chapter 8, object design: design patterns ii using uml, patterns, and java object-oriented software...

Chapter 8, Object Design:

Design Patterns II

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Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2

Recall: Why reusable Designs?

A design…

…enables flexibility to change (reusability)

…minimizes the introduction of new problems when fixing old ones (maintainability)

…allows the delivery of more functionality after an initial delivery (extensibility)

…encapsulates software engineering knowledge.


How can we describe Software Engineering Knowledge?

• Software Engineering Knowledge is not only a set of algorithms

• It also contains a catalog of patterns describing generic solutions for recurring problems• Not described in a programming language. • Description usually in natural language. A pattern is

presented in form of a schema consisting of sections of text and pictures (Drawings, UML diagrams, etc.)


Algorithm vs Pattern

• Algorithm:• A method for solving a problem using a finite sequence of

well-defined instructions for solving a problem• Starting from an initial state, the algorithm proceeds

through a series of successive states, eventually terminating in a final state

• Pattern:• „A pattern describes a problem which occurs over and over

again in our environment, and then describes the core of the solution to that problem in such a way that you can use this solution a million times over, without ever doing it the same way twice“• Christopher Alexander, A Pattern language.


Pattern Definition

Original definition (Christopher Alexander): A pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution for conflicting forces (design tradeoffs)

Examples: • The conflicting forces between a sunny room and a room that

does not overheat on on a sunny sommer afternoon• The conflicting forces between a portable and an efficient

software system.


Pattern Language and Pattern Catalogs

• Pattern Language: • A collection of patterns that forms a vocabulary for

understanding and communicating ideas• A collection of patterns and the rules to combine them into

an architectural style. Pattern languages describe software frameworks or families of related systems.

• Pattern Catalog:• A pattern catalog is a collection of related patterns. It

typically subdivides the patterns into at least a small number of broad categories and may include some amount of cross referencing between patterns.


Schemata for Describing Patterns

Alexander’s Schema (“Alexandrian Form”)A Pattern Language – Towns Buildings Construction, Christopher Alexander, Sara Ishikawa, Murray Silverstein, Vol. 2, Oxford University Press, New York, 1977

Gang of Four SchemaDesign Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Addison Wesley, October 1994

Gang of Five SchemaPattern-Oriented Software Architecture - A System of Patterns, Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Michael Stal, Wiley and Sons Ltd., 1996.


Patterns originated in Architecture

• Christopher Alexander’s Philosophy:• Buildings have been built for thousands

of years by users who where not architects

• Users know more about what they need from buildings and towns than an architect

• Good buildings are based on a set of design principles that can be described with a pattern language

Christopher Alexander * 1936 Vienna, Austria - More 200 building projects- Creator of the „Pattern language" - Professor emeritus at UCB.

Although Alexanders patterns are about architecture and urban planning, they are applicable to many other disciplines, including software development.


Alexander’s Schema (“Alexandrian Form”)

• Name of the Pattern• Picture of an example for the pattern• Context• Problem: Short description and elaborate description• Solution: Description and Diagram• Conclusion

• Sections don‘t have explicit headings, sections are separated by symbols • 3 diamonds between context and problem and after the

solution• 3 diamonds between solution and conclusion

• Keyword “Therefore” to separate the problem from the solution.


Example of a Pattern (Alexander’s Schema)

Source http://www.patternlanguage.com/leveltwo/patterns.htm (Subscription required)

http://www.patternlanguage.com/leveltwo/patterns.htm


Name of the Pattern

Picture of an example for the pattern

Problem

Separation of Problem from Solution

Solution: Description

Solution: Diagram


Context Separation between Solution and Context


More Context


Conclusion 151 Small Meeting Rooms


Design Patterns• Design Patterns are the foundation for all SE patterns

• Based on Christopher Alexander‘s patterns• Book by John Vlissedes, Erich Gamma, Ralph Johnson

and Richard Helm, also called the Gang of Four• Idea for the book at a BOF "Towards an Architecture

Handbook“ (Bruce Anderson at OOPSLA’90)

John Vlissedes•* 1961-2005•Stanford•IBM Watson Research Center

Erich Gamma •* 1961•ETH•Taligent, IBM• JUnit, Eclipse,• Jazz

Ralph Johnson•* 1955•University of Illinois, •Smalltalk, Design Patterns, Frameworks, OOPSLA veteran

Richard Helm• University of Melbourne •IBM Research, Boston Consulting Group (Australia) •Design Patterns


3 Types of Design Patterns (GoF Patterns)

• Structural Patterns• Reduce coupling between two or more classes • Introduce an abstract class to enable future extensions• Encapsulate complex structures

• Behavioral Patterns• Allow a choice between algorithms and the assignment of

responsibilies to objects (“Who does what?”)• Characterize complex control flows that are difficult to follow

at runtime

• Creational Patterns• Allow to abstract from complex instantiation processes • Make the system independent from the way its objects are

created, composed and represented.


