Switch from Activity to the recent fragment visited in BottomNavigationView - floating-action-button

Question

I am trying to make a social media app , Im using fragments within BottomNavigation (home - smart room - notifications - profile), And Im using a floating button to switch to "Add post" activity , Now Im having a problem with the back button , i want it to take me to the recent visited fragment but it always take me to the home fragment
here is my BottomNavigation activity code :
public class BottomNavigationActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_bottom_navigation);
BottomNavigationView bottomNav = findViewById(R.id.bottom_navigation_layout);
//setEnabled(false) for the 2nd index because we're using a floating button instead
bottomNav.setBackground(null);
bottomNav.getMenu().getItem(2).setEnabled(false);
bottomNav.setOnItemSelectedListener(navListener);
//start AddPostActivity class
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
startActivity(new Intent(getApplicationContext(), AddPostActivity.class));
finish();
}
});
}
//Switch between fragments
public BottomNavigationView.OnItemSelectedListener navListener =
new BottomNavigationView.OnItemSelectedListener() {
#Override
public boolean onNavigationItemSelected(#NonNull MenuItem item) {
Fragment selectedFragment = null;
switch (item.getItemId()) {
case R.id.nav_home:
selectedFragment = new HomeFragment();
break;
case R.id.nav_sr:
selectedFragment = new SmartRoomFragment();
break;
case R.id.nav_notif:
selectedFragment = new NotificationsFragment();
break;
case R.id.nav_profile:
selectedFragment = new ProfileFragment();
break;
}
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,
selectedFragment).commit();
return true;
}
};
}
I want the best solution to handle this problem , thanks in advance!