Agenda

• Understand the role of intent in design patterns.
• Use Decorators to avoid hierarchy explosion.
• Use Proxies to serve as sophisticated references.
• Use Iterators to provide sequential access to the elements of a collection.
• Use Chain of Responsibility to delegate along a linked list of objects.
• Use Strategies to pass algorithms to objects.
• Use Commands to pass requests to objects.
• Use Event Generators for asynchronous notification.
• Use the Composite Pattern to treat objects and collections of objects uniformly.
• Use Factories to delegate the production of objects.
• Use Template Methods to delegate to subclasses.

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Intent in Design Patterns

• Guideline: Understand the role of intent in design patterns.
• Many design patterns are composition for a purpose
• Instead of HAS-A: ADAPTS-A, DECORATES-A, PROXIES-A, ...
• Inheritance shows up primarily to get at polymorphism, not code reuse
• Delegation is all over the place

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Adapter Pattern

• Composition with a purpose:
      To grant existing code access to existing objects

  

• A very implementationy pattern
• Demonstrates interfaces in a composition relationship need not be compatible

  

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State Pattern

• Composition with a purpose:
      Refactor one big confusing class into many smaller, easier-to-understand classes

  

• Inheritance is for polymorphism, so front-end can switch between back-ends

  

• Purely an implementation decision
• Highlights: state machines, flyweights, objects as understandable units
• State classes are pure implementation --> small concept --> class extension

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Discussion

• Guideline: Understand the role of intent in design patterns.

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Decorator Pattern

• Composition with a purpose, again

  

• Inheritance is for polymorphism, so client can use many combinations of wrapped objects

  

• An interfacey pattern
• Highlights: IS-A is necessary but not sufficient, can modify composition relationships at run-time
• Account is exposed in the API --> large concept --> interface implementation

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Discussion

• Guideline: Use decorators to avoid hierarchy explosion.

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Proxy Pattern

• Composition with a purpose:
      To serve as a sophisticated reference

  

• Protection proxy: controls access to back-end object
• Remote proxy: grant access to back-end in different VM
• Virtual proxy: performs lazy creation of expensive back-ends
• Decorator: client wraps back-ends itself
  
• Proxy: provider wraps back-end, passes only front-end to clients
• Inheritance used for polymorphism, so front-end can be used by client instead of back-end

  

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Discussion

• Guideline: Use proxies to serve as sophisticated references.

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Iterator Pattern

• Composition with a purpose:
      To provide sequential access to the elements of a collection

  

• Iterator: Provided by collection on request
• Note that inner classes are a language short-cut for composition
• Interitance used for polymorphism, so client can use the front-end iterator

  

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Discussion

• Guideline: Use Iterators to provide sequential access to the elements of a collection.

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Chain of Responsibility Pattern

• Composition with a purpose:
      To delegate along a linked list of objects until one takes responsibility

  

• Interitance used for polymorphism, so each object in chain can delegate to the next

  

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Discussion

• Guideline: Use Chain of Responsibility to delegate along a linked list of objects.

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Strategy Pattern

• Composition with a purpose:
      To pass algorithms to objects

  

• Strategy: back-end passed to front-end by client
• Another example: layout managers
• Interitance used for polymorphism, so front-end can invoke the algorithm

  

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Discussion

• Guideline: Use Strategies to pass algorithms to objects.

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Command Pattern

• Composition with a purpose:
      To pass requests to objects

  

• Command: like Strategy, back-end passed to front-end by client
• Strategy: back-end is an algorithm that front end uses
  Command: back-end is an action that front-end performs
• Front-end can queue up requests, store histories to support undo
• Interitance used for polymorphism, so front-end can execute the request

  

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Discussion

• Guideline: Use Commands to pass requests to objects.

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Observer Pattern

• Composition with a purpose:
      For asynchronous notification

  

• Back-end passed to front-end by client
• Front-end notifies back-end listeners of events
• Interitance used for polymorphism, so front-end can notify listeners

  

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Discussion

• Guideline: Use Event Generators for asynchronous notification.

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Composite Pattern

• Composition with a purpose:
      To treat objects and collections of objects uniformly

  

• Client builds tree structure by passing back-ends to front-ends
• Interitance used for polymorphism, so collection of back-ends can be treated as a back-end

  

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Discussion

• Guideline: Use the Composite Pattern to treat objects and collections of objects uniformly.

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Factory Pattern

• Delegation with a purpose:
      To delegate the production of objects
• Static: Has0State's method getState enforces single instance
• Instance: Service UI factory methods produce UI objects
• Inheritance used (in instance case) for polymorphism, so client can ask factory object to create or provide the product object

  

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Discussion

• Guideline: Use Factories to delegate the production of objects.

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Template Method Pattern

• Inheritance with a purpose:
      To delegate to subclasses

  

• By inheriting an superclass method implementation, the subclass delegates to the superclass
• Template method allows you to delegate to subclasses
• In composition, really delegating to back-end class, or even back-end programmer

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Discussion

• Guideline: Use Template Methods to delegate to subclasses.