尝试仅允许将数字输入JTextField，将非数字设置为0

Question

有麻烦的人这是我的华氏温度转换器类，我似乎无法让我的代码识别以下，这确保只有数字输入文本字段，如果没有，设置为0：

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

我认为这是因为它在我班级中的位置。请查看我的全班（我把它放在代码的底部）：

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));


    }
}