重点

• 接口之间的桥梁，兼容接口
• 适配器继承或依赖已有对象，实现目标接口
• 过多地使用适配器，会让系统非常零乱，不易整体进行把握。比如，明明看到调用的是 A 接口，其实内部被适配成了 B 接口的实现，一个系统如果太多出现这种情况，无异于一场灾难。因此如果不是很有必要，可以不使用适配器，而是直接对系统进行重构。

适配器

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

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

public class VlcPlayer implements IOldMediaPlayer {
   @Override
   public void playVlc(String fileName) {
      System.out.println("Playing vlc file. Name: "+ fileName);      
   }
 
   @Override
   public void playMp4(String fileName) {
      //什么也不做
   }
}

public class Mp4Player implements IOldMediaPlayer {
 
   @Override
   public void playVlc(String fileName) {
      //什么也不做
   }
 
   @Override
   public void playMp4(String fileName) {
      System.out.println("Playing mp4 file. Name: "+ fileName);      
   }
}

public class MediaAdapter implements IMediaPlayer {
 
   IOldMediaPlayer oldPlayer;
 
   public MediaAdapter(String audioType){
      if(audioType.equalsIgnoreCase("vlc") ){
         oldPlayer = new VlcPlayer();       
      } else if (audioType.equalsIgnoreCase("mp4")){
         oldPlayer = new Mp4Player();
      }  
   }
 
   @Override
   public void play(String audioType, String fileName) {
      if(audioType.equalsIgnoreCase("vlc")){
         oldPlayer.playVlc(fileName);
      }else if(audioType.equalsIgnoreCase("mp4")){
         oldPlayer.playMp4(fileName);
      }
   }
}

public class AudioPlayer implements IMediaPlayer {
   MediaAdapter mediaAdapter; 
 
   @Override
   public void play(String audioType, String fileName) {    
 
      //播放 mp3 音乐文件的内置支持
      if(audioType.equalsIgnoreCase("mp3")){
         System.out.println("Playing mp3 file. Name: "+ fileName);         
      } 
      //mediaAdapter 提供了播放其他文件格式的支持
      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");
      }
   }   
}