Value not added in arraylist on checkbox click in listview.

I have a DisplayStudentName.java file which is my main activity file. There is a model class which is used to get and set data and a MyCustomAdapter.java file which extends ArrayAdapter for listview functioning.

The checkbox is clicked but value is not added in the arraylist(named data; type String).

1. DisplayStudentName.java

      public class DisplayStudentNames extends AppCompatActivity {
      String myJSON;
      private static final String TAG_RESULTS = "result";
      private static final String TAG_ROLL = "RollNo";
      private static final String TAG_NAME = "Name";
      JSONArray peoples = null;
      ListView list;
      ArrayList<String> checkedValue;
      Button b1;
      MyCustomAdapter dataAdapter = null;
      ArrayList<Student> personList=null;
      @Override
      protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_display_student_names);
      final Spinner sbranch = (Spinner) findViewById(R.id.branch);
      final Spinner ssemester = (Spinner) findViewById(R.id.semester);
      final Spinner ssubject = (Spinner) findViewById(R.id.edit_subject);
      //String branch=sbranch.getSelectedItem().toString();
      //String semester=ssemester.getSelectedItem().toString();
      //String subject=ssubject.getSelectedItem().toString();
      String branch = "cs";
      String semester = "7";
      String subject = "Soft Computing";
      b1 = (Button) findViewById(R.id.submit);
      list = (ListView) findViewById(R.id.listView);
   
      new DataFetch().execute(branch, semester, subject);
      list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
       @Override
       public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
   
           Student student = (Student) parent.getItemAtPosition(position);
           String Name = student.getName();
           //String s=(String) ((TextView) view.findViewById(R.id.roll)).getText();
           Toast.makeText(getApplicationContext(), "Clicked: " + Name, Toast.LENGTH_SHORT).show();
   
       }
   });
   //submitAttendance();
   }
   
   class DataFetch extends AsyncTask<String, String, String> {
   ArrayList<NameValuePair> params;
   private ProgressDialog pDialog;
   private String url_fetch_data = "http://192.168.1.4/AttendanceApp/fetch_data.php";
   InputStream is = null;
   String res = "";
   
   protected void onPreExecute() {
       super.onPreExecute();
   }
   
   protected String doInBackground(String... args) {
       String branch = args[0];
   
       String semester = args[1];
       String subject1 = args[2];
   
       try {
           DefaultHttpClient httpClient = new DefaultHttpClient();
           HttpPost httpPost = new HttpPost(url_fetch_data);
           params = new ArrayList<NameValuePair>(3);
           params.add(new BasicNameValuePair("branch", branch));
           params.add(new BasicNameValuePair("semester", semester));
           params.add(new BasicNameValuePair("subject", subject1));
           httpPost.setEntity(new UrlEncodedFormEntity(params));
           HttpResponse httpResponse = httpClient.execute(httpPost);
           HttpEntity httpEntity = httpResponse.getEntity();
           is = httpEntity.getContent();
           BufferedReader br = new BufferedReader(new InputStreamReader(is, "utf-8"), 8);
   
           StringBuilder total = new StringBuilder();
           String line;
           while ((line = br.readLine()) != null) {
               total.append(line);
           }
           String result = total.toString();
           return result;
       } catch (Exception e1) {
           res = "error:" + e1.getMessage().toString();
           e1.printStackTrace();
       }
       return res;
   }
   
   protected void onPostExecute(String httpResponse) {
       myJSON = httpResponse;
       showList();
   }
   }
   
   protected void showList() {
   try {
       JSONObject jsonObj = new JSONObject(myJSON);
       peoples = jsonObj.getJSONArray("result");
   
       personList = new ArrayList<Student>();
   
       for (int i = 0; i < peoples.length(); i++) {
           Student student = new Student();
           JSONObject c = peoples.getJSONObject(i);
           String roll = c.getString(TAG_ROLL);
           String name = c.getString(TAG_NAME);
           student.setName(name);
           student.setRollNo(roll);
           personList.add(student);
       }
   
       dataAdapter = new MyCustomAdapter(this, R.layout.list_item, personList);
       list.setAdapter(dataAdapter);
   
   
   
   } catch (JSONException e) {
       e.printStackTrace();
   }
   }
   

2. MyCustomAdapter.java

   public class MyCustomAdapter extends ArrayAdapter {
   public ArrayList<Student> personList=null;
   private Context context=null;
   private static LayoutInflater inflater=null;
   private ArrayList<String> data=null;
   private View vi;
   ViewHolder holder=null;
   
   public MyCustomAdapter(Context context, int resource, ArrayList<Student> personList) {
   super(context, resource, personList);
   this.context=context;
   this.personList=personList;
   }
   
   private class ViewHolder{
   TextView roll;
   TextView name;
   CheckBox check;
   }
   
   
   public View getView(int position, View convertView, ViewGroup parent){
   Log.v("ConvertView", String.valueOf(position));
   
   if(convertView==null){
       LayoutInflater vi = (LayoutInflater)context.getSystemService(
               Context.LAYOUT_INFLATER_SERVICE);
       convertView = vi.inflate(R.layout.list_item, null);
   
       holder=new ViewHolder();
       holder.roll=(TextView)convertView.findViewById(R.id.roll);
       holder.name=(TextView)convertView.findViewById(R.id.name);
       holder.check=(CheckBox)convertView.findViewById(R.id.checkBox);
       convertView.setTag(holder);
   
   }
   else{
       holder=(ViewHolder)convertView.getTag();
   }
   
   
   Student student=personList.get(position);
   holder.roll.setText(student.getRollno());
   holder.name.setText(student.getName());
   holder.check.setTag(student);
   
   holder.check.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
       @Override
       public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
           if (isChecked) {
               data.add(holder.check.getText().toString());
           } else {
               data.remove(holder.check.getText().toString());
           }
       }
   });
   
   return convertView;
   
   
   }
   

}

3. Student.java -Model class

   public class Student {
   String rollno;
   String name;
   Boolean checkbox;
   
   public  Student(){
   
   }
   public Student(String rollno, String name, Boolean status){
   super();
   this.rollno=rollno;
   this.name=name;
   this.checkbox=status;
   }
   
   public String getRollno(){
   return rollno;
   }
   
   public void setRollNo(String rollno){
   this.rollno=rollno;
   }
   
   public String getName(){
   return name;
   }
   
   public void setName(String name){
   this.name=name;
   }
   
   public Boolean isCheckbox(){
   return checkbox;
   }
   
   public void setCheckbox(boolean checkbox) {
   this.checkbox= checkbox;
   }
   

Please tell me what is wrong with my code? }