1. Lecture 6: Software Design (Part I)
Dr Valentina Plekhanova
University of Sunderland, UK

2. Design Process: Definition
The next step after requirements analysis and specification is software design.
“Design is the creative process of transforming the problem into a solution; the description of a solution is also called design.” [Pfleeger, 1998]
Design is a process of constructing preliminary models for producing an expected product/software/program.
Design process defines several models of the systems at different levels of abstraction.
Lecture 6
Valentina Plekhanova

3. Principles of Software Design: Abstraction and Decomposition
Abstraction is an expression that shows the most important information of the program without telling its implementation details.
Decomposition 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.
Lecture 6
Valentina Plekhanova

4. Principles of Software Design: Decomposition and Modularity
Every design method involves some kind of decomposition: starting with a high level depiction of the system’s key elements and creating lower level looks at how the system’s features and functions will fit together [Pfleeger, 1998].
Modularity is a property that a program consists of many independent modules. A module is a modest-sized component that performs independently specific functions.
Lecture 6
Valentina Plekhanova

5. Principles of Software Design: Encapsulation & Information Hiding
Encapsulation is the grouping of related ideas into one unit, which can thereafter be referred to by a single name.
Information hiding is the use of encapsulation to restrict from external visibility certain information or implementation decisions that are internal to the encapsulation structure.
Lecture 6
Valentina Plekhanova

6. Design Levels & Design Methods
Design -> Abstract Design & Detailed Design
Abstract Design: different methods can be used, e.g. top down design, modular design, structured design, etc.
Detailed Design, e.g. high level design, low level design.
Lecture 6
Valentina Plekhanova

7. Conceptual Design & Technical Design
This stage is concerned with planning how to build the software itself.
Conceptual design tells the customer exactly what the system will do (i.e. What? – conceptual design concentrates on the system’s functions).
Technical design allows system builders to understand the actual hardware and software needed to solve the customers’ problem (i.e. How? – technical design describes the form the system will take) [Pfleeger, 1998]
Lecture 6
Valentina Plekhanova

8. Architecture
Architecture associates the system capabilities identified in the requirements specification with the system components that will implement them.
Components are usually modules, and the architecture also describes the interconnections among them.
Lecture 6
Valentina Plekhanova

9. Cohesion
Cohesion is an internal property of a module - level of relationships between elements of the module.
We say that a module has high 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.
Lecture 6
Valentina Plekhanova

10. Coupling
Coupling 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.
Lecture 6
Valentina Plekhanova

11. Cohesion and Coupling
Cohesion and Coupling 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 engineer is to design the modules with high cohesion and low coupling.
Lecture 6
Valentina Plekhanova

12. Design Methods Top down design Bottom up design Modular design
Structured design
Object oriented design
Lecture 6
Valentina Plekhanova

13. Top-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.
Lecture 6
Valentina Plekhanova

14. Top-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.
Lecture 6
Valentina Plekhanova

15. Top-down Design
This process of top-down design (also called stepwise refinement) is a way to manage complexity.
We are able to create user-defined functions and general procedures to carry out subtasks.
Lecture 6
Valentina Plekhanova

16. Top-down Design: 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.
Lecture 6
Valentina Plekhanova

17. Top-down Design: An Example
Lecture 6
Valentina Plekhanova

18. Object-Oriented Design
Object-oriented design is a way of designing a program system by focusing on the design of objects and their integration.
An object is an encapsulation of attributes and operations (or methods); it takes inputs and performs some functions. In fact, an object is a 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).
Lecture 6
Valentina Plekhanova

19. Design Levels High Level Design Low Level Design Lecture 6
Valentina Plekhanova

20. High-level Design
One way to represent high-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 pseudocode or with structure charts, and it should be language independent.
Lecture 6
Valentina Plekhanova

21. High Level Design: Structure Chart
Lecture 6
Valentina Plekhanova

22. High Level Design
High level design documents such as structure charts show:
the overall system design
how the program is decomposed into separate modules
which modules call which other ones
how they communicate via data
Lecture 6
Valentina Plekhanova

23. High Level Design they do not show:
the order in which the modules are called
the number of times they are called if it’s more than once
the condition(s) determining optional/conditional invocation
the internal workings of the modules
Lecture 6
Valentina Plekhanova

24. High-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.
Lecture 6
Valentina Plekhanova

25. High-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.
Lecture 6
Valentina Plekhanova

26. High-level Design: Representation
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.
Lecture 6
Valentina Plekhanova

27. High-level Design: An Example
Project (a,b,e,r, z, x,…)
e,r
a,b
Task 1 (a,b,c,d)
Task 2 (e,r,t,y,p)
t,y,p
c,d
Subtask 2.1 (t,y,p,w)
Subtask 1.1
Function 2.1.A w
Lecture 6
Valentina Plekhanova

28. Week 8: 24.04.2003-28.04.2003 Project Control Session
Tutorial Time: 10 minutes for each Team
Students will present project file, particularly Schedule, plus any project documentation.
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.
Lecture 6
Valentina Plekhanova