Factory Patterns

DCS – SWC 2 Being less concrete One important OO principle is: ”Program to an interface, not an implementation” Interfaces reduces the coupling between code and concrete types Code does not need to know the concrete type of an object

DCS – SWC 3 Being less concrete Animal sleep() makeSound() lookForFood() Dog sleep() makeSound() lookForFood() Horse sleep() makeSound() lookForFood()

DCS – SWC 4 Being less concrete Animal oneAnimal = new Horse(); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): …

DCS – SWC 5 Being less concrete Animal oneAnimal = new Dog(); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): …

DCS – SWC 6 Being less concrete This is fine, but we still need to be concrete when creating an object Also, we might need to choose – at run-time – between various concrete types

DCS – SWC 7 Being less concrete Animal oneAnimal; … if (needToRide) oneAnimal = new Horse(); else if (mustBeMammal) oneAnimal = new Dog(); else oneAnimal = new Parrot(); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): …

DCS – SWC 8 Being less concrete Is anything wrong with this…? What if we need to add some new concrete types? In that case, we will need to change the code in order to include the new types ”Closed for modification, open for extension…”

DCS – SWC 9 Being less concrete We want to isolate the references to concrete types to another class One class produces concrete objects, using their concrete types Another class processes the objects, knowing only the interface The processing class can then be closed for modification

DCS – SWC 10 Being less concrete A class which produces objects is usually called a Factory Class A factory class usually has a single method: create(…) The create method often – but not always – takes a parameter, defining what concrete object to create

DCS – SWC 11 Being less concrete Animal sleep() makeSound() lookForFood() Dog sleep() makeSound() lookForFood() Horse sleep() makeSound() lookForFood() AnimalFactory Animal create(String info)

DCS – SWC 12 Being less concrete public class AnimalFactory { public Animal create(String info) { if (info.equals(”Dog”)) return new Dog(); else if (info.equals(”Horse”)) return new Horse(); else if (info.equals(”Parrot”)) return new Parrot(); else return null; }

DCS – SWC 13 Being less concrete AnimalFactory fac; … Animal oneAnimal = fac.create(”Dog”); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): …

DCS – SWC 14 Being less concrete Have I achieved something, or am I just moving code around…? With this setup, we can now parameterise the processing code further This removes the last references to concrete types

DCS – SWC 15 Being less concrete public void processAnAnimal(String type) { AnimalFactory fac = new AnimalFactory(); … Animal oneAnimal = fac.create(type); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): … } Type specifi- cation is a parameter

DCS – SWC 16 Being less concrete public void processAnAnimal (String type, AnimalFactory fac) { Animal oneAnimal = fac.create(type); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): … } Type specifi- cation and object factory are parameters

DCS – SWC 17 Being less concrete This pattern is known as Simple Factory We have separated code for producing objects, and code for processing objects Processing code only knows about the interface Fewer responsibilities per class – ”Classes should only have one reason to change”

DCS – SWC 18 Abstraction to the next level The processing code needs a parameter which carries the type information for the object being created However, we also suggested that the factory itself could be a parameter Why would we do that….?

DCS – SWC 19 Abstraction to the next level public void processAnAnimal (String type, AnimalFactory fac) { Animal oneAnimal = fac.create(type); … oneAnimal.sleep(); oneAnimal.makeSound(); oneAnimal.lookForFood(): … } Type specifi- cation and object factory are parameters

DCS – SWC 20 Abstraction to the next level Consider a word processor: –A document is composed of various typo- graphic objects, like Heading, Emphasis, and so on –All such classes implement the interface Typo –Given some input source, a piece of code must produce a list of Typo objects

DCS – SWC 21 Abstraction to the next level // Part of input processing code TypoFactory theTypoFactory; public void createDocument(DocInput in) { ArrayList doc = new ArrayList (); while (in.hasNext()) { TypoInput tyIn = in.next(); Typo typ = makeTypo(tyIn); doc.add(typ); }

DCS – SWC 22 Abstraction to the next level // Part of input processing code private Typo makeTypo(TypoInput in) { String text = in.getText(); String type = in.getType(); Typo theTypo = theTypoFactory.create(type); thetypo.addText(text); return theTypo; }

DCS – SWC 23 Abstraction to the next level // TypoFactory code private Typo create(String type) { if (type.equals(”Heading”)) return new Heading(); else if (type.equals(”Emphasis”)) return new Emphasis();... else return null; }

DCS – SWC 24 Abstraction to the next level The code processing the input does not know about concrete Typo classes – good But the code is still ”constrained”… What is a Typo object really – it is a ”binding” between a text and a certain way of formatting the text Different concrete Typo classes provide different bindings

DCS – SWC 25 Abstraction to the next level A Heading might be –Font size 24 –Bold –Calibri font An Emphasis might be –Bold –Red font color

