Android checkbox select all is not working

when click on check all button all checkboxes of listview should be checked

public class MainActivity extends Activity {

private ListView mToDoListView;
private SqlHandler mSqlHandler;
private CustomListAdapter mToDoListAdapter;
private ArrayList<ToDoItems> mToDoList;
private Button mDeleteButton;
private CheckBox mCheckBox;`enter code here`

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_to_do);
    mSqlHandler = new SqlHandler(this);
    mToDoListView = (ListView) findViewById(R.id.list_items);
    mDeleteButton = (Button) findViewById(R.id.delete_button);
    showList();
    mToDoListAdapter = new CustomListAdapter(MainActivity.this, mToDoList);
    mToDoListView.setAdapter(mToDoListAdapter);

    /** Defining checkbox click event listener **/
    OnClickListener clickListener = new OnClickListener() {
        @Override
        public void onClick(View v) {
            CheckBox chk = (CheckBox) v;
            int itemCount = mToDoListView.getCount();
            for (int i = 0; i < itemCount; i++) {
                mToDoListView.setItemChecked(i, chk.isChecked());
            }
        }
    };
    /** Defining click event listener for the listitem checkbox */
    OnItemClickListener itemClickListener = new OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
                long arg3) {
            CheckBox chk = (CheckBox) findViewById(R.id.chkAll);
            int checkedItemCount = getCheckedItemCount();

            if (mToDoListView.getCount() == checkedItemCount)
                chk.setChecked(true);
            else
                chk.setChecked(false);
        }
    };
    /** Getting reference to checkbox available in the main.xml layout */
    CheckBox chkAll = (CheckBox) findViewById(R.id.chkAll);

    /** Setting a click listener for the checkbox **/
    chkAll.setOnClickListener(clickListener);

    /** Setting a click listener for the listitem checkbox **/
    mToDoListView.setOnItemClickListener(itemClickListener);

}

/**
 *
 * Returns the number of checked items
 */
private int getCheckedItemCount() {
    int cnt = 0;
    SparseBooleanArray positions = mToDoListView.getCheckedItemPositions();
    int itemCount = mToDoListView.getCount();

    for (int i = 0; i < itemCount; i++) {
        if (positions.get(i))
            cnt++;
    }
    return cnt;
}

public void showList() {
    mToDoList = new ArrayList<ToDoItems>();
    mToDoList.clear();
    Cursor c1 = mSqlHandler.selectQuery();
    if (c1 != null && c1.getCount() != 0) {
        if (c1.moveToFirst()) {
            do {
                ToDoItems toDoListItems = new ToDoItems();

                toDoListItems.setmTaskDetails(c1.getString(c1
                        .getColumnIndex("taskDetails")));

                toDoListItems.setmDate(c1.getString(c1
                        .getColumnIndex("date")));
                mToDoList.add(toDoListItems);

            } while (c1.moveToNext());
        }
    }
    c1.close();

}

custom adapter need any changes or not public class CustomListAdapter extends BaseAdapter {

private Context mContext;
private ArrayList<ToDoItems> mToDoList;

public CustomListAdapter(Context context, ArrayList<ToDoItems> list) {

    this.mContext = context;
    this.mToDoList = list;
}

@Override
public int getCount() {

    return mToDoList.size();
}

@Override
public Object getItem(int position) {

    return mToDoList.get(position);
}

@Override
public long getItemId(int position) {

    return position;
}

private static class ViewHolder {
    TextView taskAddedDate;
    TextView taskName;
    CheckBox checkBox;
}

@Override
public View getView(final int position, View convertView, ViewGroup parent) {
    ToDoItems toDoListItems = mToDoList.get(position);
    ViewHolder viewHolder = null;
    if (convertView == null) {
        viewHolder = new ViewHolder();

        LayoutInflater inflater = (LayoutInflater) mContext
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflater.inflate(R.layout.list_row, null);
        viewHolder = new ViewHolder();

        viewHolder.taskName = (TextView) convertView
                .findViewById(R.id.todo_task);
        viewHolder.taskAddedDate = (TextView) convertView
                .findViewById(R.id.todo_date);
        viewHolder.checkBox = (CheckBox) convertView
                .findViewById(R.id.item_checkBox);

        convertView.setTag(viewHolder);

    } else {
        viewHolder = (ViewHolder) convertView.getTag();
    }
    viewHolder.taskName.setText(toDoListItems.getmTaskDetails());
    viewHolder.taskAddedDate.setText(toDoListItems.getmDate());

    return convertView;
}

}