Taxonomy of Design Patterns (23 Patterns)


Many design patterns use a combination of inheritance and

delegation

Many design patterns use a combination of inheritance and

delegation


Adapter Pattern

ClientInterface

Request()

LegacyClass

ExistingRequest()

adaptee

Adapter

Request()

Client

Delegation.Inheritance

The adapter pattern uses inheritance as well as delegation:

- Interface inheritance: Adapter inherits Request() from ClientInterface

- Delegation: Binds LegacyClass to the Adapter.


Adapter Pattern

• The adapter pattern lets classes work together that couldn’t otherwise because of incompatible interfaces• “Convert the interface of a class into another interface expected

by a client class.”• Used to provide a new interface to existing legacy components

(Interface engineering, reengineering).

• Two adapter patterns:• Class adapter:

• Uses multiple inheritance to adapt one interface to another• Object adapter:

• Uses single inheritance and delegation

• Object adapters are much more frequent. • We cover only object adapters (and call them adapters).


Taxonomy of Design Patterns


Bridge Pattern

Taxonomy inApplication Domain

InheritanceInheritance

Delegation

Taxonomy inSolution Domain


Why the Name Bridge Pattern?It provides a bridge between the Abstraction (in the application domain) and the Implementor (in the solution domain)

Taxonomy inApplication Domain

Taxonomy inSolution Domain


Using a Bridge • The bridge pattern can be used to provide multiple

implementations under the same interface• Example: Interface to a component that is incomplete, not yet

known or unavailable during testing• GetPosition() is needed by VIP, but the class Seat is only

available by two simulations (AIMSeat and SARTSeat). To switch between these, the bridge pattern can be used:

VIP

Seat

SeatImplementation

SARTSeatAIMSeat

imp

GetPosition()SetPosition()


Another use of the Bridge Pattern:Supporting multiple Database Vendors

LeagueStoreImplementorLeagueStoreimp

XML StoreImplementor

ODBC StoreImplementor

JDBC StoreImplementor

Arena


Use of the Bridge Pattern:Supporting multiple Database Vendors

LeagueStoreImplementorLeagueStoreimp

XML StoreImplementor

ODBC StoreImplementor

JDBC StoreImplementor

Arena


The Bridge Pattern allows to postpone Design Decisions to the startup time of a system• Many design decisions are made at design time

(“design window”), or at the latest, at compile time• Bind a client to one of many implementation classes of an

interface

• The bridge pattern is useful to delay this binding between client and interface implementation until run time• Usually the binding occurs at the start up of the system (e.g.

in the constructor of the interface class).


Adapter vs Bridge

• Similarities:• Both hide the details of the underlying implementation

• Difference:• The adapter pattern is geared towards making unrelated

components work together• Applied to systems that are already designed (reengineering,

interface engineering projects) • “Inheritance followed by delegation”

• A bridge, on the other hand, is used up-front in a design to let abstractions and implementations vary independently• Green field engineering of an “extensible system” • New “beasts” can be added to the “zoo” (“application and

solution domain zoo”, even if these are not known at analysis or system design time

• “Delegation followed by inheritance”.


Taxonomy of Design Patterns (23 Patterns)


Facade Pattern• Provides a unified interface to a set of classes in a subsystem

• A façade consists of a set of public operations • Each public operation is delegated to one or more operations

in the classes behind the facade• A facade defines a higher-level interface that makes the

subsystem easier to use (i.e. it abstracts out the gory details).


Subsystem Design with Façade, Adapter, Bridge

• The ideal structure of a subsystem consists of • an interface object • a set of application domain objects (entity objects) modeling

real entities or existing systems• Some of these application domain objects are interfaces

to existing systems• one or more control objects

• We can use design patterns to realize this subsystem structure

• Realization of the interface object: Facade• Provides the interface to the subsystem

• Interface to the entity objects: Adapter or Bridge• Provides the interface to an existing system (legacy system)• The existing system is not necessarily object-oriented!


