abhishek[MSFTE]
(Moderator):
Welcome to today’s chat. Our topic today is Creational Design
Patterns. Before we begin, I would like to
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more than on abhishek[MSFTE] (Moderator):
4. Please only submit your question once. Submitting your question
more than once is against the Chat Code of Conduct.
5. Please abide by the Chat Code of Conduct. abhishek[MSFTE] (Moderator):
We are pleased to welcome our Expert for today’s chat. I will
have her introduce herself now. Nasha [MVP] (Expert):
Good Evening Everybody. This is Namratha Shah here .. u can call me
Nasha. Nasha [MVP] (Expert):
I am working for NESS Technologies Mumbai .... Nasha [MVP] (Expert):
Today we are going to discuss about GOF - Creational Design Patterns.
Nasha [MVP] (Expert):
So is everyone ready ? Nasha [MVP] (Expert):
Before we go to Creational Design Patterns let me ask you one basic
question : What are design patterns? Nasha [MVP] (Expert):
please put forward wat u know or ur ideas Nasha [MVP] (Expert):
any one Nasha [MVP] (Expert):
ok Nasha [MVP] (Expert):
A design pattern is a solution to a recurring problem in an object
oriented environment. Nasha [MVP] (Expert):
yes Nasha [MVP] (Expert):
Design patterns need not necessarily be to a particular to an application
domain infact it usually addresses problems across application domains
and programming languages. Nasha [MVP] (Expert):
We will discuss today GOF Creational Patterns Nasha [MVP] (Expert):
Giving you a brief introduction about GOF. Nasha [MVP] (Expert):
Gang Of Four includes Erich Gamma, Richard Helm, Ralph Joshson and
John Vlissides. Nasha [MVP] (Expert):
These guys worked togather and analysed various object-oriented systems
and came up with 23 design patterns and gave these patterns some nomencalture. Nasha [MVP] (Expert):
These 23 patterns are sub-divided in to 3 categories Creational, Structural
and Behavioural patterns with in these categories and they are classified
as Class based or Object Based patterns. Nasha [MVP] (Expert):
Ok the obvious question now is wat are class based and object based
patterns ? Nasha [MVP] (Expert):
any one to answer that ? Nasha [MVP] (Expert):
Let me put it in simple words .. Class based are those which are based
on inheritance and object based are those which use delegation. Nasha [MVP] (Expert):
Any queries on concepts inheritance or delegation .. if yes .. pls
raise them now .. else the later part of discussion would be very
confusing. Nasha [MVP] (Expert):
everybody familiar with inheritance and delegation ? Nasha [MVP] (Expert):
inheritance is static relationship between two classes Nasha [MVP] (Expert):
on the other hand delegation is a dynamic relationship where one class
uses another class (or an object of another class) Nasha [MVP] (Expert):
agree ? Nasha [MVP] (Expert):
kool Nasha [MVP] (Expert):
Let us start our discussion on Creational design patterns Nasha [MVP] (Expert):
We will start with the question :- What are creational design patterns
? Nasha [MVP] (Expert):
hi Nasha [MVP] (Expert):
Creational Design patterns as the name says it all .. is concerned
with the process of object creation. Nasha [MVP] (Expert):
There are 5 creational design patterns :
Factory Method.
Abstract Factory.
Prototype.
Singleton.
Builder. Nasha [MVP] (Expert):
Every pattern that I will start with I will give its scope and its
intent .... after that I will take an example and discuss the pattern
.. and finally discuss its applicablity and proc-cons. Nasha [MVP] (Expert):
The First patterns to start with is Factory Method. Nasha [MVP] (Expert):
Scope : Class Nasha [MVP] (Expert):
Intent : Defines an interface for creating an object, but let subclasses
decied which class to instantiate.Factory Method lets a class defers
instantiation to subclasses. Nasha [MVP] (Expert):
When I say the scope of this pattern is Class .. I mean to say that
it is an inheritance based pattern Nasha [MVP] (Expert):
Let us take an example Nasha [MVP] (Expert):
Consider we are designing Human Resource Information System framework
which can be customzied to address the specific requirements of various
industries or organizations. In this framework we have classes such
as Nasha [MVP] (Expert):
Employee : To record basic details of an employee
EmployeeCollection : A collection of various Employee objects
EmployeeController : Controller class for Employee responsible for
maintaining Employee details.
