Introduction to Software Engineering (CEN-4010)

CEN 4010 - Ninth Lecture

Introduction to Software Engineering (CEN-Introduction to Software Engineering (CEN-4010)4010)

Instructor: Masoud Sadjadi

http://www.cs.fiu.edu/~sadjadi/

Object DesignObject Design

Ninth Lecture 2CEN 4010: Introduction to Software Engineering

AcknowledgementsAcknowledgements

Dr. Bernd Bruegge

Dr. Allen Dutoit

OvervieOverview:w:Overview

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AgendaAgenda

Overview of Object Design

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Inheritance and Delegation

Components, Frameworks, and Libraries

System and Object Design Document

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Object DesignObject Design

Object design – is the process of adding details to the requirements

analysis and making implementation decisions– The object designer must choose among different

ways to implement the analysis model with the goal to minimize execution time, memory and other measures of cost.

Requirements Analysis: – Use cases, functional and dynamic model deliver

operations for object model Object Design:

– Iterates on the models, in particular the object model and refine the models

– Object Design serves as the basis of implementation


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Object Design: Closing the GapObject Design: Closing the Gap

Custom objects

Application objects

Off-the-shelf components

Solution objects

System Problem

Machine

System design gap

Object design gap

Requirements gap


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App. vs. Solution Domain App. vs. Solution Domain ObjectsObjects

Application objects– also called domain objects.– represent concepts of the domain that are relevant

to the system.– They are identified by the application domain

specialists and by the end users.

Solution objects – represent concepts that do not have a counterpart

in the application domain.– They are identified by the developers.– Examples: Persistent data stores, user interface

objects, middleware.


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ExampleExample

IncidentReport

Requirements Analysis(Language of Application

Domain)

IncidentReportForm

Object Design(Language of Solution Domain)

Text box Menu Scrollbar


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Refining App. Domain ObjectsRefining App. Domain Objects

New objects are often needed during object design:– The use of design patterns introduces new classes

– The implementation of algorithms may necessitate objects to hold values

– New low-level operations may be needed during the decomposition of high-level operations

Example: The EraseArea() operation in a drawing program.– Conceptually very simple

– Implementation Area represented by pixels Repair () cleans up objects partially covered by the erased

area Redraw() draws objects uncovered by the erasure Draw() erases pixels in background color not covered by

other objects


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Examples of Object Design Examples of Object Design ActivitiesActivities

Identification of existing components Full definition of associations Full definition of classes

– System Design => Service – Object Design => API

Specifying the contract for each component Choosing algorithms and data structures Identifying possibilities of reuse Detection of solution-domain classes Optimization Increase of inheritance Decision on control Packaging


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Object Design Activities Object Design Activities (1)(1)

Specifying constraints

Specifying types &signatures

Identifying patterns

Adjusting patterns

Identifying missingattributes & operations

Specifying visibility

Specification

Specifying exceptions

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Identifying components

Adjusting components

Select Subsystem


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Object Design Activities Object Design Activities (1)(1)

Collapsing classes

Restructuring Optimization

Revisitinginheritance

Optimizing accesspaths

Caching complexcomputations

Delaying complexcomputations

Check Use Cases

Realizing associations


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AgendaAgenda


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Reuse MotivationReuse Motivation

Observation about Modeling of the Real World in [Gamma et al 94]

– Strict modeling of the real world leads to a system that reflects today’s realities but not necessarily tomorrow’s.

– There is a need for reusable and flexible designs.

– Design knowledge complements application domain knowledge and solution domain knowledge.


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Reuse HeuristicsReuse Heuristics

Look for existing classes in class libraries– JSAPI, JTAPI, ....

Select data structures appropriate to the algorithms– Container classes – Arrays, lists, queues, stacks, sets, trees, ...

Define new internal classes and operations only if necessary– Complex operations defined in terms of lower-level

operations might need new classes and operations


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ReuseReuse

Main goal: – Reuse knowledge and functionality from previous

experience to current problem.– Approaches: Inheritance and Delegation

Inheritance – Interface inheritance

Used for concept classifications type hierarchies

– Implementation inheritance Used for code reuse.

Delegation– An alternative to implementation inheritance.

Used for code reuse.


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AgendaAgenda


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Metamodel for InheritanceMetamodel for Inheritance

Inheritance is used to achieve two different goals: Taxonomies and Reuse.

