关于 Java swing Box 的使用问题

代码

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

public class C5Ex1_2 {
    final static int WIDTH = 400;
    final static int HEIGHT = 400;
    public C5Ex1_2() {
        JFrame jf = new JFrame("program 1");
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setSize(WIDTH, HEIGHT);
        JPanel panel = new JPanel();
        jf.setContentPane(panel);
        panel.setLayout(new BorderLayout());

        Box vbox = Box.createVerticalBox();
        // panel.setLayout(vbox);
        panel.add(vbox, BorderLayout.CENTER);
        Box hBox=Box.createHorizontalBox();
        hBox.setSize(WIDTH, 20);
        JTextField tf_out = new JTextField(15);
        tf_out.setMaximumSize(tf_out.getPreferredSize());
        hBox.add(tf_out);
        vbox.add(hBox);
        String[] names  = {
            "1", "2", "3", "%", "ON",
            "4", "5", "6", "=", "AC",
            "7", "8", "9", ".", "+/-",
            "0", "+", "-", "*", "C"
        };
       hBox = null;
        for(int i=0;i<names.length;i++) {
            if(i%5==0) {
                // if(hBox != null) {
                //     hBox.setMaximumSize(hBox.getPreferredSize());
                //     System.out.println("size:" + hBox.getSize() + ":i=" + i);

                // }

                
                System.out.println("i="+i);
                hBox = Box.createHorizontalBox();
                Dimension size = new Dimension(400, 40);
                hBox.setPreferredSize(size);
                vbox.setBackground(Color.red);
                vbox.add(hBox);
            }
            JButton btn = new JButton(names[i]);
            Dimension preferSize = new Dimension(50, 40);
            btn.setPreferredSize(preferSize);
            // btn.setSize(50, 40);
            hBox.add(btn);
            System.out.println("hBox" + hBox);
        }

        jf.setVisible(true);
    }
    public static void main(String[] args) {
        new C5Ex1_2();
    }
}

显示结果

为什么感觉 Box 越来越小了？还是 btn 设置大小不起做用？