我更喜欢总是使用内部类为特定按钮逐个添加函数。我修改了密码。希望你会发现它有帮助…..虽然我没有更多的声誉，但我不能在评论中提出任何问题。很抱歉。

import javax.swing.*;
    import java.awt.*;
    import java.awt.event.*;
    import java.util.Scanner;

    public class BalanceCalculator extends JFrame implements ActionListener
    {
    public static final int NUMBER_OF_DIGITS = 30;
    private JTextField monthlyPayments;
    private JTextField principalValue;
    private JTextField annualInterestRate;
    private JTextArea output;

    public static void main (String[] args)
    {
        BalanceCalculator aCalculator = new BalanceCalculator();
        aCalculator.setVisible(true);
    }

    public BalanceCalculator()
    {
        super("Mortgage Calculator");
        JFrame frame = new JFrame();
        Container pane = frame.getContentPane();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(700, 500);
        pane.setLayout (new GridLayout (13, 2, 2, 2));

        JPanel textPanel1 = new JPanel();
        textPanel1.setLayout(new FlowLayout(FlowLayout.LEFT));

        JLabel label1 = new JLabel("Enter the number of payments: ");
        textPanel1.add(label1);
        pane.add(textPanel1);

        monthlyPayments = new JTextField(NUMBER_OF_DIGITS);
        textPanel1.add(monthlyPayments);
        pane.add(textPanel1);

        JPanel jRadiobuttonPanel = new JPanel();
        jRadiobuttonPanel.setLayout(new FlowLayout(FlowLayout.CENTER));

        final JRadioButton monthlyButton = new JRadioButton ("Pay by monthly");
        monthlyButton.addActionListener(this);
        jRadiobuttonPanel.add(monthlyButton);
        pane.add(jRadiobuttonPanel);

        final JRadioButton biweeklyButton = new JRadioButton ("Pay by bi-weekly");
        biweeklyButton.addActionListener(this);
        jRadiobuttonPanel.add(biweeklyButton);
        pane.add(jRadiobuttonPanel);

        final JRadioButton weeklyButton = new JRadioButton ("Pay by weekly");
        weeklyButton.addActionListener(this);
        jRadiobuttonPanel.add(weeklyButton);
        pane.add(jRadiobuttonPanel);

        ButtonGroup group = new ButtonGroup();
        group.add(monthlyButton); 
        group.add(biweeklyButton);
        group.add(weeklyButton);
        monthlyButton.setSelected (true);

        JPanel textPanel2 = new JPanel();
        textPanel2.setLayout(new FlowLayout(FlowLayout.LEFT));

        JLabel label2 = new JLabel("Enter the principal: ");
        textPanel2.add(label2);
        pane.add(textPanel2);

        principalValue = new JTextField(NUMBER_OF_DIGITS);
        textPanel2.add(principalValue);
        pane.add(textPanel2);

        JPanel textPanel3 = new JPanel();
        textPanel3.setLayout(new FlowLayout(FlowLayout.LEFT));

        JLabel label3 = new JLabel("Enter the annual interest rate: ");
        textPanel3.add(label3);
        pane.add(textPanel3);

        annualInterestRate = new JTextField(NUMBER_OF_DIGITS);
        textPanel3.add(annualInterestRate);
        pane.add(textPanel3);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new FlowLayout(FlowLayout.CENTER));

        JButton calculateButton = new JButton ("Calculate");
        calculateButton.addActionListener(new ActionListener(){
                public void actionPerformed(ActionEvent evt){
                    if (monthlyButton.isSelected())
            {

                output.setText("Monthly Button");
            }

            else if (weeklyButton.isSelected())
            {
                output.setText("Weekly Button");
            }

            else if (biweeklyButton.isSelected())
            {
                output.setText("Bi weeklybutton");
            }

            else if (monthlyButton.isSelected())
            {
                monthlyPayments.setText("");
                principalValue.setText("");
                annualInterestRate.setText("");
                output.setText("");
            }

            else
            {
                //enterYourNumber.setText("Unexpected error.");
            }
                }

        });
        buttonPanel.add(calculateButton);
        pane.add(buttonPanel);

        JLabel outputs = new JLabel("Data for your mortgage: ");
        pane.add(outputs);

        JPanel results = new JPanel();
        output = new JTextArea(200, 300);
        output.setLineWrap(true); 
        output.setEditable(false); 
        output.setVisible(true);
        results.add(output);
        pane.add(results);

        JButton resetButton = new JButton ("Reset");
        resetButton.addActionListener(new ActionListener(){
                public void actionPerformed(ActionEvent evt){
                    monthlyPayments.setText(null);  
                    principalValue.setText(null);
                    annualInterestRate.setText(null);
                }

        });
        buttonPanel.add(resetButton);
        pane.add(buttonPanel);

        add(pane, BorderLayout.CENTER);

    }

    public void actionPerformed (ActionEvent e)
    {
        try
        {
            assumingCorrectNumberFormats(e);
        }
        catch (NumberFormatException e2)
        {
            monthlyPayments.setText("Error: Re-enter number please.");
        }
    }

    public void assumingCorrectNumberFormats (ActionEvent e)
    {
        monthlyPayments.addActionListener (new ActionListener ()
        {
            public void actionPerformed (ActionEvent e) {
                System.out.println("You entered: " + e.getActionCommand ());
            }
        }
                );

        String actionCommand = e.getActionCommand();
        while (actionCommand.equals("Calculate"))
        {

        }
    }
    }

assumingcorrectnumberformats（）方法中的逻辑不正确，您使用while循环检查按钮单击，在while循环中检查单选按钮单击事件。您需要分别检查单选按钮和按钮。例如：

public void assumingCorrectNumberFormats(ActionEvent e) {

        String actionCommand = e.getActionCommand();
        if (actionCommand.equals("Calculate")) {
            System.out.println("inside calculate");
            String strPrincipal = principalValue.getText();
            String strAnnualInterest = annualInterestRate.getText();
            // do calculation
            Integer result = Integer.parseInt(strPrincipal) * Integer.parseInt(strAnnualInterest);
            System.out.println("result=" + result);
            // set the result
            output.setText(String.valueOf(result));
        }
        if (actionCommand.equals("Pay by monthly")) {
            output.setText(toString());
        }
        else if (actionCommand.equals("Pay by bi-weekly")) {
            output.setText(toString());
        }
        else if (actionCommand.equals("Pay by weekly")) {
            output.setText(toString());
        }
        else if (actionCommand.equals("Reset")) {
            monthlyPayments.setText("");
            principalValue.setText("");
            annualInterestRate.setText("");
            output.setText("");
        }
    }

您可以为新单选按钮添加单独的条件。同时减少jtextarea中的行数，否则相应地更改layoutmanager。

output = new JTextArea(200, 60);