Touch and drag image in android

I am working on some example in which i want to drag the image corresponding to touch in Android. Does anybody have an idea about how I can do it?

public class TouchBall extends Activity {


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    int w=getWindowManager().getDefaultDisplay().getWidth()-25;
    int h=getWindowManager().getDefaultDisplay().getHeight()-25;

    BallView ballView=new BallView(this,w,h);
    setContentView(ballView);
}


}
public class BallView extends SurfaceView implements SurfaceHolder.Callback {

    private Bitmap bitmap ;
    private MyThread thread;
    private int x=20,y=20;int width,height;

    public BallView(Context context,int w,int h) {
        super(context);

        width=w;
        height=h;
        thread=new MyThread(getHolder(),this);
        getHolder().addCallback(this);
        setFocusable(true);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        bitmap =BitmapFactory.decodeResource(getResources(), R.drawable.ball_green);
        canvas.drawColor(Color.BLUE);//To make background 
        canvas.drawBitmap(bitmap,x-(bitmap.getWidth()/2),y-(bitmap.getHeight()/2),null);


    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        x=(int)event.getX();
        y=(int)event.getY();

        if(x<25)
                x=25;
         if(x> width)   
                x=width;
         if(y <25)
                y=25;
         if(y > 405)
                y=405;      
        return true;
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width,int height) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {

        thread.startrun(true);
        thread.start();

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {


        thread.startrun(false);
        thread.stop();

    }   
 }

thread:

public class MyThread extends Thread {

private SurfaceHolder msurfaceHolder;
private BallView mballView;
private boolean mrun =false;

public MyThread(SurfaceHolder holder, BallView ballView) {

    msurfaceHolder = holder;
    mballView=ballView;
}

public void startrun(boolean run) {

    mrun=run;
}

@Override
public void run() {

    super.run();
     Canvas canvas;
     while (mrun) {
        canvas=null;
         try {
             canvas = msurfaceHolder.lockCanvas(null);
              synchronized (msurfaceHolder) {
               mballView.onDraw(canvas);
             }
         } finally {
                 if (canvas != null) {
                 msurfaceHolder.unlockCanvasAndPost(canvas);
             }
         }
     }
  }

}