The Composite Pattern

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The Composite Pattern

• -Shalini Pradhan

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What is Composite Pattern?

• First of all design Patterns
• Composition of some patterns
• Constitutes patterns that integrate with a
  synergy
• Abstraction from a concrete recurring solution
  that solves a problem in a certain context
• Allows to build complex objects by recursively
  composing similar objects in a tree-like manner

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Definition

• Compose objects into tree structures to represent
  whole-part hierarchies. Composite lets clients
  treat individual objects and compositions of
  objects uniformly
• "Design Patterns Gamma et al.

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Structure
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Consequences

• Benefits
• It makes it easy to add new kinds of components
• It makes clients simpler
• Liabilities
• It makes it harder to restrict the type of
  components of a composite

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Implementation Issues

• Where should be child management methods be
  declared
• In Component Class
• Gives transparency since all the components can
  be treated the same
• Not safe, since clients can try to do meaningless
  things to the leaf components on run-time
• In Composite Class
• Gives Safety, since any attempt to perform a
  child operation will be caught at compile-tim
• No transparency, since now leaf and composite
  components have different interfaces

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Composite Pattern Example

• Problem A GUI system has window objects which
  contain various GUI components (widgets) such as,
  buttons, menus and text areas. A window can
  contain widget container objects which can hold
  other widgets
• Solution 1 (Not using Patterns) If we design all
  the widgets with different interfaces for
  updating the screen

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Composite Pattern Examplecontd.

• The WindowUpdate()
• Public class Window
• Button buttons
• Menu menus
• Textarea textareas
• WidgetContainer containers
• public void update()
• if (buttons ! null)
• for (int k 0 k ltbuttons.length k)
• buttonsk.draw()
• if (menus ! null)
• for (int k 0 k ltmenus.length k)
• menusk.refresh() //Similarly handle other
  widgets
• if (containers ! null)
• for (int k 0 k ltcontainers.length k)
• containersk.updateWidgets()
• If we want to add new kind of widget Modify
  WindowUpdate()

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Composite Pattern Example contd.

• Using Composite Pattern!
• Components

Component
Buttons
Menu
Widget Container
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Composite Pattern Examplecontd.

• Now WindowUpdate()
• Public class Window
• Component components
• public void update
• if (components ! Null)
• for (int k 0 klt components.length k)
• componentsk.update