Chain of Responsibility

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Chain of Responsibility

• Ian Price
• Dale Willey

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We Will Cover

• What is Chain of Responsibility
• A few examples
• The Metsker Challenges

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• Avoid coupling the sender of a request to its
  receiver by giving more than one object the
  chance to handle the request. Chain the
  receiving objects and pass the request along the
  chain until an object handles it.

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Glorified If-Else if Statement?
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Answer!
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Where can this be Used?
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Examples

• GUI help program
• TCL object naming
• TK imaging
• Try-Throw-Catch
• New Age Networking
• Different Algorithms
• Visualization Schemes

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Challenge 12.1
How does the Chain of Responsibility pattern
differ from ordinary method lookup?
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Answer 12.1

• Method lookup searches across a well-defined
  series of classes. The Chain of Responsibility
  pattern directs the search for a responsible
  method to occur across a series of objects
• The mechanics of method lookup are part of the
  Java language specification, whereas Chain of
  Responsibility is unders your control as a
  developer

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Challenge 12.2
Redraw the diagram in Figure 12.1, moving
getResponsible() to SimulatedItem and adding this
behavior to Tool. Code and picture follow
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public Engineer getResponsible(SimulatedItem
item) if (item instanceof Tool)
Tool t (Tool) item
return t.getToolCart().getResponsible()
if (item instanceof MachineComponent)
MachineComponent c (MachineComponent)
item if (c.getResponsible()
null)
if (c.getParent() ! null)
return c.getParent().getResponsible()

return null
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(No Transcript)
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Answer 12.2
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Challenge 12.3
Write getResponsible() for A.
MachineComponent B. Tool C. ToolCart
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Answer 12.3
A MachineComponent object may have an explicitly
assigned responsible person. If it doesn't, it
passes the request to its parent public Engineer
getResponsible() if (responsible ! null)
return responsible if
(parent ! null) return
parent.getResponsible() return null
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Answer 12.3 (cont)
The code for Tool.getResponsibility() reflects
the statement that "tools are always assigned to
tool carts" public Engineer getResponsible()
return toolCart.getResponsible() The
ToolCart code reflects the statement that "tool
carts have a responsible engineer" public
Engineer getResponsible() return
responsible
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Challenge 12.4
Fill in the constructors in the next to support a
design that ensures that ever Machine Component
object has a responsible engineer.
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Answer 12.4
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Challenge 12.5
Cite an example when the Chain of Responsibility
pattern might occur if the chained objects do not
form a composite.
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Answer 12.5

• A chain of on-call engineers that follows a
  standard rotation. If the primary on-call
  engineer does not answer a production support
  page in a specific amount of time, the
  notification system pages the next engineer in
  the chain.
• When users enter such information as the date of
  an event, a chain of parsers can take turns
  trying to decode the users text.

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Questions? Comments?Rants? Raves? (Actually,
wed prefer you keep the rants to yourself)