Java 尝试只允许在JTextField中输入数字，而将非数字设置为0

有麻烦的家伙，这是我的华氏摄氏度转换器类，我似乎无法让我的代码识别以下内容，以确保只有数字输入到文本字段中，如果没有，设置为0：

try {
            Double.parseDouble(fahr.getText());
        } catch (NumberFormatException e) {
            fahr.setText("0");
        }

我想这是因为它在我的课堂上的位置。请查看我的完整课程，我将其放在代码底部：

public class Temperature implements ActionListener  {

    private static JTextField fahr;
    private static JTextField cels;
    private JFrame frame;
    private JButton first;
    private JButton second;
    private double fahrVal;
    private Double fahrConv;
    private double celsVal;
    private double celsConv;
    private NumberFormat amountFormat;


    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Temperature window = new Temperature();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Temperature() {
        initialize();
        first.addActionListener((ActionListener) this);
        second.addActionListener((ActionListener) this);
    }

    private void initialize() { 
        amountFormat = NumberFormat.getNumberInstance();
        frame = new JFrame("raB eltiT");
        frame.setBounds(100, 50, 350, 350);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

        first = new JButton("...To Celsius");
        first.setBounds(130, 60, 100, 50);
        first.setBackground(Color.cyan);
        first.setActionCommand("click");
        first.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                JButton src = (JButton) ae.getSource();
                //System.out.println("Button labeled " + src.getText() + " was clicked");
            }
        });
        first.addMouseListener(new MouseAdapter() {
            public void mouseReleased(MouseEvent me) {
                 cels.setText(String.valueOf(fahrConv));
            }
        });


        frame.getContentPane().add(first);

        second = new JButton("...To Fahrenheit");
        second.setBounds(130, 230, 100, 50);
        second.setBackground(Color.pink);
        second.setActionCommand("instead");
        frame.getContentPane().add(second);
        second.addMouseListener(new MouseAdapter() {
            public void mouseReleased(MouseEvent me) {
                 fahr.setText(String.valueOf(celsConv));
            }
        });

        // Build the Fahrenheit side

        JLabel fLabel = new JLabel("Fahrenheit");
        fahr = new JTextField("32");
        fahr.setPreferredSize(new Dimension(50,50));
        fahr.setHorizontalAlignment(JTextField.CENTER);
        JPanel fPanel = new JPanel();
        fPanel.setBounds(40, 120,100,100);
        fPanel.add(fLabel);
        fPanel.add(fahr);
        frame.getContentPane().add(fPanel);


        JLabel cLabel = new JLabel("Celsius");
        cels = new JTextField("0");
        cels.setPreferredSize(new Dimension(50,50));
        cels.setHorizontalAlignment(JTextField.CENTER);
        JPanel cPanel = new JPanel();
        cPanel.setBounds(220, 120, 100,100);
        cPanel.add(cLabel);
        cPanel.add(cels);
        frame.getContentPane().add(cPanel); 


        // Put it all up on the screen

        frame.setVisible(true);  

        try {// if is number
            Double.parseDouble(fahr.getText());
        } catch (NumberFormatException e) {
            fahr.setText("0");
        }
    }
    public void actionPerformed(ActionEvent event) {
         String fahrText = fahr.getText();
         fahrVal = Double.parseDouble(fahrText);
         fahrConv = ((fahrVal - 32)*5)/9;
         fahrConv = Double.parseDouble(new DecimalFormat("##.#").format(fahrConv));

        String celsText = cels.getText();
        celsVal = Double.parseDouble(celsText);
        celsConv = 32 + (celsVal * 9 / 5);
        celsConv = Double.parseDouble(new DecimalFormat("##.#").format(celsConv));


    }
}

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问题不在于你在做什么，而在于你在何时何地做。你在打电话吗

try {// if is number
    Double.parseDouble(fahr.getText());
} catch (NumberFormatException e) {
    fahr.setText("0");
}

在创建GUI的代码中，在用户有机会将数据输入到JTextField之前，就会发生这种情况，这使得该代码毫无用处。更好的解决方案是将此代码放入一个侦听器中，当用户准备提交数据时，该侦听器会做出响应，例如在ActionListener开始时，然后调用return：

比如：

public void actionPerformed(ActionEvent event) {
    try {
        Double.parseDouble(fahr.getText());
    } catch (NumberFormatException e) {
        fahr.setText("0");
        // display a JOptionPane error message so the user knows there's been a problem
        return; // so the code below isn't called if input in error
    }
    String fahrText = fahr.getText();
    fahrVal = Double.parseDouble(fahrText);
    fahrConv = ((fahrVal - 32)*5)/9;
    fahrConv = Double.parseDouble(new DecimalFormat("##.#").format(fahrConv));

    String celsText = cels.getText();
    celsVal = Double.parseDouble(celsText);
    celsConv = 32 + (celsVal * 9 / 5);
    celsConv = Double.parseDouble(new DecimalFormat("##.#").format(celsConv));
}

其他选择：

使用JFormattedTextField 或者使用JSpinner 或者使用DocumentListener 其他无关问题：

不要将鼠标侦听器添加到JButtons，而是使用ActionListeners。否则，侦听器将无法正常工作。例如，如果按钮被禁用，MouseListener仍然会响应，而ActionListener在这种情况下不会正确响应。 避免空布局和绝对定位，因为这会导致难以增强和调试的僵化GUI，并且在一个平台上看起来还行，在所有其他平台上看起来都很糟糕。

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谢谢你，解决了这个问题，我们会考虑你的其他建议