Now that the action has been added to the navigation graph, the next step is to add some code within the main fragment to trigger the action when the Button widget is clicked. Locate the MainFragment.java file, load it into the code editor and modify the onActivityCreated() method to obtain a reference to the button instance and to configure an onClickListener instance to be called when the user clicks the button:
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import android.widget.Button;
import androidx.navigation.Navigation;
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public class MainFragment extends Fragment {
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@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
mViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
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