public Object[][] getData() {
        try {
            MCQ_DAO mcq_DAO = new MCQ_DAO();
            FIB_DAO fib_DAO = new FIB_DAO();
            Essay_DAO essay_DAO = new Essay_DAO();
            //get question
            //get question
            ArrayList<MCQ> mcqs = mcq_DAO.getList("question_ID not in (select question_ID from QuestionsByExamID where Exam_ID='" + exam_ID + "') ");
            ArrayList<Object[]> objectArraylist = new ArrayList<Object[]>();
            //store data into arraylist
            for (int i = 0; i < mcqs.size(); i++) {
                MCQ currentMcq = (MCQ) mcqs.get(i);
                Object[] col = new Object[4];
                col[0] = currentMcq.getQuestion_ID();
                col[1]= "MCQ";
                col[2] = currentMcq.getQuestion_Text();
                col[3] = false;
                objectArraylist.add(col);

            }

            ResultSet rs= fib_DAO.getList("questionID not in (select question_ID from QuestionsByExamID where Exam_ID='"+exam_ID+"') ");
            
            //store data into arraylist
            while (rs.next()) {
                Object[] col = new Object[4];
                col[0] = rs.getInt(1);
                col[1] = "FIB";
                col[2] = rs.getString(3);
                col[3] = false;
                objectArraylist.add(col);
                
            }
            
            rs= essay_DAO.getList("question_ID not in (select question_ID from QuestionsByExamID where Exam_ID='"+exam_ID+"') ");
            
            //store data into arraylist
            while (rs.next()) {
                Object[] col = new Object[4];
                col[0] = rs.getInt(1);
                col[1] = "Essay";
                col[2] = rs.getString(4);
                col[3] = false;
                objectArraylist.add(col);
                
            }
            //trun arraylist<object[]> to object[][]
            Object[][] datas = new Object[objectArraylist.size()][4];
            for (int i = 0; i < objectArraylist.size(); i++) {
                datas[i] = objectArraylist.get(i);
            }
            return datas;
        } catch (SQLException ex) {
            Logger.getLogger(ChooseQuestionsPanel.class.getName()).log(Level.SEVERE, null, ex);
        }
        return null;
    }