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Software System Architecture-Lecture 6.pptx
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- 2. What is aDesign Pattern? A design pattern deals with interactions between individual software components They are recurring solutions to common problems of design!
- 3. Strategy Define afamily of algorithms Encapsulate each one Make them interchangeable In a Strategy design pattern, you will:
- 4. You should useStrategy when: You have code with a lot of algorithms You want to use these algorithms at different times You have algorithm(s) that use data the client should not know about
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- 6. Strategy vs. Subclassing Strategy can be used in place of subclassing Strategy is more dynamic Multiple strategies can be mixed in any combination where subclassing would be difficult
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- 10. Strategy makes thiseasy! Class functionX() functionY() StrategyX functionX() ... StrategyY functionY() ...
- 11. Benefits of Strategy Eliminates conditional statements Can be more efficient than case statements Choice of implementation Client can choose among different implementations with different space and time trade-offs
- 12. Benefits of Strategy Families of related algorithms Alternative to subclassing This lets you vary the algorithm dynamically, which makes it easier to change and extend You also avoid complex inheritance structures
- 13. Drawbacks of Strategy Clients must be aware of different strategies Clients must know how strategies differ so it can select the appropriate one Communication overhead between strategy and context Sometimes the context will create and initialize parameters that are never used
- 14. Drawbacks of Strategy Increased number of objects if the algorithm differences are simple, the extra classes add extra complexity
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- 16. New is animplementation Calling “new” is certainly coding to an implementation In fact, it’s always related to a concrete class (Open for Modification) That’s fine when things are simple, but . . .
- 17. Look Out ForChange When you have several related classes, that’s probably a good sign that they might change in the future Duck duck; if (picnic) { duck = new MallardDuck(); } else if (hunting) { duck = new DecoyDuck(); } else if (inBathTub) { duck = new RubberDucky(); }
- 18. Design Principle Whatshould you try to do with code that changes?
- 19. When Building Objectsis Complex Book example - a variety of types of pizzas Think about constructing the weapons in your decorator lab
- 20. Pizza orderPizza() { Pizzapizza = new Pizza(); pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box(); return pizza; }
- 21. Pizza orderPizza(String type){ Pizza pizza; if (type.equals(“cheese”)) { pizza = new CheesePizza(); } else if (type.equals(“greek”)) { pizza = new GreekPizza(); } else if (type.equals(“pepperoni”)) { pizza = new PepperoniPizza(); } pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box(); return pizza; }
- 22. Pizza orderPizza(String type){ Pizza pizza; if (type.equals(“cheese”)) { pizza = new CheesePizza(); } else if (type.equals(“greek”)) { pizza = new GreekPizza(); } else if (type.equals(“pepperoni”)) { pizza = new PepperoniPizza(); } else if (type.equals(“sausage”)) { pizza = new SausagePizza(); } else if (type.equals(“veggie”)) { pizza = new VeggiePizza(); } pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box(); return pizza; }
- 23. Pizza orderPizza(String type){ Pizza pizza; if (type.equals(“cheese”)) { pizza = new CheesePizza(); } else if (type.equals(“greek”)) { pizza = new GreekPizza(); } else if (type.equals(“pepperoni”)) { pizza = new PepperoniPizza(); } else if (type.equals(“sausage”)) { pizza = new SausagePizza(); } else if (type.equals(“veggie”)) { pizza = new VeggiePizza(); } pizza.prepare(); pizza.bake(); pizza.cut(); pizza.box(); return pizza; } Encapsulate!
- 24. public class SimplePizzaFactory{ public Pizza createPizza(String type) { Pizza pizza; if (type.equals(“cheese”)) { pizza = new CheesePizza(); } else if (type.equals(“pepperoni”)) { pizza = new PepperoniPizza(); } else if (type.equals(“sausage”)) { pizza = new SausagePizza(); } else if (type.equals(“veggie”)) { pizza = new VeggiePizza(); } return pizza; } } Now orderPizza() is tidy
- 25. public class PizzaStore{ SimplePizzaFactory factory; public PizzaStore(SimplePizzaFactory factory) { this.factory = factory; } public Pizza orderPizza(String type) { Pizza pizza; pizza = factory.createPizza(type); pizza.prepare(); pizza.bake(); pizza.cute(); pizza.box(); return pizza; } } No new operator
- 26. Simple Factory Pullthe code that builds the instances out and put it into a separate class Identify the aspects of your application that vary and separate them from what stays the same
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- 28. Why would wedo this? Multiple clients needing same types of object Ensure consistent object initialization
- 29. The Factory Method Definition: Thefactory Method Pattern defines an interface for creating an object, but lets the subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
- 30. Factory Method Pattern <<abstract>> Creator factoryMethod() operation() <<abstract>> Product ConcreteCreator factoryMethod() ConcreteProduct abstract! IS-A HAS-A IS-A Nomore SimpleFactory class Object creation is back in our class, but … delegated to concrete classes
- 31. Dependency Inversion Principle Dependupon abstractions -- not concrete classes
- 32. Impossible Guidelines Novariable should hold a reference to a concrete class No class should derive from a concrete class No method should override an implemented method of an of its base classes.
