Unable to update JComboBox UI using a class method

Question

I am facing a problem with populating list values in a JComboBox. I've read the integer values from a file by using a class method(which is not subscribed to the ActionEvent), and successfully modified the combo box list contents. However I'm unable to see the contents of the UI get updated .

Why is it so? However when I use the same code within an action listener of a button, I can the see the combo box UI get updated with the modified list values.

How can I update a combo box UI using a simple method that reads a file and modifies the combo box list values.

public class NewJFrame extends javax.swing.JFrame{

public NewJFrame() { //Constructor to initialize the Form
        initComponents();
        }

private void initComponents() {
ButNext = new javax.swing.JButton();
QuestionList = new javax.swing.JComboBox(); 
….
…. //lines of code

QuestionList.setMaximumRowCount(10);

org.jdesktop.beansbinding.Binding binding =      org.jdesktop.beansbinding.Bindings.createAutoBinding(org.jdesktop.beansbinding.AutoBinding.UpdateStrategy.READ_WRITE, QuestionList, org.jdesktop.beansbinding.ELProperty.create("${selectedItem}"), QuestionList, org.jdesktop.beansbinding.BeanProperty.create("selectedItem"));
    bindingGroup.addBinding(binding);

QuestionList.addItemListener(new java.awt.event.ItemListener() {
public void itemStateChanged(java.awt.event.ItemEvent evt) {
            QuestionListItemStateChanged(evt);
        }
    });
QuestionList.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
            QuestionListActionPerformed(evt);
        }
    });

…
…
bindingGroup.bind();

pack();
setLocationRelativeTo(null);

} //end of initComponents



private void ButNextActionPerformed(java.awt.event.ActionEvent evt) {      // If I I click Next button on the UI then the JComboBox is populated with the vales read from the file
        ….    
        …. // lines of code                              

        QuesListContent.add(qnumber);
        QuestionList.addItem(qnumber); 
 …. // lines of code   
}


private  int defineExamstate() throws FileNotFoundException, IOException{   // If I I call this function, then on the UI  the JComboBox is NOT populated with the vales read from the file,                       
                                                                                                                      //however the values of the list is printed correctly when checked in submain() function
String answersPath=NewJFrame.SavedanswerPath; //file to be read

Scanner input1 = new Scanner(new File(answersPath));


    while(input1.hasNextLine())
    {

    String message = input1.nextLine(); //User answers

        switch (message) {
            case "START":

                startcounter+=1;
                lastquestion=0;

                break;
            case "END":

                QuesListContent.clear();
                QuestionList.removeAllItems();
                maxquestion=0;
                endcounter+=1;

                break;
            default:
                record=message.split("Q");
                lastquestion=Integer.parseInt(record[0]);
                if(!QuesListContent.contains(lastquestion)){     
                QuesListContent.add(lastquestion);
                QuestionList.addItem(lastquestion);

                }

                break;


        }
        }
}

 protected  void submain() throws FileNotFoundException, IOException, Throwable {   //Function
 …
 …. //lines of code

 int size = QuestionList.getItemCount();
 for(int i=0;i<size;i++) {
                Object element = QuestionList.getItemAt(i);
                System.out.println("Element at " + i + " = " + element);  // Able to print the content of the combo box
            }
}


// Variables declaration - do not modify                     
  private javax.swing.JButton ButNext;
  private javax.swing.JComboBox QuestionList;

} // end of class definition