Java Swing BorderLayout布局疑难解析与组件显示策略

import javax.swing.*;
import java.awt.*;

public class ProductListGUI {

    JMenu menu;
    JMenuItem about, importData, inventory, export;

    ProductListGUI() {
        JFrame f = new JFrame("Assignment 2");

        // 1. 关键修正：在添加组件之前设置JFrame的布局管理器
        f.setLayout(new BorderLayout());

        // 面板1 (CENTER区域): 仍然使用null布局来展示绝对定位的灵活性
        JPanel panel1 = new JPanel();
        panel1.setLayout(null); // 明确使用null布局
        panel1.setBackground(new Color(230, 230, 230));
        panel1.setBorder(BorderFactory.createTitledBorder("Product Details"));

        JLabel l1, l2, l3, l4;
        JTextField t1, t2, t3;
        l1 = new JLabel("ProductID");
        l1.setBounds(10, 100, 200, 30);
        t1 = new JTextField();
        t1.setBounds(100, 100, 200, 30);
        l2 = new JLabel("Name");
        l2.setBounds(10, 150, 200, 30);
        t2 = new JTextField();
        t2.setBounds(100, 150, 200, 30);
        l3 = new JLabel("Quantity");
        l3.setBounds(10, 250, 200, 30);
        t3 = new JTextField();
        t3.setBounds(100, 250, 200, 30);

        JCheckBox checkBox = new JCheckBox("Available for Next Day Delivery");
        checkBox.setBounds(10, 300, 250, 50);

        l4 = new JLabel("Item Type");
        l4.setBounds(10, 200, 200, 30);
        String[] itemType = {"Select type", "Homeware", "Hobby", "Garden"};
        JComboBox<String> dropdown = new JComboBox<>(itemType);
        dropdown.setBounds(100, 200, 120, 20);

        panel1.add(t1);
        panel1.add(l1);
        panel1.add(t2);
        panel1.add(l2);
        panel1.add(t3);
        panel1.add(l3);
        panel1.add(l4);
        panel1.add(dropdown);
        panel1.add(checkBox);

        // JList和JScrollPane (EAST区域): 移除不必要的setBounds和setLayout(null)
        JList<String> list = new JList<>();
        JScrollPane scrollPane = new JScrollPane(list, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        // scrollPane 不需要 setLayout(null) 或 setBounds，BorderLayout 会处理其大小

        // 面板3 (SOUTH区域): 移除setLayout(null)，默认使用FlowLayout排列按钮
        JPanel panel3 = new JPanel();
        // panel3 默认使用 FlowLayout，无需手动设置
        panel3.setBackground(new Color(230, 230, 230));

        JButton b1 = new JButton("New Item");
        JButton b2 = new JButton("Save");
        JButton b3 = new JButton("Delete Selected");
        b3.setEnabled(false);

        // 2. 关键修正：移除按钮的setBounds()，让FlowLayout自动布局
        panel3.add(b1);
        panel3.add(b2);
        panel3.add(b3);

        // 菜单栏
        JMenuBar mb = new JMenuBar();
        menu = new JMenu("Actions");
        about = new JMenuItem("About");
        importData = new JMenuItem("Import Data");
        inventory = new JMenuItem("Inventory");
        export = new JMenuItem("Export to CSV");
        menu.add(about);
        menu.add(importData);
        menu.add(inventory);
        menu.add(export);
        mb.add(menu);

        // 添加组件到JFrame的内容窗格
        f.getContentPane().add(panel1, BorderLayout.CENTER);
        f.getContentPane().add(scrollPane, BorderLayout.EAST);
        f.getContentPane().add(panel3, BorderLayout.SOUTH);
        f.setJMenuBar(mb);

        f.setSize(800, 600);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // 添加关闭操作
        f.setVisible(true);
    }

    public static void main(String[] args) {
        // 在事件调度线程中创建和显示GUI
        SwingUtilities.invokeLater(ProductListGUI::new);
    }
}

import javax.swing.*;
import java.awt.*;

public class TestBorderLayout {

    public static void main(String[] args) {

        JFrame frame = new JFrame("BorderLayout Test Frame");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // 设置关闭操作

        frame.setLayout(new BorderLayout()); // 设置JFrame的布局管理器
        frame.setSize(800, 600);

        // 创建并添加不同颜色的JPanel到BorderLayout的各个区域
        frame.getContentPane().add(createColoredPanel("CENTER", Color.RED), BorderLayout.CENTER);
        frame.getContentPane().add(createColoredPanel("NORTH", Color.CYAN), BorderLayout.NORTH);
        frame.getContentPane().add(createColoredPanel("EAST", Color.LIGHT_GRAY), BorderLayout.EAST);
        frame.getContentPane().add(createColoredPanel("SOUTH", Color.GREEN), BorderLayout.SOUTH);
        frame.getContentPane().add(createColoredPanel("WEST", Color.YELLOW), BorderLayout.WEST);

        frame.setVisible(true);
    }

    /**
     * 创建一个带有标题和背景色的JPanel
     * @param title 面板标题
     * @param color 背景颜色
     * @return 配置好的JPanel
     */
    private static JPanel createColoredPanel(String title, Color color) {
        JPanel jPanel = new JPanel();
        jPanel.setLayout(new BorderLayout()); // JPanel内部也使用BorderLayout来放置JLabel
        JLabel label = new JLabel(title, SwingConstants.CENTER); // 标签居中显示
        label.setFont(new Font("Arial", Font.BOLD, 24));
        jPanel.add(label, BorderLayout.CENTER); // 将标签添加到JPanel的CENTER区域，使其填充
        jPanel.setBackground(color);
        // 添加边框以便更好地观察区域划分
        jPanel.setBorder(BorderFactory.createLineBorder(Color.BLACK, 2));
        return jPanel;
    }
}