Strategy Design Pattern in java.........

Real World Example

A Strategy defines a set of algorithms that can be used interchangeably. Modes of transportation to an airport is an example of a Strategy. Several options exist such as driving one's own car, taking a taxi, an airport shuttle, a city bus, or a limousine service. For some airports, subways and helicopters are also available as a mode of transportation to the airport. Any of these modes of transportation will get a traveler to the airport, and they can be used interchangeably. The traveler must chose the Strategy based on tradeoffs between cost, convenience, and time.

http://sourcemaking.com/design_patterns/strategy

The Strategy Pattern

The Strategy pattern is known as a behavioural pattern - it's used to manage algorithms, relationships and responsibilities between objects. The definition of Strategy provided in the original Gang of Four book on Design Patterns states: 

Defines a set of encapsulated algorithms that can be swapped to carry out a specific behaviour

Now, let's take a look at the diagram definition of the Strategy pattern

Now, let's take a look at the diagram definition of the Strategy pattern.

Where Would I Use This Pattern?

The Strategy pattern is to be used where you want to choose the algorithm 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.  

So How Does It Work In Java?

Let's use the example of a file compression tool - where we create either zip or rar files. First we'll need a strategy: 

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1.//Strategy Interface

2.public interface CompressionStrategy

3.{

4.public void compressFiles(ArrayList files);

5.}

And we'll need to provide our two implementations, one for zip and one for rar

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01.public class ZipCompressionStrategy implements CompressionStrategy

02.{

03. 

04.public void compressFiles(ArrayList files)

05.{

06.//using ZIP approach

07.}

08. 

09.}

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01.public class RarCompressionStrategy implements CompressionStrategy

02.{

03. 

04.public void compressFiles(ArrayList files)

05.{

06.//using RAR approach

07.}

08. 

09.}

Our context will provide a way for the client to compress the files. Let's say that there is a preferences setting in our application that sets which compression algorithm to use. We can change our strategy using the setCompressionStrategy method in the Context.

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01.public class CompressionContext

02.{

03.private CompressionStrategy strategy;  

04. 

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

06.public void setCompressionStrategy(CompressionStrategy strategy)

07.{

08.this.strategy = strategy; 

09.}

10. 

11.//use the strategy

12.public void createArchive(ArrayList files)

13.{

14.strategy.compressFiles(files);

15.}

16. 

17.}

http://java.dzone.com/articles/design-patterns-strategy

It's obvious that all the client has to do now is pass through the files to the CompressionContext

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01.public class Client

02.{

03. 

04.public static void main(String[] args)

05.{

06.CompressionContext ctx = new CompressionContext();

07.//we could assume context is already set by preferences

08.ctx.setCompressionStrategy(new ZipCompressionStrategy());    

09.//get a list of files

10....

11.ctx.createArchive(fileList);   

12. 

13.}

14.}