Design pattern and their application

Design Patterns and their applications
Presenter: Bui Tien Hiep

1. What is Design Pattern.
2. Why Design Pattern.
3. How to use Design Pattern.
4. The principle of Design Pattern
5. UML relationship
6. Gang of Four Design Pattern
7. Example apply Design Pattern in EC
Contents

"Each pattern describes a problem which occurs over and over again in our environment, and then
describes the core of the solution to that problem, in such a way that you can use this solution a
million times over, without ever doing it the same way twice“ – Pro.Christopher Alexander,
University of California, Berkeley
What is Design Pattern?

Pattern = Name + Problem + Solution + Consequences
What is Design Pattern?

In general, a pattern has four essential elements:
1. The pattern name is a handle we can use to describe a design problem, its
solutions, and consequences in a word or two.
2. The problem describes when to apply the pattern. It explains the problem
and its context.
3. The solution describes the elements that make up the design, their
relationships, responsibilities, and collaborations.
4. The consequences are the results and trade-offs of applying the pattern.
What is Design Parttern?(2)

• Good designers do not solve every problem from first principles. They
reuse solutions.
• Practitioners do not do a good job of recording experience in software
design for others to use. Patterns help solve this problem.
Why Design Pattern?

Once you've picked a design pattern, how do you use it? Here's a step-by-step
approach to applying a design pattern effectively:
1. Read the pattern once through for an overview. Pay particular attention
to the Applicability and Consequences sections to ensure the pattern is
right for your problem.
How to Use a Design Pattern?(1)
2. Go back and study the Structure,
Participants, and Collaborations sections.
Make sure you understand the classes and
objects in the pattern and how they relate to one
another.

3. Look at the Sample Code section to see a concrete example of the pattern
in code. Studying the code helps you learn how to implement the pattern.
4. Choose names for pattern participants that are meaningful in the
application context. The names for participants in design patterns are usually
too abstract to appear directly in an application. Nevertheless, it's useful
to incorporate the participant name into the name that appears in the
application. That helps make the pattern more explicit in the
implementation.

5. Define the classes. Declare their interfaces, establish their inheritance
relationships, and define the instance variables that represent data and
object references. Identify existing classes in your application that the
pattern will affect, and modify them accordingly.
6. Define application-specific names for operations in the pattern. Here again, the
names generally depend on the application. Use the responsibilities
and collaborations associated with each operation as a guide. Also, be
consistent in your naming conventions. For example, you might use the
"Create-" prefix consistently to denote a factory method.
7. Implement the operations to carry out the responsibilities and
collaborations in the pattern. The Implementation section offers hints to
guide you in the implementation

The principle of Design Pattern
The principle of one pattern can be summarized in five main points:
1. “Separate out the things that change from those that stay the same.”
2. “Program to an interface, not an implementation”
3. “Prefer composition over inheritance”
4. “Delegate, delegate, delegate.”
5. “You ain’t gonna need it”

Intent:
• A creational pattern dealing with creating
objects without knowing their concrete
class
• It defines an interface for creating an
object, but let the subclasses decide which
class to instantiate
• Useful when concrete class of object
should be decided at runtime according to
parameters
Problem:
• A framework needs to standardize the
architectural model for a range of
applications, but allow for individual
applications to define their own domain
objects and provide for their instantiation.
Creational: Factory

Intent:
• Separate the construction of a complex
object from its representation so that the
same construction process can create
different representations.
• Parse a complex representation, create one
of several targets
Problem:
• An application needs to create the elements
of a complex aggregate. The specification
for the aggregate exists on secondary
storage and one of many representations
needs to be built in primary storage.
Creational: Builder

Creational: Prototype
Intent:
• Specify the kinds of objects to create using
a prototypical instance, and create new
objects by copying this prototype.
• Co-opt one instance of a class for use as a
breeder of all future instances.
Problem:
• Application "hard wires" the class of object
to create in each "new" expression.

Creational: Singleton
Intent:
• Ensure a class has only one instance, and
provide a global point of access to it.
• Encapsulated "just-in-time initialization" or
"initialization on first use".
Problem:
• Application needs one, and only one,
instance of an object. Additionally, lazy
initialization and global access are
necessary.

Structural: Adapter
Intent:
• Convert the interface of a class into another
interface clients expect. Adapter lets classes
work together that couldn't otherwise
because of incompatible interfaces.
• Wrap an existing class with a new interface.
• Impedance match an old component to a
new system
Problem:
• An "off the shelf" component offers
compelling functionality that you would like
to reuse, but its "view of the world" is not
compatible with the philosophy and
architecture of the system currently being
developed.

