按下向上按钮不会返回到主片段

我有一个活动Toolbar和FrameLayout- 片段容器。片段之间的导航是使用兼容性库 (v7) 中的导航抽屉完成的。

工具栏上有一个汉堡包按钮，单击它会弹出一个 NavigationView - 选择一个片段 - 它会替换主片段 ( Fragment0)。汉堡包按钮替换为向上按钮（向左箭头）。我希望当我们点击它时，我们返回到原始片段，并且当没有汉堡包按钮时侧边栏不会在滑动时爬出。但实际上，Up-button 只改变了它的外观，而没有改变行为——当你点击它时，会弹出一个侧边栏。

这是我的活动代码：

public class MainActivity extends AppCompatActivity {

private String[] mNumberTitles;
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private Toolbar toolbar;
private ActionBarDrawerToggle mDrawerToggle;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    FragmentManager fragmentManager = getSupportFragmentManager();
    Fragment fragment = fragmentManager.findFragmentById(R.id.fragment_container);
    if (fragment == null) {
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction
                .add( R.id.fragment_container, new Fragment0())
                .commit();
    }


    mNumberTitles = getResources().getStringArray(R.array.numbers_array);
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.left_drawer);

    // Set the adapter for the list view
    mDrawerList.setAdapter(new ArrayAdapter<String>(this,
            R.layout.drawer_list_item, mNumberTitles));
    // Set the list's click listener
    mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

    initToolbar();


    mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
            toolbar, R.string.drawer_open, R.string.drawer_close);

    // Set the drawer toggle as the DrawerListener
    mDrawerLayout.addDrawerListener(mDrawerToggle);
    mDrawerToggle.syncState();


    // заменяем гамбургер на Up button во второстепенных фрагментах
    fragmentManager.addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
        @Override
        public void onBackStackChanged() {
            if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
                getSupportActionBar().setDisplayHomeAsUpEnabled(true);


            } else {
                getSupportActionBar().setDisplayHomeAsUpEnabled(false);
                mDrawerToggle.syncState();
            }
        }
    });

}

private void initToolbar() {
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

}

/** Swaps fragments in the main content view */
private void selectItem(int position) {

    Fragment fragment = null;
    switch (position) {
        case 0 : fragment = new Fragment0();
            break;
        case 1 : fragment = new Fragment1();
            break;
        case 2 : fragment = new Fragment2();
            break;
        default: fragment = new Fragment0();
            break;
    }

    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.fragment_container, fragment)
            .addToBackStack(null)
            .commit();

    // Highlight the selected item, update the title, and close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mNumberTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

private void setTitle(String title) {
    if (toolbar != null)
        toolbar.setTitle(title);
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {

    switch (item.getItemId()) {
        case android.R.id.home: // it's for up button
            Log.d("!!!!!!!!!", "HOMe button is pressed");
            getSupportFragmentManager().popBackStack();
            return true;
        default:
            break;
    }
    return super.onOptionsItemSelected(item);
}


private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView parent, View view, int position, long id) {
        selectItem(position);
    }
}

}

按下向上按钮（日志为空）时，我们不会进入方法onOptionsItemSelected(MenuItem item)( )。case android.R.id.home:

我究竟做错了什么？