MainActivity.java

package ca.uwaterloo.cs349;

import android.os.Bundle;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;
import com.google.android.material.bottomnavigation.BottomNavigationView;
import androidx.appcompat.app.AppCompatActivity;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;

public class MainActivity extends AppCompatActivity {

    AppBarConfiguration appBarConfiguration;

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        return NavigationUI.navigateUp(navController, appBarConfiguration) || super.onSupportNavigateUp();
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        BottomNavigationView navView = findViewById(R.id.nav_view);
        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
                R.id.navigation_home, R.id.navigation_library, R.id.navigation_addition)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(navView, navController);

        SharedViewModel mViewModel = new ViewModelProvider(this).get(SharedViewModel.class);
        mViewModel.load(getBaseContext());
    }

    @Override
    protected void onStop() {
        super.onStop();
        SharedViewModel mViewModel = new ViewModelProvider(this).get(SharedViewModel.class);
        mViewModel.save(getBaseContext());
    }

}