Back Button (activity) Return To Fragment

I have a class that extends FragmentActivity

That accesses fragments with the following:

@Override
public Fragment getItem(int arg0) {
    // TODO Auto-generated method stub

    Fragment fragment=null;
    if (arg0 == 0) {
        fragment = new Fragment_01_Rooms();
    }
    if (arg0 == 1) {
        fragment = new Fragment_02_Contacts();
    }

    return fragment;
}

Now the Fragment called is public class Fragment_02_Contacts extends Fragment

Which call another class via intent with the following:

Intent intent = new Intent(getActivity(), ActivityChatPrivate.class);
intent.putExtra("valueCONTACT", contactSelected);
startActivity(intent);

Now this class called has public class ActivityChatPrivate extends FragmentActivity

On this class I am trying to return back to Fragment_02_Contacts via the back button. HOW DO I DO THAT WITH

public void onBackPressed(){
    //some_code_here to return back to Fragment_02_Contacts 
}