Lecture 12: Implementing Decorators

CSC 313 – Advanced Programming Topics Lecture 12:Implementing Decorators

Decorator Pattern Intent • Invisiblyaugment main concept instances • Turn coffee into a double mocha with whip • Make $2 sandwich cost $2.17 • Decorators add functionality by wrapping • Original instance wrapped by new decorator • Can also have decorator wrap another decorator

Decorator Pattern Intent • Invisiblyaugment main concept instances • Turn coffee into a double mocha with whip • Make $2 sandwich cost $2.17 • Decorators add functionality by wrapping • Original instance wrapped by new decorator • Can also have decorator wrap another decorator • Violates all rules of good design • (Not an actual intent, just good fun)

Decorator Pattern Visual Pizza

Decorator Pattern Visual Anchovy Garlic Olives Pizza

Decorator Pattern Creation Beverage joe = new HouseBlend(); joe = new Mocha(joe); joe = new Mocha(joe); joe = new Whip(joe); joe = new Tall(joe); int mortgage = joe.cost();

Decorators’ Dirty Secrets • Decorators are subclasses of main class

Decorators’ Dirty Secrets • Decorators are subclasses of main class • Almost recursive

Meet the Decorator Classes • AbstractComponent • Component & decoratorsuperclass • Only type used outsidepattern • Interface or abstract class • Defines methods calledby code outside pattern

Meet the Decorator Classes • ConcreteComponent(s) • Base concept defined here • Only role always created • Instance at rootof decoration • Holds vital fields • Basic methods definedin this class

Meet the Decorator Classes • AbstractDecorator • Decorator superclass • Declares component field • Abstract class only • Re-declares allinherited methodsto make abstract

Meet the Decorator Classes • ConcreteDecorator(s) • Add fields or functionalityto ConcreteComponent • All methods definedso not abstract • Calls methodin component field • May instantiate 0 to ∞

Decorator Pattern Usage Drink martini = new Gin(); martini = new Vermouth(martini); martini = new Ice(martini); martini = martini.shake();

Decorator Pattern Usage Drink martini = new Gin(); martini = new Vermouth(martini); martini = new Ice(martini); martini = martini.shake(); = martini.pour();

Decorator Pattern Usage Beverage joe = new HouseBlend(); HBlend joe

Decorator Pattern Usage Beverage joe = new HouseBlend(); joe = new Mocha(joe); Mocha bev HBlend joe

Decorator Pattern Usage Beverage joe = new HouseBlend(); joe = new Mocha(joe); joe = new Whip(joe); Whip Mocha bev HBlend bev joe

Decorator Pattern Usage Beverage joe = new HouseBlend(); joe = new Mocha(joe); joe = new Whip(joe); joe = new Mocha(joe); Mocha Whip Mocha bev HBlend bev bev joe

Decorator Pattern Usage Beverage joe = new HouseBlend(); joe = new Mocha(joe); joe = new Whip(joe); joe = new Mocha(joe); int mortgage = joe.cost(); Mocha Whip Mocha bev HBlend bev bev joe

Decorator Example

Decorators: Good or Bad Pros: • Invisibly add to classes • Enable code reuse • Limit code written • Creates classes that are closed to modification Cons: • No reality in hierarchy • Use mangled recursion • Slow, polymorphic calls used everywhere

For Next Lecture • Lab #3 available on Angel • Asks you to implement Decorator Pattern • Have time Friday, butmay want help profiling • Two (short) readings available on web • Is this method hot or uglier than ____ Mom? • What rules of thumb exist for where to optimize? • How to express improvement so it is meaningful? • Could we compute maximum benefit from opts?

Lecture 12: Implementing Decorators