You should be fairly comfortable with Android Studio by now, so you can expect fewer videos going forward.

Here are the Views in my content_stopwatch.xml:

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:textSize="92sp"
        android:text=""
        android:id="@+id/time_view"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />
 
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/start"
        android:id="@+id/start_button"
        android:layout_below="@+id/time_view"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="20dp"
        android:onClick="onClickStart" />
 
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/stop"
        android:id="@+id/stop_button"
        android:layout_below="@+id/start_button"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"
        android:onClick="onClickStop" />
 
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/reset"
        android:id="@+id/reset_button"
        android:layout_below="@+id/stop_button"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"
        android:onClick="onClickReset" />

Extra code to add to StopWatchActivity.java (i.e., don't replace the entire file).

Member variables:

// Number of seconds displayed on the stopwatch
private int seconds = 0;
// Is the stopwatch running?
private boolean running;

Methods:

//Start the stopwatch running when the Start button is clicked.
public void onClickStart(View view) {
    running = true;
}
 
//Stop the stopwatch running when the Stop button is clicked.
    public void onClickStop(View view) {
running = false;
}
 
//Reset the stopwatch when the Reset button is clicked.
public void onClickReset(View view) {
    running = false;
    seconds = 0;
}

Here is some boilerplate repeating Handler code:

final Handler handler = new Handler();  // Make a new Handler
handler.post(new Runnable() {   // Give handler a Runnable instance to execute ASAP
    @Override                   // Redefine the Runnables's run method
    public void run() {
        /* Stuff you
         * want to do
         * goes here. */
        // Repost the Runnable (this) to run some time in the future.
        handler.postDelayed(this, MILLISECONDS_UNTIL_handler.post_IS_CALLED_AGAIN);
    }
});

To StopwatchActivity.java add the following to the end of onCreate()

runTimer();

and the runTimer() method:

private void runTimer() {
    final TextView timeView = (TextView)findViewById(R.id.time_view);
    final Handler handler = new Handler();
    handler.post(new Runnable() {
        @Override
        public void run() {
            int hours = seconds/3600;
            int minutes = (seconds%3600)/60;
            int secs = seconds%60;
            String time = String.format("%d:%02d:%02d",
                    hours, minutes, secs);
            timeView.setText(time);
            if (running) {
                seconds++;
            }
            handler.postDelayed(this, 1000);
        }
    });
}

Be sure you import android.os.Handler and android.widget.TextView.

To rotate a stock AVD, use the Ctrl+F11 shortcut to switch to the previous layout orientation and Ctrl+F12 to switch to next layout orientation.

onSaveInstanceState method to be added to StopwatchActivity:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putInt("seconds", seconds);
    savedInstanceState.putBoolean("running", running);
}

Code to be added to onCreate() just before runTimer();:

if (savedInstanceState != null) {
    seconds = savedInstanceState.getInt("seconds");
    running = savedInstanceState.getBoolean("running");
}

Additional member variable:

private boolean wasRunning;

Restore additional item in onCreate's if (savedInstanceState != null) block:

wasRunning = savedInstanceState.getBoolean("wasRunning");

Add additional item to onSaveInstanceState:

savedInstanceState.putBoolean("wasRunning", wasRunning);

New methods:

@Override
protected void onStop() {
    super.onStop();
    wasRunning = running;
    running = false;
}

<code java> @Override protected void onStart() {

  super.onStart();
  if (wasRunning) {
      running = true;
  }

} <code>