getting error while storing values of arraylist into string array for expandablelistview in android

i am trying expandablelistview ,as given in api demo expandable list view example 1, use of string array , here i am facing the problem of converting the array list to string array, because all my values are saved in arraylist, the values are added into array list in asynctask, and the value of array list are retrieving in my adapter class which extends BaseExpandableListAdapter.

here is my code

1) array list decleration,

public final ArrayList<ListItemReminderSummary> listItemList = new ArrayList<ListItemReminderSummary>();
    public final ArrayList<ListItemReminderSummaryList> listItemListSummary = new ArrayList<ListItemReminderSummaryList>();

2)adding values to array list in doinbackground()

ListItemReminderSummary listItem = new ListItemReminderSummary();
            listItem.Car_Id = Vehicle_Id;
            listItem.Car_Type = Car_Type;
            listItemList.add(listItem);


            ListItemReminderSummaryList SummaryList = new ListItemReminderSummaryList();
            SummaryList.SummaryReminder_Id = Reminder_Id;
            SummaryList.SummaryReminder_Type = Reminder_Type;

            listItemListSummary.add(SummaryList);

 @Override
    protected void onPostExecute(String result)
    {

         mAdapter = new SlowAdapter(this);
         ExpandableListView.setAdapter(mAdapter);

        dialog.dismiss();
    }

here is my adapter class

public class SlowAdapter extends BaseExpandableListAdapter {
        private LayoutInflater mInflater;
        private LoadData mContext;


         private String[] groups=(String[]) listItemList.toArray(new String[listItemList.size()]);
         private String[][] children = (String[][]) listItemListSummary.toArray(new String[listItemListSummary.size()][]);


        public SlowAdapter(LoadData loadData) {
            mContext = loadData;
            mInflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        }

        @Override
        public Object getChild(int groupPosition, int childPosition) {
            // TODO Auto-generated method stub
             return children[groupPosition][childPosition];
        }

        @Override
        public long getChildId(int groupPosition, int childPosition) {
            // TODO Auto-generated method stub
             return childPosition;
        }

        public TextView getGenericView() {
            // Layout parameters for the ExpandableListView
            AbsListView.LayoutParams lp = new AbsListView.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT, 64);

            TextView textView = new TextView(ReminderSummary.this);
            textView.setLayoutParams(lp);
            // Center the text vertically
            textView.setGravity(Gravity.CENTER_VERTICAL | Gravity.LEFT);
            // Set the text starting position
            textView.setPadding(36, 0, 0, 0);
            return textView;
        }
        @Override
        public View getChildView(int groupPosition, int childPosition,
                boolean isLastChild, View convertView, ViewGroup parent) {
              TextView textView = getGenericView();
                textView.setText(getChild(groupPosition, childPosition).toString());
                return textView;
        }

        @Override
        public int getChildrenCount(int groupPosition) {
            // TODO Auto-generated method stub
             return children[groupPosition].length;
        }

        @Override
        public Object getGroup(int groupPosition) {
            // TODO Auto-generated method stub
             return groups[groupPosition];
        }

        @Override
        public int getGroupCount() {
            // TODO Auto-generated method stub
             return groups.length;
        }

        @Override
        public long getGroupId(int groupPosition) {
            // TODO Auto-generated method stub
             return groupPosition;
        }

        @Override
        public View getGroupView(int groupPosition, boolean isExpanded,
                View convertView, ViewGroup parent) {
             TextView textView = getGenericView();
                textView.setText(getGroup(groupPosition).toString());
                return textView;
        }

        @Override
        public boolean hasStableIds() {
            // TODO Auto-generated method stub
            return true;
        }

        @Override
        public boolean isChildSelectable(int groupPosition, int childPosition) {
            // TODO Auto-generated method stub
            return true;
        }

    }

logcat

java.lang.ArrayStoreException: source[0] of type com.example.expandablelist.ListItemReminderSummary cannot be stored in destination array of type java.lang.String[]

so how to convert mt arraylist to string array, or i have to change my code to work with expandable listview,

this arraylist work for me to display values in a listview i want to make this arraylist work of expandable listview as well,

so please help me ,to get this done thank you...