Design Patterns Part IV (TIC V2:C10)

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Design PatternsPart IV (TICV2C10)

• Yingcai Xiao
• 10/01/08

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Design Patterns (What?)

• The original paper by the gang of four
• http//www.cse.msu.edu/cse870/Materials/Patterns
  /Docs/orig-patterns-paper.pdf
• Design Patterns are devices that allow designers
  to share knowledge about their design. Design
  patterns identify, name, and abstract common
  themes in object-oriented design.
• idea reuse vs. code reuse.

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Characteristics of a Design Pattern

• smart elegant solutions that would not occur to
  a novice immediately
• generic independent of specific system
  characteristics
• well-proven identified from successful real,
  object-oriented systems
• simple involve only a handful of classes

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More Characteristics

• reusable reuse at the design level, generic,
  well-documented
• object-oriented uses classes, objects,
  generalization, and polymorphism

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Constructing a Design Pattern

• name
• problem description when pattern is to be used
  and which problem it attempts to solve
• solution classes and objects, their structure,
  and dynamic collaboration
• consequences results and trade-offs of applying
  the pattern

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Types of Patterns (Purpose)

• Categorize patterns by what they do.
• creational deal with the process of object
  creation
• structural deal with the composition of classes
  or objects
• behavioral describe ways in which classes or
  objects interact and allocate responsibilities

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Types of Patterns (Scope)

• Scope specifies whether a pattern applies
  primarily to classes or to objects.
• Class scope deals with relationship between
  classes, established through inheritance.
  (static)
• Object scope deals with object relationship,
  established through inclusion and usage. (dynamic)

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Design Pattern Space
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• Example Proxy Pattern

• Proxy provides a surrogate to hide the real
  object behind.
• Applications
• remote proxy to represent a remote object
  locally for easy and efficient coding (e.g. Java
  RMI and .NET Remoting)
• protection proxy to control the access to the
  real object (e.g. a proxy server hides the real
  server behind the firewall)
• virtual proxy to defer the expansive actions
  creating the real object.
• smart reference replacement for bare pointer
  that performs additional actions when an object
  is accessed

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Example Virtual Proxyfrom the original paper by
the gang of fourusing OMT Notation (object
diagram)
Referes to (virtual representation)
Referes to
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Class Diagram of the Virtual Image Proxy
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General Structure of a Proxy
Class Diagram
Object Diagram
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Components of a Proxy

• Proxy
• maintains a reference to let proxy access the
  real subject
• provides an interface identical to Subjects so a
  proxy can be substituted for the real subject
• controls access to the real subject may be
  responsible for creating and deleting it

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More Participants

• Subject
• defines the common interface for RealSubject and
  Proxy so a Proxy can be used anywhere a
  RealSubject is expected
• RealSubject
• defines the real object that the proxy represents

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Collaborations

• Proxy forwards request to RealSubject when
  appropriate, depending on the kind of proxy

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Sequence Diagram
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Collaboration Diagram
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Consequences

• proxy pattern introduces a level of indirection
  when accessing an object
• a remote proxy can hide the fact that an object
  resides in a different address space
• a virtual proxy can perform optimizations such as
  creating an object on demand
• protection proxies and smart references allow
  additional tasks when an object is referenced

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Implementation

• A proxy can exploit the following features
• Java use interface and implementation.
• C use virtual functions and overloading the
  member access operators.
• Smalltalk use doesNotUnderstand, which supports
  automatic forwarding of requests
• Proxy doesnt have to know the type of the real
  object (upcasted to Object in Java)

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Sample Code in Java

• public class Proxy implements Subject
• RealSubject refersTo
• public void Request ( )
• if (refersTo null)
• refersTo new RealSubject ( )
• refersTo.Request ( )

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Known Uses

• Stubs in Java RMI.
• Proxy server in networking
• NEXTSTEP uses proxies as local representatives
  for objects that may be distributed
• Proxies in Smalltalk to access remote objects

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Related Patterns

• adapter provides a different interface to the
  object it adapts proxy provides the same
  interface as its subject
• decorator adds one or more responsibilities to
  an object proxy controls access to an object
• a protection proxy might be implemented exactly
  like a decorator

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Proxy Example in C
class ProxyBase public virtual void f()
0 virtual void g() 0 virtual void h()
0 virtual ProxyBase() class
Implementation public ProxyBase public
void f() cout ltlt "Implementation.f()" ltlt endl
void g() cout ltlt "Implementation.g()" ltlt
endl void h() cout ltlt "Implementation.h()"
ltlt endl
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Proxy Example in C
class Proxy public ProxyBase ProxyBase
implementation public Proxy()
implementation new Implementation()
Proxy() delete implementation // Forward
calls to the implementation void f()
implementation-gtf() void g()
implementation-gtg() void h()
implementation-gth() int main()
Proxy p p.f() p.g() p.h()
///