Object-Oriented Design and Programming (Java)

2 Topics Covered Today 2.3 Advanced Class Design –2.3.4 Design Patterns –2.3.5 Singleton Pattern –2.3.6 Strategy Pattern

3 Reference > –ISBN – > 机械工 业出版社 > – > ISBN:

4 History Patterns originated as an architectural concept by Christopher Alexander. The Timeless Way of Building – 《建筑的永恒之道》

5 Alexander’s Pattern

6 Alexander’s Pattern.

7 Alexander’s Pattern.. Patterns describe a solution so that it can be applied many times without ever being the same.

8 History In 1980s, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming.

9 History Design patterns gained popularity in computer science after the book > by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Gang of Four or GoF) was published in 1994.

10 Design Pattern Design patterns describe practical solutions to common design problems that occur repeatedly in software development. A design pattern description consists of: –Pattern name –A description of the problem that the pattern addresses –A description of the solution (for instance: class structure) –A discussion of the consequences of using the pattern

11 Classification Creational Patterns ( 创建模式 ) –deal with object creation mechanisms Structural Patterns ( 结构模式 ) –describe how classes and objects can be combined to form larger structures. Behavioral Patterns ( 行为模式 ) –identify common communication patterns between objects and realize these patterns.

12 23 Design Patterns Creational Patterns ( 创建模式 ) Abstract Factory ( 抽象工厂 ) Prototype ( 原始模型 ) Singleton ( 单例 ) Builder ( 构建器 ) Factory Method ( 工厂方法 ) Structural Patterns ( 结构模式 ) Adapter ( 适配器 ) Bridge ( 桥梁 ) Composite ( 合成 ) Decorator ( 装饰 ) Fa ç ade ( 外观 ) Flyweight ( 享元 ) Proxy ( 代理 ) Behavioral Patterns ( 行为模式 ) Chain of Responsibility ( 责任链 ) Command ( 命令 ) Iterator ( 迭代子 ) Mediator ( 调停者 ) Memento ( 备忘录 ) Observer ( 观察者 ) State ( 状态 ) Strategy ( 策略 ) Visitor ( 访问者 ) Template Method ( 模版方法 ) Interpreter ( 翻译器 )

13 Topics Covered Today 2.3 Advanced Class Design –2.3.4 Design Patterns –2.3.5 Singleton Pattern –2.3.6 Strategy Pattern

14 One of a Kind In some applications, there are classes that should only be instantiated once. For instance: –system clock of an operating system; –one accounting system in a company; –object used for logging; –catalog of a library system –…

15 Answer These Questions? How could you create a single object? –New MyClass(); What if wanted to create another MyClass object? Could it call new on MyClass again? –Yes, if it is a public class. And if not? –Well, if it is not a public class,only classes in the same package can instantiate it. But they can still instantiate it more than once.

16 Answer These Questions?. Can we do this? –Yes. I suppose it is a class that can not be instantiated because it has a private constructor. Is there ANY object that could use this private constructor? –The code in MyClass is the only code that can call it. public class MyClass { private MyClass() { } }

17 Answer These Questions?.. What does this mean? –MyClass is a class with static method. –We can call the static method like this: MyClass.getInstance(); Why did you use MyClass instead of some object name? –Well, getInstance() is a static method, in other words, it is a CLASS method. We need to use the class name to reference a static method. public class MyClass { public static MyClass getInstance() { } }

18 Answer These Questions?… What if I put things together? Now can I instantiate a MyClass? –Yes Can you think of a second way to instantiate an object? –MyClass.getInstance(); public class MyClass { private MyClass() { } public static MyClass getInstance() { return new MyClass(); }

19 Answer These Questions?…. Can you finish the code so that only One instance of MyClass is ever created?

20 Solution public class ASingletonClass { private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static singleton getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here }

21 Solution public class ASingletonClass { private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static ASingletonClass getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here }

22 Solution public class ASingletonClass { private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static ASingletonClass getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here }

23 Solution public class ASingletonClass { private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static ASingletonClass getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here }

24 Solution public class ASingletonClass{ private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static ASingletonClass getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here } Lazy instantiation （延迟实例化） is important for resource intensive objects.

