List of Design Patterns

1. Creational Patterns Abstract Factory Builder Factory Method Prototype Singleton Structural Patterns Adapter Bridge Composite (Sammy’s Slides) Decorator (Sammy’s Slides) Facade Flyweight Proxy List of Design Patterns • Behavioral Patterns • Chain of Responsibility • Command • Interpreter • Iterator • Mediator • Memento • Observer • State • Strategy • Template Method • Visitor

2. FACTORY METHOD (Class Creational) • Intent: • Define an interface for creating an object, but let subclasses decide which class to instantiate. • Factory Method lets a class defer instantiation to subclasses.

3. FACTORY METHOD (Class Creational) • Motivation: • Framework use abstract classes to define and maintain relationships between objects • Framework has to create objects as well - must instantiate classes but only knows about abstract classes - which it cannot instantiate • Factory method encapsulates knowledge of which subclass to create - moves this knowledge out of the framework

4. FACTORY METHODMotivation docs Document Application Open() Close() Save() Revert() CreateDocument() NewDocument() OpenDocument() Document* doc=CreateDocument(); docs.Add(doc); doc->Open(); MyApplication MyDocument return new MyDocument CreateDocument()

5. Applicability • Use the Factory Method pattern when • a class can´t anticipate the class of objects it must create. • a class wants its subclasses to specify the objects it creates. • classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate.

6. FACTORY METHODStructure Creator Product ... product = FactoryMethod() ... FactoryMethod() AnOperation() ConcreteCreator ConcreteProduct return new ConcreteProduct FactoryMethod()

7. Participants • Product • Defines the interface of objects the factory method creates • ConcreteProduct • Implements the product interface • Creator • Declares the factory method which returns object of type product • May contain a default implementation of the factory method • Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate Concrete Product. • ConcreteCreator • Overrides factory method to return instance of ConcreteProduct

8. Factory Pattern • Example • Car Factory produces different Car objects • Original • Different classes implement Car interface • Directly instantiate car objects • Need to modify client to change cars • Using pattern • Use carFactory class to produce car objects • Can change cars by changing carFactory

9. Factory Example class 350Z implements Car; // fast car class Ram implements Car; // truck class Accord implements Car; // family car Car fast = new 350Z(); // returns fast car public class carFactory { public static Car create(String type) { if (type.equals("fast")) return new 350Z(); if (type.equals("truck")) return new Ram(); else if (type.equals(“family”) return new Accord(); } } Car fast = carFactory.create("fast"); // returns fast car

10. SINGELTON (Object Creational) • Intent: • Ensure a class only has one instance, and provide a global point of access to it. • Motivation: • Some classes should have exactly one instance(one print spooler, one file system, one window manager) • A global variable makes an object accessible but doesn’t prohibit instantiation of multiple objects • Class should be responsible for keeping track of its sole interface

11. Applicability • Use the Singleton pattern when • there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. • when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code.

12. SINGLETONStructure Singleton return uniquelnstance static Instance() SingletonOperation() GetSingletonData() Static uniquelnstance singletonData

13. Participants and Collaborations • Singleton: • Defines an instance operation that lets clients access its unique interface • Instance is a class operation (static in Java) • May be responsible for creating its own unique instance • Collaborations: • Clients access a Singleton instance solely through Singleton’s Instance operation.

14. Singleton Example public class Employee { public static final int ID = 1234; // ID is a singleton } public final class MySingleton { // declare the unique instance of the class private static MySingleton uniq = new MySingleton(); // private constructor only accessed from this class private MySingleton() { … } // return reference to unique instance of class public static MySingleton getInstance() { return uniq; } }

15. Adapter Pattern • Definition • Convert existing interfaces to new interface • Where to use & benefits • Help match an interface • Make unrelated classes work together • Increase transparency of classes

16. Adapter Pattern • Example • Adapter from integer Set to integer Priority Queue • Original • Integer set does not support Priority Queue • Using pattern • Adapter provides interface for using Set as Priority Queue • Add needed functionality in Adapter methods

17. Adapter Example public interface PriorityQueue { // Priority Queue void add(Object o); int size(); Object removeSmallest(); }

18. Adapter Example public class PriorityQueueAdapter implements PriorityQueue { Set s; PriorityQueueAdapter(Set s) { this.s = s; } public void add(Object o) { s.add(o); } int size() { return s.size(); } public Integer removeSmallest() { Integer smallest = Integer.MAX_VALUE; Iterator it = s.iterator(); while ( it.hasNext() ) { Integer i = it.next(); if (i.compareTo(smallest) < 0) smallest = i; } s.remove(smallest); return smallest; } }

19. Observer Pattern – Classification & Applicability • A behavioral (object) pattern: • Concerns objects and their behavior. • Applicability • Vary and reuse 2 different abstractions independently. • Change to one object requires change in (one or more) other objects – whose identity is not necessarily known

20. observers Observer Update() Subject For all x in observers{ x.Update(); } attach (Observer) detach (Observer) Notify () Concrete Observer Concrete Subject subject Update() GetState() SetState() observerState subjectState observerState= subject.getState(); Observer Pattern – Structure

21. Observer Pattern - Participants • Subject • Has a list of observers; • interfaces for attaching/detaching an observer • Observer • An updating interface for objects that gets notified of changes in a subject. • ConcreteSubject • Stores state of interest to observers • Sends notification when state changes. • ConcreteObserver • Implements updating interface.

22. :ConcreteSubject :ConcreteObserver-1 :ConcreteObserver-2 SetState() Notify() Update() GetState() Update() GetState() Observer Pattern - Collaborations

23. public void addObserver(Observer o) {} Java terminology for Subject. public void deleteObserver (Observer o) {} public void notifyObservers(Object arg) {} Observer Pattern - Implementation interface Observer { void update (Observable sub, Object arg) } class Observable { … } public boolean hasChanged() {}

24. Observer Pattern - Implementation public PiChartView implements Observer { A Concrete Observer. void update(Observable sub, Object arg) { // repaint the pi-chart } } class StatsTable extends Observable{ public boolean hasChanged() { // override to decide when it is considered changed } }

25. Observer Pattern - Consequences • Abstract coupling between subject and observer. (subject need not know concrete observers) • Support for broadcast communication (all observers are notified) • Unexpected updates (observers need not know when updates occur)

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