DCS – SWC 26 Abstraction to the next level A Typo factory thus defines a set of bindings between text and formatting – a layout What if we wish to change the layout of a document? We could then just define a different Typo factory, with different bindings

DCS – SWC 27 Abstraction to the next level // Part of input processing code TypoFactoryFormalLayout theTypoFactory; public void createDocument(DocInput in) { ArrayList doc = new ArrayList (); while (in.hasNext()) { TypoInput tyIn = in.next(); Typo typ = makeTypo(tyIn); doc.add(typ); } Just change the type of the Typo factory…

DCS – SWC 28 Abstraction to the next level This solution is still quite static Changing to a different factory requires code modification Why not use interfaces once again! We could also define an interface for the factory side, making the processing code independent of a specific factory

DCS – SWC 29 Abstraction to the next level Typo addText() TypoFactory Typo create(…)

DCS – SWC 30 Abstraction to the next level TypoTypoFactory TypoFactory- FormalLayout TypoFactory- SmartLayout

DCS – SWC 31 Abstraction to the next level TypoTypoFactory TypoHeading- Formal TypoEmphasis- Formal TypoHeading- Smart TypoEmphasis- Smart

DCS – SWC 32 Abstraction to the next level TypoHeading- Formal TypoEmphasis- Formal TypoHeading- Smart TypoEmphasis- Smart TypoFactory- FormalLayout TypoFactory- SmartLayout

DCS – SWC 33 Abstraction to the next level The factory for Formal layout only knows the concrete classes TypoHeading- Formal and TypoEmphasisFormal The factory for Smart layout only knows the concrete classes TypoHeadingSmart and TypoEmphasisSmart The factory interface only knows about the Typo interface

DCS – SWC 34 Abstraction to the next level // A configurable document creator class public class DocumentCreator { TypoFactory typoFac; public DocumentCreator(TypoFactory typoFac) { this.typoFac = typoFac; } public void createDocument(DocInput in) {...} }

DCS – SWC 35 Abstraction to the next level public void createFormalDocument() { TypoFactory typoFac = new TypoFactoryFormalLayout(); DocumentCreator docCre = new DocumentCreator(typoFac); docCre.createDocument(getDocInput()); }

DCS – SWC 36 Abstraction to the next level Note that the only thing that changes between two TypoFactory implementa- tions is the create method We may include concrete methods in the Typo interface – making it an abstract class – if it makes sense This is known as the Factory Mehtod pattern

DCS – SWC 37 The Factory method pattern Product Factory create() someMethod() ConcreteFactory create() ConcreteProduct

DCS – SWC 38 The Abstract Factory Our code can now work with different concrete factories, through a Factory interface What if we need to create several types of ”products”, not just a single type? –Typo – formattings of text –Graphic – formattings of graphic objects

DCS – SWC 39 The Abstract Factory Answer seems simple: just use Factory Method pattern twice TypoTypoFactory TypoFactory- FormalLayout TypoFactory- SmartLayout GraphicGraphicFactory GraphicFactory- FormalLayout GraphicFactory- SmartLayout

DCS – SWC 40 The Abstract Factory This looks fine… …but does it reflect our intention? Would it make sense to have a document, with –text using Formal layout –graphics using Smart layout Model does not include any ”binding” between related products

DCS – SWC 41 The Abstract Factory public void createFormalDocument() { TypoFactory tFac = new TypoFactoryFormalLayout(); GraphicFactory gFac = new GraphicFactorySmartLayout(); DocumentCreator docCre = new DocumentCreator(tFac,gFac); docCre.createDocument(getDocInput()); } Oooppss!

DCS – SWC 42 The Abstract Factory A Typo and a Graphic are not – as seen from a type point-of-view – related Would be somewhat artificial – or perhaps even impossible – to introduce a common base class However, we can enforce the binding through a shared factory class!

DCS – SWC 43 The Abstract Factory DocItemFactory createTypo() createGraphic() FormalDocItemFactorySmartDocItemFactory

DCS – SWC 44 The Abstract Factory public void createFormalDocument() { DocItemFactory fac = new FormalDocItemFactory (); DocumentCreator docCre = new DocumentCreator(fac); docCre.createDocument(getDocInput()); }

DCS – SWC 45 The Abstract Factory public void createDocument(DocInput in) {... Typo aTypo = theFactory.createTypo(typoInfo);... Graphic aGraphic = theFactory.createGraphic(graphicInfo);... } Using the same factory for creating Typo and Graphic objects!

DCS – SWC 46 The Abstract Factory This pattern is known as the Abstract Factory pattern By making a creator class with several create… methods, we restrict the product combinations the client can create

DCS – SWC 47 The Abstract Factory The methods in the Abstract Factory are product-type dependent, so if we add another product, we need to change the interface of the base class This is a price we must pay for binding (formally) non-related types together Patterns are also compromises…