MainActivity.java

package com.example.myapplication;


import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentTransaction;

import android.os.Bundle;
import android.util.Log;
import android.view.MenuItem;

import com.google.android.material.bottomnavigation.BottomNavigationView;
import com.google.android.material.navigation.NavigationBarView;

public class MainActivity extends AppCompatActivity {

    BottomNavigationView bottomNavigationView;

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

        bottomNavigationView = findViewById(R.id.bottomNavigationView);
        openFragment(new HomeFragment());

        bottomNavigationView.setOnItemSelectedListener(new NavigationBarView.OnItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.profile:
                        openFragment(new ProfileFragment());
                        return true;
                    case R.id.home:
                        openFragment(new HomeFragment());
                        return true;
                    case R.id.about:
                        openFragment(new AboutFragment());
                        return true;
                }
                return false;
            }
        });


    }

    private void openFragment(Fragment fragment) {
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        //this is a helper class that replaces the container with the fragment. You can replace or add fragments.
        transaction.replace(R.id.fragmentView, fragment);
        transaction.addToBackStack(null); //if you add fragments it will be added to the backStack. If you replace the fragment it will add only the last fragment
        transaction.commit(); // commit() performs the action
    }

}