title: How to set a default fragment shown in the main activity?
body: <p>So i've encountered a small problem today. I was making a bottom navigation view in my app, and after clicking buttons, it replaces the fragment on the screen (and it works perfectly!).
  But just after launching the app, and without clicking any button, there is no fragment on the screen.
  I've realized that the fragments are shown only after clicking a button, and I'd like to have a default fragment (kalkulatorFragment).
  I've been trying my best to somehow set it up, but no success...</p>
  <pre><code>public class Main extends AppCompatActivity {
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
          navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
      }
  
      private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
              = new BottomNavigationView.OnNavigationItemSelectedListener() {
  
      @Override
      public boolean onNavigationItemSelected(@NonNull MenuItem item) {
          FragmentManager fm = getFragmentManager();
          FragmentTransaction ft = fm.beginTransaction();
  
          kalkulatorFragment kalkulator_fragment = new kalkulatorFragment();
          wzoryFragment wzory_fragment = new wzoryFragment();
          definicjeFragment definicje_fragment = new definicjeFragment();
  
              switch (item.getItemId()) {
                  case R.id.kalkulator:
                      ft.replace(android.R.id.content, kalkulator_fragment);
                      ft.commit();
                      return true;
  
                  case R.id.wzory:
                      ft.replace(android.R.id.content, wzory_fragment);
                      ft.commit();
                      return true;
  
                  case R.id.definicje:
                      ft.replace(android.R.id.content, definicje_fragment);
                      ft.commit();
                      return true;
              }
              return false;
          }
     
  </code></pre>