我有问题在导航抽屉中显示两个片段

Question

在android studio v3中设置默认导航抽屉并添加可扩展列表视图，但是在导航中选择子显示拖曳片段后，只需要显示一个片段，我不知道该怎么办，如果您有任何想法，请提出一些答案。

以下是显示该问题的屏幕截图：

public class mainpage extends AppCompatActivity {
    private AppBarConfiguration mAppBarConfiguration;
    private DrawerLayout mDrawerLayout;
    private ActionBarDrawerToggle mDrawerToggle;
    private String mActivityTitle;
    private String[] items;
    private ExpandableListView mExpandableListView;
    private ExpandableListAdapter mExpandableListAdapter;
    private List<String> mExpandableListTitle;
    private NavigationManager mNavigationManager;
    private Map<String, List<String>> mExpandableListData;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mainpage);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();

            }
        });

        mDrawerLayout = findViewById(R.id.drawer_layout);
        mActivityTitle = getTitle().toString();
        mExpandableListView = findViewById(R.id.expandableListView);
        mNavigationManager = FragmentNavigationManager.obtain(this);
        initItems();

        mExpandableListData = ExpandableListDataSource.getData(this);
        mExpandableListTitle = new ArrayList(mExpandableListData.keySet());
        addDrawerItems();
        setupDrawer();

        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);

        // Passing each menu ID as a set of Ids because each
        // menu should be considered as top level destinations.
        mAppBarConfiguration = new AppBarConfiguration.Builder(
                R.id.nav_account, R.id.nav_car, R.id.nav_Procedure,
                R.id.nav_parts, R.id.nav_oil, R.id.nav_Spare_parts, R.id.nav_reports, R.id.nav_task,
                R.id.nav_action, R.id.expandableListView)
                .setDrawerLayout(drawer)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.

        getMenuInflater().inflate(R.menu.mainpage, menu);
        return true;
    }

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


    private void initItems() {
        items = getResources().getStringArray(R.array.film_genre);
    }

    private void addDrawerItems() {
        mExpandableListAdapter = new CustomExpandableListAdapter(this, mExpandableListTitle, mExpandableListData);
        mExpandableListView.setAdapter(mExpandableListAdapter);
        mExpandableListView.setOnGroupExpandListener(new ExpandableListView.OnGroupExpandListener() {
            @Override
            public void onGroupExpand(int groupPosition) {
                getSupportActionBar().setTitle(mExpandableListTitle.get(groupPosition).toString());
            }
        });

        mExpandableListView.setOnGroupCollapseListener(new ExpandableListView.OnGroupCollapseListener() {
            @Override
            public void onGroupCollapse(int groupPosition) {
                getSupportActionBar().setTitle(R.string.film_genres);
            }
        });

        mExpandableListView.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {
            @Override
            public boolean onChildClick(ExpandableListView parent, View v,
                                        int groupPosition, int childPosition, long id) {
                String selectedItem = ((List) (mExpandableListData.get(mExpandableListTitle.get(groupPosition))))
                        .get(childPosition).toString();
                getSupportActionBar().setTitle(selectedItem);

                if (childPosition == 0) {
                    mNavigationManager.showFragmentAction(selectedItem);
                }
                else if (childPosition == 1) {
                    mNavigationManager.showViewReports(selectedItem);
                }
                else {
                    throw new IllegalArgumentException("Not supported fragment type");
                }

                mDrawerLayout.closeDrawer(GravityCompat.START);
                return false;
            }
        });
    }


    private void setupDrawer() {
        mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, R.string.drawer_open, R.string.drawer_close)



        {


            /** Called when a drawer has settled in a completely open state. */
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                getSupportActionBar().setTitle(R.string.film_genres);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }

            /** Called when a drawer has settled in a completely closed state. */
            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
                getSupportActionBar().setTitle(mActivityTitle);
                invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu()
            }
        };

        mDrawerToggle.setDrawerIndicatorEnabled(true);
        mDrawerLayout.setDrawerListener(mDrawerToggle);

    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        mDrawerToggle.onConfigurationChanged(newConfig);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        // Activate the navigation drawer toggle
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

这是来自FragmentNavigationManager的代码：

public class FragmentNavigationManager implements NavigationManager {

    private static FragmentNavigationManager sInstance;

    private FragmentManager mFragmentManager;
    private mainpage mActivity;

    public static FragmentNavigationManager obtain(mainpage activity) {
        if (sInstance == null) {
            sInstance = new FragmentNavigationManager();
        }
        sInstance.configure(activity);
        return sInstance;
    }


    private void configure(mainpage activity) {
        mActivity = activity;
        mFragmentManager = mActivity.getSupportFragmentManager();
    }

    @Override
    public void showFragmentAction(String title) {
        showFragment(FragmentAction.newInstance(title), false);
    }

    @Override
    public void showViewReports(String title) {
        showFragment(ViewReports.newInstance(title), false);
    }


    private void showFragment(Fragment fragment, boolean allowStateLoss) {


        FragmentManager fm = mFragmentManager;

        @SuppressLint("CommitTransaction")

        FragmentTransaction ft = fm.beginTransaction()
                .replace(R.id.container, fragment);


        ft.addToBackStack(null);

        if (allowStateLoss || !BuildConfig.DEBUG)
        {
            ft.commitAllowingStateLoss();
        }
        else {
            ft.commit();
        }

        fm.executePendingTransactions();



    }
}