Inheritance

InterfaceInheritance

ImplementationInheritance

Inheritancefor ReuseTaxonomy

Inheritance detectedby generalization

Inheritance detectedby specialization

Analysisactivity

Object Design


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Taxonomy ExampleTaxonomy Example

Mammal

Tiger Wolf Wale


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Implementation InheritanceImplementation Inheritance

A very similar class is already implemented that does almost the same as the desired class implementation.

Example: – I have a List class, I

need a Stack class.

– How about subclassing the Stack class from the List class and providing three methods, Push() and Pop(), Top()?

Add ()

Remove()

List

Push ()Pop()

Stack

Top()

“Already implemented”


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ProblemsProblems

Problem with implementation inheritance:– Some of the inherited operations might exhibit

unwanted behavior. – What happens if the Stack user calls Remove()

instead of Pop()?– What happens if the superclass implementation is

modified?– The subclass developer is exposed to internal

information about the superclass and this may result in tight coupling of subclass to superclass.


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Impl. vs Interface InheritanceImpl. vs Interface Inheritance

Implementation inheritance– Also called class inheritance– Goal: Extend an application’s functionality by

reusing functionality in parent class– Inherit from an existing class with some or all

operations already implemented

Interface inheritance– Also called subtyping– Inherit from an abstract class with all operations

specified, but not yet implemented


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DelegationDelegation

Delegation – is an alternative to implementation inheritance.– is as powerful as implementation inheritance.

In Delegation two objects are involved in handling a request– A receiving object delegates operations to its

delegate. – The developer can make sure that the receiving

object does not allow the client to misuse the delegate object.

Client Receiver DelegateDelegates to calls


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Delegation vs. Impl. Delegation vs. Impl. InheritanceInheritance

Inheritance: Extending a base class by a new operation or overwriting an operation.

Delegation: Catching an operation and sending it to another object.

Which of the following models is better for implementing a stack?

+Add()+Remove()

List

Stack

+Push()+Pop()+Top()

+Push()+Pop()+Top()

Stack

Add()Remove()

List


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ComparisonComparison

Delegation– Pro:

Flexibility: Any object can be replaced at run time by another one (as long as it has the same type)

– Con: Inefficiency: Objects are encapsulated.

Inheritance– Pro:

Straightforward to use Supported by many programming languages Easy to implement new functionality

– Con: Inheritance exposes a subclass to the details of its

parent class Any change in the parent class implementation forces

the subclass to change (which requires recompilation of both)


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AgendaAgenda


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Reuse CodeReuse Code

Select existing code– class libraries– frameworks– components

Adjust the class libraries, framework or components– Change the API if you have the source code.– Use the adapter or bridge pattern if you don’t have

access

What are the differences among class libraries, frameworks, and components?– They are all composed of a set of related classes.


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FrameworksFrameworks

A framework is a reusable partial application that can be specialized to produce custom applications.

Frameworks are targeted to particular technologies, such as data processing or cellular communications, or to application domains, such as user interfaces or real-time avionics.


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Key BenefitsKey Benefits

Reusability – leverages of the application domain knowledge and

prior effort of experienced developers .

Extensibility – is provided by hook methods, which are overwritten

by the application to extend the framework.– Hook methods systematically decouple the

interfaces and behaviors of an application domain from the variations required by an application in a particular context.


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Classification of FrameworksClassification of Frameworks

White-Box Frameworks– Extensibility achieved through inheritance and

dynamic binding. – Existing functionality is extended by subclassing

framework base classes and overriding predefined hook methods

– Often design patterns such as the template method pattern are used to override the hook methods.

Black-Box Frameworks – Extensibility achieved by defining interfaces for

components that can be plugged into the framework.

– Existing functionality is reused by defining components that conform to a particular interface

– These components are integrated with the framework via delegation.


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Another Classification of Another Classification of FrameworksFrameworks

Infrastructure frameworks – aim to simplify the software development process– System infrastructure frameworks are used

internally within a software project and are usually not delivered to a client.

Middleware frameworks – are used to integrate existing distributed

applications and components. – Examples: MFC, DCOM, Java RMI, WebObjects,

WebSphere, WebLogic Enterprise Application [BEA].

Enterprise application frameworks – are application specific and focus on domains– Example domains: telecommunications, avionics,

environmental modeling, manufacturing, financial engineering, enterprise business activities.