25 Solution public class ASingletonClass { private static ASingletonClass singletonInstance; // other useful instance variable here private ASingletonClass() { } public static ASingletonClass getSingletonInstance() { if(singletonInstance == null) { singletonInstance = new ASingletonClass(); } return singletonInstance; } // other useful methods here }

26 Singleton Pattern Defined The singleton pattern ensures that –only one instance of a class is created and –provides a method to access that one instance. Singleton pattern class diagram: Singleton -singletonInstance : Singleton -Singleton() +getSingletonInstance() : Singleton

27 Eagerly Created Singleton Instance public class Singleton { private static Singleton singletonInstance = new Singleton(); private Singleton() { } public static Singleton getSingletonInstance() { return singletonInstance; } JVM create the unique instance of the Singleton when class is loaded.

28 Singleton Pattern Example Class ICarnegieInfo contains the contact information for iCarnegie. Only one instance of class ICarnegieInfo can be created. ICarnegieInfo -singletonInstance : ICarnegieInfo -name: String -address: String -telephone: String -ICarnegieInfo() +getSingletonInstance() : ICarnegieInfo +getName():String +getAddress(): String +getTelephone(): String

29 Code Study Unit ICarnegieInfo.java Unit ICarnegieInfoDemo.java

30 ICarnegieInfoDemo.java import java.io.*; public class ICarnegieInfoDemo { private static PrintWriter stdOut = new PrintWriter(System.out, true); public static void main(String[] args) { ICarnegieInfo companyInfo = ICarnegieInfo.getSingletonInstance(); stdOut.println("Name: " + companyInfo.getName()); stdOut.println("Address: " + companyInfo.getAddress()); stdOut.println("Telephone: " + companyInfo.getTelephone()); }

31 Consequences The singleton pattern has the following benefits: –A singleton class can control how and when client code accesses the single instance. –A singleton class can be easily modified if requirements change and the application needs to limit the number of instances to a number other than one.

32 Topics Covered Today 2.3 Advanced Class Design –2.3.4 Design Patterns –2.3.5 Singleton Pattern –2.3.6 Strategy Pattern

33 Started with a simple SimUDuck app Joe works for a company that makes a highly successful duck pond simulation game, SimUDuck. The game can show a large variety of duck species swimming and making quacking sounds. The initial designers of the system used standard OO techniques and created one Duck superclass from which all other duck types inherit.

34 Duck Class Diagram quack() swim() display() // other duck-like methods Duck display() { // looks like a mallard } MallardDuck display() { // looks like a redhead } RedHeadDuck The display() method is abstract, since all duck subclass looks different.

35 Need the Ducks to FLY quack() swim() display() fly() // other duck-like methods Duck display() { // looks like a mallard } MallardDuck display() { // looks like a redhead } RedHeadDuck good or bad??

36 RubberDuck Duck quack() swim() display() fly() RubberDuck quack() { // override to squeak } display() RedHeadDuck display() MallardDuck display()

37 DecoyDuck quack() { // override to do nothing } display() fly() { // override to do nothing } 1. Code is duplicated across subclass. 2. Runtime behavior changes are difficult. RubberDuck quack() { // override} display() fly() { // override to do nothing }

38 Using Interface

39 Design Principle Identify the aspects of your application that vary and separate them from what stays the same. Duck Class Flying behaviors Quacking behaviors pull out what varies

40 Design Principle. Program to an interface, not an implementation. fly() > FlyBehavior fly() { // do nothing } FlyNoWay fly() { // implements duck flying } FlyWithWings

41 Program to interface Programming to an implementation would be: –Dog d = new Dog(); d.bark(); Programming to an interface/supertype would be: –Animal a = new Dog(); a.makeSound(); Assign concrete object at run time: –Animal a = getAnimal(); a.makeSound(); makeSound() > Animal makeSound() { meow(); } meow() {// meow sound} Cat makeSound() { bark();} bark() { // bark sound} Dog

42 Implementing the Duck Behavior quack() > QuackBehavior quack() { // rubber duck squesk } Squeak quack() { // implements duck quacking } Quack quack() { // do nothing } MuteQuack

