Transcript Systems Analysis and Design Allen Dennis and

Systems Analysis and
Design
Alan Dennis, Barbara Wixom, and David
Tegarden
John Wiley & Sons, Inc.
Slides by Fred Niederman
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Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
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John Wiley & Sons, Inc.
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Slide 2
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Systems Analysis and Design
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Class and Method Design
Chapter 14
Slide 3
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Systems Analysis and Design
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Key Concepts
Low-level or detailed design is
critical despite libraries and
components
Pre-existing classes need to be
understood and organized
Some, if not all code, is generally
still needed to instantiate new
classes
Slide 4
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Systems Analysis and Design
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REVISITING THE BASIC
CHARACTERISTICS OF
OBJECT-ORIENTATION
Slide 5
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Elements
Classes
Objects
Attributes
States
Methods
Messages
Slide 6
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Encapsulation
Hiding the content of the object
from outside view
Communication only through
object’s methods
Key to reusability
Slide 7
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Polymorphism
Same message triggers different
methods in different objects
Dynamic binding means specific method
is selected at run time
Implementation of dynamic binding is
language specific
Need to be very careful about run time
errors
Need to ensure semantic consistency
Slide 8
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Systems Analysis and Design
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Inheritance
Single inheritance -- one parent
class
Multiple inheritance -- multiple
parent classes
Redefinition and inheritance conflict
Most inheritance conflicts are due to
poor classification
Slide 9
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Systems Analysis and Design
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Rumbaugh’s Rules
Query operations should not be redefined
Methods that redefine inherited ones should only restrict
the semantics of the inherited ones
The underlying semantics of the inherited method should
never be changed
The signature (argument list) of the inherited method should
never be changed
Slide 10
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Systems Analysis and Design
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Additional Inheritance
Conflicts
Two inherited attributes or methods
have same name and semantics
Two inherited attributes or methods
have different name, but same
semantics
Two inherited attributes or methods
have same name and different
semantics
Slide 11
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Systems Analysis and Design
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Your Turn
A major university graduates about
10,000 students a year and the
development office wants to build a
web-based system to solicit and
track donations.
In such a system, name 3 examples of
classes that might have one subclass;
name 1 example of a subclass that
might have 2 superclasses
Slide 12
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Systems Analysis and Design
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DESIGN CRITERIA
Slide 13
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Systems Analysis and Design
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Coupling
Interdependency among
modules
Interaction coupling through
message passing
Slide 14
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Systems Analysis and Design
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Law of Demeter
Messages should be sent only by an object
to itself
to an object contained in an attribute of itself or a superclass
to an object that is passed as a parameter to the method
to an object that is created by the method
to an object that is stored in a global variable
Slide 15
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Types of Interactive Coupling
Level
Type
Good
No Direct Coupling
Data
Stamp
Control
Common or Global
Content or Pathological
Bad
Slide 16
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Systems Analysis and Design
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Cohesion
“Single-mindedness of a module”
Method cohesion
Class cohesion
Generalization/specialization
cohesion
Slide 17
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Types of Method Cohesion
Level
Type
Good
Functional
Sequential
Communicational
Procedural
Temporal or Classical
Logical
Coincidental
Bad
Slide 18
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Types of Class Cohesion
Level
Type
Good
Ideal
Mixed-role
Mixed-domain
Worse
Slide 19
Mixed-instance
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Connascence
Creating the need to change
another module as a result of
changing one
Minimize overall connascence
Minimize across encapsulation
boundaries
Maximize within encapsulation boundary
Slide 20
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Types of Connascence
Type
Name
Type or Class
Convention
Algorithm
Position
Slide 21
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OBJECT DESIGN
ACTIVITIES
Slide 22
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Additional Specification
First review the current set of
models
Sufficient but only necessary
classes to solve problem
No missing attributes or methods
No extraneous attributes or
methods
Examine visibility
Slide 23
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Systems Analysis and Design
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Signatures for each
method
Name of the method
Parameters or arguments to
pass
Type of value the method will
return to the calling method
Slide 24
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Define constraints
Pre-conditions
Post conditions
Invariants
How to handle violations
(exceptions in C++ and Java)?
Slide 25
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Systems Analysis and Design
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Identify Opportunities for
Reuse
Patterns
Framework
Class libraries
Components
Slide 26
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Restructure the Design
Factoring
Map design to current language
Normalization
Assure all inheritance
relationships support only
generalization/specialization
semantics
Slide 27
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Systems Analysis and Design
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Optimizing the Design
Review access paths
Review attributes of each class
Review direct and indirect fan-out
Consider execution order of
statements in often-used methods
Avoid recomputation by creating
derived attributes and triggers
Slide 28
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Systems Analysis and Design
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Constraints and Contracts
A set of constraints and
guarantees
Written in natural language,
structured English, pseudocode,
or formal language
Slide 29
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Systems Analysis and Design
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Invariants on a Class
Diagram
Slide 30
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Systems Analysis and Design
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Simple Contract Format
Method Name:
Class Name:
ID:
Clients (Consumers):
Associated Use Cases:
Description of Responsibilities:
Arguments Received:
Type of Value Returned:
Pre-Conditions:
Post-Conditions:
Slide 31
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METHOD SPECIFICATION
Slide 32
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Syntax
No formal syntax specification
General information
Events
Message Passing
Algorithm Specification
Slide 33
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Structured English
Slide 34
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Pseudocode Example
(Get CD-info
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Return
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PowerPoint Presentation for Dennis, Wixom & Tegarden
Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
PowerPoint Presentation for Dennis, Wixom & Tegarden
Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
PowerPoint Presentation for Dennis, Wixom & Tegarden
Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
PowerPoint Presentation for Dennis, Wixom & Tegarden
Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
PowerPoint Presentation for Dennis, Wixom & Tegarden
Systems Analysis and Design
Copyright 2001 © John Wiley & Sons, Inc. All rights reserved.
Summary
Translating the basic elements of OO analysis
into design requires attention to prevent
difficulties and conflicts in execution.
Coupling, cohesion, and connascence are
proposed for criteria for evaluating design.
The designer will look for opportunities to
optimize the design, will create constraints and
contracts, and will specify methods.
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Systems Analysis and Design
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Expanding the Domain
The C and C++ users group is a
source of unusually rich
thoughts regarding OO
development. Check them out
at:
http://www.accu.org/
http://www.accu-usa.org/
Slide 41
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