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Class libraries and FrameworksClass libraries and Frameworks

Class Libraries: – Less domain specific – Provide a smaller scope of reuse. – Class libraries are passive; no constraint on control

flow. Framework:

– Classes cooperate for a family of related applications.

– Frameworks are active; affect the flow of control. In practice, developers often use both:

– Frameworks often use class libraries internally to simplify the development of the framework.

– Framework event handlers use class libraries to perform basic tasks (e.g. string processing, file management, numerical analysis…. )


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Components and FrameworksComponents and Frameworks

Components– Self-contained instances of classes.– Plugged together to form complete applications.– Defines a cohesive set of operations– Can be used based on the syntax and semantics

of the interface. – Components may even be reused on the binary

code level. The advantage is that applications do not always

have to be recompiled when components change.

Frameworks:– Often used to develop components.– Components are often plugged into black-box

frameworks.


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AgendaAgenda


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Documenting the System Documenting the System DesignDesign

Purpose – System design is documented in the System Design

Document (SDD). It describes design goals set by the project, subsystem decomposition (with UML class diagrams), hardware/software mapping (with UML deployment diagrams), data management, access control, control flow mechanisms, and boundary conditions. The SDD is used to define interfaces between teams of developers and serve as a reference when architecture-level decisions need to be revisited.

Audience – The audience for the SDD includes the project

management, the system architects (i.e., the developers who participate in the system design), and the developers who design and implement each subsystem.

Template – http://wwwbruegge.in.tum.de/OOSE/

SystemDesignDocumentTemplate


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SDD TemplateSDD Template

Outline 1. Introduction

1.1 Purpose of the system 1.2 Design goals 1.3 Definitions, acronyms, and abbreviations 1.4 References 1.5 Overview

2. Current software architecture 3. Proposed software architecture

1. Overview 2. Subsystem decomposition 3. Hardware/software mapping 4. Persistent data management 5. Access control and security 6. Global software control 7. Boundary conditions

4. Subsystem servicesGlossary


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Documenting the Object DesignDocumenting the Object Design

Purpose – Object design is documented in the Object Design

Document (ODD). It describes object design trade-offs made by developers, guidelines they followed for subsystem interfaces, the decomposition of subsystems into packages and classes, and the class interfaces. The ODD is used to exchange interface information among teams and as a reference during testing.

Audience – The audience for the ODD includes system

architects (i.e., the developers who participate in the system design), developers who implement each subsystem, and testers.

Template– http://wwwbruegge.in.tum.de/OOSE/

ObjectDesignDocumentTemplate


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ODD Template ODD Template (1)(1)

Outline

1. Introduction1. Object design trade-offs

e.g., buy vs. build, memory space vs. response time

2. Interface documentation guidelinese.g., naming conventions, boundary cases, exception handling

mechanisms

3. Definitions, acronyms, and abbreviation

4. References

2. Packages describes the decomposition of subsystems into packages

and the file organization of the code. This includes an overview of each package, its dependencies with other packages, and its expected usage.3.

3. Class interfaces

Glossary


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Alternative Approaches for Alternative Approaches for ODDODD

Self-Contained ODD generated from model– Write and maintain a UML model and generate the

document automatically– con: redundancy with the RAD and source code,

hard to maintain consistency. ODD as extension of the RAD

– The set of application objects augmented with solution objects.

– pro: easier to maintain consistency with the RAD.– con: polluting the RAD with irrelevant information

for the user and client, also it may not be possible. ODD embedded into source code

– represent the ODD using a modeling tool.– Once it becomes stable, generate class stubs.– Complete the source code and comments.– Generate the ODD using a tool such as JavaDoc.


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RADDocumentanalysis

Analysis

Analysis model

Object design

Initial objectdesign model

Generate class stubs

Initial classstubs

ODDDocumentobject design

System design

Implementation

Commented code

Subsystemdecomposition Design goals

Embedded ODD approachEmbedded ODD approachOvervieOverview:w:Overview

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AgendaAgenda


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JavaDocJavaDoc

Add documentation comments to the source code.

A doc comment consists of characters between /** and */

When JavaDoc parses a doc comment, leading * characters on each line are discarded. First, blanks and tabs preceding the initial * characters are also discarded.

