почему я не могу зайти на страницу входа?

Question

Я хочу иметь возможность войти на страницу входа из созданного мною навигационного ящика. Но, тем не менее, после нажатия на кнопку входа я не могу попасть на страницу входа. Я искал, как другие это делали, и пытался реализовать то же самое, но почему я не смог попасть на свою страницу входа из ящика навигации? Я предполагаю, что ошибка здесь будет l ie с моей MainActivity. Вот оно,

MainActivity

package sg.edu.rp.c346.app4thnewspd;

import androidx.annotation.NonNull;
import androidx.annotation.RequiresApi;
import androidx.appcompat.app.ActionBarDrawerToggle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.app.AppCompatDelegate;
import androidx.appcompat.widget.SwitchCompat;
import androidx.core.view.GravityCompat;
import androidx.core.view.MenuItemCompat;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import androidx.swiperefreshlayout.widget.SwipeRefreshLayout;
import androidx.appcompat.widget.Toolbar;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Build;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.CompoundButton;
import android.widget.EditText;
import android.widget.Switch;
import android.widget.Toast;

import com.google.android.material.navigation.NavigationView;
import com.google.android.material.snackbar.Snackbar;

import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import sg.edu.rp.c346.app4thnewspd.Model.Articles;
import sg.edu.rp.c346.app4thnewspd.Model.Headlines;

import static sg.edu.rp.c346.app4thnewspd.R.id.fragmentContainer;

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    RecyclerView recyclerView;
    SwipeRefreshLayout swipeRefreshLayout;
    EditText etQuery;
    Button btnSearch;
    final String API_KEY = "5353a8d609b4415ab6f449f31d46926a";
    Adapter adapter;
    List<Articles>articles = new ArrayList<>();
    private String query;
    Toolbar toolbar;
    DrawerLayout drawer;
    ActionBarDrawerToggle toggle;
    private  Switch btnSwitch;
    private NavigationView nv;
    Fragment fragment;
    FragmentManager fragmentManager;
    FragmentTransaction fragmentTransaction;



    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        if (AppCompatDelegate.getDefaultNightMode()==AppCompatDelegate.MODE_NIGHT_YES){
            setTheme(R.style.DarkTheme);
        }else{
            setTheme(R.style.LightTheme);
        }
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        drawer = findViewById(R.id.drawer);

        toggle = new ActionBarDrawerToggle(this,drawer,toolbar,R.string.open,R.string.close);
        drawer.addDrawerListener(toggle);
        toggle.setDrawerIndicatorEnabled(true);
        toggle.syncState();


        nv = (NavigationView)findViewById(R.id.navi_view);
        nv.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {


                int id = item.getItemId();
               /* switch(id)
                {

                }*/


                return true;

            }
        });



        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().replace(fragmentContainer,
                    new SignInFragment()).commit();
            nv.setCheckedItem(R.id.signIn);


        }

        Menu menu = nv.getMenu();
        MenuItem menuItem = menu.findItem(R.id.Mode);
        View actionView = MenuItemCompat.getActionView(menuItem);



        btnSwitch = (Switch) actionView.findViewById(R.id.btnSwitch);
        if(AppCompatDelegate.getDefaultNightMode() == AppCompatDelegate.MODE_NIGHT_YES){
            btnSwitch.setChecked(true);
        }


        btnSwitch.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                if (isChecked)
                {
                    AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);
                    restartActivity();
                }
                else
                {
                    AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
                    restartActivity();
                }
            }
        });



        swipeRefreshLayout = findViewById(R.id.swipeRefresh);
        recyclerView = findViewById(R.id.recyclerView);

        etQuery = findViewById(R.id.etQuery);
        btnSearch = findViewById(R.id.btnSearch);



        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        final String country = getCountry();

        swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                retrieveJson("",country,API_KEY);
            }
        });

        retrieveJson("",country, API_KEY);



        btnSearch.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!etQuery.getText().toString().equals("")){
                    swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
                        @Override
                        public void onRefresh() {
                            retrieveJson(etQuery.getText().toString(),country,API_KEY);
                        }
                    });
                    retrieveJson(etQuery.getText().toString(),country,API_KEY);
                }else{
                    swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
                        @Override
                        public void onRefresh() {
                            retrieveJson("",country,API_KEY);
                        }
                    });
                    retrieveJson("",country,API_KEY);
                }
            }
        });
    }
    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        switch (menuItem.getItemId()){
            case R.id.signIn:
                getSupportFragmentManager().beginTransaction().replace(fragmentContainer,
                        new SignInFragment()).commit();
                break;
        }
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
    @Override
    public void onBackPressed() {
        if(drawer.isDrawerOpen(GravityCompat.START))
            drawer.closeDrawer(GravityCompat.START);
        else
            super.onBackPressed();
    }



    private void restartActivity() {
        Intent i = new Intent(getApplicationContext(),MainActivity.class);
        startActivity(i);
        finish();
    }


    private String getCountry() {
        Locale locale = Locale.getDefault();
        String country = locale.getCountry();
        return country.toLowerCase();
    }




    public void retrieveJson(final String query, final String country, final String apiKey){


        swipeRefreshLayout.setRefreshing(true);
        Call<Headlines>call;
        if(!etQuery.getText().toString().equals("")){
            call = ApiClient.getInstance().getApi().getSpecificData(query,apiKey); 
        }else{
            call = ApiClient.getInstance().getApi().getHeadLines(country, apiKey);
        }
        call.enqueue(new Callback<Headlines>() {
            @Override
            public void onResponse(Call<Headlines> call, Response<Headlines> response) {
                if (response.isSuccessful() && response.body().getArticles() != null){
                    swipeRefreshLayout.setRefreshing(false);
                    articles.clear();
                    articles = response.body().getArticles();
                    adapter = new Adapter(MainActivity.this,articles);
                    recyclerView.setAdapter(adapter);

                }
            }

            @Override
            public void onFailure(Call<Headlines> call, Throwable t) {
                swipeRefreshLayout.setRefreshing(false);
                Toast.makeText(MainActivity.this, t.getLocalizedMessage(), Toast.LENGTH_SHORT).show();
            }
        });
    }


}