MediaPlayer播放资源音乐出现 start called in state 0 错误

package com.example.playaudio;

import java.io.File;
import java.io.IOException;

import android.app.Activity;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

private Button Play_btn,Pause_btn,Stop_btn;
private MediaPlayer mediaplayer = new MediaPlayer();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Play_btn = (Button)findViewById(R.id.button1);
    Pause_btn = (Button)findViewById(R.id.button2);
    Stop_btn = (Button)findViewById(R.id.button3);

    initMediaPlayer();
    //播放音乐;
    Play_btn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            if (!mediaplayer.isPlaying()) {
                mediaplayer.start();
            }
        }
    });
    //暂停音乐;
    Pause_btn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            if(mediaplayer.isPlaying()){
                mediaplayer.pause();
            }
        }
    });
    //停止音乐;
    Stop_btn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            if(mediaplayer.isPlaying()){
                mediaplayer.reset();
                initMediaPlayer();
            }
        }
    });
}
//若APP停止,释放内存;
private void initMediaPlayer() {
    try {
        File file = new File(Environment.getExternalStorageDirectory(),"music.mp3");
        mediaplayer.setDataSource(file.getPath());
        mediaplayer.prepare();
    } catch (Exception e) {
        e.printStackTrace();
    }
}
@Override
protected void onDestroy() {
    super.onDestroy();
    if(mediaplayer != null){
        mediaplayer.stop();
        mediaplayer.release();
    }
}

}

//已经在AndroidMainfest 添加了权限;根目录下已有music.mp3文件;

http://blog.csdn.net/jason0539/article/details/42588395