Adapter design pattern allows you to make an existing class work with other existing class libraries without changing the code of the existing class.
Maximum times we needed that methods of the existing class should work with other existing class to work . Example :- Say we have one class its function is doing one work and we have to add some functionality by using existing library but they are not compatiable.
To solve this problem we have to create another class Adapter that inherits from the existing class while implementing the interface of the existing library(normally library gives interface to acces its functionality). The end result is that the Adapter can call the method of the existing class (since theAdapter inherits from the existing class) and can work in the existing library (since the Adapter implements the interface of the existing library).
A real life example could be a case of card reader which acts as an adapter between memory card and a laptop. You plugins the memory card into card reader and card reader into the laptop so that memory card can be read via laptop.
Below is the UML of the Adapter Design Pattern:
- The Adaptee is the existing class.
- The IInterface is the interface defined in the existing library.
- The Adapter is the class that you create, it is inherited from the Adaptee class and it implements theIInterface interface. Notice that it can call the OperationA method(inherited from the Adaptee) inside its OperationB method (implemented by the IInterface).
Example:-an audio player device can play mp3 files only and wants to use an advanced audio player capable of playing vlc and mp4 files.
Implementation:-
We want to make AudioPlayer to play other formats as well.
We're having ( library )another interface AdvancedMediaPlayer and concrete classes implementing theAdvancedMediaPlayer interface.These classes can play vlc and mp4 format files.
we've created an adapter class MediaAdapter which implements the MediaPlayer interface and uses AdvancedMediaPlayerobjects to play the required format.
AudioPlayer uses the adapter class MediaAdapter passing it the desired audio type without knowing the actual class which can play the desired format. AdapterPatternDemo, our demo class will useAudioPlayer class to play various formats.
#### Library implemented as
public interface AdvancedMediaPlayer {
public void playVlc(String fileName);
public void playMp4(String fileName);
}
public class VlcPlayer implements AdvancedMediaPlayer{
@Override
public void playVlc(String fileName) {
System.out.println("Playing vlc file. Name: "+ fileName);
}
@Override
public void playMp4(String fileName) {
//do nothing
}
}
public class Mp4Player implements AdvancedMediaPlayer{
@Override
public void playVlc(String fileName) {
//do nothing
}
@Override
public void playMp4(String fileName) {
System.out.println("Playing mp4 file. Name: "+ fileName);
}
}
##### Adapter class
public class MediaAdapter implements MediaPlayer {
AdvancedMediaPlayer advancedMusicPlayer; // not support multiple inheritance so omposed
public MediaAdapter(String audioType){
if(audioType.equalsIgnoreCase("vlc") ){
advancedMusicPlayer = new VlcPlayer();
} else if (audioType.equalsIgnoreCase("mp4")){
advancedMusicPlayer = new Mp4Player();
}
}
@Override
public void play(String audioType, String fileName) {
if(audioType.equalsIgnoreCase("vlc")){
advancedMusicPlayer.playVlc(fileName);
}else if(audioType.equalsIgnoreCase("mp4")){
advancedMusicPlayer.playMp4(fileName);
}
}
}
#### existing class
public interface MediaPlayer {
public void play(String audioType, String fileName);
}
public class AudioPlayer implements MediaPlayer {
MediaAdapter mediaAdapter;
@Override
public void play(String audioType, String fileName) {
//inbuilt support to play mp3 music files
if(audioType.equalsIgnoreCase("mp3")){
System.out.println("Playing mp3 file. Name: "+ fileName);
}
//mediaAdapter is providing support to play other file formats
else if(audioType.equalsIgnoreCase("vlc")
|| audioType.equalsIgnoreCase("mp4")){
mediaAdapter = new MediaAdapter(audioType);
mediaAdapter.play(audioType, fileName);
}
else{
System.out.println("Invalid media. "+
audioType + " format not supported");
}
}
}
public class AdapterPatternDemo {
public static void main(String[] args) {
AudioPlayer audioPlayer = new AudioPlayer();
audioPlayer.play("mp3", "beyond the horizon.mp3");
audioPlayer.play("mp4", "alone.mp4");
audioPlayer.play("vlc", "far far away.vlc");
audioPlayer.play("avi", "mind me.avi");
}
}
