16/07/2012. Design Patterns. By Võ Văn Hải Faculty of Information Technologies - HUI. Behavioral Patterns. Session objectives. Strategy.

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1 Design Patterns By Võ Văn Hải Faculty f Infrmatin Technlgies - HUI Behaviral Patterns Sessin bjectives Strategy Observer 2 1

Behaviral Patterns 3 Mtivating example - SimpleUDuck Je wrks fr a cmpany that makes a highly successful duck pnd simulatin game called SimUDuck.

2 Behaviral Patterns 3 Mtivating example - SimpleUDuck Je wrks fr a cmpany that makes a highly successful duck pnd simulatin game called SimUDuck. The game can shw a large variety f duck species swimming and making quacking sunds. Initial designers used standard OO techniques and created ne Duck super-class frm which ther duck types inherit. 4 2

3 Mtivating example First apprach: Using Inheritance 5 Mtivating example First apprach Nw we need them t fly 6 3

It desn t quack( ) r fly( ) 7 Mtivating example Secnd Apprach: Using Interface The

4 Mtivating example First apprachpatterns They just gave a dem and there were rubber duckies flying arund the screen. Hw abut DecyDuck? It desn t quack( ) r fly( ) 7 Mtivating example Secnd Apprach: Using Interface The aspects that change fr each type f duck are the methds fly() and quack() Take these methds ut f the Duck class. 8 4

Mtivating example Secnd Apprach Disadvantage f this slutin: All methds in Java interfaces are abstract. Cde has t be duplicated fr classes. Mdificatin will have t be made t mre than ne class.

5 Mtivating example Secnd Apprach Disadvantage f this slutin: All methds in Java interfaces are abstract. Cde has t be duplicated fr classes. Mdificatin will have t be made t mre than ne class. This will intrduce bugs. Design principles: Identify the aspects f the applicatin that vary and separate them frm what stays the same. Encapsulate the parts that vary. Future changes can be made withut affecting parts that d nt vary. Results in fewer unexpected cnsequences frm cde change. 9 Behaviral Patterns Mtivating example Third Apprach Separate what varies - separate what changes frm what stays cnstant. 1. We knw that fly ( ) and quack ( ) are the parts f the Duck class that vary acrss ducks. 2. T separate these behavirs frm the Duck class, we ll pull bth methds ut f the Duck class and create a new set f classes t represent each behavir. 10 5

12 Definitin Definitin Defines a family f algrithms, encapsulates each ne, and makes them interchangeable. Strategy lets the algrithm vary independently frm clients that use it.

6 12 Definitin Definitin Defines a family f algrithms, encapsulates each ne, and makes them interchangeable. Strategy lets the algrithm vary independently frm clients that use it. Use the Strategy pattern when many related classes differ nly in their behavir. Strategies prvide a way t cnfigure a class with ne f many behavirs yu need different variants f algrithm an algrithms uses data that client shuldn t knw abut a class defines many behavirs, and these appear as multiple cnditinal statements in its peratins 13 6

7 UML 14 Cde sample 15 7

8 Cde sample 16 Cde sample 17 8

9 Cde sample - excercise Add a new type f duck t the simulatr, namely, DdelDuck. A mdel duck des nt fly and quacks. Add a new fly behavir t the simulatr, namely, FlyRcketPwered, which represents flight via a rcket. Create an instance f a MdelDuck and change its behavir at runtime t be flight via a rcket. 18 Other Example 19 9

10 Behaviral Patterns 20 Mtivatin Example: Weather Frecast Our wrk: create an applicatin that uses the WeatherData bject t update three displays fr current cnditins, weather stats and the frecast

Mtivatin Example: Weather Frecast Unpacking the WeatherData class 22 Mtivatin Example: Weather Frecast The WeatherData class has getter methds that btain measurement values frm temperature, humidity

11 Mtivatin Example: Weather Frecast Unpacking the WeatherData class 22 Mtivatin Example: Weather Frecast The WeatherData class has getter methds that btain measurement values frm temperature, humidity and pressure. The class has a measurementschanged() methd that updates the three values. Three displays must be implemented: current cnditins, statistics and frecast display. System must be expandable ther display elements maybe added r remved

12 Mtivatin Example: First Apprach Hw t add r remve ther display elements withut making changes t the prgram? 24 Real wrld example: newspaper/magazine subscriptins A newspaper publisher ges int business and begins publishing newspapers. Yu subscribe t a particular publisher, and every time there s a new editin it gets delivered t yu. As lng as yu remain a subscriber, yu get new newspapers. Yu unsubscribe when yu dn t want papers anymre, and they stp being delivered. While the publisher remains in business, peple, htels, airlines and ther businesses cnstantly subscribe and unsubscribe t the newspaper. Publishers + Subscribers = 25 12

13 Intrductin Definitin: The defines a ne-t-many dependency between bjects s that when ne bject changes state, all f its dependents are ntified and updated autmatically Features: The bject that changes state is called the subject and the ther bjects are the bservers. 26 UML 27 13

14 UML - cmpnents Subject knws its bservers. Any number f Observer bjects may bserve a subject. prvides an interface fr attaching and detaching Observer bjects. CncreteSubject stres state f interest t CncreteObserver sends a ntificatin t its bservers when its state changes Observer defines an updating interface fr bjects that shuld be ntified f changes in a subject. CncreteObserver maintains a reference t a CncreteSubject bject stres state that shuld stay cnsistent with the subject's implements the Observer updating interface t keep its state cnsistent with the subject's 28 Mtivatin Example: applied t my apprach 29 14

Mtivatin Example: results Subject Observer bjects 30 Summary

15 Mtivatin Example: results Subject Observer bjects 30 Summary Strategy Defines a family f algrithms, encapsulates each ne, and makes them interchangeable. Strategy lets the algrithm vary independently frm clients that use it. Observer Defines a ne-t-many dependency between bjects s that when ne bject changes state, all f its dependents are ntified and updated autmatically 31 15

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Design Patterns. Collectional Patterns. Session objectives 11/06/2012. Introduction. Composite pattern. Iterator pattern

Design Patterns. Collectional Patterns. Session objectives 11/06/2012. Introduction. Composite pattern. Iterator pattern Design Patterns By Võ Văn Hải Faculty f Infrmatin Technlgies HUI Cllectinal Patterns Sessin bjectives Intrductin Cmpsite pattern Iteratr pattern 2 1 Intrductin Cllectinal patterns primarily: Deal with