Please help with GUI

[Javabeginner Xie]

Hi, everyone
I write the following code to display questions I read from a file
I have meeted a trouble here, which is: the program displays all the questions at a single time, I use listeners to keep track of the user inputs, but it seems that the program has just skip the listener parts. Can you tell me what has been wrong here, Please?
Thank you very much!
The code is as follow

public void sampleTest ()
        {
            JFrame mainWindow = new JFrame("This is a test");
            mainWindow.setSize(800, 600);
        
            // Centering the window
            Dimension frameSize = mainWindow.getSize();
            Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        
            mainWindow.setLocation((screenSize.width - frameSize.width)/2, (screenSize.height - frameSize.height)/2);
        
            // Add a test name
            Container contentPane = mainWindow.getContentPane();
            JLabel testTitle = new JLabel("Sample Test", JLabel.CENTER);
            contentPane.add(testTitle, BorderLayout.NORTH);
            
            JPanel centerPanel = new JPanel();
            centerPanel.setLayout(new GridLayout(questions.size(), 1));
            
            JPanel southPanel = new JPanel();
            southPanel.setLayout(new GridLayout(3,1));
            
            // Read questions from the Question arrayList
            for (i = 0; i < questions.size(); i++)
            {
                
    
                Scanner sc = new Scanner (questions.get(i).getQuestion());
               
                if (questions.get(i).getQuestionType().equals("TF"))
                {
                    // Create a new Panel to hold a true false question, and then add this panel to the centerPanel later
                    JPanel trueFalsePanel = new JPanel();
                    trueFalsePanel.setLayout(new GridLayout(4, 1));
                    
                    String questionText = sc.nextLine();
                    // fill the text fields
                    field1 = new JTextField ("" + (i+1) + ". (" + questions.get(i).getPointsValue() + " points)");
 
                    field1.setEditable(false);
                    field1.setEnabled(true);
                    trueFalsePanel.add(field1);
                    
                    field2 = new JTextField ("" + questionText);
                    field2.setEditable(false);
                    field2.setEnabled(true);
                    trueFalsePanel.add(field2);
                    
                    field3 = new JTextField ("Your answer");
                    field3.setEditable(true);
                    field3.setEnabled(true);
                    trueFalsePanel.add(field3);
                    
                    submit = new JButton("Submit Answer");
                    trueFalsePanel.add(submit);
                    
                    
                    
                    centerPanel.add(trueFalsePanel); // Add the trueFalsePanel to the centerpanel, and then later add the centerPanel to the center of the JFrame
                    ActionListener listener1 = new ActionListener(){
                        public void actionPerformed(ActionEvent e){
                            userAnswer = field3.getText();
                            if (userAnswer.equals(""))
                            {
                                userAnswer = null;
                            }
                        }
                    };
                    field3.addActionListener(listener1);
                    
                    ActionListener answerListener = new ActionListener(){
                    public void actionPerformed(ActionEvent e){
                    answer(i, userAnswer);
                    }
                    };
                    submit.addActionListener(answerListener);
                }
                
                if (questions.get(i).getQuestionType().equals("MC"))
                {
                    // Create a new Panel to hold a multiple choice question, and then add this panel to the centerPanel later
                    JPanel multipleChoicePanel = new JPanel();
                    multipleChoicePanel.setLayout(new GridLayout(8,1));
                    
                    String questionText = sc.nextLine();
                    String answerChoiceA = sc.nextLine();
                    String answerChoiceB = sc.nextLine();
                    String answerChoiceC = sc.nextLine();
                    String answerChoiceD = sc.nextLine();
                    
                    field4 = new JTextField ("" + (i+1) + ". (" + questions.get(i).getPointsValue() + " points)");
                    field4.setEditable(false);
                    field4.setEnabled(true);
                    multipleChoicePanel.add(field4);
                    
                    field5 = new JTextField ("" + questionText);
                    field5.setEditable(false);
                    field5.setEnabled(true);
                    multipleChoicePanel.add(field5);
                    
                    field6 = new JTextField ("" + answerChoiceA);
                    field6.setEditable(false);
                    field6.setEnabled(true);
                    multipleChoicePanel.add(field6);
                    
                    field7 = new JTextField ("" + answerChoiceB);
                    field7.setEditable(false);
                    field7.setEnabled(true);
                    multipleChoicePanel.add(field7);
                    
                    field8 = new JTextField ("" + answerChoiceC);
                    field8.setEditable(false);
                    field8.setEnabled(true);
                    multipleChoicePanel.add(field8);
                    
                    field9 = new JTextField ("" + answerChoiceD);
                    field9.setEditable(false);
                    field9.setEnabled(true);
                    multipleChoicePanel.add(field9);
                    
                    field10 = new JTextField ("Your answer");
                    field10.setEditable(true);
                    field10.setEnabled(true);
                    multipleChoicePanel.add(field10);
                    
                    submit = new JButton("Submit Answer");
                    multipleChoicePanel.add(submit);
                    
                    
                    ActionListener listener2 = new ActionListener(){
                        public void actionPerformed(ActionEvent e){
                            userAnswer = field10.getText();
                            if (userAnswer.equals(""))
                            {
                                userAnswer = null;
                            }
                        }
                    };
                    field10.addActionListener(listener2);
                    
                    ActionListener answerListener = new ActionListener(){
                    public void actionPerformed(ActionEvent e){
                    answer(i, userAnswer);
                    }
                    };
                    submit.addActionListener(answerListener);
                    
                    centerPanel.add(multipleChoicePanel);
                }
                
            }
            contentPane.add(centerPanel, BorderLayout.CENTER);
            contentPane.add(new JScrollPane(centerPanel), BorderLayout.CENTER);
            
            scoreExam(true);
            int scoreEarned = getScore();
            scoreEarnedField = new JTextField("Your final score is " + scoreEarned + " points");
            southPanel.add(scoreEarnedField);
            
            contentPane.add(southPanel, BorderLayout.SOUTH);
            mainWindow.setVisible(true);
        }

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