Design patterns are optimized, reusable solutions to the programming problems that we encounter every day. A design pattern is not a class or a library that we can simply plug into our system; it's much more than that. It is a template that has to be implemented in the correct situation. It's not language-specific either. A good design pattern should be implementable in most—if not all—languages, depending on the capabilities of the language. Most importantly, any design pattern can be a double-edged sword— if implemented in the wrong place, it can be disastrous and create many problems for you. However, implemented in the right place, at the right time, it can be your savior.

1. SOFTWARE DESIGN PATTERNS

2. Introduction
Often wondered, on large/complex projects if we
were to change a small part, wouldn’t be tedious to
change the entire code inorder to get the required
results?
Well, for such situations we have an adopted
concept which is “Software Design Patterns”. When
described in terms of software engineering, a design
pattern is a general reusable solution for a commonly
occurring problem within a given context of a software
design.

3. Object-oriented design patterns showcases the
relationships and interactions between classes/objects,
without specifying the final application classes or objects
that are involved. Software Design Patterns is viewed as
a structured approach for programming intermediate
between the levels of a programming paradigm and a
concrete algorithm.

4. Categories
Design patterns can be categorised into three
fundamental groups, as follows:-
1. Structural Patterns
2. Creational Patterns
3. Behavioral Patterns
Let's see in detail each of the patterns in the next slides.

5. Structural Patterns
• In Structural patterns, larger structures are formed from
individual parts, which are generally of different classes.
• The main concern associated with these patterns is with
how classes and objects are composed to form larger
structures.
• In these patterns, inheritance is used to compose
interfaces or implementations.

6. • Consider a simple example, like how multiple
inheritance mixes together two or more classes
into one. The result would be a class that
combines the properties of its parent classes.
• The usefulness of this pattern is for making
independently developed class libraries work
together.

7. Methods in Structural Patterns
• Adapter:
An adapter allows classes to work together which could
not otherwise because of incompatible interfaces.
• Bridge:
Seprates or decouples an abstraction from its
implementation allowing them to vary independently.
• Composite:
Composite divides objects into tree structures inorder
to represent part-whole hierarchies.

8. • Decorator:
Adds additional responsibilities for an object
dynamically while keeping the same interface.
• Facade:
Facade specifies a high-level interface that makes the
subsystem easier to use.
• Flyweight:
Sharing is used to support large numbers of similar
objects efficiently and effectively.

9. • Front Controller:
Usually related to the design of Web applications.
• Module:
Several related elements, like classes, singletons,
methods, globally used, are grouped into a single
conceptual entity.
• Proxy:
Provides a placeholder for a different object in order
to control access to it.

10. Creational Patterns
• Creational patterns are generally used to create objects
for suitable classes that serve as a solution for a
problem,usually when instances of several different
classes are available.
• These patterns are particularly useful while taking
advantage of polymorphism and also when there is a
need to choose between different classes at run-time
rather than compile time.

11. Methods in Creational Patterns
• Abstract Factory:
This provides an interface for creating families of
dependent objects without specifying their concrete
classes.
• Builder:
The construction of a complex object from its
representation, allowing the same construction
process to create various representations.
• Multiton:
Makes sure a class has only named instances.

12. • Factory Method:
Defines an interface to create a single object, but also
lets subclasses decide which classes to instantiate.
• Lazy initialization:
This is a tactic of delaying the creation of an object
• Object pool:
Recycles objects that are no longer in use.

13. • Prototype:
Specifies the objects for creating a prototypical
instance, and also to create new objects by copying
this prototype.
• Singleton:
Makes sure a class only has one instance, and
thence provides a global point of access to it.
• Design pattern object library:
Wraps up object management together with factory
interface including live and dead lists.

14. Behavioral Patterns
• These patterns usually deals with interactions between
objects and focuses on how objects communicates with
each other.
• They can reduce the complex flow charts into mere
interconnections between objects of various classes.
• Behavioral patterns are also used to make the algorithm
that a class uses simply another parameter that is
adjustable at run-time.

15. Methods in Behavioral Patterns
• Chain of responsibility:
Avoids coupling the sender of a request to its receiver by
giving more than one object a chance to handle the
request.
• Command:
An object is encapsulated as a request.
• Interpreter:
Represents interpreted sentences into the corresponding
language.
• Iterator:
Provides a solution to access the elements of an object
sequentially,without exposing its underlying
representation.

16. • Mediator:
Sets an object that encapsulates and describes how a
set of objects interact.
• Memento:
This method allows to capture and externalize an
object's internal state and also allowing it to be
restored to this state later.
• Null object:
Default object is provided by avoiding null references.
• Observer:
Defines a one-to-many dependency between objects,
where a state change in one of the object results in all
its dependent objects being notified and updated
automatically.

17. • Servant:
Defines the similar functionality for a group of classes.
• Specification:
Combines business logic within a boolean representation.
• State:
Allowing the objects to alter their behavior simultaneosly
when its internal state changes.
• Strategy:
Sets a family of algorithms, encapsulate each one, and
makes them interchangeable.

18. • Template method:
Skeleton of an algorithm is defined in an operation.
• Visitor:
Representing an operation to be performed on the
elements of an object structure.

19. Practice / Example
A simple example involves using the Factory Method of
the Creational Patterns.
//Gallery.cfc
interface {
public any function getPhotos();
}
An interface is used with a declaration of getPhotos()
function.

20. //UserGallery.cfc
component implements="Gallery"
{
public any function getPhotos()
{
qry=new Query(sql="select * from
usergallery",datasource="dk");
return qry.execute().getResult();
}
}
This file has a component which implements the above interface Gallery,
and also defines the function getPhotos(). Within the function various
data are selected from the usergallery table.

21. //PicasaGallery.cfc
component interface="Gallery" {
public any function getPhotos() {
qry=new Query("Select name from
PicasaGallery",datasource="dk");
return qry.execute().getResult();
}
}
Within this file, again Gallery interface has been
implemented together with the function getPhotos()
definition. In this function the name is obtained from the
PicasaGallery table.

22. //AdminGallery.cfc
component interface="Gallery" {
public any function getPhotos() {
qry=new Query("Select name from
AdminGallery",datasource="dk");
return qry.execute().getResult();
}
}
In this file, the Gallery interface has also been implemented,
and the definition of the getPhotos() function includes
getting the name from AdminGallery table.

23. //FactoryGallery.cfc
component
{
public any function getGallery(name)
{
gallery="";
if(name=="User")
{
gallery=createObject("component",
"Design_Patterns.UserGallery");
}
else if(name=="Picasa")
{
gallery=createObject("component",
"Design_Patterns.PicasaGallery");
}
(...contd)

24. else if(name=="Admin"){
gallery=createObject("component",
"Design_Patterns.AdminGallery");
}
return gallery;
}
}
FactoryGallery.cfc creates a getGallery() function which
is used to create objects of components UserGallery,
PicassaGallery, and AdminGallery.

25. //MyGallery.cfm
<cfscript>
my_gallery=createObject("component",
"Design_Patterns.FactoryGallery");
my_photos=my_gallery.getGallery("User").getPhotos();
writeDump(my_photos);
</cfscript>
The final results can be viewed in this above file. The object of
FactoryGallery is created and with it the getPhotos() function is
accessed, and the results are dumped to view the results.

26. So in short......
• If we want add additional galleries, we only need to do 2
things:
– Create a component and implement it in the Gallery
interface.
– Add the new created component into the
FactoryGallery.cfc, within the
getGallery() function.
• The advantage of using this factory method is that,
changes are only made in two places: FactoryGallery
and MyGallery. Usually the exact use is visible for
complex projects.