expListAdapter = new MyListAdapter(this, createGroupList(), createGroupList());
setListAdapter( expListAdapter );

private List createGroupList() {
      ArrayList result = new ArrayList();
      for (int i=0; i < Liste.AnzahlElemente(); i++) {    //
        result.add(Liste.getSize(i).getGroupString());
      }
      return result;
    }

    private List createChildList() {
        ArrayList result = new ArrayList();
        for(int i=0; i < Liste.getSize(); i++) {
            ArrayList secList = new ArrayList();
            for( int n = 1 ; n <= 3 ; n++ ) {
                if (Liste.getElement(i).getChildString(n).length() != 0){
                    secList.add( "- " + Liste.getElement(i).getChildString(n));
                }
            }
            result.add( secList );
        }
        return result;
}

Collections.sort(Liste.getListe(), new NameComparator());

expListAdapter.notifyDataSetInvalidated();

//expListAdapter.notifyDataSetChanged(); //same effect as notifyDataSetInvalided

public class NameComparator implements Comparator<ToDoElement>{

     public int compare(ToDoElement item1, ToDoElement item2) {
           return item1.getGroupString().compareTo(item2.getGroupString());
     }
}

public class MyListAdapter extends BaseExpandableListAdapter{

     private ArrayList<String> ueberschriften;
     private ArrayList<ArrayList<String>> stichpunkte;

     public MyListAdapter(Context context, List _ueberschriften, List _stichpunkte) {

          this.ueberschriften = (ArrayList)_ueberschriften;
          this.stichpunkte = (ArrayList)_stichpunkte;

          inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

     // [...] some other methods

     public void update(List _GroupStrings, List _ChildStrings) {
          GroupStrings = (ArrayList<String>) _GroupStrings;
          ChildStrings = (ArrayList<ArrayList<String>>) _ChildStrings;
     }
}

Collections.sort(Liste.getListe(), new NameComparator());

expListAdapter.update(createGroupList(), createChildList());

expListAdapter.notifyDataSetChanged();