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!