Design Patterns - I (Introduction)

Authoring software requires a set of rules and methodologies to take your idea to center point without any rework for future enhancements.  In Software Engineering, we have a set of principles stated by Robert Martin called SOLID principle.   SOLID word has principles as Single Responsibility, Open Close, Liskov’s Substitution, Interface Segregation and Dependency Inversion. Let me explain you about SOLID as one word 

• Single Responsibility Principle – There should be a one strong reason to change the class.

• Open/Close Principle – Entities should open only for adding new functionality and Close for any updates after testing phase.

• Liskov’s Substitution Principle – Functions accessing base class must be able to access derived class

• Interface Segregation Principle – Clients should use Interface members completely.

• Dependency Inversion Principle – High level modules and Details should depend on abstractions only.

Other Main Principles of Software design Engineering is

• Dry (Don’t Repeat Yourself): Code should occur exactly once in the system.

• KISS (Keep it Simple, Stupid!): Unnecessary complex code should avoided.

• YAGNI (You ain’t  gonna need it): Implement the things when you need them

These principles results to maintain your software code as manageable, Scalable and reusable code and results in faster development.

In point of code reusability Design Patterns are introduced.  Design patterns are introduced by Christopher Alexander in 1977, Kent Beck and Ward Cunningham in 1987.  Design patterns are considered as reusable solutions for commonly occurring problems in software designs.  Advantages of design patterns are, they provide solutions for commonly occurring problems in software designs, they can implement in any language, provide standard terminology and specific to particular scenario.  Design Patterns are categorized into 5 categories. 

1. Creational Patterns

2. Structural Patterns

3. Behavioral Patterns

4. Model – View – Controller Patterns

5. Concurrency Patterns

Creational Patterns are patterns deals with object Creations.  Creational Patterns are applicable when:

1. A system should be independent of how its objects and products are creating.

2. Two or more objects designed to use each other.

3. Instance, can be extended without modification.

4. Sub class of a class wants to access class object.

5. Reveling interfaces of a product.

6. Constructing different independent Complex objects.

7. Single instance can be access all the time.

Structure of these creational patterns contains creation followed by its concrete creator. Creation Patterns group contains 7 Design patterns.

1. Abstract Factory

2. Factory Method

3. Builder

4. Lazy Initialization

5. Object Pool

6. Prototype

7. Singleton

Structural Design Patterns are used to define entity relationships in simple ways. Structural patterns use inheritance to compose interfaces or implementations.  Structural patterns are useful to develop independent class libraries to work together.

Structural Patterns has 11 Patterns with it.

1. Adapter

2. Aggregate

3. Bridge

4. Composite

5. Decorator

6. Extensibility

7. Facade

8. Flyweight

9. Pipes and filters

10. Private Class Data

11. Proxy

Behavioral Design Patterns identifies common communication between objects and realize these patterns.  These patterns increase the flexibility of communication between the objects.

Behavioral Patterns has 19 patterns.

1. Chain of responsibility

2. Command

3. Externalize the Stack

4. Hierarchical visitor

5. Interpreter

6. Iterator

7. Mediator

8. Memento

9. Null Object

10. Observer

11. Weak reference

12. Protocol stack

13. Scheduled-task

14. Single-serving visitor

15. Specification

16. State

17. Strategy

18. Template method

19. Visitor

Model View Controller design pattern has its own identity which has 3 inter connected parts, Model, View and Controller.  We use this mainly for User Interfaces.

These Patterns mainly deals with Multi thread Programming.  They were associated with Pattern Oriented Software Architecture.  Concurrency Patterns have 11 Patterns.

1. Active Object

2. Balking pattern

3. Double checked locking pattern

4. Guarded suspension

5. Leaders/followers pattern

6. Monitor Object

7. Reactor pattern

8. Read write lock pattern

9. Scheduler pattern

10. Thread pool pattern 

11. Thread-Specific Storage

References:  

http://www.codeproject.com/Tips/654769/Software-Design-Principles http://www.dotnet-tricks.com/Tutorial/designpatterns/RPW2250513-Different-Types-of-Software-Design-Principles.html 
• http://en.wikipedia.org/wiki/Software_design_patterns