Android: how to load fragment into FrameLayout

First you have a mistake in your Fragment transaction line, according with your layout should be:

transaction.replace(R.id.contentContainer, newFragment); // not R.id.bottomBar

Second, you should use supportFragmentManager instead of fragmentManager to work with support fragments, so implement the following way:

final FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.contentContainer, newFragment);
transaction.addToBackStack(null);
transaction.commit();

in my case i solve it by using

androidx.fragment.app.FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

replace your

FragmentTransaction

            bottomNavigationView= findViewById(R.id.bottom_navigation);
           bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {



               @SuppressLint("NonConstantResourceId")
               @Override
               public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                   Fragment temp =null;
                   switch (item.getItemId()) {
                       case R.id.library:
                            temp = new LibraryFragment();

                           
                           break;
                       case R.id.search:
                           temp = new SearchFragment();
                           break;
                       case R.id.profile:
                           temp = new UserprofileFragment();
                           break;
                       default:
                   }
                  

               getSupportFragmentManager().beginTransaction().replace(R.id.fragment, temp).commit();


                        return true;
               }
           });


    getSupportFragmentManager().beginTransaction().replace(R.id.fragment,new SearchFragment()).commit();