Errata

After completing the chapter's 11 workout app, there was a small mistake when i ran the app on tablet. Here's what happened:
I choose one of the four option and then start running the stopwatch . Then i choose another option and start its stopwatch. Then i press the back button. The first stopwatch now counts 2 seconds per second instead of 1. If i repeat the same steps, it will count 3 seconds per second and so on... I guess the runnables in stopwatchfragment keep counting.
The problem can be fixed by rotating the screen. But that's not a solution. Using a flag in the stopwatchfragment was a simple solution for me. The app was running as it should after that.
Can you please take a look at this problem?
Below are the changes i did in the stopwatchfragment code :

public class StopwatchFragment extends Fragment implements View.OnClickListener{
public int flag ; //created flag that will handle the runnable < ---
...

public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View layout = inflater.inflate(R.layout.fragment_stopwatch, container, false);

flag = 1; //set flag = run and then started the runTimer < ---
runTimer(layout);
.....
}

private void runTimer(View view){
......
......
if(running){
seconds++;
}

if(flag == 1) // flag = 1 means that the handler.postdelayed has the right to continue < ---
handler.postDelayed(this,1000); //delay 1000ms (1 second)
}
});


//override onDestroyView() so the specific runnable will not count anymore. This means that the
//previous stopwatch will count normally again (after using the back button).
@Override
public void onDestroyView() {
super.onDestroyView();
flag = 0; //making flag=0 the specific handler.postDelayed will not run anymore
}