Good Design with Façade, Adapter and Bridge• A façade should be offered by all subsystems in a

software system which provide a set of services • The façade delegates requests to the appropriate components

within the subsystem. The façade usually does not have to be changed, when the components are changed

• The adapter pattern should be used to interface to existing components and legacy systems• Example: A smart card software system should use an adapter

for a smart card reader from a specific manufacturer

• The bridge pattern should be used to interface to a set of objects with a large probability of change• When the full set of objects is not completely known at

analysis or design time (-> Mock Object Pattern)• When there is a chance that a subsystem or component must

be replaced later after the system has been deployed and client programs use it in the field.


Design Example

• Subsystem 1 VIP can call on any component or class operation look in Subsystem 2 (Vehicle Subsystem).

Vehicle Subsystem

VIP

AIM

Card

SART

Seat


Realizing an Opaque Architecture with a Facade

• The Vehicle Subsystem decides exactly how it is accessed

• No need to worry about misuse by callers

• A subsystem with a façade can be used in an early integration test • We need to write only

stubs for each of the public methods in the façade.

VIP Subsystem

AIM

Card

SA/RT

Seat

Vehicle Subsystem Facade


Taxonomy of Design Pattern


Strategy Pattern

• Different algorithms exists for a specific task• We can switch between the algorithms at run time

• Examples of tasks:• Different collision strategies for objects in video games• Parsing a set of tokens into an abstract syntax tree (Bottom up,

top down)• Sorting a list of customers (Bubble sort, mergesort, quicksort)

• Different algorithms will be appropriate at different times• First build, testing the system, delivering the final product

• If we need a new algorithm, we can add it without disturbing the application or the other algorithms.


Strategy Pattern

Context

ContextInterface()

StrategyAlgorithmInterface

*

ConcreteStrategyC

AlgorithmInterface()

ConcreteStrategyB


ConcreteStrategyA


Policy decides which ConcreteStrategy is best in the current Context.

Policy


Using a Strategy Pattern to Decide between Algorithms at Runtime

Database

SelectSortAlgorithm()Sort()

* SortInterface

Sort()

BubbleSort

Sort()

QuickSort

Sort()

MergeSort

Sort()

Policy

DevelopmentTimeIsImportantExecutionTimeIsImportantSpaceIsImportant

Client


Supporting Multiple implementations of a Network Interface

NetworkInterface

open()close()send()receive()

NetworkConnection

send()receive()setNetworkInterface()

Application

Ethernet


WaveLAN


UMTS


LocationManager

Context ={Mobile, Home, Office}


Taxonomy of Design Patterns


Abstract Factory Pattern Motivation

• Consider a user interface toolkit that supports multiple looks and feel standards for different operating systems:• How can you write a single user interface and make it

portable across the different look and feel standards for these window managers?

• Consider a facility management system for an intelligent house that supports different control systems:• How can you write a single control system that is

independent from the manufacturer?

Abstract Factory

Initiation Assocation:Class ConcreteFactory2 initiates the

associated classes ProductB2 and ProductA2

AbstractProductA

ProductA1 ProductA2

AbstractProductB

ProductB1 ProductB2

AbstractFactory

CreateProductACreateProductB

Client


ConcreteFactory1


ConcreteFactory2


Applicability for Abstract Factory Pattern

• Independence from Initialization or Representation• Manufacturer Independence• Constraints on related products• Cope with upcoming change


Example: A Facility Management System for a House

LightBulb

EIBBulb LuxmateBulb

Blind

EIBBlind LuxmateBlind

IntelligentHouse HouseFactory

createBulb()createBlind()

LuxmateFactoryEIBFactory




Clues in Nonfunctional Requirements for the Use of Design Patterns

• Text: “manufacturer independent”, “device independent”,

“must support a family of products”

=> Abstract Factory Pattern• Text: “must interface with an existing object”

=> Adapter Pattern• Text: “must interface to several systems, some

of them to be developed in the future”, “ an early prototype must be demonstrated”

=>Bridge Pattern• Text: “must interface to existing set of objects”

=> Façade Pattern


Clues in Nonfunctional Requirements for use of Design Patterns (2)

• Text: “complex structure”, “must have variable depth and width”

