java - JLabel の入力、計算、更新方法は?

Question

そのため、標準的な銀行口座残高が 500 のプログラムを作成しています。このプログラムは、ユーザーにお金を引き出すか預金するかを尋ね、引き出した金額または預金した金額を計算し、現在の残高を更新します。なぜ機能しないのですか?どうすれば修正できますか?

public class MyFrame extends JFrame {

    private JPanel panel;
    private JLabel wordsLabel;
    private JLabel balanceLabel;
    private JLabel choiceLabel;
    private JTextField transactionAmount;
    private JButton depositButton;
    private JButton withdrawButton;
    private double balance;

    public MyFrame() {
        final int FIELD_WIDTH = 10;
        balance = 500;
        panel = new JPanel();
        wordsLabel = new JLabel();
        balanceLabel = new JLabel();
        choiceLabel = new JLabel();
        transactionAmount = new JTextField(FIELD_WIDTH);
        JPanel buttonPanel = new JPanel();
        ButtonGroup myGroup = new ButtonGroup();
        //panel.setLayout(new BorderLayout());
        depositButton = new JButton("Deposit");
        withdrawButton = new JButton("Withdraw");
        transactionAmount.setText("0");
        wordsLabel.setText("Welcome to Wes Banco! Your current balance is: ");
        balanceLabel.setText("500");
        choiceLabel.setText("How much would you like to deposit/withdraw?");
        panel.add(wordsLabel);
        panel.add(balanceLabel);
        panel.add(choiceLabel);
        panel.add(transactionAmount);
        myGroup.add(depositButton);
        myGroup.add(withdrawButton);
        buttonPanel.add(depositButton);
        buttonPanel.add(withdrawButton);
        panel.add(depositButton);

        ButtonListener myListener = new ButtonListener();
        depositButton.addActionListener(myListener);
        withdrawButton.addActionListener(myListener);

        panel.add(buttonPanel);
        this.add(panel);
    }

    class ButtonListener implements ActionListener {

        public void actionPerformed(ActionEvent event) {
            double amount = Double.parseDouble(transactionAmount.getText());
            if (amount == 0) {
                JOptionPane.showMessageDialog(null, "Enter an amount");
            }
            if (depositButton.isSelected()) {
                balanceLabel.setText("" + 500 + amount);
                JOptionPane.showMessageDialog(null, 
                     "You have deposited: " + amount);
            }
            if (withdrawButton.isSelected()) {
            }
        }
    }
}