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Factory Patterns. 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. Being less concrete. Animal sleep() makeSound() - PowerPoint PPT Presentation
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Factory Patterns
RHS – 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
RHS – SWC 3
Being less concrete
Animalsleep()makeSound()lookForFood()
Dogsleep()makeSound()lookForFood()
Horsesleep()makeSound()lookForFood()
RHS – SWC 4
Being less concrete
Animal oneAnimal = new Horse();
…
oneAnimal.sleep();
oneAnimal.makeSound();
oneAnimal.lookForFood():
…
RHS – SWC 5
Being less concrete
Animal oneAnimal = new Dog();
…
oneAnimal.sleep();
oneAnimal.makeSound();
oneAnimal.lookForFood():
…
RHS – 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
RHS – 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():
…
RHS – 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…”
RHS – 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
RHS – 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
RHS – SWC 11
Being less concrete
Animalsleep()makeSound()lookForFood()
Dogsleep()makeSound()lookForFood()
Horsesleep()makeSound()lookForFood()
AnimalFactoryAnimal create(String info)
RHS – SWC 12
Being less concretepublic 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;
}
}
RHS – SWC 13
Being less concrete
AnimalFactory fac;
…
Animal oneAnimal = fac.create(”Dog”);
…
oneAnimal.sleep();
oneAnimal.makeSound();
oneAnimal.lookForFood():
…
RHS – 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
RHS – 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
RHS – 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
RHS – 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”
RHS – 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….?
RHS – 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
RHS – 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
RHS – SWC 21
Abstraction to the next level
// Part of input processing code
TypoFactory theTypoFactory;
public void createDocument(DocInput in)
{
ArrayList<Typo> doc = new ArrayList<Typo>();
while (in.hasNext())
{
TypoInput tyIn = in.next();
Typo typ = makeTypo(tyIn);
doc.add(typ);
}
}
RHS – 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;
}
RHS – 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;
}
RHS – 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
RHS – 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
RHS – 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
RHS – SWC 27
Abstraction to the next level
// Part of input processing code
TypoFactoryFormalLayout theTypoFactory;
public void createDocument(DocInput in)
{
ArrayList<Typo> doc = new ArrayList<Typo>();
while (in.hasNext())
{
TypoInput tyIn = in.next();
Typo typ = makeTypo(tyIn);
doc.add(typ);
}
}
Just change the type of the Typo factory…
RHS – 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
RHS – SWC 29
Abstraction to the next level
TypoaddText()
TypoFactoryTypo create(…)
RHS – SWC 30
Abstraction to the next level
TypoTypoFactory
TypoFactory-FormalLayout
TypoFactory-SmartLayout
RHS – SWC 31
Abstraction to the next level
TypoTypoFactory
TypoHeading-Formal
TypoEmphasis-Formal
TypoHeading-Smart
TypoEmphasis-Smart
RHS – SWC 32
Abstraction to the next level
TypoHeading-Formal
TypoEmphasis-Formal
TypoHeading-Smart
TypoEmphasis-Smart
TypoFactory-FormalLayout
TypoFactory-SmartLayout
RHS – 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
RHS – 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) {...}
}
RHS – SWC 35
Abstraction to the next level
public void createFormalDocument()
{
TypoFactory typoFac = new TypoFactoryFormalLayout();
DocumentCreator docCre = new DocumentCreator(typoFac);
docCre.createDocument(getDocInput());
}
RHS – 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
RHS – SWC 37
The Factory method pattern
ProductFactorycreate()someMethod()
ConcreteFactorycreate()
ConcreteProduct
RHS – 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
RHS – SWC 39
The Abstract Factory
• Answer seems simple: just use Factory Method pattern twice
TypoTypoFactory
TypoFactory-FormalLayout
TypoFactory-SmartLayout
GraphicGraphicFactory
GraphicFactory-FormalLayout
GraphicFactory-SmartLayout
RHS – 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
RHS – 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!
RHS – 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!
RHS – SWC 43
The Abstract Factory
DocItemFactorycreateTypo()createGraphic()
FormalDocItemFactory SmartDocItemFactory
RHS – SWC 44
The Abstract Factory
public void createFormalDocument()
{
DocItemFactory fac = new FormalDocItemFactory();
DocumentCreator docCre = new DocumentCreator(fac);
docCre.createDocument(getDocInput());
}
RHS – 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!
RHS – 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
RHS – 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…