Qualification : To record employees qualification
Experience : To record employees experience Nasha [MVP] (Expert):
Employee class contains an object of Experience and qualification.
i.e.
class Employee
{
string name;
string age;
Qualification quaf;
Experience exp;
......
} Nasha [MVP] (Expert):
Since employee controller is reponsible for creating employee objects
we have a function called as newEmployee which will return us the
employee object. Nasha [MVP] (Expert): // structure of EmployeeController
class EmployeeController
{
private EmployeeCollection employees;
public void newEmployee ()
{
Employee employee = new Employee();
employees.add(employee);
}
} Nasha [MVP] (Expert):
any queries till now ? Nasha [MVP] (Expert):
please raise ur queries rite away Nasha [MVP] (Expert):
When I say that scope of this pattern is Class .. I mean that this
pattern uses Inheritance Nasha [MVP] (Expert):
All the relationships between classes are inhertance based Nasha [MVP] (Expert):
and thatz y they are static Nasha [MVP] (Expert):
ok saurabh Nasha [MVP] (Expert):
Creational patterns are related to object creation as i mentioned
before Nasha [MVP] (Expert):
behavioural patterns are concerned with how the objects interact with
each other Nasha [MVP] (Expert):
and structural patterns are concerned with composition of classes
or objects Nasha [MVP] (Expert):
I hope I have solved ur doubt Nasha [MVP] (Expert):
any more questions ? Nasha [MVP] (Expert):
i think I would like to limit this to creational patterns only Nasha [MVP] (Expert):
Is the example for Factory Method Pattern related to Employee and
Employee Controller clear Nasha [MVP] (Expert):
well no Nasha [MVP] (Expert):
You program will communicate with Employee Controller class Nasha [MVP] (Expert):
which will instantiate the object for you Nasha [MVP] (Expert):
as per the current implementation Nasha [MVP] (Expert):
coz if you c the implementation of newEmployee method Nasha [MVP] (Expert):
public void newEmployee ()
{
Employee employee = new Employee();
employees.add(employee);
} Nasha [MVP] (Expert):
we have treated Employee as a Concrete class Nasha [MVP] (Expert):
Now since we are designing a framework Nasha [MVP] (Expert):
We plan to sell our framework to an IT Firm so we need to subclass
Employee and EmployeeController and create to new classes like ITEmployee
and ITEmployeeController which are more specific Nasha [MVP] (Expert):
to their domain Nasha [MVP] (Expert):
If check out the code EmployeeController.. you will notice that newEmployee
instantiates an object of Employee and add's it to the employee collection.
But this is only possible if the Employee class is a concrete class.
Since we are creating a framework we would like our Employee class
to be an abstract class so that it can be extended ahead as per the
domain requirement. Nasha [MVP] (Expert): Q: What're the advantages for adapting this? A: we will c in a while Nasha [MVP] (Expert):
So now the issue is how to design the framework in such a way that
the newEmployee function creates the object and adds it to the collection
without making Employee class a concrete class Nasha [MVP] (Expert):
any body who can come up with a solution to that problem Nasha [MVP] (Expert): Q: The collection will hold all the employees records
that is fine with this pattern but how these objects are accessible
to my program - will you use any other design pattern for that? A: ur cleint will talk to the controller class Nasha [MVP] (Expert):
any one ? Nasha [MVP] (Expert):
is the problem clear ? Nasha [MVP] (Expert):
anything u want Nasha [MVP] (Expert):
as long as it solves the issue Nasha [MVP] (Expert):
