从弹出窗口的listview中选择已检查的项目，并在android

Question

我是Android BaseAdapter的新手，当从弹出窗口中获取选中的项目时，我面临一个空指针异常。 我想要做的事情如下 - 我想在弹出窗口中选中已检查列表视图中的选中项目，并在弹出窗口消失后在该活动屏幕上显示这些选定项目。

这是我的代码 - Dropdownlist.java

 private void initialize(){
        //data source for drop-down list
      //  final ArrayList<String> itemlist = new ArrayList<String>();

        checkSelected = new boolean[itemlist.size()];
        //initialize all values of list to 'unselected' initially
        for (int i = 0; i < checkSelected.length; i++) {
            checkSelected[i] = false;
        }
    enter code here

        final TextView tv = (TextView) findViewById(R.DropDownList.SelectBox);
        tv.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                 int len = list.getCount();
                SparseBooleanArray checked = list.getCheckedItemPositions();
                 str=Integer.toString(checked.size());
                 for (int i = 0; i < len; i++) {
                    str=Integer.toString(len);
                    // Item position in adapter
                    if (checked.get(i)) {
                         item += itemlist.get(i);
                        //item += adapter.getItem(i).toString();
                          selectedItems.add(item);
                          /*do whatever you want with the checked item */
                         }
                     //int position = checked.keyAt(i);


            selectedItems.add(adapter.getItem(position).toString());
            }

            outputStrArr = new String[selectedItems.size()];
                for (int i = 0; i < selectedItems.size(); i++) {


        outputStrArr[i] = selectedItems.get(i);
            }
     Toast.makeText(getApplicationContext(), str,             Toast.LENGTH_LONG).show();
            Intent intent = new Intent(getApplicationContext(),


                ResultActivity.class);

                // Create a bundle object
                Bundle b = new Bundle();
                b.putStringArray("selectedItems", outputStrArr);

                // Add the bundle to the intent.
                intent.putExtras(b);

                // start the ResultActivity
                startActivity(intent);

                }
        });

        //onClickListener to initiate the dropDown list
        Button createButton = (Button)findViewById(R.DropDownList.create);
        createButton.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                initiatePopUp(itemlist,tv);
            }
        });
    }
  private void initiatePopUp(ArrayList<String> itemlist, TextView tv){
        LayoutInflater inflater = (LayoutInflater)DropDownListDemo.this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        //get the pop-up window i.e.  drop-down layout
        LinearLayout layout = (LinearLayout)inflater.inflate(R.layout.pop_up_window, (ViewGroup)findViewById(R.id.PopUpView));

        //get the view to which drop-down layout is to be anchored
        RelativeLayout layout1 = (RelativeLayout)findViewById(R.id.relativeLayout1);
        pw = new PopupWindow(layout, LayoutParams.FILL_PARENT, LayoutParams.WRAP_CONTENT, true);

        //Pop-up window background cannot be null if we want the pop-up to listen touch events outside its window
        pw.setBackgroundDrawable(new BitmapDrawable());
        pw.setTouchable(true);

        //let pop-up be informed about touch events outside its window. This  should be done before setting the content of pop-up
        pw.setOutsideTouchable(true);
        pw.setHeight(LayoutParams.WRAP_CONTENT);

        //dismiss the pop-up i.e. drop-down when touched anywhere outside the pop-up
        pw.setTouchInterceptor(new OnTouchListener() {

            public boolean onTouch(View v, MotionEvent event) {
                // TODO Auto-generated method stub
                if (event.getAction() == MotionEvent.ACTION_OUTSIDE) {
                    pw.dismiss();
                    return true;                    
                }
                return false;
            }
        });

        //provide the source layout for drop-down
        pw.setContentView(layout);

        //anchor the drop-down to bottom-left corner of 'layout1'
        pw.showAsDropDown(layout1);

        //populate the drop-down list
        list = (ListView) layout.findViewById(R.DropDownList.dropDownList);
        adapter = new DropDownListAdapter(this, itemlist, tv);
        list.setAdapter(adapter);
        list.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
    }

这是DropDownListAdapter类扩展Base适配器

    @Override
public View getView(int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub

    if (convertView == null) {
        convertView = mInflater.inflate(R.layout.drop_down_list_row, null);
        holder = new ViewHolder();
        holder.tv = (TextView) convertView.findViewById(R.DropDownList.SelectOption);
        holder.chkbox = (CheckBox) convertView.findViewById(R.DropDownList.checkbox);
        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
    }

    holder.tv.setText(mListItems.get(position));

    final int position1 = position;

    //whenever the checkbox is clicked the selected values textview is updated with new selected values
    holder.chkbox.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            setText(position1);
        }
    });

    if(DropDownListDemo.checkSelected[position])
        holder.chkbox.setChecked(true);
    else
        holder.chkbox.setChecked(false);     
    return convertView;
}