如何在按下返回键时完美处理底部导航栏。

我正在设计一个底部导航栏，但是我的底部导航栏显示得并不完美。

我的MainActivity类：

public class MainActivity extends AppCompatActivity {
    private static final String SELECTED_ITEM = "selected_item";
    private BottomNavigationView bottomNavigationView;
    private Toolbar toolbar;
    private MenuItem menuItemSelected;
    private int mMenuItemSelected;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        bottomNavigationView = (BottomNavigationView) findViewById(R.id.bottom_navigation);
        
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                selectFragment(item);
                return true;
            }
        });
        
        // 默认加载第一个Fragment
        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.frameLayout, new AnnouncementFragment());
        fragmentTransaction.commit();
        
        if (savedInstanceState != null) {
            mMenuItemSelected = savedInstanceState.getInt(SELECTED_ITEM, 0);
            menuItemSelected = bottomNavigationView.getMenu().findItem(mMenuItemSelected);
        } else {
            menuItemSelected = bottomNavigationView.getMenu().getItem(0);
        }
        
        selectFragment(menuItemSelected);
    }
    
    private void selectFragment(MenuItem item) {
        Fragment fragment = null;
        Class fragmentClass;
        
        switch (item.getItemId()) {
            case R.id.action_announcement:
                fragmentClass = AnnouncementFragment.class;
                break;
            case R.id.action_menu:
                fragmentClass = MenuFragment.class;
                break;
            case R.id.action_menu_reports:
                fragmentClass = ReportFragment.class;
                break;
            case R.id.action_setting:
                fragmentClass = SettingFragment.class;
                break;
            default:
                fragmentClass = AnnouncementFragment.class;
        }
        
        try {
            fragment = (Fragment) fragmentClass.newInstance();
        } catch (Exception e) {
            e.printStackTrace();
        }
        
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction().replace(R.id.frameLayout, fragment).commit();
    }
    
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        outState.putInt(SELECTED_ITEM, mMenuItemSelected);
        super.onSaveInstanceState(outState);
    }
    
    @Override
    public void onBackPressed() {
        MenuItem homeItem = bottomNavigationView.getMenu().getItem(0);
        if (mMenuItemSelected != homeItem.getItemId()) {
            selectFragment(homeItem);
        } else {
            super.onBackPressed();
        }
    }
}

我的返回按钮也没有正常工作：

@Override
public void onBackPressed() {
    MenuItem homeItem = bottomNavigationView.getMenu().getItem(0);
    if (mMenuItemSelected != homeItem.getItemId()) {
        selectFragment(homeItem);
    } else {
        super.onBackPressed();
    }
}

我该如何解决这个问题呢？因为底部菜单在栏上的分布不均匀。如何正确维护菜单空间以避免不均匀分布。

这里我附上我在AVD上获得的结果：