android - View.SurfaceView, why its member, mSurfaceHolder, returns null from getSurface()?

I am studying Android Game Development these days. I come
across a problem about SurfaceView\SurfaceHolder. When I read the source code of
View/SurfaceView.java in android sdk 22, I am confused. following is the
code:

public class
            SurfaceView extends MockView {
 ... 
 public SurfaceHolder
            getHolder() {
 return mSurfaceHolder;
 }


            private SurfaceHolder mSurfaceHolder = new SurfaceHolder() {

            ...

 @Override

 public Surface getSurface() {
            return null; }
 @Override
 public Canvas lockCanvas() { return null;
            }
 ...


            }
}

I know,
mSurfaceHolder.getSurface()\lockCanvas are very important, but it returns null! So, I
think this mSurfaceHolder may be dealt with some other steps. But I have learned an
example about SurfaceView, but I didn't figout out any special steps to deal with
mSurfaceHolder, the example's code is as
following:

public
            class SurfaceViewTest extends Activity {

 String TAG =
            "SurfaceViewTest";
 FastRenderView surfaceView = null;


            @Override
 protected void onCreate(Bundle savedInstanceState)
            {

 super.onCreate(savedInstanceState);

            //setContentView(R.layout.activity_surface_view_test);
 surfaceView = new
            FastRenderView(this);
 setContentView(surfaceView);

            }

 @Override
 protected void onResume() {

            super.onResume();
 surfaceView.doResume();


            }

 @Override
 public boolean onCreateOptionsMenu(Menu
            menu) {
 // Inflate the menu; this adds items to the action bar if it is
            
 // present.

            getMenuInflater().inflate(R.menu.menu_surface_view_test, 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();

 //noinspection
            SimplifiableIfStatement
 if (id == R.id.action_settings) {
 return
            true;

 }

 return
            super.onOptionsItemSelected(item);
 }

 /**
 *
            customized surfaceview class
 */
 public class FastRenderView
            extends SurfaceView implements Runnable {


 boolean
            running = false;
 SurfaceHolder holder = null;
 Thread thread =
            null;

 public FastRenderView(Context context) {

            super(context);
 // holder
 // getHolder() returns a SurfaceHolder
            implementation, it
 // isn't null, but it contains nothing.
 holder
            = getHolder();

 if (holder == null)
 Log.e(TAG, "failed
            to get valid holder");

 }

 public void
            doResume() {
 thread = new Thread(this);
 thread.start();

            running = true;
 }


 public Random rand = new
            Random();
 @Override
 public void run() {
 while (running)
            {
 // from android sdk 22, SurfaceView.java, we can see, this
 //
            holder's getSurface() returns null.
 // but why? it returns null, so here it
            an exception 
 // should be thrown out!
 if
            (!holder.getSurface().isValid()) {

 continue;

            }

 Canvas canvas = holder.lockCanvas();
 if (canvas ==
            null) {
 Log.e(TAG, "get an invalid canvas to draw");

            continue;
 }
 canvas.drawRGB(
 rand.nextInt(255),
            

 rand.nextInt(255), 
 rand.nextInt(255));

            holder.unlockCanvasAndPost(canvas);

 // sleep
 try
            {
 Thread.sleep(1000);
 }
 catch(Exception e) {

            e.printStackTrace();

 }
 }
 Log.i(TAG, "running
            ...");
 }

 public void doPause() {


            running = false;
 while (true) {
 try {


            thread.join();
 } catch (Exception e) {

            e.printStackTrace();
 }
 }
 }

            }
}

Thanks
in advance!