java - JTextField 已初始化但似乎为 null

Question

我不明白我的代码中发生了什么，当我触发带有附加操作监听器的按钮时，读取 JTextField getText() 值显示为 null，即使所有字段都包含文本。此外，当我调试代码并在该行之前停止时，JTextField 对象也显示为 null，就像它从未初始化过一样。

我不确定是否可以将所有这些 JLabel 和 JTextField 保留为类成员，然后随意读取它们。

public class EditPartGUI extends JFrame {
    private JLabel manufacturerLabel;
    private JTextField manufacturerTextField;

    private JButton submit;
    private ActionListener submitListener;

    public EditPartGUI(Part part) {
        JPanel panel = new JPanel();
        this.setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));

        panel.add(initialiseField("Manufacturer: ", manufacturerLabel, part.getManufacturer(), manufacturerTextField));

        JPanel sub = new JPanel();
        submit = new JButton("Submit");
        submitListener = new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.out.println(submit().toString());
            }
        };
        submit.addActionListener(submitListener);

        sub.add(submit);
        panel.add(sub);

        this.add(panel);
    }

    public JPanel initialiseField(String label, JLabel contentLabel, String value, JTextField contentTextField) {
        JPanel contentPanel = new JPanel();
        contentLabel = new JLabel(label, JLabel.TRAILING);

        contentTextField = new JTextField(10);
        contentTextField.setText(value);
        contentLabel.setLabelFor(contentTextField);

        contentPanel.add(contentLabel);
        contentPanel.add(contentTextField);
        return contentPanel;
    }

    public Part submit() {
        Part p = new Part();
        p.setManufacturer(this.manufacturerTextField.getText()); // <---- this is where NullPointerException shows
        return p;
    }
}

Answer 1

I'm not sure whether I can keep all these JLabel and JTextField as class members and then just freely read from them.

是的，您可以，这就是您问题的解决方案。

只需使用以下内容即可

//private JTextField manufacturerTextField;
private JTextField manufacturerTextField = new JTextField();

并且不要尝试在您的initialiseField() 方法中创建文本字段。当然，您需要对标签执行相同的操作。

so I could avoid repeating the same code for each field (there are a lot more of them in my actual code).

如果您想要许多带有这些字段的面板，那么您需要创建一个自定义类来创建面板，然后文本字段和标签将成为该类的一部分，而不是您的主类。