How to move down the selector in list view when click of volume down button in android

How to move focus of list item to down item when we clicked on volume down button of android mobile.Tried this code but unable to down the focus.any help on this or any other alternative approaches to down the selector. Thanks in advance.

    super.onCreate(savedInstanceState);
    // 2. create array adapter
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, // standard row layout
                                                    // provided by android
            listItemArray);
    // 3. Call setListAdapter
    setListAdapter(adapter);
    listView = getListView();
    listView.setOnKeyListener(new OnKeyListener() {

        @Override
        public boolean onKey(View v, int keyCode, KeyEvent event) {
            // TODO Auto-generated method stub

            switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_DOWN:

                ListView list =(ListView)v;
                for (int j=0; j < list.getChildCount(); j++){
                    if(list.getChildAt(j).getBackground().equals(R.color.gray)){
                    list.getChildAt(j).setBackgroundResource(R.color.TRANSPARENT);
                    list.getChildAt(j++).setBackgroundResource(R.color.gray);
                    }else
                        list.getChildAt(0).setBackgroundResource(R.color.gray);
                }
                                    break;
            }           

            return false;
        }
    });