to implement a bottom navigation bar with different screens/pages in Flutter, you can create a structure where each bottom navigation item corresponds to a different screen.

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: BaseBottomNavBar(),
    );
  }
}

class BaseBottomNavBar extends StatefulWidget {
  @override
  _BaseBottomNavBarState createState() => _BaseBottomNavBarState();
}

class _BaseBottomNavBarState extends State<BaseBottomNavBar> {
  int _currentIndex = 0;

  final List<Widget> _screens = [
    ScreenOne(),
    ScreenTwo(),
    ScreenThree(),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Bottom Navigation Bar Example'),
      ),
      body: _screens[_currentIndex],
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: 'Screen 1',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.favorite),
            label: 'Screen 2',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.person),
            label: 'Screen 3',
          ),
        ],
      ),
    );
  }
}

class ScreenOne extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.blue,
      child: Center(
        child: Text('Screen 1'),
      ),
    );
  }
}

class ScreenTwo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.red,
      child: Center(
        child: Text('Screen 2'),
      ),
    );
  }
}

class ScreenThree extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.green,
      child: Center(
        child: Text('Screen 3'),
      ),
    );
  }
}