and i dont need to make employee Class a concrete class Nasha [MVP] (Expert):
a hint Nasha [MVP] (Expert):
ur answer lies in the scope and the intent of the pattern Nasha [MVP] (Expert):
we are discussing' Nasha [MVP] (Expert):
so wat will be the implementation of newEmployee function Nasha [MVP] (Expert):
in the ur ITEmployeeController Class Nasha [MVP] (Expert):
and ur base EmployeeController class Nasha [MVP] (Expert):
u will need to use inheritance Nasha [MVP] (Expert):
yes Ravi Nasha [MVP] (Expert):
and wat else ? Nasha [MVP] (Expert):
wat about the EmployeeController class ? Nasha [MVP] (Expert):
U will even need to create ur ITEmployeeController class from ur EmployeeController
class Nasha [MVP] (Expert):
ok it goes like this Nasha [MVP] (Expert):
As the intent says that factory pattern defers the object instantiation
to the subclasses. Nasha [MVP] (Expert):
To achieve this we change the implementation of EmployeeController Nasha [MVP] (Expert):
let c how Nasha [MVP] (Expert):
We add an abstract method called as createEmployee to the EmployeeController Nasha [MVP] (Expert):
which will return a employee object Nasha [MVP] (Expert):
And we change the implementation of newEmployee function ..... so
Instead of instantiating an object of employee we call the createEmployee
function Nasha [MVP] (Expert):
Let us view the Structure of EmployeeController Class Nasha [MVP] (Expert):
/// Structure of EmployeeController
abstract class EmployeeController
{
private EmployeeCollection employees;
abstract public Employee createEmployee();
abstract public Employee createQualification();
abstract public Employee createExperience();
public void newEmployee ()
{
Employee employee = createEmployee();
employees.add(employee);
}
} Nasha [MVP] (Expert):
Now all the classes which extend the abstract Employeecontroller class
needs to implement the CreateEmployee method (and other factory methods). Nasha [MVP] (Expert):
something like this Nasha [MVP] (Expert):
/// Structure for ITEmployeeController
class ITEmployeeController:EmployeeController
{
public Employee createEmployee()
{
ITEmployee itemp = new ITEmployee();
itemp.Qualification = createQualification();
itemp.Experience = createExperience()
return new ITEmployee();
}
public Experience createExperience()
{
return new ITExperience();
}
public Qualification createQualification()
{
return new ITQualification();
}
} Nasha [MVP] (Expert):
any confusions ? Nasha [MVP] (Expert):
here the createEmployee fuction Nasha [MVP] (Expert):
will return the employee object ..... and newEmployee function instead
of creating an Employee object ... will return the object created
by createEmployee method Nasha [MVP] (Expert):
by doing this I dont need to create my Employee class as a concrete
class Nasha [MVP] (Expert): Q: But why you choose Employee as abstract? A: so that I can extend the class to create other
classes for different domains like IT,Bank etc Nasha [MVP] (Expert):
You will notice that the ITEmployeeController provides implementation
for createEmployee() , createQualification() and createExperience().
They are the factory methods which are responsible for creating Employee,
Qualification and Experience objects or in other words ... the concrete
products. Nasha [MVP] (Expert): Q: but controllers will also extended for all types A: yes Nasha [MVP] (Expert):
let us look at the applicability of this pattern Nasha [MVP] (Expert):
and the pros and cons Nasha [MVP] (Expert): Q: Ok, my argument was all these are possible without
making Employee abstract A: how Nasha [MVP] (Expert):
When you want your subclasses to decide which class to instantiate.
