Design Patterns CheatSheet

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Brief Summary of design patterns


Factory Method: Rider –> RoadBikeCreator extends AbstractBikeCreator .create()

Abstract Factory: Rider(AbstractBikeCreator c).ride() { c.purchaseBike().ride()) ; // someone from the outside is passing the factory.

Static Factory: Rider.createBike(whichBike).ride(); // createBike is static.

Singleton: Object StringUtils {}

Builder: new BikeBuilder.setGear(.).setChain(.).build()

Prototype: Rider –> BikePrototype.clone(color=”red”) // Great I have a new red bike.


Adapter: AppLogger.warn(msg) { this.logger.log(msg, WARN) }

Decorator: SecuredFileUploader(FileUploader).upload(checkSecured(file); FileUploader.upload())

Bridge: Encrypter(hasher: Hasher).encrypt() { hasher.hash() } we separated hasher implementation which can ba a whole implementation hierarchy from our encryption, by mere of a field pointing to a different class or class hierarchy).

Composite: Node.print <- Tree.print <- Leaf.print => foreach.print // (Tree/Node/Leaf group/aggregation treated as single)

Facade: AirplaneFacade(controls, handle, break).flight(control.start, handle.stop…) // Wrap complex system in a simple interface.

Flyweight: Circle.get(color) = cache.getOrCreate(circle) // Reduce memory used with objects that share data with similar objects

Proxy: FileReaderProxy extends AbstractFileReader .read() { new } // like access control to object for example expensive resources access…


Value Object: val SUNDAY = new Integer(1) // small immutable multithreaded

null object: NullMessage,TextMessage extends AbstractMessage // define actual object representing null

Strategy: Parser.parse { filename.match('json') return JSONParser else return CSVParser } // choose algorithm from family at runtime

Command: Client –> Invoker contains Concrete Commands –> Executes..

Chain Of Responsibility: Client –> ConcreteHandler1.handleRequest() –> ConcreteHandler2…

Interpreter: Interprester.interepet { operator.match { case “+”: new Add } } // Represent a domain with a language.

Iterator: StudentIterator extends Iterator { override hasNext(); override next() }

Mediator: GoogleGroupsMembersMedaitor.sendMessage // (instead of external class keeping track of who is in members)

Memento: TextEditor.undo() { memento.getPrevState() } // undo - rollback / restore object state


State: { setState(PlayState); }; PlayState.pressed() { setState(PausedState); } // like strategy design pattern state is about how algorithm performaed state is about what action is performed.

Template Method: DataFinderExample.find { jsonDatafinder extends DataFinder findInJson, csvDataFinder find in csv } // common skeleton for algorithm different implementations deal with specifics defer algorithm impl to subsclasses.

Visitor: `Text.accept(htmlExporterVisitor) { htmlExporterVisitor.visit(this); }; htmlExporterVisitor.print(); } Not all use cases are known at design of class.

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