具有CheckBox的ListView，“全部选中”复选框的功能与此相反

我正在尝试使用带有全选按钮的复选框来实现一个列表视图。但是，当我选中所有复选框时，相反的情况发生，所有复选框都被取消选中，而当我取消选中所有复选框时，则选中了复选框。这是我的代码：

chkall = (CheckBox) findViewById(R.id.chkAll);
        //chkall.setChecked(false);
        /** Defining array adapter to store items for the listview **/
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_multiple_choice, names);

        /** Setting the arrayadapter for this listview  **/
        getListView().setAdapter(adapter);                

        /** Defining checkbox click event listener **/
        OnClickListener clickListener = new OnClickListener() {         
            @Override
            public void onClick(View v) {
                CheckBox chk = (CheckBox) v;
                int itemCount = getListView().getCount();
                for(int i=0 ; i < itemCount ; i++){
                    getListView().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) {

                int checkedItemCount = getCheckedItemCount();                               

                if(getListView().getCount()==checkedItemCount)
                    chkall.setChecked(false);
                else
                    chkall.setChecked(true);


            //      String attID =scoresDataBaseAdapter.getAttID(perd,mRowId);
                    //scoresDataBaseAdapter.insertAttScore(perd,mRowId,attID,Sname,idstud,score);
                //  Toast.makeText(getApplicationContext(), " "+arg0+" "+arg1+" "+arg2+" "+arg3, Toast.LENGTH_SHORT).show();



            }
        };      

        /** 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 **/
        getListView().setOnItemClickListener(itemClickListener);

    }

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

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

        return cnt;
    }