Mediator

1
Mediator
2
Law of Demeter

• the methods of a class should not depend in
  any way on the structure of any class, except the
  immediate (top-level) structure of their own
  class. Further, each method should send messages
  to objects belonging to a very limited set of
  classes only.
• OOAD, page 116

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Why Mediator?

• Common problem Multiple objects of same or
  different classes must communicate/interact.
  Multiple dependencies complicate objects, lead to
  spaghetti code.
• Solution Define an object that encapsulates how
  a set of objects interact and interconnect. Make
  that object the hub of communications,
  responsible for controlling and coordinating the
  interactions of clients the colleague objects.

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Analogies/Metaphors

• Air traffic control
• Stock market
• eBay

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Advantages/Uses

• Partition system into smaller objects. Simplifies
  classes/objects by placing inter-object
  communications in mediator.
• Promotes loose coupling.
• Clarify complex relationships.
• Limit subclasses.
• Improve object reusability.
• Simplify object protocols.
• Facilitates refinement by containing changes in
  mediator or single colleagues rather than
  requiring updates in all classes.

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Dangers

• Mediator can become monolithic, violating
  proscription against God or manager classes and
  making it hard to maintain.

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Sample Code

• Interface Command
• void execute()
• Class Mediator
• BtnView btnView
• BtnSearch btnSearch
• BtnBook btnBook
• LblDisplay show
• void registerView(BtnView v)
• btnView v
• )
• void registerSearch(BtnSearch s)
• btnSearch s
• void registerBook (BtnBook b)
• btnBook b
• void registerDisplay(LblDisplay d)

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continued

• void search()
• btnSearch.setEnabled(false
• btnView.setEnabled(true)
• btnBook.setEnabled(true)
• show.setText("searching...")
• class BtnView extends JButton implements Command
  
• Mediator med
• BtnView(ActionListener al, Mediator m)
• super("View")
• addActionListener(al)
• med m
• med.registerView(this)
• public void execute()
• med.view()

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continued

• super("Just start...")
• med m
• med.registerDisplay(this)
• setFont(new Font("Arial",Font.BOLD,24))
• class MediatorDemo extends JFrame implements
  ActionListener
• Mediator med new Mediator()
• MediatorDemo()
• JPanel p new JPanel()
• p.add(new BtnView(this,med))
• p.add(new BtnBook(this,med))
• p.add(new BtnSearch(this, med))
• getContentPane().add(new LblDisplay(med),
  "North")
• getContentPane().add(p, "South")
• setSize(400,200)
• setVisible(true)

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UML
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UML
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Related Patterns

• Façade provides a unified interface to a set of
  interfaces to make a subsystem easier to use, but
  Façade does not add functionality and is not
  known by the subsystem classes.
• Observer defines a one-to-many dependency
  between objects so that when one object changes
  state all dependents are notified and updated
  automatically. Observer distributes communication
  by introducing observer and subject classes
  mediator encapsulates communication between other
  objects.

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Sources

• http//www.javacamp.org/designPattern/mediator.htm
  l
• http//www.fluffycat.com//javasource/javapatterns/
  DvdLowercaseTitle.txt
• http//www.vincehuston.org/dp/mediator.html
• http//www.patterndepot.com/put/8/mediator.pdf
• Design Pattern, GoF
• OOAD, Booch et al