如何在JFrame可见的情况下将组件添加到JFrame而无需调整大小？

import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

        JButton button1 = new JButton("Button 1");
        JButton button2 = new JButton("Button 2");

        panel.add(button1);
        panel.add(button2);

        frame.add(panel);
        frame.setVisible(true);
    }
}

import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        JButton button1 = new JButton("Button 1");
        JButton button2 = new JButton("Button 2");

        button1.setBounds(10, 10, 100, 30);
        button2.setBounds(10, 50, 100, 30);

        frame.add(button1);
        frame.add(button2);

        frame.setVisible(true);
    }
}