MOD251 Modern Software Development Methods

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MOD251 Modern Software Development Methods

• Factory Composite Observer Proxy

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Content / Agenda

• Factory Pattern
• Composite Pattern
• Observer Pattern
• Proxy Pattern

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Factory Pattern

• A creational pattern
• Also known as the Factory Method pattern.
• We have discussed this earlier, so I will just
  give a brief summary.

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Factory Pattern - Solution

• The Product interface represents the
  ConcreteProducts which are created by a Factory
  defined by the FactoryIF interface on request
  from the CreationRequestor.

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Factory Pattern - Consequences

• The creation requester class is independent of
  the class of concrete product objects actually
  created.
• The set of product classes that can be
  instantiated may change dynamically.

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Factory Pattern - Implementation

• As discussed earlier
• The factory can choose which type of object to
  instantiate based on method name, parameter (DI),
  config setting or system property.
• The object can be instantiated dynamically using
  reflection.

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Factory Pattern Java API Usage

• The Java Calendar class has a factory method for
  instantiating concrete Calendars. This is a
  variant of the factory pattern where the abstract
  Calendar class represents both the products and
  the factory
• Calendar rightNow Calendar.getInstance()

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Composite Pattern

• A partitioning pattern
• Also known as the Recursive Composition pattern.

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Composite Pattern - Synopsis

• The Composite pattern allows you to build
  complex objects by recursively composing similar
  objects in a treelike manner. The Composite
  pattern also allows the objects in the tree to be
  manipulated in a consistent manner by requiring
  all of the objects in the tree to have a common
  superclass or interface.

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Composite Pattern - Context

• An application for drawing shapes. Shapes can be
  simple shapes (circles, squares, etc.) or
  composite shapes (combinations of circles,
  squares, etc.).

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Composite Pattern - Solution

• The solution shown for the graphics application
• All shapes (both simple and composite) are drawn
  using the draw() method.
• The composite shapes delegates the drawing to the
  shapes of which it is composed.
• Composite shapes can be nested.

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Composite Pattern Java API Usage

• Used in Swing to build GUIs
• Components like JLabel, JButton,
• Containers like JFrame, JPanel,

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Observer Pattern

• A behavioral pattern
• Also known as publish-subscribe.
• Closely related to the more general MVC
  architecture/pattern.

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Observer Pattern - Synopsis

• Allows objects to dynamically register
  dependencies between objects, so that an object
  will notify those objects that are dependent on
  it when its state changes.

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Observer Pattern - Context

• A GUI application where it is important that the
  GUI is constantly updated, but you dont want
  your business logic to know about you GUI.
• A multi-client application where clients must be
  notified of important events. E.g. a monitoring
  system.

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Observer Pattern - Forces

• You are implementing two otherwise independent
  classes. An instance of one will need to be able
  to notify other objects when its state changes.
  You dont want the classes to have direct
  knowledge of each other.
• You have a one-to-many dependency that may
  require an object to notify multiple objects when
  its state changes.

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Observer Pattern - Solution

• The Observer implements the ObserverIF interface
  and can be notified of events via the notify
  method.
• The Observable implements the ObservableIF
  interface and can register observers with the
  Multicaster.
• An alternative design is to let the Observable
  extend the Multicaster directly and drop the
  ObservableIF interface.

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Observer Pattern - Consequences

• The Observer pattern allows an object to deliver
  notifications to other objects without the
  objects sending or the objects receiving the
  notifications being aware of each others class.

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Observer Pattern - Implementation

• An Observable object will normally pass a
  self-reference as a parameter to an Observer
  objects notify/update method. In most cases, the
  Observer object needs access to the Observable
  object in order to act on the notification.
• If you implement the pattern by extending the
  multicaster, you may have problems with multiple
  inheritance. This can be solved by delegating and
  wrapping an inherited class.

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Observer Pattern Java API Usage

• Javas delegation event model (in Swing
  components) is a specialized form of the Observer
  pattern.
• The Java API has a class called Observable that
  implements the multicaster and an interface
  called Observer containing the update-method.
  When implementing the Observer pattern in Java
  you usually use (subclass, implement) these.

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Observer Pattern Code example
A simple example demonstrating how to use the
Java API
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Proxy Patternby Radu Marinescu
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Loading "Heavy" Objects

• Document Editor that can embed multimedia objects
• MM objects are expensive to create ? opening of
  document slow
• avoid creating expensive objects
• they are not all necessary as they are not all
  visible at the same time
• Creating each expensive object on demand !
• i.e. when image has to be displayed
• What should we put instead?
• hide the fact that we are "lazy"!
• don't complicate the document editor!

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Idea Use a Placeholder!

• create only when needed for drawing
• keeps information about the dimensions (extent)

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Basic Aspects
proxy (n. pl prox-ies) The agency for a person
who acts as a substitute for another person,
authority to act for another

• Intent
• provide a surrogate or placeholder for another
  object to control access to it
• Applicability
• whenever there is a need for a more flexible or
  sophisticated reference to an object than a
  simple pointer
• remote proxy if real object is far away
• virtual proxy if real object is expensive
• protection proxy if real object is vulnerable
• enhancement proxies (smart pointers)
• prevent accidental delete of objects (counts
  references)

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Structure
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Participants

• Proxy
• maintains a reference that lets the proxy access
  the real subject.
• provides an interface identical to Subject's
• so that proxy can be substituted for the real
  subject
• controls access to the real subject
• may be responsible for creating or deleting it
• Subject
• defines the common interface for RealSubject and
  Proxy
• RealSubject
• defines the real object that the proxy holds
  place for

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Collaborations
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Remote Proxy

• Hide real details of accessing an object
• actual object is on a remote machine (remote
  address space)
• used in RMI and CORBA

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Further Reasons to Use Proxies

• Virtual Proxy
• Creates/accesses expensive objects on demand
• may wish to delay creating an expensive object
  until it is really accessed
• It may be too expensive to keep entire state of
  the object in memory at one time
• Protection Proxy
• Provides different levels of access to original
  object
• Cache Proxy (Server Proxy)
• Multiple local clients can share results from
  expensive operations
• remote accesses or long computations

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Consequences

• Proxies introduce a level of indirection
• used differently depending on the kind of proxy
• hide different address space (remote p.)
• creation on demand (virtual p.)
• allow additional housekeeping activities
  (protection, smart pointers)
• Copy-On-Write J.Coplien92 - The "Handle Class"
  Idiom
• copying large and complicated objects is
  expensive
• use proxy to pay the price of copying only when
  the new object is modified
• Subject must be reference counted
• proxy increments counter on copy
• when modified, do copy and decrement counter
• if (counter 0) delete Subject

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References

• Robert Martin Agile Software Development
• Mark Grand Patterns in Java, Volume 1
• Radu Marinescu A presentation on Patterns in
  the course Ingineria programarii 2 (Software
  Engineering 2), The "Politehnica" University of
  Timisoara, Romania

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Next

• Package design
• Software testing