1. CompSci 280 S2 2107 Introduction to Software Development
Design Principles

2. Today’s Agenda Topics: Reading: Introduction Quality Guidelines
Design Quality
Fundamental Concepts
Reading:
Software Engineering 10th Edition, Ian Somerville
Chapter 6: Architectural Design
Software Engineering: A Practitioner’s Approach, 7/e by Roger S. Pressman
Chapter 8: Design Concept
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3. 1.Introduction Quality Guidelines
A design should exhibit an architecture that (1) has been created using recognizable architectural styles or patterns, (2) is composed of components that exhibit good design characteristics and (3) can be implemented in an evolutionary fashion
For smaller systems, design can sometimes be developed linearly.
A design should be modular; that is, the software should be logically partitioned into elements or subsystems
A design should contain distinct representations of data, architecture, interfaces, and components.
A design should lead to data structures that are appropriate for the classes to be implemented and are drawn from recognizable data patterns.
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4. 1.Introduction Quality Guidelines
A design should lead to components that exhibit independent functional characteristics.
A design should lead to interfaces that reduce the complexity of connections between components and with the external environment.
A design should be represented using a notation that effectively communicates its meaning.
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5. 1.Introduction Design Quality
Design quality is an elusive concept
Quality depends on specific organizational priorities
A “good” design may be the most efficient, the cheapest, the most maintainable, the most reliable, etc
The attributes discussed here are concerned with the clarity and maintainability of the design
Object-oriented decomposition considers the system as a set of objects
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6. 2.Fundamental Concepts (Overview)
Abstraction — data, procedure, control
Architecture — the overall structure of the software
Separation of concerns — any complex problem can be more easily handled if it is subdivided into pieces
Modularity — compartmentalization of data and function
Information Hiding — controlled interfaces
Functional independence — single-minded function and low coupling
OO design concepts — concepts such as classes and objects, inheritance, polymorphism etc.
Design Classes — provide design detail that will enable analysis classes to be implemented
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7. 2.Design Concepts Data Abstraction
An abstraction is a view or representation of an entity that includes only the attributes of significance in a particular context.
Abstraction is essential when working with complex systems.
Complex
Object
Idealised
Model
Ignore
Inessential Details
door
implemented as a data structure
manufacturer
model number
type
swing direction
inserts
lights
number
weight
opening mechanism
Instance variables:
Names : String
DateOfBirth : Date
Methods:
getSurname(): String
getFirstname (): String
getAge(): Integer
setSurname(name: String): void
Customer
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8. 2.Design Concepts Separation of Concerns
Any complex problem can be more easily handled if it is subdivided into pieces that can each be solved and/or optimized independently
A concern is a feature or behavior that is specified as part of the requirements model for the software
By separating concerns into smaller, and therefore more manageable pieces, a problem takes less effort and time to solve Separate issues (break down large problems into pieces) and concentrate on one at a time
Separate concerns into classes and methods
Separate data from UI, and UI from application logic
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9. 2.Design Concepts Modularity
Monolithic software (i.e., a large program composed of a single module) cannot be easily grasped by a software engineer.
The number of control paths, span of reference, number of variables, and overall complexity would make understanding close to impossible.
In almost all instances, you should break the design into many modules, hoping to make understanding easier and as a consequence, reduce the cost required to build the software.
optimal number
of modules
cost of
software
number of modules
module
integration
cost
module development cost
What is the "right" number of modules for a specific software design?
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10. 2.Design Concepts Advantages of Modular Systems
Easier to understand: when dealing with a module the details of other modules can be ignored (separation of concerns)
Modules can be developed & maintained independently
Separation of work: different teams for different modules
Independent testing of modules
Modules can be reused in several systems
Modules can be replaced by other modules with the same interface
Isolation between modules can prevent failure in one module to cause failure in other modules
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11. 2.Design Concepts Spaghetti Code vs. Modular System
Haphazard connections, probably grown over time
No visible cohesive groups
High coupling: high interaction between random parts
Understand it all or nothing
Modular System
High cohesion within modules
Low coupling between modules
Modules can be understood separately
Interaction between modules is well- understood and thoroughly specified
10 parts, 13 connections
10 parts, 13 connections, 3 modules
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12. 2.Design Concepts Information Hiding
Hide information that does not need to be visible in order to use a class/module/program
Too much information can be confusing: what is important for usage and what not?
Too much information can lead to undesired dependencies
If internals are visible & accessible, someone might use/change them (use something in an unintended manner)
If internals are changed then external code that relies on them might not work anymore
Allowing only restricted access gives us more flexibility
Class/module/program can be (ex)changed without breaking other parts
Many design decisions can be hidden and the system design can evolve without collapsing
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13. 2.Design Concepts Information Hiding
module
• algorithm
controlled
interface
• data structure
• details of external interface
• resource allocation policy
clients
"secret"
a specific design decision
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14. 2.Design Concepts OO Design Concepts
Design classes
Entity classes
Boundary classes
Controller classes
Inheritance—all responsibilities of a superclass is immediately inherited by all subclasses
Messages—stimulate some behavior to occur in the receiving object
Polymorphism—a characteristic that greatly reduces the effort required to extend the design
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15. 2.Design Concepts Design Classes
Analysis classes are refined during design to become entity classes
Boundary classes are developed during design to create the interface (e.g., interactive screen or printed reports) that the user sees and interacts with as the software is used.
Boundary classes are designed with the responsibility of managing the way entity objects are represented to users.
Controller classes are designed to manage
the creation or update of entity objects;
the instantiation of boundary objects as they obtain information from entity objects;
complex communication between sets of objects;
validation of data communicated between objects or between the user and the application.
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