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Chapter 8, Object Design: Reuse and Patterns I

Bernd Bruegge & Allen Dutoit Object-Oriented Software Engineering: Conquering Complex and Changing Systems 2

Object 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;

- Serves as the basis of implementation

Where is architectural design?Cf. Shaw&Garlan - OO

Hence, OOAD (but without architecture)


Examples of Object Design Activities 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 (Re-)Packaging


1. Reuse: Identification of existing solutions Use of inheritance Off-the-shelf components and additional solution

objects Design patterns

2. Interface specification Describes precisely each class interface

3. Object model restructuring Transforms the object design model to improve its

understandability and extensibility

4. Object model optimization Transforms the object design model to address

performance criteria such as response time or memory utilization.

Object Designlectures

MappingModels to Code

lecture


Outline of Today

The use of inheritance Implementation vs Interface Inheritance Delegation Documenting the Object Design JavaDoc In Appendix

In Appendix


Application Domain vs Solution Domain Objects

IncidentReport

Requirements Analysis(Language of Application

Domain)

IncidentReport

Object Design(Language of Solution Domain)

Text box Menu Scrollbar

Do the two Incident Reports have the same meaning?


The use of inheritance Inheritance is used to achieve two different goals

Description of Taxonomies Interface Specification

Identification of taxonomies Used during requirements analysis. Activity: identify application domain objects that are hierarchically related Goal: make the analysis model more understandable

Service specification Used during object design Activity: Goal: increase reusability, enhance modifiability and extensibility

Inheritance is found either by specialization or generalization


Taxonomy Example

Mammal

Tiger Wolf Wale

Is this hierarchy for enhancing understandability or reuse?

Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 9

Problem with implementation inheritance:

Some of the inherited operations might exhibit unwanted behavior.

What happens if the Stack user calls Remove() instead of Pop()?

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”

Implementation Inheritance

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

An elephant becomes a car!


Implementation Inheritance vs Interface Inheritance

Implementation inheritance Also called class inheritance Goal: Extend an applications’ 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

Hurts understandability, maintainability, reliability, …


Client Receiver DelegateDelegates to calls

Delegation as alternative to 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

Cf. notion of modularization w. import/export - ADT


Delegation instead of Implementation Inheritance

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

What should Push() and Pop() be like?

How can this be modelled in UML?Where did we discuss UML “interface”?


Comparison: Delegation vs Implementation Inheritance

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)

why?


Many design patterns use a combination of inheritance and

delegation

Many design patterns use a combination of inheritance and

delegation

Lecture on Design Patterns


Frameworks 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.

The key benefits of frameworks are reusability and extensibility. 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.

Are J2EE and the .NET framework frameworks or architectural specs?


Example: Framework for Building Web Applications

WebBrowser

RelationalDatabase

StaticHTML

WOAdaptorWebServer

WoRequest Template

WebObjectsApplication

WORequest

EOF

WebObjects


A Little Bit of Terminology: Activities Object-Oriented methodologies use these terms:

System Design Activity Decomposition into subsystems

Object Design Activity Implementation language chosen Data structures and algorithms chosen

Structured analysis/structured design uses these terms: Preliminary Design Activity

Decomposition into subsystems Data structures are chosen

Detailed Design Activity Algorithms are chosen Data structures are refined Implementation language is chosen Typically in parallel with preliminary design, not a separate activity

?


Summary

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 Component selection 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.

Lecture on Mapping Models to Code


Additional Slides


Object 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


A More Detailed View of Object Design Activities

Specifying constraints

Specifying types &signatures

Identifying patterns

Adjusting patterns

Identifying missingattributes & operations

Specifying visibility

Specification

Specifying exceptions

Reuse

Identifying components

Adjusting components

Select Subsystem


Detailed View of Object Design Activities (ctd)

Collapsing classes

Restructuring Optimization

Revisitinginheritance

Optimizing accesspaths

Caching complexcomputations

Delaying complexcomputations

Check Use Cases

Realizing associations


Web Browser(UI Framework)

Web Server

HTTP

State Profil

A1: Application Server

StateState

StateState

State

A2: Application Server

ProfilProfil

Profil

Profil

Database Server(Database Framework)


Reuse

Main goal: Reuse knowledge from previous experience to current problem Reuse functionality already available

Composition (also called Black Box Reuse) New functionality is obtained by aggregation The new object with more functionality is an aggregation of existing

components

Inheritance (also called White-box Reuse) New functionality is obtained by inheritance.

Three ways to get new functionality: Implementation inheritance Interface inheritance Delegation


Reuse 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


Reuse Concepts

Application objects versus solution objects Specification inheritance and implementation inheritance The Liskov Substitution Principle Delegation (Section 8.3.3) Delegation and inheritance in design patterns


Observation about Modeling of the Real World

[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.


Application domain vs solution domain objects

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.


Metamodel for Inheritance

Inheritance is used during analysis and object design

Inheritance

SpecificationInheritance

ImplementationInheritance

Inheritancefor ReuseTaxonomy

Inheritance detectedby generalization

Inheritance detectedby specialization

Analysisactivity

Object Design


Implementation of Application Domain Classes

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


Component Selection

Select existing off-the-shelf class libraries frameworks or 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

Architecture Driven Design


Classification of Frameworks

Frameworks can be classified by their position in the software development process.

Frameworks can also be classified by the techniques used to extend them. Whitebox frameworks Blackbox frameworks


Frameworks in the Development Process

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.


White-box and Black-Box Frameworks

Whitebox 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.

Blackbox 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.


Class 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…. )


Components and Frameworks

Components Self-contained instances of classes Plugged together to form complete applications. Blackbox that defines a cohesive set of operations, Can be used based on the syntax and semantics of the interface. Components can 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 blackbox frameworks.


Documenting the Object Design: The Object Design Document (ODD)

Object design document Same as the Requirements Analysis Document (RAD) plus... … additions to object, functional and dynamic models (from solution

domain) … navigational map for object model … Javadoc documentation for all classes

ODD Management issues Update the system models in the RAD? Should the ODD be a separate document? Who is the target audience for these documents (Customer, developer?) If time is short: Focus on the Navigational Map and Javadoc

documentation? ODD Template:

http://www.oose.org


Documenting Object Design: ODD Conventions

Each subsystem in a system provides a service (see Chapters on System Design)

Describes the set of operations provided by the subsystem

Specifying a service operation as Signature: Name of operation, fully typed parameter list and return type Abstract: Describes the operation Pre: Precondition for calling the operation Post: Postcondition describing important state after the execution of the

operation

Use JavaDoc for the specification of service operations.

Shouldn’t these be part of architectural spec?

Example: what should withdraw be like?

More In Appendix


JavaDoc

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

*/

More In Appendix


More 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


Class 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).


Constructor and Method Doc Tags

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.


Example of a Class Doc Comment

/**

* 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 {

...

}


Example of a Method Doc Comment

/**

* 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) {

...

}


Example of a Field Doc Comment

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

/**

* The X-coordinate of the window.

*

* @see window#1

*/

int x = 1263732;


Example: Specifying a Service in Java

/** 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();

....

}


Package 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


Packaging 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)

Documents

Chapter 8, Object Design: Reuse and Patterns I