最近我正在使用Java编写一个邮件系统客户端（我选择了swing来编写GUI并使用IDEA来硬编码我的GUI）。在撰写模块中，当我单击相应的按钮时，我想显示或隐藏CC和密件抄送的文本字段。

所以我在网上搜索并浏览了以下问题和文档：

• 如何使JPanel在Java中滚动？
• 如何使JPanel可滚动？
• 滚动JPanel
• Doc：JScrollPane

最后，我选择了JScrollPane来实现它。

我的简化示例代码如下（原始代码繁琐）：

import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public class Demo extends JFrame implements ActionListener {
    private static final long serialVersionUID = 1L;

    private JLabel lbl1;
    private JTextField txf1;

    private JLabel lbl2;
    private JTextField txf2;
    // container for lbl2 and txf2, which should be able to be shown or hidden
    private JPanel pnlContainer2;

    private JLabel lbl3;
    private JTextField txf3;
    // container for lbl3 and txf3
    private JPanel pnlContainer3; 

    private JButton btnShow;

    // the container I want to move when I click btnShow
    private JPanel pnlBody; 

    // the panel to hold my "cards"
    // In this example, I include it just to show what controls are on my interface.
    private JPanel pnlContent;

    private JPanel pnlContainer;

    // here, I want to use JScrollPane to make my pnlContainer scrollable
    // to adapt to my interface
    private JScrollPane scrollPane;

    public Demo() {
        init();
    }

    private void init() {
        pnlContainer = new JPanel(new CardLayout(), true);
        pnlContainer.setBounds(0, 0, 200, 180);

        pnlContent = new JPanel(null, true);
        pnlContent.setBounds(0, 0, 200, 180 + 50);

        scrollPane = new JScrollPane(pnlContent, ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
                ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        scrollPane.setBounds(0, 0, 200, 180);
        pnlContainer.add(scrollPane);

        pnlBody = new JPanel(null, true);

        lbl1 = new JLabel("lbl1");
        lbl1.setBounds(10, 20, 40, 30);

        txf1 = new JTextField();
        txf1.setBounds(60, 20, 120, 30);

        pnlContent.add(lbl1);
        pnlContent.add(txf1);

        pnlContainer2 = new JPanel(null, true);
        pnlContainer2.setBounds(0, 70, 180, 30);

        lbl2 = new JLabel("lbl2");
        lbl2.setBounds(10, 0, 40, 30);

        txf2 = new JTextField();
        txf2.setBounds(60, 0, 120, 30);

        pnlContainer2.add(lbl2);
        pnlContainer2.add(txf2);
        pnlContainer2.setVisible(false);
        pnlContent.add(pnlContainer2);

        pnlBody = new JPanel(null, true);
        pnlBody.setBounds(0, 70, 180, 90);

        pnlContainer3 = new JPanel(null, true);
        pnlContainer3.setBounds(0, 0, 180, 30);
        pnlBody.add(pnlContainer3);

        lbl3 = new JLabel("lbl3");
        lbl3.setBounds(10, 0, 40, 30);

        txf3 = new JTextField();
        txf3.setBounds(60, 0, 120, 30);

        pnlContainer3.add(lbl3);
        pnlContainer3.add(txf3);

        btnShow = new JButton("show");
        btnShow.setBounds(60, 50, 80, 30);
        btnShow.addActionListener(this);
        pnlBody.add(btnShow);

        pnlContent.add(pnlBody);

        this.add(pnlContainer);
        this.setLayout(null);
        this.setTitle("Demo");
        this.setSize(200, 200);
        this.setLocationRelativeTo(null);
        this.setVisible(true);
        this.setResizable(false);
//      ImageIcon icon = new ImageIcon("E:\\Javarepo\\Hmail\\src\\main\\resources\\assets\\hmail.png");
//      this.setIconImage(icon.getImage());
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub
        Object src = e.getSource();
        if (src instanceof JButton) {
            JButton btn = (JButton) src;
            boolean showSelected = false;
            String altText;
            if (btn == btnShow) {
                showSelected = btnShow.getText() == "show";
                altText = showSelected ? "hide" : "show";
                btnShow.setText(altText);
            }
            relayout(showSelected);
        }
    }

    private void relayout(boolean showSelected) {
        int x = pnlBody.getX();
        int y = pnlBody.getY();
        if (showSelected) {
            pnlContainer2.setVisible(true);
            pnlBody.setBounds(x, y + 50, 180, 90);
        } else {
            pnlContainer2.setVisible(false);
            pnlBody.setBounds(x, y - 50, 180, 90);
        }

    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new Demo());
    }

}

但是，无论我将JScrollPane应用到哪个JPanel，我都无法使我的界面适应我的JContainer2的隐藏和显示。
我如何修改它，或者使用什么控件来替换JScrollPane？欢迎任何建议。

这是我的平台信息：

java-version
java版本"1.8.0_212"Java（TM）SE运行时环境（build 1.8.0_212-b10）
JavaHotSpot（TM）64位服务器VM（build 25.212-b10，混合模式）

OS： win10 1909
arch：amd64