=> Composite Pattern• Text: “must provide a policy independent from

the mechanism”Strategy Pattern

• Text: “must be location transparent”

=> Proxy Pattern• Text: “must be extensible”,

“must be scalable”

=> Observer Pattern (MVC Architectural Pattern)


Summary

• Composite, Adapter, Bridge, Façade, Proxy (Structural Patterns)• Focus: Composing objects to form larger structures

• Realize new functionality from old functionality, • Provide flexibility and extensibility

• Command, Observer, Strategy, Template (Behavioral Patterns)• Focus: Algorithms and assignment of responsibilities to

objects• Avoid tight coupling to a particular solution

• Abstract Factory, Builder (Creational Patterns)• Focus: Creation of complex objects

• Hide how complex objects are created and put together


Conclusion

Design patterns • provide solutions to common problems• lead to extensible models and code• can be used as is or as examples of interface inheritance

and delegation• apply the same principles to structure and to behavior

• Design patterns solve a lot of your software development problems• Pattern-oriented development

• My favorites: Observer, Composite, Strategy and Builder.


Additional Design Heuristics

• Never use implementation inheritance, always use interface inheritance

• A subclass should never hide operations implemented in a superclass

• If you are tempted to use implementation inheritance, use delegation instead


Example: Modeling Electrical Outlets and Plugs• The Type F electrical plug is

identified by two round pins spaced 19mm apart

• A Type F outlet has two earth (ground) clips on its side, rather than a female earth contact.

• The type F electric frequency is in Hertz (Hz, cycles per second). Even if voltages are similar, a 60 Hz device may not function properly on a 50 Hz current, so check if the type F wall plug is compatible with your electronics.

• Otherwise, one needs a type F plug adaptor that can lower the internal current. Type F adapters cannot, however, change the number of cycles.


Plugs, Transformers and Adapters

• An electric plug has a prong which can be inserted into the receptable of an electrical outlet

• A power transformer allows an appliance to get a specific voltage (110 V) from an electrical outlet providing another voltage (220 V)

• A plug adapter connects appliances and electrical outlets that are incompatible with each other. One end of the adapter is attached to the plug used by the appliance, while the other end of the adapter is connected with the electrical outlet• Provide a UML model for a plug adapter that converts an

appliance using a type F electrical plug (Germany) with a Type A electrical outlet (USA).

• A power adapter is an electrical system consisting of a plug adapter and a power transformer• Model a power adapter that connects a computer notebook (of

your choice) outlet with a Type F (Germany) electrical outlet as well as Type A electrical outlet (USA).


Universal Adapters

• A universal adapter offers all the prongs available in the world and can be inserted into all the electrical outlets anywhere in the world• Model a universal adapter that can be inserted into outlets with 2 thin, 3 thin, 2 thick or 3 thick hole

receptables and that provides 2 thin, 3 thin, 2 thick and 3 thick prongs

• Many 220 V countries have converted to the EU standard of 230 V. Legacy 220 V appliances tolerate small variations above or below the rated voltage. But severe current variations can damage these appliances• How do you model severe current variations? How do you prevent them?

• Model both, the power transformer as well as the universal power adapter, in a single system model. You can choose UML or SysML as modeling language. Which one is better for the modeling task? Justify your answer.


Example of an Algorithm: Solving the Rubic Cube• Speed: Rubik’s Cube in 6 seconds

• http://www.youtube.com/watch?v=jI_zjWssn2g&feature=related

• Concurrency: 2 Rubik Cubes at the same time• http://www.youtube.com/watch?v=RW3akfdEGI8&feature=related

• Scalability: 3 Rubik Cubes in a row• http://www.youtube.com/watch?v=api7yyAoAug&feature=channel

• Scalability: 10 Rubik Cubes in a row• http://www.youtube.com/watch?v=51z0Tf76f3Y&feature=channel

• Miscellaneous categories: • Rubik cube by a 3 year old • http://www.youtube.com/watch?v=uNcf7KD3QUg&feature=related

• Rubik Cube blindfolded • http://www.youtube.com/watch?v=JCkI2qh1SF4&NR=1

• Rubik Cube by a Robot• http://www.youtube.com/watch?v=bNAnUygqOYc&feature=channel

chapter 8, object design: design patterns ii using uml, patterns, and java object-oriented software...

Documents

catalog of patterns

collection of patterns

schemadesign patterns

software frameworks

pattern languages

collection of related

final state pattern

efficient software system