How to use fragments in Android bottom navigation view

public class MainActivity extends AppCompatActivity {

    private Fragment fragment;
    private FragmentManager fragmentManager;


    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = = new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.navigation_recentcall:

                    break;
                case R.id.navigation_contact:

                    break;
                case R.id.navigation_keypad:
                    fragment = new KeypadFragment();
                    break;
            }
            final FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.replace(R.id.content, fragment).commit();
            return true;
        }

    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        fragmentManager = getSupportFragmentManager();
        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

}

public class KeypadFragment extends Fragment {
    String input_num="";
    Button btnCall,btnAdd,btnErase;
    Button[] numbutons =new Button[12];
    TextView number;

    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
    }
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle saveInstanceState){
        View view=inflater.inflate(R.layout.fragment_keypad,null);
        btnCall = (Button) findViewById(R.id.button_call);
        btnAdd = (Button) findViewById(R.id.button_add);
        btnErase = (Button) findViewById(R.id.button_erase);
        number = (TextView) findViewById(R.id.textView_input_number);
        return view;
    }
}    

btnCall = (Button) findViewById(R.id.button_call);