title: can not play mp3 file from raw folder in android
body: <p>I created raw folder inside res directory and inserted mp3 file.I try to play my mp3 file with mediaplayer.
  this is a my source -</p>
  
  <pre><code> MediaPlayer mediaPlayer=MediaPlayer.create(MainActivity.this,R.raw.music_1);
   mediaPlayer.start();
  </code></pre>
  
  <p>but when I run app I have exception.this is a my logcat exception -</p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
  </code></pre>
  
  <p>How I can solve my problem?
  If anyone knows solution,please help me.
  Thanks in advance.</p>