Java Applet 错误 - IllegalArgumentException "adding container' s 自身的父级”

Question

我必须创建一个小程序。我写的代码如下。

import java.awt.event.*;

import java.awt.*;

import javax.swing.*;

public class feedback extends JApplet

    implements ActionListener
{
    private JTextField login;

    private JTextField email;
    private JTextField comments;
    private final String SUBMIT="SUBMIT";
    private final String CLEAR="CLEAR";
    public void actionPerformed(ActionEvent e)
    {
        String command = e.getActionCommand();
        if(CLEAR.equals(command))
            {login.setText(" ");
            email.setText(" ");
            comments.setText(" ");}
        else if(SUBMIT.equals(command))
           {
            login.setText(" ");
            email.setText(" ");
            comments.setText(" ");
           }
        }
     public void start()
    {
        Container contentPane = getContentPane();
        JScrollPane sPane = new JScrollPane();
        JPanel pContPanel = new JPanel();

        pContPanel.setLayout(new GridBagLayout());

        GridBagConstraints gbc = new GridBagConstraints(3, 4, 1, 1, 0, 0, GridBagConstraints.CENTER, GridBagConstraints.HORIZONTAL, new Insets(10,10,10,10), 20, 20);

        JLabel title = new JLabel("FEEDBACK");
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.gridwidth = 3;
        pContPanel.add(title, gbc);

        JPanel panel1 = new JPanel();
        JLabel prompt = new JLabel("LOGIN");
        panel1.add(prompt, gbc);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 1;
        gbc.gridwidth = 1;
        pContPanel.add(panel1, gbc); 

        login = new JTextField(15);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 1;
        gbc.gridwidth = 2;
        pContPanel.add(login, gbc);

        JPanel panel2=new JPanel();
        JLabel print = new JLabel("EMAIL");
        panel2.add(panel2);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 2;
        gbc.gridwidth = 1;
        pContPanel.add(panel2, gbc);

        email = new JTextField(30);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 2;
        gbc.gridwidth = 2;
        pContPanel.add(email, gbc);

        JPanel panel3=new JPanel();
        JLabel ask = new JLabel("COMMENTS");
        panel3.add(panel3);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 3;
        gbc.gridwidth = 1;
        pContPanel.add(panel3, gbc);

        comments = new JTextField(50);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 3;
        gbc.gridwidth = 2;
        pContPanel.add(comments, gbc);

        JButton bSUBMIT = new JButton(SUBMIT);
        bSUBMIT.addActionListener(this);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 4;
        gbc.gridwidth = 1;
        pContPanel.add(bSUBMIT, gbc);

        JButton bCLEAR = new JButton(CLEAR);
        bCLEAR.addActionListener(this);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 4;
        pContPanel.add(bCLEAR, gbc);


         sPane.setViewportView(pContPanel);
         contentPane.add(sPane, BorderLayout.CENTER);
        }
    }

正在编译它，没有语法错误。 但是，当我使用小程序查看器(在 BlueJ 中)运行它时，它说，

异常:java.lang.IllegalArgumentException:将容器的父容器添加到自身。

你能帮我找出我的代码中的错误在哪里以及我可以通过什么方式纠正它吗？ 谢谢。

Answer 1

您正在尝试向其自身添加一个面板，例如panel2.add(panel2);

正确的程序应该是:

import java.awt.event.*;

import java.awt.*;

import javax.swing.*;

public class Feedback extends JApplet implements ActionListener
{
    private JTextField login;

    private JTextField email;
    private JTextField comments;
    private final String SUBMIT="SUBMIT";
    private final String CLEAR="CLEAR";
    public void actionPerformed(ActionEvent e)
    {
        String command = e.getActionCommand();
        if(CLEAR.equals(command))
            {login.setText(" ");
            email.setText(" ");
            comments.setText(" ");}
        else if(SUBMIT.equals(command))
           {
            login.setText(" ");
            email.setText(" ");
            comments.setText(" ");
           }
        }
     public void start()
    {
        Container contentPane = getContentPane();
        JScrollPane sPane = new JScrollPane();
        JPanel pContPanel = new JPanel();

        pContPanel.setLayout(new GridBagLayout());

        GridBagConstraints gbc = new GridBagConstraints(3, 4, 1, 1, 0, 0, GridBagConstraints.CENTER, GridBagConstraints.HORIZONTAL, new Insets(10,10,10,10), 20, 20);

        JLabel title = new JLabel("FEEDBACK");
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.gridwidth = 3;
        pContPanel.add(title, gbc);

        JPanel panel1 = new JPanel();
        JLabel prompt = new JLabel("LOGIN");
        panel1.add(prompt, gbc);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 1;
        gbc.gridwidth = 1;
        pContPanel.add(panel1, gbc);

        login = new JTextField(15);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 1;
        gbc.gridwidth = 2;
        pContPanel.add(login, gbc);

        JPanel panel2=new JPanel();
        JLabel print = new JLabel("EMAIL");
        panel2.add(print);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 2;
        gbc.gridwidth = 1;
        pContPanel.add(panel2, gbc);

        email = new JTextField(30);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 2;
        gbc.gridwidth = 2;
        pContPanel.add(email, gbc);

        JPanel panel3=new JPanel();
        JLabel ask = new JLabel("COMMENTS");
        panel3.add(ask);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 3;
        gbc.gridwidth = 1;
        pContPanel.add(panel3, gbc);

        comments = new JTextField(50);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 3;
        gbc.gridwidth = 2;
        pContPanel.add(comments, gbc);

        JButton bSUBMIT = new JButton(SUBMIT);
        bSUBMIT.addActionListener(this);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 0;
        gbc.gridy = 4;
        gbc.gridwidth = 1;
        pContPanel.add(bSUBMIT, gbc);

        JButton bCLEAR = new JButton(CLEAR);
        bCLEAR.addActionListener(this);
        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.gridx = 1;
        gbc.gridy = 4;
        pContPanel.add(bCLEAR, gbc);


         sPane.setViewportView(pContPanel);
         contentPane.add(sPane, BorderLayout.CENTER);
        }
    }