无法启动Activityinfo

Question

我正在尝试开发简单的mp3文件。显示的mp3文件已填入列表中。我的代码是：

class Mp3Filter implements FilenameFilter{
    @Override
    public boolean accept(File dir, String name)
    {
        return (name.endsWith(".mp3"));
    }
}

public class MyActivity extends ListActivity {
    private  String SD_PATH = new String("/sdcard/Music");
    private List<String> songs = new ArrayList<String>();
    private MediaPlayer mp = new MediaPlayer();

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

        Button stopPlay= (Button) findViewById(R.id.stopBtn);
        stopPlay.setOnClickListener( new View.OnClickListener() {
            @Override
            public void onClick(View view) {
              mp.stop();
            }
        });
    }

    private void updatePlaylist() {
        String TAG="CHUSS";
        File home = new File("SD_PATH");
        if(home.listFiles( new Mp3Filter()).length>0){
            for(File file : home.listFiles( new Mp3Filter())){
                songs.add(file.getName());
                Log.e(TAG,file.getName());
            }
            ArrayAdapter<String> songlist = new ArrayAdapter<String>   (this,R.layout.songs_item,songs);
            setListAdapter(songlist);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.my, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

代码中的错误

3-05 19:44:46.807    4034-4034/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.hawk.player4, PID: 4034
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.hawk.player4/com.example.hawk.player4.MyActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5021)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
            at dalvik.system.NativeStart.main(Native Method)