Adapter Pattern works as a bridge between two incompatible interfaces. This is a structural pattern that combines the capability of two different interfaces.

Adapter Pattern involves a single class which is responsible to join functionalities of independent or incompatible interfaces.

Using of adapter Pattern can be demonstrated via following 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.

Step 1 : Create interfaces for Media Player and Advanced Media Player

public interface MediaPlayer {
    void play(String audioType, String fileName);
}

public interface AdvancedMediaPlayer {
    void playVlc(String fileName);
    void playMp4(String fileName);
}

Step 2 : Create concrete classes implementing AdvancedMediaPlayer interface

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) {

    }
}

public class Mp4Player implements AdvancedMediaPlayer {
    @Override
    public void playVlc(String fileName) {

    }

    @Override
    public void playMp4(String fileName) {
        System.out.println("Playing mp4 file. Name: "+ fileName);
    }
}

Step 3 : Create adapter class implementing the MediaPlayer interface

public class MediaAdapter implements MediaPlayer {
    AdvancedMediaPlayer advancedMusicPlayer;

    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);
        }
    }
}

Step 4 : Create concrete class implementing the MediaPlayer interface

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");
        }
    }
}

Step 5 : Use the AudioPlayer to play different types of audio formats

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");
    }
}

The output will be :

Playing mp3 file. Name: beyond the horizon.mp3
Playing mp4 file. Name: alone.mp4
Playing vlc file. Name: far far away.vlc
Invalid media. avi format not supported