Google Android - это несложно

Код: Выделить всё

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    if (savedInstanceState == null) {
        // Add the fragment on initial activity setup
        mainFragment = new MainFragment();
        getSupportFragmentManager()
        .beginTransaction()
        .add(android.R.id.content, mainFragment)
        .commit();
    } else {
        // Or set the fragment from restored state info
        mainFragment = (MainFragment) getSupportFragmentManager()
        .findFragmentById(android.R.id.content);
    }
}