Dynamic Arraylist of checkboxes in Java Swing

I'm writing a GUI Java program for student registration which will retrieve available classes from database, give the user an option to choose classes and then store this in DB.

What I'm trying to do and have so far achieved partial success, is this - I created a combobox with the available majors (got that from DB), retrieved the available classes for that major and displayed checkboxes for these classes.

There are two issues with this code. 1. After selecting the major from combobox, the checkboxes aren't displayed. They appear one by one only when I hover my cursor. 2.Once I change my major in the combobox, the checkboxes are not updated, even though the console in eclipse says checkboxes for newly selected major has been created.

ArrayList<JCheckBox> checkBoxes=new ArrayList<JCheckBox>();
    //combox action listener below
    public void actionPerformed(ActionEvent e) 
    {

      //get all available classess for the selected major
        avail_class = new String[count_class];


        //get all available class ids
        avail_classid = new String[count_class];

        JCheckBox checkbox;
        int xdim = 75;
        for (int i = 0; i < count_class; i++) 
            {
                checkbox = new JCheckBox(avail_classid[i] + "-" + avail_class[i]);
                checkbox.setBackground(new Color(0, 255, 255));
                checkbox.setBounds(183, xdim, 289, 23);
                contentPane.add(checkbox);
                checkBoxes.add(checkbox);
                checkbox.setEnabled(true);

                xdim = xdim + 50;

            }


    }