Design Patterns References: Xiaoping Jia, Object-Oriented Software Development Using Java;Douglas C.Schmidt, Design Pattern Case Studies with C++

Design Patterns Generic (reusable) solutions to a recurring problem Evolved from Christopher Alexander’s patterns for architecture. “Each 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.” – Alexander et. al., A Pattern Language (Oxford U.Press,1977)

Design Patterns–a ‘Hot Topic’ OOPSLA Workshops – 17% Patterns for Software Architecture Killer Examples for Design Patterns & Objects First Patterns in Distributed Real Time & Embedded Systems Patterns for Customer Interaction & Expectation Management Software Development Process Patterns

Design Patterns–a ‘Hot Topic’ OOPSLA Tutorials – 20% Pattern-oriented Software Architectures for Networked and Concurrent Applications Patterns of Enterprise Application Architecture Patterns at Work Patterns for Writing Effective Use Cases Dungeons and Patterns! Patterns and Application Experiences for Real-time Object Request Brokers Refactoring to Patterns Object-oriented Reengineering: Patterns & Techniques Patterns for EJB Development How to Use Design Patterns in Java and .NET

Design Patterns Categories of Software Design Patterns Creational : deal with process of creation Structural : deal with static composition and structure of classes and objects Behavioral : deal with dynamic interaction among classes and objects

Design Patterns Each pattern is described as follows: Pattern name Category : creational, structural or behavioral Intent : short description of problem addressed AKA: other names for the pattern Applicability : where pattern can be applied Structure : diagram that describes participants in the pattern & relationships among them Participants : list of classes and/or objects that participate in the pattern

Singleton Pattern Category: creational Intent: ensure that a class has only one instance and provide global point of access to it Applicability: use where there must be exactly one instance of a class and it must be accessible to clients from a well-known access point (e.g. database)

Singleton Pattern Structure Singleton static getInstance() operation() getData() static Singleton theInstance data return theInstance

Singleton Pattern Participants : only one Singleton declares the unique instance of the class as a static variable and defines a static method getInstance() for clients to access the unique instance.

Singleton Pattern public class Singleton { public static Singleton getInstance() { return theInstance; } private Singleton() { //initialize instance fields //…. private static Singleton theInstance = new Singleton();

Strategy Pattern Category : behavioral Intent: define a family of algorithms, encapsulate each one and make them interchangeable

Strategy Pattern Applicability: use when many related classes differ only in behavior plotting different functions different variations of algorithm are needed sorts an algorithm uses data that clients should not know about LayoutManager in the AWT a class defines many behaviors which appear as multiple conditional statements in its methods

Structure of Strategy Pattern Context contextInterface() Strategy algorithmInterface() Concrete Strategy A algorithmInterface() Concrete Strategy C algorithmInterface() Concrete Strategy B algorithmInterface() Note: arrowheads should be unfilled

Using the Strategy Pattern Pivot Strategy getPivot() quicksort pivotStrategy.getPivot(array,lo,hi) Random Median of Three Select First Concrete Strategies Strategy pattern resolves how to extend policies for selecting a pivot value without modifying main quicksort algorithm

A tutorial presented by Steve Metsker and William Wake Dungeons & Patterns A tutorial presented by Steve Metsker and William Wake At OOPSLA 2001

Why Design Patterns? Patterns record previous successes in a reusable form Design patterns are well-worn solutions to problems at about a class level

How the Game Works Each table has a pattern master who has a map of a dungeon You must adventure through the dungeon to reclaim pearls of wisdom from a dragon Your primary helper is an Ahobbit With capricious magical abilities

Ahobbits Your Ahobbit has a magical ring that Cannot be removed You can activate by getting him/her to say “Aha” Ahobbits are enlightened only when they understand a design pattern Ahobbits learn from real-world examples in the dungeon

Adventuring Find a room in the dungeon When you enter Examine the device or contents therein Determine which pattern it shows Use the device to explain to your Ahobbit the intent of the pattern (2 guesses) When the Ahobbit says “Aha,” something magical will occur

Beginning We shall begin as one huge exploration party Before breaking into one party per table or group You stand before a door that leads to the dungeon What do you do?