Design Patterns

Desktop Application Development

Hanoi - February, 2010Duong Trong Tan, (tandt@fpt.edu.vn)

for

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Contents

• Design Issues• Design principles• What is design pattern?• Patterns classification• Commonly used patterns• Case studies

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Notice

• This discussion involves lots of questions, not answer• I assume that you are good at core Java concepts

– and these following symbols:

A

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DESIGN ISSUES Design principles What is design pattern? Patterns classification Commonly used patterns Case studies

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What is design?

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What is a good Design?

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A constant: CHANGE!

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COUPLING | COHESION

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DESIGN PRINCIPLES Design issues

What is design pattern? Patterns classification Commonly used patterns Case studies

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Design Principles

• Encapsulate what varies• Favor composition over inheritance• Programming to interfaces not to implementations• Strive for loosely coupled between objects that interact• Classes should OPEN for extension and CLOSE for modification• Depends on abstraction, not concrete classes• Don't call us, we'll call you• A class should have only one reason to change

Freeman et al., 2004

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WHAT IS DESIGN PATTERN?

Design issues Design principles

Patterns classification Commonly used patterns Case studies

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WHAT ARE TEMPLATES?

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What is a design pattern?

• A design pattern – is a general reusable solution to recurring

problem• Patterns – can specify how objects are created, how they

interact, or how they are structured– typically deal with a small number of classes, but

can be of any size

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Why patterns?

• Provide a communication framework in which ideas can be discussed at a high level

• Understand current systems or APIs better• Guide you quickly find solutions• Better design: flexible, maintainable• Build sustainable products

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Brief history of design patterns

• Idea of “pattern” in civil architecture was originated by Alexandre Christopher

• 1995: GoF published: “Design Patterns: Elements of Object-Oriented Software”

• Now: Design patterns are language-independent– Java, C#, VB, Ruby, PHP,

Smalltalk, etc.

Image courtesy to www.selectorweb.com

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PATTERNS CLASSIFICATION

Design issues Design principles What is design pattern?

Commonly used patterns Case studies

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Design Patterns

Creational

BehavioralStructural

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COMMONLY USED PATTERNS

Design issues Design principles What is design pattern?Patterns classification

Case studies

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ScenarioCounter

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Singleton

• Makes sure only one instance of an object exists within an application.

• A better solution compared to global variables • Provide global access to the object instance.• Example: Counter, Application Menu System, Application

objects, Services, Clients

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Scenario

Enter the complex world through an entry

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Façade

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.(GoF)

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The DBFacade Example

• Common JDBC uses:– Open a connection to DB– Close a connection– Get all tables from DB– Executes a SQL statement– Retrieve result from a

SELECT statement

It looks simpler

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DatabaseFacade Outlook

JDBC APIDatabaseFacade

Clientuses

Connection

Statement

ResultSet

DatabaseMetadata

ResultSetMetadata

Driver…

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ScenarioSorting Employees

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Strategy

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it

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Scenario

What does this code snippet do?

int totalSold = 0;List<Employee> sales = new LinkedList<Employee>();sales = dataSource.getAllData();

for( Employee staff : sales){totalSold += staff.getProductsSold();

}

//see above initIterator<Employee> itr = sales.iterator();while( itr.hasNext())

totalSold += itr.next().getProductsSold();

}

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What do you call this data structure?

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Can we do this?

int totalSold = 0;Tree<Employee> sales = new BinaryTree<Employee>();sales = dataSource.getAllData();

for( Employee staff : sales){totalSold += staff.getProductsSold();

}

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Iterator

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

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ScenarioReflecting latest data

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Observer

GoF

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

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MVC

Robert Eckstein, Sun

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Demo: Observer and MVC in Swing

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Command

“Let’s hide (encapsulate) the way we call (invoke) methods.”

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Example: Menus

• Consider an application menu system.• How do we de-couple our menu system from our document class?

Command Interface

Client

We can do a lot of things (actions) with a document. Each interface might be different!

Receiver

ConcreteCommandMenu contains many menu items.

Composite Pattern!

invoker

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GoF patterns

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CASE STUDIES

Design issues Design principles What is design pattern?Patterns classification Commonly used patterns

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References

• Freeman,E. , Freeman,E. , Bates, B. & Siera, K., Head First Design Patterns, O'Reilly Media, 2004.

• Erich Gamma , Richard Helm , Ralph Johnson , John Vlissides, Design Patterns: Elements of Object-Oriented Software. Addison-Wesley, Boston, 1995.

• Robert Eckstein, Java SE Application Design With MVC, Sun (http://blogs.sun.com/JavaFundamentals/entry/java_se_application_design_with )

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Resources

• http://en.wikipedia.org/wiki/Design_pattern_%28computer_science%29

• http://www.objectmentor.com/resources/articles/Principles_and_Patterns.pdf

• http://www.artima.com/lejava/articles/designprinciples.html

• http://www.oodesign.com/• http://www.javaworld.com/channel_content/j

w-patterns-index.html