Factory Method

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Factory Method Design Pattern
CSC 335 Object-Oriented Programming and Design
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OO Design PatternFactory Method

• Name Factory
• Problem A Client needs an object and it doesn't
  know which of several objects to instantiate
• Solution Let an object instantiate the correct
  object from several choices. The return type is
  an abstract class or an interface type.

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Characteristics

• A method returns an object
• The return type is an abstract class or interface
• The interface is extended by or implemented by
  two or more classes
• Is toString a factory method or just a factory?

4
BorderFactory?

• Java has a BorderFactory class to return borders
  Ever JComponent can have a border
• toBeBordered new JPanel()
• toBeBordered.setBorder
• (BorderFactory.createMatteBorder(2,1,5,9,Color
  .RED))
• getContentPane().add(toBeBordered)
• Is createMatteBorder a factory method or just a
  factory?
• How does the intent of Factory Method differ from
  BorderFactory

5
Iterators?

• The iterator methods isolate the client from
  knowing the class to instantiate
• List list Arrays.asList(new String
  "a","b","c")
• Iterator itr list.iterator()
• while(itr.hasNext())
• System.out.println(itr.next() " ") // a b
  c
• What type is itr?
• Is iterator a Factory method or just a factory?

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Oozinoz Example

• Allow customers to buy fireworks with credit
• The credit agency may be online or offline
• If online, let the credit company make the
  decision
• If not online, send request as a dialog to the
  call center
• Currently has these two classes

CreditCheckOnLine creditLimit(idString)double
CreditCheckOffLine creditLimit(idString)double
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Use Factory Method

• Make a design that allows the client to get an
  answer from either online or offline without
  worrying about which one
• Draw a UML diagram for
• interface CreditCheck with the creditLimit method
• Two classes that implement it
• A CreditCheckFactory that returns an instance of
  a class that implements CreditCheck with a method
  named createCreditCheck

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Write the method

• Assume CreditCheckFactory has method
  isAgencyOnLine to return true if the credit
  agency is available.
• Write the complete method createCreditCheck

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Product  (Page) defines the interface of objects
the factory method creates ConcreteProduct  (Ski
llsPage, EducationPage, ExperiencePage)
implements the Product interface Creator  (Documen
t) Declares the factory method, which returns an
object of type Product. ConcreteCreator  (Report,
Resume) overrides the factory method to return
an instance of a ConcreteProduct.
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From DoFactory
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Calls subclass's constructor

• abstract class Document class Resume
  extends Document
• protected ArrayList pages public void
  createPages()
• new ArrayList()
• pages.add(new
  EducationPage())
• public Document() pages.add(new
  SkillsPage())
• pages.add(new
  ExperiencePage())
• this.createPages()
• public ArrayList pages()
• return pages
• // factory Method
• abstract public void createPages()

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• // Note Document's constructor calls the
  correct Factory
• docs0 new Resume()
• docs1 new Report()
• for (int j 0 j lt docs.length j)
• // Display document pages
• Document document docsj
• System.out.println("\n" document " -------
  ")
• ArrayList pages document.pages()
• Iterator itr pages.iterator()
• while (itr.hasNext())
• System.out.println(itr.next())
• Output
• factory.Resume_at_765291 -------
• factory.SkillsPage_at_14f8dab
• factory.EducationPage_at_1ddebc3
• factory.ExperiencePage_at_a18aa2
• factory.Report_at_194ca6c -------