structural design patterns by quontra solutions
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Decorator Pattern
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
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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Command pattern
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
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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Factory Pattern
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.
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.
Singleton Pattern
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 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 syncronizedblock. This is called double locking mechanism.
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Singleton: Specific Implementation
issues
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
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