Java设计模式——适配器模式

适配器模式充当两个不兼容接口之间的桥梁， 它属于结构模式。
它涉及一个单一的类，负责连接独立或不兼容的接口功能。比如读卡器，它充当存储卡和电脑之间的适配器。我们将存储卡插入读卡器并将读卡器插入电脑，这样就可以通过电脑读取存储卡内容。

下面进行演示说明。
需求：假设我们有一台MP3播放器，但我们又希望它能播放vlc和mp4文件。
步骤：
1、创建接口MediaPlayer（媒体播放），以及一个实现类AudioPlayer（音频播放）
2、创建另外一个接口AdvancedMediaPlayer（高级媒体播放器），以及它的实现类（VLCPlayer、Mp4Player）
3、我们想让 AudioPlayer也能播放vlc和mp4。创建一个适配器类MediaAdapter，它实现了MediaPlayer接口，并使用 AdvancedMediaPlayer对象来播放所需的格式。
4、AudioPlayer 使用适配器类 MediaAdapter，并将媒体文件传递给它，并不需知道实际文件类型。

1、创建接口MediaPlayer、AdvancedMediaPlayer

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

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

2、创建AdvancedMediaPlayer的实现类

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);		
   }
}

3、创建适配器类MediaAdapter，实现接口MediaPlayer

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);
      }
   }
}

4、创建AudioPlayer类，实现接口MediaPlayer

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");
      }
   }   
}

5、测试

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");
   }
}

6、结果

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