Strategy Design Pattern | Java Example

Definition

• defines a family of algorithms,
• encapsulates each algorithm, and
• makes the algorithms interchangeable within that family

When to Use?

• To choose an appropriate algorithm/behavior at run time when there are multiple algorithms/behaviors executing the same task/operation.

UML

Real World Scenario

As depicted in the UML diagram below, we have an application that plays audio files. And we have many options to play the audio files: VLC Media Player, Window Media Player, KMPlayer. Now, the client or the main program will choose which media player to play the audio files at run time.

Figure: Strategy Design Pattern

Code Sample

MainProgram.java

import java.io.File;

public class MainProgram {

       public static void main(String[] args) {

             File mediaFile = new File("audio.mp3");

             MediaPlayer player = new MediaPlayer(new VLCMediaPlayer());

             // MediaPlayer player = new MediaPlayer(new WindowMediaPlayer());

             // MediaPlayer player = new MediaPlayer(new KMPlayer());

             player.play(mediaFile);

       }

}

MediaPlayer.java

import java.io.File;

public class MediaPlayer {

       private IPlayer player;

       public MediaPlayer(IPlayer player) {

             this.player = player;

       }

       public void play(File mediaFile) {

             player.play(mediaFile);

       }

}

IPlayer.java

import java.io.File;

public interface IPlayer {

       public void play(File mediaFile);

}

VLCMediaPlayer.java

import java.io.File;

public class VLCMediaPlayer implements IPlayer {

       @Override

       public void play(File file) {

             System.out.println("Playing via. VLC Media Player");

       }

}

WindowMediaPlayer.java

import java.io.File;

public class WindowMediaPlayer implements IPlayer {

       @Override

       public void play(File file) {

             System.out.println("Playing via. Window Media Player");

       }

}

KMPlayer.java

import java.io.File;

public class KMPlayer implements IPlayer {

       @Override

       public void play(File file) {

             System.out.println("Playing via. KMPLayer");

       }

}