Doc comments may include HTML tags Example of a doc comment:

/**

* This is a <b> doc </b> comment

*/


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More on JavaDocMore on JavaDoc

Doc comments are only recognized when placed immediately before class, interface, constructor, method or field declarations.

When you embed HTML tags within a doc comment, you should not use heading tags such as <h1> and <h2>, because JavaDoc creates an entire structured document and these structural tags interfere with the formatting of the generated document.

Class and Interface Doc Tags Constructor and Method Doc Tags


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Class and Interface Doc TagsClass and Interface Doc Tags

@author name-text– Creates an “Author” entry.

@version version-text– Creates a “Version” entry.

@see classname– Creates a hyperlink “See Also classname”

@since since-text– Adds a “Since” entry. Usually used to specify that a

feature or change exists since the release number of the software specified in the “since-text”

@deprecated deprecated-text– Adds a comment that this method can no longer be

used. Convention is to describe method that serves as replacement

– Example: @deprecated Replaced by setBounds(int, int, int, int).


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Constructor and Method Doc Constructor and Method Doc TagsTags

Can contain @see tag, @since tag, @deprecated as well as:

@param parameter-name description Adds a parameter to the "Parameters" section. The

description may be continued on the next line.

@return description Adds a "Returns" section, which contains the description of

the return value.

@exception fully-qualified-class-name description Adds a "Throws" section, which contains the name of the

exception that may be thrown by the method. The exception is linked to its class documentation.

@see classname Adds a hyperlink "See Also" entry to the method.


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Example of a Class Doc Example of a Class Doc CommentComment

/** * A class representing a window on the screen. * For example: * <pre> * Window win = new Window(parent); * win.show(); * </pre> * * @author Sami Shaio * @version %I%, %G% * @see java.awt.BaseWindow * @see java.awt.Button */ class Window extends BaseWindow { ...


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Example of a Method Doc Example of a Method Doc CommentComment

/** * Returns the character at the specified index. An

index * ranges from <code>0</code> to <code>length() -

1</code>. * * @param index the index of the desired

character. * @return the desired character. * @exception StringIndexOutOfRangeException * if the index is not in the range

<code>0</code> * to <code>length()-1</code>. * @see java.lang.Character#charValue() */ public char charAt(int index) { ... }


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Example of a Field Doc Example of a Field Doc CommentComment

A field comment can contain only the @see, @since and @deprecated tags

/**

* The X-coordinate of the window.

*

* @see window#1

*/

int x = 1263732;


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Example: Specifying a ServiceExample: Specifying a Service

/** Office is a physical structure in a building. It is possible to create an instance of a office; add an occupant; get the name and the number of occupants */

public class Office {/** Adds an occupant to the office */

* @param NAME name is a nonempty string */

public void AddOccupant(string name);

/** @Return Returns the name of the office. Requires, that Office has been initialized with a name */

public string GetName();

....

}


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Package it all upPackage it all up

Pack up design into discrete physical units that can be edited, compiled, linked, reused

Construct physical modules– Ideally use one package for each subsystem – System decomposition might not be good for

implementation. Two design principles for packaging

– Minimize coupling: Classes in client-supplier relationships are usually

loosely coupled Large number of parameters in some methods mean

strong coupling (> 4-5) Avoid global data

– Maximize cohesion: Classes closely connected by associations => same

package


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Packaging HeuristicsPackaging Heuristics

Each subsystem service is made available by one or more interface objects within the package

Start with one interface object for each subsystem service– Try to limit the number of interface operations (7+-2)

If the subsystem service has too many operations, reconsider the number of interface objects

If you have too many interface objects, reconsider the number of subsystems

Difference between interface objects and Java interfaces– Interface object : Used during requirements analysis,

system design and object design. Denotes a service or API

– Java interface: Used during implementation in Java (A Java interface may or may not implement an interface object)


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AgendaAgenda


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SummarySummary

Object design closes the gap between the requirements and the machine.

Object design is the process of adding details to the requirements analysis and making implementation decisions

Object design activities include: Identification of Reuse Identification of Inheritance and Delegation opportunities Select existing code

– Interface specification (next lecture)

– Object model restructuring

– Object model optimization Object design is documented in the Object Design

Document, which can be automatically generated from a specification using tools such as JavaDoc.


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Introduction to Software Engineering (CEN-4010)

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