43 Integrate the Duck Behavior Duck flyBehavior: FlyBehavior quackBehavior: QuackBehavior Duck() swim() display() performQuack() performFly() public abstract class Duck { QuackBehavior quackBehavior; FlyBehavior flyBehavior; public Duck() { } public void performQuack() { quackBehavior.quack(); } public void performFly() { flyBehavior.fly(); } public void swim() { System.out.println("All ducks float."); } } Each Duck has a reference to something that implements QuackBehavior interface Rather than handling the quack behavior itself, the Duck object delegates that behavior to the object referenced by quackBehavior

44 Implementing the Duck Subclass Duck flyBehavior: FlyBehavior quackBehavior: QuackBehavior Duck() swim() display() performQuack() performFly() MallardDuck MallardDuck() display() public class MallardDuck extends Duck { public MallardDuck() { quackBehavior = new Quack(); flyBehavior = new FlyWithWings(); } public void display() { System.out.println(" I am a real mallard duck "); }

45 Define a set of Fly Behaviors //FlyBehavior.java public interface FlyBehavior { void fly(); } // FlyWithWings.java public class FlyWithWings implements FlyBehavior { public void fly() { System.out.println( " I am flying! " ); } } // FlyNoWay.java public class FlyNoWay implements FlyBehavior { public void fly() { System.out.println( " I can not fly. " ); } }

46 Define a set of Quack Behaviors // QuackBehavior.java public interface QuackBehavior { void quack(); } // Quack.java public class Quack implements QuackBehavior { public void quack() { System.out.println("Quack"); } } // MuteQuack.java public class MuteQuack implements QuackBehavior { public void quack() { System.out.println(" >"); } } // Squeak.java public class Squeak implements QuackBehavior { public void quack() { System.out.println("Squeak"); } }

47 Test Class // MiniDuckSimulator public class MiniDuckSimulator { public static void main (String[] args){ Duck mallard = new MallardDuck(); mallard.performQuack(); mallard.performFly(); } Quack I am flying!

48 Setting Behavior Dynamically Duck flyBehavior: FlyBehavior quackBehavior: QuackBehavior Duck() swim() display() performQuack() performFly() setFlyBehavior() setQuackBehavior() public abstract class Duck { QuackBehavior quackBehavior; FlyBehavior flyBehavior; public Duck() { } public void performQuack() { quackBehavior.quack(); } public void performFly() { flyBehavior.fly(); } public void swim() { System.out.println("All ducks float."); } public void setFlyBehavior(FlyBehavior fb) { flyBehavior = fb; } public void setQuackBehavior(QuackBehavior qb) { quackBehavior = qb; } }

49 ModelDuck Duck flyBehavior: FlyBehavior quackBehavior: QuackBehavior Duck() swim() display() performQuack() performFly() setFlyBehavior() setQuackBehavior() ModelDuck MadolDuck() display() public class ModelDuck extends Duck { public ModelDuck() { flyBehavior = new FlyNoWay(); quackBehavior = new MuteQuack(); } public void display() { System.out.println("I am a model duck"); }

50 MiniDuckSimulator2 //MiniDuckSimulator2 public class MiniDuckSimulator2 { public static void main (String[] args) { Duck mallard = new MallardDuck(); mallard.performQuack(); mallard.performFly(); Duck model = new ModelDuck(); model.performFly(); model.performQuack(); model.setFlyBehavior(new FlyWithWings()); model.setQuackBehavior(new Squeak()); model.performFly(); model.performQuack(); } Quack I am flying! I can not fly. > I am flying! Squeak

51 SimUDuck app Class Diagram

52 Another Example: TaxCalculator

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

54 UML Class Diagram

55 Library Syatem

56 Requirement The library system could display the borrowers information with three formats: –Plain text –HTML –XML

57 Using Strategy Pattern

58 Exercise Program spec: –An action adventure game has many game characters, such as king, queen, knight and troll. Each character can make use of one weapon to fight at a time, but can change weapons at any time during the game. These weapons include knife, bow and arrow, sword, axe. Use strategy pattern to design UML class diagram for this program.

59 Summary Singleton pattern ensures that –only one instance of a class is created and –provides a method to access that one instance. Strategy pattern –define a family of algorithms, encapsulate each one, and make them interchangeable. –Strategy lets the algorithm vary independently from the clients that use it.