Structural: Brigde
Intent:
• Decouple an abstraction from its
implementation so that the two can vary
independently.
• Publish interface in an inheritance hierarchy,
and bury implementation in its own
inheritance hierarchy.
• Beyond encapsulation, to insulation
Problem:
• "Hardening of the software arteries" has
occurred by using subclassing of an abstract
base class to provide alternative
implementations. This locks in compile-time
binding between interface and
implementation. The abstraction and
implementation cannot be independently
extended or composed.

Structural: Composite
Intent:
• Compose objects into tree structures to
represent whole-part hierarchies. Composite
lets clients treat individual objects and
compositions of objects uniformly.
• Recursive composition
• "Directories contain entries, each of which
could be a directory."
• 1-to-many "has a" up the "is a" hierarchy
Problem:
• Application needs to manipulate a
hierarchical collection of "primitive" and
"composite" objects. Processing of a
primitive object is handled one way, and
processing of a composite object is handled
differently. Having to query the "type" of
each object before attempting to process it
is not desirable.

Structural: Decorator
Intent:
• Attach additional responsibilities to an object
dynamically. Decorators provide a flexible
alternative to subclassing for extending
functionality.
• Client-specified embellishment of a core
object by recursively wrapping it.
• Wrapping a gift, putting it in a box, and
wrapping the box.
Problem:
• You want to add behavior or state to
individual objects at run-time. Inheritance is
not feasible because it is static and applies
to an entire class.

Structural: Proxy
Intent:
• Provide a surrogate or placeholder for
another object to control access to it.
• Use an extra level of indirection to support
distributed, controlled, or intelligent access.
• Add a wrapper and delegation to protect the
real component from undue complexity.
Problem:
• You need to support resource-hungry
objects, and you do not want to instantiate
such objects unless and until they are
actually requested by the client

Behavioral: Chain of Responsibility
Intent:
• Avoid coupling the sender of a request to its receiver
by giving more than one object a chance to handle the
request. Chain the receiving objects and pass the
request along the chain until an object handles it
• Launch-and-leave requests with a single processing
pipeline that contains many possible handlers.
• An object-oriented linked list with recursive traversal.
Problem:
• There is a potentially variable number of "handler" or
"processing element" or "node" objects, and a stream
of requests that must be handled. Need to efficiently
process the requests without hard-wiring handler
relationships and precedence, or request-to-handler
mappings.

Behavioral: Command
Intent:
• Encapsulate a request as an object, thereby
letting you parameterize clients with different
requests, queue or log requests, and support
undoable operations.
• Promote "invocation of a method on an
object" to full object status
• An object-oriented callback.
Problem:
• Need to issue requests to objects without
knowing anything about the operation being
requested or the receiver of the request.

Behavioral: Observer
Intent:
• Define a one-to-many dependency between
objects so that when one object changes state, all
its dependents are notified and updated
automatically.
• Encapsulate the core (or common or engine)
components in a Subject abstraction, and the
variable (or optional or user interface)
components in an Observer hierarchy.
• The "View" part of Model-View-Controller.
Problem:
• A large monolithic design does not scale well as
new graphing or monitoring requirements are
levied.

Behavioral: Template
Intent:
• Define the skeleton of an algorithm in an
operation, deferring some steps to client
subclasses. Template Method lets subclasses
redefine certain steps of an algorithm without
changing the algorithm's structure.
• Base class declares algorithm 'placeholders', and
derived classes implement the placeholders.
Problem:
• Two different components have significant
similarities, but demonstrate no reuse of common
interface or implementation. If a change common
to both components becomes necessary,
duplicate effort must be expended

Specification Design Pattern
Intent:
• Validation determines if a particular object
satisfies a certain criteria.
• Selection identifies those elements of a collection
that satisfy the given criteria
Problem:
• Need to clean way to encapsulate business logic

Example apply Design Pattern in EC
Problem:
Input: The expense list of a report and the policy list of company
Output: Policy violation messages.
Solution
Design Pattern Description
Specification Chaining the business rules
together using boolean logic.
Builder Create policy validation objects

Example apply Design Pattern in EC(2)

Example apply Design Pattern in EC(3)

Reference
1. http://sourcemaking.com/design_patterns
2. Design Patterns: Elements of Reusable Object-Oriented Software
3. http://www.cs.colorado.edu/~kena/classes/5448/s11/lectures/
4. http://en.wikipedia.org/wiki/Specification_pattern

Design pattern and their application

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