1. The way how Experts do things…
Prepared & Presented by :-
Prageeth Sandakalum, Microsoft Student Champ,
Faculty of IT, University of Moratuwa.
2. What are Design Patterns
Development Issues and introduction of Design Patterns
A brief history on design patters
What are design patterns?
Types of Design Patterns
Has your code been improved ?
Implementing Design Patterns in C#
Singleton Pattern
Prototype Pattern
Façade Pattern
Decorator pattern
4. “Designing object-oriented software is hard and designing
reusable object-oriented software is even harder.”
– Erich Gamma -
Dependency Handling - Open and Close method…
Open for Extension and Closed for Modification
Immobility of the existing libraries.
Knowledge of the patterns that have worked in the past, makes
the developers lives much easier in designing newer systems.
5. 1987 - Cunningham and Beck used Alexander’s ideas to develop
a small pattern language for Smalltalk
1990 - The Gang of Four (Gamma, Helm, Johnson and Vlissides)
begin work compiling a catalog of design patterns
1991 - Bruce Anderson gives first Patterns Workshop at OOPSLA
1993 - Kent Beck and Grady Booch sponsor the first meeting of
what is now known as the Hillside Group
1994 - First Pattern Languages of Programs (PLoP) conference
1995 - The Gang of Four (GoF) publish the Design Patterns book
6. “A design pattern is a general and reusable solution to a commonly
occurring problem in software.”
- Microsoft Developer Network (MSDN) -
It is not a code that can be directly use in your application thus
We can call a pattern a template to design classes or objects in a
way that is proven to be optimized in the past
Design patterns help to develop the high-level Solutions and to
implement the Object Oriented principals
Open Close Principal :- Open for extension and closed for modifications
Dependency Inversion Principle :- Depend upon Abstractions, Not
On concretions.
7. OO Design Patterns
Creational Structural Behavioral
Behavioral Patterns
Abstract Factory Creates an instance of several families of classes
Builder Separates object construction from its representation
Factory Method Creates an instance of several derived classes
Prototype A fully initialized instance to be copied or cloned
Singleton A class of which only a single instance can exist
8. Structural Patterns
Adapter Match interfaces of different classes
Bridge Separates an object’s interface from its implementation
Composite A tree structure of simple and composite objects
Decorator Add responsibilities to objects dynamically
Facade A single class that represents an entire subsystem
Flyweight A fine-grained instance used for efficient sharing
Proxy An object representing another object
9. Behavioral Patterns
Chain of Responsibility passing a request between a set of objects
Command Encapsulate a command request as an object
Interpreter A way to include language elements in a program
Iterator Sequentially access the elements of a collection
Mediator Defines simplified communication between classes
Memento Capture and restore an object's internal state
Observer A way of notifying change to a number of classes
State Alter an object's behavior when its state changes
Strategy Encapsulates an algorithm inside a class
Template Method Defer the steps of an algorithm to a subclass
Visitor Defines a new operation to a class without change
10. These are some of the Common advantages in Design patterns.
Capture expertise and make it more accessible to non-experts in
a standard form
Facilitate communication among the developers by providing a
common language
Make it easier to reuse successful designs and avoid alternatives
that diminish reusability
Facilitate design modifications
Improve design documentation
Improve design understandability
12. ◦ Concept
Ensures that a particular class has only one object
provide a central access point to invoke that object
◦ Motivation and Problem addresses
Make sure that the client is not allowed to create
additional objects
The “User” Object
UML Class Try out Microsoft Windows basic applications
Diagram The Calculator and the Run window
13. Implementation Steps…
Create the class with public modifier
Make the constructor private… So that no body outside, can create
objects
Define a static method GetInstance() to retrieve the Object
Inside the method, use conditional statements to check whether the
object exists
If not create an object and return it
Otherwise just return the object
Then instantiate the object (make it static)
14. Some hints for advanced development
Make the GetInstance() method thread safe
Use locks, Double check the instances
Create a “readonly” object to use as the locking variable
Initialize the object on 1st declaration
private static Singleton instance=new Singleton();
Avoid sub-classing by using sealed class
public sealed class Singleton
15. ◦ Concept
Avoid repeatedly assigning same value to the
similar objects.
Allow independent behavior after the cloning ???
◦ Motivation and Problem addresses
When creating objects (i.e. employee), we need to
specify the same value repeatedly for objects
employeeObj1.CompanyName = “IFS”;
employeeObj1.CompanyPhone = “011-2845564”;
EmployeeObj2.CompanyName = “IFS”;
employeeObj2.CompanyPhone = “011-2845564”;
16. Implementation Steps…
Created an object from the given class, i.e. firstEmp : Employee
Now create the second object , i.e. secondEmp : Employee
Set the values of the old object, i.e. firstEmp.Company = “IFS”;
you can interchange the second and third steps…
Implement the GetClone() method in the referring class.
Return the cloned version of the referring object (Use appropriate
cloning methods)
Assign the returned value to the second object or vise versa., i.e.
secondEmp = firstEmp.GetClone();
17. ◦ Concept
• Wrap complicated subsystems with simpler interfaces
• Delegates client requests to appropriate subsystems
◦ Motivation and Problem addresses
The Client code become complicated with so many
objects
How Façade The client need not to know the complex architecture
Works of the sub system
Banking Applications
18. Implementation Steps…
Create the Façade class with public modifier
Define object references of all the types in the sub system
Initialize all the objects “On Demand”
Create simple methods and handle multiple subsystem methods
inside that method
Withdraw() in Façade class handles DailyWithdrowalLimitExceeded(),
GetBalance(), IssueReceipt() etc.. Methods
19. Some hints for advanced development
Encapsulate the subsystem, So that it can only be accessed through
Façade interface.
Use internal modifiers
Make the Façade Object SINGLETON ???
Most of the time only one façade object is needed
Use multiple Sub Systems within the same dll.
If the sub system is too complex… use multiple facades.
20.
21.
22. See the Restaurant Problem explained here
You create a beverage super class and all the beverages inherit from it
But it will have too much sub classes implementing the same cost
method in the super class.
Tea, PlainTea, TeaWithoutSugar, IcedTea, VanilaTea, ChocoTea etc
But each of them have very little difference on Ingredients…
Adding Milk, Adding Sugar, Adding Chocolate
To avoid unwanted sub-classing create methods in the super class for
the extra changes
AddMilk(), AddSugar(), AddChocolate()
Has the problem been Solved?
23. Still we got some problems
OCP… Open for Extension, Closed for Modification… Then how to
handle the price changes ?
More methods needed, more problems created.
What if the new ingredients were introduced ?
Concept of Decorator Pattern
Take the initial Object and decorate it in the Run time
Take the Tea Object and decorate it with Sugar Object, then the
Choco / Vanilla Objects…
24. Implementation Steps…
Create the abstract class for the Main Classes, i.e. class Main
Create the abstract class for the Decorator Classes extending the
above class, i.e. class Decorator : Main
Create the sub classes of the Main class
Create the Decorator sub classes and let the Constructor to receive
a “Main” type object
Do the operation with the received object
When ever creating an object in the client code, Use the “Main”
class as the reference
25. Thank You !!!
References
Notes from Aruna Wickrama, Teamwork @ Asharp Power Camp
Head First Design Patterns By Eric Freeman, Elisabeth Robson,
Kathy Sierra, Bert Bates.
Design Principles and Design Patterns Robert C. Martin
Software Architecture Interview Questions By Shivprasad Koirala,
Sham Sheikh
Articles from GoF (Gang of Four)
