Structural Design Patterns by Quontra Solutions

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Structural Design Patterns Online Training Classes
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Decorator Pattern
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• The Decorator Pattern is used for adding
  additional functionality to a particular object
  as opposed to a class of objects.
• A Decorator, also known as a Wrapper, is an
  object that has an interface identical to an
  object that it contains.

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• Any calls that the decorator gets, it relays to
  the object that it contains, and adds its own
  functionality along the way, either before or
  after the call.
• This gives you a lot of flexibility, since you
  can change what the decorator does at runtime, as
  opposed to having the change be static and
  determined at compile time by sub classing.

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Decorator Pattern
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• A Decorator is a concrete instance of the
  abstract class and is indistinguishable from the
  object that it contains. 
• This can be used to great advantage, as you can
  recursively nest decorators without any other
  objects being able to tell the difference,
  allowing a near infinite amount of customization.

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• Decorators add the ability to dynamically alter
  the behavior of an object.
• Good idea to use in a situation to change the
  behavior of an object repeatedly during runtime.
• It helps to add behavior or responsibilities to
  an object.

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Decorator Pattern
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Command pattern
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• A Command pattern is an object behavioral pattern
  used to achieve complete decoupling between the
  sender and the receiver.
• A sender is an object that invokes an operation,
  and a receiver is an object that receives the
  request to execute a certain operation.

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• With decoupling, the sender has no knowledge of
  the Receiver's interface.
• The term request here refers to the command that
  is to be executed.
• The Command pattern allows to vary when and how a
  request is fulfilled thus providing flexibility
  as well as extensibility.

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Command Pattern
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• Intent
• encapsulate a request in an object
• allows the parameterization of clients with
  different requests
• allows saving the requests in a queue
• Implementation
• The Client asks for a command to be executed.
• The Invoker takes the command, encapsulates it
  and places it in a queue, in case there is
  something else to do first, and the
  ConcreteCommand that is in charge of the
  requested command, sending its result to the
  Receiver.

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Command Pattern
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Factory Pattern
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• Factory Method is a creational pattern which
  helps to model an interface for creating an
  object which at creation time can let its
  subclasses decide which class to instantiate.
• Factory Pattern is responsible for
  "Manufacturing" an Object.
• It helps to instantiate the appropriate Subclass
  by creating the right Object from a group of
  related classes.

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• The Factory Pattern promotes loose coupling by
  eliminating the need to bind application-specific
  classes into the code.
• The use of factories gives the programmer the
  opportunity to abstract the specific attributes
  of an Object into specific subclasses which
  create them.
• Define an interface for creating an object, but
  let the subclasses decide which class to
  instantiate. The Factory method lets a class
  defer instantiation to subclasses.

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Factory Pattern
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Singleton Pattern

• Singleton involves only one class which is
  responsible to instantiate itself, to make sure
  it creates not more than one instance in the
  same time it provides a global point of access to
  that instance.
• The same instance can be used from everywhere,
  being impossible to invoke directly the
  constructor each time.
• Intent
• Ensure that only one instance of a class is
  created.
• Provide a global point of access to the object.

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Singleton Pattern
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• Implementation
• It involves a static member in the "Singleton"
  class, a private constructor and a static public
  method that returns a reference to the static
  member.
• The Singleton Pattern defines a getInstance
  operation which exposes the unique instance which
  is accessed by the clients.
• Constructor is not accessible from outside of the
  class.

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Singleton Specific Implementation issues
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• Thread-safe implementation Singletons can be
  used specifically in multi-threaded application
  to make sure the reads/writes are synchronized.
• Lazy instantiation Synchronization can be very
  expensive considering the performance. When the
  singleton object is already created it should be
  returned without using any synchronized block.
• Such optimization consist of checking in an
  unsynchronized block if the object is null and if
  not to check again and create it in an
  syncronized block. This is called double locking
  mechanism.

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Singleton Specific Implementation issues
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• Early instantiation using static field The
  singleton object is instantiated when the class
  is loaded and not when it is first used, due to
  the fact that the instance member is declared
  static.
• Classes loaded by different classloaders If a
  class(same name, same package) is loaded by 2
  different classloaders they represents 2
  different clasess in memory.

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• Serialization If Singleton class implements
  java.io.Serializable interface, when a singleton
  is serialized and then deserialized more than
  once, there will be multiple instances of
  Singleton created. It can be avoided by
  implementing the readResolve method.
• Abstract Factory and Factory Methods implemented
  as singletons Having 2 factories might have
  undesired effects when objects are created.
  Hence, to ensure that a factory is nique it
  should be implemented as a singleton, thus
  avoiding to instantiate the class before using it.

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Singleton Pattern
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