- 33. Impossible Guidelines The ideahere is to stay clear of classes that are likely to change String studentChecking System businessChecking Integer FederalTaxCode
- 34. What if thereare categories of Products? <<abstract>> Player HeroGunner HeroStealth HeroTank HeroPilot EnemyLevelBoss EnemyCaptain EnemyThug
- 35. When Subclasses get outof hand… •make the factory an abstract class as well •Instead of subclasses for the products, subclasses for the components of the products •The next diagram is overwhelming… breathe in, breath out
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- 37. Abstract Factory Pattern Thisdesign favors composition handy when you need to insure the quality of many similar, but slightly different versions of a class
- 38. More Diagrams pp.156/157 160/161
- 39. Summary Simple Factory Use when you have only one family of objects Factory Method Pattern Use when you have multiple families of objects Abstract Factory Use when you have multiple families of object components
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- 41. Intent Compose objectsinto tree structures to represent part-whole hierarchies. Let clients treat individual objects and compositions of objects uniformly.
- 42. Motivation Some systemlet user build complex diagrams out of simple components. A simple implementation could define classes for each concept of the components. a problem: Code that uses these classes must treat primitive and container objects differently.
- 43. Motivation(2) The keyof composite pattern is an abstract class that represents both primitives and their containers. A typical composite structures: aPicture aPicture aLine aRect aText aLine aRect
- 44. Applicability Use thispattern when You want to represent part-whole hierarchies of objects. You want clients to be able to ignore the difference between compositions of objects and individual objects. Client will treat all objects in the composite structure uniformly
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- 46. Participants Component declarethe interface for objects in the composition. implements default behavior. declares an interface for accessing and managing its child components. (opt.) defines an interface for accessing a component’s parent in the recursive structure, …
- 47. Participants(2) Leaf representsleaf objects in the composition. define behavior for primitive objects. Composite defines behavior for components having children. stores child components. implements child-related operations in the component interface. Client manipulates objects in the composition through the Component interface.
- 48. Collaborations Client usethe Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If not, the it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding.
- 49. Consequences defines classhierarchies consisting of primitive objects and composite objects. makes the client simple. makes it easier to add new kinds of components. (disadvantage) can make your design overly general.
- 50. Sample Code Equipment:computer and stereo components. class Equipment { virtual ~Equipment(); const char* Name() { return _name;} virtual Watt Power(); virtual Currency NetPrice(); virtual Currency DiscountPrice(); virtual void Add(Equipment *); virtual void Remove(Equipment *); virtual Iterator<Equipment *>* CreaterIterator(); Protected: Equipment(const char *); private: const char* _name; }
- 51. Sample Code Leafclass: class FloppyDisk : public Equipment{ public: FloppyDisk(const char*); virtual ~FloppyDisk(); virtual Watt Power(); virtual Currency NetPrice(); Virtual Currency DiscountPrice(); }
- 52. Sample Code Thebase class for equip. containing other equip. class CompositeEquipment: public Equipment{ public: virtual ~CompositeEquipment(); virtual Watt Power(); virtual Currency NetPrice(); virtual Currency DiscountPrice(); virtual void Add(Equipment *); virtual void Remove(Equipment *); virtual Iterator<Equipment *>*CreatIterator(); protected: CompositeEquipment(const char*); private: List <Equipment *> _equipment; }
- 53. Sample Code DefaultImpl. of NetPrice Currency CompositeEquipment::NetPrice() { Iterator<Equipment *> *i=CreateIterator(); Currency total = 0; for (i->First(); !i->IsDone(); i->Next() { total += i->CurrentItem()->NetPrice(); } }
- 54. Sample Code classfor computer chassis class Chassis: public CompositeEquipment{ public: Chassis(const char*); virtual ~Chassis(); virtual Watt Power(); virtual Currency NetPrice(); virtual Currency DiscountPrice(); };
- 55. Sample Code Usethe classes Cabinet * cabinet = new Cabinet(“PC Cabinet”); Chassis* chassis = new Chassis(“PC Chassis”); Cabinet->Add(chassis); Bus * bus = new Bus(“NCA Bus”); bus->Add(new Card(“16Mbs Token Ring”)); Chassis->Add(bus); Chassis->Add(new FloppyDisk(“3.5in Floppy”)); cout << ‘The net price is” <<Chassis->NetPrice() <<endl.