... ITEmployeeController decides which class to instantiate. Nasha [MVP] (Expert):
When you want to create 'static' heirarchy of objects ... static because
Factory Method is an inheritance based pattern so all the class relationships
are predefined. Nasha [MVP] (Expert):
Pros and Cons of Factory Method :- Nasha [MVP] (Expert):
On the positive side : Nasha [MVP] (Expert):
Eliminates the need for creating concrete classes in the base creator. Nasha [MVP] (Expert):
Gives the subclasses the flexibilty of creating there own objects
by by allowing them to implement 'factory method'. Nasha [MVP] (Expert):
But factory method also has a lot of demerits Nasha [MVP] (Expert):
It results in the creation of a lot of classes ... coz you will land
up creating one concrete creator per concrete product due to subclassing
for each creator. Nasha [MVP] (Expert):
Saurabh that answers ur question Nasha [MVP] (Expert):
yes this is one of the biggest demerit of FM Nasha [MVP] (Expert): Q: but controllers will also extended for all types A: yes .. thatz the demerit which I just mentioned
... that u will have to extend the controllers Nasha [MVP] (Expert): Q: I want to know what is the core problem where
i wll need to implement factory approach A: When you want your subclasses to decide which
class to instantiate Nasha [MVP] (Expert): Q: I hope this is a valid question. Isn't this Similar
to Polymorphism ? The Subclasses can control which Brother Class to
Instantiate. Right ? A: yes ... but the relationship is predefined or
static Nasha [MVP] (Expert):
the other demerits are Nasha [MVP] (Expert):
consider if you hav multiple creators than Nasha [MVP] (Expert):
all the creators will have to implement factory methods Nasha [MVP] (Expert):
and lastly Nasha [MVP] (Expert):
All relationships between the creators and products are static relationships
... thus cannot be changed at runtime. Nasha [MVP] (Expert):
In .NET there are quite a few Nasha [MVP] (Expert):
objects which implement factory method design pattern Nasha [MVP] (Expert):
one of them is WebRequest Nasha [MVP] (Expert):
This class has a static factory method called Create Nasha [MVP] (Expert):
WebRequest.Create Nasha [MVP] (Expert):
this method creates protocol-specific descendants of WebRequest using
the value of the URI passed in as argument. Nasha [MVP] (Expert):
For example, when a URI beginning with "http://" is passed,
an HttpWebRequest object is returned; when a URI beginning with "file://"
is passed, a FileWebRequest object is returned. Nasha [MVP] (Expert): Q: so controller here is webrequest? A: WebRequest is the creator Nasha [MVP] (Expert): Q: benefit? A: are the merits and demerits clear Nasha [MVP] (Expert):
EmployeeController Nasha [MVP] (Expert):
creating the EmployeeObject Nasha [MVP] (Expert):
EmployeeController or ITEmployeeController is the creator Nasha [MVP] (Expert):
Employee or ITEmployee is the product Nasha [MVP] (Expert):
and createEmployee is the FactoryMethod that we implemented Nasha [MVP] (Expert):
I hope that makes things clear Nasha [MVP] (Expert):
good Nasha [MVP] (Expert):
:) Nasha [MVP] (Expert):
any other queries Nasha [MVP] (Expert):
pls ask ? Nasha [MVP] (Expert): Q: Nasha can you give some example of Factory Method
used in .NET Framework class library? some times we used some patterns
but don't know we are using that. A: Factory method primarily is a used in creation
of frameworks . Nasha [MVP] (Expert): Q: Nasha can you give some example of Factory Method
used in .NET Framework class library? some times we used some patterns
but don't know we are using that. A: There are a lot of classes in .NET which use this
pattern like WebRequest, IEnumerable, Cryptography HttpApplicationFactory
to name a few Nasha [MVP] (Expert): Q: Nasha, Is it possible to implement polymorphism
without inheritance? A: u can use interfaces Nasha [MVP] (Expert): Q: and who was creator in our example A: EmployeeController and ITEmployeeController Nasha [MVP] (Expert): Q: or the controller for protocol A: ya ... but better refered as creator Nasha [MVP] (Expert):
There are a lot of classes in .NET which use this pattern like WebRequest,
IEnumerable, Cryptography HttpApplicationFactory to name a few Nasha [MVP] (Expert): Q: then how? A: instead of creating an abstract class create
an interface and implement that interface Nasha [MVP] (Expert): Q: then how? A: answered Nasha [MVP] (Expert): Q: I have a little query. Are the application blocks
in sync with any of the GOF patterns? I mean, are MS application blocks
implement any of GOF patterns A: yes ... they are ... although I have not gone
in to the depths of each one of them so I cant reply u rite away ...
