Composite Design Pattern

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Composite Design Pattern
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Composite Pattern

• Recurring problem
• Often complex structures are built with container
  and primitive objects. Container objects can
  contain other objects
• How can code that uses these classes treat all
  the objects in the structure identically
  sometimes, yet differently when it matters?
• Solution
• Define an abstract class that represents
  primitives and containers
• Composite was used in the View class of Smalltalk
  MVC as well as most other GUI toolkits

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General Form of Composite
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Participants

• Component
• Declares the interface for all objects in the
  composition.
• Implements default behavior, as appropriate.
• Declares an interfaces for accessing and managing
  its child components.
• Leaf Has no children it is a primitive
• Composite Defines behavior for components having
  children. Implements child-related operations of
  the Component interface

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Participants

• Component has operations, which are methods that
  apply to all components
• The component can be a Composite or a Leaf
• Component also has composite methods add( ),
  remove( ), and getChild( )
• In each methods, a Component is passed
• Component itself could throw an exception for
  these methods
• you can not add a Component to a leaf

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Participants

• Leaf implements the operations
• Composite implements composite methods
• Composite also implements the leaf operations by
  invoking the operation on its children

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Use Example Java Swing

• Java Swing has four major pieces
• Components
• Events and EventListeners
• Layouts
• Graphics
• Component utilizes the Composite pattern in
  several ways. As such, it is a useful exemplary
  of this pattern

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Menus in Java Swing

• Java menus use the Composite pattern
• JComponent is a Component (abstract)
• JMenuBar and JMenuItem are Composites that
  extends JComponent
• JMenuItem has three subclasses
• JMenu
• JRadioButtonMenuItem
• JCheckboxMenuItem
• Leafs JLabel, JTextField

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• JMenuItem menu new JMenu("Composite")
• menu.setMnemonic('C')
• JMenuItem jmi1 new JMenu("JMenu")
• JMenuItem jmiNested1 new JMenu("Nested 1")
• JMenuItem jmiNested2 new JMenu("Nested 2")
• jmi1.add(jmiNested1)
• jmi1.add(jmiNested2)
• JLabel label new JLabel("Label")
• JTextField textF new JTextField("text
  field")
• jmi1.add(label)
• jmi1.add(textF)
• // Leafs (promitives) ignored add, nothing
  happens
• label.add(new JLabel("no"))
• textF.add(new JLabel("go"))

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• JMenuItem jmi2 new JCheckBoxMenuItem("Human",
  false)
• JMenuItem jmi3 new JRadioButtonMenuItem("Compute
  r", true)
• menu.add(jmi2)
• menu.add(jmi3)
• menu.add(jmi1)
• JMenuBar menuBar new JMenuBar()
• setJMenuBar(menuBar)
• menuBar.add(menu)
• Run JMenuItemDemoComposite.java
• See demo page

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Another Composite

• Portfolio overall new Portfolio("my IRA")
• // Add two leafs to "my IRA"
• overall.add(new Stock("Seven Eleven", 500,
  9.15))
• overall.add(new BankAccount("Swiss Account",
  300000))
• // Create a tech portfolio
• Portfolio tech new Portfolio("Tech stocks")
• Stock sun new Stock("Sun", 20, 30.50)
• MutualFund highRisk
• new MutualFund("Nasdaq", 13, 45.20)
• tech.add(sun) // add leaf
• tech.add(highRisk) // add leaf
• // Add this 2nd portfolio to the overall
  portfolio
• overall.add(tech) // add Composite
• // Show all elements with indents (output next
  slide
• overall.listAll()

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Desired Output

• my IRA 305772.6
• 500 shares Seven Eleven 4575.0
• Swiss Account 300000.0
• Tech stocks 1197.6
• 20 shares Sun 610.0
• 13.0 shares Nasdaq 587.6

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Team Activity

• Let a UML Diagram capture the real world example
  of bank accounts stocks, mutual funds, and
  portfolios
• List base class as Asset (Component from slide 3)
• Let Commodity extend Asset (there are 2 commodity
  types)
• Stock extends Commodity (a leaf)
• MutualFund extends Commodity (a leaf)
• Let BankAccount extend Asset (a leaf)
• Let Portfolio extend Asset (Composite from slide
  3)
• Can store Stocks, BankAccounts, MutualFunds
  (leafs) or other Portfolios (Composite) because
  they are all assets
• Place these methods in the correct place
• listAll add getValue

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One Answer