ListView with CheckBox, Check all checkbox does the opposite

I am trying to implement a listview with checkbox with a check all button. However, when I check the check all checkbox the opposite happens, all checkboxes gets unchecked and when I uncheck the check all checkbox, the boxes gets checked. Here is my code:

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;
    }