how to navigate tabs using a button in bottom bar flutter code example

Example 1: flutter tab bar bottom bar

int _selectedIndex = 0;
static const TextStyle optionStyle = TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
static const List<Widget> _widgetOptions = <Widget>[
  Text(
    'Index 0: Home',
    style: optionStyle,
  ),
  Text(
     'Index 1: Business',
     style: optionStyle,
  ),
  Text(
     'Index 2: School',
     style: optionStyle,
  ),
];

void _onItemTapped(int index) {
  setState(() {
    _selectedIndex = index;
  });
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: const Text('BottomNavigationBar Sample'),
    ),
    body: Center(
      child: _widgetOptions.elementAt(_selectedIndex),
    ),
    bottomNavigationBar: BottomNavigationBar(
      items: const <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.business),
          label: 'Business',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.school),
          label: 'School',
        ),
      ],
      currentIndex: _selectedIndex,
      selectedItemColor: Colors.amber[800],
      onTap: _onItemTapped,
    ),
  );
}

Example 2: flutter tab bar bottom bar

int _selectedIndex = 0;
static const TextStyle optionStyle = TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
static const List<Widget> _widgetOptions = <Widget>[
  Text(
    'Index 0: Home',
    style: optionStyle,
  ),
  Text(
    'Index 1: Business',
    style: optionStyle,
  ),
  Text(
    'Index 2: School',
    style: optionStyle,
  ),
  Text(
    'Index 3: Settings',
    style: optionStyle,
  ),
];

void _onItemTapped(int index) {
  setState(() {
    _selectedIndex = index;
  });
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: const Text('BottomNavigationBar Sample'),
    ),
    body: Center(
      child: _widgetOptions.elementAt(_selectedIndex),
    ),
    bottomNavigationBar: BottomNavigationBar(
      items: const <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
          backgroundColor: Colors.red,
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.business),
          label: 'Business',
          backgroundColor: Colors.green,
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.school),
          label: 'School',
          backgroundColor: Colors.purple,
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.settings),
          label: 'Settings',
          backgroundColor: Colors.pink,
        ),
      ],
      currentIndex: _selectedIndex,
      selectedItemColor: Colors.amber[800],
      onTap: _onItemTapped,
    ),
  );
}