but all of the app blocks come with there architecture doc .. u can
refer that Nasha [MVP] (Expert): Q: Anyway anyother way other than interfaces? A: nope Nasha [MVP] (Expert): Q: I'm in strong confusion that implementation is
also a type of inheritance, right? A: when we say inheritance we mean that the we are
carrying the baggage of the base class along with us .... but when
we say implementing as in implementing an interface we provide the
implementation Nasha [MVP] (Expert): Q: Do we say controler do anything more than passing
products. A: in our current example or in general ? Nasha [MVP] (Expert): Q: Other than interface, there is also abstract class,
no? A: yes ... interface is a sp case of abstract class
... which does not have any implemented methods Nasha [MVP] (Expert): Q: Ok, thank you, you have been wonderful through
the session A: thanx for coming and participating :) abhishek[MSFTE] (Moderator):
Thank you Namratha here for joining us today on a Microsoft Community
Chat to talk about Creational Design Patterns here. Nasha [MVP] (Expert): Q: When do we use Interface and When do we use a
Class to Inherit ? A: you use classes when you have some behaviour
in your base class object to be inherited ... you use interfaces when
you need to provide implementation Nasha [MVP] (Expert):
You use classes when you have some behaviour in your base class object
to be inherited ... you use interfaces when you need to implement
behaviour and primarily for polymorphism Nasha [MVP] (Expert):
interface is a sp case of abstract class which does not have any implementation Nasha [MVP] (Expert):
any other questions ? Nasha [MVP] (Expert):
not here dear ... this is a chat for creational design patterns ...
let us stick to it Nasha [MVP] (Expert): Q: what all patterns do u believe do come in use..
in normal life projects... in ur professional project dev. experiences....
n this one is for everybody.... A: Commonly used patterns factory method, singleton,
facade, strategy,template,builder,abstractfactry Nasha [MVP] (Expert):
Some Commonly used patterns factory method, singleton, facade, strategy,template,builder,abstractfactory Nasha [MVP] (Expert):
yes Nasha [MVP] (Expert):
the next i was to discuss was abstract factory Nasha [MVP] (Expert): Q: Is Abstractfactory different from Factory Method
? A: yes Nasha [MVP] (Expert): Q: what I am trying to get here is the jist of everybody's
work experience. :D There are many creational patterns.. but we have
talked only about factory pattern... wht is so ?? A: will continue with others next time Nasha [MVP] (Expert): Q: what I am trying to get here is the jist of everybody's
work experience. :D There are many creational patterns.. but we have
talked only about factory pattern... wht is so ?? A: only 1 hr which is less :( Nasha [MVP] (Expert):
ok today coz of less time Nasha [MVP] (Expert):
I could only discuss factory method Nasha [MVP] (Expert):
4 more are remaining Nasha [MVP] (Expert): Q: Technically Who are supposed to be Aware of Patterns
? I mean Project Manager , Architect ? A: Developers and Architects Nasha [MVP] (Expert): Q: can u explain about factory method A: thatz wat we discussed today :) Nasha [MVP] (Expert): Q: Why does C# classes does not allow multiple inhertence? A: irrelevant to the current topic Nasha [MVP] (Expert):
ok guys Nasha [MVP] (Expert):
thatz all for today Nasha [MVP] (Expert):
will meet u guys soon again and continue ahead Nasha [MVP] (Expert):
I hope u guys enjoyed and gained something from this session Nasha [MVP] (Expert):
Thanx a lot for coming and investing ur time here Nasha [MVP] (Expert):
Thanks a lot Nasha [MVP] (Expert):
Thanx everyone abhishek[MSFTE] (Moderator):
Thank you all for joining the webchat today Nasha [MVP] (Expert):
if u guys have any questions post them on UG abhishek[MSFTE] (Moderator):
We are now ending the chat due to time restrictions abhishek[MSFTE] (Moderator):
you can find more information on the Patterns at the Microsoft site:
www.microsoft.com/practices Nasha [MVP] (Expert):
bye guys abhishek[MSFTE] (Moderator):
We would also encourage you to participate in the local User Groups
a list of which is available at: http://www.microsoft.com/india/communities/usergroups/default.aspx abhishek[MSFTE] (Moderator):
We hope you join us for our next chat on May 25 2005 abhishek[MSFTE] (Moderator):
We would also store a transcript of this chat on http://www.microsoft.com/india/communities/chat/default.aspx abhishek[MSFTE] (Moderator):
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We eagerly look forward to hearing from you on how we can help you
better achieve your potential abhishek[MSFTE] (Moderator):
Thanks all!
