Android : swap between grid and list fragments

The main section of my screen layout shows a set of items. The use can choose if a grid or a list should be shown. Therefore I implemented two fragements (for list and grid) that are then shown inside the placeholder (main section of the main layout).

But now I wonder how to reuse the fragements in order to have the last scrolling position etc. If I remember the fragment in a private field instead of re-creating it, then the view remains empty.

Any idea?

public void onShowGrid(View view) {

    // how to re-use instead of re-create
    fragmentGrid = new PreviewGridFragment();

    this.getFragmentManager().beginTransaction()
            .replace(R.id.preview_fragment, fragmentGrid)
            .addToBackStack(null).commit();
}

public void onShowList(View view) {

    // how to re-use instead of re-create
    fragmentList = new PreviewListFragment();

    this.getFragmentManager().beginTransaction()
            .replace(R.id.preview_fragment, fragmentList)
            .addToBackStack(null).commit();
}