Strategy Design Pattern

Strategy Design Pattern

Author : Milan D Ashara

Programming is a very easy task, but to write code that is maintainable needs experience and knowledge of design patterns. Design patterns are knowledge of well known coding style(writing maintainable code) for solving a particular type of problem. The important part is not learning design patterns but to indentify areas in application where design patterns can be applied.

I am going to write about simple "Strategy design pattern" one among the various design patterns.

Identify which behaviour of the class/algorithm you are writing varies at run time. These behaviour should be the instance of the class. In our example Context class has behaviour ConcreteStrategyA and ConcreteStrategyB. These behaviour can be changed at run time by using setter method. The code of above image looks like this

//Strategy Interface

public interface Strategy

{

public void execute();

}

public class ConcreteStrategyA implements Strategy

{

public void execute()

{

//execute

}

}

public class ConcreteStrategyB implements Strategy

{

public void execute()

{

//execute

}

}

public class Context

{

private Strategy strategy;

//this can be set at runtime by the application preferences

public void setStrategy(Strategy strategy)

{

this.strategy = strategy;

}

//use the strategy

public void executeStrategy()

{

strategy.execute();

}

}

public class Client

{

public static void main(String[] args)

{

Context ctx = new Context();

ctx.setStrategy(new ConcreteStrategyA());

ctx.executeStrategy();

ctx.setStrategy(new ConcreteStrategyB());

ctx.executeStrategy();

}

} 

Where it can be used..

The Strategy pattern is to be used where you want to choose the algorithm/behaviour/strategy(anything that varies at runtime) to use at runtime. A good use of the Strategy pattern would be saving files in different formats, running various sorting algorithms, or file compression.
The Strategy pattern provides a way to define a family of algorithms, encapsulate each one as an object, and make them interchangeable.

knowing design pattern is not enough. Finding areas in application where it can be applied is important.

Real World Senerio were it can be used :

Imagine a shopkeeper selling many items at his post. There are so many customer who buys different items.

Here different item are different behaviours that varies.

Shopkeeper is the our context class who has to decided at run time which customer buys which item and accordingly to that shopkeepe generate invoice for that customer.

Invoice invoice=context.generateInvoice(item);

Invoice is generated accordingly to the type of item is passed to generateInvoice method.

OO Principles covered in Strategy Design Patterns are

1. What varies should be encapsulated

2. Prefer composition over inheritance : U

3. Program to interfaces not to implementation

It is simple design patterns widely used.

Cleared Oracle Certified Java EE Web Component Developer Exam

I cleared Oracle Certified Java EE Web Component Developer Exam with 89%.

I would like to thanks to the Maker of Head First JSP & Servlet. It is very good book. Read this book fully and practice and practice to clear this exam.

Exam Topics

1. Servlet life cycle
2. HTTP Methods
3. All Listeners
4. Session Management
5. JSP
6. Scriptless: JSP Standard Action and EL
7. Custom Tag Library
8. DD
9. Filter
10. Web Security
11. J2EE Paterns : Buisness Delegate, Service locator, Front controller, Transfer Object, MVC Design


Study Head First JSP and Servlet Fully and practice and practice to clear this exam