itec315_lecture 9 about object oriented analysis and design

2
Object-Oriented Analysis and Design with the Unified Process
Objectives
 Explain how statecharts can be used to describe
system behaviors
 Use statecharts to model system behavior and
method logic
 Explain the importance of design patterns in object-
oriented design
 Briefly describe and use the singleton, adaptor, and
observer design patterns

3
Objectives (continued)
 Explain why enterprise-level systems require special
design consideration
 Apply UML deployment architectural notation to
Web-based systems
 Explain how Web services work to obtain
information on the Internet

4
Overview
 Model system and object behavior
 Statecharts
 Protect and stabilize systems
 Protection from variations and indirection
 Establish best practices in system construction
 Design patterns, or templates
 Design for enterprise-level and Web-based systems
 Distributed and networked environments

5
Modeling System Behavior and
Method Logic with Statecharts
 Statecharts were used in the requirements discipline
to capture status conditions of objects
 Statecharts are used in the design discipline to
 Define the behavioral constraints of the system
classes created during design
◘Windows menus, authentication and security classes,
error-handling classes
 Describe the method logic within class methods
◘Action-expressions contain information that help
define method logic

6
System-level Statecharts
 Designate the states and processing controls of a
system
 Which menu items are enabled or disabled
 Which windows are modal
 Which tool bars and icons are active
 Document possible options presented to users
 Use state variables to record the value of the
current state

7
Figure 9-1
Logon statechart for a computer system

8
Messages, Transitions, and
Methods
 A message causes a transition to fire
 A transition in a statechart is the execution of a
method
 A method signature is defined by the message to a
destination object
 Action expressions in a statechart define method
logic
 Describes the actions to be performed during a
transition

9
Figure 9-2
Comparison of methods, messages, and transitions

10
Action-expressions and Method
Logic
 A sequence diagram may not contain enough
internal details about a method
 Design statecharts extend requirements statecharts
 Action-expressions contain logic for later coding
 Transitions that allow the object to remain in the
same state
 Temporary states for inter-model consistency
 Decision pseudostates to indicate decision points on
a paths

11
Figure 9-4
Statechart for Order

12
Figure 9-5
Design statechart for Order

13
Design Principles-Protection From
Variations
 Parts of a system that are unlikely to change are
segregated from those that will
 Drives the multilayer design pattern
 Stable business logic can be protected from
variations in the user interface
 Changes in the business logic are isolated to the
controller class

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Design Principles-Indirection
 Indirection is an implementation of the protection
from variations principle
 Decouples classes or other system components by
placing an intermediate classes between them
 Used in many corporate security systems between
an internal network and the Internet
 A proxy server catches all incoming messages and
redistributes them to the recipients

15
Importance of Design Patterns
 Standard design templates can speed OO design
 Patterns can exist at different levels of abstraction
 At the most concrete level, a class definition with
code
 At the most abstract level, an approach to a problem
 Patterns should contain five main elements
 Pattern name, problem, solution, example, benefits
and consequences

16
Figure 9-6
Pattern description
for the controller
pattern

17
Basic Design Patterns
 The authors of Elements of Reusable Object-
Oriented Software (referred to as the Gang of
Four) developed a basic classification scheme for
patterns (Figure 9-7)
 The 23 GoF patterns are some of the most
fundamental and important patterns in use
 Scores of other patterns have been defined
 For example, both Java and .NET have sets of
enterprise patterns

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Figure 9-7
Classification of design patterns

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Singleton Pattern
 For classes that must have only one instance, but
need to be invoked from several classes and
locations within the system
 The class itself controls the creation of only one
instance
 A static variable of the class refers to the object
that is created
 A class method instantiates the object on the first
call, and returns a reference to the object on
subsequent calls

20
Figure 9-8
Singleton pattern
template

21
Adaptor Pattern
 Plugs an external class into a system
 Converts the method calls from within the system to
match the method names in the external class
 A standard solution for protection from variations
 Insulates the system from frequently changing
classes
 An interface is frequently used to specify and
enforce the use of correct method names

22
Figure 9-10
Adapter pattern
template

23
Observer pattern
 Used to handle event-processing and reduce
coupling
 The domain class
 Allows other classes to “subscribe” as listeners
 “Publishes” the changes to the listeners
 The windows class
 Sends a reference of itself to subscribe as a listener
 Implements the method to be invoked when
notified
◘Inherits the method from the listener interface

24
Figure 9-11
Three classes in the Create new order use case

25
Figure 9-12
Implementation of the observer pattern

26
Figure 9-13
Observer pattern
template

27
Designing Enterprise-level
Systems
 Enterprise-level systems share components among
multiple people or groups in an organization
 Enterprise-level systems almost always use
multiple tiers of computers
 Can be client/server network-based or Internet
based
 Designed with deployment diagrams
 A type of implementation diagram that shows
physical components across different locations

28
Figure 9-14
Differences between client/server and Internet systems

29
UML Notation for Deployment
Diagrams
 Component
 Executable module or program
 Has an application program interface (API) that is
available to the outside world
 Node
 A physical entity at a specific location
 Frameset
 A high-level object that holds items to be displayed
by a browser

30
Figure 9-15
UML component
notation

31
Figure 9-16
UML node notation
Figure 9-17
UML extension for
frameset

32
Simple Internet Architecture
 Used for viewing static information
 The browser component executes on the client
computer
 The Internet server component executes on the
server computer
 Pages reside on the server and are sent to the
browser for display
 Program logic is inserted through scripting
languages, applets, or other controls

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Figure 9-18
Simple Internet architecture

34
Two-layer Architecture
 Primarily captures information from the user and
updates the database
 The domain layer and data access layer are usually
combined
 Input data is sent to a CGI or an application server
 The user-interface classes often contain the
business logic and data access
 Processing takes place with servlets (Java) or code
behind classes (.NET)

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Figure 9-19
Two-layer Internet
architecture

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Three-layer Architecture
 Separates domain layer and data access layers
 Necessary for systems with complex business logic
or multiple user interfaces
 Using CGI
 Provide a use case controller for each form
 Using an application server
 Java tools: Java Server Pages and servlets
 .NET tools: Common Runtime Language and code
behind classes

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Figure 9-20
Three-layer
Internet architecture

38
Web Services
 A program that provides a service via the Internet
 The service is posted in a directory so that systems
can find it
 Requests for information go to the UDDI directory
 UDDI provides the Internet address of a program
offering that service
 The application then requests the information
 All communication is based on XML to allow for
generic data exchange

39
Figure 9-21
Invoking a Web service

40
Summary
 Design statecharts
 Specify precise behavior and constraints of the
various portions of the final system
 Describe the method logic within an object
 Good design is based on two factors
 Good principles
 Standard design patterns

41
Summary (continued)
 Protection from variations
 Separates components that are likely to change
 Indirection
 Intermediate class provides an interface between
elements
 Design pattern examples
 Singleton
 Adaptor
 Observer

itec315_lecture 9 about object oriented analysis and design

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