design allows system builders to understand the actual hardware and software needed to solve the customers’ problem (i.e. HowHow? – technical design describes the form the system will take) [Pfleeger, 1998]

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ArchitectureArchitecture Architecture Architecture associates the system

capabilities identified in the requirements specification with the system components that will implement them.

ComponentsComponents are usually modules, and the architecture also describes the interconnections among them.

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CohesionCohesion CohesionCohesion is an internal property of a module -

level of relationships between elements of the module.

We say that a module has high cohesionhigh cohesion if all of its elements (e.g. procedures, statements) are related strongly in a logical way.

They cooperate to achieve a common goal, which is the function of the module.

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CouplingCoupling CouplingCoupling characterises interrelationship

among modules in a software, i.e. represents a module’s relationship to other modules.

With low coupling we can analyse, modify, test, and reuse modules separately.

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Cohesion and CouplingCohesion and Coupling CohesionCohesion and CouplingCoupling are used for

measurement of independence of elements/modules in a design, i.e. assessment of design quality [Yourdon and Constantine, 1978]

The goal of software engineergoal of software engineer is to design the modules with high cohesion and low coupling.

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Design MethodsDesign Methods Top down designTop down design Bottom up designBottom up design Modular designModular design Structured designStructured design Object oriented designObject oriented design

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Top-down DesignTop-down Design Top-down design supports the abstract and

decomposition principle. By this design method, a system is first

abstracted as a high level unit (e.g. module, process, procedure, function), and then decomposed into low level units, possibly integrated together using some constructs.

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Top-down Software DesignTop-down Software Design We begin by considering the problem to be

computerised as a whole and identify its major components.

For each component we then do the same – identify what its major sub-components are

Each of the sub-components can then be broken down into its sub-components and so on, until we reach a level whereby the individual pieces can be understood and designed without any difficulty.

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Top-down DesignTop-down Design This process of top-down designtop-down design (also

called stepwise refinementstepwise refinement) is a way to manage complexity.

We are able to create user-defined functions and general procedures to carry out subtasks.

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Top-down Design:Top-down Design: Reasons for Use Reasons for Use Systemic way of breaking a large

system/problem into the smaller manageable parts.

Easier to understand the system/problem. Easier to test these smaller parts.

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Top-down Design: An ExampleTop-down Design: An Example

Project

Task 1 Task 3 Task 4Task 2

Subtask 1.1 Subtask 2.1 Subtask 2.2 Subtask 4.1 Subtask 4.2 Subtask 4.3

Subtask 2.2.1 Subtask 4.2.1

Subtask 4.2.1.1

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Object-Oriented DesignObject-Oriented Design Object-oriented designObject-oriented design is a way of designing a

program system by focusing on the design of objects and their integration.

An objectAn object is an encapsulation of attributes and operations (or methods); it takes inputs and performs some functions. In fact, an object is a In fact, an object is a kind of module.kind of module.

An object-oriented program can be regarded as a network of objects that interact with each other by means of sending and receiving messages (which are similar to calling methods in Java).

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Design LevelsDesign Levels High Level DesignHigh Level Design Low Level DesignLow Level Design

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High-level DesignHigh-level Design One way to represent high-level designhigh-level design is

to use a hierarchical structure chart where each box on the chart represents a component or subtask of the program.

A high-level algorithm may be presented either in pseudocodepseudocode or with structure structure chartscharts, and it should be language independent.

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High Level Design: Structure ChartHigh Level Design: Structure Chart

A

DA

B

C

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High Level DesignHigh Level Design High level design documents such as High level design documents such as

structure charts structure charts showshow: : the overall system designthe overall system design how the program is decomposed into separate how the program is decomposed into separate

modulesmodules which modules call which other oneswhich modules call which other ones how they communicate via datahow they communicate via data

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High Level DesignHigh Level Design they do they do notnot show:show: the order in which the modules are calledthe order in which the modules are called the number of times they are called if it’s more the number of times they are called if it’s more

than oncethan once the condition(s) determining optional/conditional the condition(s) determining optional/conditional

invocationinvocation the internal workings of the modulesthe internal workings of the modules

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High-level DesignHigh-level Design High-level program design involves the

identification of components of our program - task, subtasks and decisions about how information flows between them.

We need to make decisions about the type of data to be used, names for variables and where to declare them (local, form-level, module or global variables), and what type of data structures and external data storage requirements are required.

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High-level DesignHigh-level Design At the end of high-level design we will have decided the

forms, objects, procedures and functions needed and where code needs to be added to carry out the work of the program.

All of these decisions may take several passes through the design process, each time round, adding more detail.

It is a process of spiralling down from the requirement level to the code level - though at this stage we do not have to worry about the details of the low-level code for the procedures and functions.

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High-level Design: High-level Design: RepresentationRepresentation We can represent out high-level design

decisions by the use of a structure chart which act as a 'picture' of all the software 'parts' making up the program.

Additional information can be placed on a structure chart to show the flow of data between the different parts.

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High-level Design: High-level Design: An ExampleAn Example

Project (a,b,e,r, z, x,…)

Task 1 (a,b,c,d)

a,ba,b

c,dc,d

Subtask 1.1

Task 2 (e,r,t,y,p)

e,re,r

Subtask 2.1 (t,y,p,w)

t,y,pt,y,p

Function 2.1.Aw

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Week 8:Week 8: 24.04.2003-28.04.2003 24.04.2003-28.04.2003Project Control SessionProject Control Session Tutorial Time: 10 minutes for each Team Tutorial Time: 10 minutes for each Team Students will present project file, particularly

ScheduleSchedule, plus any project documentationdocumentation. Students will describe where they are in the

project and any problems encountered. During the discussion reviewers will ask to see

evidence of deliverables for any tasks that are complete to determine whether they have in fact been done.